Bump version numbers for 1.4.12 release.

Add Snowflake pluggable transport

See merge request briar/briar!1714

.gitlab-ci.yml

@@ -98,7 +98,7 @@ bridge test:
       allow_failure: true
   script:
     - OPTIONAL_TESTS=org.briarproject.bramble.plugin.tor.BridgeTest ./gradlew --info bramble-java:test --tests BridgeTest
-  timeout: 3h
+  timeout: 4h
 
 mailbox integration test:
   extends: .optional_tests
@@ -118,11 +118,3 @@ mailbox integration test:
     - cd "$CI_PROJECT_DIR"
     - bramble-core/src/test/bash/wait-for-mailbox.sh
     - OPTIONAL_TESTS=org.briarproject.bramble.mailbox.MailboxIntegrationTest ./gradlew --info bramble-core:test --tests MailboxIntegrationTest
-
-pre_release_tests:
-  extends: .optional_tests
-  script:
-    - OPTIONAL_TESTS=org.briarproject.bramble.plugin.tor.BridgeTest ./gradlew --info bramble-java:test --tests BridgeTest
-  timeout: 3h
-  only:
-    - tags

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+Folder-Description:
+===================
+* `briar-*`: Specifically for the Briar app (Phone/Desktop/Headless)
+* `bramble-*`: The protocol stack - not necessarily Briar-dependent
+
+---
+
+* `*-api`: public stuff that can be referenced from other packages and modules - mostly interfaces + a few utility classes
+* `*-core`: implementations of api that are portable across Android/Desktop/Headless
+* `*-java`: implementations of api that are specific to Desktop & Headless

bramble-android/build.gradle

@@ -15,8 +15,8 @@ android {
 	defaultConfig {
 		minSdkVersion 16
 		targetSdkVersion 30
-		versionCode 10408
+		versionCode 10412
-		versionName "1.4.8"
+		versionName "1.4.12"
 		consumerProguardFiles 'proguard-rules.txt'
 
 		testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
@@ -42,8 +42,10 @@ configurations {
 
 dependencies {
 	implementation project(path: ':bramble-core', configuration: 'default')
+	implementation 'androidx.annotation:annotation:1.5.0'
 	tor "org.briarproject:tor-android:$tor_version"
 	tor "org.briarproject:obfs4proxy-android:$obfs4proxy_version"
+	tor "org.briarproject:snowflake-android:$snowflake_version"
 
 	annotationProcessor "com.google.dagger:dagger-compiler:$dagger_version"
 

bramble-android/proguard-rules.txt

@@ -18,3 +18,7 @@
 -dontnote com.google.common.**
 
 -dontwarn com.fasterxml.jackson.databind.ext.Java7SupportImpl
+
+# Keep all Jackson-serialisable classes and their members
+-keep interface com.fasterxml.jackson.databind.annotation.JsonSerialize
+-keep @com.fasterxml.jackson.databind.annotation.JsonSerialize class * { *; }

bramble-android/src/main/java/org/briarproject/bramble/BrambleAndroidModule.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble;
 
 import org.briarproject.bramble.battery.AndroidBatteryModule;
+import org.briarproject.bramble.io.DnsModule;
 import org.briarproject.bramble.network.AndroidNetworkModule;
 import org.briarproject.bramble.plugin.tor.CircumventionModule;
 import org.briarproject.bramble.reporting.ReportingModule;
@@ -18,6 +19,7 @@ import dagger.Module;
 		AndroidTaskSchedulerModule.class,
 		AndroidWakefulIoExecutorModule.class,
 		CircumventionModule.class,
+		DnsModule.class,
 		ReportingModule.class,
 		SocksModule.class
 })

bramble-android/src/main/java/org/briarproject/bramble/plugin/tor/AndroidTorPlugin.java

@@ -31,6 +31,8 @@ import java.util.zip.ZipInputStream;
 
 import javax.net.SocketFactory;
 
+import androidx.annotation.ChecksSdkIntAtLeast;
+
 import static android.os.Build.VERSION.SDK_INT;
 import static java.util.Arrays.asList;
 import static java.util.logging.Level.INFO;
@@ -45,13 +47,14 @@ class AndroidTorPlugin extends TorPlugin {
 
 	private static final String TOR_LIB_NAME = "libtor.so";
 	private static final String OBFS4_LIB_NAME = "libobfs4proxy.so";
+	private static final String SNOWFLAKE_LIB_NAME = "libsnowflake.so";
 
 	private static final Logger LOG =
 			getLogger(AndroidTorPlugin.class.getName());
 
 	private final Application app;
 	private final AndroidWakeLock wakeLock;
-	private final File torLib, obfs4Lib;
+	private final File torLib, obfs4Lib, snowflakeLib;
 
 	AndroidTorPlugin(Executor ioExecutor,
 			Executor wakefulIoExecutor,
@@ -83,6 +86,7 @@ class AndroidTorPlugin extends TorPlugin {
 		String nativeLibDir = app.getApplicationInfo().nativeLibraryDir;
 		torLib = new File(nativeLibDir, TOR_LIB_NAME);
 		obfs4Lib = new File(nativeLibDir, OBFS4_LIB_NAME);
+		snowflakeLib = new File(nativeLibDir, SNOWFLAKE_LIB_NAME);
 	}
 
 	@Override
@@ -108,6 +112,12 @@ class AndroidTorPlugin extends TorPlugin {
 		if (!enable) wakeLock.release();
 	}
 
+	@Override
+	@ChecksSdkIntAtLeast(api = 25)
+	protected boolean canVerifyLetsEncryptCerts() {
+		return SDK_INT >= 25;
+	}
+
 	@Override
 	public void stop() {
 		super.stop();
@@ -124,39 +134,43 @@ class AndroidTorPlugin extends TorPlugin {
 		return obfs4Lib.exists() ? obfs4Lib : super.getObfs4ExecutableFile();
 	}
 
+	@Override
+	protected File getSnowflakeExecutableFile() {
+		return snowflakeLib.exists()
+				? snowflakeLib : super.getSnowflakeExecutableFile();
+	}
+
 	@Override
 	protected void installTorExecutable() throws IOException {
-		File extracted = super.getTorExecutableFile();
+		installExecutable(super.getTorExecutableFile(), torLib, TOR_LIB_NAME);
-		if (torLib.exists()) {
-			// If an older version left behind a Tor binary, delete it
-			if (extracted.exists()) {
-				if (extracted.delete()) LOG.info("Deleted Tor binary");
-				else LOG.info("Failed to delete Tor binary");
-			}
-		} else if (SDK_INT < 29) {
-			// The binary wasn't extracted at install time. Try to extract it
-			extractLibraryFromApk(TOR_LIB_NAME, extracted);
-		} else {
-			// No point extracting the binary, we won't be allowed to execute it
-			throw new FileNotFoundException(torLib.getAbsolutePath());
-		}
 	}
 
 	@Override
 	protected void installObfs4Executable() throws IOException {
-		File extracted = super.getObfs4ExecutableFile();
+		installExecutable(super.getObfs4ExecutableFile(), obfs4Lib,
-		if (obfs4Lib.exists()) {
+				OBFS4_LIB_NAME);
-			// If an older version left behind an obfs4 binary, delete it
+	}
+
+	@Override
+	protected void installSnowflakeExecutable() throws IOException {
+		installExecutable(super.getSnowflakeExecutableFile(), snowflakeLib,
+				SNOWFLAKE_LIB_NAME);
+	}
+
+	private void installExecutable(File extracted, File lib, String libName)
+			throws IOException {
+		if (lib.exists()) {
+			// If an older version left behind a binary, delete it
 			if (extracted.exists()) {
-				if (extracted.delete()) LOG.info("Deleted obfs4 binary");
+				if (extracted.delete()) LOG.info("Deleted old binary");
-				else LOG.info("Failed to delete obfs4 binary");
+				else LOG.info("Failed to delete old binary");
 			}
 		} else if (SDK_INT < 29) {
 			// The binary wasn't extracted at install time. Try to extract it
-			extractLibraryFromApk(OBFS4_LIB_NAME, extracted);
+			extractLibraryFromApk(libName, extracted);
 		} else {
 			// No point extracting the binary, we won't be allowed to execute it
-			throw new FileNotFoundException(obfs4Lib.getAbsolutePath());
+			throw new FileNotFoundException(lib.getAbsolutePath());
 		}
 	}
 

bramble-android/witness.gradle

@@ -1,5 +1,6 @@
 dependencyVerification {
     verify = [
+        'androidx.annotation:annotation:1.5.0:annotation-1.5.0.jar:261fb7c0210858500bab66d34354972a75166ab4182add283780b05513d6ec4a',
         'cglib:cglib:3.2.8:cglib-3.2.8.jar:3f64de999ecc5595dc84ca8ff0879d8a34c8623f9ef3c517a53ed59023fcb9db',
         'com.android.tools.analytics-library:protos:30.0.3:protos-30.0.3.jar:f62b89dcd9de719c6a7b7e15fb1dd20e45b57222e675cf633607bd0ed6bca7e7',
         'com.android.tools.analytics-library:shared:30.0.3:shared-30.0.3.jar:05aa9ba3cc890354108521fdf99802565aae5dd6ca44a6ac8bb8d594d1c1cd15',
@@ -87,8 +88,9 @@ dependencyVerification {
         'org.apache.httpcomponents:httpmime:4.5.6:httpmime-4.5.6.jar:0b2b1102c18d3c7e05a77214b9b7501a6f6056174ae5604e0e256776eda7553e',
         'org.bouncycastle:bcpkix-jdk15on:1.56:bcpkix-jdk15on-1.56.jar:7043dee4e9e7175e93e0b36f45b1ec1ecb893c5f755667e8b916eb8dd201c6ca',
         'org.bouncycastle:bcprov-jdk15on:1.56:bcprov-jdk15on-1.56.jar:963e1ee14f808ffb99897d848ddcdb28fa91ddda867eb18d303e82728f878349',
-        'org.briarproject:obfs4proxy-android:0.0.12:obfs4proxy-android-0.0.12.jar:84159d2a4668abc40e3fccaa1f6fa0c04892863f9eb80a866ac8928d9f9a7e89',
+        'org.briarproject:obfs4proxy-android:0.0.14:obfs4proxy-android-0.0.14.jar:ad9b1ee4757b05867a19e993147bbb018bddd1f26ce3da746d5f037d5991a8c8',
-        'org.briarproject:tor-android:0.4.5.12-2:tor-android-0.4.5.12-2.jar:8545dbcef2bb6aa89c32bb6f8ac51f7a64bce3ae85845b3578ffdeb9b206feb9',
+        'org.briarproject:snowflake-android:2.3.1:snowflake-android-2.3.1.jar:1f83c9a070f87b7074af13627709a8b5aced5460104be7166af736b1bb73c293',
+        'org.briarproject:tor-android:0.4.5.14:tor-android-0.4.5.14.jar:7cf1beaa6c1db51fc8fac263aba9624ef289c3db29772509efcbc59f7057330a',
         'org.checkerframework:checker-compat-qual:2.5.3:checker-compat-qual-2.5.3.jar:d76b9afea61c7c082908023f0cbc1427fab9abd2df915c8b8a3e7a509bccbc6d',
         'org.checkerframework:checker-qual:2.5.2:checker-qual-2.5.2.jar:64b02691c8b9d4e7700f8ee2e742dce7ea2c6e81e662b7522c9ee3bf568c040a',
         'org.checkerframework:checker-qual:3.5.0:checker-qual-3.5.0.jar:729990b3f18a95606fc2573836b6958bcdb44cb52bfbd1b7aa9c339cff35a5a4',
@@ -129,10 +131,12 @@ dependencyVerification {
         'org.jetbrains.kotlin:kotlin-reflect:1.4.32:kotlin-reflect-1.4.32.jar:dbf19e9cdaa9c3c170f3f6f6ce3922f38dfc1d7fa1cab5b7c23a19da8b5eec5b',
         'org.jetbrains.kotlin:kotlin-stdlib-common:1.4.20:kotlin-stdlib-common-1.4.20.jar:a7112c9b3cefee418286c9c9372f7af992bd1e6e030691d52f60cb36dbec8320',
         'org.jetbrains.kotlin:kotlin-stdlib-common:1.4.32:kotlin-stdlib-common-1.4.32.jar:e1ff6f55ee9e7591dcc633f7757bac25a7edb1cc7f738b37ec652f10f66a4145',
+        'org.jetbrains.kotlin:kotlin-stdlib-common:1.7.10:kotlin-stdlib-common-1.7.10.jar:19f102efe9629f8eabc63853ad15c533e47c47f91fca09285c5bde86e59f91d4',
         'org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.4.32:kotlin-stdlib-jdk7-1.4.32.jar:5f801e75ca27d8791c14b07943c608da27620d910a8093022af57f543d5d98b6',
         'org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.4.32:kotlin-stdlib-jdk8-1.4.32.jar:adc43e54757b106e0cd7b3b7aa257dff471b61efdabe067fc02b2f57e2396262',
         'org.jetbrains.kotlin:kotlin-stdlib:1.4.20:kotlin-stdlib-1.4.20.jar:b8ab1da5cdc89cb084d41e1f28f20a42bd431538642a5741c52bbfae3fa3e656',
         'org.jetbrains.kotlin:kotlin-stdlib:1.4.32:kotlin-stdlib-1.4.32.jar:13e9fd3e69dc7230ce0fc873a92a4e5d521d179bcf1bef75a6705baac3bfecba',
+        'org.jetbrains.kotlin:kotlin-stdlib:1.7.10:kotlin-stdlib-1.7.10.jar:e771fe74250a943e8f6346713201ff1d8cb95c3a5d1a91a22b65a9e04f6a8901',
         'org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.1.0:kotlinx-metadata-jvm-0.1.0.jar:9753bb39efef35957c5c15df9a3cb769aabf2cdfa74b47afcb7760e5146be3b5',
         'org.jetbrains:annotations:13.0:annotations-13.0.jar:ace2a10dc8e2d5fd34925ecac03e4988b2c0f851650c94b8cef49ba1bd111478',
         'org.jmock:jmock-imposters:2.12.0:jmock-imposters-2.12.0.jar:3b836269745a137c9b2347e8d7c2104845b126ef04f012d6bfd94f1a7dea7b09',

bramble-api/src/main/java/org/briarproject/bramble/api/connection/ConnectionManager.java

@@ -7,6 +7,7 @@ import org.briarproject.bramble.api.plugin.TransportConnectionReader;
 import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 
 @NotNullByDefault
 public interface ConnectionManager {
@@ -16,6 +17,17 @@ public interface ConnectionManager {
 	 */
 	void manageIncomingConnection(TransportId t, TransportConnectionReader r);
 
+	/**
+	 * Manages an incoming connection from a contact via a mailbox.
+	 * <p>
+	 * This method does not mark the tag as recognised until after the data
+	 * has been read from the {@link TransportConnectionReader}, at which
+	 * point the {@link TagController} is called to decide whether the tag
+	 * should be marked as recognised.
+	 */
+	void manageIncomingConnection(TransportId t, TransportConnectionReader r,
+			TagController c);
+
 	/**
 	 * Manages an incoming connection from a contact over a duplex transport.
 	 */
@@ -34,6 +46,14 @@ public interface ConnectionManager {
 	void manageOutgoingConnection(ContactId c, TransportId t,
 			TransportConnectionWriter w);
 
+	/**
+	 * Manages an outgoing connection to a contact via a mailbox. The IDs of
+	 * any messages sent or acked are added to the given
+	 * {@link OutgoingSessionRecord}.
+	 */
+	void manageOutgoingConnection(ContactId c, TransportId t,
+			TransportConnectionWriter w, OutgoingSessionRecord sessionRecord);
+
 	/**
 	 * Manages an outgoing connection to a contact over a duplex transport.
 	 */
@@ -46,4 +66,21 @@ public interface ConnectionManager {
 	 */
 	void manageOutgoingConnection(PendingContactId p, TransportId t,
 			DuplexTransportConnection d);
+
+	/**
+	 * An interface for controlling whether a tag should be marked as
+	 * recognised.
+	 */
+	interface TagController {
+		/**
+		 * This method is only called if a tag was read from the corresponding
+		 * {@link TransportConnectionReader} and recognised.
+		 *
+		 * @param exception True if an exception was thrown while reading from
+		 * the {@link TransportConnectionReader}, after successfully reading
+		 * and recognising the tag.
+		 * @return True if the tag should be marked as recognised.
+		 */
+		boolean shouldMarkTagAsRecognised(boolean exception);
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -126,16 +126,11 @@ public interface DatabaseComponent extends TransactionManager {
 			TransportKeys k) throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
+	 * Returns true if there are any acks to send to the given contact.
-	 * contact over a transport with the given maximum latency.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(Transaction txn, ContactId c,
+	boolean containsAcksToSend(Transaction txn, ContactId c) throws DbException;
-			long maxLatency, boolean eager) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -161,6 +156,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	boolean containsIdentity(Transaction txn, AuthorId a) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given contact
+	 * over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(Transaction txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -276,6 +283,13 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	Group getGroup(Transaction txn, GroupId g) throws DbException;
 
+	/**
+	 * Returns the ID of the group containing the given message.
+	 * <p/>
+	 * Read-only.
+	 */
+	GroupId getGroupId(Transaction txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for the given group.
 	 * <p/>
@@ -342,13 +356,13 @@ public interface DatabaseComponent extends TransactionManager {
 			Metadata query) throws DbException;
 
 	/**
-	 * Returns the IDs of some messages received from the given contact that
+	 * Returns the IDs of all messages received from the given contact that
-	 * need to be acknowledged, up to the given number of messages.
+	 * need to be acknowledged.
 	 * <p/>
 	 * Read-only.
 	 */
-	Collection<MessageId> getMessagesToAck(Transaction txn, ContactId c,
+	Collection<MessageId> getMessagesToAck(Transaction txn, ContactId c)
-			int maxMessages) throws DbException;
+			throws DbException;
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
@@ -485,6 +499,8 @@ public interface DatabaseComponent extends TransactionManager {
 	 * Returns the message with the given ID for transmission to the given
 	 * contact over a transport with the given maximum latency. Returns null
 	 * if the message is no longer visible to the contact.
+	 * <p/>
+	 * Read-only if {@code markAsSent} is false.
 	 *
 	 * @param markAsSent True if the message should be marked as sent.
 	 * If false it can be marked as sent by calling
@@ -534,15 +550,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	long getNextCleanupDeadline(Transaction txn) throws DbException;
 
-	/*
+	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
+	 * message is due to be sent to the given contact over a transport with
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE if
+	 * the given latency.
-	 * no messages are scheduled to be sent.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(Transaction txn, ContactId c) throws DbException;
+	long getNextSendTime(Transaction txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.

bramble-api/src/main/java/org/briarproject/bramble/api/lifecycle/LifecycleManager.java

@@ -37,8 +37,14 @@ public interface LifecycleManager {
 	 */
 	enum LifecycleState {
 
-		STARTING, MIGRATING_DATABASE, COMPACTING_DATABASE, STARTING_SERVICES,
+		CREATED,
-		RUNNING, STOPPING;
+		STARTING,
+		MIGRATING_DATABASE,
+		COMPACTING_DATABASE,
+		STARTING_SERVICES,
+		RUNNING,
+		STOPPING,
+		STOPPED;
 
 		public boolean isAfter(LifecycleState state) {
 			return ordinal() > state.ordinal();

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxConstants.java

@@ -1,5 +1,11 @@
 package org.briarproject.bramble.api.mailbox;
 
+import org.briarproject.bramble.api.plugin.TransportId;
+
+import java.util.List;
+
+import static java.util.Collections.singletonList;
+import static java.util.concurrent.TimeUnit.DAYS;
 import static java.util.concurrent.TimeUnit.HOURS;
 import static org.briarproject.bramble.api.transport.TransportConstants.MAX_FRAME_LENGTH;
 import static org.briarproject.bramble.api.transport.TransportConstants.MAX_PAYLOAD_LENGTH;
@@ -8,6 +14,32 @@ import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENG
 
 public interface MailboxConstants {
 
+	/**
+	 * The transport ID of the mailbox plugin.
+	 */
+	TransportId ID = new TransportId("org.briarproject.bramble.mailbox");
+
+	/**
+	 * Mailbox API versions that we support as a client. This is reported to our
+	 * contacts by {@link MailboxUpdateManager}.
+	 */
+	List<MailboxVersion> CLIENT_SUPPORTS = singletonList(
+			new MailboxVersion(1, 0));
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when the server is too old to support our major version.
+	 */
+	int API_SERVER_TOO_OLD = -1;
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when we as a client are too old to support the server's major version.
+	 */
+	int API_CLIENT_TOO_OLD = -2;
+
 	/**
 	 * The maximum length of a file that can be uploaded to or downloaded from
 	 * a mailbox.
@@ -34,4 +66,8 @@ public interface MailboxConstants {
 	 */
 	long PROBLEM_MS_SINCE_LAST_SUCCESS = HOURS.toMillis(1);
 
+	/**
+	 * The maximum latency of the mailbox transport in milliseconds.
+	 */
+	long MAX_LATENCY = DAYS.toMillis(14);
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxDirectory.java

@@ -0,0 +1,22 @@
+package org.briarproject.bramble.api.mailbox;
+
+import java.io.File;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import javax.inject.Qualifier;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * Annotation for injecting the {@link File directory} where the Mailbox plugin
+ * should store its state.
+ */
+@Qualifier
+@Target({FIELD, METHOD, PARAMETER})
+@Retention(RUNTIME)
+public @interface MailboxDirectory {
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxHelper.java

@@ -0,0 +1,47 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+import java.util.TreeSet;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_CLIENT_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_SERVER_TOO_OLD;
+
+@NotNullByDefault
+public class MailboxHelper {
+
+	/**
+	 * Returns the highest major version that both client and server support
+	 * or {@link MailboxConstants#API_SERVER_TOO_OLD} if the server is too old
+	 * or {@link MailboxConstants#API_CLIENT_TOO_OLD} if the client is too old.
+	 */
+	public static int getHighestCommonMajorVersion(
+			List<MailboxVersion> client, List<MailboxVersion> server) {
+		TreeSet<Integer> clientVersions = new TreeSet<>();
+		for (MailboxVersion version : client) {
+			clientVersions.add(version.getMajor());
+		}
+		TreeSet<Integer> serverVersions = new TreeSet<>();
+		for (MailboxVersion version : server) {
+			serverVersions.add(version.getMajor());
+		}
+		for (int clientVersion : clientVersions.descendingSet()) {
+			if (serverVersions.contains(clientVersion)) return clientVersion;
+		}
+		if (clientVersions.last() < serverVersions.last()) {
+			return API_CLIENT_TOO_OLD;
+		}
+		return API_SERVER_TOO_OLD;
+	}
+
+	/**
+	 * Returns true if a client and server with the given API versions can
+	 * communicate with each other (ie, have any major API versions in common).
+	 */
+	public static boolean isClientCompatibleWithServer(
+			List<MailboxVersion> client, List<MailboxVersion> server) {
+		int common = getHighestCommonMajorVersion(client, server);
+		return common != API_CLIENT_TOO_OLD && common != API_SERVER_TOO_OLD;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxId.java

@@ -2,6 +2,7 @@ package org.briarproject.bramble.api.mailbox;
 
 import com.fasterxml.jackson.annotation.JsonValue;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -32,7 +33,7 @@ public abstract class MailboxId extends UniqueId {
 		}
 		try {
 			return fromHexString(token);
-		} catch (IllegalArgumentException e) {
+		} catch (FormatException e) {
 			throw new InvalidMailboxIdException();
 		}
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxProperties.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.NullSafety;
 
 import java.util.List;
 
@@ -11,7 +12,7 @@ import javax.annotation.concurrent.Immutable;
 @NotNullByDefault
 public class MailboxProperties {
 
-	private final String baseUrl;
+	private final String onion;
 	private final MailboxAuthToken authToken;
 	private final boolean owner;
 	private final List<MailboxVersion> serverSupports;
@@ -23,9 +24,9 @@ public class MailboxProperties {
 	/**
 	 * Constructor for properties used by the mailbox's owner.
 	 */
-	public MailboxProperties(String baseUrl, MailboxAuthToken authToken,
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
 			List<MailboxVersion> serverSupports) {
-		this.baseUrl = baseUrl;
+		this.onion = onion;
 		this.authToken = authToken;
 		this.owner = true;
 		this.serverSupports = serverSupports;
@@ -36,10 +37,10 @@ public class MailboxProperties {
 	/**
 	 * Constructor for properties used by a contact of the mailbox's owner.
 	 */
-	public MailboxProperties(String baseUrl, MailboxAuthToken authToken,
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
 			List<MailboxVersion> serverSupports, MailboxFolderId inboxId,
 			MailboxFolderId outboxId) {
-		this.baseUrl = baseUrl;
+		this.onion = onion;
 		this.authToken = authToken;
 		this.owner = false;
 		this.serverSupports = serverSupports;
@@ -47,13 +48,11 @@ public class MailboxProperties {
 		this.outboxId = outboxId;
 	}
 
-	public String getBaseUrl() {
+	/**
-		return baseUrl;
+	 * Returns the onion address of the mailbox, excluding the .onion suffix.
-	}
+	 */
-
 	public String getOnion() {
-		return baseUrl.replaceFirst("^http://", "")
+		return onion;
-				.replaceFirst("\\.onion$", "");
 	}
 
 	public MailboxAuthToken getAuthToken() {
@@ -77,4 +76,23 @@ public class MailboxProperties {
 	public MailboxFolderId getOutboxId() {
 		return outboxId;
 	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o instanceof MailboxProperties) {
+			MailboxProperties m = (MailboxProperties) o;
+			return owner == m.owner &&
+					onion.equals(m.onion) &&
+					authToken.equals(m.authToken) &&
+					NullSafety.equals(inboxId, m.inboxId) &&
+					NullSafety.equals(outboxId, m.outboxId) &&
+					serverSupports.equals(m.serverSupports);
+		}
+		return false;
+	}
+
+	@Override
+	public int hashCode() {
+		return authToken.hashCode();
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxSettingsManager.java

@@ -32,8 +32,8 @@ public interface MailboxSettingsManager {
 
 	MailboxStatus getOwnMailboxStatus(Transaction txn) throws DbException;
 
-	void recordSuccessfulConnection(Transaction txn, long now)
+	void recordSuccessfulConnection(Transaction txn, long now,
-			throws DbException;
+			List<MailboxVersion> versions) throws DbException;
 
 	void recordFailedConnectionAttempt(Transaction txn, long now)
 			throws DbException;
@@ -46,20 +46,28 @@ public interface MailboxSettingsManager {
 
 	interface MailboxHook {
 		/**
-		 * Called when Briar is paired with a mailbox
+		 * Called when Briar is paired with a mailbox.
 		 *
 		 * @param txn A read-write transaction
-		 * @param ownOnion Our new mailbox's onion (56 base32 chars)
 		 */
-		void mailboxPaired(Transaction txn, String ownOnion,
+		void mailboxPaired(Transaction txn, MailboxProperties p)
-				List<MailboxVersion> serverSupports)
 				throws DbException;
 
 		/**
-		 * Called when the mailbox is unpaired
+		 * Called when the mailbox is unpaired.
 		 *
 		 * @param txn A read-write transaction
 		 */
 		void mailboxUnpaired(Transaction txn) throws DbException;
+
+		/**
+		 * Called when we receive our mailbox's server-supported API versions.
+		 * This happens whenever we successfully check the connectivity of
+		 * our mailbox, so this hook may be called frequently.
+		 *
+		 * @param txn A read-write transaction
+		 */
+		void serverSupportedVersionsReceived(Transaction txn,
+				List<MailboxVersion> serverSupports) throws DbException;
 	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxStatus.java

@@ -2,10 +2,14 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
 import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_MS_SINCE_LAST_SUCCESS;
 import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_NUM_CONNECTION_FAILURES;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
 
 @Immutable
 @NotNullByDefault
@@ -13,12 +17,15 @@ public class MailboxStatus {
 
 	private final long lastAttempt, lastSuccess;
 	private final int attemptsSinceSuccess;
+	private final List<MailboxVersion> serverSupports;
 
 	public MailboxStatus(long lastAttempt, long lastSuccess,
-			int attemptsSinceSuccess) {
+			int attemptsSinceSuccess,
+			List<MailboxVersion> serverSupports) {
 		this.lastAttempt = lastAttempt;
 		this.lastSuccess = lastSuccess;
 		this.attemptsSinceSuccess = attemptsSinceSuccess;
+		this.serverSupports = serverSupports;
 	}
 
 	/**
@@ -60,6 +67,13 @@ public class MailboxStatus {
 		return attemptsSinceSuccess;
 	}
 
+	/**
+	 * Returns the mailbox's supported API versions.
+	 */
+	public List<MailboxVersion> getServerSupports() {
+		return serverSupports;
+	}
+
 	/**
 	 * @return true if this status indicates a problem with the mailbox.
 	 */
@@ -67,4 +81,13 @@ public class MailboxStatus {
 		return attemptsSinceSuccess >= PROBLEM_NUM_CONNECTION_FAILURES &&
 				(now - lastSuccess) >= PROBLEM_MS_SINCE_LAST_SUCCESS;
 	}
+
+	/**
+	 * @return a positive integer if the mailbox is compatible. Same result as
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}.
+	 */
+	public int getMailboxCompatibility() {
+		return getHighestCommonMajorVersion(CLIENT_SUPPORTS, serverSupports);
+	}
+
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdateManager.java

@@ -29,19 +29,35 @@ public interface MailboxUpdateManager {
 
 	/**
 	 * The number of properties required for an update message with a mailbox.
+	 * <p>
+	 * The required properties are {@link #PROP_KEY_ONION},
+	 * {@link #PROP_KEY_AUTHTOKEN}, {@link #PROP_KEY_INBOXID} and
+	 * {@link #PROP_KEY_OUTBOXID}.
 	 */
 	int PROP_COUNT = 4;
 
 	/**
-	 * The required properties of an update message with a mailbox.
+	 * The onion address of the mailbox, excluding the .onion suffix.
 	 */
 	String PROP_KEY_ONION = "onion";
+
+	/**
+	 * A bearer token for accessing the mailbox (64 hex digits).
+	 */
 	String PROP_KEY_AUTHTOKEN = "authToken";
+
+	/**
+	 * A folder ID for downloading messages (64 hex digits).
+	 */
 	String PROP_KEY_INBOXID = "inboxId";
+
+	/**
+	 * A folder ID for uploading messages (64 hex digits).
+	 */
 	String PROP_KEY_OUTBOXID = "outboxId";
 
 	/**
-	 * Length of the Onion property.
+	 * Length of the {@link #PROP_KEY_ONION} property.
 	 */
 	int PROP_ONION_LENGTH = 56;
 
@@ -63,9 +79,32 @@ public interface MailboxUpdateManager {
 	 */
 	String GROUP_KEY_SENT_CLIENT_SUPPORTS = "sentClientSupports";
 
+	/**
+	 * Key in the client's local group for storing the serverSupports list that
+	 * was last sent out, if any.
+	 */
+	String GROUP_KEY_SENT_SERVER_SUPPORTS = "sentServerSupports";
+
+	/**
+	 * Returns the latest {@link MailboxUpdate} sent to the given contact.
+	 * <p>
+	 * If we have our own mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} the contact should use for communicating with
+	 * our mailbox.
+	 */
 	MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
 			throws DbException;
 
+	/**
+	 * Returns the latest {@link MailboxUpdate} received from the given
+	 * contact, or null if no update has been received.
+	 * <p>
+	 * If the contact has a mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} we should use for communicating with the
+	 * contact's mailbox.
+	 */
 	@Nullable
 	MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
 			throws DbException;

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/MailboxPairedEvent.java

@@ -0,0 +1,36 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.Map;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when a mailbox is paired.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxPairedEvent extends Event {
+
+	private final MailboxProperties properties;
+	private final Map<ContactId, MailboxUpdateWithMailbox> localUpdates;
+
+	public MailboxPairedEvent(MailboxProperties properties,
+			Map<ContactId, MailboxUpdateWithMailbox> localUpdates) {
+		this.properties = properties;
+		this.localUpdates = localUpdates;
+	}
+
+	public MailboxProperties getProperties() {
+		return properties;
+	}
+
+	public Map<ContactId, MailboxUpdateWithMailbox> getLocalUpdates() {
+		return localUpdates;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/MailboxUnpairedEvent.java

@@ -0,0 +1,28 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.Map;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when a mailbox is unpaired.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxUnpairedEvent extends Event {
+
+	private final Map<ContactId, MailboxUpdate> localUpdates;
+
+	public MailboxUnpairedEvent(Map<ContactId, MailboxUpdate> localUpdates) {
+		this.localUpdates = localUpdates;
+	}
+
+	public Map<ContactId, MailboxUpdate> getLocalUpdates() {
+		return localUpdates;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/MailboxUpdateSentToNewContactEvent.java

@@ -0,0 +1,40 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when the first mailbox update is sent to a
+ * newly added contact, which happens in the same transaction in which the
+ * contact is added.
+ * <p>
+ * This event is not broadcast when the first mailbox update is sent to an
+ * existing contact when setting up the
+ * {@link MailboxUpdateManager mailbox update client}.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxUpdateSentToNewContactEvent extends Event {
+
+	private final ContactId contactId;
+	private final MailboxUpdate mailboxUpdate;
+
+	public MailboxUpdateSentToNewContactEvent(ContactId contactId,
+			MailboxUpdate mailboxUpdate) {
+		this.contactId = contactId;
+		this.mailboxUpdate = mailboxUpdate;
+	}
+
+	public ContactId getContactId() {
+		return contactId;
+	}
+
+	public MailboxUpdate getMailboxUpdate() {
+		return mailboxUpdate;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/DeferredSendHandler.java

@@ -1,15 +0,0 @@
-package org.briarproject.bramble.api.sync;
-
-import java.util.Collection;
-
-/**
- * An interface for holding the IDs of messages sent and acked during an
- * outgoing {@link SyncSession} so they can be recorded in the DB as sent
- * or acked at some later time.
- */
-public interface DeferredSendHandler {
-
-	void onAckSent(Collection<MessageId> acked);
-
-	void onMessageSent(MessageId sent);
-}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/OutgoingSessionRecord.java

@@ -0,0 +1,37 @@
+package org.briarproject.bramble.api.sync;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.Collection;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A container for holding the IDs of messages sent and acked during an
+ * outgoing {@link SyncSession}, so they can be recorded in the DB as sent
+ * or acked at some later time.
+ */
+@ThreadSafe
+@NotNullByDefault
+public class OutgoingSessionRecord {
+
+	private final Collection<MessageId> ackedIds = new CopyOnWriteArrayList<>();
+	private final Collection<MessageId> sentIds = new CopyOnWriteArrayList<>();
+
+	public void onAckSent(Collection<MessageId> acked) {
+		ackedIds.addAll(acked);
+	}
+
+	public void onMessageSent(MessageId sent) {
+		sentIds.add(sent);
+	}
+
+	public Collection<MessageId> getAckedIds() {
+		return ackedIds;
+	}
+
+	public Collection<MessageId> getSentIds() {
+		return sentIds;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncSessionFactory.java

@@ -12,12 +12,30 @@ import javax.annotation.Nullable;
 @NotNullByDefault
 public interface SyncSessionFactory {
 
+	/**
+	 * Creates a session for receiving data from a contact.
+	 */
 	SyncSession createIncomingSession(ContactId c, InputStream in,
 			PriorityHandler handler);
 
+	/**
+	 * Creates a session for sending data to a contact over a simplex transport.
+	 *
+	 * @param eager True if messages should be sent eagerly, ie regardless of
+	 * whether they're due for retransmission.
+	 */
 	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
 			long maxLatency, boolean eager, StreamWriter streamWriter);
 
+	/**
+	 * Creates a session for sending data to a contact via a mailbox. The IDs
+	 * of any messages sent or acked will be added to the given
+	 * {@link OutgoingSessionRecord}.
+	 */
+	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
+			long maxLatency, StreamWriter streamWriter,
+			OutgoingSessionRecord sessionRecord);
+
 	SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
 			long maxLatency, int maxIdleTime, StreamWriter streamWriter,
 			@Nullable Priority priority);

bramble-api/src/main/java/org/briarproject/bramble/api/sync/event/GroupVisibilityUpdatedEvent.java

@@ -3,6 +3,7 @@ package org.briarproject.bramble.api.sync.event;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Group.Visibility;
 
 import java.util.Collection;
 
@@ -15,12 +16,19 @@ import javax.annotation.concurrent.Immutable;
 @NotNullByDefault
 public class GroupVisibilityUpdatedEvent extends Event {
 
+	private final Visibility visibility;
 	private final Collection<ContactId> affected;
 
-	public GroupVisibilityUpdatedEvent(Collection<ContactId> affected) {
+	public GroupVisibilityUpdatedEvent(Visibility visibility,
+			Collection<ContactId> affected) {
+		this.visibility = visibility;
 		this.affected = affected;
 	}
 
+	public Visibility getVisibility() {
+		return visibility;
+	}
+
 	/**
 	 * Returns the contacts affected by the update.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/sync/event/MessageSharedEvent.java

@@ -1,9 +1,14 @@
 package org.briarproject.bramble.api.sync.event;
 
+import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Group.Visibility;
+import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.MessageId;
 
+import java.util.Map;
+
 import javax.annotation.concurrent.Immutable;
 
 /**
@@ -14,12 +19,32 @@ import javax.annotation.concurrent.Immutable;
 public class MessageSharedEvent extends Event {
 
 	private final MessageId messageId;
+	private final GroupId groupId;
+	private final Map<ContactId, Boolean> groupVisibility;
 
-	public MessageSharedEvent(MessageId message) {
+	public MessageSharedEvent(MessageId message, GroupId groupId,
+			Map<ContactId, Boolean> groupVisibility) {
 		this.messageId = message;
+		this.groupId = groupId;
+		this.groupVisibility = groupVisibility;
 	}
 
 	public MessageId getMessageId() {
 		return messageId;
 	}
+
+	public GroupId getGroupId() {
+		return groupId;
+	}
+
+	/**
+	 * Returns the IDs of all contacts for which the visibility of the
+	 * message's group is either {@link Visibility#SHARED shared} or
+	 * {@link Visibility#VISIBLE visible}. The value in the map is true if the
+	 * group is {@link Visibility#SHARED shared} or false if the group is
+	 * {@link Visibility#VISIBLE visible}.
+	 */
+	public Map<ContactId, Boolean> getGroupVisibility() {
+		return groupVisibility;
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/util/IoUtils.java

@@ -40,7 +40,7 @@ public class IoUtils {
 		}
 	}
 
-	private static void delete(File f) {
+	public static void delete(File f) {
 		if (!f.delete() && LOG.isLoggable(WARNING))
 			LOG.warning("Could not delete " + f.getAbsolutePath());
 	}

bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.util;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.UnsupportedEncodingException;
@@ -95,10 +96,10 @@ public class StringUtils {
 	/**
 	 * Converts the given hex string to a byte array.
 	 */
-	public static byte[] fromHexString(String hex) {
+	public static byte[] fromHexString(String hex) throws FormatException {
 		int len = hex.length();
 		if (len % 2 != 0)
-			throw new IllegalArgumentException("Not a hex string");
+			throw new FormatException();
 		byte[] bytes = new byte[len / 2];
 		for (int i = 0, j = 0; i < len; i += 2, j++) {
 			int high = hexDigitToInt(hex.charAt(i));
@@ -108,11 +109,11 @@ public class StringUtils {
 		return bytes;
 	}
 
-	private static int hexDigitToInt(char c) {
+	private static int hexDigitToInt(char c) throws FormatException {
 		if (c >= '0' && c <= '9') return c - '0';
 		if (c >= 'A' && c <= 'F') return c - 'A' + 10;
 		if (c >= 'a' && c <= 'f') return c - 'a' + 10;
-		throw new IllegalArgumentException("Not a hex digit: " + c);
+		throw new FormatException();
 	}
 
 	public static String trim(String s) {
@@ -130,13 +131,13 @@ public class StringUtils {
 		return MAC.matcher(mac).matches();
 	}
 
-	public static byte[] macToBytes(String mac) {
+	public static byte[] macToBytes(String mac) throws FormatException {
-		if (!MAC.matcher(mac).matches()) throw new IllegalArgumentException();
+		if (!MAC.matcher(mac).matches()) throw new FormatException();
 		return fromHexString(mac.replaceAll(":", ""));
 	}
 
-	public static String macToString(byte[] mac) {
+	public static String macToString(byte[] mac) throws FormatException {
-		if (mac.length != 6) throw new IllegalArgumentException();
+		if (mac.length != 6) throw new FormatException();
 		StringBuilder s = new StringBuilder();
 		for (byte b : mac) {
 			if (s.length() > 0) s.append(':');

bramble-api/src/test/java/org/briarproject/bramble/api/mailbox/MailboxHelperTest.java

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_CLIENT_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_SERVER_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
+import static org.junit.Assert.assertEquals;
+
+public class MailboxHelperTest {
+
+	private final Random random = new Random();
+
+	@Test
+	public void testGetHighestCommonMajorVersion() {
+		assertEquals(2, getHighestCommonMajorVersion(v(2), v(2)));
+		assertEquals(2, getHighestCommonMajorVersion(v(1, 2), v(2, 3, 4)));
+		assertEquals(2, getHighestCommonMajorVersion(v(2, 3, 4), v(2)));
+		assertEquals(2, getHighestCommonMajorVersion(v(2), v(2, 3, 4)));
+
+		assertEquals(API_CLIENT_TOO_OLD,
+				getHighestCommonMajorVersion(v(2), v(3, 4)));
+		assertEquals(API_CLIENT_TOO_OLD,
+				getHighestCommonMajorVersion(v(2), v(1, 3)));
+		assertEquals(API_SERVER_TOO_OLD,
+				getHighestCommonMajorVersion(v(3, 4, 5), v(2)));
+		assertEquals(API_SERVER_TOO_OLD,
+				getHighestCommonMajorVersion(v(1, 3), v(2)));
+	}
+
+	private List<MailboxVersion> v(int... ints) {
+		List<MailboxVersion> versions = new ArrayList<>(ints.length);
+		for (int v : ints) {
+			// minor versions should not matter
+			versions.add(new MailboxVersion(v, random.nextInt(42)));
+		}
+		return versions;
+	}
+
+}

bramble-api/src/test/java/org/briarproject/bramble/test/TestUtils.java

@@ -40,6 +40,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -50,6 +51,7 @@ import java.util.Random;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import javax.annotation.Nullable;
+import javax.crypto.Cipher;
 
 import static java.util.Arrays.asList;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_AGREEMENT_PUBLIC_KEY_BYTES;
@@ -228,14 +230,14 @@ public class TestUtils {
 
 	public static MailboxProperties getMailboxProperties(boolean owner,
 			List<MailboxVersion> serverSupports) {
-		String baseUrl = "http://" + getRandomString(56) + ".onion"; // TODO
+		String onion = getRandomString(56);
 		MailboxAuthToken authToken = new MailboxAuthToken(getRandomId());
 		if (owner) {
-			return new MailboxProperties(baseUrl, authToken, serverSupports);
+			return new MailboxProperties(onion, authToken, serverSupports);
 		}
 		MailboxFolderId inboxId = new MailboxFolderId(getRandomId());
 		MailboxFolderId outboxId = new MailboxFolderId(getRandomId());
-		return new MailboxProperties(baseUrl, authToken, serverSupports,
+		return new MailboxProperties(onion, authToken, serverSupports,
 				inboxId, outboxId);
 	}
 
@@ -335,4 +337,24 @@ public class TestUtils {
 		}
 		return false;
 	}
+
+	public static <E extends Event> E getEvent(Transaction txn,
+			Class<E> eventClass) {
+		for (CommitAction action : txn.getActions()) {
+			if (action instanceof EventAction) {
+				Event event = ((EventAction) action).getEvent();
+				if (eventClass.isInstance(event)) return eventClass.cast(event);
+			}
+		}
+		throw new AssertionError();
+	}
+
+	public static boolean isCryptoStrengthUnlimited() {
+		try {
+			return Cipher.getMaxAllowedKeyLength("AES/CBC/PKCS5Padding")
+					== Integer.MAX_VALUE;
+		} catch (NoSuchAlgorithmException e) {
+			throw new AssertionError();
+		}
+	}
 }

bramble-core/build.gradle

@@ -16,7 +16,7 @@ dependencies {
 	implementation 'org.bitlet:weupnp:0.1.4'
 	implementation 'net.i2p.crypto:eddsa:0.2.0'
 	implementation 'org.whispersystems:curve25519-java:0.5.0'
-	implementation 'org.briarproject:jtorctl:0.4'
+	implementation 'org.briarproject:jtorctl:0.5'
 
 	//noinspection GradleDependency
 	implementation "com.squareup.okhttp3:okhttp:$okhttp_version"

bramble-core/src/main/java/org/briarproject/bramble/account/AccountManagerImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.account;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.account.AccountManager;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.DecryptionException;
@@ -209,7 +210,13 @@ class AccountManagerImpl implements AccountManager {
 			LOG.warning("Failed to load encrypted database key");
 			throw new DecryptionException(INVALID_CIPHERTEXT);
 		}
-		byte[] ciphertext = fromHexString(hex);
+		byte[] ciphertext;
+		try {
+			ciphertext = fromHexString(hex);
+		} catch (FormatException e) {
+			LOG.warning("Encrypted database key has invalid format");
+			throw new DecryptionException(INVALID_CIPHERTEXT);
+		}
 		KeyStrengthener keyStrengthener = databaseConfig.getKeyStrengthener();
 		byte[] plaintext = crypto.decryptWithPassword(ciphertext, password,
 				keyStrengthener);

bramble-core/src/main/java/org/briarproject/bramble/client/ClientHelperImpl.java

@@ -52,6 +52,7 @@ import java.util.Map.Entry;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
+import static java.util.Collections.sort;
 import static org.briarproject.bramble.api.client.ContactGroupConstants.GROUP_KEY_CONTACT_ID;
 import static org.briarproject.bramble.api.identity.Author.FORMAT_VERSION;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
@@ -456,8 +457,7 @@ class ClientHelperImpl implements ClientHelper {
 		checkLength(inboxId, UniqueId.LENGTH);
 		byte[] outboxId = properties.getRaw(PROP_KEY_OUTBOXID);
 		checkLength(outboxId, UniqueId.LENGTH);
-		String baseUrl = "http://" + onion + ".onion"; // TODO
+		MailboxProperties props = new MailboxProperties(onion,
-		MailboxProperties props = new MailboxProperties(baseUrl,
 				new MailboxAuthToken(authToken), serverSupportsList,
 				new MailboxFolderId(inboxId), new MailboxFolderId(outboxId));
 		return new MailboxUpdateWithMailbox(clientSupportsList, props);
@@ -475,6 +475,8 @@ class ClientHelperImpl implements ClientHelper {
 			list.add(new MailboxVersion(element.getLong(0).intValue(),
 					element.getLong(1).intValue()));
 		}
+		// Sort the list of versions for easier comparison
+		sort(list);
 		return list;
 	}
 

bramble-core/src/main/java/org/briarproject/bramble/connection/Connection.java

@@ -54,7 +54,7 @@ abstract class Connection {
 		}
 	}
 
-	private byte[] readTag(InputStream in) throws IOException {
+	byte[] readTag(InputStream in) throws IOException {
 		byte[] tag = new byte[TAG_LENGTH];
 		read(in, tag);
 		return tag;

bramble-core/src/main/java/org/briarproject/bramble/connection/ConnectionManagerImpl.java

@@ -13,6 +13,7 @@ import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.SyncSessionFactory;
 import org.briarproject.bramble.api.transport.KeyManager;
 import org.briarproject.bramble.api.transport.StreamReaderFactory;
@@ -67,7 +68,15 @@ class ConnectionManagerImpl implements ConnectionManager {
 			TransportConnectionReader r) {
 		ioExecutor.execute(new IncomingSimplexSyncConnection(keyManager,
 				connectionRegistry, streamReaderFactory, streamWriterFactory,
-				syncSessionFactory, transportPropertyManager, t, r));
+				syncSessionFactory, transportPropertyManager, t, r, null));
+	}
+
+	@Override
+	public void manageIncomingConnection(TransportId t,
+			TransportConnectionReader r, TagController c) {
+		ioExecutor.execute(new IncomingSimplexSyncConnection(keyManager,
+				connectionRegistry, streamReaderFactory, streamWriterFactory,
+				syncSessionFactory, transportPropertyManager, t, r, c));
 	}
 
 	@Override
@@ -92,7 +101,16 @@ class ConnectionManagerImpl implements ConnectionManager {
 			TransportConnectionWriter w) {
 		ioExecutor.execute(new OutgoingSimplexSyncConnection(keyManager,
 				connectionRegistry, streamReaderFactory, streamWriterFactory,
-				syncSessionFactory, transportPropertyManager, c, t, w));
+				syncSessionFactory, transportPropertyManager, c, t, w, null));
+	}
+
+	@Override
+	public void manageOutgoingConnection(ContactId c, TransportId t,
+			TransportConnectionWriter w, OutgoingSessionRecord sessionRecord) {
+		ioExecutor.execute(new OutgoingSimplexSyncConnection(keyManager,
+				connectionRegistry, streamReaderFactory, streamWriterFactory,
+				syncSessionFactory, transportPropertyManager, c, t, w,
+				sessionRecord));
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/connection/IncomingSimplexSyncConnection.java

@@ -1,7 +1,9 @@
 package org.briarproject.bramble.connection;
 
+import org.briarproject.bramble.api.connection.ConnectionManager.TagController;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
 import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportConnectionReader;
 import org.briarproject.bramble.api.plugin.TransportId;
@@ -15,6 +17,8 @@ import org.briarproject.bramble.api.transport.StreamWriterFactory;
 
 import java.io.IOException;
 
+import javax.annotation.Nullable;
+
 import static java.util.logging.Level.WARNING;
 import static org.briarproject.bramble.util.LogUtils.logException;
 
@@ -23,6 +27,8 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 
 	private final TransportId transportId;
 	private final TransportConnectionReader reader;
+	@Nullable
+	private final TagController tagController;
 
 	IncomingSimplexSyncConnection(KeyManager keyManager,
 			ConnectionRegistry connectionRegistry,
@@ -30,33 +36,50 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 			StreamWriterFactory streamWriterFactory,
 			SyncSessionFactory syncSessionFactory,
 			TransportPropertyManager transportPropertyManager,
-			TransportId transportId, TransportConnectionReader reader) {
+			TransportId transportId,
+			TransportConnectionReader reader,
+			@Nullable TagController tagController) {
 		super(keyManager, connectionRegistry, streamReaderFactory,
 				streamWriterFactory, syncSessionFactory,
 				transportPropertyManager);
 		this.transportId = transportId;
 		this.reader = reader;
+		this.tagController = tagController;
 	}
 
 	@Override
 	public void run() {
 		// Read and recognise the tag
-		StreamContext ctx = recogniseTag(reader, transportId);
+		byte[] tag;
+		StreamContext ctx;
+		try {
+			tag = readTag(reader.getInputStream());
+			// If we have a tag controller, defer marking the tag as recognised
+			if (tagController == null) {
+				ctx = keyManager.getStreamContext(transportId, tag);
+			} else {
+				ctx = keyManager.getStreamContextOnly(transportId, tag);
+			}
+		} catch (IOException | DbException e) {
+			logException(LOG, WARNING, e);
+			onError();
+			return;
+		}
 		if (ctx == null) {
 			LOG.info("Unrecognised tag");
-			onError(false);
+			onError();
 			return;
 		}
 		ContactId contactId = ctx.getContactId();
 		if (contactId == null) {
 			LOG.warning("Received rendezvous stream, expected contact");
-			onError(true);
+			onError(tag);
 			return;
 		}
 		if (ctx.isHandshakeMode()) {
 			// TODO: Support handshake mode for contacts
 			LOG.warning("Received handshake tag, expected rotation mode");
-			onError(true);
+			onError(tag);
 			return;
 		}
 		try {
@@ -65,15 +88,33 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 					LOG.info("Ignoring priority for simplex connection");
 			// Create and run the incoming session
 			createIncomingSession(ctx, reader, handler).run();
+			// Success
+			markTagAsRecognisedIfRequired(false, tag);
 			reader.dispose(false, true);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
-			onError(true);
+			onError(tag);
 		}
 	}
 
-	private void onError(boolean recognised) {
+	private void onError() {
-		disposeOnError(reader, recognised);
+		disposeOnError(reader, false);
+	}
+
+	private void onError(byte[] tag) {
+		markTagAsRecognisedIfRequired(true, tag);
+		disposeOnError(reader, true);
+	}
+
+	private void markTagAsRecognisedIfRequired(boolean exception, byte[] tag) {
+		if (tagController != null &&
+				tagController.shouldMarkTagAsRecognised(exception)) {
+			try {
+				keyManager.markTagAsRecognised(transportId, tag);
+			} catch (DbException e) {
+				logException(LOG, WARNING, e);
+			}
+		}
 	}
 }
 

bramble-core/src/main/java/org/briarproject/bramble/connection/OutgoingSimplexSyncConnection.java

@@ -6,6 +6,7 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.SyncSession;
 import org.briarproject.bramble.api.sync.SyncSessionFactory;
 import org.briarproject.bramble.api.transport.KeyManager;
@@ -16,6 +17,8 @@ import org.briarproject.bramble.api.transport.StreamWriterFactory;
 
 import java.io.IOException;
 
+import javax.annotation.Nullable;
+
 import static java.util.logging.Level.WARNING;
 import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
 import static org.briarproject.bramble.util.LogUtils.logException;
@@ -26,6 +29,8 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 	private final ContactId contactId;
 	private final TransportId transportId;
 	private final TransportConnectionWriter writer;
+	@Nullable
+	private final OutgoingSessionRecord sessionRecord;
 
 	OutgoingSimplexSyncConnection(KeyManager keyManager,
 			ConnectionRegistry connectionRegistry,
@@ -34,13 +39,15 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 			SyncSessionFactory syncSessionFactory,
 			TransportPropertyManager transportPropertyManager,
 			ContactId contactId, TransportId transportId,
-			TransportConnectionWriter writer) {
+			TransportConnectionWriter writer,
+			@Nullable OutgoingSessionRecord sessionRecord) {
 		super(keyManager, connectionRegistry, streamReaderFactory,
 				streamWriterFactory, syncSessionFactory,
 				transportPropertyManager);
 		this.contactId = contactId;
 		this.transportId = transportId;
 		this.writer = writer;
+		this.sessionRecord = sessionRecord;
 	}
 
 	@Override
@@ -71,10 +78,16 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 		StreamWriter streamWriter = streamWriterFactory.createStreamWriter(
 				w.getOutputStream(), ctx);
 		ContactId c = requireNonNull(ctx.getContactId());
-		// Use eager retransmission if the transport is lossy and cheap
+		if (sessionRecord == null) {
-		return syncSessionFactory.createSimplexOutgoingSession(c,
+			// Use eager retransmission if the transport is lossy and cheap
-				ctx.getTransportId(), w.getMaxLatency(), w.isLossyAndCheap(),
+			return syncSessionFactory.createSimplexOutgoingSession(c,
-				streamWriter);
+					ctx.getTransportId(), w.getMaxLatency(),
+					w.isLossyAndCheap(), streamWriter);
+		} else {
+			return syncSessionFactory.createSimplexOutgoingSession(c,
+					ctx.getTransportId(), w.getMaxLatency(), streamWriter,
+					sessionRecord);
+		}
 	}
 }
 

bramble-core/src/main/java/org/briarproject/bramble/db/Database.java

@@ -163,16 +163,11 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
+	 * Returns true if there are any acks to send to the given contact.
-	 * contact over a transport with the given maximum latency.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(T txn, ContactId c, long maxLatency,
+	boolean containsAcksToSend(T txn, ContactId c) throws DbException;
-			boolean eager) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -212,6 +207,18 @@ interface Database<T> {
 	 */
 	boolean containsMessage(T txn, MessageId m) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given
+	 * contact over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(T txn, ContactId c, long maxLatency,
+			boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -313,6 +320,13 @@ interface Database<T> {
 	 */
 	Group getGroup(T txn, GroupId g) throws DbException;
 
+	/**
+	 * Returns the ID of the group containing the given message.
+	 * <p/>
+	 * Read-only.
+	 */
+	GroupId getGroupId(T txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for the given group.
 	 * <p/>
@@ -338,8 +352,11 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Returns the IDs of all contacts to which the given group's visibility is
+	 * Returns the IDs of all contacts for which the given group's visibility
-	 * either {@link Visibility VISIBLE} or {@link Visibility SHARED}.
+	 * is either {@link Visibility#SHARED shared} or
+	 * {@link Visibility#VISIBLE visible}. The value in the map is true if the
+	 * group is {@link Visibility#SHARED shared} or false if the group is
+	 * {@link Visibility#VISIBLE visible}.
 	 * <p/>
 	 * Read-only.
 	 */
@@ -580,13 +597,16 @@ interface Database<T> {
 
 	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
+	 * message is due to be sent to the given contact over a transport with
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE
+	 * the given latency.
-	 * if no messages are scheduled to be sent.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(T txn, ContactId c) throws DbException;
+	long getNextSendTime(T txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java

@@ -287,7 +287,12 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			transaction.attach(new MessageAddedEvent(m, null));
 			transaction.attach(new MessageStateChangedEvent(m.getId(), true,
 					DELIVERED));
-			if (shared) transaction.attach(new MessageSharedEvent(m.getId()));
+			if (shared) {
+				Map<ContactId, Boolean> visibility =
+						db.getGroupVisibility(txn, m.getGroupId());
+				transaction.attach(new MessageSharedEvent(m.getId(),
+						m.getGroupId(), visibility));
+			}
 		}
 		db.mergeMessageMetadata(txn, m.getId(), meta);
 	}
@@ -342,12 +347,12 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Transaction transaction, ContactId c,
+	public boolean containsAcksToSend(Transaction transaction, ContactId c)
-			long maxLatency, boolean eager) throws DbException {
+			throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
-		return db.containsAnythingToSend(txn, c, maxLatency, eager);
+		return db.containsAcksToSend(txn, c);
 	}
 
 	@Override
@@ -373,6 +378,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsIdentity(txn, a);
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Transaction transaction, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.containsMessagesToSend(txn, c, maxLatency, eager);
+	}
+
 	@Override
 	public boolean containsPendingContact(Transaction transaction,
 			PendingContactId p) throws DbException {
@@ -541,6 +555,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getGroup(txn, g);
 	}
 
+	@Override
+	public GroupId getGroupId(Transaction transaction, MessageId m)
+			throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsMessage(txn, m))
+			throw new NoSuchMessageException();
+		return db.getGroupId(txn, m);
+	}
+
 	@Override
 	public Metadata getGroupMetadata(Transaction transaction, GroupId g)
 			throws DbException {
@@ -611,11 +634,11 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 
 	@Override
 	public Collection<MessageId> getMessagesToAck(Transaction transaction,
-			ContactId c, int maxMessages) throws DbException {
+			ContactId c) throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
-		return db.getMessagesToAck(txn, c, maxMessages);
+		return db.getMessagesToAck(txn, c, Integer.MAX_VALUE);
 	}
 
 	@Override
@@ -737,7 +760,9 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	public Message getMessageToSend(Transaction transaction, ContactId c,
 			MessageId m, long maxLatency, boolean markAsSent)
 			throws DbException {
-		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		if (markAsSent && transaction.isReadOnly()) {
+			throw new IllegalArgumentException();
+		}
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
@@ -805,10 +830,10 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public long getNextSendTime(Transaction transaction, ContactId c)
+	public long getNextSendTime(Transaction transaction, ContactId c,
-			throws DbException {
+			long maxLatency) throws DbException {
 		T txn = unbox(transaction);
-		return db.getNextSendTime(txn, c);
+		return db.getNextSendTime(txn, c, maxLatency);
 	}
 
 	@Override
@@ -1016,7 +1041,8 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 				db.getGroupVisibility(txn, id).keySet();
 		db.removeGroup(txn, id);
 		transaction.attach(new GroupRemovedEvent(g));
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		transaction.attach(new GroupVisibilityUpdatedEvent(INVISIBLE,
+				affected));
 	}
 
 	@Override
@@ -1087,11 +1113,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
-		List<MessageId> visible = new ArrayList<>(acked.size());
+		db.lowerAckFlag(txn, c, acked);
-		for (MessageId m : acked) {
-			if (db.containsVisibleMessage(txn, c, m)) visible.add(m);
-		}
-		db.lowerAckFlag(txn, c, visible);
 	}
 
 	@Override
@@ -1141,7 +1163,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		else if (v == INVISIBLE) db.removeGroupVisibility(txn, c, g);
 		else db.setGroupVisibility(txn, c, g, v == SHARED);
 		List<ContactId> affected = singletonList(c);
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		transaction.attach(new GroupVisibilityUpdatedEvent(v, affected));
 	}
 
 	@Override
@@ -1174,7 +1196,9 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (db.getMessageState(txn, m) != DELIVERED)
 			throw new IllegalArgumentException("Shared undelivered message");
 		db.setMessageShared(txn, m, true);
-		transaction.attach(new MessageSharedEvent(m));
+		GroupId g = db.getGroupId(txn, m);
+		Map<ContactId, Boolean> visibility = db.getGroupVisibility(txn, g);
+		transaction.attach(new MessageSharedEvent(m, g, visibility));
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java

@@ -1147,8 +1147,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Connection txn, ContactId c,
+	public boolean containsAcksToSend(Connection txn, ContactId c)
-			long maxLatency, boolean eager) throws DbException {
+			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -1160,34 +1160,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			boolean acksToSend = rs.next();
 			rs.close();
 			ps.close();
-			if (acksToSend) return true;
+			return acksToSend;
-			if (eager) {
-				sql = "SELECT NULL from statuses"
-						+ " WHERE contactId = ? AND state = ?"
-						+ " AND groupShared = TRUE AND messageShared = TRUE"
-						+ " AND deleted = FALSE AND seen = FALSE";
-				ps = txn.prepareStatement(sql);
-				ps.setInt(1, c.getInt());
-				ps.setInt(2, DELIVERED.getValue());
-			} else {
-				long now = clock.currentTimeMillis();
-				sql = "SELECT NULL FROM statuses"
-						+ " WHERE contactId = ? AND state = ?"
-						+ " AND groupShared = TRUE AND messageShared = TRUE"
-						+ " AND deleted = FALSE AND seen = FALSE"
-						+ " AND (expiry <= ? OR maxLatency IS NULL"
-						+ " OR ? < maxLatency)";
-				ps = txn.prepareStatement(sql);
-				ps.setInt(1, c.getInt());
-				ps.setInt(2, DELIVERED.getValue());
-				ps.setLong(3, now);
-				ps.setLong(4, maxLatency);
-			}
-			rs = ps.executeQuery();
-			boolean messagesToSend = rs.next();
-			rs.close();
-			ps.close();
-			return messagesToSend;
 		} catch (SQLException e) {
 			tryToClose(rs, LOG, WARNING);
 			tryToClose(ps, LOG, WARNING);
@@ -1307,6 +1280,46 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Connection txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			if (eager) {
+				String sql = "SELECT NULL from statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+			} else {
+				long now = clock.currentTimeMillis();
+				String sql = "SELECT NULL FROM statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE"
+						+ " AND (expiry <= ? OR maxLatency IS NULL"
+						+ " OR ? < maxLatency)";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+				ps.setLong(3, now);
+				ps.setLong(4, maxLatency);
+			}
+			rs = ps.executeQuery();
+			boolean messagesToSend = rs.next();
+			rs.close();
+			ps.close();
+			return messagesToSend;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public boolean containsPendingContact(Connection txn, PendingContactId p)
 			throws DbException {
@@ -1670,6 +1683,27 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public GroupId getGroupId(Connection txn, MessageId m) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT groupId FROM messages WHERE messageId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			rs = ps.executeQuery();
+			if (!rs.next()) throw new DbStateException();
+			GroupId g = new GroupId(rs.getBytes(1));
+			rs.close();
+			ps.close();
+			return g;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Collection<Group> getGroups(Connection txn, ClientId c,
 			int majorVersion) throws DbException {
@@ -2477,12 +2511,28 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public long getNextSendTime(Connection txn, ContactId c)
+	public long getNextSendTime(Connection txn, ContactId c, long maxLatency)
 			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT expiry FROM statuses"
+			// Are any messages sendable immediately?
+			String sql = "SELECT NULL FROM statuses"
+					+ " WHERE contactId = ? AND state = ?"
+					+ " AND groupShared = TRUE AND messageShared = TRUE"
+					+ " AND deleted = FALSE AND seen = FALSE"
+					+ " AND (maxLatency IS NULL OR ? < maxLatency)";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setInt(2, DELIVERED.getValue());
+			ps.setLong(3, maxLatency);
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			rs.close();
+			ps.close();
+			if (found) return 0;
+			// When is the earliest expiry time (could be in the past)?
+			sql = "SELECT expiry FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"

bramble-core/src/main/java/org/briarproject/bramble/io/DnsModule.java

@@ -1,4 +1,4 @@
-package org.briarproject.briar.feed;
+package org.briarproject.bramble.io;
 
 import dagger.Module;
 import dagger.Provides;

bramble-core/src/main/java/org/briarproject/bramble/io/NoDns.java

@@ -1,4 +1,6 @@
-package org.briarproject.briar.feed;
+package org.briarproject.bramble.io;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.net.InetAddress;
 import java.net.UnknownHostException;
@@ -9,6 +11,7 @@ import javax.inject.Inject;
 
 import okhttp3.Dns;
 
+@NotNullByDefault
 class NoDns implements Dns {
 
 	private static final byte[] UNSPECIFIED_ADDRESS = new byte[4];

bramble-core/src/main/java/org/briarproject/bramble/lifecycle/LifecycleManagerImpl.java

@@ -18,7 +18,7 @@ import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Semaphore;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
@@ -29,10 +29,12 @@ import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.COMPACTING_DATABASE;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.CREATED;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.MIGRATING_DATABASE;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.RUNNING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STARTING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STARTING_SERVICES;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPED;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.StartResult.ALREADY_RUNNING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.StartResult.CLOCK_ERROR;
@@ -60,12 +62,11 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 	private final List<Service> services;
 	private final List<OpenDatabaseHook> openDatabaseHooks;
 	private final List<ExecutorService> executors;
-	private final Semaphore startStopSemaphore = new Semaphore(1);
 	private final CountDownLatch dbLatch = new CountDownLatch(1);
 	private final CountDownLatch startupLatch = new CountDownLatch(1);
 	private final CountDownLatch shutdownLatch = new CountDownLatch(1);
-
+	private final AtomicReference<LifecycleState> state =
-	private volatile LifecycleState state = STARTING;
+			new AtomicReference<>(CREATED);
 
 	@Inject
 	LifecycleManagerImpl(DatabaseComponent db, EventBus eventBus,
@@ -102,8 +103,8 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 
 	@Override
 	public StartResult startServices(SecretKey dbKey) {
-		if (!startStopSemaphore.tryAcquire()) {
+		if (!state.compareAndSet(CREATED, STARTING)) {
-			LOG.info("Already starting or stopping");
+			LOG.warning("Already running");
 			return ALREADY_RUNNING;
 		}
 		long now = clock.currentTimeMillis();
@@ -135,7 +136,7 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 			});
 
 			LOG.info("Starting services");
-			state = STARTING_SERVICES;
+			state.set(STARTING_SERVICES);
 			dbLatch.countDown();
 			eventBus.broadcast(new LifecycleEvent(STARTING_SERVICES));
 
@@ -148,7 +149,7 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 				}
 			}
 
-			state = RUNNING;
+			state.set(RUNNING);
 			startupLatch.countDown();
 			eventBus.broadcast(new LifecycleEvent(RUNNING));
 			return SUCCESS;
@@ -164,59 +165,58 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 		} catch (ServiceException e) {
 			logException(LOG, WARNING, e);
 			return SERVICE_ERROR;
-		} finally {
-			startStopSemaphore.release();
 		}
 	}
 
 	@Override
 	public void onDatabaseMigration() {
-		state = MIGRATING_DATABASE;
+		state.set(MIGRATING_DATABASE);
 		eventBus.broadcast(new LifecycleEvent(MIGRATING_DATABASE));
 	}
 
 	@Override
 	public void onDatabaseCompaction() {
-		state = COMPACTING_DATABASE;
+		state.set(COMPACTING_DATABASE);
 		eventBus.broadcast(new LifecycleEvent(COMPACTING_DATABASE));
 	}
 
 	@Override
 	public void stopServices() {
-		try {
+		if (!state.compareAndSet(RUNNING, STOPPING)) {
-			startStopSemaphore.acquire();
+			LOG.warning("Not running");
-		} catch (InterruptedException e) {
-			LOG.warning("Interrupted while waiting to stop services");
 			return;
 		}
-		try {
+		LOG.info("Stopping services");
-			LOG.info("Stopping services");
+		eventBus.broadcast(new LifecycleEvent(STOPPING));
-			state = STOPPING;
+		for (Service s : services) {
-			eventBus.broadcast(new LifecycleEvent(STOPPING));
+			try {
-			for (Service s : services) {
 				long start = now();
 				s.stopService();
 				if (LOG.isLoggable(FINE)) {
 					logDuration(LOG, "Stopping service "
 							+ s.getClass().getSimpleName(), start);
 				}
+			} catch (ServiceException e) {
+				logException(LOG, WARNING, e);
 			}
-			for (ExecutorService e : executors) {
+		}
-				if (LOG.isLoggable(FINE)) {
+		for (ExecutorService e : executors) {
-					LOG.fine("Stopping executor "
+			if (LOG.isLoggable(FINE)) {
-							+ e.getClass().getSimpleName());
+				LOG.fine("Stopping executor "
-				}
+						+ e.getClass().getSimpleName());
-				e.shutdownNow();
 			}
+			e.shutdownNow();
+		}
+		try {
 			long start = now();
 			db.close();
 			logDuration(LOG, "Closing database", start);
-			shutdownLatch.countDown();
+		} catch (DbException e) {
-		} catch (DbException | ServiceException e) {
 			logException(LOG, WARNING, e);
-		} finally {
-			startStopSemaphore.release();
 		}
+		state.set(STOPPED);
+		shutdownLatch.countDown();
+		eventBus.broadcast(new LifecycleEvent(STOPPED));
 	}
 
 	@Override
@@ -236,6 +236,6 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 
 	@Override
 	public LifecycleState getLifecycleState() {
-		return state;
+		return state.get();
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityChecker.java

@@ -22,10 +22,21 @@ interface ConnectivityChecker {
 	 * the check succeeds. If a check is already running then the observer is
 	 * called when the check succeeds. If a connectivity check has recently
 	 * succeeded then the observer is called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the checker is
+	 * {@link #destroy() destroyed}.
 	 */
 	void checkConnectivity(MailboxProperties properties,
 			ConnectivityObserver o);
 
+	/**
+	 * Removes an observer that was added via
+	 * {@link #checkConnectivity(MailboxProperties, ConnectivityObserver)}. If
+	 * there are no remaining observers and a connectivity check is running
+	 * then the check will be cancelled.
+	 */
+	void removeObserver(ConnectivityObserver o);
+
 	interface ConnectivityObserver {
 		void onConnectivityCheckSucceeded();
 	}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityCheckerImpl.java

@@ -80,8 +80,7 @@ abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
 						> CONNECTIVITY_CHECK_FRESHNESS_MS) {
 					// The last connectivity check is stale, start a new one
 					connectivityObservers.add(o);
-					ApiCall task =
+					ApiCall task = createConnectivityCheckTask(properties);
-							createConnectivityCheckTask(properties);
 					connectivityCheck = mailboxApiCaller.retryWithBackoff(task);
 				} else {
 					// The last connectivity check is fresh
@@ -108,4 +107,16 @@ abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
 			o.onConnectivityCheckSucceeded();
 		}
 	}
+
+	@Override
+	public void removeObserver(ConnectivityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityObservers.remove(o);
+			if (connectivityObservers.isEmpty() && connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxClient.java

@@ -0,0 +1,124 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxClient implements MailboxClient {
+
+	private static final Logger LOG =
+			getLogger(ContactMailboxClient.class.getName());
+
+	private final MailboxWorkerFactory workerFactory;
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	@Nullable
+	private MailboxWorker uploadWorker = null, downloadWorker = null;
+
+	ContactMailboxClient(MailboxWorkerFactory workerFactory,
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor) {
+		this.workerFactory = workerFactory;
+		this.connectivityChecker = connectivityChecker;
+		this.reachabilityMonitor = reachabilityMonitor;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		// Nothing to do until contact is assigned
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		MailboxWorker uploadWorker, downloadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+		if (downloadWorker != null) downloadWorker.destroy();
+		// The connectivity checker belongs to the client, so it should be
+		// destroyed. The Tor reachability monitor is shared between clients,
+		// so it should not be destroyed
+		connectivityChecker.destroy();
+	}
+
+	@Override
+	public void assignContactForUpload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for upload");
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be uploading to the outbox
+		// assigned to us by the contact
+		if (!folderId.equals(properties.getOutboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker uploadWorker = workerFactory.createUploadWorker(
+				connectivityChecker, properties, folderId, contactId);
+		synchronized (lock) {
+			if (this.uploadWorker != null) throw new IllegalStateException();
+			this.uploadWorker = uploadWorker;
+		}
+		uploadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForUpload(ContactId contactId) {
+		LOG.info("Contact deassigned for upload");
+		MailboxWorker uploadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForDownload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for download");
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be downloading from the
+		// inbox assigned to us by the contact
+		if (!folderId.equals(properties.getInboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker downloadWorker =
+				workerFactory.createDownloadWorkerForContactMailbox(
+						connectivityChecker, reachabilityMonitor, properties);
+		synchronized (lock) {
+			if (this.downloadWorker != null) throw new IllegalStateException();
+			this.downloadWorker = downloadWorker;
+		}
+		downloadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForDownload(ContactId contactId) {
+		LOG.info("Contact deassigned for download");
+		MailboxWorker downloadWorker;
+		synchronized (lock) {
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (downloadWorker != null) downloadWorker.destroy();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxConnectivityChecker.java

@@ -5,16 +5,23 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
 
-import java.io.IOException;
+import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.logging.Logger.getLogger;
 
 @ThreadSafe
 @NotNullByDefault
 class ContactMailboxConnectivityChecker extends ConnectivityCheckerImpl {
 
+	private static final Logger LOG =
+			getLogger(ContactMailboxConnectivityChecker.class.getName());
+
 	private final MailboxApi mailboxApi;
 
+	@Inject
 	ContactMailboxConnectivityChecker(Clock clock,
 			MailboxApiCaller mailboxApiCaller, MailboxApi mailboxApi) {
 		super(clock, mailboxApiCaller);
@@ -24,16 +31,13 @@ class ContactMailboxConnectivityChecker extends ConnectivityCheckerImpl {
 	@Override
 	ApiCall createConnectivityCheckTask(MailboxProperties properties) {
 		if (properties.isOwner()) throw new IllegalArgumentException();
-		return new SimpleApiCall() {
+		return new SimpleApiCall(() -> {
-			@Override
+			LOG.info("Checking connectivity of contact's mailbox");
-			void tryToCallApi() throws IOException, ApiException {
+			if (!mailboxApi.checkStatus(properties)) throw new ApiException();
-				if (!mailboxApi.checkStatus(properties)) {
+			LOG.info("Contact's mailbox is reachable");
-					throw new ApiException();
+			// Call the observers and cache the result
-				}
+			onConnectivityCheckSucceeded(clock.currentTimeMillis());
-				// Call the observers and cache the result
+		});
-				onConnectivityCheckSucceeded(clock.currentTimeMillis());
-			}
-		};
 	}
 
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxDownloadWorker.java

@@ -0,0 +1,65 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxFile;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.Collections.emptyList;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxDownloadWorker extends MailboxDownloadWorker {
+
+	ContactMailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		super(connectivityChecker, torReachabilityMonitor, mailboxApiCaller,
+				mailboxApi, mailboxFileManager, mailboxProperties);
+		if (mailboxProperties.isOwner()) throw new IllegalArgumentException();
+	}
+
+	@Override
+	protected ApiCall createApiCallForDownloadCycle() {
+		return new SimpleApiCall(this::apiCallListInbox);
+	}
+
+	private void apiCallListInbox() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing inbox");
+		MailboxFolderId folderId =
+				requireNonNull(mailboxProperties.getInboxId());
+		List<MailboxFile> files;
+		try {
+			files = mailboxApi.getFiles(mailboxProperties, folderId);
+		} catch (TolerableFailureException e) {
+			LOG.warning("Inbox folder does not exist");
+			files = emptyList();
+		}
+		if (files.isEmpty()) {
+			onDownloadCycleFinished();
+		} else {
+			Queue<FolderFile> queue = new LinkedList<>();
+			for (MailboxFile file : files) {
+				queue.add(new FolderFile(folderId, file.name));
+			}
+			downloadNextFile(queue);
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApi.java

@@ -7,7 +7,6 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -19,18 +18,9 @@ import java.util.List;
 import javax.annotation.Nonnull;
 import javax.annotation.concurrent.Immutable;
 
-import static java.util.Collections.singletonList;
-
 @NotNullByDefault
 interface MailboxApi {
 
-	/**
-	 * Mailbox API versions that we support as a client. This is reported to our
-	 * contacts by {@link MailboxUpdateManager}.
-	 */
-	List<MailboxVersion> CLIENT_SUPPORTS = singletonList(
-			new MailboxVersion(1, 0));
-
 	List<MailboxVersion> getServerSupports(MailboxProperties properties)
 			throws IOException, ApiException;
 
@@ -101,9 +91,13 @@ interface MailboxApi {
 	 * Used by owner and contacts to list their files to retrieve.
 	 * <p>
 	 * Returns 200 OK with the list of files in JSON.
+	 *
+	 * @throws TolerableFailureException if response code is 404 (folder does
+	 * not exist or client is not authorised to download from it)
 	 */
 	List<MailboxFile> getFiles(MailboxProperties properties,
-			MailboxFolderId folderId) throws IOException, ApiException;
+			MailboxFolderId folderId)
+			throws IOException, ApiException, TolerableFailureException;
 
 	/**
 	 * Used by owner and contacts to retrieve a file.
@@ -112,17 +106,22 @@ interface MailboxApi {
 	 * in the response body.
 	 *
 	 * @param file the empty file the response bytes will be written into.
+	 * @throws TolerableFailureException if response code is 404 (folder does
+	 * not exist, client is not authorised to download from folder, or file
+	 * does not exist)
 	 */
 	void getFile(MailboxProperties properties, MailboxFolderId folderId,
-			MailboxFileId fileId, File file) throws IOException, ApiException;
+			MailboxFileId fileId, File file)
+			throws IOException, ApiException, TolerableFailureException;
 
 	/**
 	 * Used by owner and contacts to delete files.
 	 * <p>
 	 * Returns 200 OK (no exception) if deletion was successful.
 	 *
-	 * @throws TolerableFailureException on 404 response,
+	 * @throws TolerableFailureException if response code is 404 (folder does
-	 * because file was most likely deleted already.
+	 * not exist, client is not authorised to download from folder, or file
+	 * does not exist)
 	 */
 	void deleteFile(MailboxProperties properties, MailboxFolderId folderId,
 			MailboxFileId fileId)

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiCaller.java

@@ -24,6 +24,8 @@ interface MailboxApiCaller {
 	 * Asynchronously calls the given API call on the {@link IoExecutor},
 	 * automatically retrying at increasing intervals until the API call
 	 * returns false or retries are cancelled.
+	 * <p>
+	 * This method is safe to call while holding a lock.
 	 *
 	 * @return A {@link Cancellable} that can be used to cancel any future
 	 * retries.

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiImpl.java

@@ -22,7 +22,6 @@ import java.io.FileOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 
 import javax.inject.Inject;
@@ -35,6 +34,7 @@ import okhttp3.Response;
 import okhttp3.ResponseBody;
 
 import static com.fasterxml.jackson.databind.MapperFeature.BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES;
+import static java.util.Collections.sort;
 import static java.util.Objects.requireNonNull;
 import static okhttp3.internal.Util.EMPTY_REQUEST;
 import static org.briarproject.bramble.util.IoUtils.copyAndClose;
@@ -42,18 +42,22 @@ import static org.briarproject.bramble.util.IoUtils.copyAndClose;
 @NotNullByDefault
 class MailboxApiImpl implements MailboxApi {
 
-	private final WeakSingletonProvider<OkHttpClient> httpClientProvider;
-	private final JsonMapper mapper = JsonMapper.builder()
-			.enable(BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES)
-			.build();
 	private static final MediaType JSON =
 			requireNonNull(MediaType.parse("application/json; charset=utf-8"));
 	private static final MediaType FILE =
 			requireNonNull(MediaType.parse("application/octet-stream"));
 
+	private final WeakSingletonProvider<OkHttpClient> httpClientProvider;
+	private final JsonMapper mapper = JsonMapper.builder()
+			.enable(BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES)
+			.build();
+	private final UrlConverter urlConverter;
+
 	@Inject
-	MailboxApiImpl(WeakSingletonProvider<OkHttpClient> httpClientProvider) {
+	MailboxApiImpl(WeakSingletonProvider<OkHttpClient> httpClientProvider,
+			UrlConverter urlConverter) {
 		this.httpClientProvider = httpClientProvider;
+		this.urlConverter = urlConverter;
 	}
 
 	@Override
@@ -78,7 +82,7 @@ class MailboxApiImpl implements MailboxApi {
 			throws IOException, ApiException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + "/setup")
+				.url(getBaseUrl(properties) + "/setup")
 				.put(EMPTY_REQUEST)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -93,7 +97,7 @@ class MailboxApiImpl implements MailboxApi {
 			if (tokenNode == null) {
 				throw new ApiException();
 			}
-			return new MailboxProperties(properties.getBaseUrl(),
+			return new MailboxProperties(properties.getOnion(),
 					MailboxAuthToken.fromString(tokenNode.textValue()),
 					parseServerSupports(node));
 		} catch (JacksonException | InvalidMailboxIdException e) {
@@ -121,6 +125,8 @@ class MailboxApiImpl implements MailboxApi {
 			if (major < 0 || minor < 0) throw new ApiException();
 			serverSupports.add(new MailboxVersion(major, minor));
 		}
+		// Sort the list of versions for easier comparison
+		sort(serverSupports);
 		return serverSupports;
 	}
 
@@ -137,7 +143,7 @@ class MailboxApiImpl implements MailboxApi {
 			throws IOException, ApiException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + "/")
+				.url(getBaseUrl(properties) + "/")
 				.delete()
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -162,7 +168,7 @@ class MailboxApiImpl implements MailboxApi {
 	public void deleteContact(MailboxProperties properties, ContactId contactId)
 			throws IOException, ApiException, TolerableFailureException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
-		String url = properties.getBaseUrl() + "/contacts/" +
+		String url = getBaseUrl(properties) + "/contacts/" +
 				contactId.getInt();
 		Request request = getRequestBuilder(properties.getAuthToken())
 				.delete()
@@ -212,9 +218,11 @@ class MailboxApiImpl implements MailboxApi {
 
 	@Override
 	public List<MailboxFile> getFiles(MailboxProperties properties,
-			MailboxFolderId folderId) throws IOException, ApiException {
+			MailboxFolderId folderId)
+			throws IOException, ApiException, TolerableFailureException {
 		String path = "/files/" + folderId;
 		Response response = sendGetRequest(properties, path);
+		if (response.code() == 404) throw new TolerableFailureException();
 		if (response.code() != 200) throw new ApiException();
 
 		ResponseBody body = response.body();
@@ -239,7 +247,7 @@ class MailboxApiImpl implements MailboxApi {
 				if (time < 1) throw new ApiException();
 				list.add(new MailboxFile(MailboxFileId.fromString(name), time));
 			}
-			Collections.sort(list);
+			sort(list);
 			return list;
 		} catch (JacksonException | InvalidMailboxIdException e) {
 			throw new ApiException();
@@ -248,9 +256,11 @@ class MailboxApiImpl implements MailboxApi {
 
 	@Override
 	public void getFile(MailboxProperties properties, MailboxFolderId folderId,
-			MailboxFileId fileId, File file) throws IOException, ApiException {
+			MailboxFileId fileId, File file)
+			throws IOException, ApiException, TolerableFailureException {
 		String path = "/files/" + folderId + "/" + fileId;
 		Response response = sendGetRequest(properties, path);
+		if (response.code() == 404) throw new TolerableFailureException();
 		if (response.code() != 200) throw new ApiException();
 
 		ResponseBody body = response.body();
@@ -266,7 +276,7 @@ class MailboxApiImpl implements MailboxApi {
 		String path = "/files/" + folderId + "/" + fileId;
 		Request request = getRequestBuilder(properties.getAuthToken())
 				.delete()
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
 		Response response = client.newCall(request).execute();
@@ -308,7 +318,7 @@ class MailboxApiImpl implements MailboxApi {
 	private Response sendGetRequest(MailboxProperties properties, String path)
 			throws IOException {
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
 		return client.newCall(request).execute();
@@ -317,7 +327,7 @@ class MailboxApiImpl implements MailboxApi {
 	private Response sendPostRequest(MailboxProperties properties, String path,
 			RequestBody body) throws IOException {
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.post(body)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -339,4 +349,7 @@ class MailboxApiImpl implements MailboxApi {
 		return (ArrayNode) arrayNode;
 	}
 
+	private String getBaseUrl(MailboxProperties properties) {
+		return urlConverter.convertOnionToBaseUrl(properties.getOnion());
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClient.java

@@ -0,0 +1,55 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxClient {
+
+	/**
+	 * Asynchronously starts the client.
+	 */
+	void start();
+
+	/**
+	 * Destroys the client and its workers, cancelling any pending tasks or
+	 * retries.
+	 */
+	void destroy();
+
+	/**
+	 * Assigns a contact to the client for upload.
+	 *
+	 * @param properties Properties for communicating with the mailbox
+	 * managed by this client.
+	 * @param folderId The ID of the folder to which files will be uploaded.
+	 */
+	void assignContactForUpload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for upload.
+	 */
+	void deassignContactForUpload(ContactId c);
+
+	/**
+	 * Assigns a contact to the client for download.
+	 *
+	 * @param properties Properties for communicating with the mailbox
+	 * managed by this client.
+	 * @param folderId The ID of the folder from which files will be
+	 * downloaded.
+	 */
+	void assignContactForDownload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for download.
+	 */
+	void deassignContactForDownload(ContactId c);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClientFactory.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface MailboxClientFactory {
+
+	/**
+	 * Creates a client for communicating with a contact's mailbox.
+	 */
+	MailboxClient createContactMailboxClient(
+			TorReachabilityMonitor reachabilityMonitor);
+
+	/**
+	 * Creates a client for communicating with our own mailbox.
+	 */
+	MailboxClient createOwnMailboxClient(
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClientFactoryImpl.java

@@ -0,0 +1,52 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.concurrent.Executor;
+
+import javax.inject.Inject;
+import javax.inject.Provider;
+
+@NotNullByDefault
+class MailboxClientFactoryImpl implements MailboxClientFactory {
+
+	private final MailboxWorkerFactory workerFactory;
+	private final TaskScheduler taskScheduler;
+	private final Executor ioExecutor;
+	private final Provider<ContactMailboxConnectivityChecker>
+			contactCheckerProvider;
+	private final Provider<OwnMailboxConnectivityChecker> ownCheckerProvider;
+
+	@Inject
+	MailboxClientFactoryImpl(MailboxWorkerFactory workerFactory,
+			TaskScheduler taskScheduler,
+			@IoExecutor Executor ioExecutor,
+			Provider<ContactMailboxConnectivityChecker> contactCheckerProvider,
+			Provider<OwnMailboxConnectivityChecker> ownCheckerProvider) {
+		this.workerFactory = workerFactory;
+		this.taskScheduler = taskScheduler;
+		this.ioExecutor = ioExecutor;
+		this.contactCheckerProvider = contactCheckerProvider;
+		this.ownCheckerProvider = ownCheckerProvider;
+	}
+
+	@Override
+	public MailboxClient createContactMailboxClient(
+			TorReachabilityMonitor reachabilityMonitor) {
+		ConnectivityChecker connectivityChecker = contactCheckerProvider.get();
+		return new ContactMailboxClient(workerFactory, connectivityChecker,
+				reachabilityMonitor);
+	}
+
+	@Override
+	public MailboxClient createOwnMailboxClient(
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		ConnectivityChecker connectivityChecker = ownCheckerProvider.get();
+		return new OwnMailboxClient(workerFactory, connectivityChecker,
+				reachabilityMonitor, taskScheduler, ioExecutor, properties);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClientManager.java

@@ -0,0 +1,667 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
+import org.briarproject.bramble.api.db.DatabaseExecutor;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.Service;
+import org.briarproject.bramble.api.lifecycle.ServiceException;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxStatus;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.MailboxPairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUnpairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUpdateSentToNewContactEvent;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.isClientCompatibleWithServer;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.TorConstants.ID;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+/**
+ * This component manages a {@link MailboxClient} for each mailbox we know
+ * about and are able to use (our own mailbox and/or contacts' mailboxes).
+ * The clients are created when we come online (i.e. when the Tor plugin
+ * becomes {@link Plugin.State#ACTIVE active}) and destroyed when we go
+ * offline.
+ * <p/>
+ * The manager keeps track of the latest {@link MailboxUpdate} sent to and
+ * received from each contact. These updates are used to decide which
+ * mailboxes the manager needs clients for, and which mailbox (if any) should
+ * be used for uploading data to and/or downloading data from each contact.
+ * <p/>
+ * The assignments of contacts to mailboxes for upload and/or download may
+ * change when the following events happen:
+ * <ul>
+ *     <li> A mailbox is paired or unpaired </li>
+ *     <li> A contact is added or removed </li>
+ *     <li> A {@link MailboxUpdate} is received from a contact </li>
+ *     <li> We discover that our own mailbox's supported API versions have
+ *     changed </li>
+ * </ul>
+ * <p/>
+ * The manager keeps its mutable state consistent with the state in the DB by
+ * using commit actions and events to update the manager's state on the event
+ * thread.
+ */
+@ThreadSafe
+@NotNullByDefault
+class MailboxClientManager implements Service, EventListener {
+
+	private static final Logger LOG =
+			getLogger(MailboxClientManager.class.getName());
+
+	private final Executor eventExecutor, dbExecutor;
+	private final TransactionManager db;
+	private final ContactManager contactManager;
+	private final PluginManager pluginManager;
+	private final MailboxSettingsManager mailboxSettingsManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
+	private final MailboxClientFactory mailboxClientFactory;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final AtomicBoolean used = new AtomicBoolean(false);
+
+	// All of the following mutable state must only be accessed on the
+	// event thread
+	private final Map<ContactId, Updates> contactUpdates = new HashMap<>();
+	private final Map<ContactId, MailboxClient> contactClients =
+			new HashMap<>();
+	@Nullable
+	private MailboxProperties ownProperties = null;
+	@Nullable
+	private MailboxClient ownClient = null;
+	private boolean online = false, handleEvents = false;
+
+	@Inject
+	MailboxClientManager(@EventExecutor Executor eventExecutor,
+			@DatabaseExecutor Executor dbExecutor,
+			TransactionManager db,
+			ContactManager contactManager,
+			PluginManager pluginManager,
+			MailboxSettingsManager mailboxSettingsManager,
+			MailboxUpdateManager mailboxUpdateManager,
+			MailboxClientFactory mailboxClientFactory,
+			TorReachabilityMonitor reachabilityMonitor) {
+		this.eventExecutor = eventExecutor;
+		this.dbExecutor = dbExecutor;
+		this.db = db;
+		this.contactManager = contactManager;
+		this.pluginManager = pluginManager;
+		this.mailboxSettingsManager = mailboxSettingsManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+		this.mailboxClientFactory = mailboxClientFactory;
+		this.reachabilityMonitor = reachabilityMonitor;
+	}
+
+	@Override
+	public void startService() throws ServiceException {
+		if (used.getAndSet(true)) throw new IllegalStateException();
+		reachabilityMonitor.start();
+		dbExecutor.execute(this::loadMailboxProperties);
+	}
+
+	@DatabaseExecutor
+	private void loadMailboxProperties() {
+		LOG.info("Loading mailbox properties");
+		try {
+			db.transaction(true, txn -> {
+				Map<ContactId, Updates> updates = new HashMap<>();
+				for (Contact c : contactManager.getContacts(txn)) {
+					MailboxUpdate local = mailboxUpdateManager
+							.getLocalUpdate(txn, c.getId());
+					MailboxUpdate remote = mailboxUpdateManager
+							.getRemoteUpdate(txn, c.getId());
+					updates.put(c.getId(), new Updates(local, remote));
+				}
+				MailboxProperties ownProps =
+						mailboxSettingsManager.getOwnMailboxProperties(txn);
+				// Use a commit action so the state in memory remains
+				// consistent with the state in the DB
+				txn.attach(() -> initialiseState(updates, ownProps));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void initialiseState(Map<ContactId, Updates> updates,
+			@Nullable MailboxProperties ownProps) {
+		contactUpdates.putAll(updates);
+		ownProperties = ownProps;
+		Plugin tor = pluginManager.getPlugin(ID);
+		if (tor != null && tor.getState() == ACTIVE) {
+			LOG.info("Online");
+			online = true;
+			createClients();
+		}
+		// Now that the mutable state has been initialised we can start
+		// handling events. This is done in a commit action so that we don't
+		// miss any changes to the DB state or handle events for any changes
+		// that were already reflected in the initial load
+		handleEvents = true;
+	}
+
+	@EventExecutor
+	private void createClients() {
+		LOG.info("Creating clients");
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isContactMailboxUsable(u.remote)) {
+				// Create and start a client for the contact's mailbox
+				MailboxClient contactClient = createAndStartClient(c);
+				// Assign the contact to the contact's mailbox for upload
+				assignContactToContactMailboxForUpload(c, contactClient, u);
+				if (!isOwnMailboxUsable(ownProperties, u.remote)) {
+					// We don't have a usable mailbox, so assign the contact to
+					// the contact's mailbox for download too
+					assignContactToContactMailboxForDownload(c,
+							contactClient, u);
+				}
+			}
+		}
+		if (ownProperties == null) return;
+		if (!isOwnMailboxUsable(ownProperties)) {
+			LOG.warning("We have a mailbox but we can't use it");
+			return;
+		}
+		// Create and start a client for our mailbox
+		createAndStartClientForOwnMailbox();
+		// Assign contacts to our mailbox for upload/download
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isOwnMailboxUsable(ownProperties, u.remote)) {
+				// Assign the contact to our mailbox for download
+				assignContactToOwnMailboxForDownload(c, u);
+				if (!isContactMailboxUsable(u.remote)) {
+					// The contact doesn't have a usable mailbox, so assign
+					// the contact to our mailbox for upload too
+					assignContactToOwnMailboxForUpload(c, u);
+				}
+			}
+		}
+	}
+
+	@Override
+	public void stopService() throws ServiceException {
+		CountDownLatch latch = new CountDownLatch(1);
+		eventExecutor.execute(() -> {
+			handleEvents = false;
+			if (online) destroyClients();
+			latch.countDown();
+		});
+		reachabilityMonitor.destroy();
+		try {
+			latch.await();
+		} catch (InterruptedException e) {
+			throw new ServiceException(e);
+		}
+	}
+
+	@EventExecutor
+	private void destroyClients() {
+		LOG.info("Destroying clients");
+		for (MailboxClient client : contactClients.values()) {
+			client.destroy();
+		}
+		contactClients.clear();
+		destroyOwnClient();
+	}
+
+	@EventExecutor
+	private void destroyOwnClient() {
+		if (ownClient != null) {
+			ownClient.destroy();
+			ownClient = null;
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (!handleEvents) return;
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorActive();
+		} else if (e instanceof TransportInactiveEvent) {
+			TransportInactiveEvent t = (TransportInactiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorInactive();
+		} else if (e instanceof MailboxPairedEvent) {
+			LOG.info("Mailbox paired");
+			MailboxPairedEvent m = (MailboxPairedEvent) e;
+			onMailboxPaired(m.getProperties(), m.getLocalUpdates());
+		} else if (e instanceof MailboxUnpairedEvent) {
+			LOG.info("Mailbox unpaired");
+			MailboxUnpairedEvent m = (MailboxUnpairedEvent) e;
+			onMailboxUnpaired(m.getLocalUpdates());
+		} else if (e instanceof MailboxUpdateSentToNewContactEvent) {
+			LOG.info("Contact added");
+			MailboxUpdateSentToNewContactEvent
+					m = (MailboxUpdateSentToNewContactEvent) e;
+			onContactAdded(m.getContactId(), m.getMailboxUpdate());
+		} else if (e instanceof ContactRemovedEvent) {
+			LOG.info("Contact removed");
+			ContactRemovedEvent c = (ContactRemovedEvent) e;
+			onContactRemoved(c.getContactId());
+		} else if (e instanceof RemoteMailboxUpdateEvent) {
+			LOG.info("Remote mailbox update");
+			RemoteMailboxUpdateEvent r = (RemoteMailboxUpdateEvent) e;
+			onRemoteMailboxUpdate(r.getContact(), r.getMailboxUpdate());
+		} else if (e instanceof OwnMailboxConnectionStatusEvent) {
+			OwnMailboxConnectionStatusEvent o =
+					(OwnMailboxConnectionStatusEvent) e;
+			onOwnMailboxConnectionStatusChanged(o.getStatus());
+		}
+	}
+
+	@EventExecutor
+	private void onTorActive() {
+		// If we checked the plugin at startup concurrently with the plugin
+		// becoming active then `online` may already be true when we receive
+		// the first TransportActiveEvent, in which case ignore it
+		if (online) return;
+		LOG.info("Online");
+		online = true;
+		createClients();
+	}
+
+	@EventExecutor
+	private void onTorInactive() {
+		// If we checked the plugin at startup concurrently with the plugin
+		// becoming inactive then `online` may already be false when we
+		// receive the first TransportInactiveEvent, in which case ignore it
+		if (!online) return;
+		LOG.info("Offline");
+		online = false;
+		destroyClients();
+	}
+
+	@EventExecutor
+	private void onMailboxPaired(MailboxProperties ownProps,
+			Map<ContactId, MailboxUpdateWithMailbox> localUpdates) {
+		for (Entry<ContactId, MailboxUpdateWithMailbox> e :
+				localUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = contactUpdates.get(c);
+			contactUpdates.put(c, new Updates(e.getValue(), u.remote));
+		}
+		ownProperties = ownProps;
+		if (!online) return;
+		if (!isOwnMailboxUsable(ownProperties)) {
+			LOG.warning("We have a mailbox but we can't use it");
+			return;
+		}
+		// Create and start a client for our mailbox
+		createAndStartClientForOwnMailbox();
+		// Assign contacts to our mailbox for upload/download
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (!isOwnMailboxUsable(ownProperties, u.remote)) {
+				// Our mailbox isn't usable for communicating with this
+				// contact, so don't assign/reassign this contact
+				continue;
+			}
+			if (isContactMailboxUsable(u.remote)) {
+				// The contact has a usable mailbox, so the contact should
+				// currently be assigned to the contact's mailbox for upload
+				// and download. Reassign the contact to our mailbox for
+				// download
+				MailboxClient contactClient =
+						requireNonNull(contactClients.get(c));
+				contactClient.deassignContactForDownload(c);
+			} else {
+				// The contact doesn't have a usable mailbox, so assign the
+				// contact to our mailbox for upload
+				assignContactToOwnMailboxForUpload(c, u);
+			}
+			assignContactToOwnMailboxForDownload(c, u);
+		}
+	}
+
+	@EventExecutor
+	private void onMailboxUnpaired(Map<ContactId, MailboxUpdate> localUpdates) {
+		for (Entry<ContactId, MailboxUpdate> e : localUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates updates = contactUpdates.get(c);
+			contactUpdates.put(c, new Updates(e.getValue(), updates.remote));
+		}
+		MailboxProperties oldOwnProperties = ownProperties;
+		ownProperties = null;
+		if (!online) return;
+		// Destroy the client for our own mailbox, if any
+		destroyOwnClient();
+		// Reassign contacts to their own mailboxes for download where possible
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isContactMailboxUsable(u.remote) &&
+					isOwnMailboxUsable(oldOwnProperties, u.remote)) {
+				// The contact should currently be assigned to our mailbox
+				// for download. Reassign the contact to the contact's
+				// mailbox for download
+				MailboxClient contactClient =
+						requireNonNull(contactClients.get(c));
+				assignContactToContactMailboxForDownload(c, contactClient, u);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onContactAdded(ContactId c, MailboxUpdate u) {
+		Updates old = contactUpdates.put(c, new Updates(u, null));
+		if (old != null) throw new IllegalStateException();
+		// We haven't yet received an update from the newly added contact,
+		// so at this stage we don't need to assign the contact to any
+		// mailboxes for upload or download
+	}
+
+	@EventExecutor
+	private void onContactRemoved(ContactId c) {
+		Updates updates = requireNonNull(contactUpdates.remove(c));
+		if (!online) return;
+		// Destroy the client for the contact's mailbox, if any
+		MailboxClient client = contactClients.remove(c);
+		if (client != null) client.destroy();
+		// If we have a mailbox and the contact is assigned to it for upload
+		// and/or download, deassign the contact
+		if (ownProperties == null) return;
+		if (isOwnMailboxUsable(ownProperties, updates.remote)) {
+			// We have a usable mailbox, so the contact should currently be
+			// assigned to our mailbox for download. Deassign the contact from
+			// our mailbox for download
+			requireNonNull(ownClient).deassignContactForDownload(c);
+			if (!isContactMailboxUsable(updates.remote)) {
+				// The contact doesn't have a usable mailbox, so the contact
+				// should currently be assigned to our mailbox for upload.
+				// Deassign the contact from our mailbox for upload
+				requireNonNull(ownClient).deassignContactForUpload(c);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onRemoteMailboxUpdate(ContactId c, MailboxUpdate remote) {
+		Updates old = contactUpdates.get(c);
+		MailboxUpdate oldRemote = old.remote;
+		Updates u = new Updates(old.local, remote);
+		contactUpdates.put(c, u);
+		if (!online) return;
+		// What may have changed?
+		// * Contact's mailbox may be usable now, was unusable before
+		// * Contact's mailbox may be unusable now, was usable before
+		// * Contact's mailbox may have been replaced
+		// * Contact's mailbox may have changed its API versions
+		// * Contact may be able to use our mailbox now, was unable before
+		// * Contact may be unable to use our mailbox now, was able before
+		boolean wasContactMailboxUsable = isContactMailboxUsable(oldRemote);
+		boolean isContactMailboxUsable = isContactMailboxUsable(remote);
+		boolean wasOwnMailboxUsable =
+				isOwnMailboxUsable(ownProperties, oldRemote);
+		boolean isOwnMailboxUsable = isOwnMailboxUsable(ownProperties, remote);
+
+		// Create/destroy/replace the client for the contact's mailbox if needed
+		MailboxClient contactClient = null;
+		boolean clientReplaced = false;
+		if (isContactMailboxUsable) {
+			if (wasContactMailboxUsable) {
+				MailboxProperties oldProps = getMailboxProperties(oldRemote);
+				MailboxProperties newProps = getMailboxProperties(remote);
+				if (oldProps.equals(newProps)) {
+					// The contact previously had a usable mailbox, now has
+					// a usable mailbox, it's the same mailbox, and its API
+					// versions haven't changed. Keep using the existing client
+					contactClient = requireNonNull(contactClients.get(c));
+				} else {
+					// The contact previously had a usable mailbox and now has
+					// a usable mailbox, but either it's a new mailbox or its
+					// API versions have changed. Replace the client
+					requireNonNull(contactClients.remove(c)).destroy();
+					contactClient = createAndStartClient(c);
+					clientReplaced = true;
+				}
+			} else {
+				// The client didn't previously have a usable mailbox but now
+				// has one. Create and start a client
+				contactClient = createAndStartClient(c);
+			}
+		} else if (wasContactMailboxUsable) {
+			// The client previously had a usable mailbox but no longer does.
+			// Destroy the existing client
+			requireNonNull(contactClients.remove(c)).destroy();
+		}
+
+		boolean wasAssignedToOwnMailboxForUpload =
+				wasOwnMailboxUsable && !wasContactMailboxUsable;
+		boolean willBeAssignedToOwnMailboxForUpload =
+				isOwnMailboxUsable && !isContactMailboxUsable;
+
+		boolean wasAssignedToContactMailboxForDownload =
+				!wasOwnMailboxUsable && wasContactMailboxUsable;
+		boolean willBeAssignedToContactMailboxForDownload =
+				!isOwnMailboxUsable && isContactMailboxUsable;
+
+		// Deassign the contact for upload/download if needed
+		if (wasAssignedToOwnMailboxForUpload &&
+				!willBeAssignedToOwnMailboxForUpload) {
+			requireNonNull(ownClient).deassignContactForUpload(c);
+		}
+		if (wasOwnMailboxUsable && !isOwnMailboxUsable) {
+			requireNonNull(ownClient).deassignContactForDownload(c);
+		}
+		// If the client for the contact's mailbox was replaced or destroyed
+		// above then we don't need to deassign the contact for download
+		if (wasAssignedToContactMailboxForDownload &&
+				!willBeAssignedToContactMailboxForDownload &&
+				!clientReplaced && isContactMailboxUsable) {
+			requireNonNull(contactClient).deassignContactForDownload(c);
+		}
+		// We never need to deassign the contact from the contact's mailbox for
+		// upload: this would only be needed if the contact's mailbox were no
+		// longer usable, in which case the client would already have been
+		// destroyed above. Thanks to the linter for spotting this
+
+		// Assign the contact for upload/download if needed
+		if (!wasAssignedToOwnMailboxForUpload &&
+				willBeAssignedToOwnMailboxForUpload) {
+			assignContactToOwnMailboxForUpload(c, u);
+		}
+		if (!wasOwnMailboxUsable && isOwnMailboxUsable) {
+			assignContactToOwnMailboxForDownload(c, u);
+		}
+		if ((!wasContactMailboxUsable || clientReplaced) &&
+				isContactMailboxUsable) {
+			assignContactToContactMailboxForUpload(c, contactClient, u);
+		}
+		if ((!wasAssignedToContactMailboxForDownload || clientReplaced) &&
+				willBeAssignedToContactMailboxForDownload) {
+			assignContactToContactMailboxForDownload(c, contactClient, u);
+		}
+	}
+
+	@EventExecutor
+	private void onOwnMailboxConnectionStatusChanged(MailboxStatus status) {
+		if (!online || ownProperties == null) return;
+		List<MailboxVersion> oldServerSupports =
+				ownProperties.getServerSupports();
+		List<MailboxVersion> newServerSupports = status.getServerSupports();
+		if (!oldServerSupports.equals(newServerSupports)) {
+			LOG.info("Our mailbox's supported API versions have changed");
+			// This potentially affects every assignment of contacts to
+			// mailboxes for upload and download, so just rebuild the clients
+			// and assignments from scratch
+			destroyClients();
+			createClients();
+		}
+	}
+
+	@EventExecutor
+	private void createAndStartClientForOwnMailbox() {
+		if (ownClient != null) throw new IllegalStateException();
+		ownClient = mailboxClientFactory.createOwnMailboxClient(
+				reachabilityMonitor, requireNonNull(ownProperties));
+		ownClient.start();
+	}
+
+	@EventExecutor
+	private MailboxClient createAndStartClient(ContactId c) {
+		MailboxClient client = mailboxClientFactory
+				.createContactMailboxClient(reachabilityMonitor);
+		MailboxClient old = contactClients.put(c, client);
+		if (old != null) throw new IllegalStateException();
+		client.start();
+		return client;
+	}
+
+	@EventExecutor
+	private void assignContactToOwnMailboxForDownload(ContactId c, Updates u) {
+		MailboxProperties localProps = getMailboxProperties(u.local);
+		requireNonNull(ownClient).assignContactForDownload(c,
+				requireNonNull(ownProperties),
+				requireNonNull(localProps.getOutboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToOwnMailboxForUpload(ContactId c, Updates u) {
+		MailboxProperties localProps = getMailboxProperties(u.local);
+		requireNonNull(ownClient).assignContactForUpload(c,
+				requireNonNull(ownProperties),
+				requireNonNull(localProps.getInboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToContactMailboxForDownload(ContactId c,
+			MailboxClient contactClient, Updates u) {
+		MailboxProperties remoteProps =
+				getMailboxProperties(requireNonNull(u.remote));
+		contactClient.assignContactForDownload(c, remoteProps,
+				requireNonNull(remoteProps.getInboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToContactMailboxForUpload(ContactId c,
+			MailboxClient contactClient, Updates u) {
+		MailboxProperties remoteProps =
+				getMailboxProperties(requireNonNull(u.remote));
+		contactClient.assignContactForUpload(c, remoteProps,
+				requireNonNull(remoteProps.getOutboxId()));
+	}
+
+	/**
+	 * Returns the {@link MailboxProperties} included in the given update,
+	 * which must be a {@link MailboxUpdateWithMailbox}.
+	 */
+	private MailboxProperties getMailboxProperties(MailboxUpdate update) {
+		if (!(update instanceof MailboxUpdateWithMailbox)) {
+			throw new IllegalArgumentException();
+		}
+		MailboxUpdateWithMailbox mailbox = (MailboxUpdateWithMailbox) update;
+		return mailbox.getMailboxProperties();
+	}
+
+	/**
+	 * Returns true if we can use our own mailbox to communicate with the
+	 * contact that sent the given update.
+	 */
+	private boolean isOwnMailboxUsable(
+			@Nullable MailboxProperties ownProperties,
+			@Nullable MailboxUpdate remote) {
+		if (ownProperties == null || remote == null) return false;
+		return isMailboxUsable(remote.getClientSupports(),
+				ownProperties.getServerSupports());
+	}
+
+	/**
+	 * Returns true if we can use the contact's mailbox to communicate with
+	 * the contact that sent the given update.
+	 */
+	private boolean isContactMailboxUsable(@Nullable MailboxUpdate remote) {
+		if (remote instanceof MailboxUpdateWithMailbox) {
+			MailboxUpdateWithMailbox remoteMailbox =
+					(MailboxUpdateWithMailbox) remote;
+			return isMailboxUsable(remoteMailbox.getClientSupports(),
+					remoteMailbox.getMailboxProperties().getServerSupports());
+		}
+		return false;
+	}
+
+	/**
+	 * Returns true if we can communicate with a contact that has the given
+	 * client-supported API versions via a mailbox with the given
+	 * server-supported API versions.
+	 */
+	private boolean isMailboxUsable(List<MailboxVersion> contactClient,
+			List<MailboxVersion> server) {
+		return isClientCompatibleWithServer(contactClient, server)
+				&& isClientCompatibleWithServer(CLIENT_SUPPORTS, server);
+	}
+
+	/**
+	 * Returns true if our client-supported API versions are compatible with
+	 * our own mailbox's server-supported API versions.
+	 */
+	@SuppressWarnings("BooleanMethodIsAlwaysInverted")
+	private boolean isOwnMailboxUsable(MailboxProperties ownProperties) {
+		return isClientCompatibleWithServer(CLIENT_SUPPORTS,
+				ownProperties.getServerSupports());
+	}
+
+	/**
+	 * A container for the latest {@link MailboxUpdate updates} sent to and
+	 * received from a given contact.
+	 */
+	private static class Updates {
+
+		/**
+		 * The latest update sent to the contact.
+		 */
+		private final MailboxUpdate local;
+
+		/**
+		 * The latest update received from the contact, or null if no update
+		 * has been received.
+		 */
+		@Nullable
+		private final MailboxUpdate remote;
+
+		private Updates(MailboxUpdate local, @Nullable MailboxUpdate remote) {
+			this.local = local;
+			this.remote = remote;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxDownloadWorker.java

@@ -0,0 +1,250 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxFileId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+import org.briarproject.bramble.mailbox.TorReachabilityMonitor.TorReachabilityObserver;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Queue;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class MailboxDownloadWorker implements MailboxWorker,
+		ConnectivityObserver, TorReachabilityObserver {
+
+	/**
+	 * When the worker is started it waits for a connectivity check, then
+	 * starts its first download cycle: checking for files to download,
+	 * downloading and deleting the files, and checking again until all files
+	 * have been downloaded and deleted.
+	 * <p>
+	 * The worker then waits for our Tor hidden service to be reachable before
+	 * starting its second download cycle. This ensures that if a contact
+	 * tried and failed to connect to our hidden service before it was
+	 * reachable, and therefore uploaded a file to the mailbox instead, we'll
+	 * find the file in the second download cycle.
+	 */
+	protected enum State {
+		CREATED,
+		CONNECTIVITY_CHECK,
+		DOWNLOAD_CYCLE_1,
+		WAITING_FOR_TOR,
+		DOWNLOAD_CYCLE_2,
+		FINISHED,
+		DESTROYED
+	}
+
+	protected static final Logger LOG =
+			getLogger(MailboxDownloadWorker.class.getName());
+
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor torReachabilityMonitor;
+	protected final MailboxApiCaller mailboxApiCaller;
+	protected final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	protected final MailboxProperties mailboxProperties;
+	protected final Object lock = new Object();
+
+	@GuardedBy("lock")
+	protected State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	protected Cancellable apiCall = null;
+
+	/**
+	 * Creates the API call that starts the worker's download cycle.
+	 */
+	protected abstract ApiCall createApiCallForDownloadCycle();
+
+	MailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		this.connectivityChecker = connectivityChecker;
+		this.torReachabilityMonitor = torReachabilityMonitor;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxProperties = mailboxProperties;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			// Don't allow the worker to be reused
+			if (state != State.CREATED) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable apiCall;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+		}
+		if (apiCall != null) apiCall.cancel();
+		connectivityChecker.removeObserver(this);
+		torReachabilityMonitor.removeObserver(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.DOWNLOAD_CYCLE_1;
+			// Start first download cycle
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					createApiCallForDownloadCycle());
+		}
+	}
+
+	void onDownloadCycleFinished() {
+		boolean addObserver = false;
+		synchronized (lock) {
+			if (state == State.DOWNLOAD_CYCLE_1) {
+				LOG.info("First download cycle finished");
+				state = State.WAITING_FOR_TOR;
+				apiCall = null;
+				addObserver = true;
+			} else if (state == State.DOWNLOAD_CYCLE_2) {
+				LOG.info("Second download cycle finished");
+				state = State.FINISHED;
+				apiCall = null;
+			}
+		}
+		if (addObserver) {
+			// Avoid leaking observer in case destroy() is called concurrently
+			// before observer is added
+			torReachabilityMonitor.addOneShotObserver(this);
+			boolean destroyed;
+			synchronized (lock) {
+				destroyed = state == State.DESTROYED;
+			}
+			if (destroyed) torReachabilityMonitor.removeObserver(this);
+		}
+	}
+
+	void downloadNextFile(Queue<FolderFile> queue) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			if (queue.isEmpty()) {
+				// Check for files again, as new files may have arrived while
+				// we were downloading
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						createApiCallForDownloadCycle());
+			} else {
+				FolderFile file = queue.remove();
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						new SimpleApiCall(() ->
+								apiCallDownloadFile(file, queue)));
+			}
+		}
+	}
+
+	private void apiCallDownloadFile(FolderFile file, Queue<FolderFile> queue)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Downloading file");
+		File tempFile = mailboxFileManager.createTempFileForDownload();
+		try {
+			mailboxApi.getFile(mailboxProperties, file.folderId, file.fileId,
+					tempFile);
+		} catch (IOException | ApiException e) {
+			if (!tempFile.delete()) {
+				LOG.warning("Failed to delete temporary file");
+			}
+			throw e;
+		} catch (TolerableFailureException e) {
+			// File not found - continue to the next file
+			LOG.warning("File does not exist");
+			if (!tempFile.delete()) {
+				LOG.warning("Failed to delete temporary file");
+			}
+			downloadNextFile(queue);
+			return;
+		}
+		mailboxFileManager.handleDownloadedFile(tempFile);
+		deleteFile(file, queue);
+	}
+
+	private void deleteFile(FolderFile file, Queue<FolderFile> queue) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					new SimpleApiCall(() -> apiCallDeleteFile(file, queue)));
+		}
+	}
+
+	private void apiCallDeleteFile(FolderFile file, Queue<FolderFile> queue)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		try {
+			mailboxApi.deleteFile(mailboxProperties, file.folderId,
+					file.fileId);
+		} catch (TolerableFailureException e) {
+			// File not found - continue to the next file
+			LOG.warning("File does not exist");
+		}
+		downloadNextFile(queue);
+	}
+
+	@Override
+	public void onTorReachable() {
+		LOG.info("Our Tor hidden service is reachable");
+		synchronized (lock) {
+			if (state != State.WAITING_FOR_TOR) return;
+			state = State.DOWNLOAD_CYCLE_2;
+			// Start second download cycle
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					createApiCallForDownloadCycle());
+		}
+	}
+
+	// Package access for testing
+	static class FolderFile {
+
+		final MailboxFolderId folderId;
+		final MailboxFileId fileId;
+
+		FolderFile(MailboxFolderId folderId, MailboxFileId fileId) {
+			this.folderId = folderId;
+			this.fileId = fileId;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxFileManager.java

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+
+import java.io.File;
+import java.io.IOException;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxFileManager {
+
+	/**
+	 * Creates an empty file for storing a download.
+	 */
+	File createTempFileForDownload() throws IOException;
+
+	/**
+	 * Creates a file to be uploaded to the given contact and writes any
+	 * waiting data to the file. The IDs of any messages sent or acked will
+	 * be added to the given {@link OutgoingSessionRecord}.
+	 */
+	File createAndWriteTempFileForUpload(ContactId contactId,
+			OutgoingSessionRecord sessionRecord) throws IOException;
+
+	/**
+	 * Handles a file that has been downloaded. The file should be created
+	 * with {@link #createTempFileForDownload()}.
+	 */
+	void handleDownloadedFile(File f);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxFileManagerImpl.java

@@ -0,0 +1,271 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.connection.ConnectionManager;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.mailbox.MailboxDirectory;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.TransportConnectionReader;
+import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.RUNNING;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.ID;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.plugin.file.FileConstants.PROP_PATH;
+import static org.briarproject.bramble.util.IoUtils.delete;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class MailboxFileManagerImpl implements MailboxFileManager, EventListener {
+
+	private static final Logger LOG =
+			getLogger(MailboxFileManagerImpl.class.getName());
+
+	// Package access for testing
+	static final String DOWNLOAD_DIR_NAME = "downloads";
+	static final String UPLOAD_DIR_NAME = "uploads";
+
+	private final Executor ioExecutor;
+	private final PluginManager pluginManager;
+	private final ConnectionManager connectionManager;
+	private final LifecycleManager lifecycleManager;
+	private final File mailboxDir;
+	private final EventBus eventBus;
+	private final CountDownLatch orphanLatch = new CountDownLatch(1);
+
+	@Inject
+	MailboxFileManagerImpl(@IoExecutor Executor ioExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			LifecycleManager lifecycleManager,
+			@MailboxDirectory File mailboxDir,
+			EventBus eventBus) {
+		this.ioExecutor = ioExecutor;
+		this.pluginManager = pluginManager;
+		this.connectionManager = connectionManager;
+		this.lifecycleManager = lifecycleManager;
+		this.mailboxDir = mailboxDir;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public File createTempFileForDownload() throws IOException {
+		return createTempFile(DOWNLOAD_DIR_NAME);
+	}
+
+	@Override
+	public File createAndWriteTempFileForUpload(ContactId contactId,
+			OutgoingSessionRecord sessionRecord) throws IOException {
+		File f = createTempFile(UPLOAD_DIR_NAME);
+		// We shouldn't reach this point until the plugin has been started
+		SimplexPlugin plugin =
+				(SimplexPlugin) requireNonNull(pluginManager.getPlugin(ID));
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, f.getAbsolutePath());
+		TransportConnectionWriter writer = plugin.createWriter(p);
+		if (writer == null) {
+			delete(f);
+			throw new IOException();
+		}
+		MailboxFileWriter decorated = new MailboxFileWriter(writer);
+		LOG.info("Writing file for upload");
+		connectionManager.manageOutgoingConnection(contactId, ID, decorated,
+				sessionRecord);
+		if (decorated.awaitDisposal()) {
+			// An exception was thrown during the session - delete the file
+			delete(f);
+			throw new IOException();
+		}
+		return f;
+	}
+
+	private File createTempFile(String dirName) throws IOException {
+		// Wait for orphaned files to be handled before creating new files
+		try {
+			orphanLatch.await();
+		} catch (InterruptedException e) {
+			throw new IOException(e);
+		}
+		File dir = createDirectoryIfNeeded(dirName);
+		return File.createTempFile("mailbox", ".tmp", dir);
+	}
+
+	private File createDirectoryIfNeeded(String name) throws IOException {
+		File dir = new File(mailboxDir, name);
+		//noinspection ResultOfMethodCallIgnored
+		dir.mkdirs();
+		if (!dir.isDirectory()) {
+			throw new IOException("Failed to create directory '" + name + "'");
+		}
+		return dir;
+	}
+
+	@Override
+	public void handleDownloadedFile(File f) {
+		// We shouldn't reach this point until the plugin has been started
+		SimplexPlugin plugin =
+				(SimplexPlugin) requireNonNull(pluginManager.getPlugin(ID));
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, f.getAbsolutePath());
+		TransportConnectionReader reader = plugin.createReader(p);
+		if (reader == null) {
+			LOG.warning("Failed to create reader for downloaded file");
+			return;
+		}
+		TransportConnectionReader decorated = new MailboxFileReader(reader, f);
+		LOG.info("Reading downloaded file");
+		connectionManager.manageIncomingConnection(ID, decorated,
+				exception -> isHandlingComplete(exception, true));
+	}
+
+	private boolean isHandlingComplete(boolean exception, boolean recognised) {
+		// If we've successfully read the file then we're done
+		if (!exception && recognised) return true;
+		// If the app is shutting down we may get spurious IO exceptions
+		// due to executors being shut down. Leave the file in the download
+		// directory and we'll try to read it again at the next startup
+		return !lifecycleManager.getLifecycleState().isAfter(RUNNING);
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		// Wait for the transport to become active before handling orphaned
+		// files so that we can get the plugin from the plugin manager
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) {
+				ioExecutor.execute(this::handleOrphanedFiles);
+				eventBus.removeListener(this);
+			}
+		}
+	}
+
+	/**
+	 * This method is called at startup, as soon as the plugin is started, to
+	 * delete any files that were left in the upload directory at the last
+	 * shutdown and handle any files that were left in the download directory.
+	 */
+	@IoExecutor
+	private void handleOrphanedFiles() {
+		try {
+			File uploadDir = createDirectoryIfNeeded(UPLOAD_DIR_NAME);
+			File[] orphanedUploads = uploadDir.listFiles();
+			if (orphanedUploads != null) {
+				for (File f : orphanedUploads) delete(f);
+			}
+			File downloadDir = createDirectoryIfNeeded(DOWNLOAD_DIR_NAME);
+			File[] orphanedDownloads = downloadDir.listFiles();
+			// Now that we've got the list of orphaned downloads, new files
+			// can be created in the download directory
+			orphanLatch.countDown();
+			if (orphanedDownloads != null) {
+				for (File f : orphanedDownloads) handleDownloadedFile(f);
+			}
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	private class MailboxFileReader implements TransportConnectionReader {
+
+		private final TransportConnectionReader delegate;
+		private final File file;
+
+		private MailboxFileReader(TransportConnectionReader delegate,
+				File file) {
+			this.delegate = delegate;
+			this.file = file;
+		}
+
+		@Override
+		public InputStream getInputStream() throws IOException {
+			return delegate.getInputStream();
+		}
+
+		@Override
+		public void dispose(boolean exception, boolean recognised)
+				throws IOException {
+			delegate.dispose(exception, recognised);
+			if (isHandlingComplete(exception, recognised)) {
+				LOG.info("Deleting downloaded file");
+				delete(file);
+			}
+		}
+	}
+
+	private static class MailboxFileWriter
+			implements TransportConnectionWriter {
+
+		private final TransportConnectionWriter delegate;
+		private final BlockingQueue<Boolean> disposalResult =
+				new ArrayBlockingQueue<>(1);
+
+		private MailboxFileWriter(TransportConnectionWriter delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public long getMaxLatency() {
+			return delegate.getMaxLatency();
+		}
+
+		@Override
+		public int getMaxIdleTime() {
+			return delegate.getMaxIdleTime();
+		}
+
+		@Override
+		public boolean isLossyAndCheap() {
+			return delegate.isLossyAndCheap();
+		}
+
+		@Override
+		public OutputStream getOutputStream() throws IOException {
+			return delegate.getOutputStream();
+		}
+
+		@Override
+		public void dispose(boolean exception) throws IOException {
+			delegate.dispose(exception);
+			disposalResult.add(exception);
+		}
+
+		/**
+		 * Waits for the delegate to be disposed and returns true if an
+		 * exception occurred.
+		 */
+		private boolean awaitDisposal() {
+			try {
+				return disposalResult.take();
+			} catch (InterruptedException e) {
+				LOG.info("Interrupted while waiting for disposal");
+				return true;
+			}
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxManagerImpl.java

@@ -9,10 +9,12 @@ import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxStatus;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 
 import java.io.IOException;
+import java.util.List;
 import java.util.concurrent.Executor;
 import java.util.logging.Logger;
 
@@ -98,34 +100,35 @@ class MailboxManagerImpl implements MailboxManager {
 
 	@Override
 	public boolean checkConnection() {
-		boolean success;
+		List<MailboxVersion> versions = null;
 		try {
 			MailboxProperties props = db.transactionWithNullableResult(true,
 					mailboxSettingsManager::getOwnMailboxProperties);
 			if (props == null) throw new DbException();
-			success = api.checkStatus(props);
+			versions = api.getServerSupports(props);
 		} catch (DbException e) {
 			logException(LOG, WARNING, e);
 			// we don't treat this is a failure to record
 			return false;
 		} catch (IOException | MailboxApi.ApiException e) {
 			// we record this as a failure
-			success = false;
 			logException(LOG, WARNING, e);
 		}
 		try {
-			recordCheckResult(success);
+			recordCheckResult(versions);
 		} catch (DbException e) {
 			logException(LOG, WARNING, e);
 		}
-		return success;
+		return versions != null;
 	}
 
-	private void recordCheckResult(boolean success) throws DbException {
+	private void recordCheckResult(@Nullable List<MailboxVersion> versions)
+			throws DbException {
 		long now = clock.currentTimeMillis();
 		db.transaction(false, txn -> {
-			if (success) {
+			if (versions != null) {
-				mailboxSettingsManager.recordSuccessfulConnection(txn, now);
+				mailboxSettingsManager
+						.recordSuccessfulConnection(txn, now, versions);
 			} else {
 				mailboxSettingsManager.recordFailedConnectionAttempt(txn, now);
 			}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxModule.java

@@ -4,16 +4,22 @@ import org.briarproject.bramble.api.FeatureFlags;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.db.DatabaseExecutor;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.mailbox.MailboxManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.plugin.PluginManager;
 import org.briarproject.bramble.api.sync.validation.ValidationManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 
 import java.util.List;
+import java.util.concurrent.Executor;
 
 import javax.inject.Inject;
 import javax.inject.Singleton;
@@ -21,10 +27,10 @@ import javax.inject.Singleton;
 import dagger.Module;
 import dagger.Provides;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
 import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.CLIENT_ID;
 import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MAJOR_VERSION;
 import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MINOR_VERSION;
-import static org.briarproject.bramble.mailbox.MailboxApi.CLIENT_SUPPORTS;
 
 @Module
 public class MailboxModule {
@@ -34,6 +40,10 @@ public class MailboxModule {
 		MailboxUpdateValidator mailboxUpdateValidator;
 		@Inject
 		MailboxUpdateManager mailboxUpdateManager;
+		@Inject
+		MailboxFileManager mailboxFileManager;
+		@Inject
+		MailboxClientManager mailboxClientManager;
 	}
 
 	@Provides
@@ -49,13 +59,19 @@ public class MailboxModule {
 	}
 
 	@Provides
+	@Singleton
 	MailboxSettingsManager provideMailboxSettingsManager(
 			MailboxSettingsManagerImpl mailboxSettingsManager) {
 		return mailboxSettingsManager;
 	}
 
 	@Provides
-	MailboxApi providesMailboxApi(MailboxApiImpl mailboxApi) {
+	UrlConverter provideUrlConverter(UrlConverterImpl urlConverter) {
+		return urlConverter;
+	}
+
+	@Provides
+	MailboxApi provideMailboxApi(MailboxApiImpl mailboxApi) {
 		return mailboxApi;
 	}
 
@@ -101,4 +117,65 @@ public class MailboxModule {
 		}
 		return mailboxUpdateManager;
 	}
+
+	@Provides
+	@Singleton
+	MailboxFileManager provideMailboxFileManager(FeatureFlags featureFlags,
+			EventBus eventBus, MailboxFileManagerImpl mailboxFileManager) {
+		if (featureFlags.shouldEnableMailbox()) {
+			eventBus.addListener(mailboxFileManager);
+		}
+		return mailboxFileManager;
+	}
+
+	@Provides
+	MailboxWorkerFactory provideMailboxWorkerFactory(
+			MailboxWorkerFactoryImpl mailboxWorkerFactory) {
+		return mailboxWorkerFactory;
+	}
+
+	@Provides
+	MailboxClientFactory provideMailboxClientFactory(
+			MailboxClientFactoryImpl mailboxClientFactory) {
+		return mailboxClientFactory;
+	}
+
+	@Provides
+	MailboxApiCaller provideMailboxApiCaller(
+			MailboxApiCallerImpl mailboxApiCaller) {
+		return mailboxApiCaller;
+	}
+
+	@Provides
+	@Singleton
+	TorReachabilityMonitor provideTorReachabilityMonitor(
+			TorReachabilityMonitorImpl reachabilityMonitor) {
+		return reachabilityMonitor;
+	}
+
+	@Provides
+	@Singleton
+	MailboxClientManager provideMailboxClientManager(
+			@EventExecutor Executor eventExecutor,
+			@DatabaseExecutor Executor dbExecutor,
+			TransactionManager db,
+			ContactManager contactManager,
+			PluginManager pluginManager,
+			MailboxSettingsManager mailboxSettingsManager,
+			MailboxUpdateManager mailboxUpdateManager,
+			MailboxClientFactory mailboxClientFactory,
+			TorReachabilityMonitor reachabilityMonitor,
+			FeatureFlags featureFlags,
+			LifecycleManager lifecycleManager,
+			EventBus eventBus) {
+		MailboxClientManager manager = new MailboxClientManager(eventExecutor,
+				dbExecutor, db, contactManager, pluginManager,
+				mailboxSettingsManager, mailboxUpdateManager,
+				mailboxClientFactory, reachabilityMonitor);
+		if (featureFlags.shouldEnableMailbox()) {
+			lifecycleManager.registerService(manager);
+			eventBus.addListener(manager);
+		}
+		return manager;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxPairingTaskImpl.java

@@ -120,7 +120,8 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		db.transaction(false, txn -> {
 			mailboxSettingsManager
 					.setOwnMailboxProperties(txn, ownerProperties);
-			mailboxSettingsManager.recordSuccessfulConnection(txn, time);
+			mailboxSettingsManager.recordSuccessfulConnection(txn, time,
+					ownerProperties.getServerSupports());
 			// A (possibly new) mailbox is paired. Reset message retransmission
 			// timers for contacts who doesn't have their own mailbox. This way,
 			// data stranded on our old mailbox will be re-uploaded to our new.
@@ -177,10 +178,9 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		LOG.info("QR code is valid");
 		byte[] onionPubKey = Arrays.copyOfRange(bytes, 1, 33);
 		String onion = crypto.encodeOnion(onionPubKey);
-		String baseUrl = "http://" + onion + ".onion"; // TODO
 		byte[] tokenBytes = Arrays.copyOfRange(bytes, 33, 65);
 		MailboxAuthToken setupToken = new MailboxAuthToken(tokenBytes);
-		return new MailboxProperties(baseUrl, setupToken, new ArrayList<>());
+		return new MailboxProperties(onion, setupToken, new ArrayList<>());
 	}
 
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxSettingsManagerImpl.java

@@ -20,12 +20,13 @@ import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
+import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;
 
+import static java.util.Collections.emptyList;
 import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 
-@Immutable
+@ThreadSafe
 @NotNullByDefault
 class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 
@@ -60,15 +61,7 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		String onion = s.get(SETTINGS_KEY_ONION);
 		String token = s.get(SETTINGS_KEY_TOKEN);
 		if (isNullOrEmpty(onion) || isNullOrEmpty(token)) return null;
-		int[] ints = s.getIntArray(SETTINGS_KEY_SERVER_SUPPORTS);
+		List<MailboxVersion> serverSupports = parseServerSupports(s);
-		// We know we were paired, so we must have proper serverSupports
-		if (ints == null || ints.length == 0 || ints.length % 2 != 0) {
-			throw new DbException();
-		}
-		List<MailboxVersion> serverSupports = new ArrayList<>();
-		for (int i = 0; i < ints.length - 1; i += 2) {
-			serverSupports.add(new MailboxVersion(ints[i], ints[i + 1]));
-		}
 		try {
 			MailboxAuthToken tokenId = MailboxAuthToken.fromString(token);
 			return new MailboxProperties(onion, tokenId, serverSupports);
@@ -81,19 +74,13 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 	public void setOwnMailboxProperties(Transaction txn, MailboxProperties p)
 			throws DbException {
 		Settings s = new Settings();
-		s.put(SETTINGS_KEY_ONION, p.getBaseUrl());
+		s.put(SETTINGS_KEY_ONION, p.getOnion());
 		s.put(SETTINGS_KEY_TOKEN, p.getAuthToken().toString());
 		List<MailboxVersion> serverSupports = p.getServerSupports();
-		int[] ints = new int[serverSupports.size() * 2];
+		encodeServerSupports(serverSupports, s);
-		int i = 0;
-		for (MailboxVersion v : serverSupports) {
-			ints[i++] = v.getMajor();
-			ints[i++] = v.getMinor();
-		}
-		s.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS, ints);
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
 		for (MailboxHook hook : hooks) {
-			hook.mailboxPaired(txn, p.getOnion(), p.getServerSupports());
+			hook.mailboxPaired(txn, p);
 		}
 	}
 
@@ -102,6 +89,10 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		Settings s = new Settings();
 		s.put(SETTINGS_KEY_ONION, "");
 		s.put(SETTINGS_KEY_TOKEN, "");
+		s.put(SETTINGS_KEY_ATTEMPTS, "");
+		s.put(SETTINGS_KEY_LAST_ATTEMPT, "");
+		s.put(SETTINGS_KEY_LAST_SUCCESS, "");
+		s.put(SETTINGS_KEY_SERVER_SUPPORTS, "");
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
 		for (MailboxHook hook : hooks) {
 			hook.mailboxUnpaired(txn);
@@ -115,33 +106,58 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		long lastAttempt = s.getLong(SETTINGS_KEY_LAST_ATTEMPT, -1);
 		long lastSuccess = s.getLong(SETTINGS_KEY_LAST_SUCCESS, -1);
 		int attempts = s.getInt(SETTINGS_KEY_ATTEMPTS, 0);
-		return new MailboxStatus(lastAttempt, lastSuccess, attempts);
+		List<MailboxVersion> serverSupports;
+		try {
+			serverSupports = parseServerSupports(s);
+		} catch (DbException e) {
+			serverSupports = emptyList();
+		}
+		return new MailboxStatus(lastAttempt, lastSuccess, attempts,
+				serverSupports);
 	}
 
 	@Override
-	public void recordSuccessfulConnection(Transaction txn, long now)
+	public void recordSuccessfulConnection(Transaction txn, long now,
-			throws DbException {
+			List<MailboxVersion> versions) throws DbException {
+		// if we no longer have a paired mailbox, return
+		Settings oldSettings =
+				settingsManager.getSettings(txn, SETTINGS_NAMESPACE);
+		String onion = oldSettings.get(SETTINGS_KEY_ONION);
+		String token = oldSettings.get(SETTINGS_KEY_TOKEN);
+		if (isNullOrEmpty(onion) || isNullOrEmpty(token)) return;
 		Settings s = new Settings();
+		// record the successful connection
 		s.putLong(SETTINGS_KEY_LAST_ATTEMPT, now);
 		s.putLong(SETTINGS_KEY_LAST_SUCCESS, now);
 		s.putInt(SETTINGS_KEY_ATTEMPTS, 0);
+		encodeServerSupports(versions, s);
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
-		MailboxStatus status = new MailboxStatus(now, now, 0);
+		for (MailboxHook hook : hooks) {
+			hook.serverSupportedVersionsReceived(txn, versions);
+		}
+		// broadcast status event
+		MailboxStatus status = new MailboxStatus(now, now, 0, versions);
 		txn.attach(new OwnMailboxConnectionStatusEvent(status));
 	}
 
 	@Override
 	public void recordFailedConnectionAttempt(Transaction txn, long now)
 			throws DbException {
+		// if we no longer have a paired mailbox, return
 		Settings oldSettings =
 				settingsManager.getSettings(txn, SETTINGS_NAMESPACE);
+		String onion = oldSettings.get(SETTINGS_KEY_ONION);
+		String token = oldSettings.get(SETTINGS_KEY_TOKEN);
+		if (isNullOrEmpty(onion) || isNullOrEmpty(token)) return;
 		int newAttempts = 1 + oldSettings.getInt(SETTINGS_KEY_ATTEMPTS, 0);
 		long lastSuccess = oldSettings.getLong(SETTINGS_KEY_LAST_SUCCESS, 0);
 		Settings newSettings = new Settings();
 		newSettings.putLong(SETTINGS_KEY_LAST_ATTEMPT, now);
 		newSettings.putInt(SETTINGS_KEY_ATTEMPTS, newAttempts);
 		settingsManager.mergeSettings(txn, newSettings, SETTINGS_NAMESPACE);
-		MailboxStatus status = new MailboxStatus(now, lastSuccess, newAttempts);
+		List<MailboxVersion> serverSupports = parseServerSupports(oldSettings);
+		MailboxStatus status = new MailboxStatus(now, lastSuccess, newAttempts,
+				serverSupports);
 		txn.attach(new OwnMailboxConnectionStatusEvent(status));
 		if (status.hasProblem(now)) txn.attach(new MailboxProblemEvent());
 	}
@@ -165,4 +181,30 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		if (isNullOrEmpty(filename)) return null;
 		return filename;
 	}
+
+	private void encodeServerSupports(List<MailboxVersion> serverSupports,
+			Settings s) {
+		int[] ints = new int[serverSupports.size() * 2];
+		int i = 0;
+		for (MailboxVersion v : serverSupports) {
+			ints[i++] = v.getMajor();
+			ints[i++] = v.getMinor();
+		}
+		s.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS, ints);
+	}
+
+	private List<MailboxVersion> parseServerSupports(Settings s)
+			throws DbException {
+		if (!s.containsKey(SETTINGS_KEY_SERVER_SUPPORTS)) return emptyList();
+		int[] ints = s.getIntArray(SETTINGS_KEY_SERVER_SUPPORTS);
+		// We know we were paired, so we must have proper serverSupports
+		if (ints == null || ints.length == 0 || ints.length % 2 != 0) {
+			throw new DbException();
+		}
+		List<MailboxVersion> serverSupports = new ArrayList<>();
+		for (int i = 0; i < ints.length - 1; i += 2) {
+			serverSupports.add(new MailboxVersion(ints[i], ints[i + 1]));
+		}
+		return serverSupports;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUpdateManagerImpl.java

@@ -25,6 +25,9 @@ import org.briarproject.bramble.api.mailbox.MailboxUpdate;
 import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
 import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.MailboxPairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUnpairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUpdateSentToNewContactEvent;
 import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.Group;
@@ -38,6 +41,7 @@ import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
 
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -45,6 +49,9 @@ import java.util.Map.Entry;
 import javax.annotation.Nullable;
 import javax.inject.Inject;
 
+import static java.util.Collections.emptyList;
+import static org.briarproject.bramble.api.data.BdfDictionary.NULL_VALUE;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
 import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 
 @NotNullByDefault
@@ -116,7 +123,7 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 			db.addGroup(txn, localGroup);
 			// Set things up for any pre-existing contacts
 			for (Contact c : db.getContacts(txn)) {
-				addingContact(txn, c);
+				addingContact(txn, c, false);
 			}
 		}
 
@@ -132,6 +139,17 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 
 	@Override
 	public void addingContact(Transaction txn, Contact c) throws DbException {
+		addingContact(txn, c, true);
+	}
+
+	/**
+	 * @param attachEvent True if a {@link MailboxUpdateSentToNewContactEvent}
+	 * should be attached to the transaction. We should only do this when
+	 * adding a new contact, not when setting up this client for an existing
+	 * contact.
+	 */
+	private void addingContact(Transaction txn, Contact c, boolean attachEvent)
+			throws DbException {
 		// Create a group to share with the contact
 		Group g = getContactGroup(c);
 		db.addGroup(txn, g);
@@ -143,13 +161,17 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 		clientHelper.setContactId(txn, g.getId(), c.getId());
 		MailboxProperties ownProps =
 				mailboxSettingsManager.getOwnMailboxProperties(txn);
+		MailboxUpdate u;
 		if (ownProps != null) {
 			// We are paired, create and send props to the newly added contact
-			createAndSendUpdateWithMailbox(txn, c, ownProps.getServerSupports(),
+			u = createAndSendUpdateWithMailbox(txn, c,
-					ownProps.getOnion());
+					ownProps.getServerSupports(), ownProps.getOnion());
 		} else {
 			// Not paired, but we still want to get our clientSupports sent
-			sendUpdateNoMailbox(txn, c);
+			u = sendUpdateNoMailbox(txn, c);
+		}
+		if (attachEvent) {
+			txn.attach(new MailboxUpdateSentToNewContactEvent(c.getId(), u));
 		}
 	}
 
@@ -159,18 +181,103 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 	}
 
 	@Override
-	public void mailboxPaired(Transaction txn, String ownOnion,
+	public void mailboxPaired(Transaction txn, MailboxProperties p)
-			List<MailboxVersion> serverSupports) throws DbException {
+			throws DbException {
+		Map<ContactId, MailboxUpdateWithMailbox> localUpdates = new HashMap<>();
 		for (Contact c : db.getContacts(txn)) {
-			createAndSendUpdateWithMailbox(txn, c, serverSupports, ownOnion);
+			MailboxUpdateWithMailbox u = createAndSendUpdateWithMailbox(txn, c,
+					p.getServerSupports(), p.getOnion());
+			localUpdates.put(c.getId(), u);
+		}
+		txn.attach(new MailboxPairedEvent(p, localUpdates));
+		// Store the list of server-supported versions
+		try {
+			storeSentServerSupports(txn, p.getServerSupports());
+		} catch (FormatException e) {
+			throw new DbException();
 		}
 	}
 
 	@Override
 	public void mailboxUnpaired(Transaction txn) throws DbException {
+		Map<ContactId, MailboxUpdate> localUpdates = new HashMap<>();
 		for (Contact c : db.getContacts(txn)) {
-			sendUpdateNoMailbox(txn, c);
+			MailboxUpdate u = sendUpdateNoMailbox(txn, c);
+			localUpdates.put(c.getId(), u);
 		}
+		txn.attach(new MailboxUnpairedEvent(localUpdates));
+		// Remove the list of server-supported versions
+		try {
+			BdfDictionary meta = BdfDictionary.of(new BdfEntry(
+					GROUP_KEY_SENT_SERVER_SUPPORTS, NULL_VALUE));
+			clientHelper.mergeGroupMetadata(txn, localGroup.getId(), meta);
+		} catch (FormatException e) {
+			throw new DbException();
+		}
+	}
+
+	@Override
+	public void serverSupportedVersionsReceived(Transaction txn,
+			List<MailboxVersion> serverSupports) throws DbException {
+		try {
+			List<MailboxVersion> oldServerSupports =
+					loadSentServerSupports(txn);
+			if (serverSupports.equals(oldServerSupports)) return;
+			storeSentServerSupports(txn, serverSupports);
+			for (Contact c : db.getContacts(txn)) {
+				Group contactGroup = getContactGroup(c);
+				LatestUpdate latest =
+						findLatest(txn, contactGroup.getId(), true);
+				// This method should only be called when we have a mailbox,
+				// in which case we should have sent a local update to every
+				// contact
+				if (latest == null) throw new DbException();
+				BdfList body =
+						clientHelper.getMessageAsList(txn, latest.messageId);
+				MailboxUpdate oldUpdate = parseUpdate(body);
+				if (!oldUpdate.hasMailbox()) throw new DbException();
+				MailboxUpdateWithMailbox newUpdate = updateServerSupports(
+						(MailboxUpdateWithMailbox) oldUpdate, serverSupports);
+				storeMessageReplaceLatest(txn, contactGroup.getId(), newUpdate,
+						latest);
+			}
+		} catch (FormatException e) {
+			throw new DbException();
+		}
+	}
+
+	private void storeSentServerSupports(Transaction txn,
+			List<MailboxVersion> serverSupports)
+			throws DbException, FormatException {
+		BdfDictionary meta = BdfDictionary.of(new BdfEntry(
+				GROUP_KEY_SENT_SERVER_SUPPORTS,
+				encodeSupportsList(serverSupports)));
+		clientHelper.mergeGroupMetadata(txn, localGroup.getId(), meta);
+	}
+
+	private List<MailboxVersion> loadSentServerSupports(Transaction txn)
+			throws DbException, FormatException {
+		BdfDictionary meta = clientHelper.getGroupMetadataAsDictionary(txn,
+				localGroup.getId());
+		BdfList serverSupports =
+				meta.getOptionalList(GROUP_KEY_SENT_SERVER_SUPPORTS);
+		if (serverSupports == null) return emptyList();
+		return clientHelper.parseMailboxVersionList(serverSupports);
+	}
+
+	/**
+	 * Returns a new {@link MailboxUpdateWithMailbox} that updates the list
+	 * of server-supported API versions in the given
+	 * {@link MailboxUpdateWithMailbox}.
+	 */
+	private MailboxUpdateWithMailbox updateServerSupports(
+			MailboxUpdateWithMailbox old, List<MailboxVersion> serverSupports) {
+		MailboxProperties oldProps = old.getMailboxProperties();
+		MailboxProperties newProps = new MailboxProperties(oldProps.getOnion(),
+				oldProps.getAuthToken(), serverSupports,
+				requireNonNull(oldProps.getInboxId()),
+				requireNonNull(oldProps.getOutboxId()));
+		return new MailboxUpdateWithMailbox(old.getClientSupports(), newProps);
 	}
 
 	@Override
@@ -239,19 +346,19 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 	 * supported Mailbox API version(s). All of which the contact needs to
 	 * communicate with our Mailbox.
 	 */
-	private void createAndSendUpdateWithMailbox(Transaction txn, Contact c,
+	private MailboxUpdateWithMailbox createAndSendUpdateWithMailbox(
-			List<MailboxVersion> serverSupports, String ownOnion)
+			Transaction txn, Contact c, List<MailboxVersion> serverSupports,
-			throws DbException {
+			String ownOnion) throws DbException {
-		String baseUrl = "http://" + ownOnion + ".onion"; // TODO
+		MailboxProperties properties = new MailboxProperties(ownOnion,
-		MailboxProperties properties = new MailboxProperties(baseUrl,
 				new MailboxAuthToken(crypto.generateUniqueId().getBytes()),
 				serverSupports,
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()),
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()));
-		MailboxUpdate u =
+		MailboxUpdateWithMailbox u =
 				new MailboxUpdateWithMailbox(clientSupports, properties);
 		Group g = getContactGroup(c);
 		storeMessageReplaceLatest(txn, g.getId(), u);
+		return u;
 	}
 
 	/**
@@ -260,11 +367,12 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 	 * Mailbox that they can use. It still includes the list of Mailbox API
 	 * version(s) that we support as a client.
 	 */
-	private void sendUpdateNoMailbox(Transaction txn, Contact c)
+	private MailboxUpdate sendUpdateNoMailbox(Transaction txn, Contact c)
 			throws DbException {
 		Group g = getContactGroup(c);
 		MailboxUpdate u = new MailboxUpdate(clientSupports);
 		storeMessageReplaceLatest(txn, g.getId(), u);
+		return u;
 	}
 
 	@Nullable
@@ -289,21 +397,27 @@ class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 			MailboxUpdate u) throws DbException {
 		try {
 			LatestUpdate latest = findLatest(txn, g, true);
-			long version = latest == null ? 1 : latest.version + 1;
+			storeMessageReplaceLatest(txn, g, u, latest);
-			Message m = clientHelper.createMessage(g, clock.currentTimeMillis(),
-					encodeProperties(version, u));
-			BdfDictionary meta = new BdfDictionary();
-			meta.put(MSG_KEY_VERSION, version);
-			meta.put(MSG_KEY_LOCAL, true);
-			clientHelper.addLocalMessage(txn, m, meta, true, false);
-			if (latest != null) {
-				db.removeMessage(txn, latest.messageId);
-			}
 		} catch (FormatException e) {
 			throw new DbException(e);
 		}
 	}
 
+	private void storeMessageReplaceLatest(Transaction txn, GroupId g,
+			MailboxUpdate u, @Nullable LatestUpdate latest)
+			throws DbException, FormatException {
+		long version = latest == null ? 1 : latest.version + 1;
+		Message m = clientHelper.createMessage(g, clock.currentTimeMillis(),
+				encodeProperties(version, u));
+		BdfDictionary meta = new BdfDictionary();
+		meta.put(MSG_KEY_VERSION, version);
+		meta.put(MSG_KEY_LOCAL, true);
+		clientHelper.addLocalMessage(txn, m, meta, true, false);
+		if (latest != null) {
+			db.removeMessage(txn, latest.messageId);
+		}
+	}
+
 	@Nullable
 	private LatestUpdate findLatest(Transaction txn, GroupId g, boolean local)
 			throws DbException, FormatException {

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUploadWorker.java

@@ -0,0 +1,464 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.connection.ConnectionRegistry;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.event.ContactConnectedEvent;
+import org.briarproject.bramble.api.plugin.event.ContactDisconnectedEvent;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+import org.briarproject.bramble.api.sync.event.GroupVisibilityUpdatedEvent;
+import org.briarproject.bramble.api.sync.event.MessageSharedEvent;
+import org.briarproject.bramble.api.sync.event.MessageToAckEvent;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.system.TaskScheduler;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.lang.Boolean.TRUE;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.MAX_LATENCY;
+import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
+import static org.briarproject.bramble.util.IoUtils.delete;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class MailboxUploadWorker implements MailboxWorker, ConnectivityObserver,
+		EventListener {
+
+	/**
+	 * When the worker is started it checks for data to send. If data is ready
+	 * to send, the worker waits for a connectivity check, then writes and
+	 * uploads a file and checks again for data to send.
+	 * <p>
+	 * If data is due to be sent at some time in the future, the worker
+	 * schedules a wakeup for that time and also listens for events indicating
+	 * that new data may be ready to send.
+	 * <p>
+	 * If there's no data to send, the worker listens for events indicating
+	 * that new data may be ready to send.
+	 * <p>
+	 * Whenever we're directly connected to the contact, the worker doesn't
+	 * check for data to send or start connectivity checks until the contact
+	 * disconnects. However, if the worker has already started writing and
+	 * uploading a file when the contact connects, the worker will finish the
+	 * upload.
+	 */
+	private enum State {
+		CREATED,
+		CONNECTED_TO_CONTACT,
+		CHECKING_FOR_DATA,
+		WAITING_FOR_DATA,
+		CONNECTIVITY_CHECK,
+		WRITING_UPLOADING,
+		DESTROYED
+	}
+
+	private static final Logger LOG =
+			getLogger(MailboxUploadWorker.class.getName());
+
+	/**
+	 * When we're waiting for data to send and an event indicates that new data
+	 * may have become available, wait this long before checking the DB. This
+	 * should help to avoid creating lots of small files when several acks or
+	 * messages become available to send in a short period (eg when reading a
+	 * file downloaded from a mailbox).
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CHECK_DELAY_MS = 5_000;
+
+	/**
+	 * How long to wait before retrying when an exception occurs while writing
+	 * a file.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long RETRY_DELAY_MS = MINUTES.toMillis(1);
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final Clock clock;
+	private final TaskScheduler taskScheduler;
+	private final EventBus eventBus;
+	private final ConnectionRegistry connectionRegistry;
+	private final ConnectivityChecker connectivityChecker;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	private final MailboxProperties mailboxProperties;
+	private final MailboxFolderId folderId;
+	private final ContactId contactId;
+
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable wakeupTask = null, checkTask = null, apiCall = null;
+
+	@GuardedBy("lock")
+	@Nullable
+	private File file = null;
+
+	MailboxUploadWorker(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			Clock clock,
+			TaskScheduler taskScheduler,
+			EventBus eventBus,
+			ConnectionRegistry connectionRegistry,
+			ConnectivityChecker connectivityChecker,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties,
+			MailboxFolderId folderId,
+			ContactId contactId) {
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.clock = clock;
+		this.taskScheduler = taskScheduler;
+		this.eventBus = eventBus;
+		this.connectionRegistry = connectionRegistry;
+		this.connectivityChecker = connectivityChecker;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxProperties = mailboxProperties;
+		this.folderId = folderId;
+		this.contactId = contactId;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			// Don't allow the worker to be reused
+			if (state != State.CREATED) return;
+			state = State.CHECKING_FOR_DATA;
+		}
+		ioExecutor.execute(this::checkForDataToSend);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable wakeupTask, checkTask, apiCall;
+		File file;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			wakeupTask = this.wakeupTask;
+			this.wakeupTask = null;
+			checkTask = this.checkTask;
+			this.checkTask = null;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+			file = this.file;
+			this.file = null;
+		}
+		if (wakeupTask != null) wakeupTask.cancel();
+		if (checkTask != null) checkTask.cancel();
+		if (apiCall != null) apiCall.cancel();
+		if (file != null) delete(file);
+		connectivityChecker.removeObserver(this);
+		eventBus.removeListener(this);
+	}
+
+	@IoExecutor
+	private void checkForDataToSend() {
+		synchronized (lock) {
+			checkTask = null;
+			if (state != State.CHECKING_FOR_DATA) return;
+			// Check whether we're directly connected to the contact. Calling
+			// this while holding the lock isn't ideal, but it avoids races
+			if (connectionRegistry.isConnected(contactId)) {
+				state = State.CONNECTED_TO_CONTACT;
+				return;
+			}
+		}
+		LOG.info("Checking for data to send");
+		try {
+			db.transaction(true, txn -> {
+				long nextSendTime;
+				if (db.containsAcksToSend(txn, contactId)) {
+					nextSendTime = 0L;
+				} else {
+					nextSendTime = db.getNextSendTime(txn, contactId,
+							MAX_LATENCY);
+				}
+				// Handle the result on the event executor to avoid races with
+				// incoming events
+				txn.attach(() -> handleNextSendTime(nextSendTime));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void handleNextSendTime(long nextSendTime) {
+		if (nextSendTime == Long.MAX_VALUE) {
+			// Nothing is sendable now or due to be sent in the future. Wait
+			// for an event indicating that new data may be ready to send
+			waitForDataToSend();
+		} else {
+			// Work out the delay until data's ready to send (may be negative)
+			long delay = nextSendTime - clock.currentTimeMillis();
+			if (delay > 0) {
+				// Schedule a wakeup when data will be ready to send. If an
+				// event is received in the meantime indicating that new data
+				// may be ready to send, we'll cancel the wakeup
+				scheduleWakeup(delay);
+			} else {
+				// Data is ready to send now
+				checkConnectivity();
+			}
+		}
+	}
+
+	@EventExecutor
+	private void waitForDataToSend() {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.WAITING_FOR_DATA;
+			LOG.info("Waiting for data to send");
+		}
+	}
+
+	@EventExecutor
+	private void scheduleWakeup(long delay) {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.WAITING_FOR_DATA;
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Scheduling wakeup in " + delay + " ms");
+			}
+			wakeupTask = taskScheduler.schedule(this::wakeUp, ioExecutor,
+					delay, MILLISECONDS);
+		}
+	}
+
+	@IoExecutor
+	private void wakeUp() {
+		LOG.info("Woke up");
+		synchronized (lock) {
+			wakeupTask = null;
+			if (state != State.WAITING_FOR_DATA) return;
+			state = State.CHECKING_FOR_DATA;
+		}
+		checkForDataToSend();
+	}
+
+	@EventExecutor
+	private void checkConnectivity() {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		LOG.info("Checking connectivity");
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.WRITING_UPLOADING;
+		}
+		ioExecutor.execute(this::writeAndUploadFile);
+	}
+
+	@IoExecutor
+	private void writeAndUploadFile() {
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+		}
+		OutgoingSessionRecord sessionRecord = new OutgoingSessionRecord();
+		File file;
+		try {
+			file = mailboxFileManager.createAndWriteTempFileForUpload(
+					contactId, sessionRecord);
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+			// Try again after a delay
+			synchronized (lock) {
+				if (state != State.WRITING_UPLOADING) return;
+				state = State.CHECKING_FOR_DATA;
+				checkTask = taskScheduler.schedule(this::checkForDataToSend,
+						ioExecutor, RETRY_DELAY_MS, MILLISECONDS);
+			}
+			return;
+		}
+		boolean deleteFile = false;
+		synchronized (lock) {
+			if (state == State.WRITING_UPLOADING) {
+				this.file = file;
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						new SimpleApiCall(() -> apiCallUploadFile(file,
+								sessionRecord)));
+			} else {
+				deleteFile = true;
+			}
+		}
+		if (deleteFile) delete(file);
+	}
+
+	@IoExecutor
+	private void apiCallUploadFile(File file,
+			OutgoingSessionRecord sessionRecord)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+		}
+		LOG.info("Uploading file");
+		mailboxApi.addFile(mailboxProperties, folderId, file);
+		markMessagesSentOrAcked(sessionRecord);
+		delete(file);
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+			state = State.CHECKING_FOR_DATA;
+			apiCall = null;
+			this.file = null;
+		}
+		checkForDataToSend();
+	}
+
+	private void markMessagesSentOrAcked(OutgoingSessionRecord sessionRecord) {
+		Collection<MessageId> acked = sessionRecord.getAckedIds();
+		Collection<MessageId> sent = sessionRecord.getSentIds();
+		try {
+			db.transaction(false, txn -> {
+				if (!acked.isEmpty()) {
+					db.setAckSent(txn, contactId, acked);
+				}
+				if (!sent.isEmpty()) {
+					db.setMessagesSent(txn, contactId, sent, MAX_LATENCY);
+				}
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof MessageToAckEvent) {
+			MessageToAckEvent m = (MessageToAckEvent) e;
+			if (m.getContactId().equals(contactId)) {
+				LOG.info("Message to ack");
+				onDataToSend();
+			}
+		} else if (e instanceof MessageSharedEvent) {
+			MessageSharedEvent m = (MessageSharedEvent) e;
+			// If the contact is present in the map (ie the value is not null)
+			// and the value is true, the message's group is shared with the
+			// contact and therefore the message may now be sendable
+			if (m.getGroupVisibility().get(contactId) == TRUE) {
+				LOG.info("Message shared");
+				onDataToSend();
+			}
+		} else if (e instanceof GroupVisibilityUpdatedEvent) {
+			GroupVisibilityUpdatedEvent g = (GroupVisibilityUpdatedEvent) e;
+			if (g.getVisibility() == SHARED &&
+					g.getAffectedContacts().contains(contactId)) {
+				LOG.info("Group shared");
+				onDataToSend();
+			}
+		} else if (e instanceof ContactConnectedEvent) {
+			ContactConnectedEvent c = (ContactConnectedEvent) e;
+			if (c.getContactId().equals(contactId)) {
+				LOG.info("Contact connected");
+				onContactConnected();
+			}
+		} else if (e instanceof ContactDisconnectedEvent) {
+			ContactDisconnectedEvent c = (ContactDisconnectedEvent) e;
+			if (c.getContactId().equals(contactId)) {
+				LOG.info("Contact disconnected");
+				onContactDisconnected();
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onDataToSend() {
+		Cancellable wakeupTask;
+		synchronized (lock) {
+			if (state != State.WAITING_FOR_DATA) return;
+			state = State.CHECKING_FOR_DATA;
+			wakeupTask = this.wakeupTask;
+			this.wakeupTask = null;
+			// Delay the check to avoid creating lots of small files
+			checkTask = taskScheduler.schedule(this::checkForDataToSend,
+					ioExecutor, CHECK_DELAY_MS, MILLISECONDS);
+		}
+		// If we had scheduled a wakeup when data was due to be sent, cancel it
+		if (wakeupTask != null) wakeupTask.cancel();
+	}
+
+	@EventExecutor
+	private void onContactConnected() {
+		Cancellable wakeupTask = null, checkTask = null;
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			// If we're checking for data to send, waiting for data to send,
+			// or checking connectivity then wait until we disconnect from
+			// the contact before proceeding. If we're writing or uploading
+			// a file then continue
+			if (state == State.CHECKING_FOR_DATA ||
+					state == State.WAITING_FOR_DATA ||
+					state == State.CONNECTIVITY_CHECK) {
+				state = State.CONNECTED_TO_CONTACT;
+				wakeupTask = this.wakeupTask;
+				this.wakeupTask = null;
+				checkTask = this.checkTask;
+				this.checkTask = null;
+			}
+		}
+		if (wakeupTask != null) wakeupTask.cancel();
+		if (checkTask != null) checkTask.cancel();
+	}
+
+	@EventExecutor
+	private void onContactDisconnected() {
+		synchronized (lock) {
+			if (state != State.CONNECTED_TO_CONTACT) return;
+			state = State.CHECKING_FOR_DATA;
+		}
+		ioExecutor.execute(this::checkForDataToSend);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorker.java

@@ -0,0 +1,23 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A worker that downloads files from a contact's mailbox.
+ */
+@ThreadSafe
+@NotNullByDefault
+interface MailboxWorker {
+
+	/**
+	 * Asynchronously starts the worker.
+	 */
+	void start();
+
+	/**
+	 * Destroys the worker and cancels any pending tasks or retries.
+	 */
+	void destroy();
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorkerFactory.java

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxWorkerFactory {
+
+	MailboxWorker createUploadWorker(ConnectivityChecker connectivityChecker,
+			MailboxProperties properties, MailboxFolderId folderId,
+			ContactId contactId);
+
+	MailboxWorker createDownloadWorkerForContactMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties);
+
+	MailboxWorker createDownloadWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties);
+
+	MailboxWorker createContactListWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorkerFactoryImpl.java

@@ -0,0 +1,101 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.connection.ConnectionRegistry;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class MailboxWorkerFactoryImpl implements MailboxWorkerFactory {
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final Clock clock;
+	private final TaskScheduler taskScheduler;
+	private final EventBus eventBus;
+	private final ConnectionRegistry connectionRegistry;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
+
+	@Inject
+	MailboxWorkerFactoryImpl(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			Clock clock,
+			TaskScheduler taskScheduler,
+			EventBus eventBus,
+			ConnectionRegistry connectionRegistry,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxUpdateManager mailboxUpdateManager) {
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.clock = clock;
+		this.taskScheduler = taskScheduler;
+		this.eventBus = eventBus;
+		this.connectionRegistry = connectionRegistry;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+	}
+
+	@Override
+	public MailboxWorker createUploadWorker(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties, MailboxFolderId folderId,
+			ContactId contactId) {
+		MailboxUploadWorker worker = new MailboxUploadWorker(ioExecutor, db,
+				clock, taskScheduler, eventBus, connectionRegistry,
+				connectivityChecker, mailboxApiCaller, mailboxApi,
+				mailboxFileManager, properties, folderId, contactId);
+		eventBus.addListener(worker);
+		return worker;
+	}
+
+	@Override
+	public MailboxWorker createDownloadWorkerForContactMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		return new ContactMailboxDownloadWorker(connectivityChecker,
+				reachabilityMonitor, mailboxApiCaller, mailboxApi,
+				mailboxFileManager, properties);
+	}
+
+	@Override
+	public MailboxWorker createDownloadWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		return new OwnMailboxDownloadWorker(connectivityChecker,
+				reachabilityMonitor, mailboxApiCaller, mailboxApi,
+				mailboxFileManager, properties);
+	}
+
+	@Override
+	public MailboxWorker createContactListWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties) {
+		OwnMailboxContactListWorker worker = new OwnMailboxContactListWorker(
+				ioExecutor, db, eventBus, connectivityChecker, mailboxApiCaller,
+				mailboxApi, mailboxUpdateManager, properties);
+		eventBus.addListener(worker);
+		return worker;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxClient.java

@@ -0,0 +1,222 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.TaskScheduler;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.concurrent.TimeUnit.HOURS;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxClient implements MailboxClient, ConnectivityObserver {
+
+	/**
+	 * How often to check our own mailbox's connectivity.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CONNECTIVITY_CHECK_INTERVAL_MS = HOURS.toMillis(1);
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxClient.class.getName());
+
+	private final MailboxWorkerFactory workerFactory;
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final TaskScheduler taskScheduler;
+	private final Executor ioExecutor;
+	private final MailboxProperties properties;
+	private final MailboxWorker contactListWorker;
+	private final Object lock = new Object();
+
+	/**
+	 * Upload workers: one worker per contact assigned for upload.
+	 */
+	@GuardedBy("lock")
+	private final Map<ContactId, MailboxWorker> uploadWorkers = new HashMap<>();
+
+	/**
+	 * Download worker: shared between all contacts assigned for download.
+	 * Null if no contacts are assigned for download.
+	 */
+	@GuardedBy("lock")
+	@Nullable
+	private MailboxWorker downloadWorker = null;
+
+	/**
+	 * IDs of contacts assigned for download, so that we know when to
+	 * create/destroy the download worker.
+	 */
+	@GuardedBy("lock")
+	private final Set<ContactId> assignedForDownload = new HashSet<>();
+
+	/**
+	 * Scheduled task for periodically checking whether the mailbox is
+	 * reachable.
+	 */
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable connectivityTask = null;
+
+	@GuardedBy("lock")
+	private boolean destroyed = false;
+
+	OwnMailboxClient(MailboxWorkerFactory workerFactory,
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			TaskScheduler taskScheduler,
+			@IoExecutor Executor ioExecutor,
+			MailboxProperties properties) {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		this.workerFactory = workerFactory;
+		this.connectivityChecker = connectivityChecker;
+		this.reachabilityMonitor = reachabilityMonitor;
+		this.taskScheduler = taskScheduler;
+		this.ioExecutor = ioExecutor;
+		this.properties = properties;
+		contactListWorker = workerFactory.createContactListWorkerForOwnMailbox(
+				connectivityChecker, properties);
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		checkConnectivity();
+		contactListWorker.start();
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		List<MailboxWorker> uploadWorkers;
+		MailboxWorker downloadWorker;
+		Cancellable connectivityTask;
+		synchronized (lock) {
+			uploadWorkers = new ArrayList<>(this.uploadWorkers.values());
+			this.uploadWorkers.clear();
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+			connectivityTask = this.connectivityTask;
+			this.connectivityTask = null;
+			destroyed = true;
+		}
+		// Destroy the workers (with apologies to Mr Marx and Mr Engels)
+		for (MailboxWorker worker : uploadWorkers) worker.destroy();
+		if (downloadWorker != null) downloadWorker.destroy();
+		// If a connectivity check is scheduled, cancel it
+		if (connectivityTask != null) connectivityTask.cancel();
+		contactListWorker.destroy();
+		// The connectivity checker belongs to the client, so it should be
+		// destroyed. The Tor reachability monitor is shared between clients,
+		// so it should not be destroyed
+		connectivityChecker.destroy();
+	}
+
+	@Override
+	public void assignContactForUpload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for upload");
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		MailboxWorker uploadWorker = workerFactory.createUploadWorker(
+				connectivityChecker, properties, folderId, contactId);
+		synchronized (lock) {
+			MailboxWorker old = uploadWorkers.put(contactId, uploadWorker);
+			if (old != null) throw new IllegalStateException();
+		}
+		uploadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForUpload(ContactId contactId) {
+		LOG.info("Contact deassigned for upload");
+		MailboxWorker uploadWorker;
+		synchronized (lock) {
+			uploadWorker = uploadWorkers.remove(contactId);
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForDownload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for download");
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		// Create a download worker if we don't already have one. The worker
+		// will use the API to discover which folders have files to download,
+		// so it doesn't need to track the set of assigned contacts
+		MailboxWorker toStart = null;
+		synchronized (lock) {
+			if (!assignedForDownload.add(contactId)) {
+				throw new IllegalStateException();
+			}
+			if (downloadWorker == null) {
+				toStart = workerFactory.createDownloadWorkerForOwnMailbox(
+						connectivityChecker, reachabilityMonitor, properties);
+				downloadWorker = toStart;
+			}
+		}
+		if (toStart != null) toStart.start();
+	}
+
+	@Override
+	public void deassignContactForDownload(ContactId contactId) {
+		LOG.info("Contact deassigned for download");
+		// If there are no more contacts assigned for download, destroy the
+		// download worker
+		MailboxWorker toDestroy = null;
+		synchronized (lock) {
+			if (!assignedForDownload.remove(contactId)) {
+				throw new IllegalStateException();
+			}
+			if (assignedForDownload.isEmpty()) {
+				toDestroy = downloadWorker;
+				downloadWorker = null;
+			}
+		}
+		if (toDestroy != null) toDestroy.destroy();
+	}
+
+	private void checkConnectivity() {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityTask = null;
+		}
+		connectivityChecker.checkConnectivity(properties, this);
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		boolean removeObserver;
+		synchronized (lock) {
+			removeObserver = destroyed;
+		}
+		if (removeObserver) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityTask = taskScheduler.schedule(this::checkConnectivity,
+					ioExecutor, CONNECTIVITY_CHECK_INTERVAL_MS, MILLISECONDS);
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxConnectivityChecker.java

@@ -4,14 +4,17 @@ import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.TransactionManager;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
 
 import java.io.IOException;
+import java.util.List;
 import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
 
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
@@ -28,6 +31,7 @@ class OwnMailboxConnectivityChecker extends ConnectivityCheckerImpl {
 	private final TransactionManager db;
 	private final MailboxSettingsManager mailboxSettingsManager;
 
+	@Inject
 	OwnMailboxConnectivityChecker(Clock clock,
 			MailboxApiCaller mailboxApiCaller,
 			MailboxApi mailboxApi,
@@ -55,11 +59,13 @@ class OwnMailboxConnectivityChecker extends ConnectivityCheckerImpl {
 	private boolean checkConnectivityAndStoreResult(
 			MailboxProperties properties) throws DbException {
 		try {
-			if (!mailboxApi.checkStatus(properties)) throw new ApiException();
+			LOG.info("Checking whether own mailbox is reachable");
+			List<MailboxVersion> serverSupports =
+					mailboxApi.getServerSupports(properties);
 			LOG.info("Own mailbox is reachable");
 			long now = clock.currentTimeMillis();
 			db.transaction(false, txn -> mailboxSettingsManager
-					.recordSuccessfulConnection(txn, now));
+					.recordSuccessfulConnection(txn, now, serverSupports));
 			// Call the observers and cache the result
 			onConnectivityCheckSucceeded(now);
 			return false; // Don't retry

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxContactListWorker.java

@@ -0,0 +1,369 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.event.ContactAddedEvent;
+import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.NoSuchContactException;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxContact;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.Set;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxContactListWorker
+		implements MailboxWorker, ConnectivityObserver, EventListener {
+
+	/**
+	 * When the worker is started it waits for a connectivity check, then
+	 * fetches the remote contact list and compares it to the local contact
+	 * list.
+	 * <p>
+	 * Any contacts that are missing from the remote list are added to the
+	 * mailbox's contact list, while any contacts that are missing from the
+	 * local list are removed from the mailbox's contact list.
+	 * <p>
+	 * Once the remote contact list has been brought up to date, the worker
+	 * waits for events indicating that contacts have been added or removed.
+	 * Each time an event is received, the worker updates the mailbox's
+	 * contact list and then goes back to waiting.
+	 */
+	private enum State {
+		CREATED,
+		CONNECTIVITY_CHECK,
+		FETCHING_CONTACT_LIST,
+		UPDATING_CONTACT_LIST,
+		WAITING_FOR_CHANGES,
+		DESTROYED
+	}
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxContactListWorker.class.getName());
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final EventBus eventBus;
+	private final ConnectivityChecker connectivityChecker;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxUpdateManager mailboxUpdateManager;
+	private final MailboxProperties mailboxProperties;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable apiCall = null;
+
+	/**
+	 * A queue of updates waiting to be applied to the remote contact list.
+	 */
+	@GuardedBy("lock")
+	private final Queue<Update> updates = new LinkedList<>();
+
+	OwnMailboxContactListWorker(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			EventBus eventBus,
+			ConnectivityChecker connectivityChecker,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxUpdateManager mailboxUpdateManager,
+			MailboxProperties mailboxProperties) {
+		if (!mailboxProperties.isOwner()) throw new IllegalArgumentException();
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.connectivityChecker = connectivityChecker;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+		this.mailboxProperties = mailboxProperties;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			if (state != State.CREATED) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable apiCall;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+		}
+		if (apiCall != null) apiCall.cancel();
+		connectivityChecker.removeObserver(this);
+		eventBus.removeListener(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.FETCHING_CONTACT_LIST;
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					new SimpleApiCall(this::apiCallFetchContactList));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallFetchContactList() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+		}
+		LOG.info("Fetching remote contact list");
+		Collection<ContactId> remote =
+				mailboxApi.getContacts(mailboxProperties);
+		ioExecutor.execute(() -> loadLocalContactList(remote));
+	}
+
+	@IoExecutor
+	private void loadLocalContactList(Collection<ContactId> remote) {
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+			apiCall = null;
+		}
+		LOG.info("Loading local contact list");
+		try {
+			db.transaction(true, txn -> {
+				Collection<Contact> local = db.getContacts(txn);
+				// Handle the result on the event executor to avoid races with
+				// incoming events
+				txn.attach(() -> reconcileContactLists(local, remote));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void reconcileContactLists(Collection<Contact> local,
+			Collection<ContactId> remote) {
+		Set<ContactId> localIds = new HashSet<>();
+		for (Contact c : local) localIds.add(c.getId());
+		remote = new HashSet<>(remote);
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+			for (ContactId c : localIds) {
+				if (!remote.contains(c)) updates.add(new Update(true, c));
+			}
+			for (ContactId c : remote) {
+				if (!localIds.contains(c)) updates.add(new Update(false, c));
+			}
+			if (updates.isEmpty()) {
+				LOG.info("Contact list is up to date");
+				state = State.WAITING_FOR_CHANGES;
+			} else {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info(updates.size() + " updates to apply");
+				}
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	@IoExecutor
+	private void updateContactList() {
+		Update update;
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			update = updates.poll();
+			if (update == null) {
+				LOG.info("No more updates to process");
+				state = State.WAITING_FOR_CHANGES;
+				apiCall = null;
+				return;
+			}
+		}
+		if (update.add) loadMailboxProperties(update.contactId);
+		else removeContact(update.contactId);
+	}
+
+	@IoExecutor
+	private void loadMailboxProperties(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Loading mailbox properties for contact");
+		try {
+			MailboxUpdate mailboxUpdate = db.transactionWithResult(true, txn ->
+					mailboxUpdateManager.getLocalUpdate(txn, c));
+			if (mailboxUpdate instanceof MailboxUpdateWithMailbox) {
+				addContact(c, (MailboxUpdateWithMailbox) mailboxUpdate);
+			} else {
+				// Our own mailbox was concurrently unpaired. This worker will
+				// be destroyed soon, so we can stop here
+				LOG.info("Own mailbox was unpaired");
+			}
+		} catch (NoSuchContactException e) {
+			// Contact was removed concurrently. Move on to the next update.
+			// Later we may process a removal update for this contact, which
+			// was never added to the mailbox's contact list. The removal API
+			// call should fail safely with a TolerableFailureException
+			LOG.info("No such contact");
+			updateContactList();
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@IoExecutor
+	private void addContact(ContactId c, MailboxUpdateWithMailbox withMailbox) {
+		MailboxProperties props = withMailbox.getMailboxProperties();
+		MailboxContact contact = new MailboxContact(c, props.getAuthToken(),
+				requireNonNull(props.getInboxId()),
+				requireNonNull(props.getOutboxId()));
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallAddContact(contact)));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallAddContact(MailboxContact contact)
+			throws IOException, ApiException, TolerableFailureException {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Adding contact to remote contact list");
+		mailboxApi.addContact(mailboxProperties, contact);
+		updateContactList();
+	}
+
+	@IoExecutor
+	private void removeContact(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallRemoveContact(c)));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallRemoveContact(ContactId c)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Removing contact from remote contact list");
+		try {
+			mailboxApi.deleteContact(mailboxProperties, c);
+		} catch (TolerableFailureException e) {
+			// Catch this so we can continue to the next update
+			LOG.warning("Contact does not exist");
+		}
+		updateContactList();
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof ContactAddedEvent) {
+			LOG.info("Contact added");
+			onContactAdded(((ContactAddedEvent) e).getContactId());
+		} else if (e instanceof ContactRemovedEvent) {
+			LOG.info("Contact removed");
+			onContactRemoved(((ContactRemovedEvent) e).getContactId());
+		}
+	}
+
+	@EventExecutor
+	private void onContactAdded(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST &&
+					state != State.WAITING_FOR_CHANGES) {
+				return;
+			}
+			updates.add(new Update(true, c));
+			if (state == State.WAITING_FOR_CHANGES) {
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onContactRemoved(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST &&
+					state != State.WAITING_FOR_CHANGES) {
+				return;
+			}
+			updates.add(new Update(false, c));
+			if (state == State.WAITING_FOR_CHANGES) {
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	/**
+	 * An update that should be applied to the remote contact list.
+	 */
+	private static class Update {
+
+		/**
+		 * True if the contact should be added, false if the contact should be
+		 * removed.
+		 */
+		private final boolean add;
+		private final ContactId contactId;
+
+		private Update(boolean add, ContactId contactId) {
+			this.add = add;
+			this.contactId = contactId;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxDownloadWorker.java

@@ -0,0 +1,148 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxFile;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.Collections.shuffle;
+import static java.util.logging.Level.INFO;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxDownloadWorker extends MailboxDownloadWorker {
+
+	/**
+	 * The maximum number of files that will be downloaded before checking
+	 * again for folders with available files. This ensures that if a file
+	 * arrives during a download cycle, its folder will be checked within a
+	 * reasonable amount of time even if some other folder has a very large
+	 * number of files.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final int MAX_ROUND_ROBIN_FILES = 1000;
+
+	OwnMailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		super(connectivityChecker, torReachabilityMonitor, mailboxApiCaller,
+				mailboxApi, mailboxFileManager, mailboxProperties);
+		if (!mailboxProperties.isOwner()) throw new IllegalArgumentException();
+	}
+
+	@Override
+	protected ApiCall createApiCallForDownloadCycle() {
+		return new SimpleApiCall(this::apiCallListFolders);
+	}
+
+	private void apiCallListFolders() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing folders with available files");
+		List<MailboxFolderId> folders =
+				mailboxApi.getFolders(mailboxProperties);
+		if (folders.isEmpty()) onDownloadCycleFinished();
+		else listNextFolder(new LinkedList<>(folders), new HashMap<>());
+	}
+
+	/**
+	 * Removes the next folder from `queue` and starts a task to list the
+	 * files in the folder and add them to `available`.
+	 */
+	private void listNextFolder(Queue<MailboxFolderId> queue,
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			MailboxFolderId folder = queue.remove();
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallListFolder(folder, queue, available)));
+		}
+	}
+
+	private void apiCallListFolder(MailboxFolderId folder,
+			Queue<MailboxFolderId> queue,
+			Map<MailboxFolderId, Queue<MailboxFile>> available)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing folder");
+		try {
+			List<MailboxFile> files =
+					mailboxApi.getFiles(mailboxProperties, folder);
+			if (!files.isEmpty()) {
+				available.put(folder, new LinkedList<>(files));
+			}
+		} catch (TolerableFailureException e) {
+			LOG.warning("Folder does not exist");
+		}
+		if (queue.isEmpty()) {
+			LOG.info("Finished listing folders");
+			if (available.isEmpty()) onDownloadCycleFinished();
+			else createDownloadQueue(available);
+		} else {
+			listNextFolder(queue, available);
+		}
+	}
+
+	/**
+	 * Visits the given folders in round-robin order to create a queue of up to
+	 * {@link #MAX_ROUND_ROBIN_FILES} to download.
+	 */
+	private void createDownloadQueue(
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		if (LOG.isLoggable(INFO)) {
+			LOG.info(available.size() + " folders have available files");
+		}
+		Queue<FolderFile> queue = createRoundRobinQueue(available);
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Downloading " + queue.size() + " files");
+		}
+		downloadNextFile(queue);
+	}
+
+	// Package access for testing
+	Queue<FolderFile> createRoundRobinQueue(
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		List<MailboxFolderId> roundRobin = new ArrayList<>(available.keySet());
+		// Shuffle the folders so we don't always favour the same folders
+		shuffle(roundRobin);
+		Queue<FolderFile> queue = new LinkedList<>();
+		while (queue.size() < MAX_ROUND_ROBIN_FILES && !available.isEmpty()) {
+			Iterator<MailboxFolderId> it = roundRobin.iterator();
+			while (queue.size() < MAX_ROUND_ROBIN_FILES && it.hasNext()) {
+				MailboxFolderId folder = it.next();
+				Queue<MailboxFile> files = available.get(folder);
+				MailboxFile file = files.remove();
+				queue.add(new FolderFile(folder, file.name));
+				if (files.isEmpty()) {
+					available.remove(folder);
+					it.remove();
+				}
+			}
+		}
+		return queue;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/SimpleApiCall.java

@@ -16,17 +16,20 @@ import static org.briarproject.bramble.util.LogUtils.logException;
  * Convenience class for making simple API calls that don't return values.
  */
 @NotNullByDefault
-public abstract class SimpleApiCall implements ApiCall {
+class SimpleApiCall implements ApiCall {
 
 	private static final Logger LOG = getLogger(SimpleApiCall.class.getName());
 
-	abstract void tryToCallApi()
+	private final Attempt attempt;
-			throws IOException, ApiException, TolerableFailureException;
+
+	SimpleApiCall(Attempt attempt) {
+		this.attempt = attempt;
+	}
 
 	@Override
 	public boolean callApi() {
 		try {
-			tryToCallApi();
+			attempt.tryToCallApi();
 			return false; // Succeeded, don't retry
 		} catch (IOException | ApiException e) {
 			logException(LOG, WARNING, e);
@@ -36,4 +39,17 @@ public abstract class SimpleApiCall implements ApiCall {
 			return false; // Failed tolerably, don't retry
 		}
 	}
+
+	interface Attempt {
+
+		/**
+		 * Makes a single attempt to call an API endpoint. If this method
+		 * throws an {@link IOException} or an {@link ApiException}, the call
+		 * will be retried. If it throws a {@link TolerableFailureException}
+		 * or returns without throwing an exception, the call will not be
+		 * retried.
+		 */
+		void tryToCallApi()
+				throws IOException, ApiException, TolerableFailureException;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java

@@ -0,0 +1,51 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.concurrent.TimeUnit.MINUTES;
+
+@ThreadSafe
+@NotNullByDefault
+interface TorReachabilityMonitor {
+
+	/**
+	 * How long the Tor plugin needs to be continuously
+	 * {@link Plugin.State#ACTIVE active} before we assume our contacts can
+	 * reach our hidden service.
+	 */
+	long REACHABILITY_PERIOD_MS = MINUTES.toMillis(10);
+
+	/**
+	 * Starts the monitor.
+	 */
+	void start();
+
+	/**
+	 * Destroys the monitor.
+	 */
+	void destroy();
+
+	/**
+	 * Adds an observer that will be called when our Tor hidden service becomes
+	 * reachable. If our hidden service is already reachable, the observer is
+	 * called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the monitor is
+	 * {@link #destroy() destroyed}.
+	 */
+	void addOneShotObserver(TorReachabilityObserver o);
+
+	/**
+	 * Removes an observer that was added via
+	 * {@link #addOneShotObserver(TorReachabilityObserver)}.
+	 */
+	void removeObserver(TorReachabilityObserver o);
+
+	interface TorReachabilityObserver {
+
+		void onTorReachable();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java

@@ -0,0 +1,137 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.TorConstants.ID;
+
+@ThreadSafe
+@NotNullByDefault
+class TorReachabilityMonitorImpl
+		implements TorReachabilityMonitor, EventListener {
+
+	private final Executor ioExecutor;
+	private final TaskScheduler taskScheduler;
+	private final PluginManager pluginManager;
+	private final EventBus eventBus;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private boolean reachable = false, destroyed = false;
+
+	@GuardedBy("lock")
+	private final List<TorReachabilityObserver> observers = new ArrayList<>();
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable task = null;
+
+	@Inject
+	TorReachabilityMonitorImpl(
+			@IoExecutor Executor ioExecutor,
+			TaskScheduler taskScheduler,
+			PluginManager pluginManager,
+			EventBus eventBus) {
+		this.ioExecutor = ioExecutor;
+		this.taskScheduler = taskScheduler;
+		this.pluginManager = pluginManager;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public void start() {
+		eventBus.addListener(this);
+		Plugin plugin = pluginManager.getPlugin(ID);
+		if (plugin != null && plugin.getState() == ACTIVE) onTorActive();
+	}
+
+	@Override
+	public void destroy() {
+		eventBus.removeListener(this);
+		synchronized (lock) {
+			destroyed = true;
+			if (task != null) task.cancel();
+			task = null;
+			observers.clear();
+		}
+	}
+
+	@Override
+	public void addOneShotObserver(TorReachabilityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (reachable) callNow = true;
+			else observers.add(o);
+		}
+		if (callNow) o.onTorReachable();
+	}
+
+	@Override
+	public void removeObserver(TorReachabilityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			observers.remove(o);
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorActive();
+		} else if (e instanceof TransportInactiveEvent) {
+			TransportInactiveEvent t = (TransportInactiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorInactive();
+		}
+	}
+
+	private void onTorActive() {
+		synchronized (lock) {
+			if (destroyed || task != null) return;
+			task = taskScheduler.schedule(this::onTorReachable, ioExecutor,
+					REACHABILITY_PERIOD_MS, MILLISECONDS);
+		}
+	}
+
+	private void onTorInactive() {
+		synchronized (lock) {
+			reachable = false;
+			if (task != null) task.cancel();
+			task = null;
+		}
+	}
+
+	@IoExecutor
+	private void onTorReachable() {
+		List<TorReachabilityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			reachable = true;
+			observers = new ArrayList<>(this.observers);
+			this.observers.clear();
+			task = null;
+		}
+		for (TorReachabilityObserver o : observers) o.onTorReachable();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/UrlConverter.java

@@ -0,0 +1,17 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+/**
+ * An interface for converting an onion address to an HTTP URL, allowing the
+ * conversion to be customised for integration tests.
+ */
+@NotNullByDefault
+interface UrlConverter {
+
+   /**
+    * Converts a raw onion address, excluding the .onion suffix, into an
+    * HTTP URL.
+    */
+   String convertOnionToBaseUrl(String onion);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/UrlConverterImpl.java

@@ -0,0 +1,18 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.inject.Inject;
+
+@NotNullByDefault
+class UrlConverterImpl implements UrlConverter {
+
+	@Inject
+	UrlConverterImpl() {
+	}
+
+	@Override
+	public String convertOnionToBaseUrl(String onion) {
+		return "http://" + onion + ".onion";
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/plugin/bluetooth/AbstractBluetoothPlugin.java

@@ -427,7 +427,13 @@ abstract class AbstractBluetoothPlugin<S, SS> implements BluetoothPlugin,
 		BdfList descriptor = new BdfList();
 		descriptor.add(TRANSPORT_ID_BLUETOOTH);
 		String address = getBluetoothAddress();
-		if (address != null) descriptor.add(macToBytes(address));
+		if (address != null) {
+			try {
+				descriptor.add(macToBytes(address));
+			} catch (FormatException e) {
+				throw new RuntimeException();
+			}
+		}
 		return new BluetoothKeyAgreementListener(descriptor, ss);
 	}
 

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/AbstractRemovableDrivePlugin.java

@@ -67,6 +67,7 @@ abstract class AbstractRemovableDrivePlugin implements SimplexPlugin {
 	public void start() {
 		callback.mergeLocalProperties(
 				new TransportProperties(singletonMap(PROP_SUPPORTED, "true")));
+		callback.pluginStateChanged(ACTIVE);
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FilePlugin.java

@@ -11,6 +11,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.io.OutputStream;
 import java.util.logging.Logger;
 
 import static java.util.logging.Level.WARNING;
@@ -29,11 +30,6 @@ abstract class FilePlugin implements SimplexPlugin {
 	protected final PluginCallback callback;
 	protected final long maxLatency;
 
-	protected abstract void writerFinished(File f, boolean exception);
-
-	protected abstract void readerFinished(File f, boolean exception,
-			boolean recognised);
-
 	FilePlugin(PluginCallback callback, long maxLatency) {
 		this.callback = callback;
 		this.maxLatency = maxLatency;
@@ -50,9 +46,8 @@ abstract class FilePlugin implements SimplexPlugin {
 		String path = p.get(PROP_PATH);
 		if (isNullOrEmpty(path)) return null;
 		try {
-			File file = new File(path);
+			FileInputStream in = new FileInputStream(path);
-			FileInputStream in = new FileInputStream(file);
+			return new TransportInputStreamReader(in);
-			return new FileTransportReader(file, in, this);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
 			return null;
@@ -70,8 +65,8 @@ abstract class FilePlugin implements SimplexPlugin {
 				LOG.info("Failed to create file");
 				return null;
 			}
-			FileOutputStream out = new FileOutputStream(file);
+			OutputStream out = new FileOutputStream(file);
-			return new FileTransportWriter(file, out, this);
+			return new TransportOutputStreamWriter(this, out);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
 			return null;

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FileTransportReader.java

@@ -1,39 +0,0 @@
-package org.briarproject.bramble.plugin.file;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.TransportConnectionReader;
-
-import java.io.File;
-import java.io.InputStream;
-import java.util.logging.Logger;
-
-import static java.util.logging.Level.WARNING;
-import static org.briarproject.bramble.util.IoUtils.tryToClose;
-
-@NotNullByDefault
-class FileTransportReader implements TransportConnectionReader {
-
-	private static final Logger LOG =
-			Logger.getLogger(FileTransportReader.class.getName());
-
-	private final File file;
-	private final InputStream in;
-	private final FilePlugin plugin;
-
-	FileTransportReader(File file, InputStream in, FilePlugin plugin) {
-		this.file = file;
-		this.in = in;
-		this.plugin = plugin;
-	}
-
-	@Override
-	public InputStream getInputStream() {
-		return in;
-	}
-
-	@Override
-	public void dispose(boolean exception, boolean recognised) {
-		tryToClose(in, LOG, WARNING);
-		plugin.readerFinished(file, exception, recognised);
-	}
-}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FileTransportWriter.java

@@ -1,54 +0,0 @@
-package org.briarproject.bramble.plugin.file;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
-
-import java.io.File;
-import java.io.OutputStream;
-import java.util.logging.Logger;
-
-import static java.util.logging.Level.WARNING;
-import static org.briarproject.bramble.util.IoUtils.tryToClose;
-
-@NotNullByDefault
-class FileTransportWriter implements TransportConnectionWriter {
-
-	private static final Logger LOG =
-			Logger.getLogger(FileTransportWriter.class.getName());
-
-	private final File file;
-	private final OutputStream out;
-	private final FilePlugin plugin;
-
-	FileTransportWriter(File file, OutputStream out, FilePlugin plugin) {
-		this.file = file;
-		this.out = out;
-		this.plugin = plugin;
-	}
-
-	@Override
-	public long getMaxLatency() {
-		return plugin.getMaxLatency();
-	}
-
-	@Override
-	public int getMaxIdleTime() {
-		return plugin.getMaxIdleTime();
-	}
-
-	@Override
-	public boolean isLossyAndCheap() {
-		return plugin.isLossyAndCheap();
-	}
-
-	@Override
-	public OutputStream getOutputStream() {
-		return out;
-	}
-
-	@Override
-	public void dispose(boolean exception) {
-		tryToClose(out, LOG, WARNING);
-		plugin.writerFinished(file, exception);
-	}
-}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/MailboxPlugin.java

@@ -0,0 +1,73 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.Pair;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.ConnectionHandler;
+import org.briarproject.bramble.api.plugin.PluginCallback;
+import org.briarproject.bramble.api.plugin.PluginException;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.util.Collection;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.ID;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+
+@NotNullByDefault
+class MailboxPlugin extends FilePlugin {
+
+	MailboxPlugin(PluginCallback callback, long maxLatency) {
+		super(callback, maxLatency);
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxIdleTime() {
+		// Unused for simplex transports
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void start() throws PluginException {
+		callback.pluginStateChanged(ACTIVE);
+	}
+
+	@Override
+	public void stop() throws PluginException {
+	}
+
+	@Override
+	public State getState() {
+		return ACTIVE;
+	}
+
+	@Override
+	public int getReasonsDisabled() {
+		return 0;
+	}
+
+	@Override
+	public boolean shouldPoll() {
+		return false;
+	}
+
+	@Override
+	public int getPollingInterval() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void poll(
+			Collection<Pair<TransportProperties, ConnectionHandler>> properties) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public boolean isLossyAndCheap() {
+		return false;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/MailboxPluginFactory.java

@@ -0,0 +1,37 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginCallback;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
+
+import javax.annotation.Nullable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.ID;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.MAX_LATENCY;
+
+@NotNullByDefault
+public class MailboxPluginFactory implements SimplexPluginFactory {
+
+	@Inject
+	MailboxPluginFactory() {
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public long getMaxLatency() {
+		return MAX_LATENCY;
+	}
+
+	@Nullable
+	@Override
+	public SimplexPlugin createPlugin(PluginCallback callback) {
+		return new MailboxPlugin(callback, MAX_LATENCY);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/RemovableDriveManagerImpl.java

@@ -106,7 +106,8 @@ class RemovableDriveManagerImpl
 	@Override
 	public boolean isWriterTaskNeeded(ContactId c) throws DbException {
 		return db.transactionWithResult(true, txn ->
-				db.containsAnythingToSend(txn, c, MAX_LATENCY, true));
+				db.containsAcksToSend(txn, c) ||
+						db.containsMessagesToSend(txn, c, MAX_LATENCY, true));
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/plugin/tcp/LanTcpPlugin.java

@@ -285,7 +285,7 @@ class LanTcpPlugin extends TcpPlugin {
 				if (ip.length == 16) addrs.add(InetAddress.getByAddress(ip));
 			}
 			return addrs;
-		} catch (IllegalArgumentException | UnknownHostException e) {
+		} catch (FormatException | UnknownHostException e) {
 			return emptyList();
 		}
 	}

bramble-core/src/main/java/org/briarproject/bramble/plugin/tor/CircumventionProvider.java

@@ -12,7 +12,8 @@ public interface CircumventionProvider {
 		DEFAULT_OBFS4,
 		NON_DEFAULT_OBFS4,
 		VANILLA,
-		MEEK
+		MEEK,
+		SNOWFLAKE
 	}
 
 	/**
@@ -27,7 +28,7 @@ public interface CircumventionProvider {
 	 * Countries where bridge connections are likely to work.
 	 * Should be a subset of {@link #BLOCKED} and the union of
 	 * {@link #DEFAULT_BRIDGES}, {@link #NON_DEFAULT_BRIDGES} and
-	 * {@link #MEEK_BRIDGES}.
+	 * {@link #DPI_BRIDGES}.
 	 */
 	String[] BRIDGES = {"BY", "CN", "EG", "IR", "RU", "TM", "VE"};
 
@@ -41,13 +42,14 @@ public interface CircumventionProvider {
 	 * Countries where non-default obfs4 or vanilla bridges are likely to work.
 	 * Should be a subset of {@link #BRIDGES}.
 	 */
-	String[] NON_DEFAULT_BRIDGES = {"BY", "RU", "TM"};
+	String[] NON_DEFAULT_BRIDGES = {"BY", "RU"};
 
 	/**
-	 * Countries where obfs4 and vanilla bridges won't work and meek is needed.
+	 * Countries where vanilla bridges are blocked via DPI but non-default
-	 * Should be a subset of {@link #BRIDGES}.
+	 * obfs4 bridges, meek and snowflake may work. Should be a subset of
+	 * {@link #BRIDGES}.
 	 */
-	String[] MEEK_BRIDGES = {"CN", "IR"};
+	String[] DPI_BRIDGES = {"CN", "IR", "TM"};
 
 	/**
 	 * Returns true if vanilla Tor connections are blocked in the given country.
@@ -68,6 +70,6 @@ public interface CircumventionProvider {
 	List<BridgeType> getSuitableBridgeTypes(String countryCode);
 
 	@IoExecutor
-	List<String> getBridges(BridgeType type);
+	List<String> getBridges(BridgeType type, String countryCode,
-
+			boolean letsEncrypt);
 }

bramble-core/src/main/java/org/briarproject/bramble/plugin/tor/CircumventionProviderImpl.java

@@ -7,18 +7,20 @@ import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
 import java.util.Scanner;
 import java.util.Set;
+import java.util.TreeMap;
 
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
 import static java.util.Arrays.asList;
-import static java.util.Collections.singletonList;
 import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
 import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.DEFAULT_OBFS4;
 import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.MEEK;
 import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.NON_DEFAULT_OBFS4;
+import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.SNOWFLAKE;
 import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.VANILLA;
 
 @Immutable
@@ -26,6 +28,8 @@ import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeTy
 class CircumventionProviderImpl implements CircumventionProvider {
 
 	private final static String BRIDGE_FILE_NAME = "bridges";
+	private final static String SNOWFLAKE_PARAMS_FILE_NAME = "snowflake-params";
+	private final static String DEFAULT_COUNTRY_CODE = "ZZ";
 
 	private static final Set<String> BLOCKED_IN_COUNTRIES =
 			new HashSet<>(asList(BLOCKED));
@@ -35,8 +39,8 @@ class CircumventionProviderImpl implements CircumventionProvider {
 			new HashSet<>(asList(DEFAULT_BRIDGES));
 	private static final Set<String> NON_DEFAULT_BRIDGE_COUNTRIES =
 			new HashSet<>(asList(NON_DEFAULT_BRIDGES));
-	private static final Set<String> MEEK_COUNTRIES =
+	private static final Set<String> DPI_COUNTRIES =
-			new HashSet<>(asList(MEEK_BRIDGES));
+			new HashSet<>(asList(DPI_BRIDGES));
 
 	@Inject
 	CircumventionProviderImpl() {
@@ -58,8 +62,8 @@ class CircumventionProviderImpl implements CircumventionProvider {
 			return asList(DEFAULT_OBFS4, VANILLA);
 		} else if (NON_DEFAULT_BRIDGE_COUNTRIES.contains(countryCode)) {
 			return asList(NON_DEFAULT_OBFS4, VANILLA);
-		} else if (MEEK_COUNTRIES.contains(countryCode)) {
+		} else if (DPI_COUNTRIES.contains(countryCode)) {
-			return singletonList(MEEK);
+			return asList(NON_DEFAULT_OBFS4, MEEK, SNOWFLAKE);
 		} else {
 			return asList(DEFAULT_OBFS4, VANILLA);
 		}
@@ -67,7 +71,8 @@ class CircumventionProviderImpl implements CircumventionProvider {
 
 	@Override
 	@IoExecutor
-	public List<String> getBridges(BridgeType type) {
+	public List<String> getBridges(BridgeType type, String countryCode,
+			boolean letsEncrypt) {
 		InputStream is = requireNonNull(getClass().getClassLoader()
 				.getResourceAsStream(BRIDGE_FILE_NAME));
 		Scanner scanner = new Scanner(is);
@@ -80,10 +85,45 @@ class CircumventionProviderImpl implements CircumventionProvider {
 					(type == VANILLA && line.startsWith("v ")) ||
 					(type == MEEK && line.startsWith("m "))) {
 				bridges.add(line.substring(2));
+			} else if (type == SNOWFLAKE && line.startsWith("s ")) {
+				String params = getSnowflakeParams(countryCode, letsEncrypt);
+				bridges.add(line.substring(2) + " " + params);
 			}
 		}
 		scanner.close();
 		return bridges;
 	}
 
+	// Package access for testing
+	@SuppressWarnings("WeakerAccess")
+	String getSnowflakeParams(String countryCode, boolean letsEncrypt) {
+		Map<String, String> params = loadSnowflakeParams();
+		if (countryCode.isEmpty()) countryCode = DEFAULT_COUNTRY_CODE;
+		// If we have parameters for this country code, return them
+		String value = params.get(makeKey(countryCode, letsEncrypt));
+		if (value != null) return value;
+		// Return the default parameters
+		value = params.get(makeKey(DEFAULT_COUNTRY_CODE, letsEncrypt));
+		return requireNonNull(value);
+	}
+
+	private Map<String, String> loadSnowflakeParams() {
+		InputStream is = requireNonNull(getClass().getClassLoader()
+				.getResourceAsStream(SNOWFLAKE_PARAMS_FILE_NAME));
+		Scanner scanner = new Scanner(is);
+		Map<String, String> params = new TreeMap<>();
+		while (scanner.hasNextLine()) {
+			String line = scanner.nextLine();
+			if (line.length() < 5) continue;
+			String key = line.substring(0, 4); // Country code, space, digit
+			String value = line.substring(5);
+			params.put(key, value);
+		}
+		scanner.close();
+		return params;
+	}
+
+	private String makeKey(String countryCode, boolean letsEncrypt) {
+		return countryCode + " " + (letsEncrypt ? "1" : "0");
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/plugin/tor/TorPlugin.java

@@ -65,6 +65,7 @@ import javax.annotation.concurrent.ThreadSafe;
 import javax.net.SocketFactory;
 
 import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
 import static java.util.Collections.singletonList;
 import static java.util.Collections.singletonMap;
 import static java.util.logging.Level.INFO;
@@ -93,9 +94,8 @@ import static org.briarproject.bramble.api.plugin.TorConstants.PROP_ONION_V3;
 import static org.briarproject.bramble.api.plugin.TorConstants.REASON_BATTERY;
 import static org.briarproject.bramble.api.plugin.TorConstants.REASON_COUNTRY_BLOCKED;
 import static org.briarproject.bramble.api.plugin.TorConstants.REASON_MOBILE_DATA;
-import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.DEFAULT_OBFS4;
 import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.MEEK;
-import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.NON_DEFAULT_OBFS4;
+import static org.briarproject.bramble.plugin.tor.CircumventionProvider.BridgeType.SNOWFLAKE;
 import static org.briarproject.bramble.plugin.tor.TorRendezvousCrypto.SEED_BYTES;
 import static org.briarproject.bramble.util.IoUtils.copyAndClose;
 import static org.briarproject.bramble.util.IoUtils.tryToClose;
@@ -107,7 +107,7 @@ import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 @ParametersNotNullByDefault
 abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 
-	private static final Logger LOG = getLogger(TorPlugin.class.getName());
+	protected static final Logger LOG = getLogger(TorPlugin.class.getName());
 
 	private static final String[] EVENTS = {
 			"CIRC",
@@ -124,7 +124,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	private static final int COOKIE_POLLING_INTERVAL_MS = 200;
 	private static final Pattern ONION_V3 = Pattern.compile("[a-z2-7]{56}");
 
-	private final Executor ioExecutor, wakefulIoExecutor;
+	protected final Executor ioExecutor;
+	private final Executor wakefulIoExecutor;
 	private final Executor connectionStatusExecutor;
 	private final NetworkManager networkManager;
 	private final LocationUtils locationUtils;
@@ -212,6 +213,10 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		return new File(torDirectory, "obfs4proxy");
 	}
 
+	protected File getSnowflakeExecutableFile() {
+		return new File(torDirectory, "snowflake");
+	}
+
 	@Override
 	public TransportId getId() {
 		return TorConstants.ID;
@@ -238,8 +243,14 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 		// Load the settings
 		settings = callback.getSettings();
-		// Install or update the assets if necessary
+		try {
-		if (!assetsAreUpToDate()) installAssets();
+			// Install or update the assets if necessary
+			if (!assetsAreUpToDate()) installAssets();
+			// Start from the default config every time
+			extract(getConfigInputStream(), configFile);
+		} catch (IOException e) {
+			throw new PluginException(e);
+		}
 		if (cookieFile.exists() && !cookieFile.delete())
 			LOG.warning("Old auth cookie not deleted");
 		// Start a new Tor process
@@ -254,34 +265,15 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		Map<String, String> env = pb.environment();
 		env.put("HOME", torDirectory.getAbsolutePath());
 		pb.directory(torDirectory);
+		pb.redirectErrorStream(true);
 		try {
 			torProcess = pb.start();
 		} catch (SecurityException | IOException e) {
 			throw new PluginException(e);
 		}
-		// Log the process's standard output until it detaches
-		if (LOG.isLoggable(INFO)) {
-			Scanner stdout = new Scanner(torProcess.getInputStream());
-			Scanner stderr = new Scanner(torProcess.getErrorStream());
-			while (stdout.hasNextLine() || stderr.hasNextLine()) {
-				if (stdout.hasNextLine()) {
-					LOG.info(stdout.nextLine());
-				}
-				if (stderr.hasNextLine()) {
-					LOG.info(stderr.nextLine());
-				}
-			}
-			stdout.close();
-			stderr.close();
-		}
 		try {
-			// Wait for the process to detach or exit
+			// Wait for the Tor process to start
-			int exit = torProcess.waitFor();
+			waitForTorToStart(torProcess);
-			if (exit != 0) {
-				if (LOG.isLoggable(WARNING))
-					LOG.warning("Tor exited with value " + exit);
-				throw new PluginException();
-			}
 			// Wait for the auth cookie file to be created/updated
 			long start = clock.currentTimeMillis();
 			while (cookieFile.length() < 32) {
@@ -320,7 +312,7 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			info = controlConnection.getInfo("status/circuit-established");
 			if ("1".equals(info)) {
 				LOG.info("Tor has already built a circuit");
-				state.getAndSetCircuitBuilt(true);
+				state.setCircuitBuilt(true);
 			}
 		} catch (TorNotRunningException e) {
 			throw new RuntimeException(e);
@@ -339,25 +331,21 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		return doneFile.lastModified() > getLastUpdateTime();
 	}
 
-	private void installAssets() throws PluginException {
+	private void installAssets() throws IOException {
-		try {
+		// The done file may already exist from a previous installation
-			// The done file may already exist from a previous installation
+		//noinspection ResultOfMethodCallIgnored
-			//noinspection ResultOfMethodCallIgnored
+		doneFile.delete();
-			doneFile.delete();
+		// The GeoIP file may exist from a previous installation - we can
-			// The GeoIP file may exist from a previous installation - we can
+		// save some space by deleting it.
-			// save some space by deleting it.
+		// TODO: Remove after a reasonable migration period
-			// TODO: Remove after a reasonable migration period
+		//  (added 2022-03-29)
-			//  (added 2022-03-29)
+		//noinspection ResultOfMethodCallIgnored
-			//noinspection ResultOfMethodCallIgnored
+		geoIpFile.delete();
-			geoIpFile.delete();
+		installTorExecutable();
-			installTorExecutable();
+		installObfs4Executable();
-			installObfs4Executable();
+		installSnowflakeExecutable();
-			extract(getConfigInputStream(), configFile);
+		if (!doneFile.createNewFile())
-			if (!doneFile.createNewFile())
+			LOG.warning("Failed to create done file");
-				LOG.warning("Failed to create done file");
-		} catch (IOException e) {
-			throw new PluginException(e);
-		}
 	}
 
 	protected void extract(InputStream in, File dest) throws IOException {
@@ -381,23 +369,35 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		if (!obfs4File.setExecutable(true, true)) throw new IOException();
 	}
 
+	protected void installSnowflakeExecutable() throws IOException {
+		if (LOG.isLoggable(INFO))
+			LOG.info("Installing snowflake binary for " + architecture);
+		File snowflakeFile = getSnowflakeExecutableFile();
+		extract(getSnowflakeInputStream(), snowflakeFile);
+		if (!snowflakeFile.setExecutable(true, true)) throw new IOException();
+	}
+
 	private InputStream getTorInputStream() throws IOException {
-		InputStream in = resourceProvider
+		return getZipInputStream("tor");
-				.getResourceInputStream("tor_" + architecture, ".zip");
-		ZipInputStream zin = new ZipInputStream(in);
-		if (zin.getNextEntry() == null) throw new IOException();
-		return zin;
 	}
 
 	private InputStream getObfs4InputStream() throws IOException {
+		return getZipInputStream("obfs4proxy");
+	}
+
+	private InputStream getSnowflakeInputStream() throws IOException {
+		return getZipInputStream("snowflake");
+	}
+
+	private InputStream getZipInputStream(String basename) throws IOException {
 		InputStream in = resourceProvider
-				.getResourceInputStream("obfs4proxy_" + architecture, ".zip");
+				.getResourceInputStream(basename + "_" + architecture, ".zip");
 		ZipInputStream zin = new ZipInputStream(in);
 		if (zin.getNextEntry() == null) throw new IOException();
 		return zin;
 	}
 
-	private static void append(StringBuilder strb, String name, int value) {
+	private static void append(StringBuilder strb, String name, Object value) {
 		strb.append(name);
 		strb.append(" ");
 		strb.append(value);
@@ -405,13 +405,23 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	}
 
 	private InputStream getConfigInputStream() {
+		File dataDirectory = new File(torDirectory, ".tor");
 		StringBuilder strb = new StringBuilder();
 		append(strb, "ControlPort", torControlPort);
 		append(strb, "CookieAuthentication", 1);
+		append(strb, "DataDirectory", dataDirectory.getAbsolutePath());
 		append(strb, "DisableNetwork", 1);
 		append(strb, "RunAsDaemon", 1);
 		append(strb, "SafeSocks", 1);
 		append(strb, "SocksPort", torSocksPort);
+		strb.append("GeoIPFile\n");
+		strb.append("GeoIPv6File\n");
+		append(strb, "ConnectionPadding", 0);
+		String obfs4Path = getObfs4ExecutableFile().getAbsolutePath();
+		append(strb, "ClientTransportPlugin obfs4 exec", obfs4Path);
+		append(strb, "ClientTransportPlugin meek_lite exec", obfs4Path);
+		String snowflakePath = getSnowflakeExecutableFile().getAbsolutePath();
+		append(strb, "ClientTransportPlugin snowflake exec", snowflakePath);
 		//noinspection CharsetObjectCanBeUsed
 		return new ByteArrayInputStream(
 				strb.toString().getBytes(Charset.forName("UTF-8")));
@@ -442,6 +452,23 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 	}
 
+	protected void waitForTorToStart(Process torProcess)
+			throws InterruptedException, PluginException {
+		Scanner stdout = new Scanner(torProcess.getInputStream());
+		// Log the first line of stdout (contains Tor and library versions)
+		if (stdout.hasNextLine()) LOG.info(stdout.nextLine());
+		// Read the process's stdout (and redirected stderr) until it detaches
+		while (stdout.hasNextLine()) stdout.nextLine();
+		stdout.close();
+		// Wait for the process to detach or exit
+		int exit = torProcess.waitFor();
+		if (exit != 0) {
+			if (LOG.isLoggable(WARNING))
+				LOG.warning("Tor exited with value " + exit);
+			throw new PluginException();
+		}
+	}
+
 	private void bind() {
 		ioExecutor.execute(() -> {
 			// If there's already a port number stored in config, reuse it
@@ -544,7 +571,7 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	}
 
 	protected void enableNetwork(boolean enable) throws IOException {
-		state.enableNetwork(enable);
+		if (!state.enableNetwork(enable)) return; // Unchanged
 		try {
 			controlConnection.setConf("DisableNetwork", enable ? "0" : "1");
 		} catch (TorNotRunningException e) {
@@ -552,34 +579,37 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 	}
 
-	private void enableBridges(boolean enable, List<BridgeType> bridgeTypes)
+	private void enableBridges(List<BridgeType> bridgeTypes, String countryCode)
 			throws IOException {
+		if (!state.setBridgeTypes(bridgeTypes)) return; // Unchanged
 		try {
-			if (enable) {
+			if (bridgeTypes.isEmpty()) {
+				controlConnection.setConf("UseBridges", "0");
+				controlConnection.resetConf(singletonList("Bridge"));
+			} else {
 				Collection<String> conf = new ArrayList<>();
 				conf.add("UseBridges 1");
-				File obfs4File = getObfs4ExecutableFile();
+				boolean letsEncrypt = canVerifyLetsEncryptCerts();
-				if (bridgeTypes.contains(MEEK)) {
-					conf.add("ClientTransportPlugin meek_lite exec " +
-							obfs4File.getAbsolutePath());
-				}
-				if (bridgeTypes.contains(DEFAULT_OBFS4) ||
-						bridgeTypes.contains(NON_DEFAULT_OBFS4)) {
-					conf.add("ClientTransportPlugin obfs4 exec " +
-							obfs4File.getAbsolutePath());
-				}
 				for (BridgeType bridgeType : bridgeTypes) {
-					conf.addAll(circumventionProvider.getBridges(bridgeType));
+					conf.addAll(circumventionProvider
+							.getBridges(bridgeType, countryCode, letsEncrypt));
 				}
 				controlConnection.setConf(conf);
-			} else {
-				controlConnection.setConf("UseBridges", "0");
 			}
 		} catch (TorNotRunningException e) {
 			throw new RuntimeException(e);
 		}
 	}
 
+	/**
+	 * Returns true if this device can verify Let's Encrypt certificates signed
+	 * with the IdentTrust DST Root X3 certificate, which expired at the end of
+	 * September 2021.
+	 */
+	protected boolean canVerifyLetsEncryptCerts() {
+		return true;
+	}
+
 	@Override
 	public void stop() {
 		ServerSocket ss = state.setStopped();
@@ -587,7 +617,6 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		if (controlSocket != null && controlConnection != null) {
 			try {
 				LOG.info("Stopping Tor");
-				controlConnection.setConf("DisableNetwork", "1");
 				controlConnection.shutdownTor("TERM");
 				controlSocket.close();
 			} catch (TorNotRunningException e) {
@@ -755,7 +784,7 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	public void circuitStatus(String status, String id, String path) {
 		// In case of races between receiving CIRCUIT_ESTABLISHED and setting
 		// DisableNetwork, set our circuitBuilt flag if not already set
-		if (status.equals("BUILT") && !state.getAndSetCircuitBuilt(true)) {
+		if (status.equals("BUILT") && state.setCircuitBuilt(true)) {
 			LOG.info("Circuit built");
 			backoff.reset();
 		}
@@ -802,6 +831,13 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 	}
 
+	@Override
+	public void controlConnectionClosed() {
+		if (state.isTorRunning()) {
+			throw new RuntimeException("Control connection closed");
+		}
+	}
+
 	private String removeSeverity(String msg) {
 		return msg.replaceFirst("[^ ]+ ", "");
 	}
@@ -812,12 +848,12 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			state.setBootstrapped();
 			backoff.reset();
 		} else if (msg.startsWith("CIRCUIT_ESTABLISHED")) {
-			if (!state.getAndSetCircuitBuilt(true)) {
+			if (state.setCircuitBuilt(true)) {
 				LOG.info("Circuit built");
 				backoff.reset();
 			}
 		} else if (msg.startsWith("CIRCUIT_NOT_ESTABLISHED")) {
-			if (state.getAndSetCircuitBuilt(false)) {
+			if (state.setCircuitBuilt(false)) {
 				LOG.info("Circuit not built");
 				// TODO: Disable and re-enable network to prompt Tor to rebuild
 				//  its guard/bridge connections? This will also close any
@@ -908,10 +944,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			}
 
 			int reasonsDisabled = 0;
-			boolean enableNetwork = false, enableBridges = false;
+			boolean enableNetwork = false, enableConnectionPadding = false;
-			boolean enableConnectionPadding = false;
+			List<BridgeType> bridgeTypes = emptyList();
-			List<BridgeType> bridgeTypes =
-					circumventionProvider.getSuitableBridgeTypes(country);
 
 			if (!online) {
 				LOG.info("Disabling network, device is offline");
@@ -940,8 +974,12 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 					enableNetwork = true;
 					if (network == PREF_TOR_NETWORK_WITH_BRIDGES ||
 							(automatic && bridgesWork)) {
-						if (ipv6Only) bridgeTypes = singletonList(MEEK);
+						if (ipv6Only) {
-						enableBridges = true;
+							bridgeTypes = asList(MEEK, SNOWFLAKE);
+						} else {
+							bridgeTypes = circumventionProvider
+									.getSuitableBridgeTypes(country);
+						}
 						if (LOG.isLoggable(INFO)) {
 							LOG.info("Using bridge types " + bridgeTypes);
 						}
@@ -961,9 +999,9 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 
 			try {
 				if (enableNetwork) {
-					enableBridges(enableBridges, bridgeTypes);
+					enableBridges(bridgeTypes, country);
 					enableConnectionPadding(enableConnectionPadding);
-					useIpv6(ipv6Only);
+					enableIpv6(ipv6Only);
 				}
 				enableNetwork(enableNetwork);
 			} catch (IOException e) {
@@ -973,6 +1011,7 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 	}
 
 	private void enableConnectionPadding(boolean enable) throws IOException {
+		if (!state.enableConnectionPadding(enable)) return; // Unchanged
 		try {
 			controlConnection.setConf("ConnectionPadding", enable ? "1" : "0");
 		} catch (TorNotRunningException e) {
@@ -980,10 +1019,11 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 	}
 
-	private void useIpv6(boolean ipv6Only) throws IOException {
+	private void enableIpv6(boolean enable) throws IOException {
+		if (!state.enableIpv6(enable)) return; // Unchanged
 		try {
-			controlConnection.setConf("ClientUseIPv4", ipv6Only ? "0" : "1");
+			controlConnection.setConf("ClientUseIPv4", enable ? "0" : "1");
-			controlConnection.setConf("ClientUseIPv6", ipv6Only ? "1" : "0");
+			controlConnection.setConf("ClientUseIPv6", enable ? "1" : "0");
 		} catch (TorNotRunningException e) {
 			throw new RuntimeException(e);
 		}
@@ -998,6 +1038,8 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 				stopped = false,
 				networkInitialised = false,
 				networkEnabled = false,
+				paddingEnabled = false,
+				ipv6Enabled = false,
 				bootstrapped = false,
 				circuitBuilt = false,
 				settingsChecked = false;
@@ -1012,6 +1054,9 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		@GuardedBy("this")
 		private int orConnectionsConnected = 0;
 
+		@GuardedBy("this")
+		private List<BridgeType> bridgeTypes = emptyList();
+
 		private synchronized void setStarted() {
 			started = true;
 			callback.pluginStateChanged(getState());
@@ -1032,28 +1077,66 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 		}
 
 		private synchronized void setBootstrapped() {
+			boolean wasBootstrapped = bootstrapped;
 			bootstrapped = true;
-			callback.pluginStateChanged(getState());
+			if (!wasBootstrapped) callback.pluginStateChanged(getState());
 		}
 
-		private synchronized boolean getAndSetCircuitBuilt(boolean built) {
+		/**
-			boolean old = circuitBuilt;
+		 * Sets the `circuitBuilt` flag and returns true if the flag has
+		 * changed.
+		 */
+		private synchronized boolean setCircuitBuilt(boolean built) {
+			if (built == circuitBuilt) return false; // Unchanged
 			circuitBuilt = built;
-			if (built != old) callback.pluginStateChanged(getState());
+			callback.pluginStateChanged(getState());
-			return old;
+			return true; // Changed
 		}
 
-		private synchronized void enableNetwork(boolean enable) {
+		/**
+		 * Sets the `networkEnabled` flag and returns true if the flag has
+		 * changed.
+		 */
+		private synchronized boolean enableNetwork(boolean enable) {
+			boolean wasInitialised = networkInitialised;
+			boolean wasEnabled = networkEnabled;
 			networkInitialised = true;
 			networkEnabled = enable;
 			if (!enable) circuitBuilt = false;
-			callback.pluginStateChanged(getState());
+			if (!wasInitialised || enable != wasEnabled) {
+				callback.pluginStateChanged(getState());
+			}
+			return enable != wasEnabled;
 		}
 
-		private synchronized void setReasonsDisabled(int reasonsDisabled) {
+		/**
+		 * Sets the `paddingEnabled` flag and returns true if the flag has
+		 * changed. Doesn't affect getState().
+		 */
+		private synchronized boolean enableConnectionPadding(boolean enable) {
+			if (enable == paddingEnabled) return false; // Unchanged
+			paddingEnabled = enable;
+			return true; // Changed
+		}
+
+		/**
+		 * Sets the `ipv6Enabled` flag and returns true if the flag has
+		 * changed. Doesn't affect getState().
+		 */
+		private synchronized boolean enableIpv6(boolean enable) {
+			if (enable == ipv6Enabled) return false; // Unchanged
+			ipv6Enabled = enable;
+			return true; // Changed
+		}
+
+		private synchronized void setReasonsDisabled(int reasons) {
+			boolean wasChecked = settingsChecked;
 			settingsChecked = true;
-			this.reasonsDisabled = reasonsDisabled;
+			int oldReasons = reasonsDisabled;
-			callback.pluginStateChanged(getState());
+			reasonsDisabled = reasons;
+			if (!wasChecked || reasons != oldReasons) {
+				callback.pluginStateChanged(getState());
+			}
 		}
 
 		// Doesn't affect getState()
@@ -1068,6 +1151,17 @@ abstract class TorPlugin implements DuplexPlugin, EventHandler, EventListener {
 			if (serverSocket == ss) serverSocket = null;
 		}
 
+		/**
+		 * Sets the list of bridge types being used and returns true if the
+		 * list has changed. The list is empty if bridges are disabled.
+		 * Doesn't affect getState().
+		 */
+		private synchronized boolean setBridgeTypes(List<BridgeType> types) {
+			if (types.equals(bridgeTypes)) return false; // Unchanged
+			bridgeTypes = types;
+			return true; // Changed
+		}
+
 		private synchronized State getState() {
 			if (!started || stopped || !settingsChecked) {
 				return STARTING_STOPPING;

bramble-core/src/main/java/org/briarproject/bramble/sync/DuplexOutgoingSession.java

@@ -44,12 +44,14 @@ import java.util.logging.Logger;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 
+import static java.lang.Boolean.TRUE;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPING;
 import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
+import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.SUPPORTED_VERSIONS;
@@ -232,11 +234,19 @@ class DuplexOutgoingSession implements SyncSession, EventListener {
 			ContactRemovedEvent c = (ContactRemovedEvent) e;
 			if (c.getContactId().equals(contactId)) interrupt();
 		} else if (e instanceof MessageSharedEvent) {
-			generateOffer();
+			MessageSharedEvent m = (MessageSharedEvent) e;
+			// If the contact is present in the map (ie the value is not null)
+			// and the value is true, the message's group is shared with the
+			// contact and therefore the message may now be sendable
+			if (m.getGroupVisibility().get(contactId) == TRUE) {
+				generateOffer();
+			}
 		} else if (e instanceof GroupVisibilityUpdatedEvent) {
 			GroupVisibilityUpdatedEvent g = (GroupVisibilityUpdatedEvent) e;
-			if (g.getAffectedContacts().contains(contactId))
+			if (g.getVisibility() == SHARED &&
+					g.getAffectedContacts().contains(contactId)) {
 				generateOffer();
+			}
 		} else if (e instanceof MessageRequestedEvent) {
 			if (((MessageRequestedEvent) e).getContactId().equals(contactId))
 				generateBatch();
@@ -310,7 +320,8 @@ class DuplexOutgoingSession implements SyncSession, EventListener {
 							Collection<Message> batch =
 									db.generateRequestedBatch(txn, contactId,
 											BATCH_CAPACITY, maxLatency);
-							setNextSendTime(db.getNextSendTime(txn, contactId));
+							setNextSendTime(db.getNextSendTime(txn, contactId,
+									maxLatency));
 							return batch;
 						});
 				if (LOG.isLoggable(INFO))
@@ -353,7 +364,8 @@ class DuplexOutgoingSession implements SyncSession, EventListener {
 				Offer o = db.transactionWithNullableResult(false, txn -> {
 					Offer offer = db.generateOffer(txn, contactId,
 							MAX_MESSAGE_IDS, maxLatency);
-					setNextSendTime(db.getNextSendTime(txn, contactId));
+					setNextSendTime(db.getNextSendTime(txn, contactId,
+							maxLatency));
 					return offer;
 				});
 				if (LOG.isLoggable(INFO))

bramble-core/src/main/java/org/briarproject/bramble/sync/MailboxOutgoingSession.java

@@ -7,14 +7,16 @@ import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.sync.Ack;
-import org.briarproject.bramble.api.sync.DeferredSendHandler;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.SyncRecordWriter;
 import org.briarproject.bramble.api.transport.StreamWriter;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collection;
+import java.util.List;
 import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
@@ -29,7 +31,7 @@ import static org.briarproject.bramble.api.sync.SyncConstants.MESSAGE_HEADER_LEN
 
 /**
  * A {@link SimplexOutgoingSession} for sending and acking messages via a
- * mailbox. The session uses a {@link DeferredSendHandler} to record the IDs
+ * mailbox. The session uses a {@link OutgoingSessionRecord} to record the IDs
  * of the messages sent and acked during the session so that they can be
  * recorded in the DB as sent or acked after the file has been successfully
  * uploaded to the mailbox.
@@ -41,7 +43,7 @@ class MailboxOutgoingSession extends SimplexOutgoingSession {
 	private static final Logger LOG =
 			getLogger(MailboxOutgoingSession.class.getName());
 
-	private final DeferredSendHandler deferredSendHandler;
+	private final OutgoingSessionRecord sessionRecord;
 	private final long initialCapacity;
 
 	MailboxOutgoingSession(DatabaseComponent db,
@@ -51,36 +53,42 @@ class MailboxOutgoingSession extends SimplexOutgoingSession {
 			long maxLatency,
 			StreamWriter streamWriter,
 			SyncRecordWriter recordWriter,
-			DeferredSendHandler deferredSendHandler,
+			OutgoingSessionRecord sessionRecord,
 			long capacity) {
 		super(db, eventBus, contactId, transportId, maxLatency, streamWriter,
 				recordWriter);
-		this.deferredSendHandler = deferredSendHandler;
+		this.sessionRecord = sessionRecord;
 		this.initialCapacity = capacity;
 	}
 
 	@Override
 	void sendAcks() throws DbException, IOException {
-		while (!isInterrupted()) {
+		List<MessageId> idsToAck = loadMessageIdsToAck();
-			Collection<MessageId> idsToAck = loadMessageIdsToAck();
+		int idsSent = 0;
-			if (idsToAck.isEmpty()) break;
+		while (idsSent < idsToAck.size() && !isInterrupted()) {
-			recordWriter.writeAck(new Ack(idsToAck));
+			int idsRemaining = idsToAck.size() - idsSent;
-			deferredSendHandler.onAckSent(idsToAck);
+			long capacity = getRemainingCapacity();
+			long idCapacity =
+					(capacity - RECORD_HEADER_BYTES) / MessageId.LENGTH;
+			if (idCapacity == 0) break; // Out of capacity
+			int idsInRecord = (int) min(idCapacity, MAX_MESSAGE_IDS);
+			int idsToSend = min(idsRemaining, idsInRecord);
+			List<MessageId> acked =
+					idsToAck.subList(idsSent, idsSent + idsToSend);
+			recordWriter.writeAck(new Ack(acked));
+			sessionRecord.onAckSent(acked);
 			LOG.info("Sent ack");
+			idsSent += idsToSend;
 		}
 	}
 
-	private Collection<MessageId> loadMessageIdsToAck() throws DbException {
+	private List<MessageId> loadMessageIdsToAck() throws DbException {
-		long idCapacity = (getRemainingCapacity() - RECORD_HEADER_BYTES)
-				/ MessageId.LENGTH;
-		if (idCapacity <= 0) return emptyList(); // Out of capacity
-		int maxMessageIds = (int) min(idCapacity, MAX_MESSAGE_IDS);
 		Collection<MessageId> ids = db.transactionWithResult(true, txn ->
-				db.getMessagesToAck(txn, contactId, maxMessageIds));
+				db.getMessagesToAck(txn, contactId));
 		if (LOG.isLoggable(INFO)) {
 			LOG.info(ids.size() + " messages to ack");
 		}
-		return ids;
+		return new ArrayList<>(ids);
 	}
 
 	private long getRemainingCapacity() {
@@ -96,7 +104,7 @@ class MailboxOutgoingSession extends SimplexOutgoingSession {
 					db.getMessageToSend(txn, contactId, m, maxLatency, false));
 			if (message == null) continue; // No longer shared
 			recordWriter.writeMessage(message);
-			deferredSendHandler.onMessageSent(m);
+			sessionRecord.onMessageSent(m);
 			LOG.info("Sent message");
 		}
 	}

bramble-core/src/main/java/org/briarproject/bramble/sync/SyncSessionFactoryImpl.java

@@ -6,6 +6,7 @@ import org.briarproject.bramble.api.db.DatabaseExecutor;
 import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.Priority;
 import org.briarproject.bramble.api.sync.PriorityHandler;
 import org.briarproject.bramble.api.sync.SyncRecordReader;
@@ -25,6 +26,8 @@ import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.MAX_FILE_PAYLOAD_BYTES;
+
 @Immutable
 @NotNullByDefault
 class SyncSessionFactoryImpl implements SyncSessionFactory {
@@ -73,6 +76,18 @@ class SyncSessionFactoryImpl implements SyncSessionFactory {
 		}
 	}
 
+	@Override
+	public SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
+			long maxLatency, StreamWriter streamWriter,
+			OutgoingSessionRecord sessionRecord) {
+		OutputStream out = streamWriter.getOutputStream();
+		SyncRecordWriter recordWriter =
+				recordWriterFactory.createRecordWriter(out);
+		return new MailboxOutgoingSession(db, eventBus, c, t, maxLatency,
+				streamWriter, recordWriter, sessionRecord,
+				MAX_FILE_PAYLOAD_BYTES);
+	}
+
 	@Override
 	public SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
 			long maxLatency, int maxIdleTime, StreamWriter streamWriter,

bramble-core/src/main/resources/bridges

@@ -11,20 +11,26 @@ d Bridge obfs4 209.148.46.65:443 74FAD13168806246602538555B5521A0383A1875 cert=s
 d Bridge obfs4 146.57.248.225:22 10A6CD36A537FCE513A322361547444B393989F0 cert=K1gDtDAIcUfeLqbstggjIw2rtgIKqdIhUlHp82XRqNSq/mtAjp1BIC9vHKJ2FAEpGssTPw iat-mode=0
 d Bridge obfs4 45.145.95.6:27015 C5B7CD6946FF10C5B3E89691A7D3F2C122D2117C cert=TD7PbUO0/0k6xYHMPW3vJxICfkMZNdkRrb63Zhl5j9dW3iRGiCx0A7mPhe5T2EDzQ35+Zw iat-mode=0
 d Bridge obfs4 51.222.13.177:80 5EDAC3B810E12B01F6FD8050D2FD3E277B289A08 cert=2uplIpLQ0q9+0qMFrK5pkaYRDOe460LL9WHBvatgkuRr/SL31wBOEupaMMJ6koRE6Ld0ew iat-mode=0
-n Bridge obfs4 46.226.107.197:10300 A38FD6BDFD902882F5F5B9B7CCC95602A20B0BC4 cert=t8tA9q2AeGlmp/dO6oW9bkY5RqqmvqjArCEM9wjJoDnk6XtnaejkF0JTA7VamdyOzcvuBg iat-mode=0
 n Bridge obfs4 185.181.11.86:443 A961609729E7FDF520B4E81F1F1B8FA1045285C3 cert=e5faG9Zk4Ni+e7z2YgGfevyKPQlMvkVGi4ublSsHYjaBovKeNXpOhbeFxzbZZoAzxAoGUQ iat-mode=0
-n Bridge obfs4 85.242.211.221:8042 A36A938DD7FDB8BACC846BA326EE0BA0D89A9252 cert=1AN6Pt1eFca3Y/WYD2TGAU3Al9cO4eouXE9SX63s66Z/ks3tVmgQ5GeXi1B5DOvx6Il7Zw iat-mode=0
-n Bridge obfs4 74.104.165.202:9002 EF432018A6AA5D970B2F84E39CD30A147030141C cert=PhppfUusY85dHGvWtGTybZ1fED4DtbHmALkNMIOIYrAz1B4xN7/2a5gyiZe1epju1BOHVg iat-mode=0
-n Bridge obfs4 70.34.242.31:443 7F026956402CDFF4BCBA8E11EE9C50E3FE0A2B72 cert=hP/KU7JATSfWH3HwS5Er/YLT0J+bRO3+s2fWx2yirrgf37EyrWvm/BQshoNje8WfUm6CBw iat-mode=0
 n Bridge obfs4 93.95.226.151:41185 460B0CFFC0CF1D965F3DE064E08BA1915E7C916A cert=inluPzp5Jp5OzZar1eQb4dcQ/YlAj/v0kHAUCoCr3rmLt03+pVuVTjoH4mRy4+acXpn+Gw iat-mode=0
 n Bridge obfs4 120.29.217.52:5223 40FE3DB9800272F9CDC76422F8ED7883280EE96D cert=/71PS4l8c/XJ4DIItlH9xMqNvPFg2RUTrHvPlQWh48u5et8h/yyyjCcYphUadDsfBWpaGQ iat-mode=0
-n Bridge obfs4 83.97.179.29:1199 D83068BFAA28E71DB024B786E1E803BE14257127 cert=IduGtt05tM59Xmvo0oVNWgIRgY4OGPJjFP+y2oa6RMDHQBL/GRyFOOgX70iiazNAIJNkPw iat-mode=0
-n Bridge obfs4 207.181.244.13:40132 37FE8D782F5DD2BAEEDAAB8257B701344676B6DD cert=f5Hbfn3ToMzH170cV8DfLly3vRynveidfOfDcbradIDtbLDX15V2yQ8eEH2CPKQJmQR2Hg iat-mode=0
-n Bridge obfs4 76.255.201.112:8888 96CF36C2ECCFB7376AB6BE905BECD2C2AE8AEFCD cert=+q0pjaiM0JMqHL/BKqCRD+pjflaw/S406eUDF7CnFgamvQW3l2HVLJhQ6uX9P8zff0PLGg iat-mode=0
-n Bridge obfs4 65.108.159.114:14174 E1AD374BA9F34BD98862D128AC54D40C7DC138AE cert=YMkxMSBN2OOd99AkJpFaEUyKkdqZgJt9oVJEgO1QnT37n/Vc2yR4nhx4k4VkPLfEP1f4eg iat-mode=0
 n Bridge obfs4 185.177.207.138:8443 53716FE26F23C8C6A71A2BC5D9D8DC64747278C7 cert=6jcYVilMEzxdsWghSrQFpYYJlkkir/GPIXw/EnddUK3S8ToVpMG8u1SwMOEdEs735RrMHw iat-mode=0
-n Bridge obfs4 176.123.2.253:1933 B855D141CE6C4DE0F7EA4AAED83EBA8373FA8191 cert=1rOoSaRagc6PPR//paIl+ukv1N+xWKCdBXMFxK0k/moEwH0lk5bURBrUDzIX35fVzaiicQ iat-mode=0
+n Bridge obfs4 87.121.72.109:9002 C8081D4731C953FA4AE166946E72B29153351E34 cert=bikAqxKV6Ch5gFCBTdPI28VeShYa1ZgkLmvc7YZNLWFsFZoaCULL/3AQKjpIfvSiJs5jGQ iat-mode=0
-v Bridge 135.181.113.164:54444 74AF4CCA614C454B7D3E81FF8BACD78CEBC7D7DE
+n Bridge obfs4 70.34.249.113:443 F441B16ABB1055794C2CE01821FC05047B2C8CFC cert=MauLNoyq8EwjY4Qe0oASYzs2hXdSjNgy+BtP9oo1naHhRsyKTtAZzeNv08RnzWjMJrTwcg iat-mode=0
+n Bridge obfs4 104.168.68.90:443 ED55B3C321E44EA7E50EF568C8A63CF75E89A58C cert=fgonxDvltTp8nmcOE9sUG94eOAALxETVVXAwnTZJLPpf7rjPuTp+abKl4VyFkxfcLRr5KQ iat-mode=0
+n Bridge obfs4 158.247.207.151:443 6170ADBBB6C1859A8E7E4416BB8AB3AF471AE47F cert=Od4izlwLnXcq7LMSOJtnZLtklaUn+X+gPcBwN7RUEkk9rqxRRYNHW7as8g6+jheDsazxAQ iat-mode=0
+n Bridge obfs4 45.142.181.131:42069 6EBCF6B02DA2B982F4080A7119D737366AFB74FA cert=9HyWH/BCwWzNirZdTQtluCgJk+gFhpOqydIuyQ1iDvpnqsAynKF+zcPE/INZFefm86UlBg iat-mode=0
+n Bridge obfs4 85.214.28.204:47111 78A36E46BB082A471848239D3F4390A8F8C6084D cert=96sr3eaUFBDu4wFVAQIfNFElh0UNuutJ/3/Fh2Vu2PHfacQ8bwfY02bwG351U8BZpLnfUQ iat-mode=0
+n Bridge obfs4 152.67.77.101:4096 B82DB9CDDF887AB8A859420E07DF298E30AF8A6E cert=21OWn3yFo+hulmQNAOtF5uwwOqWtdT5PrLhk8BG9DpOd0/k5DEkQEYPyDdXbS9nZ0E5BJA iat-mode=0
+n Bridge obfs4 82.39.132.97:6969 F505EF4C41C77FFDC0C440C122A02129FBE25823 cert=bwNWuL7UYB9aiKajE1gkffylYx/EM4FjSZxIJ0pVT/xaR21xXlIdaXw7l+EYmC4nVIh2HQ iat-mode=0
+n Bridge obfs4 185.103.252.72:443 75F15E9339FF572F88F5588D429FEA379744BC53 cert=nOZ/SaRE3L1dChvjfe0Ks/wM/F8iFhwd3g2G5zgtcLB8x+wiZRWCwjRrbbiQyb3Gh2mxRQ iat-mode=0
+n Bridge obfs4 185.177.207.13:22662 928C1E4289A01F34C8FB423FC32C0E77EE0F8736 cert=p9L6+25s8bnfkye1ZxFeAE4mAGY7DH4Gaj7dxngIIzP9BtqrHHwZXdjMK0RVIQ34C7aqZw iat-mode=2
+n Bridge obfs4 207.181.229.55:40132 37FE8D782F5DD2BAEEDAAB8257B701344676B6DD cert=f5Hbfn3ToMzH170cV8DfLly3vRynveidfOfDcbradIDtbLDX15V2yQ8eEH2CPKQJmQR2Hg iat-mode=0
+n Bridge obfs4 76.255.201.112:8888 96CF36C2ECCFB7376AB6BE905BECD2C2AE8AEFCD cert=+q0pjaiM0JMqHL/BKqCRD+pjflaw/S406eUDF7CnFgamvQW3l2HVLJhQ6uX9P8zff0PLGg iat-mode=0
 v Bridge 92.243.15.235:9001 477EAD3C04036B48235F1F27FC91420A286A4B7F
-v Bridge 77.96.91.103:443 ED000A75B79A58F1D83A4D1675C2A9395B71BE8E
+v Bridge 213.108.108.145:17674 A39C0FE47963B6E8CFE9815549864DE544935A31
-m Bridge meek_lite 192.0.2.2:2 97700DFE9F483596DDA6264C4D7DF7641E1E39CE url=https://meek.azureedge.net/ front=ajax.aspnetcdn.com
+v Bridge 185.189.195.124:8199 A1F3EE78F9C2343668E68FEB84358A4C742831A5
+v Bridge 213.196.191.96:9060 05E222E2A8C234234FE0CEB58B08A93B8FC360DB
+v Bridge 75.100.92.154:22815 465E990FA8A752DDE861EDF31E42454ACC037AB4
+m Bridge meek_lite 192.0.2.2:80 97700DFE9F483596DDA6264C4D7DF7641E1E39CE url=https://meek.azureedge.net/ front=ajax.aspnetcdn.com
+s Bridge snowflake 192.0.2.3:80 2B280B23E1107BB62ABFC40DDCC8824814F80A72

bramble-core/src/main/resources/snowflake-params

@@ -0,0 +1,4 @@
+ZZ 1 url=https://snowflake-broker.torproject.net.global.prod.fastly.net/ front=cdn.sstatic.net ice=stun:stun.l.google.com:19302,stun:stun.voip.blackberry.com:3478,stun:stun.altar.com.pl:3478,stun:stun.antisip.com:3478,stun:stun.bluesip.net:3478,stun:stun.dus.net:3478,stun:stun.epygi.com:3478,stun:stun.sonetel.com:3478,stun:stun.sonetel.net:3478,stun:stun.stunprotocol.org:3478,stun:stun.uls.co.za:3478,stun:stun.voipgate.com:3478,stun:stun.voys.nl:3478
+ZZ 0 url=https://snowflake-broker.azureedge.net/ front=ajax.aspnetcdn.com ice=stun:stun.l.google.com:19302,stun:stun.voip.blackberry.com:3478,stun:stun.altar.com.pl:3478,stun:stun.antisip.com:3478,stun:stun.bluesip.net:3478,stun:stun.dus.net:3478,stun:stun.epygi.com:3478,stun:stun.sonetel.com:3478,stun:stun.sonetel.net:3478,stun:stun.stunprotocol.org:3478,stun:stun.uls.co.za:3478,stun:stun.voipgate.com:3478,stun:stun.voys.nl:3478
+TM 1 url=https://snowflake-broker.azureedge.net/ front=ajax.aspnetcdn.com ice=stun:206.53.159.130:3479,stun:176.119.42.11:3479,stun:94.23.17.185:3479,stun:217.74.179.29:3479,stun:83.125.8.47:3479,stun:23.253.102.137:3479,stun:52.26.251.34:3479,stun:52.26.251.34:3479,stun:18.191.223.12:3479,stun:154.73.34.8:3479,stun:185.125.180.70:3479,stun:195.35.115.37:3479
+TM 0 url=https://snowflake-broker.azureedge.net/ front=ajax.aspnetcdn.com ice=stun:206.53.159.130:3479,stun:176.119.42.11:3479,stun:94.23.17.185:3479,stun:217.74.179.29:3479,stun:83.125.8.47:3479,stun:23.253.102.137:3479,stun:52.26.251.34:3479,stun:52.26.251.34:3479,stun:18.191.223.12:3479,stun:154.73.34.8:3479,stun:185.125.180.70:3479,stun:195.35.115.37:3479

bramble-core/src/test/java/org/briarproject/bramble/connection/ConnectionRegistryImplTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.connection;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
 import org.briarproject.bramble.api.connection.InterruptibleConnection;
 import org.briarproject.bramble.api.contact.ContactId;
@@ -57,6 +58,10 @@ public class ConnectionRegistryImplTest extends BrambleMockTestCase {
 	private final Priority high =
 			new Priority(fromHexString("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF"));
 
+	public ConnectionRegistryImplTest() throws FormatException {
+		// required for throws declaration
+	}
+
 	@Test
 	public void testRegisterMultipleConnections() {
 		context.checking(new Expectations() {{

bramble-core/src/test/java/org/briarproject/bramble/contact/ContactExchangeIntegrationTestComponent.java

@@ -10,6 +10,8 @@ import org.briarproject.bramble.api.identity.IdentityManager;
 import org.briarproject.bramble.api.lifecycle.IoExecutor;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.test.BrambleCoreIntegrationTestModule;
+import org.briarproject.bramble.test.TestDnsModule;
+import org.briarproject.bramble.test.TestSocksModule;
 
 import java.util.concurrent.Executor;
 
@@ -20,7 +22,9 @@ import dagger.Component;
 @Singleton
 @Component(modules = {
 		BrambleCoreIntegrationTestModule.class,
-		BrambleCoreModule.class
+		BrambleCoreModule.class,
+		TestDnsModule.class,
+		TestSocksModule.class
 })
 interface ContactExchangeIntegrationTestComponent
 		extends BrambleCoreIntegrationTestEagerSingletons {

bramble-core/src/test/java/org/briarproject/bramble/contact/ContactManagerImplTest.java

@@ -17,7 +17,6 @@ import org.briarproject.bramble.api.identity.LocalAuthor;
 import org.briarproject.bramble.api.transport.KeyManager;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.briarproject.bramble.test.DbExpectations;
-import org.junit.Before;
 import org.junit.Test;
 
 import java.util.Collection;
@@ -63,13 +62,9 @@ public class ContactManagerImplTest extends BrambleMockTestCase {
 	private final long timestamp = System.currentTimeMillis();
 	private final boolean alice = new Random().nextBoolean();
 
-	private ContactManagerImpl contactManager;
+	private final ContactManagerImpl contactManager =
-
+			new ContactManagerImpl(db, keyManager, identityManager,
-	@Before
+					pendingContactFactory);
-	public void setUp() {
-		contactManager = new ContactManagerImpl(db, keyManager,
-				identityManager, pendingContactFactory);
-	}
 
 	@Test
 	public void testAddContact() throws Exception {

bramble-core/src/test/java/org/briarproject/bramble/crypto/Blake2bDigestTest.java

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.crypto;
 
 import org.bouncycastle.crypto.digests.Blake2bDigest;
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.briarproject.bramble.util.StringUtils;
 import org.junit.Test;
@@ -47,7 +48,7 @@ public class Blake2bDigestTest extends BrambleTestCase {
 	};
 
 	@Test
-	public void testDigestWithKeyedTestVectors() {
+	public void testDigestWithKeyedTestVectors() throws FormatException {
 		for (String[] keyedTestVector : KEYED_TEST_VECTORS) {
 			byte[] input = StringUtils.fromHexString(keyedTestVector[0]);
 			byte[] key = StringUtils.fromHexString(keyedTestVector[1]);
@@ -63,7 +64,8 @@ public class Blake2bDigestTest extends BrambleTestCase {
 	}
 
 	@Test
-	public void testDigestWithKeyedTestVectorsAndRandomUpdate() {
+	public void testDigestWithKeyedTestVectorsAndRandomUpdate()
+			throws FormatException {
 		Random random = new Random();
 		for (int i = 0; i < 100; i++) {
 			for (String[] keyedTestVector : KEYED_TEST_VECTORS) {
@@ -138,7 +140,7 @@ public class Blake2bDigestTest extends BrambleTestCase {
 	}
 
 	@Test
-	public void runSelfTest() {
+	public void runSelfTest() throws FormatException {
 		Blake2bDigest testDigest = new Blake2bDigest(256);
 		byte[] md = new byte[64];
 

bramble-core/src/test/java/org/briarproject/bramble/crypto/KeyAgreementTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.crypto.AgreementPublicKey;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.KeyPair;
@@ -83,7 +84,7 @@ public class KeyAgreementTest extends BrambleTestCase {
 	}
 
 	@Test
-	public void testRfc7748TestVector() {
+	public void testRfc7748TestVector() throws FormatException {
 		byte[] aPriv = parsePrivateKey(ALICE_PRIVATE);
 		byte[] aPub = fromHexString(ALICE_PUBLIC);
 		byte[] bPriv = parsePrivateKey(BOB_PRIVATE);
@@ -97,7 +98,8 @@ public class KeyAgreementTest extends BrambleTestCase {
 	}
 
 	@Test
-	public void testDerivesSameSharedSecretFromEquivalentPublicKey() {
+	public void testDerivesSameSharedSecretFromEquivalentPublicKey()
+			throws FormatException {
 		byte[] aPub = fromHexString(ALICE_PUBLIC);
 		byte[] bPriv = parsePrivateKey(BOB_PRIVATE);
 		byte[] sharedSecret = fromHexString(SHARED_SECRET);
@@ -168,7 +170,7 @@ public class KeyAgreementTest extends BrambleTestCase {
 		return crypto.deriveSharedSecret(label, publicKey, keyPair, inputs);
 	}
 
-	private byte[] parsePrivateKey(String hex) {
+	private byte[] parsePrivateKey(String hex) throws FormatException {
 		// Private keys need to be clamped because curve25519-java does the
 		// clamping at key generation time, not multiplication time
 		return AgreementKeyParser.clamp(fromHexString(hex));

bramble-core/src/test/java/org/briarproject/bramble/crypto/XSalsa20Poly1305AuthenticatedCipherTest.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.crypto;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.test.BrambleTestCase;
 import org.briarproject.bramble.util.StringUtils;
@@ -15,11 +16,11 @@ public class XSalsa20Poly1305AuthenticatedCipherTest extends BrambleTestCase {
 
 	// Test vectors from the NaCl paper
 	// http://cr.yp.to/highspeed/naclcrypto-20090310.pdf
-	private static final byte[] TEST_KEY = StringUtils.fromHexString(
+	private final byte[] TEST_KEY = StringUtils.fromHexString(
 			"1b27556473e985d462cd51197a9a46c76009549eac6474f206c4ee0844f68389");
-	private static final byte[] TEST_IV = StringUtils.fromHexString(
+	private final byte[] TEST_IV = StringUtils.fromHexString(
 			"69696ee955b62b73cd62bda875fc73d68219e0036b7a0b37");
-	private static final byte[] TEST_PLAINTEXT = StringUtils.fromHexString(
+	private final byte[] TEST_PLAINTEXT = StringUtils.fromHexString(
 					"be075fc53c81f2d5cf141316" +
 					"ebeb0c7b5228c52a4c62cbd4" +
 					"4b66849b64244ffce5ecbaaf" +
@@ -31,7 +32,7 @@ public class XSalsa20Poly1305AuthenticatedCipherTest extends BrambleTestCase {
 					"048977eb48f59ffd4924ca1c" +
 					"60902e52f0a089bc76897040" +
 					"e082f937763848645e0705");
-	private static final byte[] TEST_CIPHERTEXT = StringUtils.fromHexString(
+	private final byte[] TEST_CIPHERTEXT = StringUtils.fromHexString(
 					"f3ffc7703f9400e52a7dfb4b" +
 					"3d3305d98e993b9f48681273" +
 					"c29650ba32fc76ce48332ea7" +
@@ -46,6 +47,10 @@ public class XSalsa20Poly1305AuthenticatedCipherTest extends BrambleTestCase {
 					"a43d14a6599b1f654cb45a74" +
 					"e355a5");
 
+	public XSalsa20Poly1305AuthenticatedCipherTest() throws FormatException {
+		// required for throws declaration
+	}
+
 	@Test
 	public void testEncrypt() throws Exception {
 		SecretKey k = new SecretKey(TEST_KEY);

bramble-core/src/test/java/org/briarproject/bramble/data/BdfReaderImplTest.java

@@ -618,11 +618,12 @@ public class BdfReaderImplTest extends BrambleTestCase {
 		r.readDictionary();
 	}
 
-	private void setContents(String hex) {
+	private void setContents(String hex) throws FormatException {
 		setContents(hex, DEFAULT_MAX_BUFFER_SIZE);
 	}
 
-	private void setContents(String hex, int maxBufferSize) {
+	private void setContents(String hex, int maxBufferSize)
+			throws FormatException {
 		ByteArrayInputStream in = new ByteArrayInputStream(fromHexString(hex));
 		r = new BdfReaderImpl(in, DEFAULT_NESTED_LIMIT, maxBufferSize);
 	}