Add transport keys for reintroduced contact.

Add DB method for checking whether there's anything to send

See merge request briar/briar!1485

.gitignore

@@ -18,6 +18,7 @@ local.properties
 
 # Android Studio
 .idea/*
+!.idea/inspectionProfiles/
 !.idea/runConfigurations/
 !.idea/codeStyleSettings.xml
 !.idea/codeStyles

.gitlab-ci.yml

@@ -1,30 +1,79 @@
 image: briar/ci-image-android:latest
 
 stages:
-- test
-- optional_tests
-- check_reproducibility
+  - test
+  - optional_tests
+  - check_reproducibility
 
-test:
-  stage: test
+workflow:
+  # when to create a CI pipeline
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: '$CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS'
+      when: never # avoids duplicate jobs for branch and MR
+    - if: '$CI_COMMIT_BRANCH'
+    - if: '$CI_COMMIT_TAG'
+
+.base-test:
   before_script:
     - set -e
     - export GRADLE_USER_HOME=$PWD/.gradle
-
   cache:
+    key: "$CI_COMMIT_REF_SLUG"
     paths:
       - .gradle/wrapper
       - .gradle/caches
-
-  script:
-    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom animalSnifferMain animalSnifferTest
-    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom check compileOfficialDebugAndroidTestSources compileScreenshotDebugAndroidTestSources
-
   after_script:
-    # these file change every time but should not be cached
+    # these file change every time and should not be cached
     - rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
     - rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/
 
+test:
+  extends: .base-test
+  stage: test
+  script:
+    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom animalSnifferMain animalSnifferTest
+    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom check
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      when: always
+    - when: always
+
+android test:
+  extends: .base-test
+  stage: optional_tests
+  image: briar/ci-image-android-emulator:latest
+  script:
+    # start emulator first, so it can fail early
+    - start-emulator.sh
+    # run normal and screenshot tests together (exclude Large tests)
+    - ./gradlew -Djava.security.egd=file:/dev/urandom connectedAndroidTest -Pandroid.testInstrumentationRunnerArguments.package=org.briarproject.briar.android -Pandroid.testInstrumentationRunnerArguments.notAnnotation=androidx.test.filters.LargeTest
+  after_script:
+    - adb pull /sdcard/Pictures/screenshots
+  artifacts:
+    name: "${CI_PROJECT_PATH}_${CI_JOB_STAGE}_${CI_COMMIT_REF_NAME}_${CI_COMMIT_SHA}"
+    paths:
+      - kernel.log
+      - logcat.txt
+      - briar-android/build/reports/androidTests/connected/flavors/*
+      - screenshots
+    expire_in: 3 days
+    when: on_failure
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "schedule"'
+      when: on_success
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      changes:
+        - briar-android/**/*
+      when: manual
+      allow_failure: true
+    - if: '$CI_COMMIT_TAG == null'
+      when: manual
+      allow_failure: true
+  retry:
+    max: 1
+  tags:
+    - kvm
 
 test_reproducible:
   stage: check_reproducibility
@@ -40,6 +89,7 @@ test_reproducible:
     - export GRADLE_USER_HOME=$PWD/.gradle
 
   cache:
+    key: "$CI_COMMIT_REF_SLUG"
     paths:
       - .gradle/wrapper
       - .gradle/caches
@@ -52,11 +102,15 @@ test_reproducible:
     - rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
     - rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/
 
-manual_tests:
+bridge test:
   extends: .optional_tests
-  when: manual
-  except:
-    - tags
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "schedule"'
+      when: on_success
+      allow_failure: true
+    - if: '$CI_COMMIT_TAG == null'
+      when: manual
+      allow_failure: true
 
 pre_release_tests:
   extends: .optional_tests

.idea/inspectionProfiles/Project_Default.xml

@@ -0,0 +1,14 @@
+<component name="InspectionProjectProfileManager">
+  <profile version="1.0">
+    <option name="myName" value="Project Default" />
+    <inspection_tool class="MissingOverrideAnnotation" enabled="true" level="WARNING" enabled_by_default="true">
+      <option name="ignoreObjectMethods" value="true" />
+      <option name="ignoreAnonymousClassMethods" value="false" />
+    </inspection_tool>
+    <inspection_tool class="WeakerAccess" enabled="true" level="WARNING" enabled_by_default="true">
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_MEMBERS" value="true" />
+      <option name="SUGGEST_PACKAGE_LOCAL_FOR_TOP_CLASSES" value="true" />
+      <option name="SUGGEST_PRIVATE_FOR_INNERS" value="true" />
+    </inspection_tool>
+  </profile>
+</component>

.idea/runConfigurations/Instrumentation_Tests__destroys_DB_.xml

@@ -0,0 +1,51 @@
+<component name="ProjectRunConfigurationManager">
+  <configuration default="false" name="Instrumentation Tests (destroys DB)" type="AndroidTestRunConfigurationType" factoryName="Android Instrumented Tests">
+    <module name="briar.briar-android" />
+    <option name="TESTING_TYPE" value="1" />
+    <option name="METHOD_NAME" value="" />
+    <option name="CLASS_NAME" value="" />
+    <option name="PACKAGE_NAME" value="org.briarproject.briar.android" />
+    <option name="INSTRUMENTATION_RUNNER_CLASS" value="" />
+    <option name="EXTRA_OPTIONS" value="-e notAnnotation androidx.test.filters.LargeTest" />
+    <option name="INCLUDE_GRADLE_EXTRA_OPTIONS" value="true" />
+    <option name="CLEAR_LOGCAT" value="false" />
+    <option name="SHOW_LOGCAT_AUTOMATICALLY" value="false" />
+    <option name="SKIP_NOOP_APK_INSTALLATIONS" value="true" />
+    <option name="FORCE_STOP_RUNNING_APP" value="true" />
+    <option name="TARGET_SELECTION_MODE" value="DEVICE_AND_SNAPSHOT_COMBO_BOX" />
+    <option name="DEBUGGER_TYPE" value="Auto" />
+    <Auto>
+      <option name="USE_JAVA_AWARE_DEBUGGER" value="false" />
+      <option name="SHOW_STATIC_VARS" value="true" />
+      <option name="WORKING_DIR" value="" />
+      <option name="TARGET_LOGGING_CHANNELS" value="lldb process:gdb-remote packets" />
+      <option name="SHOW_OPTIMIZED_WARNING" value="true" />
+    </Auto>
+    <Hybrid>
+      <option name="USE_JAVA_AWARE_DEBUGGER" value="false" />
+      <option name="SHOW_STATIC_VARS" value="true" />
+      <option name="WORKING_DIR" value="" />
+      <option name="TARGET_LOGGING_CHANNELS" value="lldb process:gdb-remote packets" />
+      <option name="SHOW_OPTIMIZED_WARNING" value="true" />
+    </Hybrid>
+    <Java />
+    <Native>
+      <option name="USE_JAVA_AWARE_DEBUGGER" value="false" />
+      <option name="SHOW_STATIC_VARS" value="true" />
+      <option name="WORKING_DIR" value="" />
+      <option name="TARGET_LOGGING_CHANNELS" value="lldb process:gdb-remote packets" />
+      <option name="SHOW_OPTIMIZED_WARNING" value="true" />
+    </Native>
+    <Profilers>
+      <option name="ADVANCED_PROFILING_ENABLED" value="false" />
+      <option name="STARTUP_PROFILING_ENABLED" value="false" />
+      <option name="STARTUP_CPU_PROFILING_ENABLED" value="false" />
+      <option name="STARTUP_CPU_PROFILING_CONFIGURATION_NAME" value="Sample Java Methods" />
+      <option name="STARTUP_NATIVE_MEMORY_PROFILING_ENABLED" value="false" />
+      <option name="NATIVE_MEMORY_SAMPLE_RATE_BYTES" value="2048" />
+    </Profilers>
+    <method v="2">
+      <option name="Android.Gradle.BeforeRunTask" enabled="true" />
+    </method>
+  </configuration>
+</component>

bramble-android/build.gradle

@@ -6,13 +6,17 @@ apply from: 'witness.gradle'
 
 android {
 	compileSdkVersion 30
-	buildToolsVersion '30.0.2'
+	buildToolsVersion '30.0.3'
+
+	packagingOptions {
+		doNotStrip '**/*.so'
+	}
 
 	defaultConfig {
 		minSdkVersion 16
-		targetSdkVersion 29
-		versionCode 10216
-		versionName "1.2.16"
+		targetSdkVersion 30
+		versionCode 10303
+		versionName "1.3.3"
 		consumerProguardFiles 'proguard-rules.txt'
 
 		testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

bramble-android/src/main/java/org/briarproject/bramble/plugin/bluetooth/AndroidBluetoothPlugin.java

@@ -59,8 +59,8 @@ import static org.briarproject.bramble.util.PrivacyUtils.scrubMacAddress;
 
 @MethodsNotNullByDefault
 @ParametersNotNullByDefault
-class AndroidBluetoothPlugin
-		extends BluetoothPlugin<BluetoothSocket, BluetoothServerSocket> {
+class AndroidBluetoothPlugin extends
+		AbstractBluetoothPlugin<BluetoothSocket, BluetoothServerSocket> {
 
 	private static final Logger LOG =
 			getLogger(AndroidBluetoothPlugin.class.getName());
@@ -75,6 +75,7 @@ class AndroidBluetoothPlugin
 
 	// Non-null if the plugin started successfully
 	private volatile BluetoothAdapter adapter = null;
+	private volatile boolean stopDiscoverAndConnect;
 
 	AndroidBluetoothPlugin(BluetoothConnectionLimiter connectionLimiter,
 			BluetoothConnectionFactory<BluetoothSocket> connectionFactory,
@@ -175,6 +176,11 @@ class AndroidBluetoothPlugin
 		} catch (IOException e) {
 			IoUtils.tryToClose(s, LOG, WARNING);
 			throw e;
+		} catch (NullPointerException e) {
+			// BluetoothSocket#connect() may throw an NPE under unknown
+			// circumstances
+			IoUtils.tryToClose(s, LOG, WARNING);
+			throw new IOException(e);
 		}
 	}
 
@@ -182,22 +188,40 @@ class AndroidBluetoothPlugin
 	@Nullable
 	DuplexTransportConnection discoverAndConnect(String uuid) {
 		if (adapter == null) return null;
-		for (String address : discoverDevices()) {
-			try {
-				if (LOG.isLoggable(INFO))
-					LOG.info("Connecting to " + scrubMacAddress(address));
-				return connectTo(address, uuid);
-			} catch (IOException e) {
-				if (LOG.isLoggable(INFO)) {
-					LOG.info("Could not connect to "
-							+ scrubMacAddress(address));
+		if (!discoverSemaphore.tryAcquire()) {
+			LOG.info("Discover already running");
+			return null;
+		}
+		try {
+			stopDiscoverAndConnect = false;
+			for (String address : discoverDevices()) {
+				if (stopDiscoverAndConnect) {
+					break;
+				}
+				try {
+					if (LOG.isLoggable(INFO))
+						LOG.info("Connecting to " + scrubMacAddress(address));
+					return connectTo(address, uuid);
+				} catch (IOException e) {
+					if (LOG.isLoggable(INFO)) {
+						LOG.info("Could not connect to "
+								+ scrubMacAddress(address));
+					}
 				}
 			}
+		} finally {
+			discoverSemaphore.release();
 		}
 		LOG.info("Could not connect to any devices");
 		return null;
 	}
 
+	@Override
+	public void stopDiscoverAndConnect() {
+		stopDiscoverAndConnect = true;
+		adapter.cancelDiscovery();
+	}
+
 	private Collection<String> discoverDevices() {
 		List<String> addresses = new ArrayList<>();
 		BlockingQueue<Intent> intents = new LinkedBlockingQueue<>();

bramble-android/src/main/java/org/briarproject/bramble/plugin/bluetooth/AndroidBluetoothPluginFactory.java

@@ -47,7 +47,7 @@ public class AndroidBluetoothPluginFactory implements DuplexPluginFactory {
 	private final BackoffFactory backoffFactory;
 
 	@Inject
-	public AndroidBluetoothPluginFactory(@IoExecutor Executor ioExecutor,
+	AndroidBluetoothPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
 			AndroidExecutor androidExecutor,
 			AndroidWakeLockManager wakeLockManager,

bramble-android/src/main/java/org/briarproject/bramble/plugin/file/AndroidRemovableDrivePlugin.java

@@ -0,0 +1,42 @@
+package org.briarproject.bramble.plugin.file;
+
+import android.app.Application;
+import android.net.Uri;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.PROP_URI;
+import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
+
+@Immutable
+@NotNullByDefault
+class AndroidRemovableDrivePlugin extends RemovableDrivePlugin {
+
+	private final Application app;
+
+	AndroidRemovableDrivePlugin(Application app, int maxLatency) {
+		super(maxLatency);
+		this.app = app;
+	}
+
+	@Override
+	InputStream openInputStream(TransportProperties p) throws IOException {
+		String uri = p.get(PROP_URI);
+		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
+		return app.getContentResolver().openInputStream(Uri.parse(uri));
+	}
+
+	@Override
+	OutputStream openOutputStream(TransportProperties p) throws IOException {
+		String uri = p.get(PROP_URI);
+		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
+		return app.getContentResolver().openOutputStream(Uri.parse(uri));
+	}
+}

bramble-android/src/main/java/org/briarproject/bramble/plugin/file/AndroidRemovableDrivePluginFactory.java

@@ -0,0 +1,47 @@
+package org.briarproject.bramble.plugin.file;
+
+import android.app.Application;
+
+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.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.ID;
+
+@Immutable
+@NotNullByDefault
+public class AndroidRemovableDrivePluginFactory implements
+		SimplexPluginFactory {
+
+	private static final int MAX_LATENCY = (int) DAYS.toMillis(14);
+
+	private final Application app;
+
+	@Inject
+	AndroidRemovableDrivePluginFactory(Application app) {
+		this.app = app;
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxLatency() {
+		return MAX_LATENCY;
+	}
+
+	@Nullable
+	@Override
+	public SimplexPlugin createPlugin(PluginCallback callback) {
+		return new AndroidRemovableDrivePlugin(app, MAX_LATENCY);
+	}
+}

bramble-android/src/main/java/org/briarproject/bramble/plugin/tcp/AndroidLanTcpPluginFactory.java

@@ -37,7 +37,7 @@ public class AndroidLanTcpPluginFactory implements DuplexPluginFactory {
 	private final Application app;
 
 	@Inject
-	public AndroidLanTcpPluginFactory(@IoExecutor Executor ioExecutor,
+	AndroidLanTcpPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
 			EventBus eventBus,
 			BackoffFactory backoffFactory,

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

@@ -58,7 +58,7 @@ public class AndroidTorPluginFactory implements DuplexPluginFactory {
 	private final File torDirectory;
 
 	@Inject
-	public AndroidTorPluginFactory(@IoExecutor Executor ioExecutor,
+	AndroidTorPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
 			Application app,
 			NetworkManager networkManager,

bramble-android/src/main/java/org/briarproject/bramble/util/AndroidUtils.java

@@ -114,12 +114,8 @@ public class AndroidUtils {
 
 	/**
 	 * Returns an array of supported content types for image attachments.
-	 * GIFs can't be compressed on API < 24 so they're not supported.
-	 * <p>
-	 * TODO: Remove this restriction when large message support is added
 	 */
 	public static String[] getSupportedImageContentTypes() {
-		if (SDK_INT < 24) return new String[] {"image/jpeg", "image/png"};
-		else return new String[] {"image/jpeg", "image/png", "image/gif"};
+		return new String[] {"image/jpeg", "image/png", "image/gif"};
 	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/Consumer.java

@@ -0,0 +1,9 @@
+package org.briarproject.bramble.api;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public interface Consumer<T> {
+
+	void accept(T t);
+}

bramble-api/src/main/java/org/briarproject/bramble/api/FeatureFlags.java

@@ -10,4 +10,6 @@ public interface FeatureFlags {
 	boolean shouldEnableProfilePictures();
 
 	boolean shouldEnableDisappearingMessages();
+
+	boolean shouldEnableConnectViaBluetooth();
 }

bramble-api/src/main/java/org/briarproject/bramble/api/client/BdfIncomingMessageHook.java

@@ -32,28 +32,31 @@ public abstract class BdfIncomingMessageHook implements IncomingMessageHook {
 
 	/**
 	 * Called once for each incoming message that passes validation.
+	 * <p>
+	 * If an unexpected exception occurs while handling data that is assumed
+	 * to be valid (e.g. locally created metadata), it may be sensible to
+	 * rethrow the unexpected exception as a DbException so that delivery is
+	 * attempted again at next startup. This will allow delivery to succeed if
+	 * the unexpected exception was caused by a bug that has subsequently been
+	 * fixed.
 	 *
 	 * @param txn A read-write transaction
-	 * @return Whether or not this message should be shared
-	 * @throws DbException Should only be used for real database errors.
-	 * If this is thrown, delivery will be attempted again at next startup,
-	 * whereas if a FormatException is thrown, the message will be permanently
-	 * invalidated.
-	 * @throws FormatException Use this for any non-database error
-	 * that occurs while handling remotely created data.
-	 * This includes errors that occur while handling locally created data
-	 * in a context controlled by remotely created data
-	 * (for example, parsing the metadata of a dependency
-	 * of an incoming message).
-	 * Never rethrow DbException as FormatException!
+	 * @throws DbException if a database error occurs while delivering the
+	 * message. Delivery will be attempted again at next startup. Throwing
+	 * this exception has the same effect as returning
+	 * {@link DeliveryAction#DEFER}.
+	 * @throws FormatException if the message is invalid in the context of its
+	 * dependencies. The message and any dependents will be marked as invalid
+	 * and deleted along with their metadata. Throwing this exception has the
+	 * same effect as returning {@link DeliveryAction#REJECT}.
 	 */
-	protected abstract boolean incomingMessage(Transaction txn, Message m,
-			BdfList body, BdfDictionary meta) throws DbException,
-			FormatException;
+	protected abstract DeliveryAction incomingMessage(Transaction txn,
+			Message m, BdfList body, BdfDictionary meta)
+			throws DbException, FormatException;
 
 	@Override
-	public boolean incomingMessage(Transaction txn, Message m, Metadata meta)
-			throws DbException, InvalidMessageException {
+	public DeliveryAction incomingMessage(Transaction txn, Message m,
+			Metadata meta) throws DbException, InvalidMessageException {
 		try {
 			BdfList body = clientHelper.toList(m);
 			BdfDictionary metaDictionary = metadataParser.parse(meta);

bramble-api/src/main/java/org/briarproject/bramble/api/contact/event/ContactAliasChangedEvent.java

@@ -0,0 +1,35 @@
+package org.briarproject.bramble.api.contact.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when the alias for a contact changed.
+ */
+@Immutable
+@NotNullByDefault
+public class ContactAliasChangedEvent extends Event {
+
+	private final ContactId contactId;
+	@Nullable
+	private final String alias;
+
+	public ContactAliasChangedEvent(ContactId contactId,
+			@Nullable String alias) {
+		this.contactId = contactId;
+		this.alias = alias;
+	}
+
+	public ContactId getContactId() {
+		return contactId;
+	}
+
+	@Nullable
+	public String getAlias() {
+		return alias;
+	}
+}

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

@@ -118,6 +118,18 @@ public interface DatabaseComponent extends TransactionManager {
 	KeySetId addTransportKeys(Transaction txn, PendingContactId p,
 			TransportKeys k) throws DbException;
 
+	/**
+	 * Returns true if there are any acks or 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 containsAnythingToSend(Transaction txn, ContactId c,
+			int maxLatency, boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given contact for the given
 	 * local pseudonym.
@@ -150,6 +162,16 @@ public interface DatabaseComponent extends TransactionManager {
 	boolean containsPendingContact(Transaction txn, PendingContactId p)
 			throws DbException;
 
+	/**
+	 * Returns true if the database contains keys for communicating with the
+	 * given contact over the given transport. Handshake mode and rotation mode
+	 * keys are included, whether activated or not.
+	 * <p/>
+	 * Read-only.
+	 */
+	boolean containsTransportKeys(Transaction txn, ContactId c, TransportId t)
+			throws DbException;
+
 	/**
 	 * Deletes the message with the given ID. Unlike
 	 * {@link #removeMessage(Transaction, MessageId)}, the message ID,
@@ -180,6 +202,18 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<Message> generateBatch(Transaction txn, ContactId c,
 			int maxLength, int maxLatency) throws DbException;
 
+	/**
+	 * Returns a batch of messages for the given contact containing the
+	 * messages with the given IDs, for transmission over a transport with
+	 * the given maximum latency.
+	 * <p/>
+	 * If any of the given messages are not in the database or are not visible
+	 * to the contact, they are omitted from the batch without throwing an
+	 * exception.
+	 */
+	Collection<Message> generateBatch(Transaction txn, ContactId c,
+			Collection<MessageId> ids, int maxLatency) throws DbException;
+
 	/**
 	 * Returns an offer for the given contact for transmission over a
 	 * transport with the given maximum latency, or null if there are no
@@ -426,6 +460,27 @@ public interface DatabaseComponent extends TransactionManager {
 	MessageStatus getMessageStatus(Transaction txn, ContactId c, MessageId m)
 			throws DbException;
 
+	/**
+	 * Returns the IDs of all messages that are eligible to be sent to the
+	 * given contact, together with their raw lengths. This may include
+	 * messages that have already been sent and are not yet due for
+	 * retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Map<MessageId, Integer> getUnackedMessagesToSend(Transaction txn,
+			ContactId c) throws DbException;
+
+	/**
+	 * Returns the total length, including headers, of all messages that are
+	 * eligible to be sent to the given contact. This may include messages
+	 * that have already been sent and are not yet due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	long getUnackedMessageBytesToSend(Transaction txn, ContactId c)
+			throws DbException;
+
 	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
 	 * message is due to be deleted, or {@link #NO_CLEANUP_DEADLINE}
@@ -483,6 +538,16 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<TransportKeySet> getTransportKeys(Transaction txn, TransportId t)
 			throws DbException;
 
+	/**
+	 * Returns the contact IDs and transport IDs for which the DB contains
+	 * at least one set of transport keys. Handshake mode and rotation mode
+	 * keys are included, whether activated or not.
+	 * <p/>
+	 * Read-only.
+	 */
+	Map<ContactId, Collection<TransportId>> getTransportsWithKeys(
+			Transaction txn) throws DbException;
+
 	/**
 	 * Increments the outgoing stream counter for the given transport keys.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/identity/Author.java

@@ -57,6 +57,7 @@ public class Author implements Nameable {
 	/**
 	 * Returns the author's name.
 	 */
+	@Override
 	public String getName() {
 		return name;
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/TransportConnectionWriter.java

@@ -22,6 +22,11 @@ public interface TransportConnectionWriter {
 	 */
 	int getMaxIdleTime();
 
+	/**
+	 * Returns true if the transport is lossy and cheap.
+	 */
+	boolean isLossyAndCheap();
+
 	/**
 	 * Returns an output stream for writing to the transport connection.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/duplex/AbstractDuplexTransportConnection.java

@@ -79,6 +79,11 @@ public abstract class AbstractDuplexTransportConnection
 			return plugin.getMaxIdleTime();
 		}
 
+		@Override
+		public boolean isLossyAndCheap() {
+			return false;
+		}
+
 		@Override
 		public OutputStream getOutputStream() throws IOException {
 			return AbstractDuplexTransportConnection.this.getOutputStream();

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/file/FileConstants.java

@@ -1,4 +1,4 @@
-package org.briarproject.bramble.api.plugin;
+package org.briarproject.bramble.api.plugin.file;
 
 public interface FileConstants {
 

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/file/RemovableDriveConstants.java

@@ -0,0 +1,11 @@
+package org.briarproject.bramble.api.plugin.file;
+
+import org.briarproject.bramble.api.plugin.TransportId;
+
+public interface RemovableDriveConstants {
+
+	TransportId ID = new TransportId("org.briarproject.bramble.drive");
+
+	String PROP_PATH = "path";
+	String PROP_URI = "uri";
+}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/file/RemovableDriveManager.java

@@ -0,0 +1,46 @@
+package org.briarproject.bramble.api.plugin.file;
+
+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.properties.TransportProperties;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface RemovableDriveManager {
+
+	/**
+	 * Returns the currently running reader task, or null if no reader task
+	 * is running.
+	 */
+	@Nullable
+	RemovableDriveTask getCurrentReaderTask();
+
+	/**
+	 * Returns the currently running writer task,  or null if no writer task
+	 * is running.
+	 */
+	@Nullable
+	RemovableDriveTask getCurrentWriterTask();
+
+	/**
+	 * Starts and returns a reader task, reading from a stream described by
+	 * the given transport properties. If a reader task is already running,
+	 * it will be returned and the argument will be ignored.
+	 */
+	RemovableDriveTask startReaderTask(TransportProperties p);
+
+	/**
+	 * Starts and returns a writer task for the given contact, writing to
+	 * a stream described by the given transport properties. If a writer task
+	 * is already running, it will be returned and the arguments will be
+	 * ignored.
+	 */
+	RemovableDriveTask startWriterTask(ContactId c, TransportProperties p);
+
+	/**
+	 * Returns true if there is anything to send to the given contact.
+	 */
+	boolean isWriterTaskNeeded(ContactId c) throws DbException;
+}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/file/RemovableDriveTask.java

@@ -0,0 +1,65 @@
+package org.briarproject.bramble.api.plugin.file;
+
+import org.briarproject.bramble.api.Consumer;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+@NotNullByDefault
+public interface RemovableDriveTask extends Runnable {
+
+	/**
+	 * Returns the {@link TransportProperties} that were used for creating
+	 * this task.
+	 */
+	TransportProperties getTransportProperties();
+
+	/**
+	 * Adds an observer to the task. The observer will be notified of state
+	 * changes on the event thread. If the task has already finished, the
+	 * observer will be notified of its final state.
+	 */
+	void addObserver(Consumer<State> observer);
+
+	/**
+	 * Removes an observer from the task.
+	 */
+	void removeObserver(Consumer<State> observer);
+
+	class State {
+
+		private final long done, total;
+		private final boolean finished, success;
+
+		public State(long done, long total, boolean finished, boolean success) {
+			this.done = done;
+			this.total = total;
+			this.finished = finished;
+			this.success = success;
+		}
+
+		/**
+		 * Returns the total length in bytes of the messages read or written
+		 * so far, or zero if the total is unknown.
+		 */
+		public long getDone() {
+			return done;
+		}
+
+		/**
+		 * Returns the total length in bytes of the messages that will have
+		 * been read or written when the task is complete, or zero if the
+		 * total is unknown.
+		 */
+		public long getTotal() {
+			return total;
+		}
+
+		public boolean isFinished() {
+			return finished;
+		}
+
+		public boolean isSuccess() {
+			return success;
+		}
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/simplex/SimplexPlugin.java

@@ -15,6 +15,12 @@ import javax.annotation.Nullable;
 @NotNullByDefault
 public interface SimplexPlugin extends Plugin {
 
+	/**
+	 * Returns true if the transport is likely to lose streams and the cost of
+	 * transmitting redundant copies of data is cheap.
+	 */
+	boolean isLossyAndCheap();
+
 	/**
 	 * Attempts to create and return a reader for the given transport
 	 * properties. Returns null if a reader cannot be created.

bramble-api/src/main/java/org/briarproject/bramble/api/reporting/ReportingConstants.java

@@ -21,5 +21,6 @@ public interface ReportingConstants {
 	 * Hidden service address for reporting crashes and feedback to the
 	 * developers.
 	 */
-	String DEV_ONION_ADDRESS = "cwqmubyvnig3wag3.onion";
+	String DEV_ONION_ADDRESS =
+			"b2nkt5doeamvdmjzfz7g42hk5vdtlnktlgzhel2bgjcc4v4jhnx2qrqd.onion";
 }

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

@@ -16,7 +16,7 @@ public interface SyncSessionFactory {
 			PriorityHandler handler);
 
 	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
-			int maxLatency, StreamWriter streamWriter);
+			int maxLatency, boolean eager, StreamWriter streamWriter);
 
 	SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
 			int maxLatency, int maxIdleTime, StreamWriter streamWriter,

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

@@ -18,11 +18,13 @@ public class MessagesSentEvent extends Event {
 
 	private final ContactId contactId;
 	private final Collection<MessageId> messageIds;
+	private final long totalLength;
 
 	public MessagesSentEvent(ContactId contactId,
-			Collection<MessageId> messageIds) {
+			Collection<MessageId> messageIds, long totalLength) {
 		this.contactId = contactId;
 		this.messageIds = messageIds;
+		this.totalLength = totalLength;
 	}
 
 	public ContactId getContactId() {
@@ -32,4 +34,8 @@ public class MessagesSentEvent extends Event {
 	public Collection<MessageId> getMessageIds() {
 		return messageIds;
 	}
+
+	public long getTotalLength() {
+		return totalLength;
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/validation/IncomingMessageHook.java

@@ -10,23 +10,54 @@ public interface IncomingMessageHook {
 
 	/**
 	 * Called once for each incoming message that passes validation.
+	 * <p>
+	 * If an unexpected exception occurs while handling data that is assumed
+	 * to be valid (e.g. locally created metadata), it may be sensible to
+	 * rethrow the unexpected exception as a DbException so that delivery is
+	 * attempted again at next startup. This will allow delivery to succeed if
+	 * the unexpected exception was caused by a bug that has subsequently been
+	 * fixed.
 	 *
 	 * @param txn A read-write transaction
-	 * @return Whether or not this message should be shared
-	 * @throws DbException Should only be used for real database errors.
-	 * If this is thrown, delivery will be attempted again at next startup,
-	 * whereas if an InvalidMessageException is thrown,
-	 * the message will be permanently invalidated.
-	 * @throws InvalidMessageException for any non-database error
-	 * that occurs while handling remotely created data.
-	 * This includes errors that occur while handling locally created data
-	 * in a context controlled by remotely created data
-	 * (for example, parsing the metadata of a dependency
-	 * of an incoming message).
-	 * Throwing this will delete the incoming message and its metadata
-	 * marking it as invalid in the database.
-	 * Never rethrow DbException as InvalidMessageException!
+	 * @throws DbException if a database error occurs while delivering the
+	 * message. Delivery will be attempted again at next startup. Throwing
+	 * this exception has the same effect as returning
+	 * {@link DeliveryAction#DEFER}.
+	 * @throws InvalidMessageException if the message is invalid in the context
+	 * of its dependencies. The message and any dependents will be marked as
+	 * invalid and deleted along with their metadata. Throwing this exception
+	 * has the same effect as returning {@link DeliveryAction#REJECT}.
 	 */
-	boolean incomingMessage(Transaction txn, Message m, Metadata meta)
+	DeliveryAction incomingMessage(Transaction txn, Message m, Metadata meta)
 			throws DbException, InvalidMessageException;
+
+	enum DeliveryAction {
+
+		/**
+		 * The message and any dependent messages will be moved to the
+		 * {@link MessageState#INVALID INVALID state} and deleted, along with
+		 * their metadata.
+		 */
+		REJECT,
+
+		/**
+		 * The message will be moved to the
+		 * {@link MessageState#PENDING PENDING state}. Delivery will be
+		 * attempted again at next startup.
+		 */
+		DEFER,
+
+		/**
+		 * The message will be moved to the
+		 * {@link MessageState#DELIVERED DELIVERED state} and shared.
+		 */
+		ACCEPT_SHARE,
+
+		/**
+		 * The message will be moved to the
+		 * {@link MessageState#DELIVERED DELIVERED state} and will not be
+		 * shared.
+		 */
+		ACCEPT_DO_NOT_SHARE
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/validation/MessageState.java

@@ -1,8 +1,33 @@
 package org.briarproject.bramble.api.sync.validation;
 
+import org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction;
+
 public enum MessageState {
 
-	UNKNOWN(0), INVALID(1), PENDING(2), DELIVERED(3);
+	/**
+	 * A remote message that has not yet been validated.
+	 */
+	UNKNOWN(0),
+
+	/**
+	 * A remote message that has failed validation, has been
+	 * {@link DeliveryAction#REJECT rejected} by the local sync client, or
+	 * depends on another message that has failed validation or been rejected.
+	 */
+	INVALID(1),
+
+	/**
+	 * A remote message that has passed validation and is awaiting delivery to
+	 * the local sync client. The message will not be delivered until all its
+	 * dependencies have been validated and delivered.
+	 */
+	PENDING(2),
+
+	/**
+	 * A local message, or a remote message that has passed validation and
+	 * been delivered to the local sync client.
+	 */
+	DELIVERED(3);
 
 	private final int value;
 

bramble-api/src/main/java/org/briarproject/bramble/api/transport/KeyManager.java

@@ -22,8 +22,24 @@ public interface KeyManager {
 
 	/**
 	 * Derives and stores a set of rotation mode transport keys for
-	 * communicating with the given contact over each transport and returns the
-	 * key set IDs.
+	 * communicating with the given contact over the given transport and
+	 * returns the key set ID, or null if the transport is not supported.
+	 * <p/>
+	 * {@link StreamContext StreamContexts} for the contact can be created
+	 * after this method has returned.
+	 *
+	 * @param alice True if the local party is Alice
+	 * @param active Whether the derived keys can be used for outgoing streams
+	 */
+	@Nullable
+	KeySetId addRotationKeys(Transaction txn, ContactId c, TransportId t,
+			SecretKey rootKey, long timestamp, boolean alice,
+			boolean active) throws DbException;
+
+	/**
+	 * Derives and stores a set of rotation mode transport keys for
+	 * communicating with the given contact over each supported transport and
+	 * returns the key set IDs.
 	 * <p/>
 	 * {@link StreamContext StreamContexts} for the contact can be created
 	 * after this method has returned.

bramble-api/src/main/java/org/briarproject/bramble/api/transport/agreement/TransportKeyAgreementManager.java

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.transport.agreement;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+
+@NotNullByDefault
+public interface TransportKeyAgreementManager {
+
+	/**
+	 * The unique ID of the transport key agreement client.
+	 */
+	ClientId CLIENT_ID =
+			new ClientId("org.briarproject.bramble.transport.agreement");
+
+	/**
+	 * The current major version of the transport key agreement client.
+	 */
+	int MAJOR_VERSION = 0;
+
+	/**
+	 * The current minor version of the transport key agreement client.
+	 */
+	int MINOR_VERSION = 0;
+}

bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreEagerSingletons.java

@@ -11,6 +11,7 @@ import org.briarproject.bramble.properties.PropertiesModule;
 import org.briarproject.bramble.rendezvous.RendezvousModule;
 import org.briarproject.bramble.sync.validation.ValidationModule;
 import org.briarproject.bramble.transport.TransportModule;
+import org.briarproject.bramble.transport.agreement.TransportKeyAgreementModule;
 import org.briarproject.bramble.versioning.VersioningModule;
 
 public interface BrambleCoreEagerSingletons {
@@ -33,6 +34,8 @@ public interface BrambleCoreEagerSingletons {
 
 	void inject(RendezvousModule.EagerSingletons init);
 
+	void inject(TransportKeyAgreementModule.EagerSingletons init);
+
 	void inject(TransportModule.EagerSingletons init);
 
 	void inject(ValidationModule.EagerSingletons init);
@@ -51,6 +54,7 @@ public interface BrambleCoreEagerSingletons {
 			c.inject(new RendezvousModule.EagerSingletons());
 			c.inject(new PluginModule.EagerSingletons());
 			c.inject(new PropertiesModule.EagerSingletons());
+			c.inject(new TransportKeyAgreementModule.EagerSingletons());
 			c.inject(new TransportModule.EagerSingletons());
 			c.inject(new ValidationModule.EagerSingletons());
 			c.inject(new VersioningModule.EagerSingletons());

bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreModule.java

@@ -23,6 +23,7 @@ import org.briarproject.bramble.settings.SettingsModule;
 import org.briarproject.bramble.sync.SyncModule;
 import org.briarproject.bramble.sync.validation.ValidationModule;
 import org.briarproject.bramble.transport.TransportModule;
+import org.briarproject.bramble.transport.agreement.TransportKeyAgreementModule;
 import org.briarproject.bramble.versioning.VersioningModule;
 
 import dagger.Module;
@@ -49,6 +50,7 @@ import dagger.Module;
 		RendezvousModule.class,
 		SettingsModule.class,
 		SyncModule.class,
+		TransportKeyAgreementModule.class,
 		TransportModule.class,
 		ValidationModule.class,
 		VersioningModule.class

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

@@ -71,8 +71,10 @@ 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
 		return syncSessionFactory.createSimplexOutgoingSession(c,
-				ctx.getTransportId(), w.getMaxLatency(), streamWriter);
+				ctx.getTransportId(), w.getMaxLatency(), w.isLossyAndCheap(),
+				streamWriter);
 	}
 }
 

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

@@ -68,6 +68,13 @@ interface Database<T> {
 	 */
 	void close() throws DbException;
 
+	/**
+	 * Returns true if the dirty flag was set while opening the database,
+	 * indicating that the database has not been shut down properly the last
+	 * time it was closed and some data could be lost.
+	 */
+	boolean wasDirtyOnInitialisation();
+
 	/**
 	 * Starts a new transaction and returns an object representing it.
 	 */
@@ -155,6 +162,18 @@ interface Database<T> {
 	KeySetId addTransportKeys(T txn, PendingContactId p, TransportKeys k)
 			throws DbException;
 
+	/**
+	 * Returns true if there are any acks or 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 containsAnythingToSend(T txn, ContactId c, int maxLatency,
+			boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given contact for the given
 	 * local pseudonym.
@@ -208,6 +227,16 @@ interface Database<T> {
 	 */
 	boolean containsTransport(T txn, TransportId t) throws DbException;
 
+	/**
+	 * Returns true if the database contains keys for communicating with the
+	 * given contact over the given transport. Handshake mode and rotation mode
+	 * keys are included, whether activated or not.
+	 * <p/>
+	 * Read-only.
+	 */
+	boolean containsTransportKeys(T txn, ContactId c, TransportId t)
+			throws DbException;
+
 	/**
 	 * Returns true if the database contains the given message, the message is
 	 * shared, and the visibility of the message's group to the given contact
@@ -469,11 +498,37 @@ interface Database<T> {
 	 * Returns the IDs of some messages that are eligible to be sent to the
 	 * given contact, up to the given total length.
 	 * <p/>
+	 * Unlike {@link #getUnackedMessagesToSend(Object, ContactId)} this method
+	 * does not return messages that have already been sent unless they are
+	 * due for retransmission.
+	 * <p/>
 	 * Read-only.
 	 */
 	Collection<MessageId> getMessagesToSend(T txn, ContactId c, int maxLength,
 			int maxLatency) throws DbException;
 
+	/**
+	 * Returns the IDs of all messages that are eligible to be sent to the
+	 * given contact, together with their raw lengths.
+	 * <p/>
+	 * Unlike {@link #getMessagesToSend(Object, ContactId, int, int)} this
+	 * method may return messages that have already been sent and are not yet
+	 * due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Map<MessageId, Integer> getUnackedMessagesToSend(T txn, ContactId c)
+			throws DbException;
+
+	/**
+	 * Returns the total length, including headers, of all messages that are
+	 * eligible to be sent to the given contact. This may include messages
+	 * that have already been sent and are not yet due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	long getUnackedMessageBytesToSend(T txn, ContactId c) throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>
@@ -573,6 +628,16 @@ interface Database<T> {
 	Collection<TransportKeySet> getTransportKeys(T txn, TransportId t)
 			throws DbException;
 
+	/**
+	 * Returns the contact IDs and transport IDs for which the DB contains
+	 * at least one set of transport keys. Handshake mode and rotation mode
+	 * keys are included, whether activated or not.
+	 * <p/>
+	 * Read-only.
+	 */
+	Map<ContactId, Collection<TransportId>> getTransportsWithKeys(T txn)
+			throws DbException;
+
 	/**
 	 * Increments the outgoing stream counter for the given transport keys.
 	 */

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

@@ -6,6 +6,7 @@ import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.PendingContact;
 import org.briarproject.bramble.api.contact.PendingContactId;
 import org.briarproject.bramble.api.contact.event.ContactAddedEvent;
+import org.briarproject.bramble.api.contact.event.ContactAliasChangedEvent;
 import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
 import org.briarproject.bramble.api.contact.event.ContactVerifiedEvent;
 import org.briarproject.bramble.api.contact.event.PendingContactAddedEvent;
@@ -340,6 +341,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.addTransportKeys(txn, p, k);
 	}
 
+	@Override
+	public boolean containsAnythingToSend(Transaction transaction, ContactId c,
+			int maxLatency, boolean eager) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.containsAnythingToSend(txn, c, maxLatency, eager);
+	}
+
 	@Override
 	public boolean containsContact(Transaction transaction, AuthorId remote,
 			AuthorId local) throws DbException {
@@ -370,6 +380,13 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsPendingContact(txn, p);
 	}
 
+	@Override
+	public boolean containsTransportKeys(Transaction transaction, ContactId c,
+			TransportId t) throws DbException {
+		T txn = unbox(transaction);
+		return db.containsTransportKeys(txn, c, t);
+	}
+
 	@Override
 	public void deleteMessage(Transaction transaction, MessageId m)
 			throws DbException {
@@ -414,14 +431,43 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
 				db.getMessagesToSend(txn, c, maxLength, maxLatency);
+		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
-			messages.add(db.getMessage(txn, m));
+			Message message = db.getMessage(txn, m);
+			totalLength += message.getRawLength();
+			messages.add(message);
 			db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
 		}
 		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
-		transaction.attach(new MessagesSentEvent(c, ids));
+		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
+		return messages;
+	}
+
+	@Override
+	public Collection<Message> generateBatch(Transaction transaction,
+			ContactId c, Collection<MessageId> ids, int maxLatency)
+			throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		long totalLength = 0;
+		List<Message> messages = new ArrayList<>(ids.size());
+		List<MessageId> sentIds = new ArrayList<>(ids.size());
+		for (MessageId m : ids) {
+			if (db.containsVisibleMessage(txn, c, m)) {
+				Message message = db.getMessage(txn, m);
+				totalLength += message.getRawLength();
+				messages.add(message);
+				sentIds.add(m);
+				db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
+			}
+		}
+		if (messages.isEmpty()) return messages;
+		db.lowerRequestedFlag(txn, c, sentIds);
+		transaction.attach(new MessagesSentEvent(c, sentIds, totalLength));
 		return messages;
 	}
 
@@ -466,14 +512,17 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
 				db.getRequestedMessagesToSend(txn, c, maxLength, maxLatency);
+		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
-			messages.add(db.getMessage(txn, m));
+			Message message = db.getMessage(txn, m);
+			totalLength += message.getRawLength();
+			messages.add(message);
 			db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
 		}
 		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
-		transaction.attach(new MessagesSentEvent(c, ids));
+		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
 	}
 
@@ -691,6 +740,25 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return status;
 	}
 
+	@Override
+	public Map<MessageId, Integer> getUnackedMessagesToSend(
+			Transaction transaction,
+			ContactId c) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getUnackedMessagesToSend(txn, c);
+	}
+
+	@Override
+	public long getUnackedMessageBytesToSend(Transaction transaction,
+			ContactId c) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getUnackedMessageBytesToSend(txn, c);
+	}
+
 	@Override
 	public Map<MessageId, MessageState> getMessageDependencies(
 			Transaction transaction, MessageId m) throws DbException {
@@ -764,6 +832,13 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getTransportKeys(txn, t);
 	}
 
+	@Override
+	public Map<ContactId, Collection<TransportId>> getTransportsWithKeys(
+			Transaction transaction) throws DbException {
+		T txn = unbox(transaction);
+		return db.getTransportsWithKeys(txn);
+	}
+
 	@Override
 	public void incrementStreamCounter(Transaction transaction, TransportId t,
 			KeySetId k) throws DbException {
@@ -1013,6 +1088,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
+		transaction.attach(new ContactAliasChangedEvent(c, alias));
 		db.setContactAlias(txn, c, alias);
 	}
 

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

@@ -37,4 +37,10 @@ interface DatabaseConstants {
 	 * has passed since the last compaction.
 	 */
 	long MAX_COMPACTION_INTERVAL_MS = DAYS.toMillis(30);
+
+	/**
+	 * The {@link Settings} key under which the flag is stored indicating
+	 * whether the database is marked as dirty.
+	 */
+	String DIRTY_KEY = "dirty";
 }

bramble-core/src/main/java/org/briarproject/bramble/db/H2Database.java

@@ -84,9 +84,14 @@ class H2Database extends JdbcDatabase {
 	@Override
 	public void close() throws DbException {
 		// H2 will close the database when the last connection closes
+		Connection c = null;
 		try {
+			c = createConnection();
 			super.closeAllConnections();
+			setDirty(c, false);
+			c.close();
 		} catch (SQLException e) {
+			tryToClose(c, LOG, WARNING);
 			throw new DbException(e);
 		}
 	}

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

@@ -81,6 +81,7 @@ class HyperSqlDatabase extends JdbcDatabase {
 		try {
 			super.closeAllConnections();
 			c = createConnection();
+			setDirty(c, false);
 			s = c.createStatement();
 			s.executeQuery("SHUTDOWN");
 			s.close();

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

@@ -51,6 +51,7 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -83,6 +84,7 @@ import static org.briarproject.bramble.api.sync.validation.MessageState.DELIVERE
 import static org.briarproject.bramble.api.sync.validation.MessageState.PENDING;
 import static org.briarproject.bramble.api.sync.validation.MessageState.UNKNOWN;
 import static org.briarproject.bramble.db.DatabaseConstants.DB_SETTINGS_NAMESPACE;
+import static org.briarproject.bramble.db.DatabaseConstants.DIRTY_KEY;
 import static org.briarproject.bramble.db.DatabaseConstants.LAST_COMPACTED_KEY;
 import static org.briarproject.bramble.db.DatabaseConstants.MAX_COMPACTION_INTERVAL_MS;
 import static org.briarproject.bramble.db.DatabaseConstants.SCHEMA_VERSION_KEY;
@@ -343,6 +345,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			"CREATE INDEX IF NOT EXISTS statusesByContactIdTimestamp"
 					+ " ON statuses (contactId, timestamp)";
 
+	private static final String
+			INDEX_STATUSES_BY_CONTACT_ID_TX_COUNT_TIMESTAMP =
+			"CREATE INDEX IF NOT EXISTS statusesByContactIdTxCountTimestamp"
+					+ " ON statuses (contactId, txCount, timestamp)";
+
 	private static final String INDEX_MESSAGES_BY_CLEANUP_DEADLINE =
 			"CREATE INDEX IF NOT EXISTS messagesByCleanupDeadline"
 					+ " ON messages (cleanupDeadline)";
@@ -365,9 +372,14 @@ abstract class JdbcDatabase implements Database<Connection> {
 	@GuardedBy("connectionsLock")
 	private boolean closed = false;
 
+	private volatile boolean wasDirtyOnInitialisation = false;
+
 	protected abstract Connection createConnection()
 			throws DbException, SQLException;
 
+	// Used exclusively during open to compact the database after schema
+	// migrations or after DatabaseConstants#MAX_COMPACTION_INTERVAL_MS has
+	// elapsed
 	protected abstract void compactAndClose() throws DbException;
 
 	JdbcDatabase(DatabaseTypes databaseTypes, MessageFactory messageFactory,
@@ -392,13 +404,19 @@ abstract class JdbcDatabase implements Database<Connection> {
 		try {
 			if (reopen) {
 				Settings s = getSettings(txn, DB_SETTINGS_NAMESPACE);
+				wasDirtyOnInitialisation = isDirty(s);
 				compact = migrateSchema(txn, s, listener) || isCompactionDue(s);
 			} else {
+				wasDirtyOnInitialisation = false;
 				createTables(txn);
 				initialiseSettings(txn);
 				compact = false;
 			}
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("db dirty? " + wasDirtyOnInitialisation);
+			}
 			createIndexes(txn);
+			setDirty(txn, true);
 			commitTransaction(txn);
 		} catch (DbException e) {
 			abortTransaction(txn);
@@ -425,6 +443,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean wasDirtyOnInitialisation() {
+		return wasDirtyOnInitialisation;
+	}
+
 	/**
 	 * Compares the schema version stored in the database with the schema
 	 * version used by the current code and applies any suitable migrations to
@@ -500,6 +523,16 @@ abstract class JdbcDatabase implements Database<Connection> {
 		mergeSettings(txn, s, DB_SETTINGS_NAMESPACE);
 	}
 
+	private boolean isDirty(Settings s) {
+		return s.getBoolean(DIRTY_KEY, false);
+	}
+
+	protected void setDirty(Connection txn, boolean dirty) throws DbException {
+		Settings s = new Settings();
+		s.putBoolean(DIRTY_KEY, dirty);
+		mergeSettings(txn, s, DB_SETTINGS_NAMESPACE);
+	}
+
 	private void initialiseSettings(Connection txn) throws DbException {
 		Settings s = new Settings();
 		s.putInt(SCHEMA_VERSION_KEY, CODE_SCHEMA_VERSION);
@@ -543,6 +576,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			s.executeUpdate(INDEX_MESSAGE_DEPENDENCIES_BY_DEPENDENCY_ID);
 			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_GROUP_ID);
 			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_TIMESTAMP);
+			s.executeUpdate(INDEX_STATUSES_BY_CONTACT_ID_TX_COUNT_TIMESTAMP);
 			s.executeUpdate(INDEX_MESSAGES_BY_CLEANUP_DEADLINE);
 			s.close();
 		} catch (SQLException e) {
@@ -1093,6 +1127,55 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean containsAnythingToSend(Connection txn, ContactId c,
+			int maxLatency, boolean eager) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT NULL FROM statuses"
+					+ " WHERE contactId = ? AND ack = TRUE";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			rs = ps.executeQuery();
+			boolean acksToSend = rs.next();
+			rs.close();
+			ps.close();
+			if (acksToSend) return true;
+			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();
+				long eta = now + maxLatency;
+				sql = "SELECT NULL FROM statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE"
+						+ " AND (expiry <= ? OR eta > ?)";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+				ps.setLong(3, now);
+				ps.setLong(4, eta);
+			}
+			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 containsContact(Connection txn, AuthorId remote,
 			AuthorId local) throws DbException {
@@ -1250,6 +1333,29 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean containsTransportKeys(Connection txn, ContactId c,
+			TransportId t) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT NULL FROM outgoingKeys"
+					+ " WHERE contactId = ? AND transportId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setString(2, t.getString());
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			rs.close();
+			ps.close();
+			return found;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public boolean containsVisibleMessage(Connection txn, ContactId c,
 			MessageId m) throws DbException {
@@ -2178,6 +2284,63 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public Map<MessageId, Integer> getUnackedMessagesToSend(Connection txn,
+			ContactId c) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT length, messageId FROM statuses"
+					+ " WHERE contactId = ? AND state = ?"
+					+ " AND groupShared = TRUE AND messageShared = TRUE"
+					+ " AND deleted = FALSE AND seen = FALSE"
+					+ " ORDER BY txCount, timestamp";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setInt(2, DELIVERED.getValue());
+			rs = ps.executeQuery();
+			Map<MessageId, Integer> results = new LinkedHashMap<>();
+			while (rs.next()) {
+				int length = rs.getInt(1);
+				MessageId id = new MessageId(rs.getBytes(2));
+				results.put(id, length);
+			}
+			rs.close();
+			ps.close();
+			return results;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
+	@Override
+	public long getUnackedMessageBytesToSend(Connection txn, ContactId c)
+			throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT SUM(length) 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());
+			rs = ps.executeQuery();
+			rs.next();
+			long total = rs.getInt(1);
+			rs.close();
+			ps.close();
+			return total;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Collection<MessageId> getMessagesToValidate(Connection txn)
 			throws DbException {
@@ -2547,6 +2710,38 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public Map<ContactId, Collection<TransportId>> getTransportsWithKeys(
+			Connection txn) throws DbException {
+		Statement s = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT DISTINCT contactId, transportId"
+					+ " FROM outgoingKeys";
+			s = txn.createStatement();
+			rs = s.executeQuery(sql);
+			Map<ContactId, Collection<TransportId>> ids = new HashMap<>();
+			while (rs.next()) {
+				ContactId c = new ContactId(rs.getInt(1));
+				TransportId t = new TransportId(rs.getString(2));
+				Collection<TransportId> transportIds = ids.get(c);
+				if (transportIds == null) {
+					transportIds = new ArrayList<>();
+					ids.put(c, transportIds);
+				}
+				transportIds.add(t);
+			}
+			rs.close();
+			s.close();
+			return ids;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(s, LOG, WARNING);
+			tryToClose(s, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public void incrementStreamCounter(Connection txn, TransportId t,
 			KeySetId k) throws DbException {

bramble-core/src/main/java/org/briarproject/bramble/keyagreement/KeyAgreementTransport.java

@@ -34,12 +34,12 @@ class KeyAgreementTransport {
 			Logger.getLogger(KeyAgreementTransport.class.getName());
 
 	// Accept records with current protocol version, known record type
-	private static Predicate<Record> ACCEPT = r ->
+	private static final Predicate<Record> ACCEPT = r ->
 			r.getProtocolVersion() == PROTOCOL_VERSION &&
 					isKnownRecordType(r.getRecordType());
 
 	// Ignore records with current protocol version, unknown record type
-	private static Predicate<Record> IGNORE = r ->
+	private static final Predicate<Record> IGNORE = r ->
 			r.getProtocolVersion() == PROTOCOL_VERSION &&
 					!isKnownRecordType(r.getRecordType());
 

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

@@ -0,0 +1,623 @@
+package org.briarproject.bramble.plugin.bluetooth;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.Multiset;
+import org.briarproject.bramble.api.Pair;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.keyagreement.KeyAgreementConnection;
+import org.briarproject.bramble.api.keyagreement.KeyAgreementListener;
+import org.briarproject.bramble.api.keyagreement.event.KeyAgreementListeningEvent;
+import org.briarproject.bramble.api.keyagreement.event.KeyAgreementStoppedListeningEvent;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
+import org.briarproject.bramble.api.plugin.Backoff;
+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.plugin.duplex.DuplexTransportConnection;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.properties.event.RemoteTransportPropertiesUpdatedEvent;
+import org.briarproject.bramble.api.rendezvous.KeyMaterialSource;
+import org.briarproject.bramble.api.rendezvous.RendezvousEndpoint;
+import org.briarproject.bramble.api.settings.Settings;
+import org.briarproject.bramble.api.settings.event.SettingsUpdatedEvent;
+
+import java.io.IOException;
+import java.security.SecureRandom;
+import java.util.Collection;
+import java.util.UUID;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Semaphore;
+import java.util.concurrent.atomic.AtomicBoolean;
+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.keyagreement.KeyAgreementConstants.TRANSPORT_ID_BLUETOOTH;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_ADDRESS_IS_REFLECTED;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_EVER_CONNECTED;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_PLUGIN_ENABLE;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.ID;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.PREF_ADDRESS_IS_REFLECTED;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.PREF_EVER_CONNECTED;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.PROP_ADDRESS;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.PROP_UUID;
+import static org.briarproject.bramble.api.plugin.BluetoothConstants.UUID_BYTES;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.Plugin.State.DISABLED;
+import static org.briarproject.bramble.api.plugin.Plugin.State.INACTIVE;
+import static org.briarproject.bramble.api.plugin.Plugin.State.STARTING_STOPPING;
+import static org.briarproject.bramble.api.properties.TransportPropertyConstants.REFLECTED_PROPERTY_PREFIX;
+import static org.briarproject.bramble.util.LogUtils.logException;
+import static org.briarproject.bramble.util.PrivacyUtils.scrubMacAddress;
+import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
+import static org.briarproject.bramble.util.StringUtils.macToBytes;
+import static org.briarproject.bramble.util.StringUtils.macToString;
+
+@MethodsNotNullByDefault
+@ParametersNotNullByDefault
+abstract class AbstractBluetoothPlugin<S, SS> implements BluetoothPlugin,
+		EventListener {
+
+	private static final Logger LOG =
+			getLogger(AbstractBluetoothPlugin.class.getName());
+
+	private final BluetoothConnectionLimiter connectionLimiter;
+	final BluetoothConnectionFactory<S> connectionFactory;
+
+	private final Executor ioExecutor, wakefulIoExecutor;
+	private final SecureRandom secureRandom;
+	private final Backoff backoff;
+	private final PluginCallback callback;
+	private final int maxLatency, maxIdleTime;
+	private final AtomicBoolean used = new AtomicBoolean(false);
+	private final AtomicBoolean everConnected = new AtomicBoolean(false);
+
+	protected final PluginState state = new PluginState();
+	protected final Semaphore discoverSemaphore = new Semaphore(1);
+
+	private volatile String contactConnectionsUuid = null;
+
+	abstract void initialiseAdapter() throws IOException;
+
+	abstract boolean isAdapterEnabled();
+
+	/**
+	 * Returns the local Bluetooth address, or null if no valid address can
+	 * be found.
+	 */
+	@Nullable
+	abstract String getBluetoothAddress();
+
+	abstract SS openServerSocket(String uuid) throws IOException;
+
+	abstract void tryToClose(@Nullable SS ss);
+
+	abstract DuplexTransportConnection acceptConnection(SS ss)
+			throws IOException;
+
+	abstract boolean isValidAddress(String address);
+
+	abstract DuplexTransportConnection connectTo(String address, String uuid)
+			throws IOException;
+
+	@Nullable
+	abstract DuplexTransportConnection discoverAndConnect(String uuid);
+
+	AbstractBluetoothPlugin(BluetoothConnectionLimiter connectionLimiter,
+			BluetoothConnectionFactory<S> connectionFactory,
+			Executor ioExecutor,
+			Executor wakefulIoExecutor,
+			SecureRandom secureRandom,
+			Backoff backoff,
+			PluginCallback callback,
+			int maxLatency,
+			int maxIdleTime) {
+		this.connectionLimiter = connectionLimiter;
+		this.connectionFactory = connectionFactory;
+		this.ioExecutor = ioExecutor;
+		this.wakefulIoExecutor = wakefulIoExecutor;
+		this.secureRandom = secureRandom;
+		this.backoff = backoff;
+		this.callback = callback;
+		this.maxLatency = maxLatency;
+		this.maxIdleTime = maxIdleTime;
+	}
+
+	void onAdapterEnabled() {
+		LOG.info("Bluetooth enabled");
+		// We may not have been able to get the local address before
+		ioExecutor.execute(this::updateProperties);
+		if (getState() == INACTIVE) bind();
+	}
+
+	void onAdapterDisabled() {
+		LOG.info("Bluetooth disabled");
+		connectionLimiter.allConnectionsClosed();
+		// The server socket may not have been closed automatically
+		SS ss = state.clearServerSocket();
+		if (ss != null) {
+			LOG.info("Closing server socket");
+			tryToClose(ss);
+		}
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxLatency() {
+		return maxLatency;
+	}
+
+	@Override
+	public int getMaxIdleTime() {
+		return maxIdleTime;
+	}
+
+	@Override
+	public void start() throws PluginException {
+		if (used.getAndSet(true)) throw new IllegalStateException();
+		Settings settings = callback.getSettings();
+		boolean enabledByUser = settings.getBoolean(PREF_PLUGIN_ENABLE,
+				DEFAULT_PREF_PLUGIN_ENABLE);
+		everConnected.set(settings.getBoolean(PREF_EVER_CONNECTED,
+				DEFAULT_PREF_EVER_CONNECTED));
+		state.setStarted(enabledByUser);
+		try {
+			initialiseAdapter();
+		} catch (IOException e) {
+			throw new PluginException(e);
+		}
+		updateProperties();
+		if (enabledByUser && isAdapterEnabled()) bind();
+	}
+
+	private void bind() {
+		ioExecutor.execute(() -> {
+			if (getState() != INACTIVE) return;
+			// Bind a server socket to accept connections from contacts
+			SS ss;
+			try {
+				ss = openServerSocket(contactConnectionsUuid);
+			} catch (IOException e) {
+				logException(LOG, WARNING, e);
+				return;
+			}
+			if (!state.setServerSocket(ss)) {
+				LOG.info("Closing redundant server socket");
+				tryToClose(ss);
+				return;
+			}
+			backoff.reset();
+			acceptContactConnections(ss);
+		});
+	}
+
+	private void updateProperties() {
+		TransportProperties p = callback.getLocalProperties();
+		String address = p.get(PROP_ADDRESS);
+		String uuid = p.get(PROP_UUID);
+		Settings s = callback.getSettings();
+		boolean isReflected = s.getBoolean(PREF_ADDRESS_IS_REFLECTED,
+				DEFAULT_PREF_ADDRESS_IS_REFLECTED);
+		boolean changed = false;
+		if (address == null || isReflected) {
+			address = getBluetoothAddress();
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Local address " + scrubMacAddress(address));
+			}
+			if (address == null) {
+				if (everConnected.get()) {
+					address = getReflectedAddress();
+					if (LOG.isLoggable(INFO)) {
+						LOG.info("Reflected address " +
+								scrubMacAddress(address));
+					}
+					if (address != null) {
+						changed = true;
+						isReflected = true;
+					}
+				}
+			} else {
+				changed = true;
+				isReflected = false;
+			}
+		}
+		if (uuid == null) {
+			byte[] random = new byte[UUID_BYTES];
+			secureRandom.nextBytes(random);
+			uuid = UUID.nameUUIDFromBytes(random).toString();
+			changed = true;
+		}
+		contactConnectionsUuid = uuid;
+		if (changed) {
+			p = new TransportProperties();
+			// If we previously used a reflected address and there's no longer
+			// a reflected address with enough votes to be used, we'll continue
+			// to use the old reflected address until there's a new winner
+			if (address != null) p.put(PROP_ADDRESS, address);
+			p.put(PROP_UUID, uuid);
+			callback.mergeLocalProperties(p);
+			s = new Settings();
+			s.putBoolean(PREF_ADDRESS_IS_REFLECTED, isReflected);
+			callback.mergeSettings(s);
+		}
+	}
+
+	@Nullable
+	private String getReflectedAddress() {
+		// Count the number of votes for each reflected address
+		String key = REFLECTED_PROPERTY_PREFIX + PROP_ADDRESS;
+		Multiset<String> votes = new Multiset<>();
+		for (TransportProperties p : callback.getRemoteProperties()) {
+			String address = p.get(key);
+			if (address != null && isValidAddress(address)) votes.add(address);
+		}
+		// If an address gets more than half of the votes, accept it
+		int total = votes.getTotal();
+		for (String address : votes.keySet()) {
+			if (votes.getCount(address) * 2 > total) return address;
+		}
+		return null;
+	}
+
+	private void acceptContactConnections(SS ss) {
+		while (true) {
+			DuplexTransportConnection conn;
+			try {
+				conn = acceptConnection(ss);
+			} catch (IOException e) {
+				// This is expected when the server socket is closed
+				LOG.info("Server socket closed");
+				state.clearServerSocket();
+				return;
+			}
+			LOG.info("Connection received");
+			connectionLimiter.connectionOpened(conn);
+			backoff.reset();
+			setEverConnected();
+			callback.handleConnection(conn);
+		}
+	}
+
+	private void setEverConnected() {
+		if (!everConnected.getAndSet(true)) {
+			ioExecutor.execute(() -> {
+				Settings s = new Settings();
+				s.putBoolean(PREF_EVER_CONNECTED, true);
+				callback.mergeSettings(s);
+				// Contacts may already have sent a reflected address
+				updateProperties();
+			});
+		}
+	}
+
+	@Override
+	public void stop() {
+		SS ss = state.setStopped();
+		tryToClose(ss);
+	}
+
+	@Override
+	public State getState() {
+		return state.getState();
+	}
+
+	@Override
+	public int getReasonsDisabled() {
+		return state.getReasonsDisabled();
+	}
+
+	@Override
+	public boolean shouldPoll() {
+		return true;
+	}
+
+	@Override
+	public int getPollingInterval() {
+		return backoff.getPollingInterval();
+	}
+
+	@Override
+	public void poll(Collection<Pair<TransportProperties, ConnectionHandler>>
+			properties) {
+		if (getState() != ACTIVE) return;
+		backoff.increment();
+		for (Pair<TransportProperties, ConnectionHandler> p : properties) {
+			connect(p.getFirst(), p.getSecond());
+		}
+	}
+
+	private void connect(TransportProperties p, ConnectionHandler h) {
+		String address = p.get(PROP_ADDRESS);
+		if (isNullOrEmpty(address)) return;
+		String uuid = p.get(PROP_UUID);
+		if (isNullOrEmpty(uuid)) return;
+		wakefulIoExecutor.execute(() -> {
+			DuplexTransportConnection d = createConnection(p);
+			if (d != null) {
+				backoff.reset();
+				setEverConnected();
+				h.handleConnection(d);
+			}
+		});
+	}
+
+	@Nullable
+	private DuplexTransportConnection connect(String address, String uuid) {
+		// Validate the address
+		if (!isValidAddress(address)) {
+			if (LOG.isLoggable(WARNING))
+				// Not scrubbing here to be able to figure out the problem
+				LOG.warning("Invalid address " + address);
+			return null;
+		}
+		// Validate the UUID
+		try {
+			//noinspection ResultOfMethodCallIgnored
+			UUID.fromString(uuid);
+		} catch (IllegalArgumentException e) {
+			if (LOG.isLoggable(WARNING)) LOG.warning("Invalid UUID " + uuid);
+			return null;
+		}
+		if (LOG.isLoggable(INFO))
+			LOG.info("Connecting to " + scrubMacAddress(address));
+		try {
+			DuplexTransportConnection conn = connectTo(address, uuid);
+			if (LOG.isLoggable(INFO))
+				LOG.info("Connected to " + scrubMacAddress(address));
+			return conn;
+		} catch (IOException e) {
+			if (LOG.isLoggable(INFO))
+				LOG.info("Could not connect to " + scrubMacAddress(address));
+			return null;
+		}
+	}
+
+	@Override
+	public DuplexTransportConnection createConnection(TransportProperties p) {
+		if (getState() != ACTIVE) return null;
+		if (!connectionLimiter.canOpenContactConnection()) return null;
+		String address = p.get(PROP_ADDRESS);
+		if (isNullOrEmpty(address)) return null;
+		String uuid = p.get(PROP_UUID);
+		if (isNullOrEmpty(uuid)) return null;
+		DuplexTransportConnection conn = connect(address, uuid);
+		if (conn != null) connectionLimiter.connectionOpened(conn);
+		return conn;
+	}
+
+	@Override
+	public boolean supportsKeyAgreement() {
+		return true;
+	}
+
+	@Override
+	public KeyAgreementListener createKeyAgreementListener(byte[] commitment) {
+		if (getState() != ACTIVE) return null;
+		// No truncation necessary because COMMIT_LENGTH = 16
+		String uuid = UUID.nameUUIDFromBytes(commitment).toString();
+		if (LOG.isLoggable(INFO)) LOG.info("Key agreement UUID " + uuid);
+		// Bind a server socket for receiving key agreement connections
+		SS ss;
+		try {
+			ss = openServerSocket(uuid);
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+			return null;
+		}
+		if (getState() != ACTIVE) {
+			tryToClose(ss);
+			return null;
+		}
+		BdfList descriptor = new BdfList();
+		descriptor.add(TRANSPORT_ID_BLUETOOTH);
+		String address = getBluetoothAddress();
+		if (address != null) descriptor.add(macToBytes(address));
+		return new BluetoothKeyAgreementListener(descriptor, ss);
+	}
+
+	@Override
+	public DuplexTransportConnection createKeyAgreementConnection(
+			byte[] commitment, BdfList descriptor) {
+		if (getState() != ACTIVE) return null;
+		// No truncation necessary because COMMIT_LENGTH = 16
+		String uuid = UUID.nameUUIDFromBytes(commitment).toString();
+		DuplexTransportConnection conn;
+		if (descriptor.size() == 1) {
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Discovering address for key agreement UUID " +
+						uuid);
+			}
+			conn = discoverAndConnect(uuid);
+		} else {
+			String address;
+			try {
+				address = parseAddress(descriptor);
+			} catch (FormatException e) {
+				LOG.info("Invalid address in key agreement descriptor");
+				return null;
+			}
+			if (LOG.isLoggable(INFO))
+				LOG.info("Connecting to key agreement UUID " + uuid);
+			conn = connect(address, uuid);
+		}
+		if (conn != null) {
+			connectionLimiter.connectionOpened(conn);
+			setEverConnected();
+		}
+		return conn;
+	}
+
+	private String parseAddress(BdfList descriptor) throws FormatException {
+		byte[] mac = descriptor.getRaw(1);
+		if (mac.length != 6) throw new FormatException();
+		return macToString(mac);
+	}
+
+	@Override
+	public boolean isDiscovering() {
+		return discoverSemaphore.availablePermits() == 0;
+	}
+
+	@Override
+	public DuplexTransportConnection discoverAndConnectForSetup(String uuid) {
+		DuplexTransportConnection conn = discoverAndConnect(uuid);
+		if (conn != null) {
+			connectionLimiter.connectionOpened(conn);
+			setEverConnected();
+		}
+		return conn;
+	}
+
+	@Override
+	public boolean supportsRendezvous() {
+		return false;
+	}
+
+	@Override
+	public RendezvousEndpoint createRendezvousEndpoint(KeyMaterialSource k,
+			boolean alice, ConnectionHandler incoming) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof SettingsUpdatedEvent) {
+			SettingsUpdatedEvent s = (SettingsUpdatedEvent) e;
+			if (s.getNamespace().equals(ID.getString()))
+				ioExecutor.execute(() -> onSettingsUpdated(s.getSettings()));
+		} else if (e instanceof KeyAgreementListeningEvent) {
+			ioExecutor.execute(connectionLimiter::keyAgreementStarted);
+		} else if (e instanceof KeyAgreementStoppedListeningEvent) {
+			ioExecutor.execute(connectionLimiter::keyAgreementEnded);
+		} else if (e instanceof RemoteTransportPropertiesUpdatedEvent) {
+			RemoteTransportPropertiesUpdatedEvent r =
+					(RemoteTransportPropertiesUpdatedEvent) e;
+			if (r.getTransportId().equals(ID)) {
+				ioExecutor.execute(this::updateProperties);
+			}
+		}
+	}
+
+	@IoExecutor
+	private void onSettingsUpdated(Settings settings) {
+		boolean enabledByUser = settings.getBoolean(PREF_PLUGIN_ENABLE,
+				DEFAULT_PREF_PLUGIN_ENABLE);
+		SS ss = state.setEnabledByUser(enabledByUser);
+		State s = getState();
+		if (ss != null) {
+			LOG.info("Disabled by user, closing server socket");
+			tryToClose(ss);
+		} else if (s == INACTIVE) {
+			if (isAdapterEnabled()) {
+				LOG.info("Enabled by user, opening server socket");
+				bind();
+			} else {
+				LOG.info("Enabled by user but adapter is disabled");
+			}
+		}
+	}
+
+	private class BluetoothKeyAgreementListener extends KeyAgreementListener {
+
+		private final SS ss;
+
+		private BluetoothKeyAgreementListener(BdfList descriptor, SS ss) {
+			super(descriptor);
+			this.ss = ss;
+		}
+
+		@Override
+		public KeyAgreementConnection accept() throws IOException {
+			DuplexTransportConnection conn = acceptConnection(ss);
+			if (LOG.isLoggable(INFO)) LOG.info(ID + ": Incoming connection");
+			connectionLimiter.connectionOpened(conn);
+			return new KeyAgreementConnection(conn, ID);
+		}
+
+		@Override
+		public void close() {
+			tryToClose(ss);
+		}
+	}
+
+	@ThreadSafe
+	@NotNullByDefault
+	private class PluginState {
+
+		@GuardedBy("this")
+		private boolean started = false,
+				stopped = false,
+				enabledByUser = false;
+
+		@GuardedBy("this")
+		@Nullable
+		private SS serverSocket = null;
+
+		private synchronized void setStarted(boolean enabledByUser) {
+			started = true;
+			this.enabledByUser = enabledByUser;
+			callback.pluginStateChanged(getState());
+		}
+
+		@Nullable
+		private synchronized SS setStopped() {
+			stopped = true;
+			SS ss = serverSocket;
+			serverSocket = null;
+			callback.pluginStateChanged(getState());
+			return ss;
+		}
+
+		@Nullable
+		private synchronized SS setEnabledByUser(boolean enabledByUser) {
+			this.enabledByUser = enabledByUser;
+			SS ss = null;
+			if (!enabledByUser) {
+				ss = serverSocket;
+				serverSocket = null;
+			}
+			callback.pluginStateChanged(getState());
+			return ss;
+		}
+
+		private synchronized boolean setServerSocket(SS ss) {
+			if (stopped || serverSocket != null) return false;
+			serverSocket = ss;
+			callback.pluginStateChanged(getState());
+			return true;
+		}
+
+		@Nullable
+		private synchronized SS clearServerSocket() {
+			SS ss = serverSocket;
+			serverSocket = null;
+			callback.pluginStateChanged(getState());
+			return ss;
+		}
+
+		private synchronized State getState() {
+			if (!started || stopped) return STARTING_STOPPING;
+			if (!enabledByUser) return DISABLED;
+			return serverSocket == null ? INACTIVE : ACTIVE;
+		}
+
+		private synchronized int getReasonsDisabled() {
+			return getState() == DISABLED ? REASON_USER : 0;
+		}
+	}
+}

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

@@ -1,606 +1,18 @@
 package org.briarproject.bramble.plugin.bluetooth;
 
-import org.briarproject.bramble.api.FormatException;
-import org.briarproject.bramble.api.Multiset;
-import org.briarproject.bramble.api.Pair;
-import org.briarproject.bramble.api.data.BdfList;
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.event.EventListener;
-import org.briarproject.bramble.api.keyagreement.KeyAgreementConnection;
-import org.briarproject.bramble.api.keyagreement.KeyAgreementListener;
-import org.briarproject.bramble.api.keyagreement.event.KeyAgreementListeningEvent;
-import org.briarproject.bramble.api.keyagreement.event.KeyAgreementStoppedListeningEvent;
-import org.briarproject.bramble.api.lifecycle.IoExecutor;
-import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
-import org.briarproject.bramble.api.plugin.Backoff;
-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.plugin.duplex.DuplexPlugin;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
-import org.briarproject.bramble.api.properties.TransportProperties;
-import org.briarproject.bramble.api.properties.event.RemoteTransportPropertiesUpdatedEvent;
-import org.briarproject.bramble.api.rendezvous.KeyMaterialSource;
-import org.briarproject.bramble.api.rendezvous.RendezvousEndpoint;
-import org.briarproject.bramble.api.settings.Settings;
-import org.briarproject.bramble.api.settings.event.SettingsUpdatedEvent;
-
-import java.io.IOException;
-import java.security.SecureRandom;
-import java.util.Collection;
-import java.util.UUID;
-import java.util.concurrent.Executor;
-import java.util.concurrent.atomic.AtomicBoolean;
-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.keyagreement.KeyAgreementConstants.TRANSPORT_ID_BLUETOOTH;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_ADDRESS_IS_REFLECTED;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_EVER_CONNECTED;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.DEFAULT_PREF_PLUGIN_ENABLE;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.ID;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.PREF_ADDRESS_IS_REFLECTED;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.PREF_EVER_CONNECTED;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.PROP_ADDRESS;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.PROP_UUID;
-import static org.briarproject.bramble.api.plugin.BluetoothConstants.UUID_BYTES;
-import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
-import static org.briarproject.bramble.api.plugin.Plugin.State.DISABLED;
-import static org.briarproject.bramble.api.plugin.Plugin.State.INACTIVE;
-import static org.briarproject.bramble.api.plugin.Plugin.State.STARTING_STOPPING;
-import static org.briarproject.bramble.api.properties.TransportPropertyConstants.REFLECTED_PROPERTY_PREFIX;
-import static org.briarproject.bramble.util.LogUtils.logException;
-import static org.briarproject.bramble.util.PrivacyUtils.scrubMacAddress;
-import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
-import static org.briarproject.bramble.util.StringUtils.macToBytes;
-import static org.briarproject.bramble.util.StringUtils.macToString;
+@NotNullByDefault
+public interface BluetoothPlugin extends DuplexPlugin {
 
-@MethodsNotNullByDefault
-@ParametersNotNullByDefault
-abstract class BluetoothPlugin<S, SS> implements DuplexPlugin, EventListener {
-
-	private static final Logger LOG =
-			getLogger(BluetoothPlugin.class.getName());
-
-	final BluetoothConnectionLimiter connectionLimiter;
-	final BluetoothConnectionFactory<S> connectionFactory;
-
-	private final Executor ioExecutor, wakefulIoExecutor;
-	private final SecureRandom secureRandom;
-	private final Backoff backoff;
-	private final PluginCallback callback;
-	private final int maxLatency, maxIdleTime;
-	private final AtomicBoolean used = new AtomicBoolean(false);
-	private final AtomicBoolean everConnected = new AtomicBoolean(false);
-
-	protected final PluginState state = new PluginState();
-
-	private volatile String contactConnectionsUuid = null;
-
-	abstract void initialiseAdapter() throws IOException;
-
-	abstract boolean isAdapterEnabled();
-
-	/**
-	 * Returns the local Bluetooth address, or null if no valid address can
-	 * be found.
-	 */
-	@Nullable
-	abstract String getBluetoothAddress();
-
-	abstract SS openServerSocket(String uuid) throws IOException;
-
-	abstract void tryToClose(@Nullable SS ss);
-
-	abstract DuplexTransportConnection acceptConnection(SS ss)
-			throws IOException;
-
-	abstract boolean isValidAddress(String address);
-
-	abstract DuplexTransportConnection connectTo(String address, String uuid)
-			throws IOException;
+	boolean isDiscovering();
 
 	@Nullable
-	abstract DuplexTransportConnection discoverAndConnect(String uuid);
+	DuplexTransportConnection discoverAndConnectForSetup(String uuid);
 
-	BluetoothPlugin(BluetoothConnectionLimiter connectionLimiter,
-			BluetoothConnectionFactory<S> connectionFactory,
-			Executor ioExecutor,
-			Executor wakefulIoExecutor,
-			SecureRandom secureRandom,
-			Backoff backoff,
-			PluginCallback callback,
-			int maxLatency,
-			int maxIdleTime) {
-		this.connectionLimiter = connectionLimiter;
-		this.connectionFactory = connectionFactory;
-		this.ioExecutor = ioExecutor;
-		this.wakefulIoExecutor = wakefulIoExecutor;
-		this.secureRandom = secureRandom;
-		this.backoff = backoff;
-		this.callback = callback;
-		this.maxLatency = maxLatency;
-		this.maxIdleTime = maxIdleTime;
-	}
-
-	void onAdapterEnabled() {
-		LOG.info("Bluetooth enabled");
-		// We may not have been able to get the local address before
-		ioExecutor.execute(this::updateProperties);
-		if (getState() == INACTIVE) bind();
-	}
-
-	void onAdapterDisabled() {
-		LOG.info("Bluetooth disabled");
-		connectionLimiter.allConnectionsClosed();
-		// The server socket may not have been closed automatically
-		SS ss = state.clearServerSocket();
-		if (ss != null) {
-			LOG.info("Closing server socket");
-			tryToClose(ss);
-		}
-	}
-
-	@Override
-	public TransportId getId() {
-		return ID;
-	}
-
-	@Override
-	public int getMaxLatency() {
-		return maxLatency;
-	}
-
-	@Override
-	public int getMaxIdleTime() {
-		return maxIdleTime;
-	}
-
-	@Override
-	public void start() throws PluginException {
-		if (used.getAndSet(true)) throw new IllegalStateException();
-		Settings settings = callback.getSettings();
-		boolean enabledByUser = settings.getBoolean(PREF_PLUGIN_ENABLE,
-				DEFAULT_PREF_PLUGIN_ENABLE);
-		everConnected.set(settings.getBoolean(PREF_EVER_CONNECTED,
-				DEFAULT_PREF_EVER_CONNECTED));
-		state.setStarted(enabledByUser);
-		try {
-			initialiseAdapter();
-		} catch (IOException e) {
-			throw new PluginException(e);
-		}
-		updateProperties();
-		if (enabledByUser && isAdapterEnabled()) bind();
-	}
-
-	private void bind() {
-		ioExecutor.execute(() -> {
-			if (getState() != INACTIVE) return;
-			// Bind a server socket to accept connections from contacts
-			SS ss;
-			try {
-				ss = openServerSocket(contactConnectionsUuid);
-			} catch (IOException e) {
-				logException(LOG, WARNING, e);
-				return;
-			}
-			if (!state.setServerSocket(ss)) {
-				LOG.info("Closing redundant server socket");
-				tryToClose(ss);
-				return;
-			}
-			backoff.reset();
-			acceptContactConnections(ss);
-		});
-	}
-
-	private void updateProperties() {
-		TransportProperties p = callback.getLocalProperties();
-		String address = p.get(PROP_ADDRESS);
-		String uuid = p.get(PROP_UUID);
-		Settings s = callback.getSettings();
-		boolean isReflected = s.getBoolean(PREF_ADDRESS_IS_REFLECTED,
-				DEFAULT_PREF_ADDRESS_IS_REFLECTED);
-		boolean changed = false;
-		if (address == null || isReflected) {
-			address = getBluetoothAddress();
-			if (LOG.isLoggable(INFO)) {
-				LOG.info("Local address " + scrubMacAddress(address));
-			}
-			if (address == null) {
-				if (everConnected.get()) {
-					address = getReflectedAddress();
-					if (LOG.isLoggable(INFO)) {
-						LOG.info("Reflected address " +
-								scrubMacAddress(address));
-					}
-					if (address != null) {
-						changed = true;
-						isReflected = true;
-					}
-				}
-			} else {
-				changed = true;
-				isReflected = false;
-			}
-		}
-		if (uuid == null) {
-			byte[] random = new byte[UUID_BYTES];
-			secureRandom.nextBytes(random);
-			uuid = UUID.nameUUIDFromBytes(random).toString();
-			changed = true;
-		}
-		contactConnectionsUuid = uuid;
-		if (changed) {
-			p = new TransportProperties();
-			// If we previously used a reflected address and there's no longer
-			// a reflected address with enough votes to be used, we'll continue
-			// to use the old reflected address until there's a new winner
-			if (address != null) p.put(PROP_ADDRESS, address);
-			p.put(PROP_UUID, uuid);
-			callback.mergeLocalProperties(p);
-			s = new Settings();
-			s.putBoolean(PREF_ADDRESS_IS_REFLECTED, isReflected);
-			callback.mergeSettings(s);
-		}
-	}
-
-	@Nullable
-	private String getReflectedAddress() {
-		// Count the number of votes for each reflected address
-		String key = REFLECTED_PROPERTY_PREFIX + PROP_ADDRESS;
-		Multiset<String> votes = new Multiset<>();
-		for (TransportProperties p : callback.getRemoteProperties()) {
-			String address = p.get(key);
-			if (address != null && isValidAddress(address)) votes.add(address);
-		}
-		// If an address gets more than half of the votes, accept it
-		int total = votes.getTotal();
-		for (String address : votes.keySet()) {
-			if (votes.getCount(address) * 2 > total) return address;
-		}
-		return null;
-	}
-
-	private void acceptContactConnections(SS ss) {
-		while (true) {
-			DuplexTransportConnection conn;
-			try {
-				conn = acceptConnection(ss);
-			} catch (IOException e) {
-				// This is expected when the server socket is closed
-				LOG.info("Server socket closed");
-				state.clearServerSocket();
-				return;
-			}
-			LOG.info("Connection received");
-			connectionLimiter.connectionOpened(conn);
-			backoff.reset();
-			setEverConnected();
-			callback.handleConnection(conn);
-		}
-	}
-
-	private void setEverConnected() {
-		if (!everConnected.getAndSet(true)) {
-			ioExecutor.execute(() -> {
-				Settings s = new Settings();
-				s.putBoolean(PREF_EVER_CONNECTED, true);
-				callback.mergeSettings(s);
-				// Contacts may already have sent a reflected address
-				updateProperties();
-			});
-		}
-	}
-
-	@Override
-	public void stop() {
-		SS ss = state.setStopped();
-		tryToClose(ss);
-	}
-
-	@Override
-	public State getState() {
-		return state.getState();
-	}
-
-	@Override
-	public int getReasonsDisabled() {
-		return state.getReasonsDisabled();
-	}
-
-	@Override
-	public boolean shouldPoll() {
-		return true;
-	}
-
-	@Override
-	public int getPollingInterval() {
-		return backoff.getPollingInterval();
-	}
-
-	@Override
-	public void poll(Collection<Pair<TransportProperties, ConnectionHandler>>
-			properties) {
-		if (getState() != ACTIVE) return;
-		backoff.increment();
-		for (Pair<TransportProperties, ConnectionHandler> p : properties) {
-			connect(p.getFirst(), p.getSecond());
-		}
-	}
-
-	private void connect(TransportProperties p, ConnectionHandler h) {
-		String address = p.get(PROP_ADDRESS);
-		if (isNullOrEmpty(address)) return;
-		String uuid = p.get(PROP_UUID);
-		if (isNullOrEmpty(uuid)) return;
-		wakefulIoExecutor.execute(() -> {
-			DuplexTransportConnection d = createConnection(p);
-			if (d != null) {
-				backoff.reset();
-				setEverConnected();
-				h.handleConnection(d);
-			}
-		});
-	}
-
-	@Nullable
-	private DuplexTransportConnection connect(String address, String uuid) {
-		// Validate the address
-		if (!isValidAddress(address)) {
-			if (LOG.isLoggable(WARNING))
-				// Not scrubbing here to be able to figure out the problem
-				LOG.warning("Invalid address " + address);
-			return null;
-		}
-		// Validate the UUID
-		try {
-			//noinspection ResultOfMethodCallIgnored
-			UUID.fromString(uuid);
-		} catch (IllegalArgumentException e) {
-			if (LOG.isLoggable(WARNING)) LOG.warning("Invalid UUID " + uuid);
-			return null;
-		}
-		if (LOG.isLoggable(INFO))
-			LOG.info("Connecting to " + scrubMacAddress(address));
-		try {
-			DuplexTransportConnection conn = connectTo(address, uuid);
-			if (LOG.isLoggable(INFO))
-				LOG.info("Connected to " + scrubMacAddress(address));
-			return conn;
-		} catch (IOException e) {
-			if (LOG.isLoggable(INFO))
-				LOG.info("Could not connect to " + scrubMacAddress(address));
-			return null;
-		}
-	}
-
-	@Override
-	public DuplexTransportConnection createConnection(TransportProperties p) {
-		if (getState() != ACTIVE) return null;
-		if (!connectionLimiter.canOpenContactConnection()) return null;
-		String address = p.get(PROP_ADDRESS);
-		if (isNullOrEmpty(address)) return null;
-		String uuid = p.get(PROP_UUID);
-		if (isNullOrEmpty(uuid)) return null;
-		DuplexTransportConnection conn = connect(address, uuid);
-		if (conn != null) connectionLimiter.connectionOpened(conn);
-		return conn;
-	}
-
-	@Override
-	public boolean supportsKeyAgreement() {
-		return true;
-	}
-
-	@Override
-	public KeyAgreementListener createKeyAgreementListener(byte[] commitment) {
-		if (getState() != ACTIVE) return null;
-		// No truncation necessary because COMMIT_LENGTH = 16
-		String uuid = UUID.nameUUIDFromBytes(commitment).toString();
-		if (LOG.isLoggable(INFO)) LOG.info("Key agreement UUID " + uuid);
-		// Bind a server socket for receiving key agreement connections
-		SS ss;
-		try {
-			ss = openServerSocket(uuid);
-		} catch (IOException e) {
-			logException(LOG, WARNING, e);
-			return null;
-		}
-		if (getState() != ACTIVE) {
-			tryToClose(ss);
-			return null;
-		}
-		BdfList descriptor = new BdfList();
-		descriptor.add(TRANSPORT_ID_BLUETOOTH);
-		String address = getBluetoothAddress();
-		if (address != null) descriptor.add(macToBytes(address));
-		return new BluetoothKeyAgreementListener(descriptor, ss);
-	}
-
-	@Override
-	public DuplexTransportConnection createKeyAgreementConnection(
-			byte[] commitment, BdfList descriptor) {
-		if (getState() != ACTIVE) return null;
-		// No truncation necessary because COMMIT_LENGTH = 16
-		String uuid = UUID.nameUUIDFromBytes(commitment).toString();
-		DuplexTransportConnection conn;
-		if (descriptor.size() == 1) {
-			if (LOG.isLoggable(INFO)) {
-				LOG.info("Discovering address for key agreement UUID " +
-						uuid);
-			}
-			conn = discoverAndConnect(uuid);
-		} else {
-			String address;
-			try {
-				address = parseAddress(descriptor);
-			} catch (FormatException e) {
-				LOG.info("Invalid address in key agreement descriptor");
-				return null;
-			}
-			if (LOG.isLoggable(INFO))
-				LOG.info("Connecting to key agreement UUID " + uuid);
-			conn = connect(address, uuid);
-		}
-		if (conn != null) {
-			connectionLimiter.connectionOpened(conn);
-			setEverConnected();
-		}
-		return conn;
-	}
-
-	private String parseAddress(BdfList descriptor) throws FormatException {
-		byte[] mac = descriptor.getRaw(1);
-		if (mac.length != 6) throw new FormatException();
-		return macToString(mac);
-	}
-
-	@Override
-	public boolean supportsRendezvous() {
-		return false;
-	}
-
-	@Override
-	public RendezvousEndpoint createRendezvousEndpoint(KeyMaterialSource k,
-			boolean alice, ConnectionHandler incoming) {
-		throw new UnsupportedOperationException();
-	}
-
-	@Override
-	public void eventOccurred(Event e) {
-		if (e instanceof SettingsUpdatedEvent) {
-			SettingsUpdatedEvent s = (SettingsUpdatedEvent) e;
-			if (s.getNamespace().equals(ID.getString()))
-				ioExecutor.execute(() -> onSettingsUpdated(s.getSettings()));
-		} else if (e instanceof KeyAgreementListeningEvent) {
-			ioExecutor.execute(connectionLimiter::keyAgreementStarted);
-		} else if (e instanceof KeyAgreementStoppedListeningEvent) {
-			ioExecutor.execute(connectionLimiter::keyAgreementEnded);
-		} else if (e instanceof RemoteTransportPropertiesUpdatedEvent) {
-			RemoteTransportPropertiesUpdatedEvent r =
-					(RemoteTransportPropertiesUpdatedEvent) e;
-			if (r.getTransportId().equals(ID)) {
-				ioExecutor.execute(this::updateProperties);
-			}
-		}
-	}
-
-	@IoExecutor
-	private void onSettingsUpdated(Settings settings) {
-		boolean enabledByUser = settings.getBoolean(PREF_PLUGIN_ENABLE,
-				DEFAULT_PREF_PLUGIN_ENABLE);
-		SS ss = state.setEnabledByUser(enabledByUser);
-		State s = getState();
-		if (ss != null) {
-			LOG.info("Disabled by user, closing server socket");
-			tryToClose(ss);
-		} else if (s == INACTIVE) {
-			if (isAdapterEnabled()) {
-				LOG.info("Enabled by user, opening server socket");
-				bind();
-			} else {
-				LOG.info("Enabled by user but adapter is disabled");
-			}
-		}
-	}
-
-	private class BluetoothKeyAgreementListener extends KeyAgreementListener {
-
-		private final SS ss;
-
-		private BluetoothKeyAgreementListener(BdfList descriptor, SS ss) {
-			super(descriptor);
-			this.ss = ss;
-		}
-
-		@Override
-		public KeyAgreementConnection accept() throws IOException {
-			DuplexTransportConnection conn = acceptConnection(ss);
-			if (LOG.isLoggable(INFO)) LOG.info(ID + ": Incoming connection");
-			connectionLimiter.connectionOpened(conn);
-			return new KeyAgreementConnection(conn, ID);
-		}
-
-		@Override
-		public void close() {
-			tryToClose(ss);
-		}
-	}
-
-	@ThreadSafe
-	@NotNullByDefault
-	protected class PluginState {
-
-		@GuardedBy("this")
-		private boolean started = false,
-				stopped = false,
-				enabledByUser = false;
-
-		@GuardedBy("this")
-		@Nullable
-		private SS serverSocket = null;
-
-		synchronized void setStarted(boolean enabledByUser) {
-			started = true;
-			this.enabledByUser = enabledByUser;
-			callback.pluginStateChanged(getState());
-		}
-
-		@Nullable
-		synchronized SS setStopped() {
-			stopped = true;
-			SS ss = serverSocket;
-			serverSocket = null;
-			callback.pluginStateChanged(getState());
-			return ss;
-		}
-
-		@Nullable
-		synchronized SS setEnabledByUser(boolean enabledByUser) {
-			this.enabledByUser = enabledByUser;
-			SS ss = null;
-			if (!enabledByUser) {
-				ss = serverSocket;
-				serverSocket = null;
-			}
-			callback.pluginStateChanged(getState());
-			return ss;
-		}
-
-		synchronized boolean setServerSocket(SS ss) {
-			if (stopped || serverSocket != null) return false;
-			serverSocket = ss;
-			callback.pluginStateChanged(getState());
-			return true;
-		}
-
-		@Nullable
-		synchronized SS clearServerSocket() {
-			SS ss = serverSocket;
-			serverSocket = null;
-			callback.pluginStateChanged(getState());
-			return ss;
-		}
-
-		synchronized State getState() {
-			if (!started || stopped) return STARTING_STOPPING;
-			if (!enabledByUser) return DISABLED;
-			return serverSocket == null ? INACTIVE : ACTIVE;
-		}
-
-		synchronized int getReasonsDisabled() {
-			return getState() == DISABLED ? REASON_USER : 0;
-		}
-	}
+	void stopDiscoverAndConnect();
 }

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

@@ -0,0 +1,119 @@
+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.TransportConnectionReader;
+import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.Collection;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.Immutable;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.ID;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@Immutable
+@NotNullByDefault
+abstract class AbstractRemovableDrivePlugin implements SimplexPlugin {
+
+	private static final Logger LOG =
+			getLogger(AbstractRemovableDrivePlugin.class.getName());
+
+	private final int maxLatency;
+
+	abstract InputStream openInputStream(TransportProperties p)
+			throws IOException;
+
+	abstract OutputStream openOutputStream(TransportProperties p)
+			throws IOException;
+
+	AbstractRemovableDrivePlugin(int maxLatency) {
+		this.maxLatency = maxLatency;
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxLatency() {
+		return maxLatency;
+	}
+
+	@Override
+	public int getMaxIdleTime() {
+		// Unused for simplex transports
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void start() {
+	}
+
+	@Override
+	public void stop() {
+	}
+
+	@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 true;
+	}
+
+	@Override
+	public TransportConnectionReader createReader(TransportProperties p) {
+		try {
+			return new TransportInputStreamReader(openInputStream(p));
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+			return null;
+		}
+	}
+
+	@Override
+	public TransportConnectionWriter createWriter(TransportProperties p) {
+		try {
+			return new TransportOutputStreamWriter(this, openOutputStream(p));
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+			return null;
+		}
+	}
+}

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

@@ -15,8 +15,8 @@ import java.util.logging.Logger;
 
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
-import static org.briarproject.bramble.api.plugin.FileConstants.PROP_PATH;
 import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.file.FileConstants.PROP_PATH;
 import static org.briarproject.bramble.util.LogUtils.logException;
 import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 

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

@@ -36,6 +36,11 @@ class FileTransportWriter implements TransportConnectionWriter {
 		return plugin.getMaxIdleTime();
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return plugin.isLossyAndCheap();
+	}
+
 	@Override
 	public OutputStream getOutputStream() {
 		return out;

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

@@ -0,0 +1,102 @@
+package org.briarproject.bramble.plugin.file;
+
+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.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveManager;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+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 org.briarproject.bramble.plugin.file.RemovableDrivePluginFactory.MAX_LATENCY;
+
+@ThreadSafe
+@NotNullByDefault
+class RemovableDriveManagerImpl
+		implements RemovableDriveManager, RemovableDriveTaskRegistry {
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final RemovableDriveTaskFactory taskFactory;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private RemovableDriveTask reader = null;
+	@GuardedBy("lock")
+	private RemovableDriveTask writer = null;
+
+	@Inject
+	RemovableDriveManagerImpl(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db, RemovableDriveTaskFactory taskFactory) {
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.taskFactory = taskFactory;
+	}
+
+	@Nullable
+	@Override
+	public RemovableDriveTask getCurrentReaderTask() {
+		synchronized (lock) {
+			return reader;
+		}
+	}
+
+	@Nullable
+	@Override
+	public RemovableDriveTask getCurrentWriterTask() {
+		synchronized (lock) {
+			return writer;
+		}
+	}
+
+	@Override
+	public RemovableDriveTask startReaderTask(TransportProperties p) {
+		RemovableDriveTask created;
+		synchronized (lock) {
+			if (reader != null) return reader;
+			reader = created = taskFactory.createReader(this, p);
+		}
+		ioExecutor.execute(created);
+		return created;
+	}
+
+	@Override
+	public RemovableDriveTask startWriterTask(ContactId c,
+			TransportProperties p) {
+		RemovableDriveTask created;
+		synchronized (lock) {
+			if (writer != null) return writer;
+			writer = created = taskFactory.createWriter(this, c, p);
+		}
+		ioExecutor.execute(created);
+		return created;
+	}
+
+	@Override
+	public boolean isWriterTaskNeeded(ContactId c) throws DbException {
+		return db.transactionWithResult(true, txn ->
+				db.containsAnythingToSend(txn, c, MAX_LATENCY, true));
+	}
+
+	@Override
+	public void removeReader(RemovableDriveTask task) {
+		synchronized (lock) {
+			if (reader == task) reader = null;
+		}
+	}
+
+	@Override
+	public void removeWriter(RemovableDriveTask task) {
+		synchronized (lock) {
+			if (writer == task) writer = null;
+		}
+	}
+}

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

@@ -0,0 +1,25 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.plugin.file.RemovableDriveManager;
+
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+@Module
+public class RemovableDriveModule {
+
+	@Provides
+	@Singleton
+	RemovableDriveManager provideRemovableDriveManager(
+			RemovableDriveManagerImpl removableDriveManager) {
+		return removableDriveManager;
+	}
+
+	@Provides
+	RemovableDriveTaskFactory provideTaskFactory(
+			RemovableDriveTaskFactoryImpl taskFactory) {
+		return taskFactory;
+	}
+}

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

@@ -0,0 +1,38 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+
+import javax.annotation.concurrent.Immutable;
+
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.PROP_PATH;
+import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
+
+@Immutable
+@NotNullByDefault
+class RemovableDrivePlugin extends AbstractRemovableDrivePlugin {
+
+	RemovableDrivePlugin(int maxLatency) {
+		super(maxLatency);
+	}
+
+	@Override
+	InputStream openInputStream(TransportProperties p) throws IOException {
+		String path = p.get(PROP_PATH);
+		if (isNullOrEmpty(path)) throw new IllegalArgumentException();
+		return new FileInputStream(path);
+	}
+
+	@Override
+	OutputStream openOutputStream(TransportProperties p) throws IOException {
+		String path = p.get(PROP_PATH);
+		if (isNullOrEmpty(path)) throw new IllegalArgumentException();
+		return new FileOutputStream(path);
+	}
+}

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

@@ -0,0 +1,41 @@
+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.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.ID;
+
+@Immutable
+@NotNullByDefault
+public class RemovableDrivePluginFactory implements SimplexPluginFactory {
+
+	static final int MAX_LATENCY = (int) DAYS.toMillis(14);
+
+	@Inject
+	RemovableDrivePluginFactory() {
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxLatency() {
+		return MAX_LATENCY;
+	}
+
+	@Nullable
+	@Override
+	public SimplexPlugin createPlugin(PluginCallback callback) {
+		return new RemovableDrivePlugin(MAX_LATENCY);
+	}
+}

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

@@ -0,0 +1,69 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.connection.ConnectionManager;
+import org.briarproject.bramble.api.event.EventBus;
+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.properties.TransportProperties;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.ID;
+
+@NotNullByDefault
+class RemovableDriveReaderTask extends RemovableDriveTaskImpl {
+
+	private final static Logger LOG =
+			getLogger(RemovableDriveReaderTask.class.getName());
+
+	RemovableDriveReaderTask(
+			Executor eventExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			EventBus eventBus,
+			RemovableDriveTaskRegistry registry,
+			TransportProperties transportProperties) {
+		super(eventExecutor, pluginManager, connectionManager, eventBus,
+				registry, transportProperties);
+	}
+
+	@Override
+	public void run() {
+		TransportConnectionReader r =
+				getPlugin().createReader(transportProperties);
+		if (r == null) {
+			LOG.warning("Failed to create reader");
+			registry.removeReader(this);
+			setSuccess(false);
+			return;
+		}
+		connectionManager.manageIncomingConnection(ID, new DecoratedReader(r));
+	}
+
+	private class DecoratedReader implements TransportConnectionReader {
+
+		private final TransportConnectionReader delegate;
+
+		private DecoratedReader(TransportConnectionReader delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public InputStream getInputStream() throws IOException {
+			return delegate.getInputStream();
+		}
+
+		@Override
+		public void dispose(boolean exception, boolean recognised)
+				throws IOException {
+			delegate.dispose(exception, recognised);
+			registry.removeReader(RemovableDriveReaderTask.this);
+			setSuccess(!exception && recognised);
+		}
+	}
+}

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

@@ -0,0 +1,16 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+@NotNullByDefault
+interface RemovableDriveTaskFactory {
+
+	RemovableDriveTask createReader(RemovableDriveTaskRegistry registry,
+			TransportProperties p);
+
+	RemovableDriveTask createWriter(RemovableDriveTaskRegistry registry,
+			ContactId c, TransportProperties p);
+}

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

@@ -0,0 +1,55 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.connection.ConnectionManager;
+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.event.EventExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class RemovableDriveTaskFactoryImpl implements RemovableDriveTaskFactory {
+
+	private final DatabaseComponent db;
+	private final Executor eventExecutor;
+	private final PluginManager pluginManager;
+	private final ConnectionManager connectionManager;
+	private final EventBus eventBus;
+
+	@Inject
+	RemovableDriveTaskFactoryImpl(
+			DatabaseComponent db,
+			@EventExecutor Executor eventExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			EventBus eventBus) {
+		this.db = db;
+		this.eventExecutor = eventExecutor;
+		this.pluginManager = pluginManager;
+		this.connectionManager = connectionManager;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public RemovableDriveTask createReader(RemovableDriveTaskRegistry registry,
+			TransportProperties p) {
+		return new RemovableDriveReaderTask(eventExecutor, pluginManager,
+				connectionManager, eventBus, registry, p);
+	}
+
+	@Override
+	public RemovableDriveTask createWriter(RemovableDriveTaskRegistry registry,
+			ContactId c, TransportProperties p) {
+		return new RemovableDriveWriterTask(db, eventExecutor, pluginManager,
+				connectionManager, eventBus, registry, c, p);
+	}
+}

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

@@ -0,0 +1,116 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.Consumer;
+import org.briarproject.bramble.api.connection.ConnectionManager;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.lang.Math.min;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.plugin.file.RemovableDriveConstants.ID;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class RemovableDriveTaskImpl implements RemovableDriveTask {
+
+	private final Executor eventExecutor;
+	private final PluginManager pluginManager;
+	final ConnectionManager connectionManager;
+	final EventBus eventBus;
+	final RemovableDriveTaskRegistry registry;
+	final TransportProperties transportProperties;
+
+	private final Object lock = new Object();
+	@GuardedBy("lock")
+	private final List<Consumer<State>> observers = new ArrayList<>();
+	@GuardedBy("lock")
+	private State state = new State(0, 0, false, false);
+
+	RemovableDriveTaskImpl(
+			Executor eventExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			EventBus eventBus,
+			RemovableDriveTaskRegistry registry,
+			TransportProperties transportProperties) {
+		this.eventExecutor = eventExecutor;
+		this.pluginManager = pluginManager;
+		this.connectionManager = connectionManager;
+		this.eventBus = eventBus;
+		this.registry = registry;
+		this.transportProperties = transportProperties;
+	}
+
+	@Override
+	public TransportProperties getTransportProperties() {
+		return transportProperties;
+	}
+
+	@Override
+	public void addObserver(Consumer<State> o) {
+		State state;
+		synchronized (lock) {
+			observers.add(o);
+			state = this.state;
+		}
+		if (state.isFinished()) {
+			eventExecutor.execute(() -> o.accept(state));
+		}
+	}
+
+	@Override
+	public void removeObserver(Consumer<State> o) {
+		synchronized (lock) {
+			observers.remove(o);
+		}
+	}
+
+	SimplexPlugin getPlugin() {
+		return (SimplexPlugin) requireNonNull(pluginManager.getPlugin(ID));
+	}
+
+	void setTotal(long total) {
+		synchronized (lock) {
+			state = new State(state.getDone(), total, state.isFinished(),
+					state.isSuccess());
+			notifyObservers();
+		}
+	}
+
+	void addDone(long done) {
+		synchronized (lock) {
+			// Done and total come from different sources; make them consistent
+			done = min(state.getDone() + done, state.getTotal());
+			state = new State(done, state.getTotal(), state.isFinished(),
+					state.isSuccess());
+			notifyObservers();
+		}
+	}
+
+	void setSuccess(boolean success) {
+		synchronized (lock) {
+			state = new State(state.getDone(), state.getTotal(), true, success);
+			notifyObservers();
+		}
+	}
+
+	@GuardedBy("lock")
+	private void notifyObservers() {
+		List<Consumer<State>> observers = new ArrayList<>(this.observers);
+		State state = this.state;
+		eventExecutor.execute(() -> {
+			for (Consumer<State> o : observers) o.accept(state);
+		});
+	}
+}

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

@@ -0,0 +1,12 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+
+@NotNullByDefault
+interface RemovableDriveTaskRegistry {
+
+	void removeReader(RemovableDriveTask task);
+
+	void removeWriter(RemovableDriveTask task);
+}

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

@@ -0,0 +1,126 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.connection.ConnectionManager;
+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.EventListener;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.sync.event.MessagesSentEvent;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+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.plugin.file.RemovableDriveConstants.ID;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@NotNullByDefault
+class RemovableDriveWriterTask extends RemovableDriveTaskImpl
+		implements EventListener {
+
+	private static final Logger LOG =
+			getLogger(RemovableDriveWriterTask.class.getName());
+
+	private final DatabaseComponent db;
+	private final ContactId contactId;
+
+	RemovableDriveWriterTask(
+			DatabaseComponent db,
+			Executor eventExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			EventBus eventBus,
+			RemovableDriveTaskRegistry registry,
+			ContactId contactId,
+			TransportProperties transportProperties) {
+		super(eventExecutor, pluginManager, connectionManager, eventBus,
+				registry, transportProperties);
+		this.db = db;
+		this.contactId = contactId;
+	}
+
+	@Override
+	public void run() {
+		SimplexPlugin plugin = getPlugin();
+		TransportConnectionWriter w = plugin.createWriter(transportProperties);
+		if (w == null) {
+			LOG.warning("Failed to create writer");
+			registry.removeWriter(this);
+			setSuccess(false);
+			return;
+		}
+		try {
+			setTotal(db.transactionWithResult(true, txn ->
+					db.getUnackedMessageBytesToSend(txn, contactId)));
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			registry.removeWriter(this);
+			setSuccess(false);
+			return;
+		}
+		eventBus.addListener(this);
+		connectionManager.manageOutgoingConnection(contactId, ID,
+				new DecoratedWriter(w));
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof MessagesSentEvent) {
+			MessagesSentEvent m = (MessagesSentEvent) e;
+			if (contactId.equals(m.getContactId())) {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info(m.getMessageIds().size() + " messages sent");
+				}
+				addDone(m.getTotalLength());
+			}
+		}
+	}
+
+	private class DecoratedWriter implements TransportConnectionWriter {
+
+		private final TransportConnectionWriter delegate;
+
+		private DecoratedWriter(TransportConnectionWriter delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public int 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);
+			registry.removeWriter(RemovableDriveWriterTask.this);
+			eventBus.removeListener(RemovableDriveWriterTask.this);
+			setSuccess(!exception);
+		}
+	}
+}

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

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportConnectionReader;
+
+import java.io.InputStream;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.IoUtils.tryToClose;
+
+@NotNullByDefault
+class TransportInputStreamReader implements TransportConnectionReader {
+
+	private static final Logger LOG =
+			getLogger(TransportInputStreamReader.class.getName());
+
+	private final InputStream in;
+
+	TransportInputStreamReader(InputStream in) {
+		this.in = in;
+	}
+
+	@Override
+	public InputStream getInputStream() {
+		return in;
+	}
+
+	@Override
+	public void dispose(boolean exception, boolean recognised) {
+		tryToClose(in, LOG, WARNING);
+	}
+}

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

@@ -0,0 +1,52 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+
+import java.io.OutputStream;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.IoUtils.tryToClose;
+
+@NotNullByDefault
+class TransportOutputStreamWriter implements TransportConnectionWriter {
+
+	private static final Logger LOG =
+			getLogger(TransportOutputStreamWriter.class.getName());
+
+	private final SimplexPlugin plugin;
+	private final OutputStream out;
+
+	TransportOutputStreamWriter(SimplexPlugin plugin, OutputStream out) {
+		this.plugin = plugin;
+		this.out = out;
+	}
+
+	@Override
+	public int 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);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/properties/TransportPropertyManagerImpl.java

@@ -44,6 +44,7 @@ import static org.briarproject.bramble.api.properties.TransportPropertyConstants
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MSG_KEY_TRANSPORT_ID;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MSG_KEY_VERSION;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.REFLECTED_PROPERTY_PREFIX;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 
 @Immutable
@@ -115,8 +116,8 @@ class TransportPropertyManagerImpl implements TransportPropertyManager,
 	}
 
 	@Override
-	public boolean incomingMessage(Transaction txn, Message m, Metadata meta)
-			throws DbException, InvalidMessageException {
+	public DeliveryAction incomingMessage(Transaction txn, Message m,
+			Metadata meta) throws DbException, InvalidMessageException {
 		try {
 			// Find the latest update for this transport, if any
 			BdfDictionary d = metadataParser.parse(meta);
@@ -131,14 +132,14 @@ class TransportPropertyManagerImpl implements TransportPropertyManager,
 					// We've already received a newer update - delete this one
 					db.deleteMessage(txn, m.getId());
 					db.deleteMessageMetadata(txn, m.getId());
-					return false;
+					return ACCEPT_DO_NOT_SHARE;
 				}
 			}
 			txn.attach(new RemoteTransportPropertiesUpdatedEvent(t));
 		} catch (FormatException e) {
 			throw new InvalidMessageException(e);
 		}
-		return false;
+		return ACCEPT_DO_NOT_SHARE;
 	}
 
 	@Override

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

@@ -15,6 +15,7 @@ import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
 import org.briarproject.bramble.api.sync.Ack;
 import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.bramble.api.sync.SyncRecordWriter;
 import org.briarproject.bramble.api.sync.SyncSession;
 import org.briarproject.bramble.api.sync.Versions;
@@ -22,7 +23,11 @@ import org.briarproject.bramble.api.sync.event.CloseSyncConnectionsEvent;
 import org.briarproject.bramble.api.transport.StreamWriter;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Executor;
 import java.util.concurrent.LinkedBlockingQueue;
@@ -61,6 +66,7 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 	private final ContactId contactId;
 	private final TransportId transportId;
 	private final int maxLatency;
+	private final boolean eager;
 	private final StreamWriter streamWriter;
 	private final SyncRecordWriter recordWriter;
 	private final AtomicInteger outstandingQueries;
@@ -70,7 +76,7 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 
 	SimplexOutgoingSession(DatabaseComponent db, Executor dbExecutor,
 			EventBus eventBus, ContactId contactId, TransportId transportId,
-			int maxLatency, StreamWriter streamWriter,
+			int maxLatency, boolean eager, StreamWriter streamWriter,
 			SyncRecordWriter recordWriter) {
 		this.db = db;
 		this.dbExecutor = dbExecutor;
@@ -78,6 +84,7 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 		this.contactId = contactId;
 		this.transportId = transportId;
 		this.maxLatency = maxLatency;
+		this.eager = eager;
 		this.streamWriter = streamWriter;
 		this.recordWriter = recordWriter;
 		outstandingQueries = new AtomicInteger(2); // One per type of record
@@ -92,8 +99,9 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 			// Send our supported protocol versions
 			recordWriter.writeVersions(new Versions(SUPPORTED_VERSIONS));
 			// Start a query for each type of record
-			dbExecutor.execute(new GenerateAck());
-			dbExecutor.execute(new GenerateBatch());
+			dbExecutor.execute(this::generateAck);
+			if (eager) dbExecutor.execute(this::loadUnackedMessageIds);
+			else dbExecutor.execute(this::generateBatch);
 			// Write records until interrupted or no more records to write
 			try {
 				while (!interrupted) {
@@ -138,81 +146,110 @@ class SimplexOutgoingSession implements SyncSession, EventListener {
 		}
 	}
 
-	private class GenerateAck implements Runnable {
-
-		@DatabaseExecutor
-		@Override
-		public void run() {
-			if (interrupted) return;
-			try {
-				Ack a = db.transactionWithNullableResult(false, txn ->
-						db.generateAck(txn, contactId, MAX_MESSAGE_IDS));
-				if (LOG.isLoggable(INFO))
-					LOG.info("Generated ack: " + (a != null));
-				if (a == null) decrementOutstandingQueries();
-				else writerTasks.add(new WriteAck(a));
-			} catch (DbException e) {
-				logException(LOG, WARNING, e);
-				interrupt();
+	@DatabaseExecutor
+	private void loadUnackedMessageIds() {
+		if (interrupted) return;
+		try {
+			Map<MessageId, Integer> ids = db.transactionWithResult(true, txn ->
+					db.getUnackedMessagesToSend(txn, contactId));
+			if (LOG.isLoggable(INFO)) {
+				LOG.info(ids.size() + " unacked messages to send");
 			}
+			if (ids.isEmpty()) decrementOutstandingQueries();
+			else dbExecutor.execute(() -> generateEagerBatch(ids));
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			interrupt();
 		}
 	}
 
-	private class WriteAck implements ThrowingRunnable<IOException> {
-
-		private final Ack ack;
-
-		private WriteAck(Ack ack) {
-			this.ack = ack;
+	@DatabaseExecutor
+	private void generateEagerBatch(Map<MessageId, Integer> ids) {
+		if (interrupted) return;
+		// Take some message IDs from `ids` to form a batch
+		Collection<MessageId> batchIds = new ArrayList<>();
+		long totalLength = 0;
+		Iterator<Entry<MessageId, Integer>> it = ids.entrySet().iterator();
+		while (it.hasNext()) {
+			// Check whether the next message will fit in the batch
+			Entry<MessageId, Integer> e = it.next();
+			int length = e.getValue();
+			if (totalLength + length > MAX_RECORD_PAYLOAD_BYTES) break;
+			// Add the message to the batch
+			it.remove();
+			batchIds.add(e.getKey());
+			totalLength += length;
 		}
-
-		@IoExecutor
-		@Override
-		public void run() throws IOException {
-			if (interrupted) return;
-			recordWriter.writeAck(ack);
-			LOG.info("Sent ack");
-			dbExecutor.execute(new GenerateAck());
+		if (batchIds.isEmpty()) throw new AssertionError();
+		try {
+			Collection<Message> batch =
+					db.transactionWithResult(false, txn ->
+							db.generateBatch(txn, contactId, batchIds,
+									maxLatency));
+			writerTasks.add(() -> writeEagerBatch(batch, ids));
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			interrupt();
 		}
 	}
 
-	private class GenerateBatch implements Runnable {
+	@IoExecutor
+	private void writeEagerBatch(Collection<Message> batch,
+			Map<MessageId, Integer> ids) throws IOException {
+		if (interrupted) return;
+		for (Message m : batch) recordWriter.writeMessage(m);
+		LOG.info("Sent eager batch");
+		if (ids.isEmpty()) decrementOutstandingQueries();
+		else dbExecutor.execute(() -> generateEagerBatch(ids));
+	}
 
-		@DatabaseExecutor
-		@Override
-		public void run() {
-			if (interrupted) return;
-			try {
-				Collection<Message> b =
-						db.transactionWithNullableResult(false, txn ->
-								db.generateBatch(txn, contactId,
-										MAX_RECORD_PAYLOAD_BYTES, maxLatency));
-				if (LOG.isLoggable(INFO))
-					LOG.info("Generated batch: " + (b != null));
-				if (b == null) decrementOutstandingQueries();
-				else writerTasks.add(new WriteBatch(b));
-			} catch (DbException e) {
-				logException(LOG, WARNING, e);
-				interrupt();
-			}
+	@DatabaseExecutor
+	private void generateAck() {
+		if (interrupted) return;
+		try {
+			Ack a = db.transactionWithNullableResult(false, txn ->
+					db.generateAck(txn, contactId, MAX_MESSAGE_IDS));
+			if (LOG.isLoggable(INFO))
+				LOG.info("Generated ack: " + (a != null));
+			if (a == null) decrementOutstandingQueries();
+			else writerTasks.add(() -> writeAck(a));
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			interrupt();
 		}
 	}
 
-	private class WriteBatch implements ThrowingRunnable<IOException> {
+	@IoExecutor
+	private void writeAck(Ack ack) throws IOException {
+		if (interrupted) return;
+		recordWriter.writeAck(ack);
+		LOG.info("Sent ack");
+		dbExecutor.execute(this::generateAck);
+	}
 
-		private final Collection<Message> batch;
-
-		private WriteBatch(Collection<Message> batch) {
-			this.batch = batch;
+	@DatabaseExecutor
+	private void generateBatch() {
+		if (interrupted) return;
+		try {
+			Collection<Message> b =
+					db.transactionWithNullableResult(false, txn ->
+							db.generateBatch(txn, contactId,
+									MAX_RECORD_PAYLOAD_BYTES, maxLatency));
+			if (LOG.isLoggable(INFO))
+				LOG.info("Generated batch: " + (b != null));
+			if (b == null) decrementOutstandingQueries();
+			else writerTasks.add(() -> writeBatch(b));
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			interrupt();
 		}
+	}
 
-		@IoExecutor
-		@Override
-		public void run() throws IOException {
-			if (interrupted) return;
-			for (Message m : batch) recordWriter.writeMessage(m);
-			LOG.info("Sent batch");
-			dbExecutor.execute(new GenerateBatch());
-		}
+	@IoExecutor
+	private void writeBatch(Collection<Message> batch) throws IOException {
+		if (interrupted) return;
+		for (Message m : batch) recordWriter.writeMessage(m);
+		LOG.info("Sent batch");
+		dbExecutor.execute(this::generateBatch);
 	}
 }

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

@@ -60,12 +60,12 @@ class SyncSessionFactoryImpl implements SyncSessionFactory {
 
 	@Override
 	public SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
-			int maxLatency, StreamWriter streamWriter) {
+			int maxLatency, boolean eager, StreamWriter streamWriter) {
 		OutputStream out = streamWriter.getOutputStream();
 		SyncRecordWriter recordWriter =
 				recordWriterFactory.createRecordWriter(out);
 		return new SimplexOutgoingSession(db, dbExecutor, eventBus, c, t,
-				maxLatency, streamWriter, recordWriter);
+				maxLatency, eager, streamWriter, recordWriter);
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/sync/validation/ValidationManagerImpl.java

@@ -20,6 +20,7 @@ import org.briarproject.bramble.api.sync.MessageContext;
 import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.bramble.api.sync.event.MessageAddedEvent;
 import org.briarproject.bramble.api.sync.validation.IncomingMessageHook;
+import org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction;
 import org.briarproject.bramble.api.sync.validation.MessageState;
 import org.briarproject.bramble.api.sync.validation.MessageValidator;
 import org.briarproject.bramble.api.sync.validation.ValidationManager;
@@ -40,6 +41,10 @@ import javax.inject.Inject;
 
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.DEFER;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.REJECT;
 import static org.briarproject.bramble.api.sync.validation.MessageState.DELIVERED;
 import static org.briarproject.bramble.api.sync.validation.MessageState.INVALID;
 import static org.briarproject.bramble.api.sync.validation.MessageState.PENDING;
@@ -185,16 +190,19 @@ class ValidationManagerImpl implements ValidationManager, Service,
 						int majorVersion = g.getMajorVersion();
 						Metadata meta =
 								db.getMessageMetadataForValidator(txn, id);
-						DeliveryResult result =
+						DeliveryAction action =
 								deliverMessage(txn, m, c, majorVersion, meta);
-						if (result.valid) {
-							addPendingDependents(txn, id, pending);
-							if (result.share) {
-								db.setMessageShared(txn, id);
-								toShare.addAll(states.keySet());
-							}
-						} else {
+						if (action == REJECT) {
+							invalidateMessage(txn, id);
 							addDependentsToInvalidate(txn, id, invalidate);
+						} else if (action == ACCEPT_SHARE) {
+							db.setMessageState(txn, m.getId(), DELIVERED);
+							addPendingDependents(txn, id, pending);
+							db.setMessageShared(txn, id);
+							toShare.addAll(states.keySet());
+						} else if (action == ACCEPT_DO_NOT_SHARE) {
+							db.setMessageState(txn, m.getId(), DELIVERED);
+							addPendingDependents(txn, id, pending);
 						}
 					}
 				}
@@ -275,16 +283,21 @@ class ValidationManagerImpl implements ValidationManager, Service,
 					Metadata meta = context.getMetadata();
 					db.mergeMessageMetadata(txn, id, meta);
 					if (allDelivered) {
-						DeliveryResult result =
+						DeliveryAction action =
 								deliverMessage(txn, m, c, majorVersion, meta);
-						if (result.valid) {
-							addPendingDependents(txn, id, pending);
-							if (result.share) {
-								db.setMessageShared(txn, id);
-								toShare.addAll(dependencies);
-							}
-						} else {
+						if (action == REJECT) {
+							invalidateMessage(txn, id);
 							addDependentsToInvalidate(txn, id, invalidate);
+						} else if (action == DEFER) {
+							db.setMessageState(txn, id, PENDING);
+						} else if (action == ACCEPT_SHARE) {
+							db.setMessageState(txn, id, DELIVERED);
+							addPendingDependents(txn, id, pending);
+							db.setMessageShared(txn, id);
+							toShare.addAll(dependencies);
+						} else if (action == ACCEPT_DO_NOT_SHARE) {
+							db.setMessageState(txn, id, DELIVERED);
+							addPendingDependents(txn, id, pending);
 						}
 					} else {
 						db.setMessageState(txn, id, PENDING);
@@ -304,23 +317,21 @@ class ValidationManagerImpl implements ValidationManager, Service,
 	}
 
 	@DatabaseExecutor
-	private DeliveryResult deliverMessage(Transaction txn, Message m,
-			ClientId c, int majorVersion, Metadata meta) throws DbException {
-		// Deliver the message to the client if it's registered a hook
-		boolean shareMsg = false;
+	private DeliveryAction deliverMessage(Transaction txn, Message m,
+			ClientId c, int majorVersion, Metadata meta) {
+		// Deliver the message to the client if it has registered a hook
 		ClientMajorVersion cv = new ClientMajorVersion(c, majorVersion);
 		IncomingMessageHook hook = hooks.get(cv);
-		if (hook != null) {
-			try {
-				shareMsg = hook.incomingMessage(txn, m, meta);
-			} catch (InvalidMessageException e) {
-				logException(LOG, INFO, e);
-				invalidateMessage(txn, m.getId());
-				return new DeliveryResult(false, false);
-			}
+		if (hook == null) return ACCEPT_DO_NOT_SHARE;
+		try {
+			return hook.incomingMessage(txn, m, meta);
+		} catch (DbException e) {
+			logException(LOG, INFO, e);
+			return DEFER;
+		} catch (InvalidMessageException e) {
+			logException(LOG, INFO, e);
+			return REJECT;
 		}
-		db.setMessageState(txn, m.getId(), DELIVERED);
-		return new DeliveryResult(true, shareMsg);
 	}
 
 	@DatabaseExecutor
@@ -447,14 +458,4 @@ class ValidationManagerImpl implements ValidationManager, Service,
 			logException(LOG, WARNING, e);
 		}
 	}
-
-	private static class DeliveryResult {
-
-		private final boolean valid, share;
-
-		private DeliveryResult(boolean valid, boolean share) {
-			this.valid = valid;
-			this.share = share;
-		}
-	}
 }

bramble-core/src/main/java/org/briarproject/bramble/transport/KeyManagerImpl.java

@@ -51,7 +51,6 @@ class KeyManagerImpl implements KeyManager, Service, EventListener {
 	private final DatabaseComponent db;
 	private final Executor dbExecutor;
 	private final PluginConfig pluginConfig;
-	private final TransportKeyManagerFactory transportKeyManagerFactory;
 	private final TransportCrypto transportCrypto;
 
 	private final ConcurrentHashMap<TransportId, TransportKeyManager> managers;
@@ -61,34 +60,39 @@ class KeyManagerImpl implements KeyManager, Service, EventListener {
 	KeyManagerImpl(DatabaseComponent db,
 			@DatabaseExecutor Executor dbExecutor,
 			PluginConfig pluginConfig,
-			TransportKeyManagerFactory transportKeyManagerFactory,
-			TransportCrypto transportCrypto) {
+			TransportCrypto transportCrypto,
+			TransportKeyManagerFactory transportKeyManagerFactory) {
 		this.db = db;
 		this.dbExecutor = dbExecutor;
 		this.pluginConfig = pluginConfig;
-		this.transportKeyManagerFactory = transportKeyManagerFactory;
 		this.transportCrypto = transportCrypto;
 		managers = new ConcurrentHashMap<>();
+		for (SimplexPluginFactory f : pluginConfig.getSimplexFactories()) {
+			TransportKeyManager m = transportKeyManagerFactory.
+					createTransportKeyManager(f.getId(), f.getMaxLatency());
+			managers.put(f.getId(), m);
+		}
+		for (DuplexPluginFactory f : pluginConfig.getDuplexFactories()) {
+			TransportKeyManager m = transportKeyManagerFactory.
+					createTransportKeyManager(f.getId(), f.getMaxLatency());
+			managers.put(f.getId(), m);
+		}
 	}
 
 	@Override
 	public void startService() throws ServiceException {
 		if (used.getAndSet(true)) throw new IllegalStateException();
-		Map<TransportId, Integer> transports = new HashMap<>();
-		for (SimplexPluginFactory f : pluginConfig.getSimplexFactories())
-			transports.put(f.getId(), f.getMaxLatency());
-		for (DuplexPluginFactory f : pluginConfig.getDuplexFactories())
-			transports.put(f.getId(), f.getMaxLatency());
 		try {
 			db.transaction(false, txn -> {
-				for (Entry<TransportId, Integer> e : transports.entrySet())
-					db.addTransport(txn, e.getKey(), e.getValue());
-				for (Entry<TransportId, Integer> e : transports.entrySet()) {
-					TransportKeyManager m = transportKeyManagerFactory
-							.createTransportKeyManager(e.getKey(),
-									e.getValue());
-					managers.put(e.getKey(), m);
-					m.start(txn);
+				for (SimplexPluginFactory f :
+						pluginConfig.getSimplexFactories()) {
+					db.addTransport(txn, f.getId(), f.getMaxLatency());
+					managers.get(f.getId()).start(txn);
+				}
+				for (DuplexPluginFactory f :
+						pluginConfig.getDuplexFactories()) {
+					db.addTransport(txn, f.getId(), f.getMaxLatency());
+					managers.get(f.getId()).start(txn);
 				}
 			});
 		} catch (DbException e) {
@@ -101,9 +105,17 @@ class KeyManagerImpl implements KeyManager, Service, EventListener {
 	}
 
 	@Override
-	public Map<TransportId, KeySetId> addRotationKeys(
-			Transaction txn, ContactId c, SecretKey rootKey, long timestamp,
-			boolean alice, boolean active) throws DbException {
+	public KeySetId addRotationKeys(Transaction txn, ContactId c,
+			TransportId t, SecretKey rootKey, long timestamp, boolean alice,
+			boolean active) throws DbException {
+		return withManager(t, m ->
+				m.addRotationKeys(txn, c, rootKey, timestamp, alice, active));
+	}
+
+	@Override
+	public Map<TransportId, KeySetId> addRotationKeys(Transaction txn,
+			ContactId c, SecretKey rootKey, long timestamp, boolean alice,
+			boolean active) throws DbException {
 		Map<TransportId, KeySetId> ids = new HashMap<>();
 		for (Entry<TransportId, TransportKeyManager> e : managers.entrySet()) {
 			TransportId t = e.getKey();
@@ -137,7 +149,7 @@ class KeyManagerImpl implements KeyManager, Service, EventListener {
 			PendingContactId p, PublicKey theirPublicKey, KeyPair ourKeyPair)
 			throws DbException, GeneralSecurityException {
 		SecretKey staticMasterKey = transportCrypto
-					.deriveStaticMasterKey(theirPublicKey, ourKeyPair);
+				.deriveStaticMasterKey(theirPublicKey, ourKeyPair);
 		SecretKey rootKey =
 				transportCrypto.deriveHandshakeRootKey(staticMasterKey, true);
 		boolean alice = transportCrypto.isAlice(theirPublicKey, ourKeyPair);

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoder.java

@@ -0,0 +1,22 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+
+@NotNullByDefault
+interface MessageEncoder {
+
+	Message encodeKeyMessage(GroupId contactGroupId,
+			TransportId transportId, PublicKey publicKey);
+
+	Message encodeActivateMessage(GroupId contactGroupId,
+			TransportId transportId, MessageId previousMessageId);
+
+	BdfDictionary encodeMessageMetadata(TransportId transportId,
+			MessageType type, boolean local);
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoderImpl.java

@@ -0,0 +1,77 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_IS_SESSION;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_LOCAL;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+
+@Immutable
+@NotNullByDefault
+class MessageEncoderImpl implements MessageEncoder {
+
+	private final ClientHelper clientHelper;
+	private final Clock clock;
+
+	@Inject
+	MessageEncoderImpl(ClientHelper clientHelper, Clock clock) {
+		this.clientHelper = clientHelper;
+		this.clock = clock;
+	}
+
+	@Override
+	public Message encodeKeyMessage(GroupId contactGroupId,
+			TransportId transportId, PublicKey publicKey) {
+		BdfList body = BdfList.of(
+				KEY.getValue(),
+				transportId.getString(),
+				publicKey.getEncoded());
+		return encodeMessage(contactGroupId, body);
+	}
+
+	@Override
+	public Message encodeActivateMessage(GroupId contactGroupId,
+			TransportId transportId, MessageId previousMessageId) {
+		BdfList body = BdfList.of(
+				ACTIVATE.getValue(),
+				transportId.getString(),
+				previousMessageId);
+		return encodeMessage(contactGroupId, body);
+	}
+
+	@Override
+	public BdfDictionary encodeMessageMetadata(TransportId transportId,
+			MessageType type, boolean local) {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_IS_SESSION, false),
+				new BdfEntry(MSG_KEY_TRANSPORT_ID, transportId.getString()),
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, type.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, local));
+	}
+
+	private Message encodeMessage(GroupId contactGroupId, BdfList body) {
+		try {
+			return clientHelper.createMessage(contactGroupId,
+					clock.currentTimeMillis(), clientHelper.toByteArray(body));
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageType.java

@@ -0,0 +1,29 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum MessageType {
+
+	KEY(0),
+	ACTIVATE(1);
+
+	private final int value;
+
+	MessageType(int value) {
+		this.value = value;
+	}
+
+	int getValue() {
+		return value;
+	}
+
+	static MessageType fromValue(int value) throws FormatException {
+		for (MessageType t : values()) if (t.value == value) return t;
+		throw new FormatException();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/Session.java

@@ -0,0 +1,58 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class Session {
+
+	private final State state;
+	@Nullable
+	private final MessageId lastLocalMessageId;
+	@Nullable
+	private final KeyPair localKeyPair;
+	@Nullable
+	private final Long localTimestamp;
+	@Nullable
+	private final KeySetId keySetId;
+
+	Session(State state, @Nullable MessageId lastLocalMessageId,
+			@Nullable KeyPair localKeyPair, @Nullable Long localTimestamp,
+			@Nullable KeySetId keySetId) {
+		this.state = state;
+		this.lastLocalMessageId = lastLocalMessageId;
+		this.localKeyPair = localKeyPair;
+		this.localTimestamp = localTimestamp;
+		this.keySetId = keySetId;
+	}
+
+	State getState() {
+		return state;
+	}
+
+	@Nullable
+	MessageId getLastLocalMessageId() {
+		return lastLocalMessageId;
+	}
+
+	@Nullable
+	KeyPair getLocalKeyPair() {
+		return localKeyPair;
+	}
+
+	@Nullable
+	Long getLocalTimestamp() {
+		return localTimestamp;
+	}
+
+	@Nullable
+	KeySetId getKeySetId() {
+		return keySetId;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoder.java

@@ -0,0 +1,13 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+
+@NotNullByDefault
+interface SessionEncoder {
+
+	BdfDictionary encodeSession(Session s, TransportId transportId);
+
+	BdfDictionary getSessionQuery(TransportId transportId);
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoderImpl.java

@@ -0,0 +1,68 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.data.BdfDictionary.NULL_VALUE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_IS_SESSION;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_KEY_SET_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PRIVATE_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionEncoderImpl implements SessionEncoder {
+
+	@Inject
+	SessionEncoderImpl() {
+	}
+
+	@Override
+	public BdfDictionary encodeSession(Session s, TransportId transportId) {
+		BdfDictionary meta = new BdfDictionary();
+		meta.put(MSG_KEY_IS_SESSION, true);
+		meta.put(MSG_KEY_TRANSPORT_ID, transportId.getString());
+		meta.put(SESSION_KEY_STATE, s.getState().getValue());
+		putNullable(meta, SESSION_KEY_LAST_LOCAL_MESSAGE_ID,
+				s.getLastLocalMessageId());
+		KeyPair localKeyPair = s.getLocalKeyPair();
+		if (localKeyPair == null) {
+			meta.put(SESSION_KEY_LOCAL_PUBLIC_KEY, NULL_VALUE);
+			meta.put(SESSION_KEY_LOCAL_PRIVATE_KEY, NULL_VALUE);
+		} else {
+			meta.put(SESSION_KEY_LOCAL_PUBLIC_KEY,
+					localKeyPair.getPublic().getEncoded());
+			meta.put(SESSION_KEY_LOCAL_PRIVATE_KEY,
+					localKeyPair.getPrivate().getEncoded());
+		}
+		putNullable(meta, SESSION_KEY_LOCAL_TIMESTAMP, s.getLocalTimestamp());
+		KeySetId keySetId = s.getKeySetId();
+		if (keySetId == null) meta.put(SESSION_KEY_KEY_SET_ID, NULL_VALUE);
+		else meta.put(SESSION_KEY_KEY_SET_ID, keySetId.getInt());
+		return meta;
+	}
+
+	@Override
+	public BdfDictionary getSessionQuery(TransportId transportId) {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_IS_SESSION, true),
+				new BdfEntry(MSG_KEY_TRANSPORT_ID, transportId.getString()));
+	}
+
+	private void putNullable(BdfDictionary meta, String key,
+			@Nullable Object o) {
+		meta.put(key, o == null ? NULL_VALUE : o);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParser.java

@@ -0,0 +1,11 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface SessionParser {
+
+	Session parseSession(BdfDictionary meta) throws FormatException;
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParserImpl.java

@@ -0,0 +1,67 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_KEY_SET_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PRIVATE_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionParserImpl implements SessionParser {
+
+	private final TransportKeyAgreementCrypto crypto;
+
+	@Inject
+	SessionParserImpl(TransportKeyAgreementCrypto crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public Session parseSession(BdfDictionary meta) throws FormatException {
+		State state =
+				State.fromValue(meta.getLong(SESSION_KEY_STATE).intValue());
+
+		MessageId lastLocalMessageId = null;
+		byte[] lastLocalMessageIdBytes =
+				meta.getOptionalRaw(SESSION_KEY_LAST_LOCAL_MESSAGE_ID);
+		if (lastLocalMessageIdBytes != null) {
+			lastLocalMessageId = new MessageId(lastLocalMessageIdBytes);
+		}
+
+		KeyPair localKeyPair = null;
+		byte[] localPublicKeyBytes =
+				meta.getOptionalRaw(SESSION_KEY_LOCAL_PUBLIC_KEY);
+		byte[] localPrivateKeyBytes =
+				meta.getOptionalRaw(SESSION_KEY_LOCAL_PRIVATE_KEY);
+		if (localPublicKeyBytes != null && localPrivateKeyBytes != null) {
+			PublicKey pub = crypto.parsePublicKey(localPublicKeyBytes);
+			PrivateKey priv = crypto.parsePrivateKey(localPrivateKeyBytes);
+			localKeyPair = new KeyPair(pub, priv);
+		}
+
+		Long localTimestamp = meta.getOptionalLong(SESSION_KEY_LOCAL_TIMESTAMP);
+
+		KeySetId keySetId = null;
+		Long keySetIdLong = meta.getOptionalLong(SESSION_KEY_KEY_SET_ID);
+		if (keySetIdLong != null) {
+			keySetId = new KeySetId(keySetIdLong.intValue());
+		}
+
+		return new Session(state, lastLocalMessageId, localKeyPair,
+				localTimestamp, keySetId);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/State.java

@@ -0,0 +1,43 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum State {
+
+	/**
+	 * We've sent a key message and are awaiting the contact's key message.
+	 */
+	AWAIT_KEY(0),
+
+	/**
+	 * We've exchanged key messages, derived the transport keys and sent an
+	 * activate message, and now we're awaiting the contact's activate message.
+	 */
+	AWAIT_ACTIVATE(1),
+
+	/**
+	 * We've exchanged key messages and activate messages, and have derived and
+	 * activated the transport keys. This is the end state.
+	 */
+	ACTIVATED(2);
+
+	private final int value;
+
+	State(int value) {
+		this.value = value;
+	}
+
+	int getValue() {
+		return value;
+	}
+
+	static State fromValue(int value) throws FormatException {
+		for (State s : values()) if (s.value == value) return s;
+		throw new FormatException();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementConstants.java

@@ -0,0 +1,27 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface TransportKeyAgreementConstants {
+
+	String MSG_KEY_IS_SESSION = "isSession";
+	String MSG_KEY_MESSAGE_TYPE = "messageType";
+	String MSG_KEY_TRANSPORT_ID = "transportId";
+	String MSG_KEY_PUBLIC_KEY = "publicKey";
+	String MSG_KEY_LOCAL = "local";
+
+	String SESSION_KEY_STATE = "state";
+	String SESSION_KEY_LAST_LOCAL_MESSAGE_ID = "lastLocalMessageId";
+	String SESSION_KEY_LOCAL_PUBLIC_KEY = "localPublicKey";
+	String SESSION_KEY_LOCAL_PRIVATE_KEY = "localPrivateKey";
+	String SESSION_KEY_LOCAL_TIMESTAMP = "localTimestamp";
+	String SESSION_KEY_KEY_SET_ID = "keySetId";
+
+	/**
+	 * Label for deriving the root key from key pairs.
+	 */
+	String ROOT_KEY_LABEL =
+			"org.briarproject.bramble.transport.agreement/ROOT_KEY";
+
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCrypto.java

@@ -0,0 +1,23 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.GeneralSecurityException;
+
+@NotNullByDefault
+interface TransportKeyAgreementCrypto {
+
+	KeyPair generateKeyPair();
+
+	SecretKey deriveRootKey(KeyPair localKeyPair, PublicKey remotePublicKey)
+			throws GeneralSecurityException;
+
+	PublicKey parsePublicKey(byte[] encoded) throws FormatException;
+
+	PrivateKey parsePrivateKey(byte[] encoded) throws FormatException;
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCryptoImpl.java

@@ -0,0 +1,66 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.GeneralSecurityException;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.ROOT_KEY_LABEL;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementCryptoImpl implements TransportKeyAgreementCrypto {
+
+	private final CryptoComponent crypto;
+
+	@Inject
+	TransportKeyAgreementCryptoImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public KeyPair generateKeyPair() {
+		return crypto.generateAgreementKeyPair();
+	}
+
+	@Override
+	public SecretKey deriveRootKey(KeyPair localKeyPair,
+			PublicKey remotePublicKey) throws GeneralSecurityException {
+		byte[] theirPublic = remotePublicKey.getEncoded();
+		byte[] ourPublic = localKeyPair.getPublic().getEncoded();
+		boolean alice = compare(ourPublic, theirPublic) < 0;
+		byte[][] inputs = {
+				alice ? ourPublic : theirPublic,
+				alice ? theirPublic : ourPublic
+		};
+		return crypto.deriveSharedSecret(ROOT_KEY_LABEL, remotePublicKey,
+				localKeyPair, inputs);
+	}
+
+	@Override
+	public PublicKey parsePublicKey(byte[] encoded) throws FormatException {
+		try {
+			return crypto.getAgreementKeyParser().parsePublicKey(encoded);
+		} catch (GeneralSecurityException e) {
+			throw new FormatException();
+		}
+	}
+
+	@Override
+	public PrivateKey parsePrivateKey(byte[] encoded) throws FormatException {
+		try {
+			return crypto.getAgreementKeyParser().parsePrivateKey(encoded);
+		} catch (GeneralSecurityException e) {
+			throw new FormatException();
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImpl.java

@@ -0,0 +1,409 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.BdfIncomingMessageHook;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.client.ContactGroupFactory;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager.ContactHook;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataParser;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Metadata;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager.OpenDatabaseHook;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginConfig;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Group.Visibility;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeyManager;
+import org.briarproject.bramble.api.transport.KeySetId;
+import org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
+
+import java.security.GeneralSecurityException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.lang.Math.min;
+import static java.util.Collections.singletonMap;
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.DEFER;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.REJECT;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.State.ACTIVATED;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementManagerImpl extends BdfIncomingMessageHook
+		implements TransportKeyAgreementManager, OpenDatabaseHook, ContactHook,
+		ClientVersioningHook {
+
+	private static final Logger LOG =
+			getLogger(TransportKeyAgreementManagerImpl.class.getName());
+
+	private final ContactGroupFactory contactGroupFactory;
+	private final ClientVersioningManager clientVersioningManager;
+	private final IdentityManager identityManager;
+	private final KeyManager keyManager;
+	private final MessageEncoder messageEncoder;
+	private final SessionEncoder sessionEncoder;
+	private final SessionParser sessionParser;
+	private final TransportKeyAgreementCrypto crypto;
+
+	private final List<TransportId> transports;
+	private final Group localGroup;
+
+	@Inject
+	TransportKeyAgreementManagerImpl(
+			DatabaseComponent db,
+			ClientHelper clientHelper,
+			MetadataParser metadataParser,
+			ContactGroupFactory contactGroupFactory,
+			ClientVersioningManager clientVersioningManager,
+			IdentityManager identityManager,
+			KeyManager keyManager,
+			MessageEncoder messageEncoder,
+			SessionEncoder sessionEncoder,
+			SessionParser sessionParser,
+			TransportKeyAgreementCrypto crypto,
+			PluginConfig config) {
+		super(db, clientHelper, metadataParser);
+		this.contactGroupFactory = contactGroupFactory;
+		this.clientVersioningManager = clientVersioningManager;
+		this.identityManager = identityManager;
+		this.keyManager = keyManager;
+		this.messageEncoder = messageEncoder;
+		this.sessionEncoder = sessionEncoder;
+		this.sessionParser = sessionParser;
+		this.crypto = crypto;
+		transports = new ArrayList<>();
+		for (DuplexPluginFactory duplex : config.getDuplexFactories()) {
+			transports.add(duplex.getId());
+		}
+		for (SimplexPluginFactory simplex : config.getSimplexFactories()) {
+			transports.add(simplex.getId());
+		}
+		localGroup = contactGroupFactory.createLocalGroup(CLIENT_ID,
+				MAJOR_VERSION);
+	}
+
+	@Override
+	public void onDatabaseOpened(Transaction txn) throws DbException {
+		Collection<Contact> contacts = db.getContacts(txn);
+		if (!db.containsGroup(txn, localGroup.getId())) {
+			db.addGroup(txn, localGroup);
+			// Set things up for any pre-existing contacts
+			for (Contact c : contacts) addingContact(txn, c);
+		}
+		// Find any contacts and transports that need keys
+		Map<ContactId, Collection<TransportId>> transportsWithKeys =
+				db.getTransportsWithKeys(txn);
+		for (Contact c : contacts) {
+			Collection<TransportId> withKeys =
+					transportsWithKeys.get(c.getId());
+			for (TransportId t : transports) {
+				if (withKeys == null || !withKeys.contains(t)) {
+					// We need keys for this contact and transport
+					GroupId contactGroupId = getContactGroup(c).getId();
+					SavedSession ss = loadSession(txn, contactGroupId, t);
+					if (ss == null) {
+						// Start a session by sending our key message
+						startSession(txn, contactGroupId, t);
+					}
+				}
+			}
+		}
+	}
+
+	@Override
+	public void addingContact(Transaction txn, Contact c) throws DbException {
+		// Create a group to share with the contact
+		Group g = getContactGroup(c);
+		db.addGroup(txn, g);
+		// Attach the contact ID to the group
+		clientHelper.setContactId(txn, g.getId(), c.getId());
+		// Apply the client's visibility to the contact group
+		Visibility client = clientVersioningManager.getClientVisibility(txn,
+				c.getId(), CLIENT_ID, MAJOR_VERSION);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), client);
+	}
+
+	@Override
+	public void removingContact(Transaction txn, Contact c) throws DbException {
+		db.removeGroup(txn, getContactGroup(c));
+	}
+
+	@Override
+	public void onClientVisibilityChanging(Transaction txn, Contact c,
+			Visibility v) throws DbException {
+		// Apply the client's visibility to the contact group
+		Group g = getContactGroup(c);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), v);
+	}
+
+	@Override
+	protected DeliveryAction incomingMessage(Transaction txn, Message m,
+			BdfList body, BdfDictionary meta)
+			throws DbException, FormatException {
+		MessageType type = MessageType.fromValue(
+				meta.getLong(MSG_KEY_MESSAGE_TYPE).intValue());
+		TransportId t = new TransportId(meta.getString(MSG_KEY_TRANSPORT_ID));
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Received " + type + " message for " + t);
+		}
+		if (!transports.contains(t)) {
+			// Defer handling the message until we support the transport
+			return DEFER;
+		}
+		SavedSession ss = loadSession(txn, m.getGroupId(), t);
+		if (type == KEY) return handleKeyMessage(txn, t, m, meta, ss);
+		else if (type == ACTIVATE) return handleActivateMessage(txn, t, ss);
+		else throw new AssertionError();
+	}
+
+	private DeliveryAction handleKeyMessage(Transaction txn, TransportId t,
+			Message m, BdfDictionary meta, @Nullable SavedSession ss)
+			throws DbException, FormatException {
+		ContactId c = clientHelper.getContactId(txn, m.getGroupId());
+		boolean haveKeys = db.containsTransportKeys(txn, c, t);
+		if (ss == null) {
+			if (haveKeys) {
+				// We have keys but no session, so we must have derived keys
+				// when adding the contact. If the contact didn't support
+				// the transport when they added us, they wouldn't have
+				// derived keys at that time. If they later added support for
+				// the transport then they would have started a session, so a
+				// key message is valid in this case
+				return handleKeyMessageForNewSession(txn, c, t, m, meta);
+			} else {
+				// We don't have keys, so we should have created a session at
+				// startup
+				throw new IllegalStateException();
+			}
+		} else if (ss.session.getState() == AWAIT_KEY) {
+			if (haveKeys) {
+				// We have keys, so we shouldn't be in the AWAIT_KEY state,
+				// even if the contact didn't derive keys when adding us and
+				// later started a session
+				throw new IllegalStateException();
+			} else {
+				// This is the key message we're waiting for
+				return handleKeyMessageForExistingSession(txn, c, t, m, meta,
+						ss);
+			}
+		} else {
+			return REJECT; // Not valid in this state
+		}
+	}
+
+	private DeliveryAction handleActivateMessage(Transaction txn,
+			TransportId t, @Nullable SavedSession ss) throws DbException {
+		if (ss != null && ss.session.getState() == AWAIT_ACTIVATE) {
+			// Activate the keys and finish the session
+			KeySetId keySetId = requireNonNull(ss.session.getKeySetId());
+			keyManager.activateKeys(txn, singletonMap(t, keySetId));
+			Session session = new Session(ACTIVATED,
+					ss.session.getLastLocalMessageId(), null, null, null);
+			saveSession(txn, t, ss.storageId, session);
+			return ACCEPT_DO_NOT_SHARE;
+		} else {
+			return REJECT; // Not valid in this state
+		}
+	}
+
+	private DeliveryAction handleKeyMessageForNewSession(Transaction txn,
+			ContactId c, TransportId t, Message m, BdfDictionary meta)
+			throws DbException, FormatException {
+		KeyPair localKeyPair = crypto.generateKeyPair();
+		PublicKey remotePublicKey =
+				crypto.parsePublicKey(meta.getRaw(MSG_KEY_PUBLIC_KEY));
+		Message keyMessage = sendKeyMessage(txn, m.getGroupId(), t,
+				localKeyPair.getPublic());
+		long minTimestamp = min(keyMessage.getTimestamp(), m.getTimestamp());
+		SecretKey rootKey;
+		try {
+			rootKey = crypto.deriveRootKey(localKeyPair, remotePublicKey);
+		} catch (GeneralSecurityException e) {
+			return REJECT; // Invalid public key
+		}
+		boolean alice = isLocalPartyAlice(txn, db.getContact(txn, c));
+		KeySetId keySetId = keyManager.addRotationKeys(txn, c, t, rootKey,
+				minTimestamp, alice, false);
+		Message activateMessage =
+				sendActivateMessage(txn, m.getGroupId(), t, keyMessage.getId());
+		Session session = new Session(AWAIT_ACTIVATE, activateMessage.getId(),
+				null, null, keySetId);
+		saveNewSession(txn, m.getGroupId(), t, session);
+		return ACCEPT_DO_NOT_SHARE;
+	}
+
+	private DeliveryAction handleKeyMessageForExistingSession(Transaction txn,
+			ContactId c, TransportId t, Message m, BdfDictionary meta,
+			SavedSession ss) throws DbException, FormatException {
+		KeyPair localKeyPair = requireNonNull(ss.session.getLocalKeyPair());
+		PublicKey remotePublicKey =
+				crypto.parsePublicKey(meta.getRaw(MSG_KEY_PUBLIC_KEY));
+		long localTimestamp = requireNonNull(ss.session.getLocalTimestamp());
+		long minTimestamp = min(localTimestamp, m.getTimestamp());
+		SecretKey rootKey;
+		try {
+			rootKey = crypto.deriveRootKey(localKeyPair, remotePublicKey);
+		} catch (GeneralSecurityException e) {
+			return REJECT; // Invalid public key
+		}
+		boolean alice = isLocalPartyAlice(txn, db.getContact(txn, c));
+		KeySetId keySetId = keyManager.addRotationKeys(txn, c, t, rootKey,
+				minTimestamp, alice, false);
+		MessageId previousMessageId =
+				requireNonNull(ss.session.getLastLocalMessageId());
+		Message activateMessage =
+				sendActivateMessage(txn, m.getGroupId(), t, previousMessageId);
+		Session session = new Session(AWAIT_ACTIVATE, activateMessage.getId(),
+				null, null, keySetId);
+		saveSession(txn, t, ss.storageId, session);
+		return ACCEPT_DO_NOT_SHARE;
+	}
+
+	private void startSession(Transaction txn, GroupId contactGroupId,
+			TransportId t) throws DbException {
+		KeyPair localKeyPair = crypto.generateKeyPair();
+		Message keyMessage = sendKeyMessage(txn, contactGroupId, t,
+				localKeyPair.getPublic());
+		Session session = new Session(AWAIT_KEY, keyMessage.getId(),
+				localKeyPair, keyMessage.getTimestamp(), null);
+		saveNewSession(txn, contactGroupId, t, session);
+	}
+
+	@Nullable
+	private SavedSession loadSession(Transaction txn, GroupId contactGroupId,
+			TransportId t) throws DbException {
+		try {
+			BdfDictionary query = sessionEncoder.getSessionQuery(t);
+			Collection<MessageId> ids =
+					clientHelper.getMessageIds(txn, contactGroupId, query);
+			if (ids.size() > 1) throw new DbException();
+			if (ids.isEmpty()) {
+				if (LOG.isLoggable(INFO)) LOG.info("No session for " + t);
+				return null;
+			}
+			MessageId storageId = ids.iterator().next();
+			BdfDictionary bdfSession =
+					clientHelper.getMessageMetadataAsDictionary(txn, storageId);
+			Session session = sessionParser.parseSession(bdfSession);
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Loaded session in state " + session.getState()
+						+ " for " + t);
+			}
+			return new SavedSession(session, storageId);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		}
+	}
+
+	private void saveNewSession(Transaction txn, GroupId contactGroupId,
+			TransportId t, Session session) throws DbException {
+		Message m =
+				clientHelper.createMessageForStoringMetadata(contactGroupId);
+		db.addLocalMessage(txn, m, new Metadata(), false, false);
+		MessageId storageId = m.getId();
+		saveSession(txn, t, storageId, session);
+	}
+
+	private void saveSession(Transaction txn, TransportId t,
+			MessageId storageId, Session session) throws DbException {
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Saving session in state " + session.getState()
+					+ " for " + t);
+		}
+		BdfDictionary meta = sessionEncoder.encodeSession(session, t);
+		try {
+			clientHelper.mergeMessageMetadata(txn, storageId, meta);
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+
+	private Message sendKeyMessage(Transaction txn, GroupId contactGroupId,
+			TransportId t, PublicKey publicKey) throws DbException {
+		Message m = messageEncoder.encodeKeyMessage(contactGroupId, t,
+				publicKey);
+		sendMessage(txn, t, m, KEY);
+		return m;
+	}
+
+	private Message sendActivateMessage(Transaction txn,
+			GroupId contactGroupId, TransportId t, MessageId previousMessageId)
+			throws DbException {
+		Message m = messageEncoder.encodeActivateMessage(contactGroupId, t,
+				previousMessageId);
+		sendMessage(txn, t, m, ACTIVATE);
+		return m;
+	}
+
+	private void sendMessage(Transaction txn, TransportId t, Message m,
+			MessageType type) throws DbException {
+		BdfDictionary meta =
+				messageEncoder.encodeMessageMetadata(t, type, true);
+		try {
+			clientHelper.addLocalMessage(txn, m, meta, true, false);
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+
+	private Group getContactGroup(Contact c) {
+		return contactGroupFactory.createContactGroup(CLIENT_ID,
+				MAJOR_VERSION, c);
+	}
+
+	private boolean isLocalPartyAlice(Transaction txn, Contact c)
+			throws DbException {
+		Author local = identityManager.getLocalAuthor(txn);
+		Author remote = c.getAuthor();
+		return compare(local.getId().getBytes(), remote.getId().getBytes()) < 0;
+	}
+
+	private static class SavedSession {
+
+		private final Session session;
+		private final MessageId storageId;
+
+		private SavedSession(Session session, MessageId storageId) {
+			this.session = session;
+			this.storageId = storageId;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementModule.java

@@ -0,0 +1,83 @@
+package org.briarproject.bramble.transport.agreement;
+
+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.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.sync.validation.ValidationManager;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.CLIENT_ID;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MAJOR_VERSION;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MINOR_VERSION;
+
+@Module
+public class TransportKeyAgreementModule {
+
+	public static class EagerSingletons {
+		@Inject
+		TransportKeyAgreementManager transportKeyAgreementManager;
+		@Inject
+		TransportKeyAgreementValidator transportKeyAgreementValidator;
+	}
+
+	@Provides
+	@Singleton
+	TransportKeyAgreementManager provideTransportKeyAgreementManager(
+			LifecycleManager lifecycleManager,
+			ValidationManager validationManager,
+			ContactManager contactManager,
+			ClientVersioningManager clientVersioningManager,
+			TransportKeyAgreementManagerImpl transportKeyAgreementManager) {
+		lifecycleManager.registerOpenDatabaseHook(transportKeyAgreementManager);
+		validationManager.registerIncomingMessageHook(CLIENT_ID,
+				MAJOR_VERSION, transportKeyAgreementManager);
+		contactManager.registerContactHook(transportKeyAgreementManager);
+		clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
+				MINOR_VERSION, transportKeyAgreementManager);
+		return transportKeyAgreementManager;
+	}
+
+	@Provides
+	@Singleton
+	TransportKeyAgreementValidator provideTransportKeyAgreementValidator(
+			ClientHelper clientHelper, MetadataEncoder metadataEncoder,
+			Clock clock, MessageEncoder messageEncoder,
+			ValidationManager validationManager) {
+		TransportKeyAgreementValidator validator =
+				new TransportKeyAgreementValidator(clientHelper,
+						metadataEncoder, clock, messageEncoder);
+		validationManager.registerMessageValidator(CLIENT_ID, MAJOR_VERSION,
+				validator);
+		return validator;
+	}
+
+	@Provides
+	MessageEncoder provideMessageEncoder(MessageEncoderImpl messageEncoder) {
+		return messageEncoder;
+	}
+
+	@Provides
+	SessionEncoder provideSessionEncoder(SessionEncoderImpl sessionEncoder) {
+		return sessionEncoder;
+	}
+
+	@Provides
+	SessionParser provideSessionParser(SessionParserImpl sessionParser) {
+		return sessionParser;
+	}
+
+	@Provides
+	TransportKeyAgreementCrypto provideTransportKeyAgreementCrypto(
+			TransportKeyAgreementCryptoImpl transportKeyAgreementCrypto) {
+		return transportKeyAgreementCrypto;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementValidator.java

@@ -0,0 +1,77 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.BdfMessageContext;
+import org.briarproject.bramble.api.client.BdfMessageValidator;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+
+import javax.annotation.concurrent.Immutable;
+
+import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_AGREEMENT_PUBLIC_KEY_BYTES;
+import static org.briarproject.bramble.api.plugin.TransportId.MAX_TRANSPORT_ID_LENGTH;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.util.ValidationUtils.checkLength;
+import static org.briarproject.bramble.util.ValidationUtils.checkSize;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementValidator extends BdfMessageValidator {
+
+	private final MessageEncoder messageEncoder;
+
+	TransportKeyAgreementValidator(ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder, Clock clock,
+			MessageEncoder messageEncoder) {
+		super(clientHelper, metadataEncoder, clock);
+		this.messageEncoder = messageEncoder;
+	}
+
+	@Override
+	protected BdfMessageContext validateMessage(Message m, Group g,
+			BdfList body) throws FormatException {
+		MessageType type = MessageType.fromValue(body.getLong(0).intValue());
+		if (type == KEY) return validateKeyMessage(body);
+		else if (type == ACTIVATE) return validateActivateMessage(body);
+		else throw new AssertionError();
+	}
+
+	private BdfMessageContext validateKeyMessage(BdfList body)
+			throws FormatException {
+		// Message type, transport ID, public key
+		checkSize(body, 3);
+		String transportId = body.getString(1);
+		checkLength(transportId, 1, MAX_TRANSPORT_ID_LENGTH);
+		byte[] publicKey = body.getRaw(2);
+		checkLength(publicKey, 1, MAX_AGREEMENT_PUBLIC_KEY_BYTES);
+		BdfDictionary meta = messageEncoder.encodeMessageMetadata(
+				new TransportId(transportId), KEY, false);
+		meta.put(MSG_KEY_PUBLIC_KEY, publicKey);
+		return new BdfMessageContext(meta);
+	}
+
+	private BdfMessageContext validateActivateMessage(BdfList body)
+			throws FormatException {
+		// Message type, transport ID, previous message ID
+		checkSize(body, 3);
+		String transportId = body.getString(1);
+		checkLength(transportId, 1, MAX_TRANSPORT_ID_LENGTH);
+		byte[] previousMessageId = body.getRaw(2);
+		checkLength(previousMessageId, MessageId.LENGTH);
+		BdfDictionary meta = messageEncoder.encodeMessageMetadata(
+				new TransportId(transportId), ACTIVATE, false);
+		MessageId dependency = new MessageId(previousMessageId);
+		return new BdfMessageContext(meta, singletonList(dependency));
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/versioning/ClientVersioningManagerImpl.java

@@ -50,6 +50,7 @@ import static java.util.Collections.emptyList;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 import static org.briarproject.bramble.versioning.ClientVersioningConstants.MSG_KEY_LOCAL;
 import static org.briarproject.bramble.versioning.ClientVersioningConstants.MSG_KEY_UPDATE_VERSION;
 
@@ -173,8 +174,8 @@ class ClientVersioningManagerImpl implements ClientVersioningManager,
 	}
 
 	@Override
-	public boolean incomingMessage(Transaction txn, Message m, Metadata meta)
-			throws DbException, InvalidMessageException {
+	public DeliveryAction incomingMessage(Transaction txn, Message m,
+			Metadata meta) throws DbException, InvalidMessageException {
 		try {
 			// Parse the new remote update
 			Update newRemoteUpdate = parseUpdate(clientHelper.toList(m));
@@ -187,7 +188,7 @@ class ClientVersioningManagerImpl implements ClientVersioningManager,
 					&& latest.remote.updateVersion > newRemoteUpdateVersion) {
 				db.deleteMessage(txn, m.getId());
 				db.deleteMessageMetadata(txn, m.getId());
-				return false;
+				return ACCEPT_DO_NOT_SHARE;
 			}
 			// Load and parse the latest local update
 			if (latest.local == null) throw new DbException();
@@ -241,7 +242,7 @@ class ClientVersioningManagerImpl implements ClientVersioningManager,
 		} catch (FormatException e) {
 			throw new InvalidMessageException(e);
 		}
-		return false;
+		return ACCEPT_DO_NOT_SHARE;
 	}
 
 	private void storeClientVersions(Transaction txn,

bramble-core/src/main/resources/bridges

@@ -1,13 +1,12 @@
 Bridge obfs4 192.95.36.142:443 CDF2E852BF539B82BD10E27E9115A31734E378C2 cert=qUVQ0srL1JI/vO6V6m/24anYXiJD3QP2HgzUKQtQ7GRqqUvs7P+tG43RtAqdhLOALP7DJQ iat-mode=1
 Bridge obfs4 38.229.1.78:80 C8CBDB2464FC9804A69531437BCF2BE31FDD2EE4 cert=Hmyfd2ev46gGY7NoVxA9ngrPF2zCZtzskRTzoWXbxNkzeVnGFPWmrTtILRyqCTjHR+s9dg iat-mode=1
+Bridge obfs4 38.229.33.83:80 0BAC39417268B96B9F514E7F63FA6FBA1A788955 cert=VwEFpk9F/UN9JED7XpG1XOjm/O8ZCXK80oPecgWnNDZDv5pdkhq1OpbAH0wNqOT6H6BmRQ iat-mode=1
 Bridge obfs4 37.218.245.14:38224 D9A82D2F9C2F65A18407B1D2B764F130847F8B5D cert=bjRaMrr1BRiAW8IE9U5z27fQaYgOhX1UCmOpg2pFpoMvo6ZgQMzLsaTzzQNTlm7hNcb+Sg iat-mode=0
-Bridge obfs4 85.31.186.98:443 011F2599C0E9B27EE74B353155E244813763C3E5 cert=ayq0XzCwhpdysn5o0EyDUbmSOx3X/oTEbzDMvczHOdBJKlvIdHHLJGkZARtT4dcBFArPPg iat-mode=0
 Bridge obfs4 85.31.186.26:443 91A6354697E6B02A386312F68D82CF86824D3606 cert=PBwr+S8JTVZo6MPdHnkTwXJPILWADLqfMGoVvhZClMq/Urndyd42BwX9YFJHZnBB3H0XCw iat-mode=0
 Bridge obfs4 193.11.166.194:27015 2D82C2E354D531A68469ADF7F878FA6060C6BACA cert=4TLQPJrTSaDffMK7Nbao6LC7G9OW/NHkUwIdjLSS3KYf0Nv4/nQiiI8dY2TcsQx01NniOg iat-mode=0
 Bridge obfs4 193.11.166.194:27020 86AC7B8D430DAC4117E9F42C9EAED18133863AAF cert=0LDeJH4JzMDtkJJrFphJCiPqKx7loozKN7VNfuukMGfHO0Z8OGdzHVkhVAOfo1mUdv9cMg iat-mode=0
 Bridge obfs4 193.11.166.194:27025 1AE2C08904527FEA90C4C4F8C1083EA59FBC6FAF cert=ItvYZzW5tn6v3G4UnQa6Qz04Npro6e81AP70YujmK/KXwDFPTs3aHXcHp4n8Vt6w/bv8cA iat-mode=0
 Bridge obfs4 209.148.46.65:443 74FAD13168806246602538555B5521A0383A1875 cert=ssH+9rP8dG2NLDN2XuFw63hIO/9MNNinLmxQDpVa+7kTOa9/m+tGWT1SmSYpQ9uTBGa6Hw iat-mode=0
-Bridge obfs4 146.57.248.225:22 10A6CD36A537FCE513A322361547444B393989F0 cert=K1gDtDAIcUfeLqbstggjIw2rtgIKqdIhUlHp82XRqNSq/mtAjp1BIC9vHKJ2FAEpGssTPw iat-mode=0
 Bridge obfs4 45.145.95.6:27015 C5B7CD6946FF10C5B3E89691A7D3F2C122D2117C cert=TD7PbUO0/0k6xYHMPW3vJxICfkMZNdkRrb63Zhl5j9dW3iRGiCx0A7mPhe5T2EDzQ35+Zw iat-mode=0
 Bridge obfs4 51.222.13.177:80 5EDAC3B810E12B01F6FD8050D2FD3E277B289A08 cert=2uplIpLQ0q9+0qMFrK5pkaYRDOe460LL9WHBvatgkuRr/SL31wBOEupaMMJ6koRE6Ld0ew iat-mode=0
 Bridge obfs4 78.46.188.239:37356 5A2D2F4158D0453E00C7C176978D3F41D69C45DB cert=3c0SwxpOisbohNxEc4tb875RVW8eOu1opRTVXJhafaKA/PNNtI7ElQIVOVZg1AdL5bxGCw iat-mode=0

bramble-core/src/test/java/org/briarproject/bramble/db/DatabaseComponentImplTest.java

@@ -298,11 +298,11 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 			throws Exception {
 		context.checking(new Expectations() {{
 			// Check whether the contact is in the DB (which it's not)
-			exactly(18).of(database).startTransaction();
+			exactly(19).of(database).startTransaction();
 			will(returnValue(txn));
-			exactly(18).of(database).containsContact(txn, contactId);
+			exactly(19).of(database).containsContact(txn, contactId);
 			will(returnValue(false));
-			exactly(18).of(database).abortTransaction(txn);
+			exactly(19).of(database).abortTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, eventBus,
 				eventExecutor, shutdownManager);
@@ -349,7 +349,7 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 		}
 
 		try {
-			db.transaction(false, transaction ->
+			db.transaction(true, transaction ->
 					db.getContact(transaction, contactId));
 			fail();
 		} catch (NoSuchContactException expected) {
@@ -357,7 +357,15 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 		}
 
 		try {
-			db.transaction(false, transaction ->
+			db.transaction(true, transaction ->
+					db.getUnackedMessageBytesToSend(transaction, contactId));
+			fail();
+		} catch (NoSuchContactException expected) {
+			// Expected
+		}
+
+		try {
+			db.transaction(true, transaction ->
 					db.getMessageStatus(transaction, contactId, groupId));
 			fail();
 		} catch (NoSuchContactException expected) {
@@ -365,7 +373,7 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 		}
 
 		try {
-			db.transaction(false, transaction ->
+			db.transaction(true, transaction ->
 					db.getMessageStatus(transaction, contactId, messageId));
 			fail();
 		} catch (NoSuchContactException expected) {
@@ -373,7 +381,7 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 		}
 
 		try {
-			db.transaction(false, transaction ->
+			db.transaction(true, transaction ->
 					db.getGroupVisibility(transaction, contactId, groupId));
 			fail();
 		} catch (NoSuchContactException expected) {
@@ -381,7 +389,7 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 		}
 
 		try {
-			db.transaction(false, transaction ->
+			db.transaction(true, transaction ->
 					db.getSyncVersions(transaction, contactId));
 			fail();
 		} catch (NoSuchContactException expected) {

bramble-core/src/test/java/org/briarproject/bramble/db/JdbcDatabaseTest.java

@@ -41,8 +41,12 @@ import org.junit.Test;
 
 import java.io.File;
 import java.sql.Connection;
+import java.sql.Driver;
+import java.sql.DriverManager;
+import java.sql.SQLException;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Enumeration;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
@@ -53,6 +57,7 @@ import java.util.concurrent.atomic.AtomicLong;
 
 import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
+import static java.util.Collections.singleton;
 import static java.util.Collections.singletonList;
 import static java.util.Collections.singletonMap;
 import static java.util.concurrent.TimeUnit.SECONDS;
@@ -218,18 +223,13 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message, DELIVERED, true, false, null);
 
 		// The contact has not seen the message, so it should be sendable
-		Collection<MessageId> ids =
-				db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		assertOneMessageToSendEagerly(db, txn);
+		assertOneMessageToSendLazily(db, txn);
 
 		// Changing the status to seen = true should make the message unsendable
 		db.raiseSeenFlag(txn, contactId, messageId);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendEagerly(db, txn);
+		assertNothingToSendLazily(db, txn);
 
 		db.commitTransaction(txn);
 		db.close();
@@ -249,32 +249,23 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message, UNKNOWN, true, false, null);
 
 		// The message has not been validated, so it should not be sendable
-		Collection<MessageId> ids = db.getMessagesToSend(txn, contactId,
-				ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Marking the message delivered should make it sendable
 		db.setMessageState(txn, messageId, DELIVERED);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		assertOneMessageToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
 
 		// Marking the message invalid should make it unsendable
 		db.setMessageState(txn, messageId, INVALID);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Marking the message pending should make it unsendable
 		db.setMessageState(txn, messageId, PENDING);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		db.commitTransaction(txn);
 		db.close();
@@ -293,39 +284,28 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message, DELIVERED, true, false, null);
 
 		// The group is invisible, so the message should not be sendable
-		Collection<MessageId> ids = db.getMessagesToSend(txn, contactId,
-				ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Making the group visible should not make the message sendable
 		db.addGroupVisibility(txn, contactId, groupId, false);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Sharing the group should make the message sendable
 		db.setGroupVisibility(txn, contactId, groupId, true);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		assertOneMessageToSendEagerly(db, txn);
+		assertOneMessageToSendLazily(db, txn);
 
 		// Unsharing the group should make the message unsendable
 		db.setGroupVisibility(txn, contactId, groupId, false);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Making the group invisible should make the message unsendable
 		db.removeGroupVisibility(txn, contactId, groupId);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		db.commitTransaction(txn);
 		db.close();
@@ -345,18 +325,13 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message, DELIVERED, false, false, null);
 
 		// The message is not shared, so it should not be sendable
-		Collection<MessageId> ids = db.getMessagesToSend(txn, contactId,
-				ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Sharing the message should make it sendable
 		db.setMessageShared(txn, messageId, true);
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		assertOneMessageToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
 
 		db.commitTransaction(txn);
 		db.close();
@@ -376,10 +351,13 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message, DELIVERED, true, false, null);
 
 		// The message is sendable, but too large to send
+		assertOneMessageToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
 		Collection<MessageId> ids =
 				db.getMessagesToSend(txn, contactId, message.getRawLength() - 1,
 						MAX_LATENCY);
 		assertTrue(ids.isEmpty());
+
 		// The message is just the right size to send
 		ids = db.getMessagesToSend(txn, contactId, message.getRawLength(),
 				MAX_LATENCY);
@@ -401,6 +379,12 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addGroup(txn, group);
 		db.addGroupVisibility(txn, contactId, groupId, false);
 
+		// Initially there should be nothing to send
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
+
 		// Add some messages to ack
 		Message message1 = getMessage(groupId);
 		MessageId messageId1 = message1.getId();
@@ -408,6 +392,10 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addMessage(txn, message1, DELIVERED, true, false, contactId);
 
 		// Both message IDs should be returned
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
 		Collection<MessageId> ids = db.getMessagesToAck(txn, contactId, 1234);
 		assertEquals(asList(messageId, messageId1), ids);
 
@@ -415,6 +403,10 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.lowerAckFlag(txn, contactId, asList(messageId, messageId1));
 
 		// Both message IDs should have been removed
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
 		assertEquals(emptyList(), db.getMessagesToAck(txn,
 				contactId, 1234));
 
@@ -423,6 +415,10 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.raiseAckFlag(txn, contactId, messageId1);
 
 		// Both message IDs should be returned
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
 		ids = db.getMessagesToAck(txn, contactId, 1234);
 		assertEquals(asList(messageId, messageId1), ids);
 
@@ -443,22 +439,25 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.addGroupVisibility(txn, contactId, groupId, true);
 		db.addMessage(txn, message, DELIVERED, true, false, null);
 
-		// Retrieve the message from the database and mark it as sent
-		Collection<MessageId> ids = db.getMessagesToSend(txn, contactId,
-				ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		// The message should be sendable via lazy or eager retransmission
+		assertOneMessageToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
+
+		// Mark the message as sent
 		db.updateExpiryTimeAndEta(txn, contactId, messageId, MAX_LATENCY);
 
-		// The message should no longer be sendable
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		// The message should no longer be sendable via lazy retransmission,
+		// but it should still be sendable via eager retransmission
+		assertNothingToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
 
-		// Pretend that the message was acked
+		// Mark the message as acked
 		db.raiseSeenFlag(txn, contactId, messageId);
 
-		// The message still should not be sendable
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		// The message still should not be sendable via lazy or eager
+		// retransmission
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		db.commitTransaction(txn);
 		db.close();
@@ -672,7 +671,9 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		Connection txn = db.startTransaction();
 
 		// Initially there should be no transport keys in the database
+		assertFalse(db.containsTransportKeys(txn, contactId, transportId));
 		assertEquals(emptyList(), db.getTransportKeys(txn, transportId));
+		assertTrue(db.getTransportsWithKeys(txn).isEmpty());
 
 		// Add the contact, the transport and the transport keys
 		db.addIdentity(txn, identity);
@@ -683,6 +684,7 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		assertEquals(keySetId1, db.addTransportKeys(txn, contactId, keys1));
 
 		// Retrieve the transport keys
+		assertTrue(db.containsTransportKeys(txn, contactId, transportId));
 		Collection<TransportKeySet> allKeys =
 				db.getTransportKeys(txn, transportId);
 		assertEquals(2, allKeys.size());
@@ -695,6 +697,8 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 				assertKeysEquals(keys1, ks.getKeys());
 			}
 		}
+		assertEquals(singletonMap(contactId, singletonList(transportId)),
+				db.getTransportsWithKeys(txn));
 
 		// Update the transport keys
 		TransportKeys updated = createTransportKeys(timePeriod + 1, active);
@@ -706,6 +710,7 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 				null, updated1));
 
 		// Retrieve the transport keys again
+		assertTrue(db.containsTransportKeys(txn, contactId, transportId));
 		allKeys = db.getTransportKeys(txn, transportId);
 		assertEquals(2, allKeys.size());
 		for (TransportKeySet ks : allKeys) {
@@ -717,10 +722,14 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 				assertKeysEquals(updated1, ks.getKeys());
 			}
 		}
+		assertEquals(singletonMap(contactId, singletonList(transportId)),
+				db.getTransportsWithKeys(txn));
 
 		// Removing the contact should remove the transport keys
 		db.removeContact(txn, contactId);
+		assertFalse(db.containsTransportKeys(txn, contactId, transportId));
 		assertEquals(emptyList(), db.getTransportKeys(txn, transportId));
+		assertTrue(db.getTransportsWithKeys(txn).isEmpty());
 
 		db.commitTransaction(txn);
 		db.close();
@@ -1921,11 +1930,8 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		assertTrue(db.containsVisibleMessage(txn, contactId, messageId));
 
 		// The message should be sendable
-		Collection<MessageId> ids = db.getMessagesToSend(txn, contactId,
-				ONE_MEGABYTE, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertEquals(singletonList(messageId), ids);
+		assertOneMessageToSendLazily(db, txn);
+		assertOneMessageToSendEagerly(db, txn);
 
 		// The message should be available
 		Message m = db.getMessage(txn, messageId);
@@ -1941,10 +1947,8 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		assertTrue(db.containsVisibleMessage(txn, contactId, messageId));
 
 		// The message should not be sendable
-		ids = db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
-		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
-		assertTrue(ids.isEmpty());
+		assertNothingToSendLazily(db, txn);
+		assertNothingToSendEagerly(db, txn);
 
 		// Requesting the message should throw an exception
 		try {
@@ -2346,6 +2350,67 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 		db.close();
 	}
 
+
+	@Test
+	public void testShutdownGracefully() throws Exception {
+		Database<Connection> db = open(false);
+		db.close();
+		open(true);
+		assertFalse(db.wasDirtyOnInitialisation());
+	}
+
+	@Test
+	public void testShutdownDirty() throws Exception {
+		Database<Connection> db = open(false);
+
+		// We want to simulate a dirty shutdown here which would normally be
+		// caused by an empty battery or by force closing the Android app.
+		// As there is no obvious way to simulate this, we're artificially
+		// causing an SqlException during close() here by unloading the JDBC
+		// drivers.
+		List<String> unloadedDrivers = unloadDrivers();
+
+		try {
+			db.close();
+			fail();
+		} catch (Exception e) {
+			// continue
+			e.printStackTrace();
+		}
+
+		// Reloading drivers to continue so that we're able to work with the
+		// database again.
+		reloadDrivers(unloadedDrivers);
+
+		db = open(true);
+		assertTrue(db.wasDirtyOnInitialisation());
+	}
+
+	@Test
+	public void testShutdownDirtyThenGracefully() throws Exception {
+		Database<Connection> db = open(false);
+
+		// Simulating a dirty shutdown here, look at #testShutdownDirty for
+		// details.
+		List<String> unloadedDrivers = unloadDrivers();
+
+		try {
+			db.close();
+			fail();
+		} catch (Exception e) {
+			// continue
+		}
+
+		reloadDrivers(unloadedDrivers);
+
+		db = open(true);
+		assertTrue(db.wasDirtyOnInitialisation());
+
+		db.close();
+		db = open(true);
+		assertFalse(db.wasDirtyOnInitialisation());
+	}
+
 	@Test
 	public void testCleanupTimer() throws Exception {
 		long duration = 60_000;
@@ -2482,11 +2547,80 @@ public abstract class JdbcDatabaseTest extends BrambleTestCase {
 				rootKey, alice);
 	}
 
+	private List<String> unloadDrivers() {
+		Enumeration<Driver> drivers = DriverManager.getDrivers();
+		List<String> unloaded = new ArrayList<>();
+		while (drivers.hasMoreElements()) {
+			Driver d = drivers.nextElement();
+			try {
+				DriverManager.deregisterDriver(d);
+				unloaded.add(d.getClass().getName());
+			} catch (SQLException e) {
+				e.printStackTrace();
+				fail();
+			}
+		}
+		return unloaded;
+	}
+
+	private void reloadDrivers(List<String> unloadedDrivers)
+			throws ClassNotFoundException, IllegalAccessException,
+			InstantiationException, SQLException {
+		for (String driverName : unloadedDrivers) {
+			DriverManager.registerDriver(
+					(Driver) Class.forName(driverName).newInstance());
+		}
+	}
+
 	@After
 	public void tearDown() {
 		deleteTestDirectory(testDir);
 	}
 
+	private void assertNothingToSendLazily(Database<Connection> db,
+			Connection txn) throws Exception {
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		Collection<MessageId> ids =
+				db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
+		assertTrue(ids.isEmpty());
+		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
+		assertTrue(ids.isEmpty());
+	}
+
+	private void assertOneMessageToSendLazily(Database<Connection> db,
+			Connection txn) throws Exception {
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, false));
+		Collection<MessageId> ids =
+				db.getMessagesToSend(txn, contactId, ONE_MEGABYTE, MAX_LATENCY);
+		assertEquals(singletonList(messageId), ids);
+		ids = db.getMessagesToOffer(txn, contactId, 100, MAX_LATENCY);
+		assertEquals(singletonList(messageId), ids);
+	}
+
+	private void assertNothingToSendEagerly(Database<Connection> db,
+			Connection txn) throws Exception {
+		assertFalse(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
+		Map<MessageId, Integer> unacked =
+				db.getUnackedMessagesToSend(txn, contactId);
+		assertTrue(unacked.isEmpty());
+		assertEquals(0, db.getUnackedMessageBytesToSend(txn, contactId));
+	}
+
+	private void assertOneMessageToSendEagerly(Database<Connection> db,
+			Connection txn) throws Exception {
+		assertTrue(
+				db.containsAnythingToSend(txn, contactId, MAX_LATENCY, true));
+		Map<MessageId, Integer> unacked =
+				db.getUnackedMessagesToSend(txn, contactId);
+		assertEquals(singleton(messageId), unacked.keySet());
+		assertEquals(message.getRawLength(), unacked.get(messageId).intValue());
+		assertEquals(message.getRawLength(),
+				db.getUnackedMessageBytesToSend(txn, contactId));
+	}
+
 	private static class StoppedClock implements Clock {
 
 		private final long time;

bramble-core/src/test/java/org/briarproject/bramble/plugin/file/RemovableDriveIntegrationTest.java

@@ -0,0 +1,168 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.Identity;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveTask;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.sync.event.MessageStateChangedEvent;
+import org.briarproject.bramble.test.BrambleTestCase;
+import org.briarproject.bramble.test.TestDatabaseConfigModule;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.io.File;
+import java.util.concurrent.CountDownLatch;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.api.plugin.file.FileConstants.PROP_PATH;
+import static org.briarproject.bramble.api.sync.validation.MessageState.DELIVERED;
+import static org.briarproject.bramble.test.TestUtils.deleteTestDirectory;
+import static org.briarproject.bramble.test.TestUtils.getSecretKey;
+import static org.briarproject.bramble.test.TestUtils.getTestDirectory;
+import static org.junit.Assert.assertTrue;
+
+public class RemovableDriveIntegrationTest extends BrambleTestCase {
+
+	private static final int TIMEOUT_MS = 5_000;
+
+	private final File testDir = getTestDirectory();
+	private final File aliceDir = new File(testDir, "alice");
+	private final File bobDir = new File(testDir, "bob");
+
+	private final SecretKey rootKey = getSecretKey();
+	private final long timestamp = System.currentTimeMillis();
+
+	private RemovableDriveIntegrationTestComponent alice, bob;
+
+	@Before
+	public void setUp() {
+		assertTrue(testDir.mkdirs());
+		alice = DaggerRemovableDriveIntegrationTestComponent.builder()
+				.testDatabaseConfigModule(
+						new TestDatabaseConfigModule(aliceDir)).build();
+		RemovableDriveIntegrationTestComponent.Helper
+				.injectEagerSingletons(alice);
+		bob = DaggerRemovableDriveIntegrationTestComponent.builder()
+				.testDatabaseConfigModule(
+						new TestDatabaseConfigModule(bobDir)).build();
+		RemovableDriveIntegrationTestComponent.Helper
+				.injectEagerSingletons(bob);
+	}
+
+	@Test
+	public void testWriteAndRead() throws Exception {
+		// Create the identities
+		Identity aliceIdentity =
+				alice.getIdentityManager().createIdentity("Alice");
+		Identity bobIdentity = bob.getIdentityManager().createIdentity("Bob");
+		// Set up the devices and get the contact IDs
+		ContactId bobId = setUp(alice, aliceIdentity,
+				bobIdentity.getLocalAuthor(), true);
+		ContactId aliceId = setUp(bob, bobIdentity,
+				aliceIdentity.getLocalAuthor(), false);
+		// Sync Alice's client versions and transport properties
+		read(bob, write(alice, bobId), 2);
+		// Sync Bob's client versions and transport properties
+		read(alice, write(bob, aliceId), 2);
+	}
+
+	private ContactId setUp(RemovableDriveIntegrationTestComponent device,
+			Identity local, Author remote, boolean alice) throws Exception {
+		// Add an identity for the user
+		IdentityManager identityManager = device.getIdentityManager();
+		identityManager.registerIdentity(local);
+		// Start the lifecycle manager
+		LifecycleManager lifecycleManager = device.getLifecycleManager();
+		lifecycleManager.startServices(getSecretKey());
+		lifecycleManager.waitForStartup();
+		// Add the other user as a contact
+		ContactManager contactManager = device.getContactManager();
+		return contactManager.addContact(remote, local.getId(), rootKey,
+				timestamp, alice, true, true);
+	}
+
+	@SuppressWarnings("SameParameterValue")
+	private void read(RemovableDriveIntegrationTestComponent device,
+			File file, int deliveries) throws Exception {
+		// Listen for message deliveries
+		MessageDeliveryListener listener =
+				new MessageDeliveryListener(deliveries);
+		device.getEventBus().addListener(listener);
+		// Read the incoming stream
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, file.getAbsolutePath());
+		RemovableDriveTask reader =
+				device.getRemovableDriveManager().startReaderTask(p);
+		CountDownLatch disposedLatch = new CountDownLatch(1);
+		reader.addObserver(state -> {
+			if (state.isFinished()) disposedLatch.countDown();
+		});
+		// Wait for the messages to be delivered
+		assertTrue(listener.delivered.await(TIMEOUT_MS, MILLISECONDS));
+		// Clean up the listener
+		device.getEventBus().removeListener(listener);
+		// Wait for the reader to be disposed
+		disposedLatch.await(TIMEOUT_MS, MILLISECONDS);
+	}
+
+	private File write(RemovableDriveIntegrationTestComponent device,
+			ContactId contactId) throws Exception {
+		// Write the outgoing stream to a file
+		File file = File.createTempFile("sync", ".tmp", testDir);
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, file.getAbsolutePath());
+		RemovableDriveTask writer = device.getRemovableDriveManager()
+				.startWriterTask(contactId, p);
+		CountDownLatch disposedLatch = new CountDownLatch(1);
+		writer.addObserver(state -> {
+			if (state.isFinished()) disposedLatch.countDown();
+		});
+		// Wait for the writer to be disposed
+		disposedLatch.await(TIMEOUT_MS, MILLISECONDS);
+		// Return the file containing the stream
+		return file;
+	}
+
+	private void tearDown(RemovableDriveIntegrationTestComponent device)
+			throws Exception {
+		// Stop the lifecycle manager
+		LifecycleManager lifecycleManager = device.getLifecycleManager();
+		lifecycleManager.stopServices();
+		lifecycleManager.waitForShutdown();
+	}
+
+	@After
+	public void tearDown() throws Exception {
+		// Tear down the devices
+		tearDown(alice);
+		tearDown(bob);
+		deleteTestDirectory(testDir);
+	}
+
+	@NotNullByDefault
+	private static class MessageDeliveryListener implements EventListener {
+
+		private final CountDownLatch delivered;
+
+		private MessageDeliveryListener(int deliveries) {
+			delivered = new CountDownLatch(deliveries);
+		}
+
+		@Override
+		public void eventOccurred(Event e) {
+			if (e instanceof MessageStateChangedEvent) {
+				MessageStateChangedEvent m = (MessageStateChangedEvent) e;
+				if (m.getState().equals(DELIVERED)) delivered.countDown();
+			}
+		}
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/plugin/file/RemovableDriveIntegrationTestComponent.java

@@ -0,0 +1,53 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.BrambleCoreEagerSingletons;
+import org.briarproject.bramble.BrambleCoreModule;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.plugin.file.RemovableDriveManager;
+import org.briarproject.bramble.battery.DefaultBatteryManagerModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
+import org.briarproject.bramble.system.DefaultWakefulIoExecutorModule;
+import org.briarproject.bramble.system.TimeTravelModule;
+import org.briarproject.bramble.test.TestDatabaseConfigModule;
+import org.briarproject.bramble.test.TestSecureRandomModule;
+
+import javax.inject.Singleton;
+
+import dagger.Component;
+
+@Singleton
+@Component(modules = {
+		BrambleCoreModule.class,
+		DefaultBatteryManagerModule.class,
+		DefaultEventExecutorModule.class,
+		DefaultWakefulIoExecutorModule.class,
+		TestDatabaseConfigModule.class,
+		RemovableDriveIntegrationTestModule.class,
+		RemovableDriveModule.class,
+		TestSecureRandomModule.class,
+		TimeTravelModule.class
+})
+interface RemovableDriveIntegrationTestComponent
+		extends BrambleCoreEagerSingletons {
+
+	ContactManager getContactManager();
+
+	EventBus getEventBus();
+
+	IdentityManager getIdentityManager();
+
+	LifecycleManager getLifecycleManager();
+
+	RemovableDriveManager getRemovableDriveManager();
+
+	class Helper {
+
+		public static void injectEagerSingletons(
+				RemovableDriveIntegrationTestComponent c) {
+			BrambleCoreEagerSingletons.Helper.injectEagerSingletons(c);
+		}
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/plugin/file/RemovableDriveIntegrationTestModule.java

@@ -1,61 +1,81 @@
-package org.briarproject.bramble.plugin;
+package org.briarproject.bramble.plugin.file;
 
+import org.briarproject.bramble.api.FeatureFlags;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.BluetoothConstants;
-import org.briarproject.bramble.api.plugin.LanTcpConstants;
 import org.briarproject.bramble.api.plugin.PluginConfig;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
-import org.briarproject.bramble.plugin.bluetooth.JavaBluetoothPluginFactory;
-import org.briarproject.bramble.plugin.modem.ModemPluginFactory;
-import org.briarproject.bramble.plugin.tcp.LanTcpPluginFactory;
-import org.briarproject.bramble.plugin.tcp.WanTcpPluginFactory;
 
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 
+import javax.inject.Singleton;
+
 import dagger.Module;
 import dagger.Provides;
 
-import static java.util.Arrays.asList;
 import static java.util.Collections.emptyList;
+import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
-import static java.util.Collections.singletonMap;
 
 @Module
-public class DesktopPluginModule extends PluginModule {
+class RemovableDriveIntegrationTestModule {
 
 	@Provides
-	PluginConfig getPluginConfig(JavaBluetoothPluginFactory bluetooth,
-			ModemPluginFactory modem, LanTcpPluginFactory lan,
-			WanTcpPluginFactory wan) {
+	@Singleton
+	PluginConfig providePluginConfig(RemovableDrivePluginFactory drive) {
 		@NotNullByDefault
 		PluginConfig pluginConfig = new PluginConfig() {
 
 			@Override
 			public Collection<DuplexPluginFactory> getDuplexFactories() {
-				return asList(bluetooth, modem, lan, wan);
-			}
-
-			@Override
-			public Collection<SimplexPluginFactory> getSimplexFactories() {
 				return emptyList();
 			}
 
+			@Override
+			public Collection<SimplexPluginFactory> getSimplexFactories() {
+				return singletonList(drive);
+			}
+
 			@Override
 			public boolean shouldPoll() {
-				return true;
+				return false;
 			}
 
 			@Override
 			public Map<TransportId, List<TransportId>> getTransportPreferences() {
-				// Prefer LAN to Bluetooth
-				return singletonMap(BluetoothConstants.ID,
-						singletonList(LanTcpConstants.ID));
+				return emptyMap();
 			}
+
 		};
 		return pluginConfig;
 	}
+
+	@Provides
+	FeatureFlags provideFeatureFlags() {
+		return new FeatureFlags() {
+
+			@Override
+			public boolean shouldEnableImageAttachments() {
+				return true;
+			}
+
+			@Override
+			public boolean shouldEnableProfilePictures() {
+				return true;
+			}
+
+			@Override
+			public boolean shouldEnableDisappearingMessages() {
+				return true;
+			}
+
+			@Override
+			public boolean shouldEnableConnectViaBluetooth() {
+				return true;
+			}
+		};
+	}
 }

bramble-core/src/test/java/org/briarproject/bramble/properties/TransportPropertyManagerImplTest.java

@@ -43,6 +43,7 @@ import static org.briarproject.bramble.api.properties.TransportPropertyConstants
 import static org.briarproject.bramble.api.properties.TransportPropertyManager.CLIENT_ID;
 import static org.briarproject.bramble.api.properties.TransportPropertyManager.MAJOR_VERSION;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 import static org.briarproject.bramble.test.TestUtils.getContact;
 import static org.briarproject.bramble.test.TestUtils.getGroup;
 import static org.briarproject.bramble.test.TestUtils.getMessage;
@@ -230,7 +231,8 @@ public class TransportPropertyManagerImplTest extends BrambleMockTestCase {
 		}});
 
 		TransportPropertyManagerImpl t = createInstance();
-		assertFalse(t.incomingMessage(txn, message, meta));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				t.incomingMessage(txn, message, meta));
 		assertTrue(hasEvent(txn, RemoteTransportPropertiesUpdatedEvent.class));
 	}
 
@@ -269,7 +271,8 @@ public class TransportPropertyManagerImplTest extends BrambleMockTestCase {
 		}});
 
 		TransportPropertyManagerImpl t = createInstance();
-		assertFalse(t.incomingMessage(txn, message, meta));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				t.incomingMessage(txn, message, meta));
 		assertTrue(hasEvent(txn, RemoteTransportPropertiesUpdatedEvent.class));
 	}
 
@@ -308,7 +311,8 @@ public class TransportPropertyManagerImplTest extends BrambleMockTestCase {
 		}});
 
 		TransportPropertyManagerImpl t = createInstance();
-		assertFalse(t.incomingMessage(txn, message, meta));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				t.incomingMessage(txn, message, meta));
 		assertFalse(hasEvent(txn, RemoteTransportPropertiesUpdatedEvent.class));
 	}
 

bramble-core/src/test/java/org/briarproject/bramble/sync/SimplexOutgoingSessionTest.java

@@ -17,9 +17,14 @@ import org.briarproject.bramble.test.DbExpectations;
 import org.briarproject.bramble.test.ImmediateExecutor;
 import org.junit.Test;
 
+import java.util.LinkedHashMap;
+import java.util.Map;
 import java.util.concurrent.Executor;
 
+import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.api.record.Record.MAX_RECORD_PAYLOAD_BYTES;
+import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_IDS;
 import static org.briarproject.bramble.test.TestUtils.getContactId;
 import static org.briarproject.bramble.test.TestUtils.getMessage;
@@ -39,14 +44,19 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 	private final Executor dbExecutor = new ImmediateExecutor();
 	private final ContactId contactId = getContactId();
 	private final TransportId transportId = getTransportId();
-	private final Message message = getMessage(new GroupId(getRandomId()));
-	private final MessageId messageId = message.getId();
+	private final Ack ack =
+			new Ack(singletonList(new MessageId(getRandomId())));
+	private final Message message = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
+	private final Message message1 = getMessage(new GroupId(getRandomId()),
+			MAX_MESSAGE_BODY_LENGTH);
 
 	@Test
 	public void testNothingToSend() throws Exception {
 		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
 				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
-				streamWriter, recordWriter);
+				false, streamWriter, recordWriter);
+
 		Transaction noAckTxn = new Transaction(null, false);
 		Transaction noMsgTxn = new Transaction(null, false);
 
@@ -63,8 +73,8 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			// No messages to send
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(noMsgTxn));
-			oneOf(db).generateBatch(with(noMsgTxn), with(contactId),
-					with(any(int.class)), with(MAX_LATENCY));
+			oneOf(db).generateBatch(noMsgTxn, contactId,
+					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
 			will(returnValue(null));
 			// Send the end of stream marker
 			oneOf(streamWriter).sendEndOfStream();
@@ -76,11 +86,44 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 	}
 
 	@Test
-	public void testSomethingToSend() throws Exception {
-		Ack ack = new Ack(singletonList(messageId));
+	public void testNothingToSendEagerly() throws Exception {
 		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
 				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
-				streamWriter, recordWriter);
+				true, streamWriter, recordWriter);
+
+		Transaction noAckTxn = new Transaction(null, false);
+		Transaction noIdsTxn = new Transaction(null, true);
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// No acks to send
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(noAckTxn));
+			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(null));
+			// No messages to send
+			oneOf(db).transactionWithResult(with(true),
+					withDbCallable(noIdsTxn));
+			oneOf(db).getUnackedMessagesToSend(noIdsTxn, contactId);
+			will(returnValue(emptyMap()));
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
+
+	@Test
+	public void testSomethingToSend() throws Exception {
+		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
+				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
+				false, streamWriter, recordWriter);
+
 		Transaction ackTxn = new Transaction(null, false);
 		Transaction noAckTxn = new Transaction(null, false);
 		Transaction msgTxn = new Transaction(null, false);
@@ -100,8 +143,8 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			// One message to send
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(msgTxn));
-			oneOf(db).generateBatch(with(msgTxn), with(contactId),
-					with(any(int.class)), with(MAX_LATENCY));
+			oneOf(db).generateBatch(msgTxn, contactId,
+					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
 			will(returnValue(singletonList(message)));
 			oneOf(recordWriter).writeMessage(message);
 			// No more acks
@@ -112,8 +155,8 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			// No more messages
 			oneOf(db).transactionWithNullableResult(with(false),
 					withNullableDbCallable(noMsgTxn));
-			oneOf(db).generateBatch(with(noMsgTxn), with(contactId),
-					with(any(int.class)), with(MAX_LATENCY));
+			oneOf(db).generateBatch(noMsgTxn, contactId,
+					MAX_RECORD_PAYLOAD_BYTES, MAX_LATENCY);
 			will(returnValue(null));
 			// Send the end of stream marker
 			oneOf(streamWriter).sendEndOfStream();
@@ -123,4 +166,63 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 
 		session.run();
 	}
+
+	@Test
+	public void testSomethingToSendEagerly() throws Exception {
+		SimplexOutgoingSession session = new SimplexOutgoingSession(db,
+				dbExecutor, eventBus, contactId, transportId, MAX_LATENCY,
+				true, streamWriter, recordWriter);
+
+		Map<MessageId, Integer> unacked = new LinkedHashMap<>();
+		unacked.put(message.getId(), message.getRawLength());
+		unacked.put(message1.getId(), message1.getRawLength());
+
+		Transaction ackTxn = new Transaction(null, false);
+		Transaction noAckTxn = new Transaction(null, false);
+		Transaction idsTxn = new Transaction(null, true);
+		Transaction msgTxn = new Transaction(null, false);
+		Transaction msgTxn1 = new Transaction(null, false);
+
+		context.checking(new DbExpectations() {{
+			// Add listener
+			oneOf(eventBus).addListener(session);
+			// Send the protocol versions
+			oneOf(recordWriter).writeVersions(with(any(Versions.class)));
+			// One ack to send
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(ackTxn));
+			oneOf(db).generateAck(ackTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(ack));
+			oneOf(recordWriter).writeAck(ack);
+			// No more acks
+			oneOf(db).transactionWithNullableResult(with(false),
+					withNullableDbCallable(noAckTxn));
+			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
+			will(returnValue(null));
+			// Two messages to send
+			oneOf(db).transactionWithResult(with(true), withDbCallable(idsTxn));
+			oneOf(db).getUnackedMessagesToSend(idsTxn, contactId);
+			will(returnValue(unacked));
+			// Send the first message
+			oneOf(db).transactionWithResult(with(false),
+					withDbCallable(msgTxn));
+			oneOf(db).generateBatch(msgTxn, contactId,
+					singletonList(message.getId()), MAX_LATENCY);
+			will(returnValue(singletonList(message)));
+			oneOf(recordWriter).writeMessage(message);
+			// Send the second message
+			oneOf(db).transactionWithResult(with(false),
+					withDbCallable(msgTxn1));
+			oneOf(db).generateBatch(msgTxn1, contactId,
+					singletonList(message1.getId()), MAX_LATENCY);
+			will(returnValue(singletonList(message1)));
+			oneOf(recordWriter).writeMessage(message1);
+			// Send the end of stream marker
+			oneOf(streamWriter).sendEndOfStream();
+			// Remove listener
+			oneOf(eventBus).removeListener(session);
+		}});
+
+		session.run();
+	}
 }

bramble-core/src/test/java/org/briarproject/bramble/sync/validation/ValidationManagerImplTest.java

@@ -31,6 +31,8 @@ import static java.util.Arrays.asList;
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonList;
 import static java.util.Collections.singletonMap;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_SHARE;
 import static org.briarproject.bramble.api.sync.validation.MessageState.DELIVERED;
 import static org.briarproject.bramble.api.sync.validation.MessageState.INVALID;
 import static org.briarproject.bramble.api.sync.validation.MessageState.PENDING;
@@ -111,7 +113,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the first message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId);
@@ -167,7 +169,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(new Metadata()));
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn, messageId);
@@ -187,7 +189,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(metadata));
 			// Deliver the dependent
 			oneOf(hook).incomingMessage(txn1, message2, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId2, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId2);
@@ -247,7 +249,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(true));
+			will(returnValue(ACCEPT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId);
@@ -367,7 +369,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId);
@@ -432,7 +434,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId);
@@ -602,7 +604,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// The message has two pending dependents: 1 and 2
 			oneOf(db).getMessageDependents(txn1, messageId);
@@ -622,7 +624,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(metadata));
 			// Deliver message 1
 			oneOf(hook).incomingMessage(txn2, message1, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn2, messageId1, DELIVERED);
 			// Message 1 has one pending dependent: 3
 			oneOf(db).getMessageDependents(txn2, messageId1);
@@ -642,7 +644,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(metadata));
 			// Deliver message 2
 			oneOf(hook).incomingMessage(txn3, message2, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn3, messageId2, DELIVERED);
 			// Message 2 has one pending dependent: 3 (same dependent as 1)
 			oneOf(db).getMessageDependents(txn3, messageId2);
@@ -662,6 +664,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(metadata));
 			// Deliver message 3
 			oneOf(hook).incomingMessage(txn4, message3, metadata);
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn4, messageId3, DELIVERED);
 			// Message 3 has one pending dependent: 4
 			oneOf(db).getMessageDependents(txn4, messageId3);
@@ -685,7 +688,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			will(returnValue(metadata));
 			// Deliver message 4
 			oneOf(hook).incomingMessage(txn6, message4, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn6, messageId4, DELIVERED);
 			// Message 4 has no pending dependents
 			oneOf(db).getMessageDependents(txn6, messageId4);
@@ -717,7 +720,7 @@ public class ValidationManagerImplTest extends BrambleMockTestCase {
 			oneOf(db).mergeMessageMetadata(txn1, messageId, metadata);
 			// Deliver the message
 			oneOf(hook).incomingMessage(txn1, message, metadata);
-			will(returnValue(false));
+			will(returnValue(ACCEPT_DO_NOT_SHARE));
 			oneOf(db).setMessageState(txn1, messageId, DELIVERED);
 			// Get any pending dependents
 			oneOf(db).getMessageDependents(txn1, messageId);

bramble-core/src/test/java/org/briarproject/bramble/test/BrambleCoreIntegrationTestModule.java

@@ -38,6 +38,11 @@ public class BrambleCoreIntegrationTestModule {
 			public boolean shouldEnableDisappearingMessages() {
 				return true;
 			}
+
+			@Override
+			public boolean shouldEnableConnectViaBluetooth() {
+				return true;
+			}
 		};
 	}
 }

bramble-core/src/test/java/org/briarproject/bramble/test/TestDuplexTransportConnection.java

@@ -25,7 +25,7 @@ public class TestDuplexTransportConnection
 	@SuppressWarnings("WeakerAccess")
 	public TestDuplexTransportConnection(InputStream in, OutputStream out) {
 		reader = new TestTransportConnectionReader(in);
-		writer = new TestTransportConnectionWriter(out);
+		writer = new TestTransportConnectionWriter(out, false);
 	}
 
 	@Override

bramble-core/src/test/java/org/briarproject/bramble/test/TestTransportConnectionWriter.java

@@ -15,10 +15,13 @@ public class TestTransportConnectionWriter
 		implements TransportConnectionWriter {
 
 	private final OutputStream out;
+	private final boolean lossyAndCheap;
 	private final CountDownLatch disposed = new CountDownLatch(1);
 
-	public TestTransportConnectionWriter(OutputStream out) {
+	public TestTransportConnectionWriter(OutputStream out,
+			boolean lossyAndCheap) {
 		this.out = out;
+		this.lossyAndCheap = lossyAndCheap;
 	}
 
 	public CountDownLatch getDisposedLatch() {
@@ -35,6 +38,11 @@ public class TestTransportConnectionWriter
 		return 60_000;
 	}
 
+	@Override
+	public boolean isLossyAndCheap() {
+		return lossyAndCheap;
+	}
+
 	@Override
 	public OutputStream getOutputStream() {
 		return out;

bramble-core/src/test/java/org/briarproject/bramble/transport/KeyManagerImplTest.java

@@ -25,6 +25,7 @@ import java.util.Collection;
 import java.util.Map;
 import java.util.Random;
 
+import static java.util.Collections.emptyList;
 import static java.util.Collections.singletonList;
 import static java.util.Collections.singletonMap;
 import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
@@ -71,8 +72,7 @@ public class KeyManagerImplTest extends BrambleMockTestCase {
 	private final SecretKey rootKey = getSecretKey();
 	private final Random random = new Random();
 
-	private final KeyManagerImpl keyManager = new KeyManagerImpl(db, executor,
-			pluginConfig, transportKeyManagerFactory, transportCrypto);
+	private KeyManagerImpl keyManager;
 
 	@Before
 	public void testStartService() throws Exception {
@@ -83,18 +83,25 @@ public class KeyManagerImplTest extends BrambleMockTestCase {
 				singletonList(pluginFactory);
 		int maxLatency = 1337;
 
-		context.checking(new DbExpectations() {{
-			oneOf(pluginConfig).getSimplexFactories();
+		context.checking(new Expectations() {{
+			allowing(pluginConfig).getSimplexFactories();
 			will(returnValue(factories));
-			oneOf(pluginFactory).getId();
+			allowing(pluginFactory).getId();
 			will(returnValue(transportId));
-			oneOf(pluginFactory).getMaxLatency();
+			allowing(pluginFactory).getMaxLatency();
 			will(returnValue(maxLatency));
-			oneOf(db).addTransport(txn, transportId, maxLatency);
+			allowing(pluginConfig).getDuplexFactories();
+			will(returnValue(emptyList()));
 			oneOf(transportKeyManagerFactory)
 					.createTransportKeyManager(transportId, maxLatency);
 			will(returnValue(transportKeyManager));
-			oneOf(pluginConfig).getDuplexFactories();
+		}});
+
+		keyManager = new KeyManagerImpl(db, executor,
+				pluginConfig, transportCrypto, transportKeyManagerFactory);
+
+		context.checking(new DbExpectations() {{
+			oneOf(db).addTransport(txn, transportId, maxLatency);
 			oneOf(db).transaction(with(false), withDbRunnable(txn));
 			oneOf(transportKeyManager).start(txn);
 		}});
@@ -235,4 +242,37 @@ public class KeyManagerImplTest extends BrambleMockTestCase {
 		keyManager.eventOccurred(event);
 		executor.runUntilIdle();
 	}
+
+	@Test
+	public void testAddMultipleRotationKeySets() throws Exception {
+		long timestamp = System.currentTimeMillis();
+		boolean alice = random.nextBoolean();
+		boolean active = random.nextBoolean();
+
+		context.checking(new Expectations() {{
+			oneOf(transportKeyManager).addRotationKeys(txn, contactId,
+					rootKey, timestamp, alice, active);
+			will(returnValue(keySetId));
+		}});
+
+		assertEquals(singletonMap(transportId, keySetId),
+				keyManager.addRotationKeys(txn, contactId, rootKey, timestamp,
+						alice, active));
+	}
+
+	@Test
+	public void testAddSingleRotationKeySet() throws Exception {
+		long timestamp = System.currentTimeMillis();
+		boolean alice = random.nextBoolean();
+		boolean active = random.nextBoolean();
+
+		context.checking(new Expectations() {{
+			oneOf(transportKeyManager).addRotationKeys(txn, contactId,
+					rootKey, timestamp, alice, active);
+			will(returnValue(keySetId));
+		}});
+
+		assertEquals(keySetId, keyManager.addRotationKeys(txn, contactId,
+				transportId, rootKey, timestamp, alice, active));
+	}
 }

bramble-core/src/test/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImplTest.java

@@ -0,0 +1,554 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.client.ContactGroupFactory;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataParser;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.Metadata;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.identity.LocalAuthor;
+import org.briarproject.bramble.api.plugin.PluginConfig;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.transport.KeyManager;
+import org.briarproject.bramble.api.transport.KeySetId;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.CaptureArgumentAction;
+import org.jmock.Expectations;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import static java.lang.Math.min;
+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 org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.DEFER;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.REJECT;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.CLIENT_ID;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MAJOR_VERSION;
+import static org.briarproject.bramble.test.TestUtils.getAgreementPrivateKey;
+import static org.briarproject.bramble.test.TestUtils.getAgreementPublicKey;
+import static org.briarproject.bramble.test.TestUtils.getContact;
+import static org.briarproject.bramble.test.TestUtils.getGroup;
+import static org.briarproject.bramble.test.TestUtils.getLocalAuthor;
+import static org.briarproject.bramble.test.TestUtils.getMessage;
+import static org.briarproject.bramble.test.TestUtils.getSecretKey;
+import static org.briarproject.bramble.test.TestUtils.getTransportId;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.State.ACTIVATED;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+public class TransportKeyAgreementManagerImplTest extends BrambleMockTestCase {
+
+	private final DatabaseComponent db = context.mock(DatabaseComponent.class);
+	private final ClientHelper clientHelper = context.mock(ClientHelper.class);
+	private final MetadataParser metadataParser =
+			context.mock(MetadataParser.class);
+	private final ContactGroupFactory contactGroupFactory =
+			context.mock(ContactGroupFactory.class);
+	private final ClientVersioningManager clientVersioningManager =
+			context.mock(ClientVersioningManager.class);
+	private final IdentityManager identityManager =
+			context.mock(IdentityManager.class);
+	private final KeyManager keyManager = context.mock(KeyManager.class);
+	private final MessageEncoder messageEncoder =
+			context.mock(MessageEncoder.class);
+	private final SessionEncoder sessionEncoder =
+			context.mock(SessionEncoder.class);
+	private final SessionParser sessionParser =
+			context.mock(SessionParser.class);
+	private final TransportKeyAgreementCrypto crypto =
+			context.mock(TransportKeyAgreementCrypto.class);
+	private final PluginConfig pluginConfig = context.mock(PluginConfig.class);
+	private final SimplexPluginFactory simplexFactory =
+			context.mock(SimplexPluginFactory.class);
+	private final DuplexPluginFactory duplexFactory =
+			context.mock(DuplexPluginFactory.class);
+
+	private final TransportId simplexTransportId = getTransportId();
+	private final TransportId duplexTransportId = getTransportId();
+	private final Group localGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+	private final Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+	private final Contact contact = getContact();
+	private final LocalAuthor localAuthor = getLocalAuthor();
+	private final boolean alice = compare(localAuthor.getId().getBytes(),
+			contact.getAuthor().getId().getBytes()) < 0;
+	private final KeyPair localKeyPair =
+			new KeyPair(getAgreementPublicKey(), getAgreementPrivateKey());
+	private final PublicKey remotePublicKey = getAgreementPublicKey();
+	private final SecretKey rootKey = getSecretKey();
+	private final KeySetId keySetId = new KeySetId(123);
+
+	private final Message storageMessage = getMessage(contactGroup.getId());
+	private final Message localKeyMessage = getMessage(contactGroup.getId());
+	private final Message localActivateMessage =
+			getMessage(contactGroup.getId());
+	private final Message remoteKeyMessage = getMessage(contactGroup.getId());
+	private final Message remoteActivateMessage =
+			getMessage(contactGroup.getId());
+	private final long localTimestamp = localKeyMessage.getTimestamp();
+	private final long remoteTimestamp = remoteKeyMessage.getTimestamp();
+
+	// These query and metadata dictionaries are handled by the manager without
+	// inspecting their contents, so we can use empty dictionaries for testing
+	private final BdfDictionary sessionQuery = new BdfDictionary();
+	private final BdfDictionary sessionMeta = new BdfDictionary();
+	private final BdfDictionary localKeyMeta = new BdfDictionary();
+	private final BdfDictionary localActivateMeta = new BdfDictionary();
+
+	// The manager doesn't use the incoming message body, so it can be empty
+	private final BdfList remoteMessageBody = new BdfList();
+
+	private final BdfDictionary remoteKeyMeta = BdfDictionary.of(
+			new BdfEntry(MSG_KEY_MESSAGE_TYPE, KEY.getValue()),
+			new BdfEntry(MSG_KEY_TRANSPORT_ID,
+					simplexTransportId.getString()),
+			new BdfEntry(MSG_KEY_PUBLIC_KEY, remotePublicKey.getEncoded()));
+
+	private final BdfDictionary remoteActivateMeta = BdfDictionary.of(
+			new BdfEntry(MSG_KEY_MESSAGE_TYPE, ACTIVATE.getValue()),
+			new BdfEntry(MSG_KEY_TRANSPORT_ID,
+					simplexTransportId.getString()));
+
+	private TransportKeyAgreementManagerImpl manager;
+
+	@Before
+	public void setUp() {
+		context.checking(new Expectations() {{
+			oneOf(pluginConfig).getSimplexFactories();
+			will(returnValue(singletonList(simplexFactory)));
+			oneOf(simplexFactory).getId();
+			will(returnValue(simplexTransportId));
+			oneOf(pluginConfig).getDuplexFactories();
+			will(returnValue(singletonList(duplexFactory)));
+			oneOf(duplexFactory).getId();
+			will(returnValue(duplexTransportId));
+			oneOf(contactGroupFactory)
+					.createLocalGroup(CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(localGroup));
+		}});
+
+		manager = new TransportKeyAgreementManagerImpl(db, clientHelper,
+				metadataParser, contactGroupFactory, clientVersioningManager,
+				identityManager, keyManager, messageEncoder, sessionEncoder,
+				sessionParser, crypto, pluginConfig);
+	}
+
+	@Test
+	public void testCreatesContactGroupAtStartupIfLocalGroupDoesNotExist()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group doesn't exist so we need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(false));
+			oneOf(db).addGroup(txn, localGroup);
+			// Create the contact group and set it up
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(db).addGroup(txn, contactGroup);
+			oneOf(clientHelper)
+					.setContactId(txn, contactGroup.getId(), contact.getId());
+			oneOf(clientVersioningManager).getClientVisibility(txn,
+					contact.getId(), CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(VISIBLE));
+			oneOf(db).setGroupVisibility(txn, contact.getId(),
+					contactGroup.getId(), VISIBLE);
+			// We already have keys for both transports
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					asList(simplexTransportId, duplexTransportId))));
+		}});
+
+		manager.onDatabaseOpened(txn);
+	}
+
+	@Test
+	public void testDoesNotCreateContactGroupAtStartupIfLocalGroupExists()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group exists so we don't need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(true));
+			// We already have keys for both transports
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					asList(simplexTransportId, duplexTransportId))));
+		}});
+
+		manager.onDatabaseOpened(txn);
+	}
+
+	@Test
+	public void testStartsSessionAtStartup() throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group exists so we don't need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(true));
+			// We need keys for the simplex transport
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					singletonList(duplexTransportId))));
+			// Get the contact group ID
+			oneOf(contactGroupFactory)
+					.createContactGroup(CLIENT_ID, MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+		}});
+
+		// Check whether a session exists - it doesn't
+		expectSessionDoesNotExist(txn);
+		// Generate the local key pair
+		expectGenerateLocalKeyPair();
+		// Send a key message
+		expectSendKeyMessage(txn);
+		// Save the session
+		expectCreateStorageMessage(txn);
+		AtomicReference<Session> savedSession = expectSaveSession(txn);
+
+		manager.onDatabaseOpened(txn);
+
+		assertEquals(AWAIT_KEY, savedSession.get().getState());
+		assertEquals(localKeyMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertEquals(localKeyPair, savedSession.get().getLocalKeyPair());
+		assertEquals(Long.valueOf(localTimestamp),
+				savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testDefersMessageIfTransportIsNotSupported() throws Exception {
+		Transaction txn = new Transaction(null, false);
+		TransportId unknownTransportId = getTransportId();
+		BdfDictionary meta = new BdfDictionary(remoteKeyMeta);
+		meta.put(MSG_KEY_TRANSPORT_ID, unknownTransportId.getString());
+
+		assertEquals(DEFER, manager.incomingMessage(txn, remoteKeyMessage,
+				remoteMessageBody, meta));
+	}
+
+	@Test
+	public void testAcceptsKeyMessageInAwaitKeyState() throws Exception {
+		Transaction txn = new Transaction(null, false);
+		Session loadedSession = new Session(AWAIT_KEY,
+				localKeyMessage.getId(), localKeyPair, localTimestamp, null);
+
+		// Check whether a session exists - it does
+		expectLoadSession(txn, loadedSession);
+		// Load the contact ID
+		expectLoadContactId(txn);
+		// Check whether we already have keys - we don't
+		expectKeysExist(txn, false);
+		// Parse the remote public key
+		expectParseRemotePublicKey();
+		// Derive and store the transport keys
+		expectDeriveAndStoreTransportKeys(txn);
+		// Send an activate message
+		expectSendActivateMessage(txn);
+		// Save the session
+		AtomicReference<Session> savedSession = expectSaveSession(txn);
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+
+		assertEquals(AWAIT_ACTIVATE, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertEquals(keySetId, savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testAcceptsKeyMessageIfWeHaveTransportKeysButNoSession()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		// Check whether a session exists - it doesn't
+		expectSessionDoesNotExist(txn);
+		// Load the contact ID
+		expectLoadContactId(txn);
+		// Check whether we already have keys - we do
+		expectKeysExist(txn, true);
+		// Generate the local key pair
+		expectGenerateLocalKeyPair();
+		// Parse the remote public key
+		expectParseRemotePublicKey();
+		// Send a key message
+		expectSendKeyMessage(txn);
+		// Derive and store the transport keys
+		expectDeriveAndStoreTransportKeys(txn);
+		// Send an activate message
+		expectSendActivateMessage(txn);
+		// Save the session
+		expectCreateStorageMessage(txn);
+		AtomicReference<Session> savedSession = expectSaveSession(txn);
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+
+		assertEquals(AWAIT_ACTIVATE, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertEquals(keySetId, savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testRejectsKeyMessageInAwaitActivateState() throws Exception {
+		Session loadedSession = new Session(AWAIT_ACTIVATE,
+				localActivateMessage.getId(), null, null, keySetId);
+		testRejectsKeyMessageWithExistingSession(loadedSession);
+	}
+
+	@Test
+	public void testRejectsKeyMessageInActivatedState() throws Exception {
+		Session loadedSession = new Session(ACTIVATED,
+				localActivateMessage.getId(), null, null, null);
+		testRejectsKeyMessageWithExistingSession(loadedSession);
+	}
+
+	private void testRejectsKeyMessageWithExistingSession(Session loadedSession)
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		// Check whether a session exists - it does
+		expectLoadSession(txn, loadedSession);
+		// Load the contact ID
+		expectLoadContactId(txn);
+		// Check whether we already have keys - we don't
+		expectKeysExist(txn, false);
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+	}
+
+	@Test
+	public void testAcceptsActivateMessageInAwaitActivateState()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+		Session loadedSession = new Session(AWAIT_ACTIVATE,
+				localActivateMessage.getId(), null, null, keySetId);
+
+		// Check whether a session exists - it does
+		expectLoadSession(txn, loadedSession);
+
+		// Activate the transport keys
+		context.checking(new Expectations() {{
+			oneOf(keyManager).activateKeys(txn,
+					singletonMap(simplexTransportId, keySetId));
+		}});
+
+		// Save the session
+		AtomicReference<Session> savedSession = expectSaveSession(txn);
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+
+		assertEquals(ACTIVATED, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testRejectsActivateMessageWithNoSession() throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		// Check whether a session exists - it doesn't
+		expectSessionDoesNotExist(txn);
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+	}
+
+	@Test
+	public void testRejectsActivateMessageInAwaitKeyState() throws Exception {
+		Session loadedSession = new Session(AWAIT_KEY,
+				localKeyMessage.getId(), localKeyPair, localTimestamp, null);
+		testRejectsActivateMessageWithExistingSession(loadedSession);
+	}
+
+	@Test
+	public void testRejectsActivateMessageInActivatedState() throws Exception {
+		Session loadedSession = new Session(ACTIVATED,
+				localActivateMessage.getId(), null, null, null);
+		testRejectsActivateMessageWithExistingSession(loadedSession);
+	}
+
+	private void testRejectsActivateMessageWithExistingSession(
+			Session loadedSession) throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		// Check whether a session exists - it does
+		expectLoadSession(txn, loadedSession);
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+	}
+
+	private void expectSessionDoesNotExist(Transaction txn) throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(emptyList()));
+		}});
+	}
+
+	private void expectLoadSession(Transaction txn, Session loadedSession)
+			throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(singletonList(storageMessage.getId())));
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					storageMessage.getId());
+			will(returnValue(sessionMeta));
+			oneOf(sessionParser).parseSession(sessionMeta);
+			will(returnValue(loadedSession));
+		}});
+	}
+
+	private void expectSendKeyMessage(Transaction txn) throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(messageEncoder).encodeKeyMessage(contactGroup.getId(),
+					simplexTransportId, localKeyPair.getPublic());
+			will(returnValue(localKeyMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, KEY, true);
+			will(returnValue(localKeyMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localKeyMessage,
+					localKeyMeta, true, false);
+		}});
+	}
+
+	private void expectSendActivateMessage(Transaction txn) throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(messageEncoder).encodeActivateMessage(contactGroup.getId(),
+					simplexTransportId, localKeyMessage.getId());
+			will(returnValue(localActivateMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, ACTIVATE, true);
+			will(returnValue(localActivateMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localActivateMessage,
+					localActivateMeta, true, false);
+		}});
+	}
+
+	private void expectCreateStorageMessage(Transaction txn) throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(clientHelper)
+					.createMessageForStoringMetadata(contactGroup.getId());
+			will(returnValue(storageMessage));
+			oneOf(db).addLocalMessage(txn, storageMessage, new Metadata(),
+					false, false);
+		}});
+	}
+
+	private AtomicReference<Session> expectSaveSession(Transaction txn)
+			throws Exception {
+		AtomicReference<Session> savedSession = new AtomicReference<>();
+
+		context.checking(new Expectations() {{
+			oneOf(sessionEncoder).encodeSession(with(any(Session.class)),
+					with(simplexTransportId));
+			will(doAll(
+					new CaptureArgumentAction<>(savedSession, Session.class, 0),
+					returnValue(sessionMeta)));
+			oneOf(clientHelper).mergeMessageMetadata(txn,
+					storageMessage.getId(), sessionMeta);
+		}});
+
+		return savedSession;
+	}
+
+	private void expectLoadContactId(Transaction txn) throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(clientHelper).getContactId(txn, contactGroup.getId());
+			will(returnValue(contact.getId()));
+		}});
+	}
+
+	private void expectGenerateLocalKeyPair() {
+		context.checking(new Expectations() {{
+			oneOf(crypto).generateKeyPair();
+			will(returnValue(localKeyPair));
+		}});
+	}
+
+	private void expectParseRemotePublicKey() throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(crypto).parsePublicKey(remotePublicKey.getEncoded());
+			will(returnValue(remotePublicKey));
+		}});
+	}
+
+	private void expectDeriveAndStoreTransportKeys(Transaction txn)
+			throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(crypto).deriveRootKey(localKeyPair, remotePublicKey);
+			will(returnValue(rootKey));
+			oneOf(db).getContact(txn, contact.getId());
+			will(returnValue(contact));
+			oneOf(identityManager).getLocalAuthor(txn);
+			will(returnValue(localAuthor));
+			oneOf(keyManager).addRotationKeys(txn, contact.getId(),
+					simplexTransportId, rootKey,
+					min(localTimestamp, remoteTimestamp), alice, false);
+			will(returnValue(keySetId));
+		}});
+	}
+
+	private void expectKeysExist(Transaction txn, boolean exist)
+			throws Exception {
+		context.checking(new Expectations() {{
+			oneOf(db).containsTransportKeys(txn, contact.getId(),
+					simplexTransportId);
+			will(returnValue(exist));
+		}});
+	}
+}

bramble-core/src/test/java/org/briarproject/bramble/versioning/ClientVersioningManagerImplTest.java

@@ -31,6 +31,7 @@ import static java.util.Collections.singletonMap;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 import static org.briarproject.bramble.api.versioning.ClientVersioningManager.CLIENT_ID;
 import static org.briarproject.bramble.api.versioning.ClientVersioningManager.MAJOR_VERSION;
 import static org.briarproject.bramble.test.TestUtils.getClientId;
@@ -41,7 +42,6 @@ import static org.briarproject.bramble.test.TestUtils.getRandomId;
 import static org.briarproject.bramble.versioning.ClientVersioningConstants.MSG_KEY_LOCAL;
 import static org.briarproject.bramble.versioning.ClientVersioningConstants.MSG_KEY_UPDATE_VERSION;
 import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
 
 public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
@@ -419,7 +419,8 @@ public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
 		ClientVersioningManagerImpl c = createInstance();
 		c.registerClient(clientId, 123, 234, hook);
-		assertFalse(c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
 	}
 
 	@Test
@@ -464,7 +465,8 @@ public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
 		ClientVersioningManagerImpl c = createInstance();
 		c.registerClient(clientId, 123, 234, hook);
-		assertFalse(c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
 	}
 
 	@Test
@@ -496,7 +498,8 @@ public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
 		ClientVersioningManagerImpl c = createInstance();
 		c.registerClient(clientId, 123, 234, hook);
-		assertFalse(c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
 	}
 
 	@Test
@@ -579,7 +582,8 @@ public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
 		ClientVersioningManagerImpl c = createInstance();
 		c.registerClient(clientId, 123, 234, hook);
-		assertFalse(c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
 	}
 
 	@Test
@@ -649,7 +653,8 @@ public class ClientVersioningManagerImplTest extends BrambleMockTestCase {
 
 		ClientVersioningManagerImpl c = createInstance();
 		c.registerClient(clientId, 123, 234, hook);
-		assertFalse(c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
+		assertEquals(ACCEPT_DO_NOT_SHARE,
+				c.incomingMessage(txn, newRemoteUpdate, new Metadata()));
 	}
 
 	@Test

bramble-java/build.gradle

@@ -16,7 +16,7 @@ dependencies {
 	implementation fileTree(dir: 'libs', include: '*.jar')
 	implementation 'net.java.dev.jna:jna:4.5.2'
 	implementation 'net.java.dev.jna:jna-platform:4.5.2'
-	tor 'org.briarproject:tor:0.3.5.13@zip'
+	tor 'org.briarproject:tor:0.3.5.13-1@zip'
 	tor 'org.briarproject:obfs4proxy:0.0.12-dev-40245c4a@zip'
 
 	annotationProcessor 'com.google.dagger:dagger-compiler:2.24'

bramble-java/src/main/java/org/briarproject/bramble/plugin/bluetooth/JavaBluetoothPlugin.java

@@ -25,8 +25,8 @@ import static org.briarproject.bramble.util.StringUtils.isValidMac;
 
 @MethodsNotNullByDefault
 @ParametersNotNullByDefault
-class JavaBluetoothPlugin
-		extends BluetoothPlugin<StreamConnection, StreamConnectionNotifier> {
+class JavaBluetoothPlugin extends
+		AbstractBluetoothPlugin<StreamConnection, StreamConnectionNotifier> {
 
 	private static final Logger LOG =
 			getLogger(JavaBluetoothPlugin.class.getName());
@@ -108,6 +108,11 @@ class JavaBluetoothPlugin
 		return null; // TODO
 	}
 
+	@Override
+	public void stopDiscoverAndConnect() {
+		// TODO
+	}
+
 	private String makeUrl(String address, String uuid) {
 		return "btspp://" + address + ":" + uuid + ";name=RFCOMM";
 	}

bramble-java/src/main/java/org/briarproject/bramble/plugin/tor/UnixTorPluginFactory.java

@@ -25,6 +25,7 @@ import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 import javax.net.SocketFactory;
 
+import static java.util.logging.Level.INFO;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.util.OsUtils.isLinux;
 
@@ -54,7 +55,7 @@ public class UnixTorPluginFactory implements DuplexPluginFactory {
 	private final File torDirectory;
 
 	@Inject
-	public UnixTorPluginFactory(@IoExecutor Executor ioExecutor,
+	UnixTorPluginFactory(@IoExecutor Executor ioExecutor,
 			@IoExecutor Executor wakefulIoExecutor,
 			NetworkManager networkManager,
 			LocationUtils locationUtils,
@@ -96,8 +97,15 @@ public class UnixTorPluginFactory implements DuplexPluginFactory {
 		String architecture = null;
 		if (isLinux()) {
 			String arch = System.getProperty("os.arch");
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("System's os.arch is " + arch);
+			}
 			if (arch.equals("amd64")) {
 				architecture = "linux-x86_64";
+			} else if (arch.equals("aarch64")) {
+				architecture = "linux-aarch64";
+			} else if (arch.equals("arm")) {
+				architecture = "linux-armhf";
 			}
 		}
 		if (architecture == null) {
@@ -105,6 +113,10 @@ public class UnixTorPluginFactory implements DuplexPluginFactory {
 			return null;
 		}
 
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("The selected architecture for Tor is " + architecture);
+		}
+
 		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
 				MAX_POLLING_INTERVAL, BACKOFF_BASE);
 		TorRendezvousCrypto torRendezvousCrypto = new TorRendezvousCryptoImpl();

bramble-java/src/test/java/org/briarproject/bramble/plugin/tor/BridgeTest.java

@@ -23,8 +23,10 @@ import org.junit.runners.Parameterized;
 import org.junit.runners.Parameterized.Parameters;
 
 import java.io.File;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.logging.Logger;
 
 import javax.inject.Inject;
@@ -45,15 +47,22 @@ import static org.junit.Assume.assumeTrue;
 public class BridgeTest extends BrambleTestCase {
 
 	@Parameters
-	public static Iterable<String> data() {
+	public static Iterable<Params> data() {
 		BrambleJavaIntegrationTestComponent component =
 				DaggerBrambleJavaIntegrationTestComponent.builder().build();
 		BrambleCoreIntegrationTestEagerSingletons.Helper
 				.injectEagerSingletons(component);
-		return component.getCircumventionProvider().getBridges(false);
+		// Share a failure counter among all the test instances
+		AtomicInteger failures = new AtomicInteger(0);
+		List<String> bridges =
+				component.getCircumventionProvider().getBridges(false);
+		List<Params> states = new ArrayList<>(bridges.size());
+		for (String bridge : bridges) states.add(new Params(bridge, failures));
+		return states;
 	}
 
 	private final static long TIMEOUT = SECONDS.toMillis(60);
+	private final static int NUM_FAILURES_ALLOWED = 1;
 
 	private final static Logger LOG = getLogger(BridgeTest.class.getName());
 
@@ -80,11 +89,13 @@ public class BridgeTest extends BrambleTestCase {
 
 	private final File torDir = getTestDirectory();
 	private final String bridge;
+	private final AtomicInteger failures;
 
 	private UnixTorPluginFactory factory;
 
-	public BridgeTest(String bridge) {
-		this.bridge = bridge;
+	public BridgeTest(Params params) {
+		bridge = params.bridge;
+		failures = params.failures;
 	}
 
 	@Before
@@ -152,10 +163,24 @@ public class BridgeTest extends BrambleTestCase {
 				clock.sleep(500);
 			}
 			if (plugin.getState() != ACTIVE) {
-				fail("Could not connect to Tor within timeout.");
+				LOG.warning("Could not connect to Tor within timeout");
+				if (failures.incrementAndGet() > NUM_FAILURES_ALLOWED) {
+					fail(failures.get() + " bridges are unreachable");
+				}
 			}
 		} finally {
 			plugin.stop();
 		}
 	}
+
+	private static class Params {
+
+		private final String bridge;
+		private final AtomicInteger failures;
+
+		private Params(String bridge, AtomicInteger failures) {
+			this.bridge = bridge;
+			this.failures = failures;
+		}
+	}
 }

bramble-java/witness.gradle

@@ -24,7 +24,7 @@ dependencyVerification {
         'org.apache.ant:ant:1.9.4:ant-1.9.4.jar:649ae0730251de07b8913f49286d46bba7b92d47c5f332610aa426c4f02161d8',
         'org.beanshell:bsh:1.3.0:bsh-1.3.0.jar:9b04edc75d19db54f1b4e8b5355e9364384c6cf71eb0a1b9724c159d779879f8',
         'org.briarproject:obfs4proxy:0.0.12-dev-40245c4a:obfs4proxy-0.0.12-dev-40245c4a.zip:172029e7058b3a83ac93ac4991a44bf76e16ce8d46f558f5836d57da3cb3a766',
-        'org.briarproject:tor:0.3.5.13:tor-0.3.5.13.zip:1c5f0b821ee2aadb0ea04aa96caab3ca0a08370cce8de81c2dfe04d172f8a2a0',
+        'org.briarproject:tor:0.3.5.13-1:tor-0.3.5.13-1.zip:ef35c16bf8dc1f4c75ed71d9f55e4514f383d124ec96b859aca647c990927c99',
         '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.codehaus.mojo:animal-sniffer-annotations:1.17:animal-sniffer-annotations-1.17.jar:92654f493ecfec52082e76354f0ebf87648dc3d5cec2e3c3cdb947c016747a53',