Bump version numbers for 1.4.12 release.

Add Snowflake pluggable transport

See merge request briar/briar!1714

.gitlab-ci.yml

@@ -32,9 +32,9 @@ test:
   extends: .base-test
   stage: test
   script:
-    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom animalSnifferMain animalSnifferTest
+    - ./gradlew -Djava.security.egd=file:/dev/urandom animalSnifferMain animalSnifferTest
-    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom :briar-headless:linuxJars
+    - ./gradlew -Djava.security.egd=file:/dev/urandom assembleOfficialDebug :briar-headless:linuxJars
-    - ./gradlew --no-daemon -Djava.security.egd=file:/dev/urandom compileOfficialDebugAndroidTestSources compileScreenshotDebugAndroidTestSources check
+    - ./gradlew -Djava.security.egd=file:/dev/urandom compileOfficialDebugAndroidTestSources compileScreenshotDebugAndroidTestSources check
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       when: always
@@ -62,7 +62,7 @@ android test:
     when: on_failure
   rules:
     - if: '$CI_PIPELINE_SOURCE == "schedule"'
-      when: on_success
+      when: manual
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
       changes:
         - briar-android/**/*
@@ -85,35 +85,36 @@ test_reproducible:
 
 .optional_tests:
   stage: optional_tests
-  before_script:
+  extends: .base-test
-    - set -e
-    - export GRADLE_USER_HOME=$PWD/.gradle
-
-  cache:
-    key: "$CI_COMMIT_REF_SLUG"
-    paths:
-      - .gradle/wrapper
-      - .gradle/caches
-
-  script:
-    - OPTIONAL_TESTS=org.briarproject.bramble.plugin.tor.BridgeTest ./gradlew --info bramble-java:test --tests BridgeTest
-
-  after_script:
-    # these file change every time but should not be cached
-    - rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
-    - rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/
 
 bridge test:
   extends: .optional_tests
   rules:
     - if: '$CI_PIPELINE_SOURCE == "schedule"'
       when: on_success
-      allow_failure: true
+      allow_failure: false
     - if: '$CI_COMMIT_TAG == null'
       when: manual
       allow_failure: true
+  script:
+    - OPTIONAL_TESTS=org.briarproject.bramble.plugin.tor.BridgeTest ./gradlew --info bramble-java:test --tests BridgeTest
+  timeout: 4h
 
-pre_release_tests:
+mailbox integration test:
   extends: .optional_tests
-  only:
+  rules:
-    - tags
+    - if: '$CI_PIPELINE_SOURCE == "schedule"'
+      when: on_success
+    - if: '$CI_COMMIT_TAG == null'
+      when: manual
+      allow_failure: true # TODO figure out how not to allow failure while leaving this optional
+  script:
+    # start mailbox
+    - cd /opt && git clone --depth 1 https://code.briarproject.org/briar/briar-mailbox.git briar-mailbox
+    - cd briar-mailbox
+    - mkdir -p /root/.local/share # create directory that mailbox (currently) expects to exist
+    - ./gradlew run --args="--debug --setup-token 54686973206973206120736574757020746f6b656e20666f722042726961722e" &
+    # run mailbox integration test once mailbox has started
+    - cd "$CI_PROJECT_DIR"
+    - bramble-core/src/test/bash/wait-for-mailbox.sh
+    - OPTIONAL_TESTS=org.briarproject.bramble.mailbox.MailboxIntegrationTest ./gradlew --info bramble-core:test --tests MailboxIntegrationTest

CONTRIBUTING.md

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

README.md

@@ -22,6 +22,15 @@ our site.
 
 [Wiki](https://code.briarproject.org/briar/briar/-/wikis/home)
 
+## Reproducible builds
+
+We provide [docker images](https://code.briarproject.org/briar/briar-reproducer#briar-reproducer)
+to ease the task of verifying that the published APK binaries
+include nothing but our publicly available source code.
+
+You can either use those images or use them as a blueprint to build your own environment
+for reproduction.
+
 ## Donate
 
 [![Donate using Liberapay](https://briarproject.org/img/liberapay.svg)](https://liberapay.com/Briar/donate) [![Flattr this](https://briarproject.org/img/flattr-badge-large.png "Flattr this")](https://flattr.com/t/592836/)   

bramble-android/build.gradle

@@ -15,8 +15,8 @@ android {
 	defaultConfig {
 		minSdkVersion 16
 		targetSdkVersion 30
-		versionCode 10403
+		versionCode 10412
-		versionName "1.4.3"
+		versionName "1.4.12"
 		consumerProguardFiles 'proguard-rules.txt'
 
 		testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
@@ -42,8 +42,10 @@ configurations {
 
 dependencies {
 	implementation project(path: ':bramble-core', configuration: 'default')
+	implementation 'androidx.annotation:annotation:1.5.0'
 	tor "org.briarproject:tor-android:$tor_version"
-	tor "org.briarproject:obfs4proxy-android:$obfs4proxy_version@zip"
+	tor "org.briarproject:obfs4proxy-android:$obfs4proxy_version"
+	tor "org.briarproject:snowflake-android:$snowflake_version"
 
 	annotationProcessor "com.google.dagger:dagger-compiler:$dagger_version"
 
@@ -53,7 +55,7 @@ dependencies {
 	testImplementation "junit:junit:$junit_version"
 	testImplementation "org.jmock:jmock:$jmock_version"
 	testImplementation "org.jmock:jmock-junit4:$jmock_version"
-	testImplementation "org.jmock:jmock-legacy:$jmock_version"
+	testImplementation "org.jmock:jmock-imposters:$jmock_version"
 }
 
 def torBinariesDir = 'src/main/res/raw'
@@ -70,11 +72,6 @@ clean.dependsOn cleanTorBinaries
 
 task unpackTorBinaries {
 	doLast {
-		copy {
-			from configurations.tor.collect { zipTree(it) }
-			into torBinariesDir
-			include 'geoip.zip'
-		}
 		configurations.tor.each { outer ->
 			zipTree(outer).each { inner ->
 				if (inner.name.endsWith('_arm_pie.zip')) {

bramble-android/proguard-rules.txt

@@ -1,6 +1,8 @@
--keep,includedescriptorclasses class org.briarproject.** { *; }
+# Keep the H2 classes that are loaded via reflection
-
+-keep class org.h2.Driver { *; }
--keep class org.h2.** { *; }
+-keep class org.h2.engine.Engine { *; }
+-keep class org.h2.store.fs.** { *; }
+# Don't warn about unused dependencies of H2 classes
 -dontwarn org.h2.**
 -dontnote org.h2.**
 
@@ -15,5 +17,8 @@
 -dontwarn sun.misc.Unsafe
 -dontnote com.google.common.**
 
-# UPnP library isn't used
+-dontwarn com.fasterxml.jackson.databind.ext.Java7SupportImpl
--dontwarn org.bitlet.weupnp.**
+
+# Keep all Jackson-serialisable classes and their members
+-keep interface com.fasterxml.jackson.databind.annotation.JsonSerialize
+-keep @com.fasterxml.jackson.databind.annotation.JsonSerialize class * { *; }

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

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

bramble-android/src/main/java/org/briarproject/bramble/network/AndroidNetworkManager.java

@@ -11,7 +11,10 @@ import android.net.LinkAddress;
 import android.net.LinkProperties;
 import android.net.Network;
 import android.net.NetworkInfo;
+import android.net.wifi.WifiInfo;
+import android.net.wifi.WifiManager;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.lifecycle.Service;
@@ -21,7 +24,6 @@ import org.briarproject.bramble.api.network.event.NetworkStatusEvent;
 import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 
 import java.net.Inet4Address;
 import java.net.InetAddress;
@@ -38,6 +40,7 @@ import javax.annotation.Nullable;
 import javax.inject.Inject;
 
 import static android.content.Context.CONNECTIVITY_SERVICE;
+import static android.content.Context.WIFI_SERVICE;
 import static android.content.Intent.ACTION_SCREEN_OFF;
 import static android.content.Intent.ACTION_SCREEN_ON;
 import static android.net.ConnectivityManager.CONNECTIVITY_ACTION;
@@ -111,15 +114,37 @@ class AndroidNetworkManager implements NetworkManager, Service {
 
 	@Override
 	public NetworkStatus getNetworkStatus() {
-		NetworkInfo net = connectivityManager.getActiveNetworkInfo();
+		// https://issuetracker.google.com/issues/175055271
-		boolean connected = net != null && net.isConnected();
+		try {
-		boolean wifi = false, ipv6Only = false;
+			NetworkInfo net = connectivityManager.getActiveNetworkInfo();
-		if (connected) {
+			boolean connected = net != null && net.isConnected();
-			wifi = net.getType() == TYPE_WIFI;
+			boolean wifi = false, ipv6Only = false;
-			if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
+			if (connected) {
-			else ipv6Only = areAllAvailableNetworksIpv6Only();
+				wifi = net.getType() == TYPE_WIFI;
+				if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
+				else ipv6Only = areAllAvailableNetworksIpv6Only();
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
+			// Without the ConnectivityManager we can't detect whether we have
+			// internet access. Assume we do, which is probably less harmful
+			// than assuming we don't. Likewise, assume the connection is
+			// IPv6-only. Fall back to the WifiManager to detect whether we
+			// have a wifi connection.
+			LOG.info("ConnectivityManager is broken, guessing connectivity");
+			boolean connected = true, wifi = false, ipv6Only = true;
+			WifiManager wm = (WifiManager) app.getSystemService(WIFI_SERVICE);
+			if (wm != null) {
+				WifiInfo info = wm.getConnectionInfo();
+				if (info != null && info.getIpAddress() != 0) {
+					LOG.info("Connected to wifi");
+					wifi = true;
+					ipv6Only = false;
+				}
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
 		}
-		return new NetworkStatus(connected, wifi, ipv6Only);
 	}
 
 	/**
@@ -130,23 +155,29 @@ class AndroidNetworkManager implements NetworkManager, Service {
 	 */
 	@TargetApi(23)
 	private boolean isActiveNetworkIpv6Only() {
-		Network net = connectivityManager.getActiveNetwork();
+		// https://issuetracker.google.com/issues/175055271
-		if (net == null) {
+		try {
-			LOG.info("No active network");
+			Network net = connectivityManager.getActiveNetwork();
+			if (net == null) {
+				LOG.info("No active network");
+				return false;
+			}
+			LinkProperties props = connectivityManager.getLinkProperties(net);
+			if (props == null) {
+				LOG.info("No link properties for active network");
+				return false;
+			}
+			boolean hasIpv6Unicast = false;
+			for (LinkAddress linkAddress : props.getLinkAddresses()) {
+				InetAddress addr = linkAddress.getAddress();
+				if (addr instanceof Inet4Address) return false;
+				if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
+			}
+			return hasIpv6Unicast;
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 			return false;
 		}
-		LinkProperties props = connectivityManager.getLinkProperties(net);
-		if (props == null) {
-			LOG.info("No link properties for active network");
-			return false;
-		}
-		boolean hasIpv6Unicast = false;
-		for (LinkAddress linkAddress : props.getLinkAddresses()) {
-			InetAddress addr = linkAddress.getAddress();
-			if (addr instanceof Inet4Address) return false;
-			if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
-		}
-		return hasIpv6Unicast;
 	}
 
 	/**

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

@@ -32,13 +32,22 @@ class AndroidRemovableDrivePlugin extends RemovableDrivePlugin {
 	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));
+		try {
+			return app.getContentResolver().openInputStream(Uri.parse(uri));
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 
 	@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));
+		try {
+			return app.getContentResolver()
+					.openOutputStream(Uri.parse(uri), "wt");
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 }

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

@@ -175,16 +175,24 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	@TargetApi(21)
 	@Nullable
 	private InetAddress getWifiClientIpv6Address() {
-		for (Network net : connectivityManager.getAllNetworks()) {
+		// https://issuetracker.google.com/issues/175055271
-			NetworkCapabilities caps =
+		try {
-					connectivityManager.getNetworkCapabilities(net);
+			for (Network net : connectivityManager.getAllNetworks()) {
-			if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) continue;
+				NetworkCapabilities caps =
-			LinkProperties props = connectivityManager.getLinkProperties(net);
+						connectivityManager.getNetworkCapabilities(net);
-			if (props == null) continue;
+				if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) {
-			for (LinkAddress linkAddress : props.getLinkAddresses()) {
+					continue;
-				InetAddress addr = linkAddress.getAddress();
+				}
-				if (isIpv6LinkLocalAddress(addr)) return addr;
+				LinkProperties props =
+						connectivityManager.getLinkProperties(net);
+				if (props == null) continue;
+				for (LinkAddress linkAddress : props.getLinkAddresses()) {
+					InetAddress addr = linkAddress.getAddress();
+					if (isIpv6LinkLocalAddress(addr)) return addr;
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		return null;
 	}
@@ -227,12 +235,17 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	// network's socket factory may try to connect via another network
 	private SocketFactory getSocketFactory() {
 		if (SDK_INT < 21) return SocketFactory.getDefault();
-		for (Network net : connectivityManager.getAllNetworks()) {
+		// https://issuetracker.google.com/issues/175055271
-			NetworkCapabilities caps =
+		try {
-					connectivityManager.getNetworkCapabilities(net);
+			for (Network net : connectivityManager.getAllNetworks()) {
-			if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
+				NetworkCapabilities caps =
-				return net.getSocketFactory();
+						connectivityManager.getNetworkCapabilities(net);
+				if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
+					return net.getSocketFactory();
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		LOG.warning("Could not find suitable socket factory");
 		return SocketFactory.getDefault();

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

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

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

@@ -11,62 +11,35 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.Backoff;
 import org.briarproject.bramble.api.plugin.BackoffFactory;
 import org.briarproject.bramble.api.plugin.PluginCallback;
-import org.briarproject.bramble.api.plugin.TorConstants;
 import org.briarproject.bramble.api.plugin.TorControlPort;
 import org.briarproject.bramble.api.plugin.TorDirectory;
 import org.briarproject.bramble.api.plugin.TorSocksPort;
-import org.briarproject.bramble.api.plugin.TransportId;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPlugin;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.system.AndroidWakeLockManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.LocationUtils;
 import org.briarproject.bramble.api.system.ResourceProvider;
 import org.briarproject.bramble.api.system.WakefulIoExecutor;
-import org.briarproject.bramble.util.AndroidUtils;
 
 import java.io.File;
 import java.util.concurrent.Executor;
-import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 import javax.net.SocketFactory;
 
+import static org.briarproject.bramble.util.AndroidUtils.getSupportedArchitectures;
+
 @Immutable
 @NotNullByDefault
-public class AndroidTorPluginFactory implements DuplexPluginFactory {
+public class AndroidTorPluginFactory extends TorPluginFactory {
 
-	private static final Logger LOG =
-			Logger.getLogger(AndroidTorPluginFactory.class.getName());
-
-	private static final int MAX_LATENCY = 30 * 1000; // 30 seconds
-	private static final int MAX_IDLE_TIME = 30 * 1000; // 30 seconds
-	private static final int MIN_POLLING_INTERVAL = 60 * 1000; // 1 minute
-	private static final int MAX_POLLING_INTERVAL = 10 * 60 * 1000; // 10 mins
-	private static final double BACKOFF_BASE = 1.2;
-
-	private final Executor ioExecutor, wakefulIoExecutor;
 	private final Application app;
-	private final NetworkManager networkManager;
-	private final LocationUtils locationUtils;
-	private final EventBus eventBus;
-	private final SocketFactory torSocketFactory;
-	private final BackoffFactory backoffFactory;
-	private final ResourceProvider resourceProvider;
-	private final CircumventionProvider circumventionProvider;
-	private final BatteryManager batteryManager;
 	private final AndroidWakeLockManager wakeLockManager;
-	private final Clock clock;
-	private final File torDirectory;
-	private int torSocksPort;
-	private int torControlPort;
-	private final CryptoComponent crypto;
 
 	@Inject
 	AndroidTorPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
-			Application app,
 			NetworkManager networkManager,
 			LocationUtils locationUtils,
 			EventBus eventBus,
@@ -75,80 +48,43 @@ public class AndroidTorPluginFactory implements DuplexPluginFactory {
 			ResourceProvider resourceProvider,
 			CircumventionProvider circumventionProvider,
 			BatteryManager batteryManager,
-			AndroidWakeLockManager wakeLockManager,
 			Clock clock,
+			CryptoComponent crypto,
 			@TorDirectory File torDirectory,
 			@TorSocksPort int torSocksPort,
 			@TorControlPort int torControlPort,
-			CryptoComponent crypto) {
+			Application app,
-		this.ioExecutor = ioExecutor;
+			AndroidWakeLockManager wakeLockManager) {
-		this.wakefulIoExecutor = wakefulIoExecutor;
+		super(ioExecutor, wakefulIoExecutor, networkManager, locationUtils,
+				eventBus, torSocketFactory, backoffFactory, resourceProvider,
+				circumventionProvider, batteryManager, clock, crypto,
+				torDirectory, torSocksPort, torControlPort);
 		this.app = app;
-		this.networkManager = networkManager;
-		this.locationUtils = locationUtils;
-		this.eventBus = eventBus;
-		this.torSocketFactory = torSocketFactory;
-		this.backoffFactory = backoffFactory;
-		this.resourceProvider = resourceProvider;
-		this.circumventionProvider = circumventionProvider;
-		this.batteryManager = batteryManager;
 		this.wakeLockManager = wakeLockManager;
-		this.clock = clock;
-		this.torDirectory = torDirectory;
-		this.torSocksPort = torSocksPort;
-		this.torControlPort = torControlPort;
-		this.crypto = crypto;
 	}
 
+	@Nullable
 	@Override
-	public TransportId getId() {
+	String getArchitectureForTorBinary() {
-		return TorConstants.ID;
+		for (String abi : getSupportedArchitectures()) {
-	}
+			if (abi.startsWith("x86_64")) return "x86_64_pie";
-
+			else if (abi.startsWith("x86")) return "x86_pie";
-	@Override
+			else if (abi.startsWith("arm64")) return "arm64_pie";
-	public long getMaxLatency() {
+			else if (abi.startsWith("armeabi")) return "arm_pie";
-		return MAX_LATENCY;
-	}
-
-	@Override
-	public DuplexPlugin createPlugin(PluginCallback callback) {
-
-		// Check that we have a Tor binary for this architecture
-		String architecture = null;
-		for (String abi : AndroidUtils.getSupportedArchitectures()) {
-			if (abi.startsWith("x86_64")) {
-				architecture = "x86_64";
-				break;
-			} else if (abi.startsWith("x86")) {
-				architecture = "x86";
-				break;
-			} else if (abi.startsWith("arm64")) {
-				architecture = "arm64";
-				break;
-			} else if (abi.startsWith("armeabi")) {
-				architecture = "arm";
-				break;
-			}
 		}
-		if (architecture == null) {
+		return null;
-			LOG.info("Tor is not supported on this architecture");
+	}
-			return null;
-		}
-		// Use position-independent executable
-		architecture += "_pie";
 
-		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
+	@Override
-				MAX_POLLING_INTERVAL, BACKOFF_BASE);
+	TorPlugin createPluginInstance(Backoff backoff,
-		TorRendezvousCrypto torRendezvousCrypto =
+			TorRendezvousCrypto torRendezvousCrypto, PluginCallback callback,
-				new TorRendezvousCryptoImpl(crypto);
+			String architecture) {
-		AndroidTorPlugin plugin = new AndroidTorPlugin(ioExecutor,
+		return new AndroidTorPlugin(ioExecutor,
 				wakefulIoExecutor, app, networkManager, locationUtils,
 				torSocketFactory, clock, resourceProvider,
 				circumventionProvider, batteryManager, wakeLockManager,
 				backoff, torRendezvousCrypto, callback, architecture,
 				MAX_LATENCY, MAX_IDLE_TIME, torDirectory, torSocksPort,
 				torControlPort);
-		eventBus.addListener(plugin);
-		return plugin;
 	}
 }

bramble-android/src/main/java/org/briarproject/bramble/system/AndroidTaskScheduler.java

@@ -8,6 +8,7 @@ import android.content.Intent;
 import android.os.Process;
 import android.os.SystemClock;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.lifecycle.Service;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.AlarmListener;
@@ -116,10 +117,12 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		long dueMillis = now + MILLISECONDS.convert(delay, unit);
 		Runnable wakeful = () ->
 				wakeLockManager.executeWakefully(task, executor, "TaskHandoff");
-		Future<?> check = scheduleCheckForDueTasks(delay, unit);
+		// Acquire the lock before scheduling the check to ensure the check
-		ScheduledTask s = new ScheduledTask(wakeful, dueMillis, check,
+		// doesn't access the task queue before the task has been added
-				cancelled);
+		ScheduledTask s;
 		synchronized (lock) {
+			Future<?> check = scheduleCheckForDueTasks(delay, unit);
+			s = new ScheduledTask(wakeful, dueMillis, check, cancelled);
 			tasks.add(s);
 		}
 		return s;
@@ -136,6 +139,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		return schedule(wrapped, executor, delay, unit, cancelled);
 	}
 
+	@GuardedBy("lock")
 	private Future<?> scheduleCheckForDueTasks(long delay, TimeUnit unit) {
 		Runnable wakeful = () -> wakeLockManager.runWakefully(
 				this::runDueTasks, "TaskScheduler");
@@ -206,7 +210,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		private final Future<?> check;
 		private final AtomicBoolean cancelled;
 
-		public ScheduledTask(Runnable task, long dueMillis,
+		private ScheduledTask(Runnable task, long dueMillis,
 				Future<?> check, AtomicBoolean cancelled) {
 			this.task = task;
 			this.dueMillis = dueMillis;

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

@@ -4,6 +4,7 @@ import android.annotation.SuppressLint;
 import android.bluetooth.BluetoothAdapter;
 import android.content.Context;
 import android.os.Build;
+import android.os.Looper;
 import android.provider.Settings;
 
 import org.briarproject.bramble.api.Pair;
@@ -134,4 +135,8 @@ public class AndroidUtils {
 			return null;
 		}
 	}
+
+	public static boolean isUiThread() {
+		return Looper.myLooper() == Looper.getMainLooper();
+	}
 }

bramble-android/src/test/java/org/briarproject/bramble/account/AndroidAccountManagerTest.java

@@ -9,7 +9,7 @@ import org.briarproject.bramble.api.db.DatabaseConfig;
 import org.briarproject.bramble.api.identity.IdentityManager;
 import org.briarproject.bramble.test.BrambleMockTestCase;
 import org.jmock.Expectations;
-import org.jmock.lib.legacy.ClassImposteriser;
+import org.jmock.imposters.ByteBuddyClassImposteriser;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -44,7 +44,7 @@ public class AndroidAccountManagerTest extends BrambleMockTestCase {
 	private AndroidAccountManager accountManager;
 
 	public AndroidAccountManagerTest() {
-		context.setImposteriser(ClassImposteriser.INSTANCE);
+		context.setImposteriser(ByteBuddyClassImposteriser.INSTANCE);
 		app = context.mock(Application.class);
 		applicationInfo = new ApplicationInfo();
 		applicationInfo.dataDir = testDir.getAbsolutePath();

bramble-android/witness.gradle

@@ -1,5 +1,6 @@
 dependencyVerification {
     verify = [
+        'androidx.annotation:annotation:1.5.0:annotation-1.5.0.jar:261fb7c0210858500bab66d34354972a75166ab4182add283780b05513d6ec4a',
         'cglib:cglib:3.2.8:cglib-3.2.8.jar:3f64de999ecc5595dc84ca8ff0879d8a34c8623f9ef3c517a53ed59023fcb9db',
         'com.android.tools.analytics-library:protos:30.0.3:protos-30.0.3.jar:f62b89dcd9de719c6a7b7e15fb1dd20e45b57222e675cf633607bd0ed6bca7e7',
         'com.android.tools.analytics-library:shared:30.0.3:shared-30.0.3.jar:05aa9ba3cc890354108521fdf99802565aae5dd6ca44a6ac8bb8d594d1c1cd15',
@@ -87,8 +88,9 @@ dependencyVerification {
         'org.apache.httpcomponents:httpmime:4.5.6:httpmime-4.5.6.jar:0b2b1102c18d3c7e05a77214b9b7501a6f6056174ae5604e0e256776eda7553e',
         'org.bouncycastle:bcpkix-jdk15on:1.56:bcpkix-jdk15on-1.56.jar:7043dee4e9e7175e93e0b36f45b1ec1ecb893c5f755667e8b916eb8dd201c6ca',
         'org.bouncycastle:bcprov-jdk15on:1.56:bcprov-jdk15on-1.56.jar:963e1ee14f808ffb99897d848ddcdb28fa91ddda867eb18d303e82728f878349',
-        'org.briarproject:obfs4proxy-android:0.0.12-dev-40245c4a:obfs4proxy-android-0.0.12-dev-40245c4a.zip:8ab05a8f8391be2cb5ab2b665c281a06d9e3a756bd0f95a40a36ca927866ea82',
+        'org.briarproject:obfs4proxy-android:0.0.14:obfs4proxy-android-0.0.14.jar:ad9b1ee4757b05867a19e993147bbb018bddd1f26ce3da746d5f037d5991a8c8',
-        'org.briarproject:tor-android:0.3.5.17:tor-android-0.3.5.17.jar:1888afc10a26b93d00a010ea27bf0b1b162a6d524688b08b98d70d14dc363b54',
+        'org.briarproject:snowflake-android:2.3.1:snowflake-android-2.3.1.jar:1f83c9a070f87b7074af13627709a8b5aced5460104be7166af736b1bb73c293',
+        'org.briarproject:tor-android:0.4.5.14:tor-android-0.4.5.14.jar:7cf1beaa6c1db51fc8fac263aba9624ef289c3db29772509efcbc59f7057330a',
         'org.checkerframework:checker-compat-qual:2.5.3:checker-compat-qual-2.5.3.jar:d76b9afea61c7c082908023f0cbc1427fab9abd2df915c8b8a3e7a509bccbc6d',
         'org.checkerframework:checker-qual:2.5.2:checker-qual-2.5.2.jar:64b02691c8b9d4e7700f8ee2e742dce7ea2c6e81e662b7522c9ee3bf568c040a',
         'org.checkerframework:checker-qual:3.5.0:checker-qual-3.5.0.jar:729990b3f18a95606fc2573836b6958bcdb44cb52bfbd1b7aa9c339cff35a5a4',
@@ -129,10 +131,12 @@ dependencyVerification {
         'org.jetbrains.kotlin:kotlin-reflect:1.4.32:kotlin-reflect-1.4.32.jar:dbf19e9cdaa9c3c170f3f6f6ce3922f38dfc1d7fa1cab5b7c23a19da8b5eec5b',
         'org.jetbrains.kotlin:kotlin-stdlib-common:1.4.20:kotlin-stdlib-common-1.4.20.jar:a7112c9b3cefee418286c9c9372f7af992bd1e6e030691d52f60cb36dbec8320',
         'org.jetbrains.kotlin:kotlin-stdlib-common:1.4.32:kotlin-stdlib-common-1.4.32.jar:e1ff6f55ee9e7591dcc633f7757bac25a7edb1cc7f738b37ec652f10f66a4145',
+        'org.jetbrains.kotlin:kotlin-stdlib-common:1.7.10:kotlin-stdlib-common-1.7.10.jar:19f102efe9629f8eabc63853ad15c533e47c47f91fca09285c5bde86e59f91d4',
         'org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.4.32:kotlin-stdlib-jdk7-1.4.32.jar:5f801e75ca27d8791c14b07943c608da27620d910a8093022af57f543d5d98b6',
         'org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.4.32:kotlin-stdlib-jdk8-1.4.32.jar:adc43e54757b106e0cd7b3b7aa257dff471b61efdabe067fc02b2f57e2396262',
         'org.jetbrains.kotlin:kotlin-stdlib:1.4.20:kotlin-stdlib-1.4.20.jar:b8ab1da5cdc89cb084d41e1f28f20a42bd431538642a5741c52bbfae3fa3e656',
         'org.jetbrains.kotlin:kotlin-stdlib:1.4.32:kotlin-stdlib-1.4.32.jar:13e9fd3e69dc7230ce0fc873a92a4e5d521d179bcf1bef75a6705baac3bfecba',
+        'org.jetbrains.kotlin:kotlin-stdlib:1.7.10:kotlin-stdlib-1.7.10.jar:e771fe74250a943e8f6346713201ff1d8cb95c3a5d1a91a22b65a9e04f6a8901',
         'org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.1.0:kotlinx-metadata-jvm-0.1.0.jar:9753bb39efef35957c5c15df9a3cb769aabf2cdfa74b47afcb7760e5146be3b5',
         'org.jetbrains:annotations:13.0:annotations-13.0.jar:ace2a10dc8e2d5fd34925ecac03e4988b2c0f851650c94b8cef49ba1bd111478',
         'org.jmock:jmock-imposters:2.12.0:jmock-imposters-2.12.0.jar:3b836269745a137c9b2347e8d7c2104845b126ef04f012d6bfd94f1a7dea7b09',

bramble-api/build.gradle

@@ -9,11 +9,11 @@ apply from: 'witness.gradle'
 dependencies {
 	implementation "com.google.dagger:dagger:$dagger_version"
 	implementation 'com.google.code.findbugs:jsr305:3.0.2'
+	implementation "com.fasterxml.jackson.core:jackson-annotations:$jackson_version"
 
 	testImplementation "junit:junit:$junit_version"
 	testImplementation "org.jmock:jmock:$jmock_version"
 	testImplementation "org.jmock:jmock-junit4:$jmock_version"
-	testImplementation "org.jmock:jmock-legacy:$jmock_version"
 
 	signature 'org.codehaus.mojo.signature:java16:1.1@signature'
 }

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

@@ -0,0 +1,6 @@
+package org.briarproject.bramble.api;
+
+public interface Cancellable {
+
+	void cancel();
+}

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

@@ -10,4 +10,12 @@ public interface FeatureFlags {
 	boolean shouldEnableProfilePictures();
 
 	boolean shouldEnableDisappearingMessages();
+
+	boolean shouldEnableMailbox();
+
+	boolean shouldEnablePrivateGroupsInCore();
+
+	boolean shouldEnableForumsInCore();
+
+	boolean shouldEnableBlogsInCore();
 }

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

@@ -1,8 +1,14 @@
 package org.briarproject.bramble.api;
 
+import org.briarproject.bramble.util.StringUtils;
+
+import java.util.ArrayList;
 import java.util.Hashtable;
+import java.util.List;
 import java.util.Map;
 
+import javax.annotation.Nullable;
+
 public abstract class StringMap extends Hashtable<String, String> {
 
 	protected StringMap(Map<String, String> m) {
@@ -52,4 +58,31 @@ public abstract class StringMap extends Hashtable<String, String> {
 	public void putLong(String key, long value) {
 		put(key, String.valueOf(value));
 	}
+
+	@Nullable
+	public int[] getIntArray(String key) {
+		String s = get(key);
+		if (s == null) return null;
+		// Handle empty string because "".split(",") returns {""}
+		if (s.length() == 0) return new int[0];
+		String[] intStrings = s.split(",");
+		int[] ints = new int[intStrings.length];
+		try {
+			for (int i = 0; i < ints.length; i++) {
+				ints[i] = Integer.parseInt(intStrings[i]);
+			}
+		} catch (NumberFormatException e) {
+			return null;
+		}
+		return ints;
+	}
+
+	public void putIntArray(String key, int[] value) {
+		List<String> intStrings = new ArrayList<>();
+		for (int integer : value) {
+			intStrings.add(String.valueOf(integer));
+		}
+		// Puts empty string if input array value is empty
+		put(key, StringUtils.join(intStrings, ","));
+	}
 }

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

@@ -6,14 +6,14 @@ import javax.annotation.concurrent.ThreadSafe;
 
 @ThreadSafe
 @NotNullByDefault
-public abstract class UniqueId extends Bytes {
+public class UniqueId extends Bytes {
 
 	/**
 	 * The length of a unique identifier in bytes.
 	 */
 	public static final int LENGTH = 32;
 
-	protected UniqueId(byte[] id) {
+	public UniqueId(byte[] id) {
 		super(id);
 		if (id.length != LENGTH) throw new IllegalArgumentException();
 	}

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

@@ -1,4 +1,4 @@
-package org.briarproject.briar.feed;
+package org.briarproject.bramble.api;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -13,7 +13,7 @@ import javax.inject.Provider;
  * collected.
  */
 @NotNullByDefault
-abstract class WeakSingletonProvider<T> implements Provider<T> {
+public abstract class WeakSingletonProvider<T> implements Provider<T> {
 
 	private final Object lock = new Object();
 	@GuardedBy("lock")
@@ -31,5 +31,5 @@ abstract class WeakSingletonProvider<T> implements Provider<T> {
 		}
 	}
 
-	abstract T createInstance();
+	public abstract T createInstance();
 }

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

@@ -9,6 +9,8 @@ import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -18,6 +20,7 @@ import org.briarproject.bramble.api.sync.MessageId;
 
 import java.security.GeneralSecurityException;
 import java.util.Collection;
+import java.util.List;
 import java.util.Map;
 
 @NotNullByDefault
@@ -123,6 +126,19 @@ public interface ClientHelper {
 	Map<TransportId, TransportProperties> parseAndValidateTransportPropertiesMap(
 			BdfDictionary properties) throws FormatException;
 
+	/**
+	 * Parse and validate the elements of a Mailbox update message.
+	 *
+	 * @return the parsed update message
+	 * @throws FormatException if the message elements are invalid
+	 */
+	MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
+			BdfList serverSupports, BdfDictionary properties)
+			throws FormatException;
+
+	List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException;
+
 	/**
 	 * Retrieves the contact ID from the group metadata of the given contact
 	 * group.

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

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

bramble-api/src/main/java/org/briarproject/bramble/api/contact/ContactManager.java

@@ -113,6 +113,26 @@ public interface ContactManager {
 	 */
 	String getHandshakeLink(Transaction txn) throws DbException;
 
+	/**
+	 * Creates a {@link PendingContact} from the given handshake link and
+	 * alias, adds it to the database and returns it.
+	 *
+	 * @param link The handshake link received from the pending contact
+	 * @param alias The alias the user has given this pending contact
+	 * @throws UnsupportedVersionException If the link uses a format version
+	 * that is not supported
+	 * @throws FormatException If the link is invalid
+	 * @throws GeneralSecurityException If the pending contact's handshake
+	 * public key is invalid
+	 * @throws ContactExistsException If a contact with the same handshake
+	 * public key already exists
+	 * @throws PendingContactExistsException If a pending contact with the same
+	 * handshake public key already exists
+	 */
+	PendingContact addPendingContact(Transaction txn, String link, String alias)
+			throws DbException, FormatException, GeneralSecurityException,
+			ContactExistsException, PendingContactExistsException;
+
 	/**
 	 * Creates a {@link PendingContact} from the given handshake link and
 	 * alias, adds it to the database and returns it.
@@ -146,11 +166,24 @@ public interface ContactManager {
 	Collection<Pair<PendingContact, PendingContactState>> getPendingContacts()
 			throws DbException;
 
+	/**
+	 * Returns a list of {@link PendingContact PendingContacts} and their
+	 * {@link PendingContactState states}.
+	 */
+	Collection<Pair<PendingContact, PendingContactState>> getPendingContacts(Transaction txn)
+			throws DbException;
+
 	/**
 	 * Removes a {@link PendingContact}.
 	 */
 	void removePendingContact(PendingContactId p) throws DbException;
 
+	/**
+	 * Removes a {@link PendingContact}.
+	 */
+	void removePendingContact(Transaction txn, PendingContactId p)
+			throws DbException;
+
 	/**
 	 * Returns the contact with the given ID.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/contact/PendingContactId.java

@@ -3,7 +3,6 @@ package org.briarproject.bramble.api.contact;
 import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
-import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 
 /**
@@ -17,9 +16,4 @@ public class PendingContactId extends UniqueId {
 	public PendingContactId(byte[] id) {
 		super(id);
 	}
-
-	@Override
-	public boolean equals(@Nullable Object o) {
-		return o instanceof PendingContactId && super.equals(o);
-	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/crypto/CryptoComponent.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.api.crypto;
 
+import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.security.GeneralSecurityException;
@@ -10,6 +11,8 @@ import javax.annotation.Nullable;
 @NotNullByDefault
 public interface CryptoComponent {
 
+	UniqueId generateUniqueId();
+
 	SecretKey generateSecretKey();
 
 	SecureRandom getSecureRandom();
@@ -172,9 +175,11 @@ public interface CryptoComponent {
 	String asciiArmour(byte[] b, int lineLength);
 
 	/**
-	 * Encode the onion/hidden service address given its public key. As
+	 * Encode the Onion given its public key. Specified here:
-	 * specified here: https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt?id=29245fd5#n2135
+	 * https://gitweb.torproject.org/torspec.git/tree/rend-spec-v3.txt?id=29245fd5#n2135
+	 *
+	 * @return the encoded onion, base32 chars
 	 */
-	String encodeOnionAddress(byte[] publicKey);
+	String encodeOnion(byte[] publicKey);
 
 }

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

@@ -33,11 +33,18 @@ import java.util.List;
 import java.util.Map;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
 /**
  * Encapsulates the database implementation and exposes high-level operations
  * to other components.
+ * <p>
+ * With the exception of the {@link #open(SecretKey, MigrationListener)} and
+ * {@link #close()} methods, which must not be called concurrently, the
+ * database can be accessed from any thread. See {@link TransactionManager}
+ * for locking behaviour.
  */
+@ThreadSafe
 @NotNullByDefault
 public interface DatabaseComponent extends TransactionManager {
 
@@ -119,16 +126,11 @@ public interface DatabaseComponent extends TransactionManager {
 			TransportKeys k) throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
+	 * Returns true if there are any acks to send to the given contact.
-	 * contact over a transport with the given maximum latency.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(Transaction txn, ContactId c,
+	boolean containsAcksToSend(Transaction txn, ContactId c) throws DbException;
-			long maxLatency, boolean eager) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -154,6 +156,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	boolean containsIdentity(Transaction txn, AuthorId a) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given contact
+	 * over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(Transaction txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -193,26 +207,15 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
+	 * Returns a batch of messages for the given contact, for transmission over
-	 * less than or equal to the given length, for transmission over a
+	 * a transport with the given maximum latency. The total length of the
-	 * transport with the given maximum latency. Returns null if there are no
+	 * messages, including record headers, will be no more than the given
-	 * sendable messages that fit in the given length.
+	 * capacity. Returns null if there are no sendable messages that would fit
+	 * in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long 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, long maxLatency) throws DbException;
 
 	/**
 	 * Returns an offer for the given contact for transmission over a
@@ -232,15 +235,16 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
+	 * Returns a batch of messages for the given contact, for transmission over
-	 * less than or equal to the given length, for transmission over a
+	 * a transport with the given maximum latency. Only messages that have been
-	 * transport with the given maximum latency. Only messages that have been
+	 * requested by the contact are returned. The total length of the messages,
-	 * requested by the contact are returned. Returns null if there are no
+	 * including record headers, will be no more than the given capacity.
-	 * sendable messages that fit in the given length.
+	 * Returns null if there are no sendable messages that have been requested
+	 * by the contact and would fit in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateRequestedBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns the contact with the given ID.
@@ -279,6 +283,13 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	Group getGroup(Transaction txn, GroupId g) throws DbException;
 
+	/**
+	 * Returns the ID of the group containing the given message.
+	 * <p/>
+	 * Read-only.
+	 */
+	GroupId getGroupId(Transaction txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for the given group.
 	 * <p/>
@@ -344,6 +355,30 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<MessageId> getMessageIds(Transaction txn, GroupId g,
 			Metadata query) throws DbException;
 
+	/**
+	 * Returns the IDs of all messages received from the given contact that
+	 * need to be acknowledged.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToAck(Transaction txn, ContactId c)
+			throws DbException;
+
+	/**
+	 * Returns the IDs of some messages that are eligible to be sent to the
+	 * given contact over a transport with the given maximum latency. The total
+	 * length of the messages including record headers will be no more than the
+	 * given capacity.
+	 * <p/>
+	 * Unlike {@link #getUnackedMessagesToSend(Transaction, ContactId)} this
+	 * method does not return messages that have already been sent unless they
+	 * are due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToSend(Transaction txn, ContactId c,
+			long capacity, long maxLatency) throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>
@@ -460,21 +495,38 @@ public interface DatabaseComponent extends TransactionManager {
 	MessageStatus getMessageStatus(Transaction txn, ContactId c, MessageId m)
 			throws DbException;
 
+	/**
+	 * Returns the message with the given ID for transmission to the given
+	 * contact over a transport with the given maximum latency. Returns null
+	 * if the message is no longer visible to the contact.
+	 * <p/>
+	 * Read-only if {@code markAsSent} is false.
+	 *
+	 * @param markAsSent True if the message should be marked as sent.
+	 * If false it can be marked as sent by calling
+	 * {@link #setMessagesSent(Transaction, ContactId, Collection, long)}.
+	 */
+	@Nullable
+	Message getMessageToSend(Transaction txn, ContactId c, MessageId m,
+			long maxLatency, boolean markAsSent) 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
+	 * given contact.
-	 * messages that have already been sent and are not yet due for
+	 * <p>
-	 * retransmission.
+	 * Unlike {@link #getMessagesToSend(Transaction, ContactId, long, long)}
+	 * this method may return messages that have already been sent and are
+	 * not yet due for retransmission.
 	 * <p/>
 	 * Read-only.
 	 */
-	Map<MessageId, Integer> getUnackedMessagesToSend(Transaction txn,
+	Collection<MessageId> getUnackedMessagesToSend(Transaction txn,
 			ContactId c) throws DbException;
 
 	/**
-	 * Reset the transmission count, expiry time and ETA of all messages that
+	 * Resets the transmission count, expiry time and max latency of all messages
-	 * are eligible to be sent to the given contact. This includes messages that
+	 * that are eligible to be sent to the given contact. This includes messages
-	 * have already been sent and are not yet due for retransmission.
+	 * that have already been sent and are not yet due for retransmission.
 	 */
 	void resetUnackedMessagesToSend(Transaction txn, ContactId c)
 			throws DbException;
@@ -498,15 +550,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	long getNextCleanupDeadline(Transaction txn) throws DbException;
 
-	/*
+	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
+	 * message is due to be sent to the given contact over a transport with
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE if
+	 * the given latency.
-	 * no messages are scheduled to be sent.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(Transaction txn, ContactId c) throws DbException;
+	long getNextSendTime(Transaction txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.
@@ -648,6 +703,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void removeTransportKeys(Transaction txn, TransportId t, KeySetId k)
 			throws DbException;
 
+	/**
+	 * Records an ack for the given messages as having been sent to the given
+	 * contact.
+	 */
+	void setAckSent(Transaction txn, ContactId c, Collection<MessageId> acked)
+			throws DbException;
+
 	/**
 	 * Sets the cleanup timer duration for the given message. This does not
 	 * start the message's cleanup timer.
@@ -694,6 +756,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void setMessageState(Transaction txn, MessageId m, MessageState state)
 			throws DbException;
 
+	/**
+	 * Records the given messages as having been sent to the given contact
+	 * over a transport with the given maximum latency.
+	 */
+	void setMessagesSent(Transaction txn, ContactId c,
+			Collection<MessageId> sent, long maxLatency) throws DbException;
+
 	/**
 	 * Adds dependencies for a message
 	 */

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

@@ -18,6 +18,10 @@ import static java.lang.annotation.RetentionPolicy.RUNTIME;
  * submitted, tasks are not run concurrently, and submitting a task will never
  * block. Tasks must not run indefinitely. Tasks submitted during shutdown are
  * discarded.
+ * <p>
+ * It is not mandatory to use this executor for database tasks. The database
+ * can be accessed from any thread, but this executor's guarantee that tasks
+ * are run in the order they're submitted may be useful in some cases.
  */
 @Qualifier
 @Target({FIELD, METHOD, PARAMETER})

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

@@ -45,6 +45,9 @@ public class Transaction {
 	/**
 	 * Attaches an event to be broadcast when the transaction has been
 	 * committed. The event will be broadcast on the {@link EventExecutor}.
+	 * Events and {@link #attach(Runnable) tasks} are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Event e) {
 		if (actions == null) actions = new ArrayList<>();
@@ -54,6 +57,9 @@ public class Transaction {
 	/**
 	 * Attaches a task to be executed when the transaction has been
 	 * committed. The task will be run on the {@link EventExecutor}.
+	 * {@link #attach(Event) Events} and tasks are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Runnable r) {
 		if (actions == null) actions = new ArrayList<>();

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

@@ -1,51 +1,95 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
+/**
+ * An interface for managing database transactions.
+ * <p>
+ * Read-only transactions may access the database concurrently. Read-write
+ * transactions access the database exclusively, so starting a read-only or
+ * read-write transaction will block until there are no read-write
+ * transactions in progress.
+ * <p>
+ * Failing to {@link #endTransaction(Transaction) end} a transaction will
+ * prevent other callers from accessing the database, so it is recommended to
+ * use the {@link #transaction(boolean, DbRunnable)},
+ * {@link #transactionWithResult(boolean, DbCallable)} and
+ * {@link #transactionWithNullableResult(boolean, NullableDbCallable)} methods
+ * where possible, which handle committing or aborting the transaction on the
+ * caller's behalf.
+ * <p>
+ * Transactions are not reentrant, i.e. it is not permitted to start a
+ * transaction on a thread that already has a transaction in progress.
+ */
+@ThreadSafe
 @NotNullByDefault
 public interface TransactionManager {
 
 	/**
-	 * Starts a new transaction and returns an object representing it.
+	 * Starts a new transaction and returns an object representing it. This
-	 * <p/>
+	 * method acquires the database lock, which is held until
-	 * This method acquires locks, so it must not be called while holding a
+	 * {@link #endTransaction(Transaction)} is called.
-	 * lock.
 	 *
-	 * @param readOnly true if the transaction will only be used for reading.
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	Transaction startTransaction(boolean readOnly) throws DbException;
 
 	/**
 	 * Commits a transaction to the database.
+	 * {@link #endTransaction(Transaction)} must be called to release the
+	 * database lock.
 	 */
 	void commitTransaction(Transaction txn) throws DbException;
 
 	/**
-	 * Ends a transaction. If the transaction has not been committed,
+	 * Ends a transaction. If the transaction has not been committed by
-	 * it will be aborted. If the transaction has been committed,
+	 * calling {@link #commitTransaction(Transaction)}, it is aborted and the
-	 * any events attached to the transaction are broadcast.
+	 * database lock is released.
-	 * The database lock will be released in either case.
+	 * <p>
+	 * If the transaction has been committed, any
+	 * {@link Transaction#attach events} attached to the transaction are
+	 * broadcast and any {@link Transaction#attach(Runnable) tasks} attached
+	 * to the transaction are submitted to the {@link EventExecutor}. The
+	 * database lock is then released.
 	 */
 	void endTransaction(Transaction txn);
 
 	/**
-	 * Runs the given task within a transaction.
+	 * Runs the given task within a transaction. The database lock is held
+	 * while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<E extends Exception> void transaction(boolean readOnly,
 			DbRunnable<E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task.
+	 * task. The database lock is held while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<R, E extends Exception> R transactionWithResult(boolean readOnly,
 			DbCallable<R, E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task, which may be null.
+	 * task, which may be null. The database lock is held while running the
+	 * task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	@Nullable
 	<R, E extends Exception> R transactionWithNullableResult(boolean readOnly,

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

@@ -21,9 +21,4 @@ public class AuthorId extends UniqueId {
 	public AuthorId(byte[] id) {
 		super(id);
 	}
-
-	@Override
-	public boolean equals(Object o) {
-		return o instanceof AuthorId && super.equals(o);
-	}
 }

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

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

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

@@ -0,0 +1,8 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public class InvalidMailboxIdException extends Exception {
+
+}

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

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+public class MailboxAuthToken extends MailboxId {
+	public MailboxAuthToken(byte[] id) {
+		super(id);
+	}
+
+	/**
+	 * Creates a {@link MailboxAuthToken} from the given string.
+	 *
+	 * @throws InvalidMailboxIdException if token is not valid.
+	 */
+	public static MailboxAuthToken fromString(@Nullable String token)
+			throws InvalidMailboxIdException {
+		return new MailboxAuthToken(bytesFromString(token));
+	}
+}

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

@@ -0,0 +1,73 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.plugin.TransportId;
+
+import java.util.List;
+
+import static java.util.Collections.singletonList;
+import static java.util.concurrent.TimeUnit.DAYS;
+import static java.util.concurrent.TimeUnit.HOURS;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_FRAME_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_PAYLOAD_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.STREAM_HEADER_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
+
+public interface MailboxConstants {
+
+	/**
+	 * The transport ID of the mailbox plugin.
+	 */
+	TransportId ID = new TransportId("org.briarproject.bramble.mailbox");
+
+	/**
+	 * Mailbox API versions that we support as a client. This is reported to our
+	 * contacts by {@link MailboxUpdateManager}.
+	 */
+	List<MailboxVersion> CLIENT_SUPPORTS = singletonList(
+			new MailboxVersion(1, 0));
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when the server is too old to support our major version.
+	 */
+	int API_SERVER_TOO_OLD = -1;
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when we as a client are too old to support the server's major version.
+	 */
+	int API_CLIENT_TOO_OLD = -2;
+
+	/**
+	 * The maximum length of a file that can be uploaded to or downloaded from
+	 * a mailbox.
+	 */
+	int MAX_FILE_BYTES = 1024 * 1024;
+
+	/**
+	 * The maximum length of the plaintext payload of a file, such that the
+	 * ciphertext is no more than {@link #MAX_FILE_BYTES}.
+	 */
+	int MAX_FILE_PAYLOAD_BYTES =
+			(MAX_FILE_BYTES - TAG_LENGTH - STREAM_HEADER_LENGTH)
+					/ MAX_FRAME_LENGTH * MAX_PAYLOAD_LENGTH;
+
+	/**
+	 * The number of connection failures
+	 * that indicate a problem with the mailbox.
+	 */
+	int PROBLEM_NUM_CONNECTION_FAILURES = 5;
+
+	/**
+	 * The time in milliseconds since the last connection success
+	 * that need to pass to indicates a problem with the mailbox.
+	 */
+	long PROBLEM_MS_SINCE_LAST_SUCCESS = HOURS.toMillis(1);
+
+	/**
+	 * The maximum latency of the mailbox transport in milliseconds.
+	 */
+	long MAX_LATENCY = DAYS.toMillis(14);
+}

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

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

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

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+public class MailboxFileId extends MailboxId {
+	public MailboxFileId(byte[] id) {
+		super(id);
+	}
+
+	/**
+	 * Creates a {@link MailboxFileId} from the given string.
+	 *
+	 * @throws IllegalArgumentException if token is not valid.
+	 */
+	public static MailboxFileId fromString(@Nullable String token)
+			throws InvalidMailboxIdException {
+		return new MailboxFileId(bytesFromString(token));
+	}
+}

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

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+public class MailboxFolderId extends MailboxId {
+	public MailboxFolderId(byte[] id) {
+		super(id);
+	}
+
+	/**
+	 * Creates a {@link MailboxFolderId} from the given string.
+	 *
+	 * @throws IllegalArgumentException if token is not valid.
+	 */
+	public static MailboxFolderId fromString(@Nullable String token)
+			throws InvalidMailboxIdException {
+		return new MailboxFolderId(bytesFromString(token));
+	}
+}

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

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

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

@@ -0,0 +1,50 @@
+package org.briarproject.bramble.api.mailbox;
+
+import com.fasterxml.jackson.annotation.JsonValue;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.UniqueId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.Locale;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static org.briarproject.bramble.util.StringUtils.fromHexString;
+import static org.briarproject.bramble.util.StringUtils.toHexString;
+
+@ThreadSafe
+@NotNullByDefault
+public abstract class MailboxId extends UniqueId {
+	MailboxId(byte[] id) {
+		super(id);
+	}
+
+	/**
+	 * Returns valid {@link MailboxId} bytes from the given string.
+	 *
+	 * @throws InvalidMailboxIdException if token is not valid.
+	 */
+	static byte[] bytesFromString(@Nullable String token)
+			throws InvalidMailboxIdException {
+		if (token == null || token.length() != 64) {
+			throw new InvalidMailboxIdException();
+		}
+		try {
+			return fromHexString(token);
+		} catch (FormatException e) {
+			throw new InvalidMailboxIdException();
+		}
+	}
+
+	/**
+	 * Returns the string representation expected by the mailbox API.
+	 * Also used for serialization.
+	 */
+	@Override
+	@JsonValue
+	public String toString() {
+		return toHexString(getBytes()).toLowerCase(Locale.US);
+	}
+}

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

@@ -0,0 +1,56 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
+
+import javax.annotation.Nullable;
+
+public interface MailboxManager {
+
+	/**
+	 * @return true if a mailbox is already paired.
+	 */
+	boolean isPaired(Transaction txn) throws DbException;
+
+	/**
+	 * @return the current status of the mailbox.
+	 */
+	MailboxStatus getMailboxStatus(Transaction txn) throws DbException;
+
+	/**
+	 * Returns the currently running pairing task,
+	 * or null if no pairing task is running.
+	 */
+	@Nullable
+	MailboxPairingTask getCurrentPairingTask();
+
+	/**
+	 * Starts and returns a pairing task. If a pairing task is already running,
+	 * it will be returned and the argument will be ignored.
+	 *
+	 * @param qrCodePayload The ISO-8859-1 encoded bytes of the mailbox QR code.
+	 */
+	MailboxPairingTask startPairingTask(String qrCodePayload);
+
+	/**
+	 * Can be used by the UI to test the mailbox connection.
+	 *
+	 * @return true (success) or false (error).
+	 * A {@link OwnMailboxConnectionStatusEvent} might be broadcast with a new
+	 * {@link MailboxStatus}.
+	 */
+	boolean checkConnection();
+
+	/**
+	 * Unpairs the owner's mailbox and tries to wipe it.
+	 * As this makes a network call, it should be run on the {@link IoExecutor}.
+	 *
+	 * @return true if we could wipe the mailbox, false if we couldn't.
+	 * It is advised to inform the user to wipe the mailbox themselves,
+	 * if we failed to wipe it.
+	 */
+	@IoExecutor
+	boolean unPair() throws DbException;
+}

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

@@ -0,0 +1,25 @@
+package org.briarproject.bramble.api.mailbox;
+
+public abstract class MailboxPairingState {
+
+	public static class QrCodeReceived extends MailboxPairingState {
+	}
+
+	public static class Pairing extends MailboxPairingState {
+	}
+
+	public static class Paired extends MailboxPairingState {
+	}
+
+	public static class InvalidQrCode extends MailboxPairingState {
+	}
+
+	public static class MailboxAlreadyPaired extends MailboxPairingState {
+	}
+
+	public static class ConnectionError extends MailboxPairingState {
+	}
+
+	public static class UnexpectedError extends MailboxPairingState {
+	}
+}

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

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.Consumer;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+public interface MailboxPairingTask extends Runnable {
+
+	/**
+	 * Adds an observer to the task. The observer will be notified on the
+	 * event thread of the current state of the task and any subsequent state
+	 * changes.
+	 */
+	void addObserver(Consumer<MailboxPairingState> observer);
+
+	/**
+	 * Removes an observer from the task.
+	 */
+	void removeObserver(Consumer<MailboxPairingState> observer);
+
+}

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

@@ -1,32 +1,98 @@
 package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.nullsafety.NullSafety;
 
+import java.util.List;
+
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
 public class MailboxProperties {
 
-	private final String onionAddress, authToken;
+	private final String onion;
+	private final MailboxAuthToken authToken;
 	private final boolean owner;
+	private final List<MailboxVersion> serverSupports;
+	@Nullable
+	private final MailboxFolderId inboxId; // Null for own mailbox
+	@Nullable
+	private final MailboxFolderId outboxId; // Null for own mailbox
 
-	public MailboxProperties(String onionAddress, String authToken,
+	/**
-			boolean owner) {
+	 * Constructor for properties used by the mailbox's owner.
-		this.onionAddress = onionAddress;
+	 */
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
+			List<MailboxVersion> serverSupports) {
+		this.onion = onion;
 		this.authToken = authToken;
-		this.owner = owner;
+		this.owner = true;
+		this.serverSupports = serverSupports;
+		this.inboxId = null;
+		this.outboxId = null;
 	}
 
-	public String getOnionAddress() {
+	/**
-		return onionAddress;
+	 * Constructor for properties used by a contact of the mailbox's owner.
+	 */
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
+			List<MailboxVersion> serverSupports, MailboxFolderId inboxId,
+			MailboxFolderId outboxId) {
+		this.onion = onion;
+		this.authToken = authToken;
+		this.owner = false;
+		this.serverSupports = serverSupports;
+		this.inboxId = inboxId;
+		this.outboxId = outboxId;
 	}
 
-	public String getAuthToken() {
+	/**
+	 * Returns the onion address of the mailbox, excluding the .onion suffix.
+	 */
+	public String getOnion() {
+		return onion;
+	}
+
+	public MailboxAuthToken getAuthToken() {
 		return authToken;
 	}
 
 	public boolean isOwner() {
 		return owner;
 	}
+
+	public List<MailboxVersion> getServerSupports() {
+		return serverSupports;
+	}
+
+	@Nullable
+	public MailboxFolderId getInboxId() {
+		return inboxId;
+	}
+
+	@Nullable
+	public MailboxFolderId getOutboxId() {
+		return outboxId;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o instanceof MailboxProperties) {
+			MailboxProperties m = (MailboxProperties) o;
+			return owner == m.owner &&
+					onion.equals(m.onion) &&
+					authToken.equals(m.authToken) &&
+					NullSafety.equals(inboxId, m.inboxId) &&
+					NullSafety.equals(outboxId, m.outboxId) &&
+					serverSupports.equals(m.serverSupports);
+		}
+		return false;
+	}
+
+	@Override
+	public int hashCode() {
+		return authToken.hashCode();
+	}
 }

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

@@ -1,15 +1,26 @@
 package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.Nullable;
 
 @NotNullByDefault
 public interface MailboxSettingsManager {
 
+	/**
+	 * Registers a hook to be called when a mailbox has been paired or unpaired.
+	 * This method should be called before
+	 * {@link LifecycleManager#startServices(SecretKey)}.
+	 */
+	void registerMailboxHook(MailboxHook hook);
+
 	@Nullable
 	MailboxProperties getOwnMailboxProperties(Transaction txn)
 			throws DbException;
@@ -17,10 +28,12 @@ public interface MailboxSettingsManager {
 	void setOwnMailboxProperties(Transaction txn, MailboxProperties p)
 			throws DbException;
 
+	void removeOwnMailboxProperties(Transaction txn) throws DbException;
+
 	MailboxStatus getOwnMailboxStatus(Transaction txn) throws DbException;
 
-	void recordSuccessfulConnection(Transaction txn, long now)
+	void recordSuccessfulConnection(Transaction txn, long now,
-			throws DbException;
+			List<MailboxVersion> versions) throws DbException;
 
 	void recordFailedConnectionAttempt(Transaction txn, long now)
 			throws DbException;
@@ -30,4 +43,31 @@ public interface MailboxSettingsManager {
 
 	@Nullable
 	String getPendingUpload(Transaction txn, ContactId id) throws DbException;
+
+	interface MailboxHook {
+		/**
+		 * Called when Briar is paired with a mailbox.
+		 *
+		 * @param txn A read-write transaction
+		 */
+		void mailboxPaired(Transaction txn, MailboxProperties p)
+				throws DbException;
+
+		/**
+		 * Called when the mailbox is unpaired.
+		 *
+		 * @param txn A read-write transaction
+		 */
+		void mailboxUnpaired(Transaction txn) throws DbException;
+
+		/**
+		 * Called when we receive our mailbox's server-supported API versions.
+		 * This happens whenever we successfully check the connectivity of
+		 * our mailbox, so this hook may be called frequently.
+		 *
+		 * @param txn A read-write transaction
+		 */
+		void serverSupportedVersionsReceived(Transaction txn,
+				List<MailboxVersion> serverSupports) throws DbException;
+	}
 }

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

@@ -2,20 +2,30 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_MS_SINCE_LAST_SUCCESS;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_NUM_CONNECTION_FAILURES;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
+
 @Immutable
 @NotNullByDefault
 public class MailboxStatus {
 
 	private final long lastAttempt, lastSuccess;
 	private final int attemptsSinceSuccess;
+	private final List<MailboxVersion> serverSupports;
 
 	public MailboxStatus(long lastAttempt, long lastSuccess,
-			int attemptsSinceSuccess) {
+			int attemptsSinceSuccess,
+			List<MailboxVersion> serverSupports) {
 		this.lastAttempt = lastAttempt;
 		this.lastSuccess = lastSuccess;
 		this.attemptsSinceSuccess = attemptsSinceSuccess;
+		this.serverSupports = serverSupports;
 	}
 
 	/**
@@ -56,4 +66,28 @@ public class MailboxStatus {
 	public int getAttemptsSinceSuccess() {
 		return attemptsSinceSuccess;
 	}
+
+	/**
+	 * Returns the mailbox's supported API versions.
+	 */
+	public List<MailboxVersion> getServerSupports() {
+		return serverSupports;
+	}
+
+	/**
+	 * @return true if this status indicates a problem with the mailbox.
+	 */
+	public boolean hasProblem(long now) {
+		return attemptsSinceSuccess >= PROBLEM_NUM_CONNECTION_FAILURES &&
+				(now - lastSuccess) >= PROBLEM_MS_SINCE_LAST_SUCCESS;
+	}
+
+	/**
+	 * @return a positive integer if the mailbox is compatible. Same result as
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}.
+	 */
+	public int getMailboxCompatibility() {
+		return getHighestCommonMajorVersion(CLIENT_SUPPORTS, serverSupports);
+	}
+
 }

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

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdate {
+	private final boolean hasMailbox;
+	private final List<MailboxVersion> clientSupports;
+
+	public MailboxUpdate(List<MailboxVersion> clientSupports) {
+		this(clientSupports, false);
+	}
+
+	MailboxUpdate(List<MailboxVersion> clientSupports, boolean hasMailbox) {
+		this.clientSupports = clientSupports;
+		this.hasMailbox = hasMailbox;
+	}
+
+	public List<MailboxVersion> getClientSupports() {
+		return clientSupports;
+	}
+
+	public boolean hasMailbox() {
+		return hasMailbox;
+	}
+}

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

@@ -0,0 +1,111 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface MailboxUpdateManager {
+
+	/**
+	 * The unique ID of the mailbox update (properties) client.
+	 */
+	ClientId CLIENT_ID =
+			new ClientId("org.briarproject.bramble.mailbox.properties");
+
+	/**
+	 * The current major version of the mailbox update (properties) client.
+	 */
+	int MAJOR_VERSION = 2;
+
+	/**
+	 * The current minor version of the mailbox update (properties) client.
+	 */
+	int MINOR_VERSION = 0;
+
+	/**
+	 * The number of properties required for an update message with a mailbox.
+	 * <p>
+	 * The required properties are {@link #PROP_KEY_ONION},
+	 * {@link #PROP_KEY_AUTHTOKEN}, {@link #PROP_KEY_INBOXID} and
+	 * {@link #PROP_KEY_OUTBOXID}.
+	 */
+	int PROP_COUNT = 4;
+
+	/**
+	 * The onion address of the mailbox, excluding the .onion suffix.
+	 */
+	String PROP_KEY_ONION = "onion";
+
+	/**
+	 * A bearer token for accessing the mailbox (64 hex digits).
+	 */
+	String PROP_KEY_AUTHTOKEN = "authToken";
+
+	/**
+	 * A folder ID for downloading messages (64 hex digits).
+	 */
+	String PROP_KEY_INBOXID = "inboxId";
+
+	/**
+	 * A folder ID for uploading messages (64 hex digits).
+	 */
+	String PROP_KEY_OUTBOXID = "outboxId";
+
+	/**
+	 * Length of the {@link #PROP_KEY_ONION} property.
+	 */
+	int PROP_ONION_LENGTH = 56;
+
+	/**
+	 * Message metadata key for the version number of a local or remote update,
+	 * as a BDF long.
+	 */
+	String MSG_KEY_VERSION = "version";
+
+	/**
+	 * Message metadata key for whether an update is local or remote, as a BDF
+	 * boolean.
+	 */
+	String MSG_KEY_LOCAL = "local";
+
+	/**
+	 * Key in the client's local group for storing the clientSupports list that
+	 * was last sent out.
+	 */
+	String GROUP_KEY_SENT_CLIENT_SUPPORTS = "sentClientSupports";
+
+	/**
+	 * Key in the client's local group for storing the serverSupports list that
+	 * was last sent out, if any.
+	 */
+	String GROUP_KEY_SENT_SERVER_SUPPORTS = "sentServerSupports";
+
+	/**
+	 * Returns the latest {@link MailboxUpdate} sent to the given contact.
+	 * <p>
+	 * If we have our own mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} the contact should use for communicating with
+	 * our mailbox.
+	 */
+	MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
+			throws DbException;
+
+	/**
+	 * Returns the latest {@link MailboxUpdate} received from the given
+	 * contact, or null if no update has been received.
+	 * <p>
+	 * If the contact has a mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} we should use for communicating with the
+	 * contact's mailbox.
+	 */
+	@Nullable
+	MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
+			throws DbException;
+}

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

@@ -0,0 +1,30 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdateWithMailbox extends MailboxUpdate {
+
+	private final MailboxProperties properties;
+
+	public MailboxUpdateWithMailbox(List<MailboxVersion> clientSupports,
+			MailboxProperties properties) {
+		super(clientSupports, true);
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		this.properties = properties;
+	}
+
+	public MailboxUpdateWithMailbox(MailboxUpdateWithMailbox o,
+			List<MailboxVersion> newClientSupports) {
+		this(newClientSupports, o.getMailboxProperties());
+	}
+
+	public MailboxProperties getMailboxProperties() {
+		return properties;
+	}
+}

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

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxVersion implements Comparable<MailboxVersion> {
+
+	private final int major;
+	private final int minor;
+
+	public MailboxVersion(int major, int minor) {
+		this.major = major;
+		this.minor = minor;
+	}
+
+	public int getMajor() {
+		return major;
+	}
+
+	public int getMinor() {
+		return minor;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o instanceof MailboxVersion) {
+			MailboxVersion v = (MailboxVersion) o;
+			return major == v.major && minor == v.minor;
+		}
+		return false;
+	}
+
+	@Override
+	public int compareTo(MailboxVersion v) {
+		int c = major - v.major;
+		if (c != 0) {
+			return c;
+		}
+		return minor - v.minor;
+	}
+}

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

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

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

@@ -0,0 +1,19 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast by {@link MailboxSettingsManager} when
+ * recording a connection failure for own Mailbox
+ * that has persistent for long enough for the mailbox owner to become active
+ * and fix the problem with the mailbox.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxProblemEvent extends Event {
+
+}

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

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

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

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

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

@@ -0,0 +1,27 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxStatus;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast by {@link MailboxSettingsManager} when
+ * recording the connection status of own Mailbox.
+ */
+@Immutable
+@NotNullByDefault
+public class OwnMailboxConnectionStatusEvent extends Event {
+
+	private final MailboxStatus status;
+
+	public OwnMailboxConnectionStatusEvent(MailboxStatus status) {
+		this.status = status;
+	}
+
+	public MailboxStatus getStatus() {
+		return status;
+	}
+}

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

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when {@link MailboxUpdate} are received
+ * from a contact.
+ */
+@Immutable
+@NotNullByDefault
+public class RemoteMailboxUpdateEvent extends Event {
+
+	private final ContactId contactId;
+	private final MailboxUpdate mailboxUpdate;
+
+	public RemoteMailboxUpdateEvent(ContactId contactId,
+			MailboxUpdate mailboxUpdate) {
+		this.contactId = contactId;
+		this.mailboxUpdate = mailboxUpdate;
+	}
+
+	public ContactId getContact() {
+		return contactId;
+	}
+
+	public MailboxUpdate getMailboxUpdate() {
+		return mailboxUpdate;
+	}
+}

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

@@ -1,5 +1,7 @@
 package org.briarproject.bramble.api.plugin;
 
+import static java.util.concurrent.TimeUnit.SECONDS;
+
 public interface TorConstants {
 
 	TransportId ID = new TransportId("org.briarproject.bramble.tor");
@@ -10,8 +12,9 @@ public interface TorConstants {
 	int DEFAULT_SOCKS_PORT = 59050;
 	int DEFAULT_CONTROL_PORT = 59051;
 
-	int CONNECT_TO_PROXY_TIMEOUT = 5000; // Milliseconds
+	int CONNECT_TO_PROXY_TIMEOUT = (int) SECONDS.toMillis(5);
-	int EXTRA_SOCKET_TIMEOUT = 30000; // Milliseconds
+	int EXTRA_CONNECT_TIMEOUT = (int) SECONDS.toMillis(120);
+	int EXTRA_SOCKET_TIMEOUT = (int) SECONDS.toMillis(30);
 
 	// Local settings (not shared with contacts)
 	String PREF_TOR_NETWORK = "network2";

bramble-api/src/main/java/org/briarproject/bramble/api/record/RecordWriter.java

@@ -12,4 +12,6 @@ public interface RecordWriter {
 	void flush() throws IOException;
 
 	void close() throws IOException;
+
+	long getBytesWritten();
 }

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

@@ -20,9 +20,4 @@ public class GroupId extends UniqueId {
 	public GroupId(byte[] id) {
 		super(id);
 	}
-
-	@Override
-	public boolean equals(Object o) {
-		return o instanceof GroupId && super.equals(o);
-	}
 }

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

@@ -27,9 +27,4 @@ public class MessageId extends UniqueId {
 	public MessageId(byte[] id) {
 		super(id);
 	}
-
-	@Override
-	public boolean equals(Object o) {
-		return o instanceof MessageId && super.equals(o);
-	}
 }

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

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

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

@@ -20,4 +20,6 @@ public interface SyncRecordWriter {
 	void writePriority(Priority p) throws IOException;
 
 	void flush() throws IOException;
+
+	long getBytesWritten();
 }

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

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

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

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

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

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

bramble-api/src/main/java/org/briarproject/bramble/api/system/TaskScheduler.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.api.system;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.util.concurrent.Executor;
@@ -16,6 +17,8 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling the task.
 	 */
 	Cancellable schedule(Runnable task, Executor executor, long delay,
 			TimeUnit unit);
@@ -27,17 +30,11 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling all future executions of
+	 * the task.
 	 */
 	Cancellable scheduleWithFixedDelay(Runnable task, Executor executor,
 			long delay, long interval, TimeUnit unit);
 
-	interface Cancellable {
-
-		/**
-		 * Cancels the task if it has not already started running. If the task
-		 * is {@link #scheduleWithFixedDelay(Runnable, Executor, long, long, TimeUnit) periodic},
-		 * all future executions of the task are cancelled.
-		 */
-		void cancel();
-	}
 }

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

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

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

@@ -26,7 +26,7 @@ public class NetworkUtils {
 			// Despite what the docs say, the return value can be null
 			//noinspection ConstantConditions
 			return ifaces == null ? emptyList() : list(ifaces);
-		} catch (SocketException e) {
+		} catch (SocketException | NullPointerException e) {
 			logException(LOG, WARNING, e);
 			return emptyList();
 		}

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

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

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

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

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

@@ -1,17 +1,46 @@
 package org.briarproject.bramble.test;
 
-import java.lang.Thread.UncaughtExceptionHandler;
+import org.junit.After;
+import org.junit.Before;
 
-import static org.junit.Assert.fail;
+import java.lang.Thread.UncaughtExceptionHandler;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
 
 public abstract class BrambleTestCase {
 
+	private static final Logger LOG =
+			getLogger(BrambleTestCase.class.getName());
+
+	@Nullable
+	protected volatile Throwable exceptionInBackgroundThread = null;
+
 	public BrambleTestCase() {
 		// Ensure exceptions thrown on worker threads cause tests to fail
 		UncaughtExceptionHandler fail = (thread, throwable) -> {
-			throwable.printStackTrace();
+			LOG.log(WARNING, "Caught unhandled exception", throwable);
-			fail();
+			exceptionInBackgroundThread = throwable;
 		};
 		Thread.setDefaultUncaughtExceptionHandler(fail);
 	}
+
+	@Before
+	public void beforeBrambleTestCase() {
+		exceptionInBackgroundThread = null;
+	}
+
+	@After
+	public void afterBrambleTestCase() {
+		Throwable thrown = exceptionInBackgroundThread;
+		if (thrown != null) {
+			LOG.log(WARNING,
+					"Background thread has thrown an exception unexpectedly",
+					thrown);
+			throw new AssertionError(thrown);
+		}
+	}
 }

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

@@ -12,10 +12,20 @@ import org.briarproject.bramble.api.crypto.PublicKey;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.crypto.SignaturePrivateKey;
 import org.briarproject.bramble.api.crypto.SignaturePublicKey;
+import org.briarproject.bramble.api.db.CommitAction;
+import org.briarproject.bramble.api.db.EventAction;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorId;
 import org.briarproject.bramble.api.identity.Identity;
 import org.briarproject.bramble.api.identity.LocalAuthor;
+import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.sync.ClientId;
@@ -25,7 +35,12 @@ import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageId;
 import org.briarproject.bramble.util.IoUtils;
 
+import java.io.ByteArrayOutputStream;
 import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -35,6 +50,9 @@ import java.util.Map;
 import java.util.Random;
 import java.util.concurrent.atomic.AtomicInteger;
 
+import javax.annotation.Nullable;
+import javax.crypto.Cipher;
+
 import static java.util.Arrays.asList;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_AGREEMENT_PUBLIC_KEY_BYTES;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_SIGNATURE_PUBLIC_KEY_BYTES;
@@ -45,6 +63,7 @@ import static org.briarproject.bramble.api.properties.TransportPropertyConstants
 import static org.briarproject.bramble.api.sync.ClientId.MAX_CLIENT_ID_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_GROUP_DESCRIPTOR_LENGTH;
 import static org.briarproject.bramble.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
+import static org.briarproject.bramble.util.IoUtils.copyAndClose;
 import static org.briarproject.bramble.util.StringUtils.getRandomString;
 
 public class TestUtils {
@@ -209,6 +228,37 @@ public class TestUtils {
 				getAgreementPublicKey(), verified);
 	}
 
+	public static MailboxProperties getMailboxProperties(boolean owner,
+			List<MailboxVersion> serverSupports) {
+		String onion = getRandomString(56);
+		MailboxAuthToken authToken = new MailboxAuthToken(getRandomId());
+		if (owner) {
+			return new MailboxProperties(onion, authToken, serverSupports);
+		}
+		MailboxFolderId inboxId = new MailboxFolderId(getRandomId());
+		MailboxFolderId outboxId = new MailboxFolderId(getRandomId());
+		return new MailboxProperties(onion, authToken, serverSupports,
+				inboxId, outboxId);
+	}
+
+	public static void writeBytes(File file, byte[] bytes)
+			throws IOException {
+		FileOutputStream outputStream = new FileOutputStream(file);
+		//noinspection TryFinallyCanBeTryWithResources
+		try {
+			outputStream.write(bytes);
+		} finally {
+			outputStream.close();
+		}
+	}
+
+	public static byte[] readBytes(File file) throws IOException {
+		ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
+		FileInputStream inputStream = new FileInputStream(file);
+		copyAndClose(inputStream, outputStream);
+		return outputStream.toByteArray();
+	}
+
 	public static double getMedian(Collection<? extends Number> samples) {
 		int size = samples.size();
 		if (size == 0) throw new IllegalArgumentException();
@@ -243,9 +293,68 @@ public class TestUtils {
 		return Math.sqrt(getVariance(samples));
 	}
 
-	public static boolean isOptionalTestEnabled(Class testClass) {
+	public static boolean isOptionalTestEnabled(Class<?> testClass) {
 		String optionalTests = System.getenv("OPTIONAL_TESTS");
 		return optionalTests != null &&
 				asList(optionalTests.split(",")).contains(testClass.getName());
 	}
+
+	public static boolean mailboxUpdateEqual(@Nullable MailboxUpdate a,
+			@Nullable MailboxUpdate b) {
+		if (a == null || b == null) {
+			return a == b;
+		}
+		if (!a.hasMailbox() && !b.hasMailbox()) {
+			return a.getClientSupports().equals(b.getClientSupports());
+		} else if (a.hasMailbox() && b.hasMailbox()) {
+			MailboxUpdateWithMailbox am = (MailboxUpdateWithMailbox) a;
+			MailboxUpdateWithMailbox bm = (MailboxUpdateWithMailbox) b;
+			return am.getClientSupports().equals(bm.getClientSupports()) &&
+					mailboxPropertiesEqual(am.getMailboxProperties(),
+							bm.getMailboxProperties());
+		}
+		return false;
+	}
+
+	public static boolean mailboxPropertiesEqual(@Nullable MailboxProperties a,
+			@Nullable MailboxProperties b) {
+		if (a == null || b == null) {
+			return a == b;
+		}
+		return a.getOnion().equals(b.getOnion()) &&
+				a.getAuthToken().equals(b.getAuthToken()) &&
+				a.isOwner() == b.isOwner() &&
+				a.getServerSupports().equals(b.getServerSupports());
+	}
+
+	public static boolean hasEvent(Transaction txn,
+			Class<? extends Event> eventClass) {
+		for (CommitAction action : txn.getActions()) {
+			if (action instanceof EventAction) {
+				Event event = ((EventAction) action).getEvent();
+				if (eventClass.isInstance(event)) return true;
+			}
+		}
+		return false;
+	}
+
+	public static <E extends Event> E getEvent(Transaction txn,
+			Class<E> eventClass) {
+		for (CommitAction action : txn.getActions()) {
+			if (action instanceof EventAction) {
+				Event event = ((EventAction) action).getEvent();
+				if (eventClass.isInstance(event)) return eventClass.cast(event);
+			}
+		}
+		throw new AssertionError();
+	}
+
+	public static boolean isCryptoStrengthUnlimited() {
+		try {
+			return Cipher.getMaxAllowedKeyLength("AES/CBC/PKCS5Padding")
+					== Integer.MAX_VALUE;
+		} catch (NoSuchAlgorithmException e) {
+			throw new AssertionError();
+		}
+	}
 }

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

@@ -0,0 +1,36 @@
+package org.briarproject.bramble.test;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.fail;
+
+public class ThreadExceptionTest extends BrambleTestCase {
+
+	@Test(expected = AssertionError.class)
+	public void testAssertionErrorMakesTestCaseFail() {
+		// This is what BrambleTestCase does, too:
+		fail();
+	}
+
+	@Test
+	public void testExceptionInThreadMakesTestCaseFail() {
+		Thread t = new Thread(() -> {
+			System.out.println("thread before exception");
+			throw new RuntimeException("boom");
+		});
+
+		t.start();
+		try {
+			t.join();
+			System.out.println("joined thread");
+		} catch (InterruptedException e) {
+			System.out.println("interrupted while joining thread");
+			fail();
+		}
+
+		assertNotNull(exceptionInBackgroundThread);
+		exceptionInBackgroundThread = null;
+	}
+
+}

bramble-api/witness.gradle

@@ -1,6 +1,7 @@
 dependencyVerification {
     verify = [
         'cglib:cglib:3.2.8:cglib-3.2.8.jar:3f64de999ecc5595dc84ca8ff0879d8a34c8623f9ef3c517a53ed59023fcb9db',
+        'com.fasterxml.jackson.core:jackson-annotations:2.13.0:jackson-annotations-2.13.0.jar:81f9724d8843e8b08f8f6c0609e7a2b030d00c34861c4ac7e2099a7235047d6f',
         'com.google.code.findbugs:annotations:3.0.1:annotations-3.0.1.jar:6b47ff0a6de0ce17cbedc3abb0828ca5bce3009d53ea47b3723ff023c4742f79',
         'com.google.code.findbugs:jsr305:3.0.2:jsr305-3.0.2.jar:766ad2a0783f2687962c8ad74ceecc38a28b9f72a2d085ee438b7813e928d0c7',
         'com.google.dagger:dagger:2.33:dagger-2.33.jar:d8798c5b8cf6b125234e33af5c6293bb9f2208ce29b57924c35b8c0be7b6bdcb',

bramble-core/build.gradle

@@ -10,12 +10,17 @@ apply from: '../dagger.gradle'
 
 dependencies {
 	implementation project(path: ':bramble-api', configuration: 'default')
-	implementation 'org.bouncycastle:bcprov-jdk15on:1.69'
+	implementation 'org.bouncycastle:bcprov-jdk15to18:1.70'
+	//noinspection GradleDependency
 	implementation 'com.h2database:h2:1.4.192' // The last version that supports Java 1.6
 	implementation 'org.bitlet:weupnp:0.1.4'
 	implementation 'net.i2p.crypto:eddsa:0.2.0'
 	implementation 'org.whispersystems:curve25519-java:0.5.0'
-	implementation 'org.briarproject:jtorctl:0.3'
+	implementation 'org.briarproject:jtorctl:0.5'
+
+	//noinspection GradleDependency
+	implementation "com.squareup.okhttp3:okhttp:$okhttp_version"
+	implementation "com.fasterxml.jackson.core:jackson-databind:$jackson_version"
 
 	annotationProcessor "com.google.dagger:dagger-compiler:$dagger_version"
 
@@ -25,7 +30,8 @@ dependencies {
 	testImplementation "junit:junit:$junit_version"
 	testImplementation "org.jmock:jmock:$jmock_version"
 	testImplementation "org.jmock:jmock-junit4:$jmock_version"
-	testImplementation "org.jmock:jmock-legacy:$jmock_version"
+	testImplementation "org.jmock:jmock-imposters:$jmock_version"
+	testImplementation "com.squareup.okhttp3:mockwebserver:4.9.3"
 
 	testAnnotationProcessor "com.google.dagger:dagger-compiler:$dagger_version"
 

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

@@ -6,6 +6,7 @@ import org.briarproject.bramble.crypto.CryptoExecutorModule;
 import org.briarproject.bramble.db.DatabaseExecutorModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.lifecycle.LifecycleModule;
+import org.briarproject.bramble.mailbox.MailboxModule;
 import org.briarproject.bramble.plugin.PluginModule;
 import org.briarproject.bramble.properties.PropertiesModule;
 import org.briarproject.bramble.rendezvous.RendezvousModule;
@@ -28,6 +29,8 @@ public interface BrambleCoreEagerSingletons {
 
 	void inject(LifecycleModule.EagerSingletons init);
 
+	void inject(MailboxModule.EagerSingletons init);
+
 	void inject(PluginModule.EagerSingletons init);
 
 	void inject(PropertiesModule.EagerSingletons init);
@@ -51,6 +54,7 @@ public interface BrambleCoreEagerSingletons {
 			c.inject(new DatabaseExecutorModule.EagerSingletons());
 			c.inject(new IdentityModule.EagerSingletons());
 			c.inject(new LifecycleModule.EagerSingletons());
+			c.inject(new MailboxModule.EagerSingletons());
 			c.inject(new RendezvousModule.EagerSingletons());
 			c.inject(new PluginModule.EagerSingletons());
 			c.inject(new PropertiesModule.EagerSingletons());

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

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

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

@@ -1,6 +1,7 @@
 package org.briarproject.bramble.client;
 
 import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
@@ -22,6 +23,12 @@ 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.AuthorFactory;
+import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -29,23 +36,33 @@ import org.briarproject.bramble.api.sync.GroupId;
 import org.briarproject.bramble.api.sync.Message;
 import org.briarproject.bramble.api.sync.MessageFactory;
 import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.util.Base32;
 
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.security.GeneralSecurityException;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
+import static java.util.Collections.sort;
 import static org.briarproject.bramble.api.client.ContactGroupConstants.GROUP_KEY_CONTACT_ID;
 import static org.briarproject.bramble.api.identity.Author.FORMAT_VERSION;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGTH;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_COUNT;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_AUTHTOKEN;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_INBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_OUTBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_ONION_LENGTH;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTY_LENGTH;
 import static org.briarproject.bramble.util.ValidationUtils.checkLength;
@@ -399,6 +416,70 @@ class ClientHelperImpl implements ClientHelper {
 		return tpMap;
 	}
 
+	@Override
+	public MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
+			BdfList serverSupports, BdfDictionary properties)
+			throws FormatException {
+		List<MailboxVersion> clientSupportsList =
+				parseMailboxVersionList(clientSupports);
+		List<MailboxVersion> serverSupportsList =
+				parseMailboxVersionList(serverSupports);
+
+		// We must always learn what Mailbox API version(s) the client supports
+		if (clientSupports.isEmpty()) {
+			throw new FormatException();
+		}
+		if (properties.isEmpty()) {
+			// No mailbox -- cannot claim to support any API versions!
+			if (!serverSupports.isEmpty()) {
+				throw new FormatException();
+			}
+			return new MailboxUpdate(clientSupportsList);
+		}
+		// Mailbox must be accompanied by the Mailbox API version(s) it supports
+		if (serverSupports.isEmpty()) {
+			throw new FormatException();
+		}
+		// Accepting more props than we need, for forward compatibility
+		if (properties.size() < PROP_COUNT) {
+			throw new FormatException();
+		}
+		String onion = properties.getString(PROP_KEY_ONION);
+		checkLength(onion, PROP_ONION_LENGTH);
+		try {
+			Base32.decode(onion, true);
+		} catch (IllegalArgumentException e) {
+			throw new FormatException();
+		}
+		byte[] authToken = properties.getRaw(PROP_KEY_AUTHTOKEN);
+		checkLength(authToken, UniqueId.LENGTH);
+		byte[] inboxId = properties.getRaw(PROP_KEY_INBOXID);
+		checkLength(inboxId, UniqueId.LENGTH);
+		byte[] outboxId = properties.getRaw(PROP_KEY_OUTBOXID);
+		checkLength(outboxId, UniqueId.LENGTH);
+		MailboxProperties props = new MailboxProperties(onion,
+				new MailboxAuthToken(authToken), serverSupportsList,
+				new MailboxFolderId(inboxId), new MailboxFolderId(outboxId));
+		return new MailboxUpdateWithMailbox(clientSupportsList, props);
+	}
+
+	@Override
+	public List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException {
+		List<MailboxVersion> list = new ArrayList<>();
+		for (int i = 0; i < bdfList.size(); i++) {
+			BdfList element = bdfList.getList(i);
+			if (element.size() != 2) {
+				throw new FormatException();
+			}
+			list.add(new MailboxVersion(element.getLong(0).intValue(),
+					element.getLong(1).intValue()));
+		}
+		// Sort the list of versions for easier comparison
+		sort(list);
+		return list;
+	}
+
 	@Override
 	public ContactId getContactId(Transaction txn, GroupId contactGroupId)
 			throws DbException {

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

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

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

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

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

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

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

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

bramble-core/src/main/java/org/briarproject/bramble/contact/ContactManagerImpl.java

@@ -131,22 +131,30 @@ class ContactManagerImpl implements ContactManager, EventListener {
 	}
 
 	@Override
-	public PendingContact addPendingContact(String link, String alias)
+	public PendingContact addPendingContact(Transaction txn, String link,
+			String alias)
 			throws DbException, FormatException, GeneralSecurityException {
 		PendingContact p =
 				pendingContactFactory.createPendingContact(link, alias);
+		AuthorId local = identityManager.getLocalAuthor(txn).getId();
+		db.addPendingContact(txn, p, local);
+		KeyPair ourKeyPair = identityManager.getHandshakeKeys(txn);
+		keyManager.addPendingContact(txn, p.getId(), p.getPublicKey(),
+				ourKeyPair);
+		return p;
+	}
+
+	@Override
+	public PendingContact addPendingContact(String link, String alias)
+			throws DbException, FormatException, GeneralSecurityException {
 		Transaction txn = db.startTransaction(false);
 		try {
-			AuthorId local = identityManager.getLocalAuthor(txn).getId();
+			PendingContact p = addPendingContact(txn, link, alias);
-			db.addPendingContact(txn, p, local);
-			KeyPair ourKeyPair = identityManager.getHandshakeKeys(txn);
-			keyManager.addPendingContact(txn, p.getId(), p.getPublicKey(),
-					ourKeyPair);
 			db.commitTransaction(txn);
+			return p;
 		} finally {
 			db.endTransaction(txn);
 		}
-		return p;
 	}
 
 	@Override
@@ -158,8 +166,14 @@ class ContactManagerImpl implements ContactManager, EventListener {
 	@Override
 	public Collection<Pair<PendingContact, PendingContactState>> getPendingContacts()
 			throws DbException {
-		Collection<PendingContact> pendingContacts =
+		return db.transactionWithResult(true, this::getPendingContacts);
-				db.transactionWithResult(true, db::getPendingContacts);
+	}
+
+	@Override
+	public Collection<Pair<PendingContact, PendingContactState>> getPendingContacts(
+			Transaction txn)
+			throws DbException {
+		Collection<PendingContact> pendingContacts = db.getPendingContacts(txn);
 		List<Pair<PendingContact, PendingContactState>> pairs =
 				new ArrayList<>(pendingContacts.size());
 		for (PendingContact p : pendingContacts) {
@@ -172,7 +186,13 @@ class ContactManagerImpl implements ContactManager, EventListener {
 
 	@Override
 	public void removePendingContact(PendingContactId p) throws DbException {
-		db.transaction(false, txn -> db.removePendingContact(txn, p));
+		db.transaction(false, txn -> removePendingContact(txn, p));
+	}
+
+	@Override
+	public void removePendingContact(Transaction txn, PendingContactId p)
+			throws DbException {
+		db.removePendingContact(txn, p);
 		states.remove(p);
 	}
 

bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoComponentImpl.java

@@ -8,6 +8,7 @@ import org.bouncycastle.crypto.CryptoException;
 import org.bouncycastle.crypto.Digest;
 import org.bouncycastle.crypto.digests.Blake2bDigest;
 import org.bouncycastle.crypto.digests.SHA3Digest;
+import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.crypto.AgreementPrivateKey;
 import org.briarproject.bramble.api.crypto.AgreementPublicKey;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
@@ -41,6 +42,7 @@ import javax.inject.Inject;
 
 import static java.lang.System.arraycopy;
 import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.KEY_TYPE_AGREEMENT;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.KEY_TYPE_SIGNATURE;
 import static org.briarproject.bramble.api.crypto.DecryptionResult.INVALID_CIPHERTEXT;
@@ -54,7 +56,7 @@ import static org.briarproject.bramble.util.LogUtils.now;
 class CryptoComponentImpl implements CryptoComponent {
 
 	private static final Logger LOG =
-			Logger.getLogger(CryptoComponentImpl.class.getName());
+			getLogger(CryptoComponentImpl.class.getName());
 
 	private static final int SIGNATURE_KEY_PAIR_BITS = 256;
 	private static final int STORAGE_IV_BYTES = 24; // 196 bits
@@ -128,6 +130,13 @@ class CryptoComponentImpl implements CryptoComponent {
 		}
 	}
 
+	@Override
+	public UniqueId generateUniqueId() {
+		byte[] b = new byte[UniqueId.LENGTH];
+		secureRandom.nextBytes(b);
+		return new UniqueId(b);
+	}
+
 	@Override
 	public SecretKey generateSecretKey() {
 		byte[] b = new byte[SecretKey.LENGTH];
@@ -449,7 +458,7 @@ class CryptoComponentImpl implements CryptoComponent {
 	}
 
 	@Override
-	public String encodeOnionAddress(byte[] publicKey) {
+	public String encodeOnion(byte[] publicKey) {
 		Digest digest = new SHA3Digest(256);
 		byte[] label = ".onion checksum".getBytes(Charset.forName("US-ASCII"));
 		digest.update(label, 0, label.length);

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

@@ -163,16 +163,11 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
+	 * Returns true if there are any acks to send to the given contact.
-	 * contact over a transport with the given maximum latency.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(T txn, ContactId c, long maxLatency,
+	boolean containsAcksToSend(T txn, ContactId c) throws DbException;
-			boolean eager) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -212,6 +207,18 @@ interface Database<T> {
 	 */
 	boolean containsMessage(T txn, MessageId m) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given
+	 * contact over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(T txn, ContactId c, long maxLatency,
+			boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -313,6 +320,13 @@ interface Database<T> {
 	 */
 	Group getGroup(T txn, GroupId g) throws DbException;
 
+	/**
+	 * Returns the ID of the group containing the given message.
+	 * <p/>
+	 * Read-only.
+	 */
+	GroupId getGroupId(T txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for the given group.
 	 * <p/>
@@ -338,8 +352,11 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Returns the IDs of all contacts to which the given group's visibility is
+	 * Returns the IDs of all contacts for which the given group's visibility
-	 * either {@link Visibility VISIBLE} or {@link Visibility SHARED}.
+	 * is either {@link Visibility#SHARED shared} or
+	 * {@link Visibility#VISIBLE visible}. The value in the map is true if the
+	 * group is {@link Visibility#SHARED shared} or false if the group is
+	 * {@link Visibility#VISIBLE visible}.
 	 * <p/>
 	 * Read-only.
 	 */
@@ -406,6 +423,12 @@ interface Database<T> {
 	Collection<MessageId> getMessageIds(T txn, GroupId g, Metadata query)
 			throws DbException;
 
+	/**
+	 * Returns the length of the given message in bytes, including the
+	 * message header.
+	 */
+	int getMessageLength(T txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for all delivered messages in the given group.
 	 * <p/>
@@ -496,7 +519,8 @@ 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.
+	 * given contact. The total length of the messages including record headers
+	 * will be no more than the given capacity.
 	 * <p/>
 	 * Unlike {@link #getUnackedMessagesToSend(Object, ContactId)} this method
 	 * does not return messages that have already been sent unless they are
@@ -504,20 +528,20 @@ interface Database<T> {
 	 * <p/>
 	 * Read-only.
 	 */
-	Collection<MessageId> getMessagesToSend(T txn, ContactId c, int maxLength,
+	Collection<MessageId> getMessagesToSend(T txn, ContactId c, long capacity,
 			long maxLatency) throws DbException;
 
 	/**
 	 * Returns the IDs of all messages that are eligible to be sent to the
-	 * given contact, together with their raw lengths.
+	 * given contact.
 	 * <p/>
-	 * Unlike {@link #getMessagesToSend(Object, ContactId, int, long)} this
+	 * Unlike {@link #getMessagesToSend(Object, ContactId, long, long)} 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)
+	Collection<MessageId> getUnackedMessagesToSend(T txn, ContactId c)
 			throws DbException;
 
 	/**
@@ -573,13 +597,16 @@ interface Database<T> {
 
 	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
+	 * message is due to be sent to the given contact over a transport with
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE
+	 * the given latency.
-	 * if no messages are scheduled to be sent.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(T txn, ContactId c) throws DbException;
+	long getNextSendTime(T txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.
@@ -598,13 +625,14 @@ interface Database<T> {
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
-	 * given contact and have been requested by the contact, up to the given
+	 * given contact and have been requested by the contact. The total length
-	 * total length.
+	 * of the messages including record headers will be no more than the given
+	 * capacity.
 	 * <p/>
 	 * Read-only.
 	 */
 	Collection<MessageId> getRequestedMessagesToSend(T txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns all settings in the given namespace.
@@ -758,9 +786,10 @@ interface Database<T> {
 	void resetExpiryTime(T txn, ContactId c, MessageId m) throws DbException;
 
 	/**
-	 * Resets the transmission count, expiry time and ETA of all messages that
+	 * Resets the transmission count, expiry time and max latency of all
-	 * are eligible to be sent to the given contact. This includes messages that
+	 * messages that are eligible to be sent to the given contact. This includes
-	 * have already been sent and are not yet due for retransmission.
+	 * messages that have already been sent and are not yet due for
+	 * retransmission.
 	 */
 	void resetUnackedMessagesToSend(T txn, ContactId c) throws DbException;
 
@@ -848,11 +877,13 @@ interface Database<T> {
 	void stopCleanupTimer(T txn, MessageId m) throws DbException;
 
 	/**
-	 * Updates the transmission count, expiry time and estimated time of arrival
+	 * Updates the transmission count, expiry time and max latency of the given
-	 * of the given message with respect to the given contact, using the latency
+	 * message with respect to the given contact.
-	 * of the transport over which it was sent.
+	 *
+	 * @param maxLatency latency of the transport over which the message was
+	 * sent.
 	 */
-	void updateExpiryTimeAndEta(T txn, ContactId c, MessageId m,
+	void updateRetransmissionData(T txn, ContactId c, MessageId m,
 			long maxLatency) throws DbException;
 
 	/**

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

@@ -75,7 +75,6 @@ import org.briarproject.bramble.api.transport.TransportKeys;
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.Executor;
@@ -87,6 +86,7 @@ import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;
 
+import static java.util.Collections.singletonList;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
@@ -287,7 +287,12 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			transaction.attach(new MessageAddedEvent(m, null));
 			transaction.attach(new MessageStateChangedEvent(m.getId(), true,
 					DELIVERED));
-			if (shared) transaction.attach(new MessageSharedEvent(m.getId()));
+			if (shared) {
+				Map<ContactId, Boolean> visibility =
+						db.getGroupVisibility(txn, m.getGroupId());
+				transaction.attach(new MessageSharedEvent(m.getId(),
+						m.getGroupId(), visibility));
+			}
 		}
 		db.mergeMessageMetadata(txn, m.getId(), meta);
 	}
@@ -342,12 +347,12 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Transaction transaction, ContactId c,
+	public boolean containsAcksToSend(Transaction transaction, ContactId c)
-			long maxLatency, boolean eager) throws DbException {
+			throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
-		return db.containsAnythingToSend(txn, c, maxLatency, eager);
+		return db.containsAcksToSend(txn, c);
 	}
 
 	@Override
@@ -373,6 +378,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsIdentity(txn, a);
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Transaction transaction, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.containsMessagesToSend(txn, c, maxLatency, eager);
+	}
+
 	@Override
 	public boolean containsPendingContact(Transaction transaction,
 			PendingContactId p) throws DbException {
@@ -424,53 +438,27 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
 			Message message = db.getMessage(txn, m);
 			totalLength += message.getRawLength();
 			messages.add(message);
-			db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
+			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
 	}
 
-	@Override
-	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, Collection<MessageId> ids, long 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;
-	}
-
 	@Nullable
 	@Override
 	public Offer generateOffer(Transaction transaction, ContactId c,
@@ -483,7 +471,7 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 				db.getMessagesToOffer(txn, c, maxMessages, maxLatency);
 		if (ids.isEmpty()) return null;
 		for (MessageId m : ids)
-			db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
+			db.updateRetransmissionData(txn, c, m, maxLatency);
 		return new Offer(ids);
 	}
 
@@ -505,22 +493,22 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateRequestedBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getRequestedMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getRequestedMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
 			Message message = db.getMessage(txn, m);
 			totalLength += message.getRawLength();
 			messages.add(message);
-			db.updateExpiryTimeAndEta(txn, c, m, maxLatency);
+			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
@@ -567,6 +555,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getGroup(txn, g);
 	}
 
+	@Override
+	public GroupId getGroupId(Transaction transaction, MessageId m)
+			throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsMessage(txn, m))
+			throw new NoSuchMessageException();
+		return db.getGroupId(txn, m);
+	}
+
 	@Override
 	public Metadata getGroupMetadata(Transaction transaction, GroupId g)
 			throws DbException {
@@ -635,6 +632,24 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageIds(txn, g, query);
 	}
 
+	@Override
+	public Collection<MessageId> getMessagesToAck(Transaction transaction,
+			ContactId c) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToAck(txn, c, Integer.MAX_VALUE);
+	}
+
+	@Override
+	public Collection<MessageId> getMessagesToSend(Transaction transaction,
+			ContactId c, long capacity, long maxLatency) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToSend(txn, c, capacity, maxLatency);
+	}
+
 	@Override
 	public Collection<MessageId> getMessagesToValidate(Transaction transaction)
 			throws DbException {
@@ -740,10 +755,31 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return status;
 	}
 
+	@Nullable
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(
+	public Message getMessageToSend(Transaction transaction, ContactId c,
-			Transaction transaction,
+			MessageId m, long maxLatency, boolean markAsSent)
-			ContactId c) throws DbException {
+			throws DbException {
+		if (markAsSent && transaction.isReadOnly()) {
+			throw new IllegalArgumentException();
+		}
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		if (!db.containsVisibleMessage(txn, c, m)) return null;
+		Message message = db.getMessage(txn, m);
+		if (markAsSent) {
+			db.updateRetransmissionData(txn, c, m, maxLatency);
+			db.lowerRequestedFlag(txn, c, singletonList(m));
+			transaction.attach(new MessagesSentEvent(c, singletonList(m),
+					message.getRawLength()));
+		}
+		return message;
+	}
+
+	@Override
+	public Collection<MessageId> getUnackedMessagesToSend(
+			Transaction transaction, ContactId c) throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
@@ -794,10 +830,10 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public long getNextSendTime(Transaction transaction, ContactId c)
+	public long getNextSendTime(Transaction transaction, ContactId c,
-			throws DbException {
+			long maxLatency) throws DbException {
 		T txn = unbox(transaction);
-		return db.getNextSendTime(txn, c);
+		return db.getNextSendTime(txn, c, maxLatency);
 	}
 
 	@Override
@@ -1005,7 +1041,8 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 				db.getGroupVisibility(txn, id).keySet();
 		db.removeGroup(txn, id);
 		transaction.attach(new GroupRemovedEvent(g));
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		transaction.attach(new GroupVisibilityUpdatedEvent(INVISIBLE,
+				affected));
 	}
 
 	@Override
@@ -1069,6 +1106,16 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.removeTransportKeys(txn, t, k);
 	}
 
+	@Override
+	public void setAckSent(Transaction transaction, ContactId c,
+			Collection<MessageId> acked) throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		db.lowerAckFlag(txn, c, acked);
+	}
+
 	@Override
 	public void setCleanupTimerDuration(Transaction transaction, MessageId m,
 			long duration) throws DbException {
@@ -1115,8 +1162,8 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (old == INVISIBLE) db.addGroupVisibility(txn, c, g, v == SHARED);
 		else if (v == INVISIBLE) db.removeGroupVisibility(txn, c, g);
 		else db.setGroupVisibility(txn, c, g, v == SHARED);
-		List<ContactId> affected = Collections.singletonList(c);
+		List<ContactId> affected = singletonList(c);
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		transaction.attach(new GroupVisibilityUpdatedEvent(v, affected));
 	}
 
 	@Override
@@ -1149,7 +1196,9 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (db.getMessageState(txn, m) != DELIVERED)
 			throw new IllegalArgumentException("Shared undelivered message");
 		db.setMessageShared(txn, m, true);
-		transaction.attach(new MessageSharedEvent(m));
+		GroupId g = db.getGroupId(txn, m);
+		Map<ContactId, Boolean> visibility = db.getGroupVisibility(txn, g);
+		transaction.attach(new MessageSharedEvent(m, g, visibility));
 	}
 
 	@Override
@@ -1163,6 +1212,28 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new MessageStateChangedEvent(m, false, state));
 	}
 
+	@Override
+	public void setMessagesSent(Transaction transaction, ContactId c,
+			Collection<MessageId> sent, long 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<MessageId> visible = new ArrayList<>(sent.size());
+		for (MessageId m : sent) {
+			if (db.containsVisibleMessage(txn, c, m)) {
+				visible.add(m);
+				totalLength += db.getMessageLength(txn, m);
+				db.updateRetransmissionData(txn, c, m, maxLatency);
+			}
+		}
+		db.lowerRequestedFlag(txn, c, visible);
+		if (!visible.isEmpty()) {
+			transaction.attach(new MessagesSentEvent(c, visible, totalLength));
+		}
+	}
+
 	@Override
 	public void addMessageDependencies(Transaction transaction,
 			Message dependent, Collection<MessageId> dependencies)

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

@@ -2,8 +2,6 @@ package org.briarproject.bramble.db;
 
 import org.briarproject.bramble.api.settings.Settings;
 
-import static java.util.concurrent.TimeUnit.DAYS;
-
 interface DatabaseConstants {
 
 	/**
@@ -25,19 +23,6 @@ interface DatabaseConstants {
 	 */
 	String SCHEMA_VERSION_KEY = "schemaVersion";
 
-	/**
-	 * The {@link Settings} key under which the time of the last database
-	 * compaction is stored.
-	 */
-	String LAST_COMPACTED_KEY = "lastCompacted";
-
-	/**
-	 * The maximum time between database compactions in milliseconds. When the
-	 * database is opened it will be compacted if more than this amount of time
-	 * 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.

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

@@ -85,12 +85,17 @@ class H2Database extends JdbcDatabase {
 	public void close() throws DbException {
 		// H2 will close the database when the last connection closes
 		Connection c = null;
+		Statement s = null;
 		try {
 			c = createConnection();
-			super.closeAllConnections();
+			closeAllConnections();
 			setDirty(c, false);
+			s = c.createStatement();
+			s.execute("SHUTDOWN COMPACT");
+			s.close();
 			c.close();
 		} catch (SQLException e) {
+			tryToClose(s, LOG, WARNING);
 			tryToClose(c, LOG, WARNING);
 			throw new DbException(e);
 		}

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

@@ -79,11 +79,11 @@ class HyperSqlDatabase extends JdbcDatabase {
 		Connection c = null;
 		Statement s = null;
 		try {
-			super.closeAllConnections();
+			closeAllConnections();
 			c = createConnection();
 			setDirty(c, false);
 			s = c.createStatement();
-			s.executeQuery("SHUTDOWN");
+			s.executeQuery("SHUTDOWN COMPACT");
 			s.close();
 			c.close();
 		} catch (SQLException e) {
@@ -106,7 +106,7 @@ class HyperSqlDatabase extends JdbcDatabase {
 		Connection c = null;
 		Statement s = null;
 		try {
-			super.closeAllConnections();
+			closeAllConnections();
 			c = createConnection();
 			s = c.createStatement();
 			s.executeQuery("SHUTDOWN COMPACT");

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

@@ -51,7 +51,6 @@ 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;
@@ -70,12 +69,14 @@ import static java.sql.Types.BOOLEAN;
 import static java.sql.Types.INTEGER;
 import static java.sql.Types.VARCHAR;
 import static java.util.Arrays.asList;
+import static java.util.logging.Level.FINE;
 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.db.DatabaseComponent.NO_CLEANUP_DEADLINE;
 import static org.briarproject.bramble.api.db.DatabaseComponent.TIMER_NOT_STARTED;
 import static org.briarproject.bramble.api.db.Metadata.REMOVE;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 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;
@@ -85,8 +86,6 @@ 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;
 import static org.briarproject.bramble.db.ExponentialBackoff.calculateExpiry;
 import static org.briarproject.bramble.db.JdbcUtils.tryToClose;
@@ -102,7 +101,12 @@ import static org.briarproject.bramble.util.LogUtils.now;
 abstract class JdbcDatabase implements Database<Connection> {
 
 	// Package access for testing
-	static final int CODE_SCHEMA_VERSION = 49;
+	static final int CODE_SCHEMA_VERSION = 50;
+
+	/**
+	 * The maximum number of idle connections to keep open.
+	 */
+	private static final int MAX_CONNECTION_POOL_SIZE = 1;
 
 	// Time period offsets for incoming transport keys
 	private static final int OFFSET_PREV = -1;
@@ -252,7 +256,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 					+ " requested BOOLEAN NOT NULL,"
 					+ " expiry BIGINT NOT NULL,"
 					+ " txCount INT NOT NULL,"
-					+ " eta BIGINT NOT NULL,"
+					+ " maxLatency BIGINT," // Null if latency was reset
 					+ " PRIMARY KEY (messageId, contactId),"
 					+ " FOREIGN KEY (messageId)"
 					+ " REFERENCES messages (messageId)"
@@ -365,7 +369,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 	private final Condition connectionsChanged = connectionsLock.newCondition();
 
 	@GuardedBy("connectionsLock")
-	private final LinkedList<Connection> connections = new LinkedList<>();
+	private final LinkedList<Connection> connectionPool = new LinkedList<>();
 
 	@GuardedBy("connectionsLock")
 	private int openConnections = 0;
@@ -378,8 +382,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			throws DbException, SQLException;
 
 	// Used exclusively during open to compact the database after schema
-	// migrations or after DatabaseConstants#MAX_COMPACTION_INTERVAL_MS has
+	// migrations or if the database was not shut down cleanly
-	// elapsed
 	protected abstract void compactAndClose() throws DbException;
 
 	JdbcDatabase(DatabaseTypes databaseTypes, MessageFactory messageFactory,
@@ -405,7 +408,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 			if (reopen) {
 				Settings s = getSettings(txn, DB_SETTINGS_NAMESPACE);
 				wasDirtyOnInitialisation = isDirty(s);
-				compact = migrateSchema(txn, s, listener) || isCompactionDue(s);
+				boolean migrated = migrateSchema(txn, s, listener);
+				compact = wasDirtyOnInitialisation || migrated;
 			} else {
 				wasDirtyOnInitialisation = false;
 				createTables(txn);
@@ -435,14 +439,6 @@ abstract class JdbcDatabase implements Database<Connection> {
 			} finally {
 				connectionsLock.unlock();
 			}
-			txn = startTransaction();
-			try {
-				storeLastCompacted(txn);
-				commitTransaction(txn);
-			} catch (DbException e) {
-				abortTransaction(txn);
-				throw e;
-			}
 		}
 	}
 
@@ -502,18 +498,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 				new Migration45_46(),
 				new Migration46_47(dbTypes),
 				new Migration47_48(),
-				new Migration48_49()
+				new Migration48_49(),
+				new Migration49_50()
 		);
 	}
 
-	private boolean isCompactionDue(Settings s) {
-		long lastCompacted = s.getLong(LAST_COMPACTED_KEY, 0);
-		long elapsed = clock.currentTimeMillis() - lastCompacted;
-		if (LOG.isLoggable(INFO))
-			LOG.info(elapsed + " ms since last compaction");
-		return elapsed > MAX_COMPACTION_INTERVAL_MS;
-	}
-
 	private void storeSchemaVersion(Connection txn, int version)
 			throws DbException {
 		Settings s = new Settings();
@@ -521,12 +510,6 @@ abstract class JdbcDatabase implements Database<Connection> {
 		mergeSettings(txn, s, DB_SETTINGS_NAMESPACE);
 	}
 
-	private void storeLastCompacted(Connection txn) throws DbException {
-		Settings s = new Settings();
-		s.putLong(LAST_COMPACTED_KEY, clock.currentTimeMillis());
-		mergeSettings(txn, s, DB_SETTINGS_NAMESPACE);
-	}
-
 	private boolean isDirty(Settings s) {
 		return s.getBoolean(DIRTY_KEY, false);
 	}
@@ -540,7 +523,6 @@ abstract class JdbcDatabase implements Database<Connection> {
 	private void initialiseSettings(Connection txn) throws DbException {
 		Settings s = new Settings();
 		s.putInt(SCHEMA_VERSION_KEY, CODE_SCHEMA_VERSION);
-		s.putLong(LAST_COMPACTED_KEY, clock.currentTimeMillis());
 		mergeSettings(txn, s, DB_SETTINGS_NAMESPACE);
 	}
 
@@ -595,7 +577,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 		connectionsLock.lock();
 		try {
 			if (closed) throw new DbClosedException();
-			txn = connections.poll();
+			txn = connectionPool.poll();
+			logConnectionCounts();
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -606,7 +589,14 @@ abstract class JdbcDatabase implements Database<Connection> {
 				txn.setAutoCommit(false);
 				connectionsLock.lock();
 				try {
+					// The DB may have been closed since the check above
+					if (closed) {
+						tryToClose(txn, LOG, WARNING);
+						throw new DbClosedException();
+					}
 					openConnections++;
+					logConnectionCounts();
+					connectionsChanged.signalAll();
 				} finally {
 					connectionsLock.unlock();
 				}
@@ -617,67 +607,91 @@ abstract class JdbcDatabase implements Database<Connection> {
 		return txn;
 	}
 
-	@Override
+	@GuardedBy("connectionsLock")
-	public void abortTransaction(Connection txn) {
+	private void logConnectionCounts() {
-		try {
+		if (LOG.isLoggable(FINE)) {
-			txn.rollback();
+			LOG.fine(openConnections + " connections open, "
-			connectionsLock.lock();
+					+ connectionPool.size() + " in pool");
-			try {
-				connections.add(txn);
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
-		} catch (SQLException e) {
-			// Try to close the connection
-			logException(LOG, WARNING, e);
-			tryToClose(txn, LOG, WARNING);
-			// Whatever happens, allow the database to close
-			connectionsLock.lock();
-			try {
-				openConnections--;
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
 		}
 	}
 
 	@Override
-	public void commitTransaction(Connection txn) throws DbException {
+	public void abortTransaction(Connection txn) {
+		// The transaction may have been aborted due to an earlier exception,
+		// so close the connection rather than returning it to the pool
 		try {
-			txn.commit();
+			txn.rollback();
 		} catch (SQLException e) {
-			throw new DbException(e);
+			logException(LOG, WARNING, e);
 		}
+		closeConnection(txn);
+	}
+
+	private void closeConnection(Connection txn) {
+		tryToClose(txn, LOG, WARNING);
 		connectionsLock.lock();
 		try {
-			connections.add(txn);
+			openConnections--;
+			logConnectionCounts();
 			connectionsChanged.signalAll();
 		} finally {
 			connectionsLock.unlock();
 		}
 	}
 
-	void closeAllConnections() throws SQLException {
+	@Override
+	public void commitTransaction(Connection txn) throws DbException {
+		// If the transaction commits successfully then return the connection
+		// to the pool, otherwise close it
+		try {
+			txn.commit();
+			returnConnectionToPool(txn);
+		} catch (SQLException e) {
+			logException(LOG, WARNING, e);
+			closeConnection(txn);
+			throw new DbException(e);
+		}
+	}
+
+	private void returnConnectionToPool(Connection txn) {
+		boolean shouldClose;
+		connectionsLock.lock();
+		try {
+			shouldClose = connectionPool.size() >= MAX_CONNECTION_POOL_SIZE;
+			if (shouldClose) openConnections--;
+			else connectionPool.add(txn);
+			logConnectionCounts();
+			connectionsChanged.signalAll();
+		} finally {
+			connectionsLock.unlock();
+		}
+		if (shouldClose) tryToClose(txn, LOG, WARNING);
+	}
+
+	void closeAllConnections() {
 		boolean interrupted = false;
 		connectionsLock.lock();
 		try {
 			closed = true;
-			for (Connection c : connections) c.close();
+			for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
-			openConnections -= connections.size();
+			openConnections -= connectionPool.size();
-			connections.clear();
+			connectionPool.clear();
 			while (openConnections > 0) {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info("Waiting for " + openConnections
+							+ " connections to be closed");
+				}
 				try {
 					connectionsChanged.await();
 				} catch (InterruptedException e) {
 					LOG.warning("Interrupted while closing connections");
 					interrupted = true;
 				}
-				for (Connection c : connections) c.close();
+				for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
-				openConnections -= connections.size();
+				openConnections -= connectionPool.size();
-				connections.clear();
+				connectionPool.clear();
 			}
+			LOG.info("All connections closed");
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -920,9 +934,10 @@ abstract class JdbcDatabase implements Database<Connection> {
 		try {
 			String sql = "INSERT INTO statuses (messageId, contactId, groupId,"
 					+ " timestamp, length, state, groupShared, messageShared,"
-					+ " deleted, ack, seen, requested, expiry, txCount, eta)"
+					+ " deleted, ack, seen, requested, expiry, txCount,"
+					+ " maxLatency)"
 					+ " VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, FALSE, 0, 0,"
-					+ " 0)";
+					+ " NULL)";
 			ps = txn.prepareStatement(sql);
 			ps.setBytes(1, m.getBytes());
 			ps.setInt(2, c.getInt());
@@ -1132,8 +1147,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Connection txn, ContactId c,
+	public boolean containsAcksToSend(Connection txn, ContactId c)
-			long maxLatency, boolean eager) throws DbException {
+			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -1145,34 +1160,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			boolean acksToSend = rs.next();
 			rs.close();
 			ps.close();
-			if (acksToSend) return true;
+			return acksToSend;
-			if (eager) {
-				sql = "SELECT NULL from statuses"
-						+ " WHERE contactId = ? AND state = ?"
-						+ " AND groupShared = TRUE AND messageShared = TRUE"
-						+ " AND deleted = FALSE AND seen = FALSE";
-				ps = txn.prepareStatement(sql);
-				ps.setInt(1, c.getInt());
-				ps.setInt(2, DELIVERED.getValue());
-			} else {
-				long now = clock.currentTimeMillis();
-				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);
@@ -1292,6 +1280,46 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Connection txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			if (eager) {
+				String sql = "SELECT NULL from statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+			} else {
+				long now = clock.currentTimeMillis();
+				String sql = "SELECT NULL FROM statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE"
+						+ " AND (expiry <= ? OR maxLatency IS NULL"
+						+ " OR ? < maxLatency)";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+				ps.setLong(3, now);
+				ps.setLong(4, maxLatency);
+			}
+			rs = ps.executeQuery();
+			boolean messagesToSend = rs.next();
+			rs.close();
+			ps.close();
+			return messagesToSend;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public boolean containsPendingContact(Connection txn, PendingContactId p)
 			throws DbException {
@@ -1655,6 +1683,27 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public GroupId getGroupId(Connection txn, MessageId m) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT groupId FROM messages WHERE messageId = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			rs = ps.executeQuery();
+			if (!rs.next()) throw new DbStateException();
+			GroupId g = new GroupId(rs.getBytes(1));
+			rs.close();
+			ps.close();
+			return g;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Collection<Group> getGroups(Connection txn, ClientId c,
 			int majorVersion) throws DbException {
@@ -1902,6 +1951,31 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public int getMessageLength(Connection txn, MessageId m)
+			throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT length from messages"
+					+ " WHERE messageId = ? AND state = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			ps.setInt(2, DELIVERED.getValue());
+			rs = ps.executeQuery();
+			if (!rs.next()) throw new DbStateException();
+			int length = rs.getInt(1);
+			if (rs.next()) throw new DbStateException();
+			rs.close();
+			ps.close();
+			return length;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Map<MessageId, Metadata> getMessageMetadata(Connection txn,
 			GroupId g) throws DbException {
@@ -2194,7 +2268,6 @@ abstract class JdbcDatabase implements Database<Connection> {
 	public Collection<MessageId> getMessagesToOffer(Connection txn,
 			ContactId c, int maxMessages, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
-		long eta = now + maxLatency;
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -2203,13 +2276,14 @@ abstract class JdbcDatabase implements Database<Connection> {
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE"
 					+ " AND seen = FALSE AND requested = FALSE"
-					+ " AND (expiry <= ? OR eta > ?)"
+					+ " AND (expiry <= ? OR maxLatency IS NULL"
+					+ " OR ? < maxLatency)"
 					+ " ORDER BY timestamp LIMIT ?";
 			ps = txn.prepareStatement(sql);
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			ps.setLong(3, now);
-			ps.setLong(4, eta);
+			ps.setLong(4, maxLatency);
 			ps.setInt(5, maxMessages);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
@@ -2250,10 +2324,9 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Collection<MessageId> getMessagesToSend(Connection txn, ContactId c,
+	public Collection<MessageId> getMessagesToSend(Connection txn,
-			int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
-		long eta = now + maxLatency;
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -2262,21 +2335,21 @@ abstract class JdbcDatabase implements Database<Connection> {
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE"
 					+ " AND seen = FALSE"
-					+ " AND (expiry <= ? OR eta > ?)"
+					+ " AND (expiry <= ? OR maxLatency IS NULL"
+					+ " OR ? < maxLatency)"
 					+ " ORDER BY timestamp";
 			ps = txn.prepareStatement(sql);
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			ps.setLong(3, now);
-			ps.setLong(4, eta);
+			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2289,12 +2362,12 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(Connection txn,
+	public Collection<MessageId> getUnackedMessagesToSend(Connection txn,
 			ContactId c) throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT length, messageId FROM statuses"
+			String sql = "SELECT messageId FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"
@@ -2303,15 +2376,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			rs = ps.executeQuery();
-			Map<MessageId, Integer> results = new LinkedHashMap<>();
+			List<MessageId> ids = new ArrayList<>();
-			while (rs.next()) {
+			while (rs.next()) ids.add(new MessageId(rs.getBytes(1)));
-				int length = rs.getInt(1);
-				MessageId id = new MessageId(rs.getBytes(2));
-				results.put(id, length);
-			}
 			rs.close();
 			ps.close();
-			return results;
+			return ids;
 		} catch (SQLException e) {
 			tryToClose(rs, LOG, WARNING);
 			tryToClose(ps, LOG, WARNING);
@@ -2424,6 +2493,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				MessageId m = new MessageId(rs.getBytes(1));
 				GroupId g = new GroupId(rs.getBytes(2));
 				Collection<MessageId> messageIds = ids.get(g);
+				//noinspection Java8MapApi
 				if (messageIds == null) {
 					messageIds = new ArrayList<>();
 					ids.put(g, messageIds);
@@ -2441,12 +2511,28 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public long getNextSendTime(Connection txn, ContactId c)
+	public long getNextSendTime(Connection txn, ContactId c, long maxLatency)
 			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT expiry FROM statuses"
+			// Are any messages sendable immediately?
+			String sql = "SELECT NULL FROM statuses"
+					+ " WHERE contactId = ? AND state = ?"
+					+ " AND groupShared = TRUE AND messageShared = TRUE"
+					+ " AND deleted = FALSE AND seen = FALSE"
+					+ " AND (maxLatency IS NULL OR ? < maxLatency)";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setInt(2, DELIVERED.getValue());
+			ps.setLong(3, maxLatency);
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			rs.close();
+			ps.close();
+			if (found) return 0;
+			// When is the earliest expiry time (could be in the past)?
+			sql = "SELECT expiry FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"
@@ -2550,9 +2636,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 
 	@Override
 	public Collection<MessageId> getRequestedMessagesToSend(Connection txn,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
-		long eta = now + maxLatency;
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -2561,21 +2646,21 @@ abstract class JdbcDatabase implements Database<Connection> {
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE"
 					+ " AND seen = FALSE AND requested = TRUE"
-					+ " AND (expiry <= ? OR eta > ?)"
+					+ " AND (expiry <= ? OR maxLatency IS NULL"
+					+ " OR ? < maxLatency)"
 					+ " ORDER BY timestamp";
 			ps = txn.prepareStatement(sql);
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			ps.setLong(3, now);
-			ps.setLong(4, eta);
+			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2729,6 +2814,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				ContactId c = new ContactId(rs.getInt(1));
 				TransportId t = new TransportId(rs.getString(2));
 				Collection<TransportId> transportIds = ids.get(c);
+				//noinspection Java8MapApi
 				if (transportIds == null) {
 					transportIds = new ArrayList<>();
 					ids.put(c, transportIds);
@@ -3298,7 +3384,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 			throws DbException {
 		PreparedStatement ps = null;
 		try {
-			String sql = "UPDATE statuses SET expiry = 0, txCount = 0, eta = 0"
+			String sql = "UPDATE statuses SET expiry = 0, txCount = 0,"
+					+ " maxLatency = NULL"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE";
@@ -3643,8 +3730,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public void updateExpiryTimeAndEta(Connection txn, ContactId c, MessageId m,
+	public void updateRetransmissionData(Connection txn, ContactId c,
-			long maxLatency) throws DbException {
+			MessageId m, long maxLatency) throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -3660,13 +3747,12 @@ abstract class JdbcDatabase implements Database<Connection> {
 			rs.close();
 			ps.close();
 			sql = "UPDATE statuses"
-					+ " SET expiry = ?, txCount = txCount + 1, eta = ?"
+					+ " SET expiry = ?, txCount = txCount + 1, maxLatency = ?"
 					+ " WHERE messageId = ? AND contactId = ?";
 			ps = txn.prepareStatement(sql);
 			long now = clock.currentTimeMillis();
-			long eta = now + maxLatency;
 			ps.setLong(1, calculateExpiry(now, maxLatency, txCount));
-			ps.setLong(2, eta);
+			ps.setLong(2, maxLatency);
 			ps.setBytes(3, m.getBytes());
 			ps.setInt(4, c.getInt());
 			int affected = ps.executeUpdate();

bramble-core/src/main/java/org/briarproject/bramble/db/Migration49_50.java

@@ -0,0 +1,45 @@
+package org.briarproject.bramble.db;
+
+import org.briarproject.bramble.api.db.DbException;
+
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.sql.Statement;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.db.JdbcUtils.tryToClose;
+
+class Migration49_50 implements Migration<Connection> {
+
+	private static final Logger LOG = getLogger(Migration49_50.class.getName());
+
+	@Override
+	public int getStartVersion() {
+		return 49;
+	}
+
+	@Override
+	public int getEndVersion() {
+		return 50;
+	}
+
+	@Override
+	public void migrate(Connection txn) throws DbException {
+		Statement s = null;
+		try {
+			s = txn.createStatement();
+			s.execute("ALTER TABLE statuses"
+					+ " ALTER COLUMN eta"
+					+ " RENAME TO maxLatency");
+			s.execute("ALTER TABLE statuses"
+					+ " ALTER COLUMN maxLatency"
+					+ " SET NULL");
+			s.execute("UPDATE statuses SET maxLatency = NULL");
+		} catch (SQLException e) {
+			tryToClose(s, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+}

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

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

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

@@ -1,18 +1,49 @@
 package org.briarproject.bramble.io;
 
+import org.briarproject.bramble.api.WeakSingletonProvider;
 import org.briarproject.bramble.api.io.TimeoutMonitor;
 
+import javax.annotation.Nonnull;
 import javax.inject.Singleton;
+import javax.net.SocketFactory;
 
 import dagger.Module;
 import dagger.Provides;
+import okhttp3.Dns;
+import okhttp3.OkHttpClient;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
 
 @Module
 public class IoModule {
 
+	private static final int CONNECT_TIMEOUT = 60_000; // Milliseconds
+
 	@Provides
 	@Singleton
 	TimeoutMonitor provideTimeoutMonitor(TimeoutMonitorImpl timeoutMonitor) {
 		return timeoutMonitor;
 	}
+
+	// Share an HTTP client instance between requests where possible, while
+	// allowing the client to be garbage-collected between requests. The
+	// provider keeps a weak reference to the last client instance and reuses
+	// the instance until it gets garbage-collected. See
+	// https://medium.com/@leandromazzuquini/if-you-are-using-okhttp-you-should-know-this-61d68e065a2b
+	@Provides
+	@Singleton
+	WeakSingletonProvider<OkHttpClient> provideOkHttpClientProvider(
+			SocketFactory torSocketFactory, Dns noDnsLookups) {
+		return new WeakSingletonProvider<OkHttpClient>() {
+			@Override
+			@Nonnull
+			public OkHttpClient createInstance() {
+				return new OkHttpClient.Builder()
+						.socketFactory(torSocketFactory)
+						.dns(noDnsLookups) // Don't make local DNS lookups
+						.connectTimeout(CONNECT_TIMEOUT, MILLISECONDS)
+						.build();
+			}
+		};
+	}
 }

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

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

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

@@ -1,10 +1,10 @@
 package org.briarproject.bramble.io;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.io.TimeoutMonitor;
 import org.briarproject.bramble.api.lifecycle.IoExecutor;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 
 import java.io.IOException;

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

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

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

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+/**
+ * An interface for calling an API endpoint with the option to retry the call.
+ */
+interface ApiCall {
+
+	/**
+	 * This method makes a synchronous call to an API endpoint and returns
+	 * true if the call should be retried, in which case the method may be
+	 * called again on the same {@link ApiCall} instance after a delay.
+	 *
+	 * @return True if the API call needs to be retried, or false if the API
+	 * call succeeded or {@link TolerableFailureException failed tolerably}.
+	 */
+	@IoExecutor
+	boolean callApi();
+}

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

@@ -0,0 +1,43 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * An interface for checking whether a mailbox is reachable.
+ */
+@ThreadSafe
+@NotNullByDefault
+interface ConnectivityChecker {
+
+	/**
+	 * Destroys the checker. Any current connectivity check is cancelled.
+	 */
+	void destroy();
+
+	/**
+	 * Starts a connectivity check if needed and calls the given observer when
+	 * the check succeeds. If a check is already running then the observer is
+	 * called when the check succeeds. If a connectivity check has recently
+	 * succeeded then the observer is called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the checker is
+	 * {@link #destroy() destroyed}.
+	 */
+	void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o);
+
+	/**
+	 * Removes an observer that was added via
+	 * {@link #checkConnectivity(MailboxProperties, ConnectivityObserver)}. If
+	 * there are no remaining observers and a connectivity check is running
+	 * then the check will be cancelled.
+	 */
+	void removeObserver(ConnectivityObserver o);
+
+	interface ConnectivityObserver {
+		void onConnectivityCheckSucceeded();
+	}
+}

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

@@ -0,0 +1,122 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
+
+	/**
+	 * If no more than this much time has elapsed since the last connectivity
+	 * check succeeded, consider the result to be fresh and don't check again.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CONNECTIVITY_CHECK_FRESHNESS_MS = 10_000;
+
+	private final Object lock = new Object();
+
+	protected final Clock clock;
+	private final MailboxApiCaller mailboxApiCaller;
+
+	@GuardedBy("lock")
+	private boolean destroyed = false;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable connectivityCheck = null;
+
+	@GuardedBy("lock")
+	private long lastConnectivityCheckSucceeded = 0;
+
+	@GuardedBy("lock")
+	private final List<ConnectivityObserver> connectivityObservers =
+			new ArrayList<>();
+
+	/**
+	 * Creates an {@link ApiCall} for checking whether the mailbox is
+	 * reachable. The {@link ApiCall} should call
+	 * {@link #onConnectivityCheckSucceeded(long)} if the check succeeds.
+	 */
+	abstract ApiCall createConnectivityCheckTask(MailboxProperties properties);
+
+	ConnectivityCheckerImpl(Clock clock, MailboxApiCaller mailboxApiCaller) {
+		this.clock = clock;
+		this.mailboxApiCaller = mailboxApiCaller;
+	}
+
+	@Override
+	public void destroy() {
+		synchronized (lock) {
+			destroyed = true;
+			connectivityObservers.clear();
+			if (connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
+
+	@Override
+	public void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (connectivityCheck == null) {
+				// No connectivity check is running
+				long now = clock.currentTimeMillis();
+				if (now - lastConnectivityCheckSucceeded
+						> CONNECTIVITY_CHECK_FRESHNESS_MS) {
+					// The last connectivity check is stale, start a new one
+					connectivityObservers.add(o);
+					ApiCall task = createConnectivityCheckTask(properties);
+					connectivityCheck = mailboxApiCaller.retryWithBackoff(task);
+				} else {
+					// The last connectivity check is fresh
+					callNow = true;
+				}
+			} else {
+				// A connectivity check is running, wait for it to succeed
+				connectivityObservers.add(o);
+			}
+		}
+		if (callNow) o.onConnectivityCheckSucceeded();
+	}
+
+	protected void onConnectivityCheckSucceeded(long now) {
+		List<ConnectivityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityCheck = null;
+			lastConnectivityCheckSucceeded = now;
+			observers = new ArrayList<>(connectivityObservers);
+			connectivityObservers.clear();
+		}
+		for (ConnectivityObserver o : observers) {
+			o.onConnectivityCheckSucceeded();
+		}
+	}
+
+	@Override
+	public void removeObserver(ConnectivityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityObservers.remove(o);
+			if (connectivityObservers.isEmpty() && connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
+}