Merge branch 'contact-exchange-refactoring' into 'master'

Contact exchange refactoring

See merge request briar/briar!1106

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

@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.contact;
+
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.db.ContactExistsException;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
+
+import java.io.IOException;
+
+@NotNullByDefault
+public interface ContactExchangeManager {
+
+	/**
+	 * Exchanges contact information with a remote peer.
+	 *
+	 * @param alice Whether the local peer takes the role of Alice
+	 * @return The newly added contact
+	 * @throws ContactExistsException If the contact already exists
+	 */
+	Contact exchangeContacts(TransportId t, DuplexTransportConnection conn,
+			SecretKey masterKey, boolean alice) throws IOException, DbException;
+}

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

@@ -93,6 +93,11 @@ public interface ContactManager {
 	 */
 	Contact getContact(ContactId c) throws DbException;
 
+	/**
+	 * Returns the contact with the given ID.
+	 */
+	Contact getContact(Transaction txn, ContactId c) throws DbException;
+
 	/**
 	 * Returns the contact with the given remoteAuthorId
 	 * that was added by the LocalAuthor with the given localAuthorId

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

@@ -1,32 +0,0 @@
-package org.briarproject.bramble.api.contact.event;
-
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.identity.Author;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import javax.annotation.Nullable;
-
-@NotNullByDefault
-public class ContactExchangeFailedEvent extends Event {
-
-	@Nullable
-	private final Author duplicateRemoteAuthor;
-
-	public ContactExchangeFailedEvent(@Nullable Author duplicateRemoteAuthor) {
-		this.duplicateRemoteAuthor = duplicateRemoteAuthor;
-	}
-
-	public ContactExchangeFailedEvent() {
-		this(null);
-	}
-
-	@Nullable
-	public Author getDuplicateRemoteAuthor() {
-		return duplicateRemoteAuthor;
-	}
-
-	public boolean wasDuplicateContact() {
-		return duplicateRemoteAuthor != null;
-	}
-
-}

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

@@ -1,20 +0,0 @@
-package org.briarproject.bramble.api.contact.event;
-
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.identity.Author;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-@NotNullByDefault
-public class ContactExchangeSucceededEvent extends Event {
-
-	private final Author remoteAuthor;
-
-	public ContactExchangeSucceededEvent(Author remoteAuthor) {
-		this.remoteAuthor = remoteAuthor;
-	}
-
-	public Author getRemoteAuthor() {
-		return remoteAuthor;
-	}
-
-}

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

@@ -1,8 +1,27 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.AuthorId;
+
 /**
  * Thrown when a duplicate contact is added to the database. This exception may
  * occur due to concurrent updates and does not indicate a database error.
  */
 public class ContactExistsException extends DbException {
+
+	private final AuthorId local;
+	private final Author remote;
+
+	public ContactExistsException(AuthorId local, Author remote) {
+		this.local = local;
+		this.remote = remote;
+	}
+
+	public AuthorId getLocalAuthorId() {
+		return local;
+	}
+
+	public Author getRemoteAuthor() {
+		return remote;
+	}
 }

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

@@ -1,16 +1,6 @@
-package org.briarproject.bramble.api.contact;
+package org.briarproject.bramble.contact;
 
-import org.briarproject.bramble.api.crypto.SecretKey;
-import org.briarproject.bramble.api.identity.LocalAuthor;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.TransportId;
-import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
-
-/**
- * A task for conducting a contact information exchange with a remote peer.
- */
-@NotNullByDefault
-public interface ContactExchangeTask {
+interface ContactExchangeConstants {
 
 	/**
 	 * The current version of the contact exchange protocol.
@@ -39,9 +29,7 @@ public interface ContactExchangeTask {
 	String BOB_NONCE_LABEL = "org.briarproject.bramble.contact/BOB_NONCE";
 
 	/**
-	 * Exchanges contact information with a remote peer.
+	 * Label for signing key binding nonces.
 	 */
-	void startExchange(LocalAuthor localAuthor, SecretKey masterKey,
-			DuplexTransportConnection conn, TransportId transportId,
-			boolean alice);
+	String SIGNING_LABEL = "org.briarproject.briar.contact/EXCHANGE";
 }

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

@@ -0,0 +1,35 @@
+package org.briarproject.bramble.contact;
+
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface ContactExchangeCrypto {
+
+	/**
+	 * Derives the header key for a contact exchange stream from the master key.
+	 *
+	 * @param alice Whether the header key is for the stream sent by Alice
+	 */
+	SecretKey deriveHeaderKey(SecretKey masterKey, boolean alice);
+
+	/**
+	 * Creates and returns a signature that proves ownership of a pseudonym.
+	 *
+	 * @param privateKey The pseudonym's signature private key
+	 * @param alice Whether the pseudonym belongs to Alice
+	 */
+	byte[] sign(PrivateKey privateKey, SecretKey masterKey, boolean alice);
+
+	/**
+	 * Verifies a signature that proves ownership of a pseudonym.
+	 *
+	 * @param publicKey The pseudonym's signature public key
+	 * @param alice Whether the pseudonym belongs to Alice
+	 * @return True if the signature is valid
+	 */
+	boolean verify(PublicKey publicKey, SecretKey masterKey, boolean alice,
+			byte[] signature);
+}

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

@@ -0,0 +1,66 @@
+package org.briarproject.bramble.contact;
+
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.GeneralSecurityException;
+
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.contact.ContactExchangeConstants.ALICE_KEY_LABEL;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.ALICE_NONCE_LABEL;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.BOB_KEY_LABEL;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.BOB_NONCE_LABEL;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.PROTOCOL_VERSION;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.SIGNING_LABEL;
+
+@NotNullByDefault
+class ContactExchangeCryptoImpl implements ContactExchangeCrypto {
+
+	private static final byte[] PROTOCOL_VERSION_BYTES =
+			new byte[] {PROTOCOL_VERSION};
+
+	private final CryptoComponent crypto;
+
+	@Inject
+	ContactExchangeCryptoImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public SecretKey deriveHeaderKey(SecretKey masterKey, boolean alice) {
+		String label = alice ? ALICE_KEY_LABEL : BOB_KEY_LABEL;
+		return crypto.deriveKey(label, masterKey, PROTOCOL_VERSION_BYTES);
+	}
+
+	@Override
+	public byte[] sign(PrivateKey privateKey, SecretKey masterKey,
+			boolean alice) {
+		byte[] nonce = deriveNonce(masterKey, alice);
+		try {
+			return crypto.sign(SIGNING_LABEL, nonce, privateKey);
+		} catch (GeneralSecurityException e) {
+			throw new AssertionError();
+		}
+	}
+
+	@Override
+	public boolean verify(PublicKey publicKey,
+			SecretKey masterKey, boolean alice, byte[] signature) {
+		byte[] nonce = deriveNonce(masterKey, alice);
+		try {
+			return crypto.verifySignature(signature, SIGNING_LABEL, nonce,
+					publicKey);
+		} catch (GeneralSecurityException e) {
+			return false;
+		}
+	}
+
+	private byte[] deriveNonce(SecretKey masterKey, boolean alice) {
+		String label = alice ? ALICE_NONCE_LABEL : BOB_NONCE_LABEL;
+		return crypto.mac(label, masterKey, PROTOCOL_VERSION_BYTES);
+	}
+}

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

@@ -3,24 +3,21 @@ package org.briarproject.bramble.contact;
 import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.Predicate;
 import org.briarproject.bramble.api.client.ClientHelper;
-import org.briarproject.bramble.api.contact.ContactExchangeTask;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactExchangeManager;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.ContactManager;
-import org.briarproject.bramble.api.contact.event.ContactExchangeFailedEvent;
-import org.briarproject.bramble.api.contact.event.ContactExchangeSucceededEvent;
-import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.crypto.PublicKey;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.data.BdfDictionary;
 import org.briarproject.bramble.api.data.BdfList;
-import org.briarproject.bramble.api.db.ContactExistsException;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.DbException;
-import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.IdentityManager;
 import org.briarproject.bramble.api.identity.LocalAuthor;
 import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
-import org.briarproject.bramble.api.plugin.ConnectionManager;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -39,28 +36,24 @@ import java.io.EOFException;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
-import java.security.GeneralSecurityException;
 import java.util.Map;
 import java.util.logging.Logger;
 
 import javax.inject.Inject;
 
-import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.contact.RecordTypes.CONTACT_INFO;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;
-import static org.briarproject.bramble.util.LogUtils.logException;
+import static org.briarproject.bramble.contact.ContactExchangeConstants.PROTOCOL_VERSION;
 import static org.briarproject.bramble.util.ValidationUtils.checkLength;
 import static org.briarproject.bramble.util.ValidationUtils.checkSize;
 
 @MethodsNotNullByDefault
 @ParametersNotNullByDefault
-class ContactExchangeTaskImpl extends Thread implements ContactExchangeTask {
+class ContactExchangeManagerImpl implements ContactExchangeManager {
 
 	private static final Logger LOG =
-			Logger.getLogger(ContactExchangeTaskImpl.class.getName());
-
-	private static final String SIGNING_LABEL_EXCHANGE =
-			"org.briarproject.briar.contact/EXCHANGE";
+			getLogger(ContactExchangeManagerImpl.class.getName());
 
 	// Accept records with current protocol version, known record type
 	private static final Predicate<Record> ACCEPT = r ->
@@ -80,190 +73,108 @@ class ContactExchangeTaskImpl extends Thread implements ContactExchangeTask {
 	private final ClientHelper clientHelper;
 	private final RecordReaderFactory recordReaderFactory;
 	private final RecordWriterFactory recordWriterFactory;
-	private final EventBus eventBus;
 	private final Clock clock;
-	private final ConnectionManager connectionManager;
 	private final ContactManager contactManager;
+	private final IdentityManager identityManager;
 	private final TransportPropertyManager transportPropertyManager;
-	private final CryptoComponent crypto;
+	private final ContactExchangeCrypto contactExchangeCrypto;
 	private final StreamReaderFactory streamReaderFactory;
 	private final StreamWriterFactory streamWriterFactory;
 
-	private volatile LocalAuthor localAuthor;
-	private volatile DuplexTransportConnection conn;
-	private volatile TransportId transportId;
-	private volatile SecretKey masterKey;
-	private volatile boolean alice;
-
 	@Inject
-	ContactExchangeTaskImpl(DatabaseComponent db, ClientHelper clientHelper,
+	ContactExchangeManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
 			RecordReaderFactory recordReaderFactory,
-			RecordWriterFactory recordWriterFactory, EventBus eventBus,
-			Clock clock, ConnectionManager connectionManager,
-			ContactManager contactManager,
+			RecordWriterFactory recordWriterFactory, Clock clock,
+			ContactManager contactManager, IdentityManager identityManager,
 			TransportPropertyManager transportPropertyManager,
-			CryptoComponent crypto, StreamReaderFactory streamReaderFactory,
+			ContactExchangeCrypto contactExchangeCrypto,
+			StreamReaderFactory streamReaderFactory,
 			StreamWriterFactory streamWriterFactory) {
 		this.db = db;
 		this.clientHelper = clientHelper;
 		this.recordReaderFactory = recordReaderFactory;
 		this.recordWriterFactory = recordWriterFactory;
-		this.eventBus = eventBus;
 		this.clock = clock;
-		this.connectionManager = connectionManager;
 		this.contactManager = contactManager;
+		this.identityManager = identityManager;
 		this.transportPropertyManager = transportPropertyManager;
-		this.crypto = crypto;
+		this.contactExchangeCrypto = contactExchangeCrypto;
 		this.streamReaderFactory = streamReaderFactory;
 		this.streamWriterFactory = streamWriterFactory;
 	}
 
 	@Override
-	public void startExchange(LocalAuthor localAuthor, SecretKey masterKey,
-			DuplexTransportConnection conn, TransportId transportId,
-			boolean alice) {
-		this.localAuthor = localAuthor;
-		this.conn = conn;
-		this.transportId = transportId;
-		this.masterKey = masterKey;
-		this.alice = alice;
-		start();
-	}
-
-	@Override
-	public void run() {
+	public Contact exchangeContacts(TransportId t,
+			DuplexTransportConnection conn, SecretKey masterKey, boolean alice)
+			throws IOException, DbException {
 		// Get the transport connection's input and output streams
-		InputStream in;
-		OutputStream out;
-		try {
-			in = conn.getReader().getInputStream();
-			out = conn.getWriter().getOutputStream();
-		} catch (IOException e) {
-			logException(LOG, WARNING, e);
-			tryToClose(conn);
-			eventBus.broadcast(new ContactExchangeFailedEvent());
-			return;
-		}
+		InputStream in = conn.getReader().getInputStream();
+		OutputStream out = conn.getWriter().getOutputStream();
 
-		// Get the local transport properties
-		Map<TransportId, TransportProperties> localProperties;
-		try {
-			localProperties = transportPropertyManager.getLocalProperties();
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			eventBus.broadcast(new ContactExchangeFailedEvent());
-			tryToClose(conn);
-			return;
-		}
+		// Get the local author and transport properties
+		LocalAuthor localAuthor = identityManager.getLocalAuthor();
+		Map<TransportId, TransportProperties> localProperties =
+				transportPropertyManager.getLocalProperties();
 
 		// Derive the header keys for the transport streams
-		SecretKey aliceHeaderKey = crypto.deriveKey(ALICE_KEY_LABEL, masterKey,
-				new byte[] {PROTOCOL_VERSION});
-		SecretKey bobHeaderKey = crypto.deriveKey(BOB_KEY_LABEL, masterKey,
-				new byte[] {PROTOCOL_VERSION});
+		SecretKey localHeaderKey =
+				contactExchangeCrypto.deriveHeaderKey(masterKey, alice);
+		SecretKey remoteHeaderKey =
+				contactExchangeCrypto.deriveHeaderKey(masterKey, !alice);
 
 		// Create the readers
-		InputStream streamReader =
-				streamReaderFactory.createContactExchangeStreamReader(in,
-						alice ? bobHeaderKey : aliceHeaderKey);
+		InputStream streamReader = streamReaderFactory
+				.createContactExchangeStreamReader(in, remoteHeaderKey);
 		RecordReader recordReader =
 				recordReaderFactory.createRecordReader(streamReader);
 
 		// Create the writers
-		StreamWriter streamWriter =
-				streamWriterFactory.createContactExchangeStreamWriter(out,
-						alice ? aliceHeaderKey : bobHeaderKey);
-		RecordWriter recordWriter =
-				recordWriterFactory
-						.createRecordWriter(streamWriter.getOutputStream());
+		StreamWriter streamWriter = streamWriterFactory
+				.createContactExchangeStreamWriter(out, localHeaderKey);
+		RecordWriter recordWriter = recordWriterFactory
+				.createRecordWriter(streamWriter.getOutputStream());
 
-		// Derive the nonces to be signed
-		byte[] aliceNonce = crypto.mac(ALICE_NONCE_LABEL, masterKey,
-				new byte[] {PROTOCOL_VERSION});
-		byte[] bobNonce = crypto.mac(BOB_NONCE_LABEL, masterKey,
-				new byte[] {PROTOCOL_VERSION});
-		byte[] localNonce = alice ? aliceNonce : bobNonce;
-		byte[] remoteNonce = alice ? bobNonce : aliceNonce;
-
-		// Sign the nonce
-		byte[] localSignature = sign(localAuthor, localNonce);
+		// Create our signature
+		byte[] localSignature = contactExchangeCrypto
+				.sign(localAuthor.getPrivateKey(), masterKey, alice);
 
 		// Exchange contact info
 		long localTimestamp = clock.currentTimeMillis();
 		ContactInfo remoteInfo;
-		try {
-			if (alice) {
-				sendContactInfo(recordWriter, localAuthor, localProperties,
-						localSignature, localTimestamp);
-				recordWriter.flush();
-				remoteInfo = receiveContactInfo(recordReader);
-			} else {
-				remoteInfo = receiveContactInfo(recordReader);
-				sendContactInfo(recordWriter, localAuthor, localProperties,
-						localSignature, localTimestamp);
-				recordWriter.flush();
-			}
-			// Send EOF on the outgoing stream
-			streamWriter.sendEndOfStream();
-			// Skip any remaining records from the incoming stream
-			recordReader.readRecord(r -> false, IGNORE);
-		} catch (IOException e) {
-			logException(LOG, WARNING, e);
-			eventBus.broadcast(new ContactExchangeFailedEvent());
-			tryToClose(conn);
-			return;
+		if (alice) {
+			sendContactInfo(recordWriter, localAuthor, localProperties,
+					localSignature, localTimestamp);
+			remoteInfo = receiveContactInfo(recordReader);
+		} else {
+			remoteInfo = receiveContactInfo(recordReader);
+			sendContactInfo(recordWriter, localAuthor, localProperties,
+					localSignature, localTimestamp);
 		}
 
+		// Send EOF on the outgoing stream
+		streamWriter.sendEndOfStream();
+
+		// Skip any remaining records from the incoming stream
+		recordReader.readRecord(r -> false, IGNORE);
+
 		// Verify the contact's signature
-		if (!verify(remoteInfo.author, remoteNonce, remoteInfo.signature)) {
+		PublicKey remotePublicKey = remoteInfo.author.getPublicKey();
+		if (!contactExchangeCrypto.verify(remotePublicKey,
+				masterKey, !alice, remoteInfo.signature)) {
 			LOG.warning("Invalid signature");
-			eventBus.broadcast(new ContactExchangeFailedEvent());
-			tryToClose(conn);
-			return;
+			throw new FormatException();
 		}
 
 		// The agreed timestamp is the minimum of the peers' timestamps
 		long timestamp = Math.min(localTimestamp, remoteInfo.timestamp);
 
-		try {
-			// Add the contact
-			ContactId contactId = addContact(remoteInfo.author, timestamp,
-					remoteInfo.properties);
-			// Reuse the connection as a transport connection
-			connectionManager.manageOutgoingConnection(contactId, transportId,
-					conn);
-			// Pseudonym exchange succeeded
-			LOG.info("Pseudonym exchange succeeded");
-			eventBus.broadcast(
-					new ContactExchangeSucceededEvent(remoteInfo.author));
-		} catch (ContactExistsException e) {
-			logException(LOG, WARNING, e);
-			tryToClose(conn);
-			eventBus.broadcast(
-					new ContactExchangeFailedEvent(remoteInfo.author));
-		} catch (DbException e) {
-			logException(LOG, WARNING, e);
-			tryToClose(conn);
-			eventBus.broadcast(new ContactExchangeFailedEvent());
-		}
-	}
+		// Add the contact
+		Contact contact = addContact(remoteInfo.author, localAuthor,
+				masterKey, timestamp, alice, remoteInfo.properties);
 
-	private byte[] sign(LocalAuthor author, byte[] nonce) {
-		try {
-			return crypto.sign(SIGNING_LABEL_EXCHANGE, nonce,
-					author.getPrivateKey());
-		} catch (GeneralSecurityException e) {
-			throw new AssertionError();
-		}
-	}
-
-	private boolean verify(Author author, byte[] nonce, byte[] signature) {
-		try {
-			return crypto.verifySignature(signature, SIGNING_LABEL_EXCHANGE,
-					nonce, author.getPublicKey());
-		} catch (GeneralSecurityException e) {
-			return false;
-		}
+		// Contact exchange succeeded
+		LOG.info("Contact exchange succeeded");
+		return contact;
 	}
 
 	private void sendContactInfo(RecordWriter recordWriter, Author author,
@@ -274,6 +185,7 @@ class ContactExchangeTaskImpl extends Thread implements ContactExchangeTask {
 		BdfList payload = BdfList.of(authorList, props, signature, timestamp);
 		recordWriter.writeRecord(new Record(PROTOCOL_VERSION, CONTACT_INFO,
 				clientHelper.toByteArray(payload)));
+		recordWriter.flush();
 		LOG.info("Sent contact info");
 	}
 
@@ -295,7 +207,8 @@ class ContactExchangeTaskImpl extends Thread implements ContactExchangeTask {
 		return new ContactInfo(author, properties, signature, timestamp);
 	}
 
-	private ContactId addContact(Author remoteAuthor, long timestamp,
+	private Contact addContact(Author remoteAuthor, LocalAuthor localAuthor,
+			SecretKey masterKey, long timestamp, boolean alice,
 			Map<TransportId, TransportProperties> remoteProperties)
 			throws DbException {
 		return db.transactionWithResult(false, txn -> {
@@ -304,20 +217,10 @@ class ContactExchangeTaskImpl extends Thread implements ContactExchangeTask {
 					true, true);
 			transportPropertyManager.addRemoteProperties(txn, contactId,
 					remoteProperties);
-			return contactId;
+			return contactManager.getContact(txn, contactId);
 		});
 	}
 
-	private void tryToClose(DuplexTransportConnection conn) {
-		try {
-			LOG.info("Closing connection");
-			conn.getReader().dispose(true, true);
-			conn.getWriter().dispose(true);
-		} catch (IOException e) {
-			logException(LOG, WARNING, e);
-		}
-	}
-
 	private static class ContactInfo {
 
 		private final Author author;

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

@@ -137,6 +137,11 @@ class ContactManagerImpl implements ContactManager {
 		return db.transactionWithResult(true, txn -> db.getContact(txn, c));
 	}
 
+	@Override
+	public Contact getContact(Transaction txn, ContactId c) throws DbException {
+		return db.getContact(txn, c);
+	}
+
 	@Override
 	public Contact getContact(AuthorId remoteAuthorId, AuthorId localAuthorId)
 			throws DbException {

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

@@ -1,6 +1,6 @@
 package org.briarproject.bramble.contact;
 
-import org.briarproject.bramble.api.contact.ContactExchangeTask;
+import org.briarproject.bramble.api.contact.ContactExchangeManager;
 import org.briarproject.bramble.api.contact.ContactManager;
 
 import javax.inject.Inject;
@@ -19,14 +19,14 @@ public class ContactModule {
 
 	@Provides
 	@Singleton
-	ContactManager getContactManager(ContactManagerImpl contactManager) {
+	ContactManager provideContactManager(ContactManagerImpl contactManager) {
 		return contactManager;
 	}
 
 	@Provides
-	ContactExchangeTask provideContactExchangeTask(
-			ContactExchangeTaskImpl contactExchangeTask) {
-		return contactExchangeTask;
+	ContactExchangeManager provideContactExchangeManager(
+			ContactExchangeManagerImpl contactExchangeManager) {
+		return contactExchangeManager;
 	}
 
 	@Provides
@@ -34,4 +34,10 @@ public class ContactModule {
 			PendingContactFactoryImpl pendingContactFactory) {
 		return pendingContactFactory;
 	}
+
+	@Provides
+	ContactExchangeCrypto provideContactExchangeCrypto(
+			ContactExchangeCryptoImpl contactExchangeCrypto) {
+		return contactExchangeCrypto;
+	}
 }

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

@@ -234,16 +234,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 
 	@Override
 	public ContactId addContact(Transaction transaction, Author remote,
-			AuthorId local, boolean verified)
-			throws DbException {
+			AuthorId local, boolean verified) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsIdentity(txn, local))
 			throw new NoSuchIdentityException();
 		if (db.containsIdentity(txn, remote.getId()))
-			throw new ContactExistsException();
+			throw new ContactExistsException(local, remote);
 		if (db.containsContact(txn, remote.getId(), local))
-			throw new ContactExistsException();
+			throw new ContactExistsException(local, remote);
 		ContactId c = db.addContact(txn, remote, local, verified);
 		transaction.attach(new ContactAddedEvent(c));
 		return c;

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

@@ -1459,7 +1459,8 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 							true));
 			fail();
 		} catch (ContactExistsException expected) {
-			// Expected
+			assertEquals(localAuthor.getId(), expected.getLocalAuthorId());
+			assertEquals(author, expected.getRemoteAuthor());
 		}
 	}
 
@@ -1488,7 +1489,8 @@ public class DatabaseComponentImplTest extends BrambleMockTestCase {
 							true));
 			fail();
 		} catch (ContactExistsException expected) {
-			// Expected
+			assertEquals(localAuthor.getId(), expected.getLocalAuthorId());
+			assertEquals(author, expected.getRemoteAuthor());
 		}
 	}
 

briar-android/src/main/java/org/briarproject/briar/android/AndroidComponent.java

@@ -8,7 +8,7 @@ import org.briarproject.bramble.BrambleCoreEagerSingletons;
 import org.briarproject.bramble.BrambleCoreModule;
 import org.briarproject.bramble.account.BriarAccountModule;
 import org.briarproject.bramble.api.account.AccountManager;
-import org.briarproject.bramble.api.contact.ContactExchangeTask;
+import org.briarproject.bramble.api.contact.ContactExchangeManager;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.crypto.CryptoExecutor;
 import org.briarproject.bramble.api.crypto.PasswordStrengthEstimator;
@@ -128,7 +128,7 @@ public interface AndroidComponent
 
 	SettingsManager settingsManager();
 
-	ContactExchangeTask contactExchangeTask();
+	ContactExchangeManager contactExchangeManager();
 
 	KeyAgreementTask keyAgreementTask();
 

briar-android/src/main/java/org/briarproject/briar/android/AppModule.java

@@ -33,6 +33,7 @@ import org.briarproject.bramble.plugin.tor.CircumventionProvider;
 import org.briarproject.bramble.util.AndroidUtils;
 import org.briarproject.bramble.util.StringUtils;
 import org.briarproject.briar.android.account.LockManagerImpl;
+import org.briarproject.briar.android.keyagreement.ContactExchangeModule;
 import org.briarproject.briar.android.viewmodel.ViewModelModule;
 import org.briarproject.briar.api.android.AndroidNotificationManager;
 import org.briarproject.briar.api.android.DozeWatchdog;
@@ -59,7 +60,7 @@ import static java.util.Collections.emptyList;
 import static org.briarproject.bramble.api.reporting.ReportingConstants.DEV_ONION_ADDRESS;
 import static org.briarproject.bramble.api.reporting.ReportingConstants.DEV_PUBLIC_KEY_HEX;
 
-@Module(includes = ViewModelModule.class)
+@Module(includes = {ContactExchangeModule.class, ViewModelModule.class})
 public class AppModule {
 
 	static class EagerSingletons {

briar-android/src/main/java/org/briarproject/briar/android/keyagreement/ContactExchangeActivity.java

@@ -1,46 +1,32 @@
 package org.briarproject.briar.android.keyagreement;
 
+import android.arch.lifecycle.ViewModelProvider;
+import android.arch.lifecycle.ViewModelProviders;
 import android.os.Bundle;
 import android.support.annotation.UiThread;
 import android.widget.Toast;
 
-import org.briarproject.bramble.api.contact.ContactExchangeTask;
-import org.briarproject.bramble.api.contact.event.ContactExchangeFailedEvent;
-import org.briarproject.bramble.api.contact.event.ContactExchangeSucceededEvent;
-import org.briarproject.bramble.api.db.DbException;
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.event.EventListener;
 import org.briarproject.bramble.api.identity.Author;
-import org.briarproject.bramble.api.identity.IdentityManager;
-import org.briarproject.bramble.api.identity.LocalAuthor;
 import org.briarproject.bramble.api.keyagreement.KeyAgreementResult;
 import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
 import org.briarproject.briar.R;
 import org.briarproject.briar.android.activity.ActivityComponent;
 
-import java.util.logging.Logger;
-
 import javax.annotation.Nullable;
 import javax.inject.Inject;
 
 import static android.widget.Toast.LENGTH_LONG;
-import static java.util.logging.Level.WARNING;
-import static org.briarproject.bramble.util.LogUtils.logException;
+import static java.util.Objects.requireNonNull;
 
 @MethodsNotNullByDefault
 @ParametersNotNullByDefault
-public class ContactExchangeActivity extends KeyAgreementActivity implements
-		EventListener {
+public class ContactExchangeActivity extends KeyAgreementActivity {
 
-	private static final Logger LOG =
-			Logger.getLogger(ContactExchangeActivity.class.getName());
+	@Inject
+	ViewModelProvider.Factory viewModelFactory;
 
-	// Fields that are accessed from background threads must be volatile
-	@Inject
-	volatile ContactExchangeTask contactExchangeTask;
-	@Inject
-	volatile IdentityManager identityManager;
+	private ContactExchangeViewModel viewModel;
 
 	@Override
 	public void injectActivity(ActivityComponent component) {
@@ -50,83 +36,50 @@ public class ContactExchangeActivity extends KeyAgreementActivity implements
 	@Override
 	public void onCreate(@Nullable Bundle state) {
 		super.onCreate(state);
-		getSupportActionBar().setTitle(R.string.add_contact_title);
-	}
-
-	@Override
-	public void onStart() {
-		super.onStart();
-		// Listen to updates from contactExchangeTask
-		eventBus.addListener(this);
-	}
-
-	@Override
-	protected void onStop() {
-		super.onStop();
-		// Stop listen to updates from contactExchangeTask
-		eventBus.addListener(this);
+		requireNonNull(getSupportActionBar())
+				.setTitle(R.string.add_contact_title);
+		viewModel = ViewModelProviders.of(this, viewModelFactory)
+				.get(ContactExchangeViewModel.class);
 	}
 
 	private void startContactExchange(KeyAgreementResult result) {
-		runOnDbThread(() -> {
-			LocalAuthor localAuthor;
-			// Load the local pseudonym
-			try {
-				localAuthor = identityManager.getLocalAuthor();
-			} catch (DbException e) {
-				logException(LOG, WARNING, e);
-				contactExchangeFailed();
-				return;
-			}
-
-			// Exchange contact details
-			contactExchangeTask.startExchange(localAuthor,
-					result.getMasterKey(), result.getConnection(),
-					result.getTransportId(), result.wasAlice());
-		});
-	}
-
-	@Override
-	public void eventOccurred(Event e) {
-		if (e instanceof ContactExchangeSucceededEvent) {
-			contactExchangeSucceeded(
-					((ContactExchangeSucceededEvent) e).getRemoteAuthor());
-		} else if (e instanceof ContactExchangeFailedEvent) {
-			ContactExchangeFailedEvent fe = (ContactExchangeFailedEvent) e;
-			if (fe.wasDuplicateContact()) {
-				duplicateContact(fe.getDuplicateRemoteAuthor());
+		viewModel.getSucceeded().observe(this, succeeded -> {
+			if (succeeded == null) return;
+			if (succeeded) {
+				Author remote = requireNonNull(viewModel.getRemoteAuthor());
+				contactExchangeSucceeded(remote);
 			} else {
-				contactExchangeFailed();
+				Author duplicate = viewModel.getDuplicateAuthor();
+				if (duplicate == null) contactExchangeFailed();
+				else duplicateContact(duplicate);
 			}
-		}
+		});
+		viewModel.startContactExchange(result.getTransportId(),
+				result.getConnection(), result.getMasterKey(),
+				result.wasAlice());
 	}
 
+	@UiThread
 	private void contactExchangeSucceeded(Author remoteAuthor) {
-		runOnUiThreadUnlessDestroyed(() -> {
-			String contactName = remoteAuthor.getName();
-			String format = getString(R.string.contact_added_toast);
-			String text = String.format(format, contactName);
-			Toast.makeText(ContactExchangeActivity.this, text, LENGTH_LONG)
-					.show();
-			supportFinishAfterTransition();
-		});
+		String contactName = remoteAuthor.getName();
+		String format = getString(R.string.contact_added_toast);
+		String text = String.format(format, contactName);
+		Toast.makeText(this, text, LENGTH_LONG).show();
+		supportFinishAfterTransition();
 	}
 
+	@UiThread
 	private void duplicateContact(Author remoteAuthor) {
-		runOnUiThreadUnlessDestroyed(() -> {
-			String contactName = remoteAuthor.getName();
-			String format = getString(R.string.contact_already_exists);
-			String text = String.format(format, contactName);
-			Toast.makeText(ContactExchangeActivity.this, text, LENGTH_LONG)
-					.show();
-			finish();
-		});
+		String contactName = remoteAuthor.getName();
+		String format = getString(R.string.contact_already_exists);
+		String text = String.format(format, contactName);
+		Toast.makeText(this, text, LENGTH_LONG).show();
+		finish();
 	}
 
+	@UiThread
 	private void contactExchangeFailed() {
-		runOnUiThreadUnlessDestroyed(() -> {
-			showErrorFragment(R.string.connection_error_explanation);
-		});
+		showErrorFragment(R.string.connection_error_explanation);
 	}
 
 	@UiThread

briar-android/src/main/java/org/briarproject/briar/android/keyagreement/ContactExchangeModule.java

@@ -0,0 +1,20 @@
+package org.briarproject.briar.android.keyagreement;
+
+import android.arch.lifecycle.ViewModel;
+
+import org.briarproject.briar.android.viewmodel.ViewModelKey;
+
+import dagger.Binds;
+import dagger.Module;
+import dagger.multibindings.IntoMap;
+
+@Module
+public abstract class ContactExchangeModule {
+
+	@Binds
+	@IntoMap
+	@ViewModelKey(ContactExchangeViewModel.class)
+	abstract ViewModel bindContactExchangeViewModel(
+			ContactExchangeViewModel contactExchangeViewModel);
+
+}

briar-android/src/main/java/org/briarproject/briar/android/keyagreement/ContactExchangeViewModel.java

@@ -0,0 +1,104 @@
+package org.briarproject.briar.android.keyagreement;
+
+import android.app.Application;
+import android.arch.lifecycle.AndroidViewModel;
+import android.arch.lifecycle.LiveData;
+import android.arch.lifecycle.MutableLiveData;
+import android.support.annotation.UiThread;
+
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactExchangeManager;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.db.ContactExistsException;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.ConnectionManager;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
+
+import java.io.IOException;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.inject.Inject;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@NotNullByDefault
+class ContactExchangeViewModel extends AndroidViewModel {
+
+	private static final Logger LOG =
+			getLogger(ContactExchangeViewModel.class.getName());
+
+	private final Executor ioExecutor;
+	private final ContactExchangeManager contactExchangeManager;
+	private final ConnectionManager connectionManager;
+	private final MutableLiveData<Boolean> succeeded = new MutableLiveData<>();
+
+	@Nullable
+	private volatile Author remoteAuthor, duplicateAuthor;
+
+	@Inject
+	ContactExchangeViewModel(Application app, @IoExecutor Executor ioExecutor,
+			ContactExchangeManager contactExchangeManager,
+			ConnectionManager connectionManager) {
+		super(app);
+		this.ioExecutor = ioExecutor;
+		this.contactExchangeManager = contactExchangeManager;
+		this.connectionManager = connectionManager;
+	}
+
+	@UiThread
+	void startContactExchange(TransportId t, DuplexTransportConnection conn,
+			SecretKey masterKey, boolean alice) {
+		ioExecutor.execute(() -> {
+			try {
+				Contact contact = contactExchangeManager.exchangeContacts(t,
+						conn, masterKey, alice);
+				// Reuse the connection as a transport connection
+				connectionManager.manageOutgoingConnection(contact.getId(),
+						t, conn);
+				remoteAuthor = contact.getAuthor();
+				succeeded.postValue(true);
+			} catch (ContactExistsException e) {
+				tryToClose(conn);
+				duplicateAuthor = e.getRemoteAuthor();
+				succeeded.postValue(false);
+			} catch (DbException | IOException e) {
+				tryToClose(conn);
+				logException(LOG, WARNING, e);
+				succeeded.postValue(false);
+			}
+		});
+	}
+
+	private void tryToClose(DuplexTransportConnection conn) {
+		try {
+			conn.getReader().dispose(true, true);
+			conn.getWriter().dispose(true);
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@UiThread
+	@Nullable
+	Author getRemoteAuthor() {
+		return remoteAuthor;
+	}
+
+	@UiThread
+	@Nullable
+	Author getDuplicateAuthor() {
+		return duplicateAuthor;
+	}
+
+	LiveData<Boolean> getSucceeded() {
+		return succeeded;
+	}
+}

briar-android/src/main/java/org/briarproject/briar/android/viewmodel/ViewModelKey.java

@@ -14,6 +14,6 @@ import dagger.MapKey;
 @Target({ElementType.METHOD})
 @Retention(RetentionPolicy.RUNTIME)
 @MapKey
-@interface ViewModelKey {
+public @interface ViewModelKey {
 	Class<? extends ViewModel> value();
 }