Separate the sync layer from its clients. #112

2026-02-15 12:19:54 +01:00 · 2015-12-21 14:36:24 +00:00
parent f5f572139a
commit 5355951466
117 changed files with 3160 additions and 3465 deletions
--- a/briar-core/src/org/briarproject/sync/AuthorFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/AuthorFactoryImpl.java
@@ -1,7 +1,6 @@
 package org.briarproject.sync;

 import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.MessageDigest;
 import org.briarproject.api.data.BdfWriter;
 import org.briarproject.api.data.BdfWriterFactory;
 import org.briarproject.api.identity.Author;
@@ -49,10 +48,8 @@ class AuthorFactoryImpl implements AuthorFactory {
 			w.writeListEnd();
 		} catch (IOException e) {
 			// Shouldn't happen with ByteArrayOutputStream
-			throw new RuntimeException();
+			throw new RuntimeException(e);
 		}
-		MessageDigest messageDigest = crypto.getMessageDigest();
-		messageDigest.update(out.toByteArray());
-		return new AuthorId(messageDigest.digest());
+		return new AuthorId(crypto.hash(AuthorId.LABEL, out.toByteArray()));
 	}
 }
--- a/briar-core/src/org/briarproject/sync/AuthorReader.java
+++ b/briar-core/src/org/briarproject/sync/AuthorReader.java
@@ -1,12 +1,10 @@
 package org.briarproject.sync;

 import org.briarproject.api.FormatException;
-import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.MessageDigest;
 import org.briarproject.api.data.BdfReader;
 import org.briarproject.api.data.ObjectReader;
 import org.briarproject.api.identity.Author;
-import org.briarproject.api.identity.AuthorId;
+import org.briarproject.api.identity.AuthorFactory;

 import java.io.IOException;

@@ -15,26 +13,18 @@ import static org.briarproject.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGT

 class AuthorReader implements ObjectReader<Author> {

-	private final MessageDigest messageDigest;
+	private final AuthorFactory authorFactory;

-	AuthorReader(CryptoComponent crypto) {
-		messageDigest = crypto.getMessageDigest();
+	AuthorReader(AuthorFactory authorFactory) {
+		this.authorFactory = authorFactory;
 	}

 	public Author readObject(BdfReader r) throws IOException {
-		// Set up the reader
-		DigestingConsumer digesting = new DigestingConsumer(messageDigest);
-		r.addConsumer(digesting);
-		// Read and digest the data
 		r.readListStart();
 		String name = r.readString(MAX_AUTHOR_NAME_LENGTH);
 		if (name.length() == 0) throw new FormatException();
 		byte[] publicKey = r.readRaw(MAX_PUBLIC_KEY_LENGTH);
 		r.readListEnd();
-		// Reset the reader
-		r.removeConsumer(digesting);
-		// Build and return the author
-		AuthorId id = new AuthorId(messageDigest.digest());
-		return new Author(id, name, publicKey);
+		return authorFactory.createAuthor(name, publicKey);
 	}
 }
--- a/briar-core/src/org/briarproject/sync/CopyingConsumer.java
+++ b/briar-core/src/org/briarproject/sync/CopyingConsumer.java
@@ -1,24 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.data.Consumer;
-
-import java.io.ByteArrayOutputStream;
-import java.io.IOException;
-
-/** A consumer that makes a copy of the bytes consumed. */
-class CopyingConsumer implements Consumer {
-
-	private final ByteArrayOutputStream out = new ByteArrayOutputStream();
-
-	public byte[] getCopy() {
-		return out.toByteArray();
-	}
-
-	public void write(byte b) throws IOException {
-		out.write(b);
-	}
-
-	public void write(byte[] b, int off, int len) throws IOException {
-		out.write(b, off, len);
-	}
-}
--- a/briar-core/src/org/briarproject/sync/CountingConsumer.java
+++ b/briar-core/src/org/briarproject/sync/CountingConsumer.java
@@ -1,34 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.FormatException;
-import org.briarproject.api.data.Consumer;
-
-import java.io.IOException;
-
-/**
- * A consumer that counts the number of bytes consumed and throws a
- * FormatException if the count exceeds a given limit.
- */
-class CountingConsumer implements Consumer {
-
-	private final long limit;
-	private long count = 0;
-
-	public CountingConsumer(long limit) {
-		this.limit = limit;
-	}
-
-	public long getCount() {
-		return count;
-	}
-
-	public void write(byte b) throws IOException {
-		count++;
-		if (count > limit) throw new FormatException();
-	}
-
-	public void write(byte[] b, int off, int len) throws IOException {
-		count += len;
-		if (count > limit) throw new FormatException();
-	}
-}
--- a/briar-core/src/org/briarproject/sync/DigestingConsumer.java
+++ b/briar-core/src/org/briarproject/sync/DigestingConsumer.java
@@ -1,22 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.crypto.MessageDigest;
-import org.briarproject.api.data.Consumer;
-
-/** A consumer that passes its input through a message digest. */
-class DigestingConsumer implements Consumer {
-
-	private final MessageDigest messageDigest;
-
-	public DigestingConsumer(MessageDigest messageDigest) {
-		this.messageDigest = messageDigest;
-	}
-
-	public void write(byte b) {
-		messageDigest.update(b);
-	}
-
-	public void write(byte[] b, int off, int len) {
-		messageDigest.update(b, off, len);
-	}
-}
--- a/briar-core/src/org/briarproject/sync/DuplexOutgoingSession.java
+++ b/briar-core/src/org/briarproject/sync/DuplexOutgoingSession.java
@@ -10,21 +10,21 @@ import org.briarproject.api.event.EventBus;
 import org.briarproject.api.event.EventListener;
 import org.briarproject.api.event.LocalSubscriptionsUpdatedEvent;
 import org.briarproject.api.event.LocalTransportsUpdatedEvent;
-import org.briarproject.api.event.MessageAddedEvent;
 import org.briarproject.api.event.MessageRequestedEvent;
 import org.briarproject.api.event.MessageToAckEvent;
 import org.briarproject.api.event.MessageToRequestEvent;
+import org.briarproject.api.event.MessageValidatedEvent;
 import org.briarproject.api.event.RemoteSubscriptionsUpdatedEvent;
 import org.briarproject.api.event.RemoteTransportsUpdatedEvent;
 import org.briarproject.api.event.ShutdownEvent;
 import org.briarproject.api.event.TransportRemovedEvent;
 import org.briarproject.api.sync.Ack;
-import org.briarproject.api.sync.MessagingSession;
 import org.briarproject.api.sync.Offer;
 import org.briarproject.api.sync.PacketWriter;
 import org.briarproject.api.sync.Request;
 import org.briarproject.api.sync.SubscriptionAck;
 import org.briarproject.api.sync.SubscriptionUpdate;
+import org.briarproject.api.sync.SyncSession;
 import org.briarproject.api.sync.TransportAck;
 import org.briarproject.api.sync.TransportUpdate;
 import org.briarproject.api.system.Clock;
@@ -39,15 +39,15 @@ import java.util.logging.Logger;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MAX_PACKET_PAYLOAD_LENGTH;

 /**
- * An outgoing {@link org.briarproject.api.sync.MessagingSession
- * MessagingSession} suitable for duplex transports. The session offers
- * messages before sending them, keeps its output stream open when there are no
- * packets to send, and reacts to events that make packets available to send.
+ * An outgoing {@link org.briarproject.api.sync.SyncSession SyncSession}
+ * suitable for duplex transports. The session offers messages before sending
+ * them, keeps its output stream open when there are no packets to send, and
+ * reacts to events that make packets available to send.
 */
-class DuplexOutgoingSession implements MessagingSession, EventListener {
+class DuplexOutgoingSession implements SyncSession, EventListener {

 	// Check for retransmittable packets once every 60 seconds
 	private static final int RETX_QUERY_INTERVAL = 60 * 1000;
@@ -161,8 +161,9 @@ class DuplexOutgoingSession implements MessagingSession, EventListener {
 		if (e instanceof ContactRemovedEvent) {
 			ContactRemovedEvent c = (ContactRemovedEvent) e;
 			if (c.getContactId().equals(contactId)) interrupt();
-		} else if (e instanceof MessageAddedEvent) {
-			dbExecutor.execute(new GenerateOffer());
+		} else if (e instanceof MessageValidatedEvent) {
+			if (((MessageValidatedEvent) e).isValid())
+				dbExecutor.execute(new GenerateOffer());
 		} else if (e instanceof LocalSubscriptionsUpdatedEvent) {
 			LocalSubscriptionsUpdatedEvent l =
 					(LocalSubscriptionsUpdatedEvent) e;
@@ -243,7 +244,7 @@ class DuplexOutgoingSession implements MessagingSession, EventListener {
 			if (interrupted) return;
 			try {
 				Collection<byte[]> b = db.generateRequestedBatch(contactId,
-						MAX_PAYLOAD_LENGTH, maxLatency);
+						MAX_PACKET_PAYLOAD_LENGTH, maxLatency);
 				if (LOG.isLoggable(INFO))
 					LOG.info("Generated batch: " + (b != null));
 				if (b != null) writerTasks.add(new WriteBatch(b));
--- a/briar-core/src/org/briarproject/sync/GroupFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/GroupFactoryImpl.java
@@ -1,52 +1,24 @@
 package org.briarproject.sync;

 import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.MessageDigest;
-import org.briarproject.api.data.BdfWriter;
-import org.briarproject.api.data.BdfWriterFactory;
+import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupFactory;
 import org.briarproject.api.sync.GroupId;

-import java.io.ByteArrayOutputStream;
-import java.io.IOException;
-
 import javax.inject.Inject;

-import static org.briarproject.api.sync.MessagingConstants.GROUP_SALT_LENGTH;
-
 class GroupFactoryImpl implements GroupFactory {

 	private final CryptoComponent crypto;
-	private final BdfWriterFactory bdfWriterFactory;

 	@Inject
-	GroupFactoryImpl(CryptoComponent crypto, BdfWriterFactory bdfWriterFactory) {
+	GroupFactoryImpl(CryptoComponent crypto) {
 		this.crypto = crypto;
-		this.bdfWriterFactory = bdfWriterFactory;
 	}

-	public Group createGroup(String name) {
-		byte[] salt = new byte[GROUP_SALT_LENGTH];
-		crypto.getSecureRandom().nextBytes(salt);
-		return createGroup(name, salt);
-	}
-
-	public Group createGroup(String name, byte[] salt) {
-		ByteArrayOutputStream out = new ByteArrayOutputStream();
-		BdfWriter w = bdfWriterFactory.createWriter(out);
-		try {
-			w.writeListStart();
-			w.writeString(name);
-			w.writeRaw(salt);
-			w.writeListEnd();
-		} catch (IOException e) {
-			// Shouldn't happen with ByteArrayOutputStream
-			throw new RuntimeException();
-		}
-		MessageDigest messageDigest = crypto.getMessageDigest();
-		messageDigest.update(out.toByteArray());
-		GroupId id = new GroupId(messageDigest.digest());
-		return new Group(id, name, salt);
+	public Group createGroup(ClientId c, byte[] descriptor) {
+		byte[] hash = crypto.hash(GroupId.LABEL, c.getBytes(), descriptor);
+		return new Group(new GroupId(hash), c, descriptor);
 	}
 }
--- a/briar-core/src/org/briarproject/sync/GroupReader.java
+++ b/briar-core/src/org/briarproject/sync/GroupReader.java
@@ -1,39 +1,31 @@
 package org.briarproject.sync;

 import org.briarproject.api.FormatException;
-import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.MessageDigest;
+import org.briarproject.api.UniqueId;
 import org.briarproject.api.data.BdfReader;
 import org.briarproject.api.data.ObjectReader;
+import org.briarproject.api.sync.ClientId;
 import org.briarproject.api.sync.Group;
-import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.GroupFactory;

 import java.io.IOException;

-import static org.briarproject.api.sync.MessagingConstants.GROUP_SALT_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_GROUP_NAME_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MAX_GROUP_DESCRIPTOR_LENGTH;

 class GroupReader implements ObjectReader<Group> {

-	private final MessageDigest messageDigest;
+	private final GroupFactory groupFactory;

-	GroupReader(CryptoComponent crypto) {
-		messageDigest = crypto.getMessageDigest();
+	GroupReader(GroupFactory groupFactory) {
+		this.groupFactory = groupFactory;
 	}

 	public Group readObject(BdfReader r) throws IOException {
-		DigestingConsumer digesting = new DigestingConsumer(messageDigest);
-		// Read and digest the data
-		r.addConsumer(digesting);
 		r.readListStart();
-		String name = r.readString(MAX_GROUP_NAME_LENGTH);
-		if (name.length() == 0) throw new FormatException();
-		byte[] salt = r.readRaw(GROUP_SALT_LENGTH);
-		if (salt.length != GROUP_SALT_LENGTH) throw new FormatException();
+		byte[] id = r.readRaw(UniqueId.LENGTH);
+		if (id.length != UniqueId.LENGTH) throw new FormatException();
+		byte[] descriptor = r.readRaw(MAX_GROUP_DESCRIPTOR_LENGTH);
 		r.readListEnd();
-		r.removeConsumer(digesting);
-		// Build and return the group
-		GroupId id = new GroupId(messageDigest.digest());
-		return new Group(id, name, salt);
+		return groupFactory.createGroup(new ClientId(id), descriptor);
 	}
 }
--- a/briar-core/src/org/briarproject/sync/IncomingSession.java
+++ b/briar-core/src/org/briarproject/sync/IncomingSession.java
@@ -13,37 +13,30 @@ import org.briarproject.api.event.ShutdownEvent;
 import org.briarproject.api.event.TransportRemovedEvent;
 import org.briarproject.api.sync.Ack;
 import org.briarproject.api.sync.Message;
-import org.briarproject.api.sync.MessageVerifier;
-import org.briarproject.api.sync.MessagingSession;
 import org.briarproject.api.sync.Offer;
 import org.briarproject.api.sync.PacketReader;
 import org.briarproject.api.sync.Request;
 import org.briarproject.api.sync.SubscriptionAck;
 import org.briarproject.api.sync.SubscriptionUpdate;
+import org.briarproject.api.sync.SyncSession;
 import org.briarproject.api.sync.TransportAck;
 import org.briarproject.api.sync.TransportUpdate;
-import org.briarproject.api.sync.UnverifiedMessage;

 import java.io.IOException;
-import java.security.GeneralSecurityException;
 import java.util.concurrent.Executor;
 import java.util.logging.Logger;

 import static java.util.logging.Level.WARNING;

-/**
- * An incoming {@link org.briarproject.api.sync.MessagingSession
- * MessagingSession}.
- */
-class IncomingSession implements MessagingSession, EventListener {
+/** An incoming {@link org.briarproject.api.sync.SyncSession SyncSession}. */
+class IncomingSession implements SyncSession, EventListener {

 	private static final Logger LOG =
 			Logger.getLogger(IncomingSession.class.getName());

 	private final DatabaseComponent db;
-	private final Executor dbExecutor, cryptoExecutor;
+	private final Executor dbExecutor;
 	private final EventBus eventBus;
-	private final MessageVerifier messageVerifier;
 	private final ContactId contactId;
 	private final TransportId transportId;
 	private final PacketReader packetReader;
@@ -51,14 +44,11 @@ class IncomingSession implements MessagingSession, EventListener {
 	private volatile boolean interrupted = false;

 	IncomingSession(DatabaseComponent db, Executor dbExecutor,
-			Executor cryptoExecutor, EventBus eventBus,
-			MessageVerifier messageVerifier, ContactId contactId,
-			TransportId transportId, PacketReader packetReader) {
+			EventBus eventBus, ContactId contactId, TransportId transportId,
+			PacketReader packetReader) {
 		this.db = db;
 		this.dbExecutor = dbExecutor;
-		this.cryptoExecutor = cryptoExecutor;
 		this.eventBus = eventBus;
-		this.messageVerifier = messageVerifier;
 		this.contactId = contactId;
 		this.transportId = transportId;
 		this.packetReader = packetReader;
@@ -73,8 +63,8 @@ class IncomingSession implements MessagingSession, EventListener {
 					Ack a = packetReader.readAck();
 					dbExecutor.execute(new ReceiveAck(a));
 				} else if (packetReader.hasMessage()) {
-					UnverifiedMessage m = packetReader.readMessage();
-					cryptoExecutor.execute(new VerifyMessage(m));
+					Message m = packetReader.readMessage();
+					dbExecutor.execute(new ReceiveMessage(m));
 				} else if (packetReader.hasOffer()) {
 					Offer o = packetReader.readOffer();
 					dbExecutor.execute(new ReceiveOffer(o));
@@ -137,25 +127,6 @@ class IncomingSession implements MessagingSession, EventListener {
 		}
 	}

-	private class VerifyMessage implements Runnable {
-
-		private final UnverifiedMessage message;
-
-		private VerifyMessage(UnverifiedMessage message) {
-			this.message = message;
-		}
-
-		public void run() {
-			try {
-				Message m = messageVerifier.verifyMessage(message);
-				dbExecutor.execute(new ReceiveMessage(m));
-			} catch (GeneralSecurityException e) {
-				if (LOG.isLoggable(WARNING)) LOG.log(WARNING, e.toString(), e);
-				interrupt();
-			}
-		}
-	}
-
 	private class ReceiveMessage implements Runnable {

 		private final Message message;
--- a/briar-core/src/org/briarproject/sync/MessageFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/MessageFactoryImpl.java
@@ -1,129 +1,39 @@
 package org.briarproject.sync;

+import com.google.inject.Inject;
+
+import org.briarproject.api.UniqueId;
 import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.MessageDigest;
-import org.briarproject.api.crypto.PrivateKey;
-import org.briarproject.api.crypto.Signature;
-import org.briarproject.api.data.BdfWriter;
-import org.briarproject.api.data.BdfWriterFactory;
-import org.briarproject.api.data.Consumer;
-import org.briarproject.api.identity.Author;
-import org.briarproject.api.sync.Group;
+import org.briarproject.api.sync.GroupId;
 import org.briarproject.api.sync.Message;
 import org.briarproject.api.sync.MessageFactory;
 import org.briarproject.api.sync.MessageId;
-import org.briarproject.util.StringUtils;
+import org.briarproject.util.ByteUtils;

-import java.io.ByteArrayOutputStream;
 import java.io.IOException;
-import java.security.GeneralSecurityException;
-import java.security.SecureRandom;

-import javax.inject.Inject;
-
-import static org.briarproject.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_BODY_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_CONTENT_TYPE_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MESSAGE_SALT_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MAX_MESSAGE_BODY_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;

 class MessageFactoryImpl implements MessageFactory {

-	private final Signature signature;
-	private final SecureRandom random;
-	private final MessageDigest messageDigest;
-	private final BdfWriterFactory bdfWriterFactory;
+	private final CryptoComponent crypto;

 	@Inject
-	MessageFactoryImpl(CryptoComponent crypto, BdfWriterFactory bdfWriterFactory) {
-		signature = crypto.getSignature();
-		random = crypto.getSecureRandom();
-		messageDigest = crypto.getMessageDigest();
-		this.bdfWriterFactory = bdfWriterFactory;
+	MessageFactoryImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
 	}

-	public Message createAnonymousMessage(MessageId parent, Group group,
-			String contentType, long timestamp, byte[] body) throws IOException,
-			GeneralSecurityException {
-		return createMessage(parent, group, null, null, contentType, timestamp,
-				body);
-	}
-
-	public Message createPseudonymousMessage(MessageId parent, Group group,
-			Author author, PrivateKey privateKey, String contentType,
-			long timestamp, byte[] body) throws IOException,
-			GeneralSecurityException {
-		return createMessage(parent, group, author, privateKey, contentType,
-				timestamp, body);
-	}
-
-	private Message createMessage(MessageId parent, Group group, Author author,
-			PrivateKey privateKey, String contentType, long timestamp,
-			byte[] body) throws IOException, GeneralSecurityException {
-		// Validate the arguments
-		if ((author == null) != (privateKey == null))
+	@Override
+	public Message createMessage(GroupId groupId, long timestamp, byte[] body)
+			throws IOException {
+		if (body.length > MAX_MESSAGE_BODY_LENGTH)
 			throw new IllegalArgumentException();
-		if (StringUtils.toUtf8(contentType).length > MAX_CONTENT_TYPE_LENGTH)
-			throw new IllegalArgumentException();
-		if (body.length > MAX_BODY_LENGTH)
-			throw new IllegalArgumentException();
-		// Serialise the message to a buffer
-		ByteArrayOutputStream out = new ByteArrayOutputStream();
-		BdfWriter w = bdfWriterFactory.createWriter(out);
-		// Initialise the consumers
-		CountingConsumer counting = new CountingConsumer(MAX_PAYLOAD_LENGTH);
-		w.addConsumer(counting);
-		Consumer digestingConsumer = new DigestingConsumer(messageDigest);
-		w.addConsumer(digestingConsumer);
-		Consumer signingConsumer = null;
-		if (privateKey != null) {
-			signature.initSign(privateKey);
-			signingConsumer = new SigningConsumer(signature);
-			w.addConsumer(signingConsumer);
-		}
-		// Write the message
-		w.writeListStart();
-		if (parent == null) w.writeNull();
-		else w.writeRaw(parent.getBytes());
-		writeGroup(w, group);
-		if (author == null) w.writeNull();
-		else writeAuthor(w, author);
-		w.writeString(contentType);
-		w.writeInteger(timestamp);
-		byte[] salt = new byte[MESSAGE_SALT_LENGTH];
-		random.nextBytes(salt);
-		w.writeRaw(salt);
-		w.writeRaw(body);
-		int bodyStart = (int) counting.getCount() - body.length;
-		// Sign the message with the author's private key, if there is one
-		if (privateKey == null) {
-			w.writeNull();
-		} else {
-			w.removeConsumer(signingConsumer);
-			byte[] sig = signature.sign();
-			if (sig.length > MAX_SIGNATURE_LENGTH)
-				throw new IllegalArgumentException();
-			w.writeRaw(sig);
-		}
-		w.writeListEnd();
-		// Hash the message, including the signature, to get the message ID
-		w.removeConsumer(digestingConsumer);
-		MessageId id = new MessageId(messageDigest.digest());
-		return new MessageImpl(id, parent, group, author, contentType,
-				timestamp, out.toByteArray(), bodyStart, body.length);
-	}
-
-	private void writeGroup(BdfWriter w, Group g) throws IOException {
-		w.writeListStart();
-		w.writeString(g.getName());
-		w.writeRaw(g.getSalt());
-		w.writeListEnd();
-	}
-
-	private void writeAuthor(BdfWriter w, Author a) throws IOException {
-		w.writeListStart();
-		w.writeString(a.getName());
-		w.writeRaw(a.getPublicKey());
-		w.writeListEnd();
+		byte[] raw = new byte[MESSAGE_HEADER_LENGTH + body.length];
+		System.arraycopy(groupId.getBytes(), 0, raw, 0, UniqueId.LENGTH);
+		ByteUtils.writeUint64(timestamp, raw, UniqueId.LENGTH);
+		System.arraycopy(body, 0, raw, MESSAGE_HEADER_LENGTH, body.length);
+		MessageId id = new MessageId(crypto.hash(MessageId.LABEL, raw));
+		return new Message(id, groupId, timestamp, raw);
 	}
 }
--- a/briar-core/src/org/briarproject/sync/MessageImpl.java
+++ b/briar-core/src/org/briarproject/sync/MessageImpl.java
@@ -1,84 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.identity.Author;
-import org.briarproject.api.sync.Group;
-import org.briarproject.api.sync.Message;
-import org.briarproject.api.sync.MessageId;
-
-import static org.briarproject.api.sync.MessagingConstants.MAX_BODY_LENGTH;
-
-/** A simple in-memory implementation of a message. */
-class MessageImpl implements Message {
-
-	private final MessageId id, parent;
-	private final Group group;
-	private final Author author;
-	private final String contentType;
-	private final long timestamp;
-	private final byte[] raw;
-	private final int bodyStart, bodyLength;
-
-	public MessageImpl(MessageId id, MessageId parent, Group group,
-			Author author, String contentType, long timestamp,
-			byte[] raw, int bodyStart, int bodyLength) {
-		if (bodyStart + bodyLength > raw.length)
-			throw new IllegalArgumentException();
-		if (bodyLength > MAX_BODY_LENGTH)
-			throw new IllegalArgumentException();
-		this.id = id;
-		this.parent = parent;
-		this.group = group;
-		this.author = author;
-		this.contentType = contentType;
-		this.timestamp = timestamp;
-		this.raw = raw;
-		this.bodyStart = bodyStart;
-		this.bodyLength = bodyLength;
-	}
-
-	public MessageId getId() {
-		return id;
-	}
-
-	public MessageId getParent() {
-		return parent;
-	}
-
-	public Group getGroup() {
-		return group;
-	}
-
-	public Author getAuthor() {
-		return author;
-	}
-
-	public String getContentType() {
-		return contentType;
-	}
-
-	public long getTimestamp() {
-		return timestamp;
-	}
-
-	public byte[] getSerialised() {
-		return raw;
-	}
-
-	public int getBodyStart() {
-		return bodyStart;
-	}
-
-	public int getBodyLength() {
-		return bodyLength;
-	}
-
-	@Override
-	public int hashCode() {
-		return id.hashCode();
-	}
-
-	@Override
-	public boolean equals(Object o) {
-		return o instanceof Message && id.equals(((Message) o).getId());
-	}
-}
--- a/briar-core/src/org/briarproject/sync/MessageReader.java
+++ b/briar-core/src/org/briarproject/sync/MessageReader.java
@@ -1,82 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.FormatException;
-import org.briarproject.api.UniqueId;
-import org.briarproject.api.data.BdfReader;
-import org.briarproject.api.data.ObjectReader;
-import org.briarproject.api.identity.Author;
-import org.briarproject.api.sync.Group;
-import org.briarproject.api.sync.MessageId;
-import org.briarproject.api.sync.UnverifiedMessage;
-
-import java.io.IOException;
-
-import static org.briarproject.api.identity.AuthorConstants.MAX_SIGNATURE_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_BODY_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_CONTENT_TYPE_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MESSAGE_SALT_LENGTH;
-
-class MessageReader implements ObjectReader<UnverifiedMessage> {
-
-	private final ObjectReader<Group> groupReader;
-	private final ObjectReader<Author> authorReader;
-
-	MessageReader(ObjectReader<Group> groupReader,
-			ObjectReader<Author> authorReader) {
-		this.groupReader = groupReader;
-		this.authorReader = authorReader;
-	}
-
-	public UnverifiedMessage readObject(BdfReader r) throws IOException {
-		CopyingConsumer copying = new CopyingConsumer();
-		CountingConsumer counting = new CountingConsumer(MAX_PAYLOAD_LENGTH);
-		r.addConsumer(copying);
-		r.addConsumer(counting);
-		// Read the start of the message
-		r.readListStart();
-		// Read the parent's message ID, if there is one
-		MessageId parent = null;
-		if (r.hasNull()) {
-			r.readNull();
-		} else {
-			byte[] b = r.readRaw(UniqueId.LENGTH);
-			if (b.length < UniqueId.LENGTH) throw new FormatException();
-			parent = new MessageId(b);
-		}
-		// Read the group
-		Group group = groupReader.readObject(r);
-		// Read the author, if there is one
-		Author author = null;
-		if (r.hasNull()) r.readNull();
-		else author = authorReader.readObject(r);
-		// Read the content type
-		String contentType = r.readString(MAX_CONTENT_TYPE_LENGTH);
-		// Read the timestamp
-		long timestamp = r.readInteger();
-		if (timestamp < 0) throw new FormatException();
-		// Read the salt
-		byte[] salt = r.readRaw(MESSAGE_SALT_LENGTH);
-		if (salt.length < MESSAGE_SALT_LENGTH) throw new FormatException();
-		// Read the message body
-		byte[] body = r.readRaw(MAX_BODY_LENGTH);
-		// Record the offset of the body within the message
-		int bodyStart = (int) counting.getCount() - body.length;
-		// Record the length of the data covered by the author's signature
-		int signedLength = (int) counting.getCount();
-		// Read the author's signature, if there is one
-		byte[] signature = null;
-		if (author == null) r.readNull();
-		else signature = r.readRaw(MAX_SIGNATURE_LENGTH);
-		// Read the end of the message
-		r.readListEnd();
-		// Reset the reader
-		r.removeConsumer(counting);
-		r.removeConsumer(copying);
-		// Build and return the unverified message
-		byte[] raw = copying.getCopy();
-		return new UnverifiedMessage(parent, group, author, contentType,
-				timestamp, raw, signature, bodyStart, body.length,
-				signedLength);
-	}
-}
--- a/briar-core/src/org/briarproject/sync/MessageVerifierImpl.java
+++ b/briar-core/src/org/briarproject/sync/MessageVerifierImpl.java
@@ -1,68 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.crypto.CryptoComponent;
-import org.briarproject.api.crypto.KeyParser;
-import org.briarproject.api.crypto.MessageDigest;
-import org.briarproject.api.crypto.PublicKey;
-import org.briarproject.api.crypto.Signature;
-import org.briarproject.api.identity.Author;
-import org.briarproject.api.sync.Message;
-import org.briarproject.api.sync.MessageId;
-import org.briarproject.api.sync.MessageVerifier;
-import org.briarproject.api.sync.UnverifiedMessage;
-import org.briarproject.api.system.Clock;
-
-import java.security.GeneralSecurityException;
-import java.util.logging.Logger;
-
-import javax.inject.Inject;
-
-import static java.util.logging.Level.INFO;
-import static org.briarproject.api.transport.TransportConstants.MAX_CLOCK_DIFFERENCE;
-
-class MessageVerifierImpl implements MessageVerifier {
-
-	private static final Logger LOG =
-			Logger.getLogger(MessageVerifierImpl.class.getName());
-
-	private final CryptoComponent crypto;
-	private final Clock clock;
-	private final KeyParser keyParser;
-
-	@Inject
-	MessageVerifierImpl(CryptoComponent crypto, Clock clock) {
-		this.crypto = crypto;
-		this.clock = clock;
-		keyParser = crypto.getSignatureKeyParser();
-	}
-
-	public Message verifyMessage(UnverifiedMessage m)
-			throws GeneralSecurityException {
-		long now = System.currentTimeMillis();
-		MessageDigest messageDigest = crypto.getMessageDigest();
-		Signature signature = crypto.getSignature();
-		// Reject the message if it's too far in the future
-		if (m.getTimestamp() > clock.currentTimeMillis() + MAX_CLOCK_DIFFERENCE)
-			throw new GeneralSecurityException();
-		// Hash the message to get the message ID
-		byte[] raw = m.getSerialised();
-		messageDigest.update(raw);
-		MessageId id = new MessageId(messageDigest.digest());
-		// Verify the author's signature, if there is one
-		Author author = m.getAuthor();
-		if (author != null) {
-			PublicKey k = keyParser.parsePublicKey(author.getPublicKey());
-			signature.initVerify(k);
-			signature.update(raw, 0, m.getSignedLength());
-			if (!signature.verify(m.getSignature()))
-				throw new GeneralSecurityException();
-		}
-		Message verified = new MessageImpl(id, m.getParent(), m.getGroup(),
-				author, m.getContentType(), m.getTimestamp(), raw,
-				m.getBodyStart(), m.getBodyLength());
-		long duration = System.currentTimeMillis() - now;
-		if (LOG.isLoggable(INFO))
-			LOG.info("Verifying message took " + duration + " ms");
-		return verified;
-	}
-}
--- a/briar-core/src/org/briarproject/sync/PacketReaderFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/PacketReaderFactoryImpl.java
@@ -1,11 +1,11 @@
 package org.briarproject.sync;

+import org.briarproject.api.crypto.CryptoComponent;
 import org.briarproject.api.data.BdfReaderFactory;
 import org.briarproject.api.data.ObjectReader;
 import org.briarproject.api.sync.PacketReader;
 import org.briarproject.api.sync.PacketReaderFactory;
 import org.briarproject.api.sync.SubscriptionUpdate;
-import org.briarproject.api.sync.UnverifiedMessage;

 import java.io.InputStream;

@@ -13,21 +13,21 @@ import javax.inject.Inject;

 class PacketReaderFactoryImpl implements PacketReaderFactory {

+	private final CryptoComponent crypto;
 	private final BdfReaderFactory bdfReaderFactory;
-	private final ObjectReader<UnverifiedMessage> messageReader;
 	private final ObjectReader<SubscriptionUpdate> subscriptionUpdateReader;

 	@Inject
-	PacketReaderFactoryImpl(BdfReaderFactory bdfReaderFactory,
-			ObjectReader<UnverifiedMessage> messageReader,
+	PacketReaderFactoryImpl(CryptoComponent crypto,
+			BdfReaderFactory bdfReaderFactory,
 			ObjectReader<SubscriptionUpdate> subscriptionUpdateReader) {
+		this.crypto = crypto;
 		this.bdfReaderFactory = bdfReaderFactory;
-		this.messageReader = messageReader;
 		this.subscriptionUpdateReader = subscriptionUpdateReader;
 	}

 	public PacketReader createPacketReader(InputStream in) {
-		return new PacketReaderImpl(bdfReaderFactory, messageReader,
+		return new PacketReaderImpl(crypto, bdfReaderFactory,
 				subscriptionUpdateReader, in);
 	}
 }
--- a/briar-core/src/org/briarproject/sync/PacketReaderImpl.java
+++ b/briar-core/src/org/briarproject/sync/PacketReaderImpl.java
@@ -4,10 +4,13 @@ import org.briarproject.api.FormatException;
 import org.briarproject.api.TransportId;
 import org.briarproject.api.TransportProperties;
 import org.briarproject.api.UniqueId;
+import org.briarproject.api.crypto.CryptoComponent;
 import org.briarproject.api.data.BdfReader;
 import org.briarproject.api.data.BdfReaderFactory;
 import org.briarproject.api.data.ObjectReader;
 import org.briarproject.api.sync.Ack;
+import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.Message;
 import org.briarproject.api.sync.MessageId;
 import org.briarproject.api.sync.Offer;
 import org.briarproject.api.sync.PacketReader;
@@ -16,7 +19,6 @@ import org.briarproject.api.sync.SubscriptionAck;
 import org.briarproject.api.sync.SubscriptionUpdate;
 import org.briarproject.api.sync.TransportAck;
 import org.briarproject.api.sync.TransportUpdate;
-import org.briarproject.api.sync.UnverifiedMessage;
 import org.briarproject.util.ByteUtils;

 import java.io.ByteArrayInputStream;
@@ -31,9 +33,6 @@ import java.util.Map;
 import static org.briarproject.api.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.api.TransportPropertyConstants.MAX_PROPERTY_LENGTH;
 import static org.briarproject.api.TransportPropertyConstants.MAX_TRANSPORT_ID_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.HEADER_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.PROTOCOL_VERSION;
 import static org.briarproject.api.sync.PacketTypes.ACK;
 import static org.briarproject.api.sync.PacketTypes.MESSAGE;
 import static org.briarproject.api.sync.PacketTypes.OFFER;
@@ -42,14 +41,18 @@ import static org.briarproject.api.sync.PacketTypes.SUBSCRIPTION_ACK;
 import static org.briarproject.api.sync.PacketTypes.SUBSCRIPTION_UPDATE;
 import static org.briarproject.api.sync.PacketTypes.TRANSPORT_ACK;
 import static org.briarproject.api.sync.PacketTypes.TRANSPORT_UPDATE;
+import static org.briarproject.api.sync.SyncConstants.MAX_PACKET_PAYLOAD_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.PACKET_HEADER_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.PROTOCOL_VERSION;

 // This class is not thread-safe
 class PacketReaderImpl implements PacketReader {

 	private enum State { BUFFER_EMPTY, BUFFER_FULL, EOF }

+	private final CryptoComponent crypto;
 	private final BdfReaderFactory bdfReaderFactory;
-	private final ObjectReader<UnverifiedMessage> messageReader;
 	private final ObjectReader<SubscriptionUpdate> subscriptionUpdateReader;
 	private final InputStream in;
 	private final byte[] header, payload;
@@ -57,24 +60,23 @@ class PacketReaderImpl implements PacketReader {
 	private State state = State.BUFFER_EMPTY;
 	private int payloadLength = 0;

-	PacketReaderImpl(BdfReaderFactory bdfReaderFactory,
-			ObjectReader<UnverifiedMessage> messageReader,
+	PacketReaderImpl(CryptoComponent crypto, BdfReaderFactory bdfReaderFactory,
 			ObjectReader<SubscriptionUpdate> subscriptionUpdateReader,
 			InputStream in) {
+		this.crypto = crypto;
 		this.bdfReaderFactory = bdfReaderFactory;
-		this.messageReader = messageReader;
 		this.subscriptionUpdateReader = subscriptionUpdateReader;
 		this.in = in;
-		header = new byte[HEADER_LENGTH];
-		payload = new byte[MAX_PAYLOAD_LENGTH];
+		header = new byte[PACKET_HEADER_LENGTH];
+		payload = new byte[MAX_PACKET_PAYLOAD_LENGTH];
 	}

 	private void readPacket() throws IOException {
-		assert state == State.BUFFER_EMPTY;
+		if (state != State.BUFFER_EMPTY) throw new IllegalStateException();
 		// Read the header
 		int offset = 0;
-		while (offset < HEADER_LENGTH) {
-			int read = in.read(header, offset, HEADER_LENGTH - offset);
+		while (offset < PACKET_HEADER_LENGTH) {
+			int read = in.read(header, offset, PACKET_HEADER_LENGTH - offset);
 			if (read == -1) {
 				if (offset > 0) throw new FormatException();
 				state = State.EOF;
@@ -86,7 +88,7 @@ class PacketReaderImpl implements PacketReader {
 		if (header[0] != PROTOCOL_VERSION) throw new FormatException();
 		// Read the payload length
 		payloadLength = ByteUtils.readUint16(header, 2);
-		if (payloadLength > MAX_PAYLOAD_LENGTH) throw new FormatException();
+		if (payloadLength > MAX_PACKET_PAYLOAD_LENGTH) throw new FormatException();
 		// Read the payload
 		offset = 0;
 		while (offset < payloadLength) {
@@ -99,7 +101,7 @@ class PacketReaderImpl implements PacketReader {

 	public boolean eof() throws IOException {
 		if (state == State.BUFFER_EMPTY) readPacket();
-		assert state != State.BUFFER_EMPTY;
+		if (state == State.BUFFER_EMPTY) throw new IllegalStateException();
 		return state == State.EOF;
 	}

@@ -109,44 +111,43 @@ class PacketReaderImpl implements PacketReader {

 	public Ack readAck() throws IOException {
 		if (!hasAck()) throw new FormatException();
-		// Set up the reader
-		InputStream bais = new ByteArrayInputStream(payload, 0, payloadLength);
-		BdfReader r = bdfReaderFactory.createReader(bais);
-		// Read the start of the payload
-		r.readListStart();
-		// Read the message IDs
-		List<MessageId> acked = new ArrayList<MessageId>();
-		r.readListStart();
-		while (!r.hasListEnd()) {
-			byte[] b = r.readRaw(UniqueId.LENGTH);
-			if (b.length != UniqueId.LENGTH)
-				throw new FormatException();
-			acked.add(new MessageId(b));
+		return new Ack(Collections.unmodifiableList(readMessageIds()));
+	}
+
+	private List<MessageId> readMessageIds() throws IOException {
+		if (payloadLength == 0) throw new FormatException();
+		if (payloadLength % UniqueId.LENGTH != 0) throw new FormatException();
+		List<MessageId> ids = new ArrayList<MessageId>();
+		for (int off = 0; off < payloadLength; off += UniqueId.LENGTH) {
+			byte[] id = new byte[UniqueId.LENGTH];
+			System.arraycopy(payload, off, id, 0, UniqueId.LENGTH);
+			ids.add(new MessageId(id));
 		}
-		if (acked.isEmpty()) throw new FormatException();
-		r.readListEnd();
-		// Read the end of the payload
-		r.readListEnd();
-		if (!r.eof()) throw new FormatException();
 		state = State.BUFFER_EMPTY;
-		// Build and return the ack
-		return new Ack(Collections.unmodifiableList(acked));
+		return ids;
 	}

 	public boolean hasMessage() throws IOException {
 		return !eof() && header[1] == MESSAGE;
 	}

-	public UnverifiedMessage readMessage() throws IOException {
+	public Message readMessage() throws IOException {
 		if (!hasMessage()) throw new FormatException();
-		// Set up the reader
-		InputStream bais = new ByteArrayInputStream(payload, 0, payloadLength);
-		BdfReader r = bdfReaderFactory.createReader(bais);
-		// Read and build the message
-		UnverifiedMessage m = messageReader.readObject(r);
-		if (!r.eof()) throw new FormatException();
+		if (payloadLength <= MESSAGE_HEADER_LENGTH) throw new FormatException();
+		// Group ID
+		byte[] id = new byte[UniqueId.LENGTH];
+		System.arraycopy(payload, 0, id, 0, UniqueId.LENGTH);
+		GroupId groupId = new GroupId(id);
+		// Timestamp
+		long timestamp = ByteUtils.readUint64(payload, UniqueId.LENGTH);
+		if (timestamp < 0) throw new FormatException();
+		// Raw message
+		byte[] raw = new byte[payloadLength];
+		System.arraycopy(payload, 0, raw, 0, payloadLength);
 		state = State.BUFFER_EMPTY;
-		return m;
+		// Message ID
+		MessageId messageId = new MessageId(crypto.hash(MessageId.LABEL, raw));
+		return new Message(messageId, groupId, timestamp, raw);
 	}

 	public boolean hasOffer() throws IOException {
@@ -155,28 +156,7 @@ class PacketReaderImpl implements PacketReader {

 	public Offer readOffer() throws IOException {
 		if (!hasOffer()) throw new FormatException();
-		// Set up the reader
-		InputStream bais = new ByteArrayInputStream(payload, 0, payloadLength);
-		BdfReader r = bdfReaderFactory.createReader(bais);
-		// Read the start of the payload
-		r.readListStart();
-		// Read the message IDs
-		List<MessageId> offered = new ArrayList<MessageId>();
-		r.readListStart();
-		while (!r.hasListEnd()) {
-			byte[] b = r.readRaw(UniqueId.LENGTH);
-			if (b.length != UniqueId.LENGTH)
-				throw new FormatException();
-			offered.add(new MessageId(b));
-		}
-		if (offered.isEmpty()) throw new FormatException();
-		r.readListEnd();
-		// Read the end of the payload
-		r.readListEnd();
-		if (!r.eof()) throw new FormatException();
-		state = State.BUFFER_EMPTY;
-		// Build and return the offer
-		return new Offer(Collections.unmodifiableList(offered));
+		return new Offer(Collections.unmodifiableList(readMessageIds()));
 	}

 	public boolean hasRequest() throws IOException {
@@ -185,28 +165,7 @@ class PacketReaderImpl implements PacketReader {

 	public Request readRequest() throws IOException {
 		if (!hasRequest()) throw new FormatException();
-		// Set up the reader
-		InputStream bais = new ByteArrayInputStream(payload, 0, payloadLength);
-		BdfReader r = bdfReaderFactory.createReader(bais);
-		// Read the start of the payload
-		r.readListStart();
-		// Read the message IDs
-		r.readListStart();
-		List<MessageId> requested = new ArrayList<MessageId>();
-		while (!r.hasListEnd()) {
-			byte[] b = r.readRaw(UniqueId.LENGTH);
-			if (b.length != UniqueId.LENGTH)
-				throw new FormatException();
-			requested.add(new MessageId(b));
-		}
-		if (requested.isEmpty()) throw new FormatException();
-		r.readListEnd();
-		// Read the end of the payload
-		r.readListEnd();
-		if (!r.eof()) throw new FormatException();
-		state = State.BUFFER_EMPTY;
-		// Build and return the request
-		return new Request(Collections.unmodifiableList(requested));
+		return new Request(Collections.unmodifiableList(readMessageIds()));
 	}

 	public boolean hasSubscriptionAck() throws IOException {
--- a/briar-core/src/org/briarproject/sync/PacketWriterImpl.java
+++ b/briar-core/src/org/briarproject/sync/PacketWriterImpl.java
@@ -1,5 +1,6 @@
 package org.briarproject.sync;

+import org.briarproject.api.UniqueId;
 import org.briarproject.api.data.BdfWriter;
 import org.briarproject.api.data.BdfWriterFactory;
 import org.briarproject.api.sync.Ack;
@@ -19,12 +20,6 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;

-import static org.briarproject.api.data.DataConstants.LIST_END_LENGTH;
-import static org.briarproject.api.data.DataConstants.LIST_START_LENGTH;
-import static org.briarproject.api.data.DataConstants.UNIQUE_ID_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.HEADER_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.PROTOCOL_VERSION;
 import static org.briarproject.api.sync.PacketTypes.ACK;
 import static org.briarproject.api.sync.PacketTypes.OFFER;
 import static org.briarproject.api.sync.PacketTypes.REQUEST;
@@ -32,6 +27,9 @@ import static org.briarproject.api.sync.PacketTypes.SUBSCRIPTION_ACK;
 import static org.briarproject.api.sync.PacketTypes.SUBSCRIPTION_UPDATE;
 import static org.briarproject.api.sync.PacketTypes.TRANSPORT_ACK;
 import static org.briarproject.api.sync.PacketTypes.TRANSPORT_UPDATE;
+import static org.briarproject.api.sync.SyncConstants.MAX_PACKET_PAYLOAD_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.PACKET_HEADER_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.PROTOCOL_VERSION;

 // This class is not thread-safe
 class PacketWriterImpl implements PacketWriter {
@@ -44,9 +42,9 @@ class PacketWriterImpl implements PacketWriter {
 	PacketWriterImpl(BdfWriterFactory bdfWriterFactory, OutputStream out) {
 		this.bdfWriterFactory = bdfWriterFactory;
 		this.out = out;
-		header = new byte[HEADER_LENGTH];
+		header = new byte[PACKET_HEADER_LENGTH];
 		header[0] = PROTOCOL_VERSION;
-		payload = new ByteArrayOutputStream(MAX_PAYLOAD_LENGTH);
+		payload = new ByteArrayOutputStream(MAX_PACKET_PAYLOAD_LENGTH);
 	}

 	public int getMaxMessagesForAck(long capacity) {
@@ -62,10 +60,9 @@ class PacketWriterImpl implements PacketWriter {
 	}

 	private int getMaxMessagesForPacket(long capacity) {
-		int payload = (int) Math.min(capacity - HEADER_LENGTH,
-				MAX_PAYLOAD_LENGTH);
-		int overhead = LIST_START_LENGTH * 2 + LIST_END_LENGTH * 2;
-		return (payload - overhead) / UNIQUE_ID_LENGTH;
+		int payload = (int) Math.min(capacity - PACKET_HEADER_LENGTH,
+				MAX_PACKET_PAYLOAD_LENGTH);
+		return payload / UniqueId.LENGTH;
 	}

 	private void writePacket(byte packetType) throws IOException {
@@ -77,13 +74,8 @@ class PacketWriterImpl implements PacketWriter {
 	}

 	public void writeAck(Ack a) throws IOException {
-		assert payload.size() == 0;
-		BdfWriter w = bdfWriterFactory.createWriter(payload);
-		w.writeListStart();
-		w.writeListStart();
-		for (MessageId m : a.getMessageIds()) w.writeRaw(m.getBytes());
-		w.writeListEnd();
-		w.writeListEnd();
+		if (payload.size() != 0) throw new IllegalStateException();
+		for (MessageId m : a.getMessageIds()) payload.write(m.getBytes());
 		writePacket(ACK);
 	}

@@ -95,29 +87,19 @@ class PacketWriterImpl implements PacketWriter {
 	}

 	public void writeOffer(Offer o) throws IOException {
-		assert payload.size() == 0;
-		BdfWriter w = bdfWriterFactory.createWriter(payload);
-		w.writeListStart();
-		w.writeListStart();
-		for (MessageId m : o.getMessageIds()) w.writeRaw(m.getBytes());
-		w.writeListEnd();
-		w.writeListEnd();
+		if (payload.size() != 0) throw new IllegalStateException();
+		for (MessageId m : o.getMessageIds()) payload.write(m.getBytes());
 		writePacket(OFFER);
 	}

 	public void writeRequest(Request r) throws IOException {
-		assert payload.size() == 0;
-		BdfWriter w = bdfWriterFactory.createWriter(payload);
-		w.writeListStart();
-		w.writeListStart();
-		for (MessageId m : r.getMessageIds()) w.writeRaw(m.getBytes());
-		w.writeListEnd();
-		w.writeListEnd();
+		if (payload.size() != 0) throw new IllegalStateException();
+		for (MessageId m : r.getMessageIds()) payload.write(m.getBytes());
 		writePacket(REQUEST);
 	}

 	public void writeSubscriptionAck(SubscriptionAck a) throws IOException {
-		assert payload.size() == 0;
+		if (payload.size() != 0) throw new IllegalStateException();
 		BdfWriter w = bdfWriterFactory.createWriter(payload);
 		w.writeListStart();
 		w.writeInteger(a.getVersion());
@@ -127,14 +109,14 @@ class PacketWriterImpl implements PacketWriter {

 	public void writeSubscriptionUpdate(SubscriptionUpdate u)
 			throws IOException {
-		assert payload.size() == 0;
+		if (payload.size() != 0) throw new IllegalStateException();
 		BdfWriter w = bdfWriterFactory.createWriter(payload);
 		w.writeListStart();
 		w.writeListStart();
 		for (Group g : u.getGroups()) {
 			w.writeListStart();
-			w.writeString(g.getName());
-			w.writeRaw(g.getSalt());
+			w.writeRaw(g.getClientId().getBytes());
+			w.writeRaw(g.getDescriptor());
 			w.writeListEnd();
 		}
 		w.writeListEnd();
@@ -144,7 +126,7 @@ class PacketWriterImpl implements PacketWriter {
 	}

 	public void writeTransportAck(TransportAck a) throws IOException {
-		assert payload.size() == 0;
+		if (payload.size() != 0) throw new IllegalStateException();
 		BdfWriter w = bdfWriterFactory.createWriter(payload);
 		w.writeListStart();
 		w.writeString(a.getId().getString());
@@ -154,7 +136,7 @@ class PacketWriterImpl implements PacketWriter {
 	}

 	public void writeTransportUpdate(TransportUpdate u) throws IOException {
-		assert payload.size() == 0;
+		if (payload.size() != 0) throw new IllegalStateException();
 		BdfWriter w = bdfWriterFactory.createWriter(payload);
 		w.writeListStart();
 		w.writeString(u.getId().getString());
--- a/briar-core/src/org/briarproject/sync/SigningConsumer.java
+++ b/briar-core/src/org/briarproject/sync/SigningConsumer.java
@@ -1,22 +0,0 @@
-package org.briarproject.sync;
-
-import org.briarproject.api.crypto.Signature;
-import org.briarproject.api.data.Consumer;
-
-/** A consumer that passes its input through a signature. */
-class SigningConsumer implements Consumer {
-
-	private final Signature signature;
-
-	public SigningConsumer(Signature signature) {
-		this.signature = signature;
-	}
-
-	public void write(byte b) {
-		signature.update(b);
-	}
-
-	public void write(byte[] b, int off, int len) {
-		signature.update(b, off, len);
-	}
-}
--- a/briar-core/src/org/briarproject/sync/SimplexOutgoingSession.java
+++ b/briar-core/src/org/briarproject/sync/SimplexOutgoingSession.java
@@ -11,10 +11,10 @@ import org.briarproject.api.event.EventListener;
 import org.briarproject.api.event.ShutdownEvent;
 import org.briarproject.api.event.TransportRemovedEvent;
 import org.briarproject.api.sync.Ack;
-import org.briarproject.api.sync.MessagingSession;
 import org.briarproject.api.sync.PacketWriter;
 import org.briarproject.api.sync.SubscriptionAck;
 import org.briarproject.api.sync.SubscriptionUpdate;
+import org.briarproject.api.sync.SyncSession;
 import org.briarproject.api.sync.TransportAck;
 import org.briarproject.api.sync.TransportUpdate;

@@ -28,15 +28,15 @@ import java.util.logging.Logger;

 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
+import static org.briarproject.api.sync.SyncConstants.MAX_PACKET_PAYLOAD_LENGTH;

 /**
- * An outgoing {@link org.briarproject.api.sync.MessagingSession
- * MessagingSession} suitable for simplex transports. The session sends
- * messages without offering them, and closes its output stream when there are
- * no more packets to send.
+ * An outgoing {@link org.briarproject.api.sync.SyncSession SyncSession}
+ * suitable for simplex transports. The session sends messages without offering
+ * them first, and closes its output stream when there are no more packets to
+ * send.
 */
-class SimplexOutgoingSession implements MessagingSession, EventListener {
+class SimplexOutgoingSession implements SyncSession, EventListener {

 	private static final Logger LOG =
 			Logger.getLogger(SimplexOutgoingSession.class.getName());
@@ -163,7 +163,7 @@ class SimplexOutgoingSession implements MessagingSession, EventListener {
 			if (interrupted) return;
 			try {
 				Collection<byte[]> b = db.generateBatch(contactId,
-						MAX_PAYLOAD_LENGTH, maxLatency);
+						MAX_PACKET_PAYLOAD_LENGTH, maxLatency);
 				if (LOG.isLoggable(INFO))
 					LOG.info("Generated batch: " + (b != null));
 				if (b == null) decrementOutstandingQueries();
--- a/briar-core/src/org/briarproject/sync/SubscriptionUpdateReader.java
+++ b/briar-core/src/org/briarproject/sync/SubscriptionUpdateReader.java
@@ -2,7 +2,6 @@ package org.briarproject.sync;

 import org.briarproject.api.FormatException;
 import org.briarproject.api.data.BdfReader;
-import org.briarproject.api.data.Consumer;
 import org.briarproject.api.data.ObjectReader;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupId;
@@ -15,8 +14,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Set;

-import static org.briarproject.api.sync.MessagingConstants.MAX_PAYLOAD_LENGTH;
-import static org.briarproject.api.sync.MessagingConstants.MAX_SUBSCRIPTIONS;
+import static org.briarproject.api.sync.SyncConstants.MAX_SUBSCRIPTIONS;

 class SubscriptionUpdateReader implements ObjectReader<SubscriptionUpdate> {

@@ -27,12 +25,7 @@ class SubscriptionUpdateReader implements ObjectReader<SubscriptionUpdate> {
 	}

 	public SubscriptionUpdate readObject(BdfReader r) throws IOException {
-		// Set up the reader
-		Consumer counting = new CountingConsumer(MAX_PAYLOAD_LENGTH);
-		r.addConsumer(counting);
-		// Read the start of the update
 		r.readListStart();
-		// Read the subscriptions, rejecting duplicates
 		List<Group> groups = new ArrayList<Group>();
 		Set<GroupId> ids = new HashSet<GroupId>();
 		r.readListStart();
@@ -42,14 +35,9 @@ class SubscriptionUpdateReader implements ObjectReader<SubscriptionUpdate> {
 			groups.add(g);
 		}
 		r.readListEnd();
-		// Read the version number
 		long version = r.readInteger();
 		if (version < 0) throw new FormatException();
-		// Read the end of the update
 		r.readListEnd();
-		// Reset the reader
-		r.removeConsumer(counting);
-		// Build and return the subscription update
 		groups = Collections.unmodifiableList(groups);
 		return new SubscriptionUpdate(groups, version);
 	}
--- a/briar-core/src/org/briarproject/sync/SyncModule.java
+++ b/briar-core/src/org/briarproject/sync/SyncModule.java
@@ -3,19 +3,18 @@ package org.briarproject.sync;
 import com.google.inject.AbstractModule;
 import com.google.inject.Provides;

-import org.briarproject.api.crypto.CryptoComponent;
 import org.briarproject.api.data.ObjectReader;
 import org.briarproject.api.identity.Author;
 import org.briarproject.api.identity.AuthorFactory;
+import org.briarproject.api.lifecycle.LifecycleManager;
 import org.briarproject.api.sync.Group;
 import org.briarproject.api.sync.GroupFactory;
 import org.briarproject.api.sync.MessageFactory;
-import org.briarproject.api.sync.MessageVerifier;
-import org.briarproject.api.sync.MessagingSessionFactory;
 import org.briarproject.api.sync.PacketReaderFactory;
 import org.briarproject.api.sync.PacketWriterFactory;
 import org.briarproject.api.sync.SubscriptionUpdate;
-import org.briarproject.api.sync.UnverifiedMessage;
+import org.briarproject.api.sync.SyncSessionFactory;
+import org.briarproject.api.sync.ValidationManager;

 import javax.inject.Singleton;

@@ -26,28 +25,20 @@ public class SyncModule extends AbstractModule {
 		bind(AuthorFactory.class).to(AuthorFactoryImpl.class);
 		bind(GroupFactory.class).to(GroupFactoryImpl.class);
 		bind(MessageFactory.class).to(MessageFactoryImpl.class);
-		bind(MessageVerifier.class).to(MessageVerifierImpl.class);
 		bind(PacketReaderFactory.class).to(PacketReaderFactoryImpl.class);
 		bind(PacketWriterFactory.class).to(PacketWriterFactoryImpl.class);
-		bind(MessagingSessionFactory.class).to(
-				MessagingSessionFactoryImpl.class).in(Singleton.class);
+		bind(SyncSessionFactory.class).to(
+				SyncSessionFactoryImpl.class).in(Singleton.class);
 	}

 	@Provides
-	ObjectReader<Author> getAuthorReader(CryptoComponent crypto) {
-		return new AuthorReader(crypto);
+	ObjectReader<Author> getAuthorReader(AuthorFactory authorFactory) {
+		return new AuthorReader(authorFactory);
 	}

 	@Provides
-	ObjectReader<Group> getGroupReader(CryptoComponent crypto) {
-		return new GroupReader(crypto);
-	}
-
-	@Provides
-	ObjectReader<UnverifiedMessage> getMessageReader(
-			ObjectReader<Group> groupReader,
-			ObjectReader<Author> authorReader) {
-		return new MessageReader(groupReader, authorReader);
+	ObjectReader<Group> getGroupReader(GroupFactory groupFactory) {
+		return new GroupReader(groupFactory);
 	}

 	@Provides
@@ -55,4 +46,11 @@ public class SyncModule extends AbstractModule {
 			ObjectReader<Group> groupReader) {
 		return new SubscriptionUpdateReader(groupReader);
 	}
+
+	@Provides @Singleton
+	ValidationManager getValidationManager(LifecycleManager lifecycleManager,
+			ValidationManagerImpl validationManager) {
+		lifecycleManager.register(validationManager);
+		return validationManager;
+	}
 }
--- a/briar-core/src/org/briarproject/sync/MessagingSessionFactoryImpl.java
+++ b/briar-core/src/org/briarproject/sync/MessagingSessionFactoryImpl.java
@@ -2,17 +2,15 @@ package org.briarproject.sync;

 import org.briarproject.api.TransportId;
 import org.briarproject.api.contact.ContactId;
-import org.briarproject.api.crypto.CryptoExecutor;
 import org.briarproject.api.db.DatabaseComponent;
 import org.briarproject.api.db.DatabaseExecutor;
 import org.briarproject.api.event.EventBus;
-import org.briarproject.api.sync.MessageVerifier;
-import org.briarproject.api.sync.MessagingSession;
-import org.briarproject.api.sync.MessagingSessionFactory;
 import org.briarproject.api.sync.PacketReader;
 import org.briarproject.api.sync.PacketReaderFactory;
 import org.briarproject.api.sync.PacketWriter;
 import org.briarproject.api.sync.PacketWriterFactory;
+import org.briarproject.api.sync.SyncSession;
+import org.briarproject.api.sync.SyncSessionFactory;
 import org.briarproject.api.system.Clock;

 import java.io.InputStream;
@@ -21,49 +19,44 @@ import java.util.concurrent.Executor;

 import javax.inject.Inject;

-class MessagingSessionFactoryImpl implements MessagingSessionFactory {
+class SyncSessionFactoryImpl implements SyncSessionFactory {

 	private final DatabaseComponent db;
-	private final Executor dbExecutor, cryptoExecutor;
-	private final MessageVerifier messageVerifier;
+	private final Executor dbExecutor;
 	private final EventBus eventBus;
 	private final Clock clock;
 	private final PacketReaderFactory packetReaderFactory;
 	private final PacketWriterFactory packetWriterFactory;

 	@Inject
-	MessagingSessionFactoryImpl(DatabaseComponent db,
-			@DatabaseExecutor Executor dbExecutor,
-			@CryptoExecutor Executor cryptoExecutor,
-			MessageVerifier messageVerifier, EventBus eventBus, Clock clock,
-			PacketReaderFactory packetReaderFactory,
+	SyncSessionFactoryImpl(DatabaseComponent db,
+			@DatabaseExecutor Executor dbExecutor, EventBus eventBus,
+			Clock clock, PacketReaderFactory packetReaderFactory,
 			PacketWriterFactory packetWriterFactory) {
 		this.db = db;
 		this.dbExecutor = dbExecutor;
-		this.cryptoExecutor = cryptoExecutor;
-		this.messageVerifier = messageVerifier;
 		this.eventBus = eventBus;
 		this.clock = clock;
 		this.packetReaderFactory = packetReaderFactory;
 		this.packetWriterFactory = packetWriterFactory;
 	}

-	public MessagingSession createIncomingSession(ContactId c, TransportId t,
+	public SyncSession createIncomingSession(ContactId c, TransportId t,
 			InputStream in) {
 		PacketReader packetReader = packetReaderFactory.createPacketReader(in);
-		return new IncomingSession(db, dbExecutor, cryptoExecutor, eventBus,
-				messageVerifier, c, t, packetReader);
+		return new IncomingSession(db, dbExecutor, eventBus, c, t,
+				packetReader);
 	}

-	public MessagingSession createSimplexOutgoingSession(ContactId c,
-			TransportId t, int maxLatency, OutputStream out) {
+	public SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
+			int maxLatency, OutputStream out) {
 		PacketWriter packetWriter = packetWriterFactory.createPacketWriter(out);
 		return new SimplexOutgoingSession(db, dbExecutor, eventBus, c, t,
 				maxLatency, packetWriter);
 	}

-	public MessagingSession createDuplexOutgoingSession(ContactId c,
-			TransportId t, int maxLatency, int maxIdleTime, OutputStream out) {
+	public SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
+			int maxLatency, int maxIdleTime, OutputStream out) {
 		PacketWriter packetWriter = packetWriterFactory.createPacketWriter(out);
 		return new DuplexOutgoingSession(db, dbExecutor, eventBus, clock, c, t,
 				maxLatency, maxIdleTime, packetWriter);
--- a/briar-core/src/org/briarproject/sync/ValidationManagerImpl.java
+++ b/briar-core/src/org/briarproject/sync/ValidationManagerImpl.java
@@ -0,0 +1,162 @@
+package org.briarproject.sync;
+
+import com.google.inject.Inject;
+
+import org.briarproject.api.UniqueId;
+import org.briarproject.api.crypto.CryptoExecutor;
+import org.briarproject.api.db.DatabaseComponent;
+import org.briarproject.api.db.DatabaseExecutor;
+import org.briarproject.api.db.DbException;
+import org.briarproject.api.db.Metadata;
+import org.briarproject.api.db.NoSuchMessageException;
+import org.briarproject.api.db.NoSuchSubscriptionException;
+import org.briarproject.api.event.Event;
+import org.briarproject.api.event.EventBus;
+import org.briarproject.api.event.EventListener;
+import org.briarproject.api.event.MessageAddedEvent;
+import org.briarproject.api.sync.ClientId;
+import org.briarproject.api.sync.GroupId;
+import org.briarproject.api.sync.Message;
+import org.briarproject.api.sync.MessageId;
+import org.briarproject.api.sync.MessageValidator;
+import org.briarproject.api.sync.ValidationManager;
+import org.briarproject.util.ByteUtils;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.WARNING;
+import static org.briarproject.api.sync.SyncConstants.MESSAGE_HEADER_LENGTH;
+
+class ValidationManagerImpl implements ValidationManager, EventListener {
+
+	private static final Logger LOG =
+			Logger.getLogger(ValidationManagerImpl.class.getName());
+
+	private final DatabaseComponent db;
+	private final Executor dbExecutor;
+	private final Executor cryptoExecutor;
+	private final EventBus eventBus;
+	private final Map<ClientId, MessageValidator> validators;
+
+	@Inject
+	ValidationManagerImpl(DatabaseComponent db,
+			@DatabaseExecutor Executor dbExecutor,
+			@CryptoExecutor Executor cryptoExecutor, EventBus eventBus) {
+		this.db = db;
+		this.dbExecutor = dbExecutor;
+		this.cryptoExecutor = cryptoExecutor;
+		this.eventBus = eventBus;
+		validators = new ConcurrentHashMap<ClientId, MessageValidator>();
+	}
+
+	@Override
+	public boolean start() {
+		eventBus.addListener(this);
+		return true;
+	}
+
+	@Override
+	public boolean stop() {
+		eventBus.removeListener(this);
+		return true;
+	}
+
+	@Override
+	public void setMessageValidator(ClientId c, MessageValidator v) {
+		validators.put(c, v);
+		getMessagesToValidate(c);
+	}
+
+	private void getMessagesToValidate(final ClientId c) {
+		dbExecutor.execute(new Runnable() {
+			public void run() {
+				try {
+					// TODO: Don't do all of this in a single DB task
+					for (MessageId id : db.getMessagesToValidate(c)) {
+						try {
+							Message m = parseMessage(id, db.getRawMessage(id));
+							validateMessage(m, c);
+						} catch (NoSuchMessageException e) {
+							LOG.info("Message removed before validation");
+						}
+					}
+				} catch (DbException e) {
+					if (LOG.isLoggable(WARNING))
+						LOG.log(WARNING, e.toString(), e);
+				}
+			}
+		});
+	}
+
+	private Message parseMessage(MessageId id, byte[] raw) {
+		if (raw.length <= MESSAGE_HEADER_LENGTH)
+			throw new IllegalArgumentException();
+		byte[] groupId = new byte[UniqueId.LENGTH];
+		System.arraycopy(raw, 0, groupId, 0, UniqueId.LENGTH);
+		long timestamp = ByteUtils.readUint64(raw, UniqueId.LENGTH);
+		return new Message(id, new GroupId(groupId), timestamp, raw);
+	}
+
+	private void validateMessage(final Message m, final ClientId c) {
+		cryptoExecutor.execute(new Runnable() {
+			public void run() {
+				MessageValidator v = validators.get(c);
+				if (v == null) {
+					LOG.warning("No validator");
+				} else {
+					Metadata meta = v.validateMessage(m);
+					storeValidationResult(m, c, meta);
+				}
+			}
+		});
+	}
+
+	private void storeValidationResult(final Message m, final ClientId c,
+			final Metadata meta) {
+		dbExecutor.execute(new Runnable() {
+			public void run() {
+				try {
+					if (meta == null) {
+						db.setMessageValidity(m, c, false);
+					} else {
+						db.mergeMessageMetadata(m.getId(), meta);
+						db.setMessageValidity(m, c, true);
+					}
+				} catch (NoSuchMessageException e) {
+					LOG.info("Message removed during validation");
+				} catch (DbException e) {
+					if (LOG.isLoggable(WARNING))
+						LOG.log(WARNING, e.toString(), e);
+				}
+			}
+		});
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof MessageAddedEvent) {
+			MessageAddedEvent m = (MessageAddedEvent) e;
+			// Validate the message if it wasn't created locally
+			if (m.getContactId() != null) loadClientId(m.getMessage());
+		}
+	}
+
+	private void loadClientId(final Message m) {
+		dbExecutor.execute(new Runnable() {
+			public void run() {
+				try {
+					ClientId c = db.getGroup(m.getGroupId()).getClientId();
+					validateMessage(m, c);
+				} catch (NoSuchSubscriptionException e) {
+					LOG.info("Group removed before validation");
+				} catch (DbException e) {
+					if (LOG.isLoggable(WARNING))
+						LOG.log(WARNING, e.toString(), e);
+				}
+			}
+		});
+	}
+}