Protocol refactoring. Each bundle now consists of a signed header and zero or more signed batches. There is no overall signature on the bundle, since the bundle's contents may need to be processed before the entire bundle has been read. The protocol does not prevent an adversary from removing batches from a bundle, reordering batches, moving them from one bundle to another, etc. However, since each batch is signed and acknowledged independently, no such guarantees are required. Bundle IDs will go away when the retransmission mechanism is changed.

2026-02-12 10:49:06 +01:00 · 2011-07-12 12:55:46 +01:00
parent 4977695a79
commit e0509db45d
28 changed files with 1198 additions and 228 deletions
--- a/api/net/sf/briar/api/db/DatabaseComponent.java
+++ b/api/net/sf/briar/api/db/DatabaseComponent.java
@@ -1,5 +1,7 @@
 package net.sf.briar.api.db;

+import java.io.IOException;
+import java.security.SignatureException;
 import java.util.Map;
 import java.util.Set;

@@ -49,7 +51,7 @@ public interface DatabaseComponent {
 	 * Generates a bundle of acknowledgements, subscriptions, and batches of
 	 * messages for the given contact.
 	 */
-	Bundle generateBundle(ContactId c, BundleBuilder bundleBuilder) throws DbException;
+	Bundle generateBundle(ContactId c, BundleBuilder bundleBuilder) throws DbException, IOException, SignatureException;

 	/** Returns the IDs of all contacts. */
 	Set<ContactId> getContacts() throws DbException;
@@ -71,7 +73,7 @@ public interface DatabaseComponent {
 	 * messages received from the given contact. Some or all of the messages
 	 * in the bundle may be stored.
 	 */
-	void receiveBundle(ContactId c, Bundle b) throws DbException;
+	void receiveBundle(ContactId c, Bundle b) throws DbException, IOException, SignatureException;

 	/** Removes a contact (and all associated state) from the database. */
 	void removeContact(ContactId c) throws DbException;
--- a/api/net/sf/briar/api/protocol/Batch.java
+++ b/api/net/sf/briar/api/protocol/Batch.java
@@ -1,16 +1,19 @@
 package net.sf.briar.api.protocol;

-/** A batch of messages up to CAPACITY bytes in total size. */
+/** A batch of messages up to MAX_SIZE bytes in total size. */
 public interface Batch {

-	public static final long CAPACITY = 1024L * 1024L;
+	public static final int MAX_SIZE = 1024 * 1024;

 	/** Returns the batch's unique identifier. */
 	BatchId getId();

-	/** Returns the size of the batch in bytes. */
+	/** Returns the size of the serialised batch in bytes. */
 	long getSize();

 	/** Returns the messages contained in the batch. */
 	Iterable<Message> getMessages();
+
+	/** Returns the sender's signature over the contents of the batch. */
+	byte[] getSignature();
 }
--- a/api/net/sf/briar/api/protocol/BatchBuilder.java
+++ b/api/net/sf/briar/api/protocol/BatchBuilder.java
@@ -1,10 +1,15 @@
 package net.sf.briar.api.protocol;

+import java.security.SignatureException;
+
 public interface BatchBuilder {

 	/** Adds a message to the batch. */
 	void addMessage(Message m);

+	/** Sets the sender's signature over the contents of the batch. */
+	void setSignature(byte[] sig);
+
 	/** Builds and returns the batch. */
-	Batch build();
+	Batch build() throws SignatureException;
 }
--- a/api/net/sf/briar/api/protocol/Bundle.java
+++ b/api/net/sf/briar/api/protocol/Bundle.java
@@ -1,28 +1,21 @@
 package net.sf.briar.api.protocol;

-import java.util.Map;
+import java.io.IOException;
+import java.security.SignatureException;

-/** A bundle of acknowledgements, subscriptions, and batches of messages. */
+/**
+ * A bundle of acknowledgements, subscriptions, transport details and batches.
+ */
 public interface Bundle {

-	/** Returns the bundle's unique identifier. */
-	BundleId getId();
+	/** Returns the size of the serialised bundle in bytes. */
+	long getSize() throws IOException;

-	/** Returns the bundle's capacity in bytes. */
-	long getCapacity();
+	/** Returns the bundle's header. */
+	Header getHeader() throws IOException, SignatureException;

-	/** Returns the bundle's size in bytes. */
-	long getSize();
-
-	/** Returns the acknowledgements contained in the bundle. */
-	Iterable<BatchId> getAcks();
-
-	/** Returns the subscriptions contained in the bundle. */
-	Iterable<GroupId> getSubscriptions();
-
-	/** Returns the transport details contained in the bundle. */
-	Map<String, String> getTransports();
-
-	/** Returns the batches of messages contained in the bundle. */
-	Iterable<Batch> getBatches();
+	/**
+	 * Returns the next batch of messages, or null if there are no more batches.
+	 */
+	Batch getNextBatch() throws IOException, SignatureException;
 }
--- a/api/net/sf/briar/api/protocol/BundleBuilder.java
+++ b/api/net/sf/briar/api/protocol/BundleBuilder.java
@@ -1,22 +1,18 @@
 package net.sf.briar.api.protocol;

+import java.io.IOException;
+
 public interface BundleBuilder {

 	/** Returns the bundle's capacity in bytes. */
-	long getCapacity();
+	long getCapacity() throws IOException;

-	/** Adds an acknowledgement to the bundle. */
-	void addAck(BatchId b);
-
-	/** Adds a subscription to the bundle. */
-	void addSubscription(GroupId g);
-
-	/** Adds a transport detail to the bundle. */
-	void addTransport(String key, String value);
+	/** Adds a header to the bundle. */
+	void addHeader(Header h) throws IOException;

 	/** Adds a batch of messages to the bundle. */
-	void addBatch(Batch b);
+	void addBatch(Batch b) throws IOException;

 	/** Builds and returns the bundle. */
-	Bundle build();
+	Bundle build() throws IOException;
 }
--- a/api/net/sf/briar/api/protocol/Header.java
+++ b/api/net/sf/briar/api/protocol/Header.java
@@ -0,0 +1,28 @@
+package net.sf.briar.api.protocol;
+
+import java.util.Map;
+import java.util.Set;
+
+/** A bundle header up to MAX_SIZE bytes in total size. */
+public interface Header {
+
+	static final int MAX_SIZE = 1024 * 1024;
+
+	// FIXME: Remove BundleId when refactoring is complete
+	BundleId getId();
+
+	/** Returns the size of the serialised header in bytes. */
+	long getSize();
+
+	/** Returns the acknowledgements contained in the header. */
+	Set<BatchId> getAcks();
+
+	/** Returns the subscriptions contained in the header. */
+	Set<GroupId> getSubscriptions();
+
+	/** Returns the transport details contained in the header. */
+	Map<String, String> getTransports();
+
+	/** Returns the sender's signature over the contents of the header. */
+	byte[] getSignature();
+}
--- a/api/net/sf/briar/api/protocol/HeaderBuilder.java
+++ b/api/net/sf/briar/api/protocol/HeaderBuilder.java
@@ -0,0 +1,24 @@
+package net.sf.briar.api.protocol;
+
+import java.io.IOException;
+import java.security.SignatureException;
+import java.util.Map;
+import java.util.Set;
+
+public interface HeaderBuilder {
+
+	/** Adds acknowledgements to the header. */
+	void addAcks(Set<BatchId> acks) throws IOException;
+
+	/** Adds subscriptions to the header. */
+	void addSubscriptions(Set<GroupId> subs) throws IOException;
+
+	/** Adds transport details to the header. */
+	void addTransports(Map<String, String> transports) throws IOException;
+
+	/** Sets the sender's signature over the contents of the header. */
+	void setSignature(byte[] sig) throws IOException;
+
+	/** Builds and returns the header. */
+	Header build() throws SignatureException;
+}
--- a/api/net/sf/briar/api/protocol/MessageParser.java
+++ b/api/net/sf/briar/api/protocol/MessageParser.java
@@ -0,0 +1,10 @@
+package net.sf.briar.api.protocol;
+
+import java.security.SignatureException;
+
+import net.sf.briar.api.serial.FormatException;
+
+public interface MessageParser {
+
+	Message parseMessage(byte[] body) throws FormatException, SignatureException;
+}
--- a/api/net/sf/briar/api/protocol/UniqueId.java
+++ b/api/net/sf/briar/api/protocol/UniqueId.java
@@ -2,7 +2,9 @@ package net.sf.briar.api.protocol;

 import java.util.Arrays;

-public abstract class UniqueId {
+import net.sf.briar.api.serial.Raw;
+
+public abstract class UniqueId implements Raw {

 	public static final int LENGTH = 32;

--- a/components/net/sf/briar/db/DatabaseComponentImpl.java
+++ b/components/net/sf/briar/db/DatabaseComponentImpl.java
@@ -10,6 +10,7 @@ import net.sf.briar.api.db.DbException;
 import net.sf.briar.api.db.Status;
 import net.sf.briar.api.protocol.AuthorId;
 import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.HeaderBuilder;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;

@@ -27,6 +28,7 @@ DatabaseCleaner.Callback {

 	protected final Database<Txn> db;
 	protected final DatabaseCleaner cleaner;
+	protected final Provider<HeaderBuilder> headerBuilderProvider;
 	protected final Provider<BatchBuilder> batchBuilderProvider;

 	private final Object spaceLock = new Object();
@@ -36,9 +38,11 @@ DatabaseCleaner.Callback {
 	private volatile boolean writesAllowed = true;

 	DatabaseComponentImpl(Database<Txn> db, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
 		this.db = db;
 		this.cleaner = cleaner;
+		this.headerBuilderProvider = headerBuilderProvider;
 		this.batchBuilderProvider = batchBuilderProvider;
 	}

--- a/components/net/sf/briar/db/ReadWriteLockDatabaseComponent.java
+++ b/components/net/sf/briar/db/ReadWriteLockDatabaseComponent.java
@@ -1,9 +1,10 @@
 package net.sf.briar.db;

+import java.io.IOException;
+import java.security.SignatureException;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
-import java.util.Map.Entry;
 import java.util.Set;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.logging.Level;
@@ -20,6 +21,8 @@ import net.sf.briar.api.protocol.BatchId;
 import net.sf.briar.api.protocol.Bundle;
 import net.sf.briar.api.protocol.BundleBuilder;
 import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.HeaderBuilder;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;

@@ -55,8 +58,9 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {

 	@Inject
 	ReadWriteLockDatabaseComponent(Database<Txn> db, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
-		super(db, cleaner, batchBuilderProvider);
+		super(db, cleaner, headerBuilderProvider, batchBuilderProvider);
 	}

 	public void close() throws DbException {
@@ -187,23 +191,22 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 	}

 	public Bundle generateBundle(ContactId c, BundleBuilder b)
-	throws DbException {
+	throws DbException, IOException, SignatureException {
 		if(LOG.isLoggable(Level.FINE)) LOG.fine("Generating bundle for " + c);
-		// Ack all batches received from c
+		HeaderBuilder h;
+		// Add acks
 		contactLock.readLock().lock();
 		try {
 			if(!containsContact(c)) throw new NoSuchContactException();
+			h = headerBuilderProvider.get();
 			messageStatusLock.writeLock().lock();
 			try {
 				Txn txn = db.startTransaction();
 				try {
-					int numAcks = 0;
-					for(BatchId ack : db.removeBatchesToAck(txn, c)) {
-						b.addAck(ack);
-						numAcks++;
-					}
+					Set<BatchId> acks = db.removeBatchesToAck(txn, c);
+					h.addAcks(acks);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numAcks + " acks");
+						LOG.fine("Added " + acks.size() + " acks");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -215,7 +218,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		} finally {
 			contactLock.readLock().unlock();
 		}
-		// Add a list of subscriptions
+		// Add subscriptions
 		contactLock.readLock().lock();
 		try {
 			if(!containsContact(c)) throw new NoSuchContactException();
@@ -223,13 +226,10 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			try {
 				Txn txn = db.startTransaction();
 				try {
-					int numSubs = 0;
-					for(GroupId g : db.getSubscriptions(txn)) {
-						b.addSubscription(g);
-						numSubs++;
-					}
+					Set<GroupId> subs = db.getSubscriptions(txn);
+					h.addSubscriptions(subs);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numSubs + " subscriptions");
+						LOG.fine("Added " + subs.size() + " subscriptions");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -249,14 +249,10 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			try {
 				Txn txn = db.startTransaction();
 				try {
-					int numTransports = 0;
 					Map<String, String> transports = db.getTransports(txn);
-					for(Entry<String, String> e : transports.entrySet()) {
-						b.addTransport(e.getKey(), e.getValue());
-						numTransports++;
-					}
+					h.addTransports(transports);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numTransports + " transports");
+						LOG.fine("Added " + transports.size() + " transports");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -268,8 +264,12 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		} finally {
 			contactLock.readLock().unlock();
 		}
-		// Add as many messages as possible to the bundle
+		// Sign the header and add it to the bundle
+		Header header = h.build();
 		long capacity = b.getCapacity();
+		capacity -= header.getSize();
+		b.addHeader(header);
+		// Add as many messages as possible to the bundle
 		while(true) {
 			Batch batch = fillBatch(c, capacity);
 			if(batch == null) break; // No more messages to send
@@ -278,7 +278,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			capacity -= size;
 			// If the batch is less than half full, stop trying - there may be
 			// more messages trickling in but we can't wait forever
-			if(size * 2 < Batch.CAPACITY) break;
+			if(size * 2 < Batch.MAX_SIZE) break;
 		}
 		Bundle bundle = b.build();
 		if(LOG.isLoggable(Level.FINE))
@@ -287,20 +287,20 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		return bundle;
 	}

-	private Batch fillBatch(ContactId c, long capacity) throws DbException {
+	private Batch fillBatch(ContactId c, long capacity) throws DbException,
+	SignatureException {
 		contactLock.readLock().lock();
 		try {
 			if(!containsContact(c)) throw new NoSuchContactException();
 			messageLock.readLock().lock();
 			try {
 				Set<MessageId> sent;
-				BatchBuilder b;
 				Batch batch;
 				messageStatusLock.readLock().lock();
 				try {
 					Txn txn = db.startTransaction();
 					try {
-						capacity = Math.min(capacity, Batch.CAPACITY);
+						capacity = Math.min(capacity, Batch.MAX_SIZE);
 						Iterator<MessageId> it =
 							db.getSendableMessages(txn, c, capacity).iterator();
 						if(!it.hasNext()) {
@@ -308,7 +308,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 							return null; // No more messages to send
 						}
 						sent = new HashSet<MessageId>();
-						b = batchBuilderProvider.get();
+						BatchBuilder b = batchBuilderProvider.get();
 						while(it.hasNext()) {
 							MessageId m = it.next();
 							b.addMessage(db.getMessage(txn, m));
@@ -319,6 +319,9 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 					} catch(DbException e) {
 						db.abortTransaction(txn);
 						throw e;
+					} catch(SignatureException e) {
+						db.abortTransaction(txn);
+						throw e;
 					}
 				} finally {
 					messageStatusLock.readLock().unlock();
@@ -438,21 +441,23 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}

-	public void receiveBundle(ContactId c, Bundle b) throws DbException {
+	public void receiveBundle(ContactId c, Bundle b) throws DbException,
+	IOException, SignatureException {
 		if(LOG.isLoggable(Level.FINE))
 			LOG.fine("Received bundle from " + c + ", "
 					+ b.getSize() + " bytes");
+		Header h;
 		// Mark all messages in acked batches as seen
 		contactLock.readLock().lock();
 		try {
 			if(!containsContact(c)) throw new NoSuchContactException();
+			h = b.getHeader();
 			messageLock.readLock().lock();
 			try {
 				messageStatusLock.writeLock().lock();
 				try {
-					int acks = 0;
-					for(BatchId ack : b.getAcks()) {
-						acks++;
+					Set<BatchId> acks = h.getAcks();
+					for(BatchId ack : acks) {
 						Txn txn = db.startTransaction();
 						try {
 							db.removeAckedBatch(txn, c, ack);
@@ -463,7 +468,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						}
 					}
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Received " + acks + " acks");
+						LOG.fine("Received " + acks.size() + " acks");
 				} finally {
 					messageStatusLock.writeLock().unlock();
 				}
@@ -481,14 +486,12 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			try {
 				Txn txn = db.startTransaction();
 				try {
+					// FIXME: Replace clearSubs and addSub with setSubs
 					db.clearSubscriptions(txn, c);
-					int subs = 0;
-					for(GroupId g : b.getSubscriptions()) {
-						subs++;
-						db.addSubscription(txn, c, g);
-					}
+					Set<GroupId> subs = h.getSubscriptions();
+					for(GroupId sub : subs) db.addSubscription(txn, c, sub);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Received " + subs + " subscriptions");
+						LOG.fine("Received " + subs.size() + " subscriptions");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -508,7 +511,11 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			try {
 				Txn txn = db.startTransaction();
 				try {
-					db.setTransports(txn, c, b.getTransports());
+					Map<String, String> transports = h.getTransports();
+					db.setTransports(txn, c, transports);
+					if(LOG.isLoggable(Level.FINE))
+						LOG.fine("Received " + transports.size()
+								+ " transports");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -522,7 +529,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 		// Store the messages
 		int batches = 0;
-		for(Batch batch : b.getBatches()) {
+		for(Batch batch = b.getNextBatch(); batch != null; batch = b.getNextBatch()) {
 			batches++;
 			waitForPermissionToWrite();
 			contactLock.readLock().lock();
@@ -579,7 +586,7 @@ class ReadWriteLockDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				try {
 					Txn txn = db.startTransaction();
 					try {
-						lost = db.addReceivedBundle(txn, c, b.getId());
+						lost = db.addReceivedBundle(txn, c, h.getId());
 						db.commitTransaction(txn);
 					} catch(DbException e) {
 						db.abortTransaction(txn);
--- a/components/net/sf/briar/db/SynchronizedDatabaseComponent.java
+++ b/components/net/sf/briar/db/SynchronizedDatabaseComponent.java
@@ -1,9 +1,10 @@
 package net.sf.briar.db;

+import java.io.IOException;
+import java.security.SignatureException;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
-import java.util.Map.Entry;
 import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
@@ -19,6 +20,8 @@ import net.sf.briar.api.protocol.BatchId;
 import net.sf.briar.api.protocol.Bundle;
 import net.sf.briar.api.protocol.BundleBuilder;
 import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.HeaderBuilder;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;

@@ -48,8 +51,9 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {

 	@Inject
 	SynchronizedDatabaseComponent(Database<Txn> db, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
-		super(db, cleaner, batchBuilderProvider);
+		super(db, cleaner, headerBuilderProvider, batchBuilderProvider);
 	}

 	public void close() throws DbException {
@@ -140,21 +144,20 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 	}

 	public Bundle generateBundle(ContactId c, BundleBuilder b)
-	throws DbException {
+	throws DbException, IOException, SignatureException {
 		if(LOG.isLoggable(Level.FINE)) LOG.fine("Generating bundle for " + c);
-		// Ack all batches received from c
+		HeaderBuilder h;
+		// Add acks
 		synchronized(contactLock) {
 			if(!containsContact(c)) throw new NoSuchContactException();
+			h = headerBuilderProvider.get();
 			synchronized(messageStatusLock) {
 				Txn txn = db.startTransaction();
 				try {
-					int numAcks = 0;
-					for(BatchId ack : db.removeBatchesToAck(txn, c)) {
-						b.addAck(ack);
-						numAcks++;
-					}
+					Set<BatchId> acks = db.removeBatchesToAck(txn, c);
+					h.addAcks(acks);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numAcks + " acks");
+						LOG.fine("Added " + acks.size() + " acks");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -162,19 +165,16 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				}
 			}
 		}
-		// Add a list of subscriptions
+		// Add subscriptions
 		synchronized(contactLock) {
 			if(!containsContact(c)) throw new NoSuchContactException();
 			synchronized(subscriptionLock) {
 				Txn txn = db.startTransaction();
 				try {
-					int numSubs = 0;
-					for(GroupId g : db.getSubscriptions(txn)) {
-						b.addSubscription(g);
-						numSubs++;
-					}
+					Set<GroupId> subs = db.getSubscriptions(txn);
+					h.addSubscriptions(subs);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numSubs + " subscriptions");
+						LOG.fine("Added " + subs.size() + " subscriptions");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -188,14 +188,10 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			synchronized(transportLock) {
 				Txn txn = db.startTransaction();
 				try {
-					int numTransports = 0;
 					Map<String, String> transports = db.getTransports(txn);
-					for(Entry<String, String> e : transports.entrySet()) {
-						b.addTransport(e.getKey(), e.getValue());
-						numTransports++;
-					}
+					h.addTransports(transports);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Added " + numTransports + " transports");
+						LOG.fine("Added " + transports.size() + " transports");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -203,8 +199,12 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				}
 			}
 		}
-		// Add as many messages as possible to the bundle
+		// Sign the header and add it to the bundle
+		Header header = h.build();
 		long capacity = b.getCapacity();
+		capacity -= header.getSize();
+		b.addHeader(header);
+		// Add as many messages as possible to the bundle
 		while(true) {
 			Batch batch = fillBatch(c, capacity);
 			if(batch == null) break; // No more messages to send
@@ -213,7 +213,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			capacity -= size;
 			// If the batch is less than half full, stop trying - there may be
 			// more messages trickling in but we can't wait forever
-			if(size * 2 < Batch.CAPACITY) break;
+			if(size * 2 < Batch.MAX_SIZE) break;
 		}
 		Bundle bundle = b.build();
 		if(LOG.isLoggable(Level.FINE))
@@ -222,14 +222,15 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		return bundle;
 	}

-	private Batch fillBatch(ContactId c, long capacity) throws DbException {
+	private Batch fillBatch(ContactId c, long capacity) throws DbException,
+	SignatureException {
 		synchronized(contactLock) {
 			if(!containsContact(c)) throw new NoSuchContactException();
 			synchronized(messageLock) {
 				synchronized(messageStatusLock) {
 					Txn txn = db.startTransaction();
 					try {
-						capacity = Math.min(capacity, Batch.CAPACITY);
+						capacity = Math.min(capacity, Batch.MAX_SIZE);
 						Iterator<MessageId> it =
 							db.getSendableMessages(txn, c, capacity).iterator();
 						if(!it.hasNext()) {
@@ -252,6 +253,9 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 					} catch(DbException e) {
 						db.abortTransaction(txn);
 						throw e;
+					} catch(SignatureException e) {
+						db.abortTransaction(txn);
+						throw e;
 					}
 				}
 			}
@@ -331,18 +335,20 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 	}

-	public void receiveBundle(ContactId c, Bundle b) throws DbException {
+	public void receiveBundle(ContactId c, Bundle b) throws DbException,
+	IOException, SignatureException {
 		if(LOG.isLoggable(Level.FINE))
 			LOG.fine("Received bundle from " + c + ", "
 					+ b.getSize() + " bytes");
+		Header h;
 		// Mark all messages in acked batches as seen
 		synchronized(contactLock) {
 			if(!containsContact(c)) throw new NoSuchContactException();
+			h = b.getHeader();
 			synchronized(messageLock) {
 				synchronized(messageStatusLock) {
-					int acks = 0;
-					for(BatchId ack : b.getAcks()) {
-						acks++;
+					Set<BatchId> acks = h.getAcks();
+					for(BatchId ack : acks) {
 						Txn txn = db.startTransaction();
 						try {
 							db.removeAckedBatch(txn, c, ack);
@@ -353,7 +359,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 						}
 					}
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Received " + acks + " acks");
+						LOG.fine("Received " + acks.size() + " acks");
 				}
 			}
 		}
@@ -363,14 +369,12 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			synchronized(subscriptionLock) {
 				Txn txn = db.startTransaction();
 				try {
+					// FIXME: Replace clearSubs and addSub with setSubs
 					db.clearSubscriptions(txn, c);
-					int subs = 0;
-					for(GroupId g : b.getSubscriptions()) {
-						subs++;
-						db.addSubscription(txn, c, g);
-					}
+					Set<GroupId> subs = h.getSubscriptions();
+					for(GroupId sub : subs) db.addSubscription(txn, c, sub);
 					if(LOG.isLoggable(Level.FINE))
-						LOG.fine("Received " + subs + " subscriptions");
+						LOG.fine("Received " + subs.size() + " subscriptions");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -384,7 +388,11 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 			synchronized(transportLock) {
 				Txn txn = db.startTransaction();
 				try {
-					db.setTransports(txn, c, b.getTransports());
+					Map<String, String> transports = h.getTransports();
+					db.setTransports(txn, c, transports);
+					if(LOG.isLoggable(Level.FINE))
+						LOG.fine("Received " + transports.size()
+								+ " transports");
 					db.commitTransaction(txn);
 				} catch(DbException e) {
 					db.abortTransaction(txn);
@@ -394,7 +402,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 		}
 		// Store the messages
 		int batches = 0;
-		for(Batch batch : b.getBatches()) {
+		for(Batch batch = b.getNextBatch(); batch != null; batch = b.getNextBatch()) {
 			batches++;
 			waitForPermissionToWrite();
 			synchronized(contactLock) {
@@ -436,7 +444,7 @@ class SynchronizedDatabaseComponent<Txn> extends DatabaseComponentImpl<Txn> {
 				synchronized(messageStatusLock) {
 					Txn txn = db.startTransaction();
 					try {
-						lost = db.addReceivedBundle(txn, c, b.getId());
+						lost = db.addReceivedBundle(txn, c, h.getId());
 						db.commitTransaction(txn);
 					} catch(DbException e) {
 						db.abortTransaction(txn);
--- a/components/net/sf/briar/protocol/BatchImpl.java
+++ b/components/net/sf/briar/protocol/BatchImpl.java
@@ -1,8 +1,6 @@
 package net.sf.briar.protocol;

-import java.util.ArrayList;
 import java.util.List;
-import java.util.Random;

 import net.sf.briar.api.protocol.Batch;
 import net.sf.briar.api.protocol.BatchId;
@@ -11,15 +9,16 @@ import net.sf.briar.api.protocol.Message;
 /** A simple in-memory implementation of a batch. */
 class BatchImpl implements Batch {

-	private final List<Message> messages = new ArrayList<Message>();
-	private BatchId id = null;
-	private long size = 0L;
+	private final BatchId id;
+	private final long size;
+	private final List<Message> messages;
+	private final byte[] signature;

-	public void seal() {
-		// FIXME: Calculate batch ID
-		byte[] b = new byte[BatchId.LENGTH];
-		new Random().nextBytes(b);
-		id = new BatchId(b);
+	BatchImpl(BatchId id, long size, List<Message> messages, byte[] signature) {
+		this.id = id;
+		this.size = size;
+		this.messages = messages;
+		this.signature = signature;
 	}

 	public BatchId getId() {
@@ -34,8 +33,7 @@ class BatchImpl implements Batch {
 		return messages;
 	}

-	public void addMessage(Message m) {
-		messages.add(m);
-		size += m.getSize();
+	public byte[] getSignature() {
+		return signature;
 	}
 }
--- a/components/net/sf/briar/protocol/BundleReader.java
+++ b/components/net/sf/briar/protocol/BundleReader.java
@@ -0,0 +1,94 @@
+package net.sf.briar.protocol;
+
+import java.io.IOException;
+import java.security.SignatureException;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import net.sf.briar.api.protocol.Batch;
+import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.BatchId;
+import net.sf.briar.api.protocol.Bundle;
+import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.HeaderBuilder;
+import net.sf.briar.api.protocol.Message;
+import net.sf.briar.api.protocol.MessageParser;
+import net.sf.briar.api.protocol.UniqueId;
+import net.sf.briar.api.serial.FormatException;
+import net.sf.briar.api.serial.Raw;
+import net.sf.briar.api.serial.Reader;
+
+import com.google.inject.Provider;
+
+/** A bundle that deserialises its contents on demand using a reader. */
+abstract class BundleReader implements Bundle {
+
+	private static enum State { START, FIRST_BATCH, MORE_BATCHES, END };
+
+	private final Reader r;
+	private final MessageParser messageParser;
+	private final Provider<HeaderBuilder> headerBuilderProvider;
+	private final Provider<BatchBuilder> batchBuilderProvider;
+	private State state = State.START;
+
+	BundleReader(Reader r, MessageParser messageParser,
+			Provider<HeaderBuilder> headerBuilderProvider,
+			Provider<BatchBuilder> batchBuilderProvider) {
+		this.r = r;
+		this.messageParser = messageParser;
+		this.headerBuilderProvider = headerBuilderProvider;
+		this.batchBuilderProvider = batchBuilderProvider;
+	}
+
+	public Header getHeader() throws IOException, SignatureException {
+		if(state != State.START) throw new IllegalStateException();
+		r.setReadLimit(Header.MAX_SIZE);
+		Set<BatchId> acks = new HashSet<BatchId>();
+		for(Raw raw : r.readList(Raw.class)) {
+			byte[] b = raw.getBytes();
+			if(b.length != UniqueId.LENGTH) throw new FormatException();
+			acks.add(new BatchId(b));
+		}
+		Set<GroupId> subs = new HashSet<GroupId>();
+		for(Raw raw : r.readList(Raw.class)) {
+			byte[] b = raw.getBytes();
+			if(b.length != UniqueId.LENGTH) throw new FormatException();
+			subs.add(new GroupId(b));
+		}
+		Map<String, String> transports = r.readMap(String.class, String.class);
+		byte[] sig = r.readRaw();
+		state = State.FIRST_BATCH;
+		HeaderBuilder h = headerBuilderProvider.get();
+		h.addAcks(acks);
+		h.addSubscriptions(subs);
+		h.addTransports(transports);
+		h.setSignature(sig);
+		return h.build();
+	}
+
+	public Batch getNextBatch() throws IOException, SignatureException {
+		if(state == State.FIRST_BATCH) {
+			r.readListStart();
+			state = State.MORE_BATCHES;
+		}
+		if(state != State.MORE_BATCHES) throw new IllegalStateException();
+		if(r.hasListEnd()) {
+			r.readListEnd();
+			state = State.END;
+			return null;
+		}
+		r.setReadLimit(Batch.MAX_SIZE);
+		List<Raw> messages = r.readList(Raw.class);
+		BatchBuilder b = batchBuilderProvider.get();
+		for(Raw r : messages) {
+			Message m = messageParser.parseMessage(r.getBytes());
+			b.addMessage(m);
+		}
+		byte[] sig = r.readRaw();
+		b.setSignature(sig);
+		return b.build();
+	}
+}
--- a/components/net/sf/briar/protocol/BundleWriter.java
+++ b/components/net/sf/briar/protocol/BundleWriter.java
@@ -0,0 +1,66 @@
+package net.sf.briar.protocol;
+
+import java.io.IOException;
+
+import net.sf.briar.api.protocol.Batch;
+import net.sf.briar.api.protocol.BatchId;
+import net.sf.briar.api.protocol.BundleBuilder;
+import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.Message;
+import net.sf.briar.api.serial.Writer;
+
+/** A bundle builder that serialises its contents using a writer. */
+abstract class BundleWriter implements BundleBuilder {
+
+	private static enum State { START, FIRST_BATCH, MORE_BATCHES, END };
+
+	private final Writer w;
+	private final long capacity;
+	private State state = State.START;
+
+	BundleWriter(Writer w, long capacity) {
+		this.w = w;
+		this.capacity = capacity;
+	}
+
+	public long getCapacity() {
+		return capacity;
+	}
+
+	public void addHeader(Header h) throws IOException {
+		if(state != State.START) throw new IllegalStateException();
+		w.writeListStart();
+		for(BatchId ack : h.getAcks()) w.writeRaw(ack);
+		w.writeListEnd();
+		w.writeListStart();
+		for(GroupId sub : h.getSubscriptions()) w.writeRaw(sub);
+		w.writeListEnd();
+		w.writeMap(h.getTransports());
+		w.writeRaw(h.getSignature());
+		state = State.FIRST_BATCH;
+	}
+
+	public void addBatch(Batch b) throws IOException {
+		if(state == State.FIRST_BATCH) {
+			w.writeListStart();
+			state = State.MORE_BATCHES;
+		}
+		if(state != State.MORE_BATCHES) throw new IllegalStateException();
+		w.writeListStart();
+		for(Message m : b.getMessages()) w.writeRaw(m.getBody());
+		w.writeListEnd();
+		w.writeRaw(b.getSignature());
+	}
+
+	void close() throws IOException {
+		if(state == State.FIRST_BATCH) {
+			w.writeListStart();
+			state = State.MORE_BATCHES;
+		}
+		if(state != State.MORE_BATCHES) throw new IllegalStateException();
+		w.writeListEnd();
+		w.close();
+		state = State.END;
+	}
+}
--- a/components/net/sf/briar/protocol/FileBundle.java
+++ b/components/net/sf/briar/protocol/FileBundle.java
@@ -0,0 +1,30 @@
+package net.sf.briar.protocol;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+
+import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.HeaderBuilder;
+import net.sf.briar.api.protocol.MessageParser;
+import net.sf.briar.api.serial.ReaderFactory;
+
+import com.google.inject.Provider;
+
+class FileBundle extends BundleReader {
+
+	private final File file;
+
+	FileBundle(File file, ReaderFactory readerFactory,
+			MessageParser messageParser,
+			Provider<HeaderBuilder> headerBuilderProvider,
+			Provider<BatchBuilder> batchBuilderProvider) throws IOException {
+		super(readerFactory.createReader(new FileInputStream(file)),
+				messageParser, headerBuilderProvider, batchBuilderProvider);
+		this.file = file;
+	}
+
+	public long getSize() throws IOException {
+		return file.length();
+	}
+}
--- a/components/net/sf/briar/protocol/FileBundleBuilder.java
+++ b/components/net/sf/briar/protocol/FileBundleBuilder.java
@@ -0,0 +1,41 @@
+package net.sf.briar.protocol;
+
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+
+import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.Bundle;
+import net.sf.briar.api.protocol.HeaderBuilder;
+import net.sf.briar.api.protocol.MessageParser;
+import net.sf.briar.api.serial.ReaderFactory;
+import net.sf.briar.api.serial.WriterFactory;
+
+import com.google.inject.Provider;
+
+public class FileBundleBuilder extends BundleWriter {
+
+	private final File file;
+	private final ReaderFactory readerFactory;
+	private final MessageParser messageParser;
+	private final Provider<HeaderBuilder> headerBuilderProvider;
+	private final Provider<BatchBuilder> batchBuilderProvider;
+
+	FileBundleBuilder(File file, long capacity, WriterFactory writerFactory,
+			ReaderFactory readerFactory, MessageParser messageParser,
+			Provider<HeaderBuilder> headerBuilderProvider,
+			Provider<BatchBuilder> batchBuilderProvider) throws IOException {
+		super(writerFactory.createWriter(new FileOutputStream(file)), capacity);
+		this.file = file;
+		this.readerFactory = readerFactory;
+		this.messageParser = messageParser;
+		this.headerBuilderProvider = headerBuilderProvider;
+		this.batchBuilderProvider = batchBuilderProvider;
+	}
+
+	public Bundle build() throws IOException {
+		super.close();
+		return new FileBundle(file, readerFactory, messageParser,
+				headerBuilderProvider, batchBuilderProvider);
+	}
+}
--- a/components/net/sf/briar/protocol/HeaderImpl.java
+++ b/components/net/sf/briar/protocol/HeaderImpl.java
@@ -0,0 +1,55 @@
+package net.sf.briar.protocol;
+
+import java.util.Map;
+import java.util.Set;
+
+import net.sf.briar.api.protocol.BatchId;
+import net.sf.briar.api.protocol.BundleId;
+import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+
+/** A simple in-memory implementation of a header. */
+class HeaderImpl implements Header {
+
+	private final BundleId id;
+	private final long size;
+	private final Set<BatchId> acks;
+	private final Set<GroupId> subscriptions;
+	private final Map<String, String> transports;
+	private final byte[] signature;
+
+	HeaderImpl(BundleId id, long size, Set<BatchId> acks,
+			Set<GroupId> subscriptions, Map<String, String> transports,
+			byte[] signature) {
+		this.id = id;
+		this.size = size;
+		this.acks = acks;
+		this.subscriptions = subscriptions;
+		this.transports = transports;
+		this.signature = signature;
+	}
+
+	public BundleId getId() {
+		return id;
+	}
+
+	public long getSize() {
+		return size;
+	}
+
+	public Set<BatchId> getAcks() {
+		return acks;
+	}
+
+	public Set<GroupId> getSubscriptions() {
+		return subscriptions;
+	}
+
+	public Map<String, String> getTransports() {
+		return transports;
+	}
+
+	public byte[] getSignature() {
+		return signature;
+	}
+}
--- a/components/net/sf/briar/protocol/MessageImpl.java
+++ b/components/net/sf/briar/protocol/MessageImpl.java
@@ -5,6 +5,7 @@ import net.sf.briar.api.protocol.GroupId;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;

+/** A simple in-memory implementation of a message. */
 public class MessageImpl implements Message {

 	private final MessageId id, parent;
@@ -53,7 +54,7 @@ public class MessageImpl implements Message {

 	@Override
 	public boolean equals(Object o) {
-		return o instanceof MessageImpl && id.equals(((MessageImpl)o).id);
+		return o instanceof Message && id.equals(((Message)o).getId());
 	}

 	@Override
--- a/components/net/sf/briar/protocol/ProtocolModule.java
+++ b/components/net/sf/briar/protocol/ProtocolModule.java
@@ -1,10 +1,8 @@
 package net.sf.briar.protocol;

-import net.sf.briar.api.protocol.Batch;
 import net.sf.briar.api.protocol.Message;

 import com.google.inject.AbstractModule;
-import com.google.inject.Provides;

 public class ProtocolModule extends AbstractModule {

@@ -12,9 +10,4 @@ public class ProtocolModule extends AbstractModule {
 	protected void configure() {
 		bind(Message.class).to(MessageImpl.class);
 	}
-
-	@Provides
-	Batch createBatch() {
-		return new BatchImpl();
-	}
 }
--- a/test/build.xml
+++ b/test/build.xml
@@ -20,6 +20,8 @@
 			<test name='net.sf.briar.i18n.FontManagerTest'/>
 			<test name='net.sf.briar.i18n.I18nTest'/>
 			<test name='net.sf.briar.invitation.InvitationWorkerTest'/>
+			<test name='net.sf.briar.protocol.BundleReaderTest'/>
+			<test name='net.sf.briar.protocol.BundleWriterTest'/>
 			<test name='net.sf.briar.serial.ReaderImplTest'/>
 			<test name='net.sf.briar.serial.WriterImplTest'/>
 			<test name='net.sf.briar.setup.SetupWorkerTest'/>
--- a/test/net/sf/briar/db/DatabaseComponentImplTest.java
+++ b/test/net/sf/briar/db/DatabaseComponentImplTest.java
@@ -7,6 +7,7 @@ import java.util.Collections;

 import net.sf.briar.api.db.DbException;
 import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.HeaderBuilder;
 import net.sf.briar.api.protocol.MessageId;
 import net.sf.briar.db.DatabaseCleaner.Callback;

@@ -24,6 +25,7 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {

 	protected abstract <T> DatabaseComponentImpl<T> createDatabaseComponentImpl(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider);

 	@Test
@@ -33,14 +35,17 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			oneOf(database).getFreeSpace();
 			will(returnValue(MIN_FREE_SPACE));
 		}});
 		Callback db = createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.checkFreeSpaceAndClean();

@@ -54,8 +59,11 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			oneOf(database).getFreeSpace();
 			will(returnValue(MIN_FREE_SPACE - 1));
@@ -69,7 +77,7 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 			will(returnValue(MIN_FREE_SPACE));
 		}});
 		Callback db = createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.checkFreeSpaceAndClean();

@@ -84,8 +92,11 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			oneOf(database).getFreeSpace();
 			will(returnValue(MIN_FREE_SPACE - 1));
@@ -101,7 +112,7 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 			will(returnValue(MIN_FREE_SPACE));
 		}});
 		Callback db = createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.checkFreeSpaceAndClean();

@@ -116,8 +127,11 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			oneOf(database).getFreeSpace();
 			will(returnValue(MIN_FREE_SPACE - 1));
@@ -135,7 +149,7 @@ public abstract class DatabaseComponentImplTest extends DatabaseComponentTest {
 			will(returnValue(MIN_FREE_SPACE));
 		}});
 		Callback db = createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.checkFreeSpaceAndClean();

--- a/test/net/sf/briar/db/DatabaseComponentTest.java
+++ b/test/net/sf/briar/db/DatabaseComponentTest.java
@@ -1,5 +1,7 @@
 package net.sf.briar.db;

+import java.io.IOException;
+import java.security.SignatureException;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
@@ -20,6 +22,8 @@ import net.sf.briar.api.protocol.Bundle;
 import net.sf.briar.api.protocol.BundleBuilder;
 import net.sf.briar.api.protocol.BundleId;
 import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.HeaderBuilder;
 import net.sf.briar.api.protocol.Message;
 import net.sf.briar.api.protocol.MessageId;
 import net.sf.briar.protocol.MessageImpl;
@@ -32,8 +36,6 @@ import com.google.inject.Provider;

 public abstract class DatabaseComponentTest extends TestCase {

-	private static final int ONE_MEGABYTE = 1024 * 1024;
-
 	protected final Object txn = new Object();
 	protected final AuthorId authorId;
 	protected final BatchId batchId;
@@ -45,6 +47,11 @@ public abstract class DatabaseComponentTest extends TestCase {
 	private final int size;
 	private final byte[] body;
 	private final Message message;
+	private final Set<ContactId> contacts;
+	private final Set<BatchId> acks;
+	private final Set<GroupId> subs;
+	private final Map<String, String> transports;
+	private final Set<MessageId> messages;

 	public DatabaseComponentTest() {
 		super();
@@ -60,26 +67,32 @@ public abstract class DatabaseComponentTest extends TestCase {
 		body = new byte[size];
 		message = new MessageImpl(messageId, MessageId.NONE, groupId, authorId,
 				timestamp, body);
+		contacts = Collections.singleton(contactId);
+		acks = Collections.singleton(batchId);
+		subs = Collections.singleton(groupId);
+		transports = Collections.singletonMap("foo", "bar");
+		messages = Collections.singleton(messageId);
 	}

 	protected abstract <T> DatabaseComponent createDatabaseComponent(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider);

 	@Test
 	public void testSimpleCalls() throws DbException {
-		final Map<String, String> transports =
-			Collections.singletonMap("foo", "bar");
 		final Map<String, String> transports1 =
 			Collections.singletonMap("foo", "bar baz");
-		final Set<GroupId> subs = Collections.singleton(groupId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			allowing(database).startTransaction();
 			will(returnValue(txn));
@@ -95,7 +108,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			will(returnValue(contactId));
 			// getContacts()
 			oneOf(database).getContacts(txn);
-			will(returnValue(Collections.singleton(contactId)));
+			will(returnValue(contacts));
 			// getTransports(contactId)
 			oneOf(database).containsContact(txn, contactId);
 			will(returnValue(true));
@@ -119,16 +132,16 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).close();
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.open(false);
 		assertEquals(Rating.UNRATED, db.getRating(authorId));
 		assertEquals(contactId, db.addContact(transports));
-		assertEquals(Collections.singleton(contactId), db.getContacts());
+		assertEquals(contacts, db.getContacts());
 		assertEquals(transports, db.getTransports(contactId));
 		db.setTransports(contactId, transports1);
 		db.subscribe(groupId);
-		assertEquals(Collections.singleton(groupId), db.getSubscriptions());
+		assertEquals(subs, db.getSubscriptions());
 		db.unsubscribe(groupId);
 		db.removeContact(contactId);
 		db.close();
@@ -138,14 +151,16 @@ public abstract class DatabaseComponentTest extends TestCase {

 	@Test
 	public void testNoParentStopsBackwardInclusion() throws DbException {
-		final Set<MessageId> messages = Collections.singleton(messageId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// setRating(Rating.GOOD)
 			allowing(database).startTransaction();
@@ -163,7 +178,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.setRating(authorId, Rating.GOOD);

@@ -172,14 +187,16 @@ public abstract class DatabaseComponentTest extends TestCase {

 	@Test
 	public void testMissingParentStopsBackwardInclusion() throws DbException {
-		final Set<MessageId> messages = Collections.singleton(messageId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// setRating(Rating.GOOD)
 			oneOf(database).startTransaction();
@@ -200,7 +217,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.setRating(authorId, Rating.GOOD);

@@ -210,14 +227,16 @@ public abstract class DatabaseComponentTest extends TestCase {
 	@Test
 	public void testChangingGroupsStopsBackwardInclusion() throws DbException {
 		final GroupId groupId1 = new GroupId(TestUtils.getRandomId());
-		final Set<MessageId> messages = Collections.singleton(messageId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// setRating(Rating.GOOD)
 			oneOf(database).startTransaction();
@@ -242,7 +261,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.setRating(authorId, Rating.GOOD);

@@ -252,14 +271,16 @@ public abstract class DatabaseComponentTest extends TestCase {
 	@Test
 	public void testUnaffectedParentStopsBackwardInclusion()
 	throws DbException {
-		final Set<MessageId> messages = Collections.singleton(messageId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// setRating(Rating.GOOD)
 			oneOf(database).startTransaction();
@@ -287,7 +308,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.setRating(authorId, Rating.GOOD);

@@ -297,14 +318,16 @@ public abstract class DatabaseComponentTest extends TestCase {
 	@Test
 	public void testAffectedParentContinuesBackwardInclusion()
 	throws DbException {
-		final Set<MessageId> messages = Collections.singleton(messageId);
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// setRating(Rating.GOOD)
 			oneOf(database).startTransaction();
@@ -334,7 +357,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.setRating(authorId, Rating.GOOD);

@@ -349,8 +372,11 @@ public abstract class DatabaseComponentTest extends TestCase {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// addLocallyGeneratedMessage(message)
 			oneOf(database).startTransaction();
@@ -360,7 +386,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.addLocallyGeneratedMessage(message);

@@ -374,8 +400,11 @@ public abstract class DatabaseComponentTest extends TestCase {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// addLocallyGeneratedMessage(message)
 			oneOf(database).startTransaction();
@@ -387,7 +416,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.addLocallyGeneratedMessage(message);

@@ -401,8 +430,11 @@ public abstract class DatabaseComponentTest extends TestCase {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// addLocallyGeneratedMessage(message)
 			oneOf(database).startTransaction();
@@ -412,7 +444,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).addMessage(txn, message);
 			will(returnValue(true));
 			oneOf(database).getContacts(txn);
-			will(returnValue(Collections.singleton(contactId)));
+			will(returnValue(contacts));
 			oneOf(database).setStatus(txn, contactId, messageId, Status.NEW);
 			// The author is unrated and there are no sendable children
 			oneOf(database).getRating(txn, authorId);
@@ -423,7 +455,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.addLocallyGeneratedMessage(message);

@@ -438,8 +470,11 @@ public abstract class DatabaseComponentTest extends TestCase {
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		context.checking(new Expectations() {{
 			// addLocallyGeneratedMessage(message)
 			oneOf(database).startTransaction();
@@ -449,7 +484,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).addMessage(txn, message);
 			will(returnValue(true));
 			oneOf(database).getContacts(txn);
-			will(returnValue(Collections.singleton(contactId)));
+			will(returnValue(contacts));
 			oneOf(database).setStatus(txn, contactId, messageId, Status.NEW);
 			// The author is rated GOOD and there are two sendable children
 			oneOf(database).getRating(txn, authorId);
@@ -463,7 +498,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.addLocallyGeneratedMessage(message);

@@ -472,14 +507,17 @@ public abstract class DatabaseComponentTest extends TestCase {

 	@Test
 	public void testGenerateBundleThrowsExceptionIfContactIsMissing()
-	throws DbException {
+	throws DbException, IOException, SignatureException {
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		final BundleBuilder bundleBuilder = context.mock(BundleBuilder.class);
 		context.checking(new Expectations() {{
 			// Check that the contact is still in the DB
@@ -490,7 +528,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		try {
 			db.generateBundle(contactId, bundleBuilder);
@@ -501,15 +539,22 @@ public abstract class DatabaseComponentTest extends TestCase {
 	}

 	@Test
-	public void testGenerateBundle() throws DbException {
+	public void testGenerateBundle() throws DbException, IOException,
+	SignatureException {
+		final long headerSize = 1234L;
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		final BundleBuilder bundleBuilder = context.mock(BundleBuilder.class);
+		final HeaderBuilder headerBuilder = context.mock(HeaderBuilder.class);
+		final Header header = context.mock(Header.class);
 		final BatchBuilder batchBuilder = context.mock(BatchBuilder.class);
 		final Batch batch = context.mock(Batch.class);
 		final Bundle bundle = context.mock(Bundle.class);
@@ -519,24 +564,33 @@ public abstract class DatabaseComponentTest extends TestCase {
 			allowing(database).commitTransaction(txn);
 			allowing(database).containsContact(txn, contactId);
 			will(returnValue(true));
-			// Add acks to the bundle
+			// Build the header
+			oneOf(headerBuilderProvider).get();
+			will(returnValue(headerBuilder));
+			// Add acks to the header
 			oneOf(database).removeBatchesToAck(txn, contactId);
-			will(returnValue(Collections.singleton(batchId)));
-			oneOf(bundleBuilder).addAck(batchId);
-			// Add subscriptions to the bundle
+			will(returnValue(acks));
+			oneOf(headerBuilder).addAcks(acks);
+			// Add subscriptions to the header
 			oneOf(database).getSubscriptions(txn);
-			will(returnValue(Collections.singleton(groupId)));
-			oneOf(bundleBuilder).addSubscription(groupId);
-			// Add transports to the bundle
+			will(returnValue(subs));
+			oneOf(headerBuilder).addSubscriptions(subs);
+			// Add transports to the header
 			oneOf(database).getTransports(txn);
-			will(returnValue(Collections.singletonMap("foo", "bar")));
-			oneOf(bundleBuilder).addTransport("foo", "bar");
-			// Prepare to add batches to the bundle
+			will(returnValue(transports));
+			oneOf(headerBuilder).addTransports(transports);
+			// Build the header
+			oneOf(headerBuilder).build();
+			will(returnValue(header));
 			oneOf(bundleBuilder).getCapacity();
-			will(returnValue((long) ONE_MEGABYTE));
-			// Add messages to the batch
-			oneOf(database).getSendableMessages(txn, contactId, Batch.CAPACITY);
-			will(returnValue(Collections.singleton(messageId)));
+			will(returnValue(1024L * 1024L));
+			oneOf(header).getSize();
+			will(returnValue(headerSize));
+			oneOf(bundleBuilder).addHeader(header);
+			// Add a batch to the bundle
+			oneOf(database).getSendableMessages(txn, contactId,
+					Batch.MAX_SIZE - headerSize);
+			will(returnValue(messages));
 			oneOf(batchBuilderProvider).get();
 			will(returnValue(batchBuilder));
 			oneOf(database).getMessage(txn, messageId);
@@ -547,8 +601,8 @@ public abstract class DatabaseComponentTest extends TestCase {
 			// Record the batch as outstanding
 			oneOf(batch).getId();
 			will(returnValue(batchId));
-			oneOf(database).addOutstandingBatch(txn, contactId, batchId,
-					Collections.singleton(messageId));
+			oneOf(database).addOutstandingBatch(
+					txn, contactId, batchId, messages);
 			// Add the batch to the bundle
 			oneOf(bundleBuilder).addBatch(batch);
 			// Check whether to add another batch
@@ -559,7 +613,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			will(returnValue(bundle));
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.generateBundle(contactId, bundleBuilder);

@@ -568,14 +622,17 @@ public abstract class DatabaseComponentTest extends TestCase {

 	@Test
 	public void testReceiveBundleThrowsExceptionIfContactIsMissing()
-	throws DbException {
+	throws DbException, IOException, SignatureException {
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		final Bundle bundle = context.mock(Bundle.class);
 		context.checking(new Expectations() {{
 			// Check that the contact is still in the DB
@@ -586,7 +643,7 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(database).commitTransaction(txn);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		try {
 			db.receiveBundle(contactId, bundle);
@@ -597,17 +654,20 @@ public abstract class DatabaseComponentTest extends TestCase {
 	}

 	@Test
-	public void testReceivedBundle() throws DbException {
-		final Map<String, String> transports =
-			Collections.singletonMap("foo", "bar");
+	public void testReceivedBundle() throws DbException, IOException,
+	SignatureException {
 		Mockery context = new Mockery();
 		@SuppressWarnings("unchecked")
 		final Database<Object> database = context.mock(Database.class);
 		final DatabaseCleaner cleaner = context.mock(DatabaseCleaner.class);
 		@SuppressWarnings("unchecked")
-		final Provider<BatchBuilder> batchBuilderProvider =
+		final Provider<HeaderBuilder> headerBuilderProvider =
 			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
 		final Bundle bundle = context.mock(Bundle.class);
+		final Header header = context.mock(Header.class);
 		final Batch batch = context.mock(Batch.class);
 		context.checking(new Expectations() {{
 			allowing(database).startTransaction();
@@ -615,22 +675,25 @@ public abstract class DatabaseComponentTest extends TestCase {
 			allowing(database).commitTransaction(txn);
 			allowing(database).containsContact(txn, contactId);
 			will(returnValue(true));
+			// Header
+			oneOf(bundle).getHeader();
+			will(returnValue(header));
 			// Acks
-			oneOf(bundle).getAcks();
-			will(returnValue(Collections.singleton(batchId)));
+			oneOf(header).getAcks();
+			will(returnValue(acks));
 			oneOf(database).removeAckedBatch(txn, contactId, batchId);
 			// Subscriptions
 			oneOf(database).clearSubscriptions(txn, contactId);
-			oneOf(bundle).getSubscriptions();
-			will(returnValue(Collections.singleton(groupId)));
+			oneOf(header).getSubscriptions();
+			will(returnValue(subs));
 			oneOf(database).addSubscription(txn, contactId, groupId);
 			// Transports
-			oneOf(bundle).getTransports();
+			oneOf(header).getTransports();
 			will(returnValue(transports));
 			oneOf(database).setTransports(txn, contactId, transports);
 			// Batches
-			oneOf(bundle).getBatches();
-			will(returnValue(Collections.singleton(batch)));
+			oneOf(bundle).getNextBatch();
+			will(returnValue(batch));
 			oneOf(batch).getMessages();
 			will(returnValue(Collections.singleton(message)));
 			oneOf(database).containsSubscription(txn, groupId);
@@ -642,15 +705,18 @@ public abstract class DatabaseComponentTest extends TestCase {
 			oneOf(batch).getId();
 			will(returnValue(batchId));
 			oneOf(database).addBatchToAck(txn, contactId, batchId);
+			// Any more batches? Nope
+			oneOf(bundle).getNextBatch();
+			will(returnValue(null));
 			// Lost batches
-			oneOf(bundle).getId();
+			oneOf(header).getId();
 			will(returnValue(bundleId));
 			oneOf(database).addReceivedBundle(txn, contactId, bundleId);
 			will(returnValue(Collections.singleton(batchId)));
 			oneOf(database).removeLostBatch(txn, contactId, batchId);
 		}});
 		DatabaseComponent db = createDatabaseComponent(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);

 		db.receiveBundle(contactId, bundle);

--- a/test/net/sf/briar/db/ReadWriteLockDatabaseComponentTest.java
+++ b/test/net/sf/briar/db/ReadWriteLockDatabaseComponentTest.java
@@ -2,6 +2,7 @@ package net.sf.briar.db;

 import net.sf.briar.api.db.DatabaseComponent;
 import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.HeaderBuilder;

 import com.google.inject.Provider;

@@ -11,16 +12,18 @@ extends DatabaseComponentImplTest {
 	@Override
 	protected <T> DatabaseComponent createDatabaseComponent(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
 		return createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);
 	}

 	@Override
 	protected <T> DatabaseComponentImpl<T> createDatabaseComponentImpl(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
 		return new ReadWriteLockDatabaseComponent<T>(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);
 	}
 }
--- a/test/net/sf/briar/db/SynchronizedDatabaseComponentTest.java
+++ b/test/net/sf/briar/db/SynchronizedDatabaseComponentTest.java
@@ -2,6 +2,7 @@ package net.sf.briar.db;

 import net.sf.briar.api.db.DatabaseComponent;
 import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.HeaderBuilder;

 import com.google.inject.Provider;

@@ -11,16 +12,18 @@ extends DatabaseComponentImplTest {
 	@Override
 	protected <T> DatabaseComponent createDatabaseComponent(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
 		return createDatabaseComponentImpl(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);
 	}

 	@Override
 	protected <T> DatabaseComponentImpl<T> createDatabaseComponentImpl(
 			Database<T> database, DatabaseCleaner cleaner,
+			Provider<HeaderBuilder> headerBuilderProvider,
 			Provider<BatchBuilder> batchBuilderProvider) {
 		return new SynchronizedDatabaseComponent<T>(database, cleaner,
-				batchBuilderProvider);
+				headerBuilderProvider, batchBuilderProvider);
 	}
 }
--- a/test/net/sf/briar/protocol/BundleReaderTest.java
+++ b/test/net/sf/briar/protocol/BundleReaderTest.java
@@ -0,0 +1,251 @@
+package net.sf.briar.protocol;
+
+import java.io.IOException;
+import java.security.SignatureException;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import junit.framework.TestCase;
+import net.sf.briar.TestUtils;
+import net.sf.briar.api.protocol.Batch;
+import net.sf.briar.api.protocol.BatchBuilder;
+import net.sf.briar.api.protocol.BatchId;
+import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.HeaderBuilder;
+import net.sf.briar.api.protocol.Message;
+import net.sf.briar.api.protocol.MessageParser;
+import net.sf.briar.api.serial.Raw;
+import net.sf.briar.api.serial.Reader;
+
+import org.jmock.Expectations;
+import org.jmock.Mockery;
+import org.junit.Test;
+
+import com.google.inject.Provider;
+
+public class BundleReaderTest extends TestCase {
+
+	private final long size = 1024L * 1024L;
+	private final BatchId ack = new BatchId(TestUtils.getRandomId());
+	private final List<Raw> rawAcks =
+		Collections.<Raw>singletonList(new TestRaw(ack.getBytes()));
+	private final Set<BatchId> acks = Collections.singleton(ack);
+	private final GroupId sub = new GroupId(TestUtils.getRandomId());
+	private final List<Raw> rawSubs =
+		Collections.<Raw>singletonList(new TestRaw(sub.getBytes()));
+	private final Set<GroupId> subs = Collections.singleton(sub);
+	private final Map<String, String> transports =
+		Collections.singletonMap("foo", "bar");
+	private final byte[] headerSig = TestUtils.getRandomId();
+	private final byte[] messageBody = new byte[123];
+	private final List<Raw> rawMessages =
+		Collections.<Raw>singletonList(new TestRaw(messageBody));
+	private final byte[] batchSig = TestUtils.getRandomId();
+
+	@Test
+	public void testGetHeader() throws IOException, SignatureException {
+		Mockery context = new Mockery();
+		final Reader reader = context.mock(Reader.class);
+		final MessageParser messageParser = context.mock(MessageParser.class);
+		@SuppressWarnings("unchecked")
+		final Provider<HeaderBuilder> headerBuilderProvider =
+			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
+		final HeaderBuilder headerBuilder = context.mock(HeaderBuilder.class);
+		final Header header = context.mock(Header.class);
+		context.checking(new Expectations() {{
+			oneOf(reader).setReadLimit(Header.MAX_SIZE);
+			oneOf(headerBuilderProvider).get();
+			will(returnValue(headerBuilder));
+			// Acks
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawAcks));
+			oneOf(headerBuilder).addAcks(acks);
+			// Subs
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawSubs));
+			oneOf(headerBuilder).addSubscriptions(subs);
+			// Transports
+			oneOf(reader).readMap(String.class, String.class);
+			will(returnValue(transports));
+			oneOf(headerBuilder).addTransports(transports);
+			// Signature
+			oneOf(reader).readRaw();
+			will(returnValue(headerSig));
+			oneOf(headerBuilder).setSignature(headerSig);
+			// Build the header
+			oneOf(headerBuilder).build();
+			will(returnValue(header));
+		}});
+		BundleReader r = createBundleReader(reader, messageParser,
+				headerBuilderProvider, batchBuilderProvider);
+
+		assertEquals(header, r.getHeader());
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testBatchBeforeHeaderThrowsException() throws IOException,
+	SignatureException {
+		Mockery context = new Mockery();
+		final Reader reader = context.mock(Reader.class);
+		final MessageParser messageParser = context.mock(MessageParser.class);
+		@SuppressWarnings("unchecked")
+		final Provider<HeaderBuilder> headerBuilderProvider =
+			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
+		BundleReader r = createBundleReader(reader, messageParser,
+				headerBuilderProvider, batchBuilderProvider);
+
+		try {
+			r.getNextBatch();
+			assertTrue(false);
+		} catch(IllegalStateException expected) {}
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testGetHeaderNoBatches() throws IOException,
+	SignatureException {
+		Mockery context = new Mockery();
+		final Reader reader = context.mock(Reader.class);
+		final MessageParser messageParser = context.mock(MessageParser.class);
+		@SuppressWarnings("unchecked")
+		final Provider<HeaderBuilder> headerBuilderProvider =
+			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
+		final HeaderBuilder headerBuilder = context.mock(HeaderBuilder.class);
+		final Header header = context.mock(Header.class);
+		context.checking(new Expectations() {{
+			oneOf(reader).setReadLimit(Header.MAX_SIZE);
+			oneOf(headerBuilderProvider).get();
+			will(returnValue(headerBuilder));
+			// Acks
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawAcks));
+			oneOf(headerBuilder).addAcks(acks);
+			// Subs
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawSubs));
+			oneOf(headerBuilder).addSubscriptions(subs);
+			// Transports
+			oneOf(reader).readMap(String.class, String.class);
+			will(returnValue(transports));
+			oneOf(headerBuilder).addTransports(transports);
+			// Signature
+			oneOf(reader).readRaw();
+			will(returnValue(headerSig));
+			oneOf(headerBuilder).setSignature(headerSig);
+			// Build the header
+			oneOf(headerBuilder).build();
+			will(returnValue(header));
+			// No batches
+			oneOf(reader).readListStart();
+			oneOf(reader).hasListEnd();
+			will(returnValue(true));
+			oneOf(reader).readListEnd();
+		}});
+		BundleReader r = createBundleReader(reader, messageParser,
+				headerBuilderProvider, batchBuilderProvider);
+
+		assertEquals(header, r.getHeader());
+		assertNull(r.getNextBatch());
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testGetHeaderOneBatch() throws IOException,
+	SignatureException {
+		Mockery context = new Mockery();
+		final Reader reader = context.mock(Reader.class);
+		final MessageParser messageParser = context.mock(MessageParser.class);
+		@SuppressWarnings("unchecked")
+		final Provider<HeaderBuilder> headerBuilderProvider =
+			context.mock(Provider.class);
+		@SuppressWarnings("unchecked")
+		final Provider<BatchBuilder> batchBuilderProvider =
+			context.mock(Provider.class, "batchBuilderProvider");
+		final HeaderBuilder headerBuilder = context.mock(HeaderBuilder.class);
+		final Header header = context.mock(Header.class);
+		final BatchBuilder batchBuilder = context.mock(BatchBuilder.class);
+		final Batch batch = context.mock(Batch.class);
+		final Message message = context.mock(Message.class);
+		context.checking(new Expectations() {{
+			oneOf(reader).setReadLimit(Header.MAX_SIZE);
+			oneOf(headerBuilderProvider).get();
+			will(returnValue(headerBuilder));
+			// Acks
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawAcks));
+			oneOf(headerBuilder).addAcks(acks);
+			// Subs
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawSubs));
+			oneOf(headerBuilder).addSubscriptions(subs);
+			// Transports
+			oneOf(reader).readMap(String.class, String.class);
+			will(returnValue(transports));
+			oneOf(headerBuilder).addTransports(transports);
+			// Signature
+			oneOf(reader).readRaw();
+			will(returnValue(headerSig));
+			oneOf(headerBuilder).setSignature(headerSig);
+			// Build the header
+			oneOf(headerBuilder).build();
+			will(returnValue(header));
+			// First batch
+			oneOf(reader).readListStart();
+			oneOf(reader).hasListEnd();
+			will(returnValue(false));
+			oneOf(reader).setReadLimit(Batch.MAX_SIZE);
+			oneOf(batchBuilderProvider).get();
+			will(returnValue(batchBuilder));
+			oneOf(reader).readList(Raw.class);
+			will(returnValue(rawMessages));
+			oneOf(messageParser).parseMessage(messageBody);
+			will(returnValue(message));
+			oneOf(batchBuilder).addMessage(message);
+			oneOf(reader).readRaw();
+			will(returnValue(batchSig));
+			oneOf(batchBuilder).setSignature(batchSig);
+			oneOf(batchBuilder).build();
+			will(returnValue(batch));
+			// No more batches
+			oneOf(reader).hasListEnd();
+			will(returnValue(true));
+			oneOf(reader).readListEnd();
+		}});
+		BundleReader r = createBundleReader(reader, messageParser,
+				headerBuilderProvider, batchBuilderProvider);
+
+		assertEquals(header, r.getHeader());
+		assertEquals(batch, r.getNextBatch());
+		assertNull(r.getNextBatch());
+
+		context.assertIsSatisfied();
+	}
+
+	private BundleReader createBundleReader(Reader reader,
+			MessageParser messageParser,
+			Provider<HeaderBuilder> headerBuilderProvider,
+			Provider<BatchBuilder> batchBuilderProvider) {
+		return new BundleReader(reader, messageParser, headerBuilderProvider,
+				batchBuilderProvider) {
+			public long getSize() {
+				return size;
+			}
+		};
+	}
+}
--- a/test/net/sf/briar/protocol/BundleWriterTest.java
+++ b/test/net/sf/briar/protocol/BundleWriterTest.java
@@ -0,0 +1,242 @@
+package net.sf.briar.protocol;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Map;
+import java.util.Set;
+
+import junit.framework.TestCase;
+import net.sf.briar.TestUtils;
+import net.sf.briar.api.protocol.Batch;
+import net.sf.briar.api.protocol.BatchId;
+import net.sf.briar.api.protocol.Bundle;
+import net.sf.briar.api.protocol.GroupId;
+import net.sf.briar.api.protocol.Header;
+import net.sf.briar.api.protocol.Message;
+import net.sf.briar.api.serial.Writer;
+
+import org.jmock.Expectations;
+import org.jmock.Mockery;
+import org.junit.Test;
+
+public class BundleWriterTest extends TestCase {
+
+	private final BatchId ack = new BatchId(TestUtils.getRandomId());
+	private final Set<BatchId> acks = Collections.singleton(ack);
+	private final GroupId sub = new GroupId(TestUtils.getRandomId());
+	private final Set<GroupId> subs = Collections.singleton(sub);
+	private final Map<String, String> transports =
+		Collections.singletonMap("foo", "bar");
+	private final byte[] headerSig = TestUtils.getRandomId();
+	private final long capacity = 1024L * 1024L;
+	private final byte[] messageBody = new byte[123];
+	private final byte[] batchSig = TestUtils.getRandomId();
+
+	@Test
+	public void testAddHeader() throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		final Header header = context.mock(Header.class);
+		context.checking(new Expectations() {{
+			// Acks
+			oneOf(writer).writeListStart();
+			oneOf(header).getAcks();
+			will(returnValue(acks));
+			oneOf(writer).writeRaw(ack);
+			oneOf(writer).writeListEnd();
+			// Subs
+			oneOf(writer).writeListStart();
+			oneOf(header).getSubscriptions();
+			will(returnValue(subs));
+			oneOf(writer).writeRaw(sub);
+			oneOf(writer).writeListEnd();
+			// Transports
+			oneOf(header).getTransports();
+			will(returnValue(transports));
+			oneOf(writer).writeMap(transports);
+			// Signature
+			oneOf(header).getSignature();
+			will(returnValue(headerSig));
+			oneOf(writer).writeRaw(headerSig);
+		}});
+		BundleWriter w = createBundleWriter(writer);
+
+		w.addHeader(header);
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testAddHeaderEmptyLists() throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		final Header header = context.mock(Header.class);
+		context.checking(new Expectations() {{
+			// Acks
+			oneOf(writer).writeListStart();
+			oneOf(header).getAcks();
+			will(returnValue(Collections.emptySet()));
+			oneOf(writer).writeListEnd();
+			// Subs
+			oneOf(writer).writeListStart();
+			oneOf(header).getSubscriptions();
+			will(returnValue(Collections.emptySet()));
+			oneOf(writer).writeListEnd();
+			// Transports
+			oneOf(header).getTransports();
+			will(returnValue(Collections.emptyMap()));
+			oneOf(writer).writeMap(Collections.emptyMap());
+			// Signature
+			oneOf(header).getSignature();
+			will(returnValue(headerSig));
+			oneOf(writer).writeRaw(headerSig);
+		}});
+		BundleWriter w = createBundleWriter(writer);
+
+		w.addHeader(header);
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testBatchBeforeHeaderThrowsException() throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		final Batch batch = context.mock(Batch.class);
+		BundleWriter w = createBundleWriter(writer);
+
+		try {
+			w.addBatch(batch);
+			assertTrue(false);
+		} catch(IllegalStateException expected) {}
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testCloseBeforeHeaderThrowsException() throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		BundleWriter w = createBundleWriter(writer);
+
+		try {
+			w.close();
+			assertTrue(false);
+		} catch(IllegalStateException expected) {}
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testCloseWithoutBatchesDoesNotThrowException()
+	throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		final Header header = context.mock(Header.class);
+		context.checking(new Expectations() {{
+			// Acks
+			oneOf(writer).writeListStart();
+			oneOf(header).getAcks();
+			will(returnValue(acks));
+			oneOf(writer).writeRaw(ack);
+			oneOf(writer).writeListEnd();
+			// Subs
+			oneOf(writer).writeListStart();
+			oneOf(header).getSubscriptions();
+			will(returnValue(subs));
+			oneOf(writer).writeRaw(sub);
+			oneOf(writer).writeListEnd();
+			// Transports
+			oneOf(header).getTransports();
+			will(returnValue(transports));
+			oneOf(writer).writeMap(transports);
+			// Signature
+			oneOf(header).getSignature();
+			will(returnValue(headerSig));
+			oneOf(writer).writeRaw(headerSig);
+			// Close - write an empty list of batches
+			oneOf(writer).writeListStart();
+			oneOf(writer).writeListEnd();
+			oneOf(writer).close();
+		}});
+		BundleWriter w = createBundleWriter(writer);
+
+		w.addHeader(header);
+		w.close();
+
+		context.assertIsSatisfied();
+	}
+
+	@Test
+	public void testAddHeaderAndTwoBatches() throws IOException {
+		Mockery context = new Mockery();
+		final Writer writer = context.mock(Writer.class);
+		final Header header = context.mock(Header.class);
+		final Batch batch = context.mock(Batch.class);
+		final Message message = context.mock(Message.class);
+		context.checking(new Expectations() {{
+			// Acks
+			oneOf(writer).writeListStart();
+			oneOf(header).getAcks();
+			will(returnValue(acks));
+			oneOf(writer).writeRaw(ack);
+			oneOf(writer).writeListEnd();
+			// Subs
+			oneOf(writer).writeListStart();
+			oneOf(header).getSubscriptions();
+			will(returnValue(subs));
+			oneOf(writer).writeRaw(sub);
+			oneOf(writer).writeListEnd();
+			// Transports
+			oneOf(header).getTransports();
+			will(returnValue(transports));
+			oneOf(writer).writeMap(transports);
+			// Signature
+			oneOf(header).getSignature();
+			will(returnValue(headerSig));
+			oneOf(writer).writeRaw(headerSig);
+			// First batch
+			oneOf(writer).writeListStart();
+			oneOf(writer).writeListStart();
+			oneOf(batch).getMessages();
+			will(returnValue(Collections.singleton(message)));
+			oneOf(message).getBody();
+			will(returnValue(messageBody));
+			oneOf(writer).writeRaw(messageBody);
+			oneOf(writer).writeListEnd();
+			oneOf(batch).getSignature();
+			will(returnValue(batchSig));
+			oneOf(writer).writeRaw(batchSig);
+			// Second batch
+			oneOf(writer).writeListStart();
+			oneOf(batch).getMessages();
+			will(returnValue(Collections.singleton(message)));
+			oneOf(message).getBody();
+			will(returnValue(messageBody));
+			oneOf(writer).writeRaw(messageBody);
+			oneOf(writer).writeListEnd();
+			oneOf(batch).getSignature();
+			will(returnValue(batchSig));
+			oneOf(writer).writeRaw(batchSig);
+			// Close
+			oneOf(writer).writeListEnd();
+			oneOf(writer).close();
+		}});
+		BundleWriter w = createBundleWriter(writer);
+
+		w.addHeader(header);
+		w.addBatch(batch);
+		w.addBatch(batch);
+		w.close();
+
+		context.assertIsSatisfied();
+	}
+
+	private BundleWriter createBundleWriter(Writer writer) {
+		return new BundleWriter(writer, capacity) {
+			public Bundle build() throws IOException {
+				return null;
+			}
+		};
+	}
+}
--- a/test/net/sf/briar/protocol/TestRaw.java
+++ b/test/net/sf/briar/protocol/TestRaw.java
@@ -0,0 +1,29 @@
+package net.sf.briar.protocol;
+
+import java.util.Arrays;
+
+import net.sf.briar.api.serial.Raw;
+
+class TestRaw implements Raw {
+
+	private final byte[] bytes;
+
+	TestRaw(byte[] bytes) {
+		this.bytes = bytes;
+	}
+
+	public byte[] getBytes() {
+		return bytes;
+	}
+
+	@Override
+	public int hashCode() {
+		return Arrays.hashCode(bytes);
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if(o instanceof Raw) return Arrays.equals(bytes, ((Raw) o).getBytes());
+		return false;
+	}
+}