Add transport key agreement client.

2026-02-12 10:49:06 +01:00 · 2021-05-17 16:24:41 +01:00
parent 6e6cadd3ad
commit e228b9fcbf
19 changed files with 1708 additions and 0 deletions
--- a/bramble-api/src/main/java/org/briarproject/bramble/api/transport/agreement/TransportKeyAgreementManager.java
+++ b/bramble-api/src/main/java/org/briarproject/bramble/api/transport/agreement/TransportKeyAgreementManager.java
@@ -0,0 +1,24 @@
+package org.briarproject.bramble.api.transport.agreement;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+
+@NotNullByDefault
+public interface TransportKeyAgreementManager {
+
+	/**
+	 * The unique ID of the transport key agreement client.
+	 */
+	ClientId CLIENT_ID =
+			new ClientId("org.briarproject.bramble.transport.agreement");
+
+	/**
+	 * The current major version of the transport key agreement client.
+	 */
+	int MAJOR_VERSION = 0;
+
+	/**
+	 * The current minor version of the transport key agreement client.
+	 */
+	int MINOR_VERSION = 0;
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreEagerSingletons.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreEagerSingletons.java
@@ -11,6 +11,7 @@ import org.briarproject.bramble.properties.PropertiesModule;
 import org.briarproject.bramble.rendezvous.RendezvousModule;
 import org.briarproject.bramble.sync.validation.ValidationModule;
 import org.briarproject.bramble.transport.TransportModule;
+import org.briarproject.bramble.transport.agreement.TransportKeyAgreementModule;
 import org.briarproject.bramble.versioning.VersioningModule;

 public interface BrambleCoreEagerSingletons {
@@ -33,6 +34,8 @@ public interface BrambleCoreEagerSingletons {

 	void inject(RendezvousModule.EagerSingletons init);

+	void inject(TransportKeyAgreementModule.EagerSingletons init);
+
 	void inject(TransportModule.EagerSingletons init);

 	void inject(ValidationModule.EagerSingletons init);
@@ -51,6 +54,7 @@ public interface BrambleCoreEagerSingletons {
 			c.inject(new RendezvousModule.EagerSingletons());
 			c.inject(new PluginModule.EagerSingletons());
 			c.inject(new PropertiesModule.EagerSingletons());
+			c.inject(new TransportKeyAgreementModule.EagerSingletons());
 			c.inject(new TransportModule.EagerSingletons());
 			c.inject(new ValidationModule.EagerSingletons());
 			c.inject(new VersioningModule.EagerSingletons());
--- a/bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreModule.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/BrambleCoreModule.java
@@ -23,6 +23,7 @@ import org.briarproject.bramble.settings.SettingsModule;
 import org.briarproject.bramble.sync.SyncModule;
 import org.briarproject.bramble.sync.validation.ValidationModule;
 import org.briarproject.bramble.transport.TransportModule;
+import org.briarproject.bramble.transport.agreement.TransportKeyAgreementModule;
 import org.briarproject.bramble.versioning.VersioningModule;

 import dagger.Module;
@@ -49,6 +50,7 @@ import dagger.Module;
 		RendezvousModule.class,
 		SettingsModule.class,
 		SyncModule.class,
+		TransportKeyAgreementModule.class,
 		TransportModule.class,
 		ValidationModule.class,
 		VersioningModule.class
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoder.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoder.java
@@ -0,0 +1,22 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+
+@NotNullByDefault
+interface MessageEncoder {
+
+	Message encodeKeyMessage(GroupId contactGroupId,
+			TransportId transportId, PublicKey publicKey);
+
+	Message encodeActivateMessage(GroupId contactGroupId,
+			TransportId transportId, MessageId previousMessageId);
+
+	BdfDictionary encodeMessageMetadata(TransportId transportId,
+			MessageType type, boolean local);
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoderImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageEncoderImpl.java
@@ -0,0 +1,77 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_IS_SESSION;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_LOCAL;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+
+@Immutable
+@NotNullByDefault
+class MessageEncoderImpl implements MessageEncoder {
+
+	private final ClientHelper clientHelper;
+	private final Clock clock;
+
+	@Inject
+	MessageEncoderImpl(ClientHelper clientHelper, Clock clock) {
+		this.clientHelper = clientHelper;
+		this.clock = clock;
+	}
+
+	@Override
+	public Message encodeKeyMessage(GroupId contactGroupId,
+			TransportId transportId, PublicKey publicKey) {
+		BdfList body = BdfList.of(
+				KEY.getValue(),
+				transportId.getString(),
+				publicKey.getEncoded());
+		return encodeMessage(contactGroupId, body);
+	}
+
+	@Override
+	public Message encodeActivateMessage(GroupId contactGroupId,
+			TransportId transportId, MessageId previousMessageId) {
+		BdfList body = BdfList.of(
+				ACTIVATE.getValue(),
+				transportId.getString(),
+				previousMessageId);
+		return encodeMessage(contactGroupId, body);
+	}
+
+	@Override
+	public BdfDictionary encodeMessageMetadata(TransportId transportId,
+			MessageType type, boolean local) {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_IS_SESSION, false),
+				new BdfEntry(MSG_KEY_TRANSPORT_ID, transportId.getString()),
+				new BdfEntry(MSG_KEY_MESSAGE_TYPE, type.getValue()),
+				new BdfEntry(MSG_KEY_LOCAL, local));
+	}
+
+	private Message encodeMessage(GroupId contactGroupId, BdfList body) {
+		try {
+			return clientHelper.createMessage(contactGroupId,
+					clock.currentTimeMillis(), clientHelper.toByteArray(body));
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageType.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/MessageType.java
@@ -0,0 +1,29 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum MessageType {
+
+	KEY(0),
+	ACTIVATE(1);
+
+	private final int value;
+
+	MessageType(int value) {
+		this.value = value;
+	}
+
+	int getValue() {
+		return value;
+	}
+
+	static MessageType fromValue(int value) throws FormatException {
+		for (MessageType t : values()) if (t.value == value) return t;
+		throw new FormatException();
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/Session.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/Session.java
@@ -0,0 +1,65 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+class Session {
+
+	private final State state;
+	@Nullable
+	private final MessageId lastLocalMessageId;
+	@Nullable
+	private final KeyPair localKeyPair;
+	@Nullable
+	private final Long localTimestamp, remoteTimestamp;
+	@Nullable
+	private final KeySetId keySetId;
+
+	Session(State state, @Nullable MessageId lastLocalMessageId,
+			@Nullable KeyPair localKeyPair,
+			@Nullable Long localTimestamp, @Nullable Long remoteTimestamp,
+			@Nullable KeySetId keySetId) {
+		this.state = state;
+		this.lastLocalMessageId = lastLocalMessageId;
+		this.localKeyPair = localKeyPair;
+		this.localTimestamp = localTimestamp;
+		this.remoteTimestamp = remoteTimestamp;
+		this.keySetId = keySetId;
+	}
+
+	State getState() {
+		return state;
+	}
+
+	@Nullable
+	MessageId getLastLocalMessageId() {
+		return lastLocalMessageId;
+	}
+
+	@Nullable
+	KeyPair getLocalKeyPair() {
+		return localKeyPair;
+	}
+
+	@Nullable
+	Long getLocalTimestamp() {
+		return localTimestamp;
+	}
+
+	@Nullable
+	Long getRemoteTimestamp() {
+		return remoteTimestamp;
+	}
+
+	@Nullable
+	KeySetId getKeySetId() {
+		return keySetId;
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoder.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoder.java
@@ -0,0 +1,13 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+
+@NotNullByDefault
+interface SessionEncoder {
+
+	BdfDictionary encodeSession(Session s, TransportId transportId);
+
+	BdfDictionary getSessionQuery(TransportId transportId);
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoderImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionEncoderImpl.java
@@ -0,0 +1,70 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.data.BdfDictionary.NULL_VALUE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_IS_SESSION;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_KEY_SET_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PRIVATE_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_REMOTE_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionEncoderImpl implements SessionEncoder {
+
+	@Inject
+	SessionEncoderImpl() {
+	}
+
+	@Override
+	public BdfDictionary encodeSession(Session s, TransportId transportId) {
+		BdfDictionary meta = new BdfDictionary();
+		meta.put(MSG_KEY_IS_SESSION, true);
+		meta.put(MSG_KEY_TRANSPORT_ID, transportId.getString());
+		meta.put(SESSION_KEY_STATE, s.getState().getValue());
+		putNullable(meta, SESSION_KEY_LAST_LOCAL_MESSAGE_ID,
+				s.getLastLocalMessageId());
+		KeyPair localKeyPair = s.getLocalKeyPair();
+		if (localKeyPair == null) {
+			meta.put(SESSION_KEY_LOCAL_PUBLIC_KEY, NULL_VALUE);
+			meta.put(SESSION_KEY_LOCAL_PRIVATE_KEY, NULL_VALUE);
+		} else {
+			meta.put(SESSION_KEY_LOCAL_PUBLIC_KEY,
+					localKeyPair.getPublic().getEncoded());
+			meta.put(SESSION_KEY_LOCAL_PRIVATE_KEY,
+					localKeyPair.getPrivate().getEncoded());
+		}
+		putNullable(meta, SESSION_KEY_LOCAL_TIMESTAMP, s.getLocalTimestamp());
+		putNullable(meta, SESSION_KEY_REMOTE_TIMESTAMP, s.getRemoteTimestamp());
+		KeySetId keySetId = s.getKeySetId();
+		if (keySetId == null) meta.put(SESSION_KEY_KEY_SET_ID, NULL_VALUE);
+		else meta.put(SESSION_KEY_KEY_SET_ID, keySetId.getInt());
+		return meta;
+	}
+
+	@Override
+	public BdfDictionary getSessionQuery(TransportId transportId) {
+		return BdfDictionary.of(
+				new BdfEntry(MSG_KEY_IS_SESSION, true),
+				new BdfEntry(MSG_KEY_TRANSPORT_ID, transportId.getString()));
+	}
+
+	private void putNullable(BdfDictionary meta, String key,
+			@Nullable Object o) {
+		meta.put(key, o == null ? NULL_VALUE : o);
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParser.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParser.java
@@ -0,0 +1,11 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface SessionParser {
+
+	Session parseSession(BdfDictionary meta) throws FormatException;
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParserImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/SessionParserImpl.java
@@ -0,0 +1,70 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeySetId;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_KEY_SET_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LAST_LOCAL_MESSAGE_ID;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PRIVATE_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_LOCAL_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_REMOTE_TIMESTAMP;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.SESSION_KEY_STATE;
+
+@Immutable
+@NotNullByDefault
+class SessionParserImpl implements SessionParser {
+
+	private final TransportKeyAgreementCrypto crypto;
+
+	@Inject
+	SessionParserImpl(TransportKeyAgreementCrypto crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public Session parseSession(BdfDictionary meta) throws FormatException {
+		State state =
+				State.fromValue(meta.getLong(SESSION_KEY_STATE).intValue());
+
+		MessageId lastLocalMessageId = null;
+		byte[] lastLocalMessageIdBytes =
+				meta.getOptionalRaw(SESSION_KEY_LAST_LOCAL_MESSAGE_ID);
+		if (lastLocalMessageIdBytes != null) {
+			lastLocalMessageId = new MessageId(lastLocalMessageIdBytes);
+		}
+
+		KeyPair localKeyPair = null;
+		byte[] localPublicKeyBytes =
+				meta.getOptionalRaw(SESSION_KEY_LOCAL_PUBLIC_KEY);
+		byte[] localPrivateKeyBytes =
+				meta.getOptionalRaw(SESSION_KEY_LOCAL_PRIVATE_KEY);
+		if (localPublicKeyBytes != null && localPrivateKeyBytes != null) {
+			PublicKey pub = crypto.parsePublicKey(localPublicKeyBytes);
+			PrivateKey priv = crypto.parsePrivateKey(localPrivateKeyBytes);
+			localKeyPair = new KeyPair(pub, priv);
+		}
+
+		Long localTimestamp = meta.getOptionalLong(SESSION_KEY_LOCAL_TIMESTAMP);
+		Long remoteTimestamp =
+				meta.getOptionalLong(SESSION_KEY_REMOTE_TIMESTAMP);
+
+		KeySetId keySetId = null;
+		Long keySetIdLong = meta.getOptionalLong(SESSION_KEY_KEY_SET_ID);
+		if (keySetIdLong != null) {
+			keySetId = new KeySetId(keySetIdLong.intValue());
+		}
+
+		return new Session(state, lastLocalMessageId, localKeyPair,
+				localTimestamp, remoteTimestamp, keySetId);
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/State.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/State.java
@@ -0,0 +1,43 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+enum State {
+
+	/**
+	 * We've sent a key message and are awaiting the contact's key message.
+	 */
+	AWAIT_KEY(0),
+
+	/**
+	 * We've exchanged key messages, derived the transport keys and sent an
+	 * activate message, and now we're awaiting the contact's activate message.
+	 */
+	AWAIT_ACTIVATE(1),
+
+	/**
+	 * We've exchanged key messages and activate messages, and have derived and
+	 * activated the transport keys. This is the end state.
+	 */
+	ACTIVATED(2);
+
+	private final int value;
+
+	State(int value) {
+		this.value = value;
+	}
+
+	int getValue() {
+		return value;
+	}
+
+	static State fromValue(int value) throws FormatException {
+		for (State s : values()) if (s.value == value) return s;
+		throw new FormatException();
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementConstants.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementConstants.java
@@ -0,0 +1,28 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+@NotNullByDefault
+interface TransportKeyAgreementConstants {
+
+	String MSG_KEY_IS_SESSION = "isSession";
+	String MSG_KEY_MESSAGE_TYPE = "messageType";
+	String MSG_KEY_TRANSPORT_ID = "transportId";
+	String MSG_KEY_PUBLIC_KEY = "publicKey";
+	String MSG_KEY_LOCAL = "local";
+
+	String SESSION_KEY_STATE = "state";
+	String SESSION_KEY_LAST_LOCAL_MESSAGE_ID = "lastLocalMessageId";
+	String SESSION_KEY_LOCAL_PUBLIC_KEY = "localPublicKey";
+	String SESSION_KEY_LOCAL_PRIVATE_KEY = "localPrivateKey";
+	String SESSION_KEY_LOCAL_TIMESTAMP = "localTimestamp";
+	String SESSION_KEY_REMOTE_TIMESTAMP = "remoteTimestamp";
+	String SESSION_KEY_KEY_SET_ID = "keySetId";
+
+	/**
+	 * Label for deriving the root key from key pairs.
+	 */
+	String ROOT_KEY_LABEL =
+			"org.briarproject.bramble.transport.agreement/ROOT_KEY";
+
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCrypto.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCrypto.java
@@ -0,0 +1,23 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.GeneralSecurityException;
+
+@NotNullByDefault
+interface TransportKeyAgreementCrypto {
+
+	KeyPair generateKeyPair();
+
+	SecretKey deriveRootKey(KeyPair localKeyPair, PublicKey remotePublicKey,
+			long timestamp) throws GeneralSecurityException;
+
+	PublicKey parsePublicKey(byte[] encoded) throws FormatException;
+
+	PrivateKey parsePrivateKey(byte[] encoded) throws FormatException;
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCryptoImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementCryptoImpl.java
@@ -0,0 +1,67 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.crypto.CryptoComponent;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PrivateKey;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.security.GeneralSecurityException;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.ROOT_KEY_LABEL;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementCryptoImpl implements TransportKeyAgreementCrypto {
+
+	private final CryptoComponent crypto;
+
+	@Inject
+	TransportKeyAgreementCryptoImpl(CryptoComponent crypto) {
+		this.crypto = crypto;
+	}
+
+	@Override
+	public KeyPair generateKeyPair() {
+		return crypto.generateAgreementKeyPair();
+	}
+
+	@Override
+	public SecretKey deriveRootKey(KeyPair localKeyPair,
+			PublicKey remotePublicKey, long timestamp)
+			throws GeneralSecurityException {
+		byte[] theirPublic = remotePublicKey.getEncoded();
+		byte[] ourPublic = localKeyPair.getPublic().getEncoded();
+		boolean alice = compare(ourPublic, theirPublic) < 0;
+		byte[][] inputs = {
+				alice ? ourPublic : theirPublic,
+				alice ? theirPublic : ourPublic
+		};
+		return crypto.deriveSharedSecret(ROOT_KEY_LABEL, remotePublicKey,
+				localKeyPair, inputs);
+	}
+
+	@Override
+	public PublicKey parsePublicKey(byte[] encoded) throws FormatException {
+		try {
+			return crypto.getAgreementKeyParser().parsePublicKey(encoded);
+		} catch (GeneralSecurityException e) {
+			throw new FormatException();
+		}
+	}
+
+	@Override
+	public PrivateKey parsePrivateKey(byte[] encoded) throws FormatException {
+		try {
+			return crypto.getAgreementKeyParser().parsePrivateKey(encoded);
+		} catch (GeneralSecurityException e) {
+			throw new FormatException();
+		}
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImpl.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImpl.java
@@ -0,0 +1,411 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.BdfIncomingMessageHook;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.client.ContactGroupFactory;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager.ContactHook;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataParser;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Metadata;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.identity.Author;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager.OpenDatabaseHook;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginConfig;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Group.Visibility;
+import org.briarproject.bramble.api.sync.GroupId;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.transport.KeyManager;
+import org.briarproject.bramble.api.transport.KeySetId;
+import org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
+
+import java.security.GeneralSecurityException;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.List;
+import java.util.Map;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.lang.Math.min;
+import static java.util.Collections.singletonMap;
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.DEFER;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.REJECT;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.State.ACTIVATED;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementManagerImpl extends BdfIncomingMessageHook
+		implements TransportKeyAgreementManager, OpenDatabaseHook, ContactHook,
+		ClientVersioningHook {
+
+	private static final Logger LOG =
+			getLogger(TransportKeyAgreementManagerImpl.class.getName());
+
+	private final ContactGroupFactory contactGroupFactory;
+	private final ClientVersioningManager clientVersioningManager;
+	private final IdentityManager identityManager;
+	private final KeyManager keyManager;
+	private final MessageEncoder messageEncoder;
+	private final SessionEncoder sessionEncoder;
+	private final SessionParser sessionParser;
+	private final TransportKeyAgreementCrypto crypto;
+
+	private final List<TransportId> transports;
+	private final Group localGroup;
+
+	@Inject
+	TransportKeyAgreementManagerImpl(
+			DatabaseComponent db,
+			ClientHelper clientHelper,
+			MetadataParser metadataParser,
+			ContactGroupFactory contactGroupFactory,
+			ClientVersioningManager clientVersioningManager,
+			IdentityManager identityManager,
+			KeyManager keyManager,
+			MessageEncoder messageEncoder,
+			SessionEncoder sessionEncoder,
+			SessionParser sessionParser,
+			TransportKeyAgreementCrypto crypto,
+			PluginConfig config) {
+		super(db, clientHelper, metadataParser);
+		this.contactGroupFactory = contactGroupFactory;
+		this.clientVersioningManager = clientVersioningManager;
+		this.identityManager = identityManager;
+		this.keyManager = keyManager;
+		this.messageEncoder = messageEncoder;
+		this.sessionEncoder = sessionEncoder;
+		this.sessionParser = sessionParser;
+		this.crypto = crypto;
+		transports = new ArrayList<>();
+		for (DuplexPluginFactory duplex : config.getDuplexFactories()) {
+			transports.add(duplex.getId());
+		}
+		for (SimplexPluginFactory simplex : config.getSimplexFactories()) {
+			transports.add(simplex.getId());
+		}
+		localGroup = contactGroupFactory.createLocalGroup(CLIENT_ID,
+				MAJOR_VERSION);
+	}
+
+	@Override
+	public void onDatabaseOpened(Transaction txn) throws DbException {
+		Collection<Contact> contacts = db.getContacts(txn);
+		if (!db.containsGroup(txn, localGroup.getId())) {
+			db.addGroup(txn, localGroup);
+			// Set things up for any pre-existing contacts
+			for (Contact c : contacts) addingContact(txn, c);
+		}
+		// Find any contacts and transports that need keys
+		Map<ContactId, Collection<TransportId>> transportsWithKeys =
+				db.getTransportsWithKeys(txn);
+		for (Contact c : contacts) {
+			Collection<TransportId> withKeys =
+					transportsWithKeys.get(c.getId());
+			for (TransportId t : transports) {
+				if (withKeys == null || !withKeys.contains(t)) {
+					// We need keys for this contact and transport
+					GroupId contactGroupId = getContactGroup(c).getId();
+					SavedSession ss = loadSession(txn, contactGroupId, t);
+					if (ss == null) {
+						// Start a session by sending our key message
+						startSession(txn, contactGroupId, t);
+					}
+				}
+			}
+		}
+	}
+
+	@Override
+	public void addingContact(Transaction txn, Contact c) throws DbException {
+		// Create a group to share with the contact
+		Group g = getContactGroup(c);
+		db.addGroup(txn, g);
+		// Attach the contact ID to the group
+		clientHelper.setContactId(txn, g.getId(), c.getId());
+		// Apply the client's visibility to the contact group
+		Visibility client = clientVersioningManager.getClientVisibility(txn,
+				c.getId(), CLIENT_ID, MAJOR_VERSION);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), client);
+	}
+
+	@Override
+	public void removingContact(Transaction txn, Contact c) throws DbException {
+		db.removeGroup(txn, getContactGroup(c));
+	}
+
+	@Override
+	public void onClientVisibilityChanging(Transaction txn, Contact c,
+			Visibility v) throws DbException {
+		// Apply the client's visibility to the contact group
+		Group g = getContactGroup(c);
+		db.setGroupVisibility(txn, c.getId(), g.getId(), v);
+	}
+
+	@Override
+	protected DeliveryAction incomingMessage(Transaction txn, Message m,
+			BdfList body, BdfDictionary meta)
+			throws DbException, FormatException {
+		MessageType type = MessageType.fromValue(
+				meta.getLong(MSG_KEY_MESSAGE_TYPE).intValue());
+		TransportId t = new TransportId(meta.getString(MSG_KEY_TRANSPORT_ID));
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Received " + type + " message for " + t);
+		}
+		if (!transports.contains(t)) {
+			// Defer handling the message until we support the transport
+			return DEFER;
+		}
+		SavedSession ss = loadSession(txn, m.getGroupId(), t);
+		if (type == KEY) return handleKeyMessage(txn, t, m, meta, ss);
+		else if (type == ACTIVATE) return handleActivateMessage(txn, t, ss);
+		else throw new AssertionError();
+	}
+
+	private DeliveryAction handleKeyMessage(Transaction txn, TransportId t,
+			Message m, BdfDictionary meta, @Nullable SavedSession ss)
+			throws DbException, FormatException {
+		ContactId c = clientHelper.getContactId(txn, m.getGroupId());
+		boolean haveKeys = db.containsTransportKeys(txn, c, t);
+		if (ss == null) {
+			if (haveKeys) {
+				// We have keys but no session, so we must have derived keys
+				// when adding the contact. If the contact didn't support
+				// the transport when they added us, they wouldn't have
+				// derived keys at that time. If they later added support for
+				// the transport then they would have started a session, so a
+				// key message is valid in this case
+				return handleKeyMessageForNewSession(txn, c, t, m, meta);
+			} else {
+				// We don't have keys, so we should have created a session at
+				// startup
+				throw new IllegalStateException();
+			}
+		} else if (ss.session.getState() == AWAIT_KEY) {
+			if (haveKeys) {
+				// We have keys, so we shouldn't be in the AWAIT_KEY state,
+				// even if the contact didn't derive keys when adding us and
+				// later started a session
+				throw new IllegalStateException();
+			} else {
+				// This is the key message we're waiting for
+				return handleKeyMessageForExistingSession(txn, c, t, m, meta,
+						ss);
+			}
+		} else {
+			return REJECT; // Not valid in this state
+		}
+	}
+
+	private DeliveryAction handleActivateMessage(Transaction txn,
+			TransportId t, @Nullable SavedSession ss) throws DbException {
+		if (ss != null && ss.session.getState() == AWAIT_ACTIVATE) {
+			// Activate the keys and finish the session
+			KeySetId keySetId = requireNonNull(ss.session.getKeySetId());
+			keyManager.activateKeys(txn, singletonMap(t, keySetId));
+			Session session = new Session(ACTIVATED,
+					ss.session.getLastLocalMessageId(), null, null, null, null);
+			saveSession(txn, t, ss.storageId, session);
+			return ACCEPT_DO_NOT_SHARE;
+		} else {
+			return REJECT; // Not valid in this state
+		}
+	}
+
+	private DeliveryAction handleKeyMessageForNewSession(Transaction txn,
+			ContactId c, TransportId t, Message m, BdfDictionary meta)
+			throws DbException, FormatException {
+		KeyPair localKeyPair = crypto.generateKeyPair();
+		PublicKey remotePublicKey =
+				crypto.parsePublicKey(meta.getRaw(MSG_KEY_PUBLIC_KEY));
+		Message keyMessage = sendKeyMessage(txn, m.getGroupId(), t,
+				localKeyPair.getPublic());
+		long minTimestamp = min(keyMessage.getTimestamp(), m.getTimestamp());
+		SecretKey rootKey;
+		try {
+			rootKey = crypto.deriveRootKey(localKeyPair, remotePublicKey,
+					minTimestamp);
+		} catch (GeneralSecurityException e) {
+			return REJECT; // Invalid public key
+		}
+		boolean alice = isLocalPartyAlice(txn, db.getContact(txn, c));
+		KeySetId keySetId = keyManager.addRotationKeys(txn, c, t, rootKey,
+				minTimestamp, alice, false);
+		Message activateMessage =
+				sendActivateMessage(txn, m.getGroupId(), t, keyMessage.getId());
+		Session session = new Session(AWAIT_ACTIVATE, activateMessage.getId(),
+				null, null, null, keySetId);
+		saveNewSession(txn, m.getGroupId(), t, session);
+		return ACCEPT_DO_NOT_SHARE;
+	}
+
+	private DeliveryAction handleKeyMessageForExistingSession(Transaction txn,
+			ContactId c, TransportId t, Message m, BdfDictionary meta,
+			SavedSession ss) throws DbException, FormatException {
+		KeyPair localKeyPair = requireNonNull(ss.session.getLocalKeyPair());
+		PublicKey remotePublicKey =
+				crypto.parsePublicKey(meta.getRaw(MSG_KEY_PUBLIC_KEY));
+		long localTimestamp = requireNonNull(ss.session.getLocalTimestamp());
+		long minTimestamp = min(localTimestamp, m.getTimestamp());
+		SecretKey rootKey;
+		try {
+			rootKey = crypto.deriveRootKey(localKeyPair, remotePublicKey,
+					minTimestamp);
+		} catch (GeneralSecurityException e) {
+			return REJECT; // Invalid public key
+		}
+		boolean alice = isLocalPartyAlice(txn, db.getContact(txn, c));
+		KeySetId keySetId = keyManager.addRotationKeys(txn, c, t, rootKey,
+				minTimestamp, alice, false);
+		MessageId previousMessageId =
+				requireNonNull(ss.session.getLastLocalMessageId());
+		Message activateMessage =
+				sendActivateMessage(txn, m.getGroupId(), t, previousMessageId);
+		Session session = new Session(AWAIT_ACTIVATE, activateMessage.getId(),
+				null, null, null, keySetId);
+		saveSession(txn, t, ss.storageId, session);
+		return ACCEPT_DO_NOT_SHARE;
+	}
+
+	private void startSession(Transaction txn, GroupId contactGroupId,
+			TransportId t) throws DbException {
+		KeyPair localKeyPair = crypto.generateKeyPair();
+		Message keyMessage = sendKeyMessage(txn, contactGroupId, t,
+				localKeyPair.getPublic());
+		Session session = new Session(AWAIT_KEY, keyMessage.getId(),
+				localKeyPair, keyMessage.getTimestamp(), null, null);
+		saveNewSession(txn, contactGroupId, t, session);
+	}
+
+	@Nullable
+	private SavedSession loadSession(Transaction txn, GroupId contactGroupId,
+			TransportId t) throws DbException {
+		try {
+			BdfDictionary query = sessionEncoder.getSessionQuery(t);
+			Collection<MessageId> ids =
+					clientHelper.getMessageIds(txn, contactGroupId, query);
+			if (ids.size() > 1) throw new DbException();
+			if (ids.isEmpty()) {
+				if (LOG.isLoggable(INFO)) LOG.info("No session for " + t);
+				return null;
+			}
+			MessageId storageId = ids.iterator().next();
+			BdfDictionary bdfSession =
+					clientHelper.getMessageMetadataAsDictionary(txn, storageId);
+			Session session = sessionParser.parseSession(bdfSession);
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Loaded session in state " + session.getState()
+						+ " for " + t);
+			}
+			return new SavedSession(session, storageId);
+		} catch (FormatException e) {
+			throw new DbException(e);
+		}
+	}
+
+	private void saveNewSession(Transaction txn, GroupId contactGroupId,
+			TransportId t, Session session) throws DbException {
+		Message m =
+				clientHelper.createMessageForStoringMetadata(contactGroupId);
+		db.addLocalMessage(txn, m, new Metadata(), false, false);
+		MessageId storageId = m.getId();
+		saveSession(txn, t, storageId, session);
+	}
+
+	private void saveSession(Transaction txn, TransportId t,
+			MessageId storageId, Session session) throws DbException {
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Saving session in state " + session.getState()
+					+ " for " + t);
+		}
+		BdfDictionary meta = sessionEncoder.encodeSession(session, t);
+		try {
+			clientHelper.mergeMessageMetadata(txn, storageId, meta);
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+
+	private Message sendKeyMessage(Transaction txn, GroupId contactGroupId,
+			TransportId t, PublicKey publicKey) throws DbException {
+		Message m = messageEncoder.encodeKeyMessage(contactGroupId, t,
+				publicKey);
+		sendMessage(txn, t, m, KEY);
+		return m;
+	}
+
+	private Message sendActivateMessage(Transaction txn,
+			GroupId contactGroupId, TransportId t, MessageId previousMessageId)
+			throws DbException {
+		Message m = messageEncoder.encodeActivateMessage(contactGroupId, t,
+				previousMessageId);
+		sendMessage(txn, t, m, ACTIVATE);
+		return m;
+	}
+
+	private void sendMessage(Transaction txn, TransportId t, Message m,
+			MessageType type) throws DbException {
+		BdfDictionary meta =
+				messageEncoder.encodeMessageMetadata(t, type, true);
+		try {
+			clientHelper.addLocalMessage(txn, m, meta, true, false);
+		} catch (FormatException e) {
+			throw new AssertionError();
+		}
+	}
+
+	private Group getContactGroup(Contact c) {
+		return contactGroupFactory.createContactGroup(CLIENT_ID,
+				MAJOR_VERSION, c);
+	}
+
+	private boolean isLocalPartyAlice(Transaction txn, Contact c)
+			throws DbException {
+		Author local = identityManager.getLocalAuthor(txn);
+		Author remote = c.getAuthor();
+		return compare(local.getId().getBytes(), remote.getId().getBytes()) < 0;
+	}
+
+	private static class SavedSession {
+
+		private final Session session;
+		private final MessageId storageId;
+
+		private SavedSession(Session session, MessageId storageId) {
+			this.session = session;
+			this.storageId = storageId;
+		}
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementModule.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementModule.java
@@ -0,0 +1,83 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.sync.validation.ValidationManager;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.CLIENT_ID;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MAJOR_VERSION;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MINOR_VERSION;
+
+@Module
+public class TransportKeyAgreementModule {
+
+	public static class EagerSingletons {
+		@Inject
+		TransportKeyAgreementManager transportKeyAgreementManager;
+		@Inject
+		TransportKeyAgreementValidator transportKeyAgreementValidator;
+	}
+
+	@Provides
+	@Singleton
+	TransportKeyAgreementManager provideTransportKeyAgreementManager(
+			LifecycleManager lifecycleManager,
+			ValidationManager validationManager,
+			ContactManager contactManager,
+			ClientVersioningManager clientVersioningManager,
+			TransportKeyAgreementManagerImpl transportKeyAgreementManager) {
+		lifecycleManager.registerOpenDatabaseHook(transportKeyAgreementManager);
+		validationManager.registerIncomingMessageHook(CLIENT_ID,
+				MAJOR_VERSION, transportKeyAgreementManager);
+		contactManager.registerContactHook(transportKeyAgreementManager);
+		clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
+				MINOR_VERSION, transportKeyAgreementManager);
+		return transportKeyAgreementManager;
+	}
+
+	@Provides
+	@Singleton
+	TransportKeyAgreementValidator provideTransportKeyAgreementValidator(
+			ClientHelper clientHelper, MetadataEncoder metadataEncoder,
+			Clock clock, MessageEncoder messageEncoder,
+			ValidationManager validationManager) {
+		TransportKeyAgreementValidator validator =
+				new TransportKeyAgreementValidator(clientHelper,
+						metadataEncoder, clock, messageEncoder);
+		validationManager.registerMessageValidator(CLIENT_ID, MAJOR_VERSION,
+				validator);
+		return validator;
+	}
+
+	@Provides
+	MessageEncoder provideMessageEncoder(MessageEncoderImpl messageEncoder) {
+		return messageEncoder;
+	}
+
+	@Provides
+	SessionEncoder provideSessionEncoder(SessionEncoderImpl sessionEncoder) {
+		return sessionEncoder;
+	}
+
+	@Provides
+	SessionParser provideSessionParser(SessionParserImpl sessionParser) {
+		return sessionParser;
+	}
+
+	@Provides
+	TransportKeyAgreementCrypto provideTransportKeyAgreementCrypto(
+			TransportKeyAgreementCryptoImpl transportKeyAgreementCrypto) {
+		return transportKeyAgreementCrypto;
+	}
+}
--- a/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementValidator.java
+++ b/bramble-core/src/main/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementValidator.java
@@ -0,0 +1,77 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.FormatException;
+import org.briarproject.bramble.api.client.BdfMessageContext;
+import org.briarproject.bramble.api.client.BdfMessageValidator;
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.system.Clock;
+
+import javax.annotation.concurrent.Immutable;
+
+import static java.util.Collections.singletonList;
+import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_AGREEMENT_PUBLIC_KEY_BYTES;
+import static org.briarproject.bramble.api.plugin.TransportId.MAX_TRANSPORT_ID_LENGTH;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.util.ValidationUtils.checkLength;
+import static org.briarproject.bramble.util.ValidationUtils.checkSize;
+
+@Immutable
+@NotNullByDefault
+class TransportKeyAgreementValidator extends BdfMessageValidator {
+
+	private final MessageEncoder messageEncoder;
+
+	TransportKeyAgreementValidator(ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder, Clock clock,
+			MessageEncoder messageEncoder) {
+		super(clientHelper, metadataEncoder, clock);
+		this.messageEncoder = messageEncoder;
+	}
+
+	@Override
+	protected BdfMessageContext validateMessage(Message m, Group g,
+			BdfList body) throws FormatException {
+		MessageType type = MessageType.fromValue(body.getLong(0).intValue());
+		if (type == KEY) return validateKeyMessage(body);
+		else if (type == ACTIVATE) return validateActivateMessage(body);
+		else throw new AssertionError();
+	}
+
+	private BdfMessageContext validateKeyMessage(BdfList body)
+			throws FormatException {
+		// Message type, transport ID, public key
+		checkSize(body, 3);
+		String transportId = body.getString(1);
+		checkLength(transportId, 1, MAX_TRANSPORT_ID_LENGTH);
+		byte[] publicKey = body.getRaw(2);
+		checkLength(publicKey, 1, MAX_AGREEMENT_PUBLIC_KEY_BYTES);
+		BdfDictionary meta = messageEncoder.encodeMessageMetadata(
+				new TransportId(transportId), KEY, false);
+		meta.put(MSG_KEY_PUBLIC_KEY, publicKey);
+		return new BdfMessageContext(meta);
+	}
+
+	private BdfMessageContext validateActivateMessage(BdfList body)
+			throws FormatException {
+		// Message type, transport ID, previous message ID
+		checkSize(body, 3);
+		String transportId = body.getString(1);
+		checkLength(transportId, 1, MAX_TRANSPORT_ID_LENGTH);
+		byte[] previousMessageId = body.getRaw(2);
+		checkLength(previousMessageId, MessageId.LENGTH);
+		BdfDictionary meta = messageEncoder.encodeMessageMetadata(
+				new TransportId(transportId), ACTIVATE, false);
+		MessageId dependency = new MessageId(previousMessageId);
+		return new BdfMessageContext(meta, singletonList(dependency));
+	}
+}
--- a/bramble-core/src/test/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImplTest.java
+++ b/bramble-core/src/test/java/org/briarproject/bramble/transport/agreement/TransportKeyAgreementManagerImplTest.java
@@ -0,0 +1,589 @@
+package org.briarproject.bramble.transport.agreement;
+
+import org.briarproject.bramble.api.client.ClientHelper;
+import org.briarproject.bramble.api.client.ContactGroupFactory;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.crypto.KeyPair;
+import org.briarproject.bramble.api.crypto.PublicKey;
+import org.briarproject.bramble.api.crypto.SecretKey;
+import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
+import org.briarproject.bramble.api.data.BdfList;
+import org.briarproject.bramble.api.data.MetadataParser;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.Metadata;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.identity.IdentityManager;
+import org.briarproject.bramble.api.identity.LocalAuthor;
+import org.briarproject.bramble.api.plugin.PluginConfig;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
+import org.briarproject.bramble.api.sync.Group;
+import org.briarproject.bramble.api.sync.Message;
+import org.briarproject.bramble.api.transport.KeyManager;
+import org.briarproject.bramble.api.transport.KeySetId;
+import org.briarproject.bramble.api.versioning.ClientVersioningManager;
+import org.briarproject.bramble.test.BrambleMockTestCase;
+import org.briarproject.bramble.test.CaptureArgumentAction;
+import org.jmock.Expectations;
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+import static java.lang.Math.min;
+import static java.util.Arrays.asList;
+import static java.util.Collections.emptyList;
+import static java.util.Collections.singletonList;
+import static java.util.Collections.singletonMap;
+import static org.briarproject.bramble.api.Bytes.compare;
+import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.DEFER;
+import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.REJECT;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.CLIENT_ID;
+import static org.briarproject.bramble.api.transport.agreement.TransportKeyAgreementManager.MAJOR_VERSION;
+import static org.briarproject.bramble.test.TestUtils.getAgreementPrivateKey;
+import static org.briarproject.bramble.test.TestUtils.getAgreementPublicKey;
+import static org.briarproject.bramble.test.TestUtils.getContact;
+import static org.briarproject.bramble.test.TestUtils.getGroup;
+import static org.briarproject.bramble.test.TestUtils.getLocalAuthor;
+import static org.briarproject.bramble.test.TestUtils.getMessage;
+import static org.briarproject.bramble.test.TestUtils.getSecretKey;
+import static org.briarproject.bramble.test.TestUtils.getTransportId;
+import static org.briarproject.bramble.transport.agreement.MessageType.ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.MessageType.KEY;
+import static org.briarproject.bramble.transport.agreement.State.ACTIVATED;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_ACTIVATE;
+import static org.briarproject.bramble.transport.agreement.State.AWAIT_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_MESSAGE_TYPE;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_PUBLIC_KEY;
+import static org.briarproject.bramble.transport.agreement.TransportKeyAgreementConstants.MSG_KEY_TRANSPORT_ID;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+public class TransportKeyAgreementManagerImplTest extends BrambleMockTestCase {
+
+	private final DatabaseComponent db = context.mock(DatabaseComponent.class);
+	private final ClientHelper clientHelper = context.mock(ClientHelper.class);
+	private final MetadataParser metadataParser =
+			context.mock(MetadataParser.class);
+	private final ContactGroupFactory contactGroupFactory =
+			context.mock(ContactGroupFactory.class);
+	private final ClientVersioningManager clientVersioningManager =
+			context.mock(ClientVersioningManager.class);
+	private final IdentityManager identityManager =
+			context.mock(IdentityManager.class);
+	private final KeyManager keyManager = context.mock(KeyManager.class);
+	private final MessageEncoder messageEncoder =
+			context.mock(MessageEncoder.class);
+	private final SessionEncoder sessionEncoder =
+			context.mock(SessionEncoder.class);
+	private final SessionParser sessionParser =
+			context.mock(SessionParser.class);
+	private final TransportKeyAgreementCrypto crypto =
+			context.mock(TransportKeyAgreementCrypto.class);
+	private final PluginConfig pluginConfig = context.mock(PluginConfig.class);
+	private final SimplexPluginFactory simplexFactory =
+			context.mock(SimplexPluginFactory.class);
+	private final DuplexPluginFactory duplexFactory =
+			context.mock(DuplexPluginFactory.class);
+
+	private final TransportId simplexTransportId = getTransportId();
+	private final TransportId duplexTransportId = getTransportId();
+	private final Group localGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+	private final Group contactGroup = getGroup(CLIENT_ID, MAJOR_VERSION);
+	private final Contact contact = getContact();
+	private final LocalAuthor localAuthor = getLocalAuthor();
+	private final boolean alice = compare(localAuthor.getId().getBytes(),
+			contact.getAuthor().getId().getBytes()) < 0;
+	private final KeyPair localKeyPair =
+			new KeyPair(getAgreementPublicKey(), getAgreementPrivateKey());
+	private final PublicKey remotePublicKey = getAgreementPublicKey();
+	private final SecretKey rootKey = getSecretKey();
+	private final KeySetId keySetId = new KeySetId(123);
+
+	private final Message storageMessage = getMessage(contactGroup.getId());
+	private final Message localKeyMessage = getMessage(contactGroup.getId());
+	private final Message localActivateMessage =
+			getMessage(contactGroup.getId());
+	private final Message remoteKeyMessage = getMessage(contactGroup.getId());
+	private final Message remoteActivateMessage =
+			getMessage(contactGroup.getId());
+	private final long localTimestamp = localKeyMessage.getTimestamp();
+	private final long remoteTimestamp = remoteKeyMessage.getTimestamp();
+
+	// These query and metadata dictionaries are handled by the manager without
+	// inspecting their contents, so we can use empty dictionaries for testing
+	private final BdfDictionary sessionQuery = new BdfDictionary();
+	private final BdfDictionary sessionMeta = new BdfDictionary();
+	private final BdfDictionary localKeyMeta = new BdfDictionary();
+	private final BdfDictionary localActivateMeta = new BdfDictionary();
+
+	// The manager doesn't use the incoming message body, so it can be empty
+	private final BdfList remoteMessageBody = new BdfList();
+
+	private final BdfDictionary remoteKeyMeta = BdfDictionary.of(
+			new BdfEntry(MSG_KEY_MESSAGE_TYPE, KEY.getValue()),
+			new BdfEntry(MSG_KEY_TRANSPORT_ID,
+					simplexTransportId.getString()),
+			new BdfEntry(MSG_KEY_PUBLIC_KEY, remotePublicKey.getEncoded()));
+
+	private final BdfDictionary remoteActivateMeta = BdfDictionary.of(
+			new BdfEntry(MSG_KEY_MESSAGE_TYPE, ACTIVATE.getValue()),
+			new BdfEntry(MSG_KEY_TRANSPORT_ID,
+					simplexTransportId.getString()));
+
+	private TransportKeyAgreementManagerImpl manager;
+
+	@Before
+	public void setUp() {
+		context.checking(new Expectations() {{
+			oneOf(pluginConfig).getSimplexFactories();
+			will(returnValue(singletonList(simplexFactory)));
+			oneOf(simplexFactory).getId();
+			will(returnValue(simplexTransportId));
+			oneOf(pluginConfig).getDuplexFactories();
+			will(returnValue(singletonList(duplexFactory)));
+			oneOf(duplexFactory).getId();
+			will(returnValue(duplexTransportId));
+			oneOf(contactGroupFactory)
+					.createLocalGroup(CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(localGroup));
+		}});
+
+		manager = new TransportKeyAgreementManagerImpl(db, clientHelper,
+				metadataParser, contactGroupFactory, clientVersioningManager,
+				identityManager, keyManager, messageEncoder, sessionEncoder,
+				sessionParser, crypto, pluginConfig);
+	}
+
+	@Test
+	public void testCreatesContactGroupAtStartupIfLocalGroupDoesNotExist()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group doesn't exist so we need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(false));
+			oneOf(db).addGroup(txn, localGroup);
+			// Create the contact group and set it up
+			oneOf(contactGroupFactory).createContactGroup(CLIENT_ID,
+					MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(db).addGroup(txn, contactGroup);
+			oneOf(clientHelper)
+					.setContactId(txn, contactGroup.getId(), contact.getId());
+			oneOf(clientVersioningManager).getClientVisibility(txn,
+					contact.getId(), CLIENT_ID, MAJOR_VERSION);
+			will(returnValue(VISIBLE));
+			oneOf(db).setGroupVisibility(txn, contact.getId(),
+					contactGroup.getId(), VISIBLE);
+			// We already have keys for both transports
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					asList(simplexTransportId, duplexTransportId))));
+		}});
+
+		manager.onDatabaseOpened(txn);
+	}
+
+	@Test
+	public void testDoesNotCreateContactGroupAtStartupIfLocalGroupExists()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group exists so we don't need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(true));
+			// We already have keys for both transports
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					asList(simplexTransportId, duplexTransportId))));
+		}});
+
+		manager.onDatabaseOpened(txn);
+	}
+
+	@Test
+	public void testStartsSessionAtStartup() throws Exception {
+		Transaction txn = new Transaction(null, false);
+		AtomicReference<Session> savedSession = new AtomicReference<>();
+
+		context.checking(new Expectations() {{
+			oneOf(db).getContacts(txn);
+			will(returnValue(singletonList(contact)));
+			// The local group exists so we don't need to create contact groups
+			oneOf(db).containsGroup(txn, localGroup.getId());
+			will(returnValue(true));
+			// We need keys for the simplex transport
+			oneOf(db).getTransportsWithKeys(txn);
+			will(returnValue(singletonMap(contact.getId(),
+					singletonList(duplexTransportId))));
+			// Check whether a session exists - it doesn't
+			oneOf(contactGroupFactory)
+					.createContactGroup(CLIENT_ID, MAJOR_VERSION, contact);
+			will(returnValue(contactGroup));
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(emptyList()));
+			// Send a key message
+			oneOf(crypto).generateKeyPair();
+			will(returnValue(localKeyPair));
+			oneOf(messageEncoder).encodeKeyMessage(contactGroup.getId(),
+					simplexTransportId, localKeyPair.getPublic());
+			will(returnValue(localKeyMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, KEY, true);
+			will(returnValue(localKeyMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localKeyMessage,
+					localKeyMeta, true, false);
+			// Save the session
+			oneOf(clientHelper)
+					.createMessageForStoringMetadata(contactGroup.getId());
+			will(returnValue(storageMessage));
+			oneOf(db).addLocalMessage(txn, storageMessage, new Metadata(),
+					false, false);
+			oneOf(sessionEncoder).encodeSession(with(any(Session.class)),
+					with(simplexTransportId));
+			will(doAll(
+					new CaptureArgumentAction<>(savedSession, Session.class, 0),
+					returnValue(sessionMeta)));
+			oneOf(clientHelper).mergeMessageMetadata(txn,
+					storageMessage.getId(), sessionMeta);
+		}});
+
+		manager.onDatabaseOpened(txn);
+
+		assertEquals(AWAIT_KEY, savedSession.get().getState());
+		assertEquals(localKeyMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertEquals(localKeyPair, savedSession.get().getLocalKeyPair());
+		assertEquals(Long.valueOf(localTimestamp),
+				savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getRemoteTimestamp());
+		assertNull(savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testDefersMessageIfTransportIsNotSupported() throws Exception {
+		Transaction txn = new Transaction(null, false);
+		TransportId unknownTransportId = getTransportId();
+		BdfDictionary meta = new BdfDictionary(remoteKeyMeta);
+		meta.put(MSG_KEY_TRANSPORT_ID, unknownTransportId.getString());
+
+		assertEquals(DEFER, manager.incomingMessage(txn, remoteKeyMessage,
+				remoteMessageBody, meta));
+	}
+
+	@Test
+	public void testAcceptsKeyMessageInAwaitKeyState() throws Exception {
+		Transaction txn = new Transaction(null, false);
+		Session loadedSession = new Session(AWAIT_KEY,
+				localKeyMessage.getId(), localKeyPair, localTimestamp,
+				null, null);
+		AtomicReference<Session> savedSession = new AtomicReference<>();
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it does
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(singletonList(storageMessage.getId())));
+			// Load the session
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					storageMessage.getId());
+			will(returnValue(sessionMeta));
+			oneOf(sessionParser).parseSession(sessionMeta);
+			will(returnValue(loadedSession));
+			// Load the contact ID
+			oneOf(clientHelper).getContactId(txn, contactGroup.getId());
+			will(returnValue(contact.getId()));
+			// Check whether we already have keys - we don't
+			oneOf(db).containsTransportKeys(txn, contact.getId(),
+					simplexTransportId);
+			will(returnValue(false));
+			// Parse the remote public key
+			oneOf(crypto).parsePublicKey(remotePublicKey.getEncoded());
+			will(returnValue(remotePublicKey));
+			// Derive and store the transport keys
+			oneOf(crypto).deriveRootKey(localKeyPair, remotePublicKey,
+					min(localTimestamp, remoteTimestamp));
+			will(returnValue(rootKey));
+			oneOf(db).getContact(txn, contact.getId());
+			will(returnValue(contact));
+			oneOf(identityManager).getLocalAuthor(txn);
+			will(returnValue(localAuthor));
+			oneOf(keyManager).addRotationKeys(txn, contact.getId(),
+					simplexTransportId, rootKey,
+					min(localTimestamp, remoteTimestamp), alice, false);
+			will(returnValue(keySetId));
+			// Send an activate message
+			oneOf(messageEncoder).encodeActivateMessage(contactGroup.getId(),
+					simplexTransportId, localKeyMessage.getId());
+			will(returnValue(localActivateMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, ACTIVATE, true);
+			will(returnValue(localActivateMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localActivateMessage,
+					localActivateMeta, true, false);
+			// Save the session
+			oneOf(sessionEncoder).encodeSession(with(any(Session.class)),
+					with(simplexTransportId));
+			will(doAll(
+					new CaptureArgumentAction<>(savedSession, Session.class, 0),
+					returnValue(sessionMeta)));
+			oneOf(clientHelper).mergeMessageMetadata(txn,
+					storageMessage.getId(), sessionMeta);
+		}});
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+
+		assertEquals(AWAIT_ACTIVATE, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getRemoteTimestamp());
+		assertEquals(keySetId, savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testAcceptsKeyMessageIfWeHaveTransportKeysButNoSession()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+		AtomicReference<Session> savedSession = new AtomicReference<>();
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it doesn't
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(emptyList()));
+			// Load the contact ID
+			oneOf(clientHelper).getContactId(txn, contactGroup.getId());
+			will(returnValue(contact.getId()));
+			// Check whether we already have keys - we do
+			oneOf(db).containsTransportKeys(txn, contact.getId(),
+					simplexTransportId);
+			will(returnValue(true));
+			// Generate the local key pair
+			oneOf(crypto).generateKeyPair();
+			will(returnValue(localKeyPair));
+			// Parse the remote public key
+			oneOf(crypto).parsePublicKey(remotePublicKey.getEncoded());
+			will(returnValue(remotePublicKey));
+			// Send a key message
+			oneOf(messageEncoder).encodeKeyMessage(contactGroup.getId(),
+					simplexTransportId, localKeyPair.getPublic());
+			will(returnValue(localKeyMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, KEY, true);
+			will(returnValue(localKeyMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localKeyMessage,
+					localKeyMeta, true, false);
+			// Derive and store the transport keys
+			oneOf(crypto).deriveRootKey(localKeyPair, remotePublicKey,
+					min(localTimestamp, remoteTimestamp));
+			will(returnValue(rootKey));
+			oneOf(db).getContact(txn, contact.getId());
+			will(returnValue(contact));
+			oneOf(identityManager).getLocalAuthor(txn);
+			will(returnValue(localAuthor));
+			oneOf(keyManager).addRotationKeys(txn, contact.getId(),
+					simplexTransportId, rootKey,
+					min(localTimestamp, remoteTimestamp), alice, false);
+			will(returnValue(keySetId));
+			// Send an activate message
+			oneOf(messageEncoder).encodeActivateMessage(contactGroup.getId(),
+					simplexTransportId, localKeyMessage.getId());
+			will(returnValue(localActivateMessage));
+			oneOf(messageEncoder)
+					.encodeMessageMetadata(simplexTransportId, ACTIVATE, true);
+			will(returnValue(localActivateMeta));
+			oneOf(clientHelper).addLocalMessage(txn, localActivateMessage,
+					localActivateMeta, true, false);
+			// Save the session
+			oneOf(clientHelper)
+					.createMessageForStoringMetadata(contactGroup.getId());
+			will(returnValue(storageMessage));
+			oneOf(db).addLocalMessage(txn, storageMessage, new Metadata(),
+					false, false);
+			oneOf(sessionEncoder).encodeSession(with(any(Session.class)),
+					with(simplexTransportId));
+			will(doAll(
+					new CaptureArgumentAction<>(savedSession, Session.class, 0),
+					returnValue(sessionMeta)));
+			oneOf(clientHelper).mergeMessageMetadata(txn,
+					storageMessage.getId(), sessionMeta);
+		}});
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+
+		assertEquals(AWAIT_ACTIVATE, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getRemoteTimestamp());
+		assertEquals(keySetId, savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testRejectsKeyMessageInAwaitActivateState() throws Exception {
+		Session loadedSession = new Session(AWAIT_ACTIVATE,
+				localActivateMessage.getId(), null, null, null, keySetId);
+		testRejectsKeyMessageWithExistingSession(loadedSession);
+	}
+
+	@Test
+	public void testRejectsKeyMessageInActivatedState() throws Exception {
+		Session loadedSession = new Session(ACTIVATED,
+				localActivateMessage.getId(), null, null, null, null);
+		testRejectsKeyMessageWithExistingSession(loadedSession);
+	}
+
+	private void testRejectsKeyMessageWithExistingSession(Session loadedSession)
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it does
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(singletonList(storageMessage.getId())));
+			// Load the session
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					storageMessage.getId());
+			will(returnValue(sessionMeta));
+			oneOf(sessionParser).parseSession(sessionMeta);
+			will(returnValue(loadedSession));
+			// Load the contact ID
+			oneOf(clientHelper).getContactId(txn, contactGroup.getId());
+			will(returnValue(contact.getId()));
+			// Check whether we already have keys - we don't
+			oneOf(db).containsTransportKeys(txn, contact.getId(),
+					simplexTransportId);
+			will(returnValue(false));
+		}});
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteKeyMessage, remoteMessageBody, remoteKeyMeta));
+	}
+
+	@Test
+	public void testAcceptsActivateMessageInAwaitActivateState()
+			throws Exception {
+		Transaction txn = new Transaction(null, false);
+		Session loadedSession = new Session(AWAIT_ACTIVATE,
+				localActivateMessage.getId(), null, null, null, keySetId);
+		AtomicReference<Session> savedSession = new AtomicReference<>();
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it does
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(singletonList(storageMessage.getId())));
+			// Load the session
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					storageMessage.getId());
+			will(returnValue(sessionMeta));
+			oneOf(sessionParser).parseSession(sessionMeta);
+			will(returnValue(loadedSession));
+			// Activate the transport keys
+			oneOf(keyManager).activateKeys(txn,
+					singletonMap(simplexTransportId, keySetId));
+			// Save the session
+			oneOf(sessionEncoder).encodeSession(with(any(Session.class)),
+					with(simplexTransportId));
+			will(doAll(
+					new CaptureArgumentAction<>(savedSession, Session.class, 0),
+					returnValue(sessionMeta)));
+			oneOf(clientHelper).mergeMessageMetadata(txn,
+					storageMessage.getId(), sessionMeta);
+		}});
+
+		assertEquals(ACCEPT_DO_NOT_SHARE, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+
+		assertEquals(ACTIVATED, savedSession.get().getState());
+		assertEquals(localActivateMessage.getId(),
+				savedSession.get().getLastLocalMessageId());
+		assertNull(savedSession.get().getLocalKeyPair());
+		assertNull(savedSession.get().getLocalTimestamp());
+		assertNull(savedSession.get().getRemoteTimestamp());
+		assertNull(savedSession.get().getKeySetId());
+	}
+
+	@Test
+	public void testRejectsActivateMessageWithNoSession() throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it doesn't
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(emptyList()));
+		}});
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+	}
+
+	@Test
+	public void testRejectsActivateMessageInAwaitKeyState() throws Exception {
+		Session loadedSession = new Session(AWAIT_KEY,
+				localActivateMessage.getId(), localKeyPair, localTimestamp,
+				null, null);
+		testRejectsActivateMessageWithExistingSession(loadedSession);
+	}
+
+	@Test
+	public void testRejectsActivateMessageInActivatedState() throws Exception {
+		Session loadedSession = new Session(ACTIVATED,
+				localActivateMessage.getId(), null, null, null, null);
+		testRejectsActivateMessageWithExistingSession(loadedSession);
+	}
+
+	private void testRejectsActivateMessageWithExistingSession(
+			Session loadedSession) throws Exception {
+		Transaction txn = new Transaction(null, false);
+
+		context.checking(new Expectations() {{
+			// Check whether a session exists - it does
+			oneOf(sessionEncoder).getSessionQuery(simplexTransportId);
+			will(returnValue(sessionQuery));
+			oneOf(clientHelper)
+					.getMessageIds(txn, contactGroup.getId(), sessionQuery);
+			will(returnValue(singletonList(storageMessage.getId())));
+			// Load the session
+			oneOf(clientHelper).getMessageMetadataAsDictionary(txn,
+					storageMessage.getId());
+			will(returnValue(sessionMeta));
+			oneOf(sessionParser).parseSession(sessionMeta);
+			will(returnValue(loadedSession));
+		}});
+
+		assertEquals(REJECT, manager.incomingMessage(txn,
+				remoteActivateMessage, remoteMessageBody, remoteActivateMeta));
+	}
+}