fix: Use stanza receival timestamps to guard against stale kex messages

This was mostly caused by Dart not copying values but referencing
them. AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.

We know make some assumptions about received key exchanges, so this
needs some field testing.

example/omemo_dart_example.dart

@@ -119,6 +119,8 @@ void main() async {
     aliceDevice.id,
     // The deserialised keys
     keys,
+    // Since the message was not delayed, we use the current time
+    DateTime.now().millisecondsSinceEpoch,
   );
 
   // All Bob has to do now is replace the OMEMO wrapper element 

lib/src/double_ratchet/double_ratchet.dart

@@ -68,6 +68,7 @@ class OmemoDoubleRatchet {
     this.sessionAd,
     this.mkSkipped, // MKSKIPPED
     this.acknowledged,
+    this.kexTimestamp,
   );
   
   factory OmemoDoubleRatchet.fromJson(Map<String, dynamic> data) {
@@ -85,6 +86,7 @@ class OmemoDoubleRatchet {
       'ik_pub': 'base/64/encoded',
       'session_ad': 'base/64/encoded',
       'acknowledged': true | false,
+      'kex_timestamp': int,
       'mkskipped': [
         {
           'key': 'base/64/encoded',
@@ -129,6 +131,7 @@ class OmemoDoubleRatchet {
       base64.decode(data['session_ad']! as String),
       mkSkipped,
       data['acknowledged']! as bool,
+      data['kex_timestamp']! as int,
     );
   }
   
@@ -160,6 +163,10 @@ class OmemoDoubleRatchet {
 
   final Map<SkippedKey, List<int>> mkSkipped;
 
+  /// The point in time at which we performed the kex exchange to create this ratchet.
+  /// Precision is milliseconds since epoch.
+  int kexTimestamp;
+
   /// Indicates whether we received an empty OMEMO message after building a session with
   /// the device. 
   bool acknowledged;
@@ -167,7 +174,7 @@ class OmemoDoubleRatchet {
   /// Create an OMEMO session using the Signed Pre Key [spk], the shared secret [sk] that
   /// was obtained using a X3DH and the associated data [ad] that was also obtained through
   /// a X3DH. [ik] refers to Bob's (the receiver's) IK public key.
-  static Future<OmemoDoubleRatchet> initiateNewSession(OmemoPublicKey spk, OmemoPublicKey ik, List<int> sk, List<int> ad) async {
+  static Future<OmemoDoubleRatchet> initiateNewSession(OmemoPublicKey spk, OmemoPublicKey ik, List<int> sk, List<int> ad, int timestamp) async {
     final dhs = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
     final dhr = spk;
     final rk  = await kdfRk(sk, await omemoDH(dhs, dhr, 0));
@@ -186,6 +193,7 @@ class OmemoDoubleRatchet {
       ad,
       {},
       false,
+      timestamp,
     );
   }
 
@@ -193,7 +201,7 @@ class OmemoDoubleRatchet {
   /// Pre Key keypair [spk], the shared secret [sk] that was obtained through a X3DH and
   /// the associated data [ad] that was also obtained through a X3DH. [ik] refers to
   /// Alice's (the initiator's) IK public key.
-  static Future<OmemoDoubleRatchet> acceptNewSession(OmemoKeyPair spk, OmemoPublicKey ik, List<int> sk, List<int> ad) async {
+  static Future<OmemoDoubleRatchet> acceptNewSession(OmemoKeyPair spk, OmemoPublicKey ik, List<int> sk, List<int> ad, int kexTimestamp) async {
     return OmemoDoubleRatchet(
       spk,
       null,
@@ -207,6 +215,7 @@ class OmemoDoubleRatchet {
       ad,
       {},
       false,
+      kexTimestamp,
     );
   }
 
@@ -233,6 +242,7 @@ class OmemoDoubleRatchet {
       'session_ad': base64.encode(sessionAd),
       'mkskipped': mkSkippedSerialised,
       'acknowledged': acknowledged,
+      'kex_timestamp': kexTimestamp,
     };
   }
   
@@ -268,18 +278,18 @@ class OmemoDoubleRatchet {
   }
 
   Future<void> _dhRatchet(OmemoMessage header) async {
-    pn = header.n!;
+    pn = ns;
     ns = 0;
     nr = 0;
     dhr = OmemoPublicKey.fromBytes(header.dhPub!, KeyPairType.x25519);
 
     final newRk = await kdfRk(rk, await omemoDH(dhs, dhr!, 0));
-    rk = newRk;
-    ckr = newRk;
+    rk = List.from(newRk);
+    ckr = List.from(newRk);
     dhs = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
     final newNewRk = await kdfRk(rk, await omemoDH(dhs, dhr!, 0));
-    rk = newNewRk;
-    cks = newNewRk;
+    rk = List.from(newNewRk);
+    cks = List.from(newNewRk);
   }
 
   /// Encrypt [plaintext] using the Double Ratchet.
@@ -313,8 +323,8 @@ class OmemoDoubleRatchet {
     }
 
     final dhPubMatches = listsEqual(
-      header.dhPub ?? <int>[],
-      await dhr?.getBytes() ?? <int>[],
+      header.dhPub!,
+      (await dhr?.getBytes()) ?? <int>[],
     );
     if (!dhPubMatches) {
       await _skipMessageKeys(header.pn!);
@@ -330,12 +340,40 @@ class OmemoDoubleRatchet {
     return decrypt(mk, ciphertext, concat([sessionAd, header.writeToBuffer()]), sessionAd);
   }
 
+  OmemoDoubleRatchet clone() {
+    return OmemoDoubleRatchet(
+      dhs,
+      dhr,
+      rk,
+      cks != null ?
+        List<int>.from(cks!) :
+        null,
+      ckr != null ?
+        List<int>.from(ckr!) :
+        null,
+      ns,
+      nr,
+      pn,
+      ik,
+      sessionAd,
+      Map<SkippedKey, List<int>>.from(mkSkipped),
+      acknowledged,
+      kexTimestamp,
+    );
+  }
+  
   @visibleForTesting
   Future<bool> equals(OmemoDoubleRatchet other) async {
-    // ignore: invalid_use_of_visible_for_testing_member
-    final dhrMatch = dhr == null ? other.dhr == null : await dhr!.equals(other.dhr!);
-    final ckrMatch = ckr == null ? other.ckr == null : listsEqual(ckr!, other.ckr!);
-    final cksMatch = cks == null ? other.cks == null : listsEqual(cks!, other.cks!);
+    final dhrMatch = dhr == null ?
+      other.dhr == null :
+      // ignore: invalid_use_of_visible_for_testing_member
+      other.dhr != null && await dhr!.equals(other.dhr!);
+    final ckrMatch = ckr == null ?
+      other.ckr == null :
+      other.ckr != null && listsEqual(ckr!, other.ckr!);
+    final cksMatch = cks == null ?
+      other.cks == null :
+      other.cks != null && listsEqual(cks!, other.cks!);
  
     // ignore: invalid_use_of_visible_for_testing_member
     final dhsMatch = await dhs.equals(other.dhs);
@@ -351,6 +389,7 @@ class OmemoDoubleRatchet {
       ns == other.ns &&
       nr == other.nr &&
       pn == other.pn &&
-      listsEqual(sessionAd, other.sessionAd);
+      listsEqual(sessionAd, other.sessionAd) &&
+      kexTimestamp == other.kexTimestamp;
   }
 }

lib/src/errors.dart

@@ -30,3 +30,16 @@ class NoDecryptionKeyException implements Exception {
 class UnknownSignedPrekeyException implements Exception {
   String errMsg() => 'Unknown Signed Prekey used.';
 }
+
+/// Triggered by the Session Manager when the received Key Exchange message does not meet
+/// the requirement that a key exchange, given that the ratchet already exists, must be
+/// sent after its creation.
+class InvalidKeyExchangeException implements Exception {
+  String errMsg() => 'The key exchange was sent before the last kex finished';
+}
+
+/// Triggered by the Session Manager when a message's sequence number is smaller than we
+/// expect it to be.
+class MessageAlreadyDecryptedException implements Exception {
+  String errMsg() => 'The message has already been decrypted';
+}

lib/src/helpers.dart

@@ -73,3 +73,7 @@ OmemoKeyPair? decodeKeyPairIfNotNull(String? pk, String? sk, KeyPairType type) {
     type,
   );
 }
+
+int getTimestamp() {
+  return DateTime.now().millisecondsSinceEpoch;
+}

lib/src/omemo/sessionmanager.dart

@@ -154,6 +154,7 @@ class OmemoSessionManager {
       bundle.ik,
       kexResult.sk,
       kexResult.ad,
+      getTimestamp(),
     );
 
     await _trustManager.onNewSession(jid, deviceId);
@@ -200,6 +201,7 @@ class OmemoSessionManager {
       OmemoPublicKey.fromBytes(kex.ik!, KeyPairType.ed25519),
       kexResult.sk,
       kexResult.ad,
+      getTimestamp(),
     );
 
     await _trustManager.onNewSession(jid, deviceId);
@@ -240,7 +242,11 @@ class OmemoSessionManager {
     final kex = <int, OmemoKeyExchange>{};
     if (newSessions != null) {
       for (final newSession in newSessions) {
-        kex[newSession.id] = await addSessionFromBundle(newSession.jid, newSession.id, newSession);
+        kex[newSession.id] = await addSessionFromBundle(
+          newSession.jid,
+          newSession.id,
+          newSession,
+        );
       }
     }
     
@@ -300,7 +306,7 @@ class OmemoSessionManager {
   /// [mapKey] with [oldRatchet].
   Future<void> _restoreRatchet(RatchetMapKey mapKey, OmemoDoubleRatchet oldRatchet) async {
     await _lock.synchronized(() {
-      _log.finest('Restoring ratchet ${mapKey.jid}:${mapKey.deviceId}');
+      _log.finest('Restoring ratchet ${mapKey.jid}:${mapKey.deviceId} to ${oldRatchet.nr}');
       _ratchetMap[mapKey] = oldRatchet;
 
       // Commit the ratchet
@@ -318,12 +324,15 @@ class OmemoSessionManager {
   /// <keys /> element with a "jid" attribute matching our own. [senderJid] refers to the
   /// bare Jid of the sender. [senderDeviceId] refers to the "sid" attribute of the
   /// <encrypted /> element.
+  /// [timestamp] refers to the time the message was sent. This might be either what the
+  /// server tells you via "XEP-0203: Delayed Delivery" or the point in time at which
+  /// you received the stanza, if no Delayed Delivery element was found.
   ///
   /// If the received message is an empty OMEMO message, i.e. there is no <payload />
   /// element, then [ciphertext] must be set to null. In this case, this function
   /// will return null as there is no message to be decrypted. This, however, is used
   /// to set up sessions or advance the ratchets.
-  Future<String?> decryptMessage(List<int>? ciphertext, String senderJid, int senderDeviceId, List<EncryptedKey> keys) async {
+  Future<String?> decryptMessage(List<int>? ciphertext, String senderJid, int senderDeviceId, List<EncryptedKey> keys, int timestamp) async {
     // Try to find a session we can decrypt with.
     var device = await getDevice();
     final rawKey = keys.firstWhereOrNull((key) => key.rid == device.id);
@@ -335,22 +344,32 @@ class OmemoSessionManager {
     final decodedRawKey = base64.decode(rawKey.value);
     OmemoAuthenticatedMessage authMessage;
     OmemoDoubleRatchet? oldRatchet;
+    OmemoMessage? message;
     if (rawKey.kex) {
       // If the ratchet already existed, we store it. If it didn't, oldRatchet will stay
       // null.
-      oldRatchet = await _getRatchet(ratchetKey);
+      final oldRatchet = (await _getRatchet(ratchetKey))?.clone();
+      final kex = OmemoKeyExchange.fromBuffer(decodedRawKey);
+      authMessage = kex.message!;
+      message = OmemoMessage.fromBuffer(authMessage.message!);
+
+      // Guard against old key exchanges
+      if (oldRatchet != null) {
+        _log.finest('KEX for existent ratchet. ${oldRatchet.pn}');
+        if (oldRatchet.kexTimestamp > timestamp) {
+          throw InvalidKeyExchangeException();
+        }
+      }
       
       // TODO(PapaTutuWawa): Only do this when we should
-      final kex = OmemoKeyExchange.fromBuffer(decodedRawKey);
       await _addSessionFromKeyExchange(
         senderJid,
         senderDeviceId,
         kex,
       );
 
-      authMessage = kex.message!;
-
       // Replace the OPK
+      // TODO(PapaTutuWawa): Replace the OPK when we know that the KEX worked
       await _deviceLock.synchronized(() async {
         device = await device.replaceOnetimePrekey(kex.pkId!);
 
@@ -359,6 +378,7 @@ class OmemoSessionManager {
       });
     } else {
       authMessage = OmemoAuthenticatedMessage.fromBuffer(decodedRawKey);
+      message = OmemoMessage.fromBuffer(authMessage.message!);
     }
     
     final devices = _deviceMap[senderJid];
@@ -369,11 +389,10 @@ class OmemoSessionManager {
       throw NoDecryptionKeyException();
     }
 
-    final message = OmemoMessage.fromBuffer(authMessage.message!);
     List<int>? keyAndHmac;
     // We can guarantee that the ratchet exists at this point in time
     final ratchet = (await _getRatchet(ratchetKey))!;
-    oldRatchet ??= ratchet ;
+    oldRatchet ??= ratchet.clone();
 
     try {
       if (rawKey.kex) {

test/double_ratchet_test.dart

@@ -84,12 +84,14 @@ void main() {
       ikBob.pk,
       resultAlice.sk,
       resultAlice.ad,
+      0,
     );
     final bobsRatchet = await OmemoDoubleRatchet.acceptNewSession(
       spkBob,
       ikAlice.pk,
       resultBob.sk,
       resultBob.ad,
+      0,
     );
 
     expect(alicesRatchet.sessionAd, bobsRatchet.sessionAd);

test/omemo_test.dart

@@ -90,6 +90,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(messagePlaintext, bobMessage);
     // The ratchet should be modified two times: Once for when the ratchet is created and
@@ -121,6 +122,7 @@ void main() {
       bobJid,
       await bobSession.getDeviceId(),
       bobResponseMessage.encryptedKeys,
+      0,
     );
     expect(bobResponseText, aliceReceivedMessage);
   });
@@ -170,6 +172,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(messagePlaintext, bobMessage);
 
@@ -189,6 +192,7 @@ void main() {
       bobJid,
       await bobSession.getDeviceId(),
       bobResponseMessage.encryptedKeys,
+      0,
     );
     expect(bobResponseText, aliceReceivedMessage);
 
@@ -245,6 +249,7 @@ void main() {
       aliceJid,
       await aliceSession1.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(messagePlaintext, bobMessage);
 
@@ -254,6 +259,7 @@ void main() {
       aliceJid,
       await aliceSession1.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(messagePlaintext, aliceMessage2);
   });
@@ -294,6 +300,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(bobMessage, null);
 
@@ -371,6 +378,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
     expect(messagePlaintext, bobMessage);
   });
@@ -437,6 +445,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      0,
     );
 
     for (var i = 0; i < 100; i++) {
@@ -456,6 +465,7 @@ void main() {
         bobJid,
         await bobSession.getDeviceId(),
         bobResponseMessage.encryptedKeys,
+        0,
       );
       expect(messageText, aliceReceivedMessage);
     }
@@ -610,6 +620,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       msg1.encryptedKeys,
+      0,
     );
     final aliceRatchet1 = aliceSession.getRatchet(
       bobJid,
@@ -634,6 +645,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       msg2.encryptedKeys,
+      getTimestamp(),
     );
     final aliceRatchet2 = aliceSession.getRatchet(
       bobJid,
@@ -653,7 +665,7 @@ void main() {
     expect(await bobRatchet1.equals(bobRatchet2), false);
   });
 
-  test('Test receiving an old message that contains a KEX', () async {
+  test('Test receiving old messages including a KEX', () async {
     const aliceJid = 'alice@server.example';
     const bobJid = 'bob@other.server.example';
     // Alice and Bob generate their sessions
@@ -668,6 +680,9 @@ void main() {
       opkAmount: 2,
     );
 
+    final bobsReceivedMessages = List<EncryptionResult>.empty(growable: true);
+    final bobsReceivedMessagesTimestamps = List<int>.empty(growable: true);
+    
     // Alice sends Bob a message
     final msg1 = await aliceSession.encryptToJid(
       bobJid,
@@ -676,12 +691,16 @@ void main() {
         await bobSession.getDeviceBundle(),
       ],
     );
+    bobsReceivedMessages.add(msg1);
+    final t1 = getTimestamp();
+    bobsReceivedMessagesTimestamps.add(t1);
 
     await bobSession.decryptMessage(
       msg1.ciphertext,
       aliceJid,
       await aliceSession.getDeviceId(),
       msg1.encryptedKeys,
+      t1,
     );
 
     // Bob responds
@@ -689,97 +708,69 @@ void main() {
       aliceJid,
       'Hello!',
     );
+
     await aliceSession.decryptMessage(
       msg2.ciphertext,
       bobJid,
       await bobSession.getDeviceId(),
       msg2.encryptedKeys,
+      getTimestamp(),
     );
     
-    // Due to some issue with the transport protocol, the first message Bob received is
-    // received again
-    try {
-      await bobSession.decryptMessage(
-        msg1.ciphertext,
+    // Send some messages between the two
+    for (var i = 0; i < 100; i++) {
+      final msg = await aliceSession.encryptToJid(
+        bobJid,
+        'Hello $i',
+      );
+      bobsReceivedMessages.add(msg);
+      final t = getTimestamp();
+      bobsReceivedMessagesTimestamps.add(t);
+      final result = await bobSession.decryptMessage(
+        msg.ciphertext,
         aliceJid,
         await aliceSession.getDeviceId(),
-        msg1.encryptedKeys,
+        msg.encryptedKeys,
+        t,
       );
-      expect(true, false);
-    } on InvalidMessageHMACException {
-      // NOOP
-    }
-
-    final msg3 = await aliceSession.encryptToJid(
-      bobJid,
-      'Are you okay?',
-    );
-    final result = await bobSession.decryptMessage(
-      msg3.ciphertext,
-      aliceJid,
-      await aliceSession.getDeviceId(),
-      msg3.encryptedKeys,
-    );
-
-    expect(result, 'Are you okay?');
-  });
-
-  test('Test receiving an old message that does not contain a KEX', () async {
-    const aliceJid = 'alice@server.example';
-    const bobJid = 'bob@other.server.example';
-    // Alice and Bob generate their sessions
-    final aliceSession = await OmemoSessionManager.generateNewIdentity(
-      aliceJid,
-      AlwaysTrustingTrustManager(),
-      opkAmount: 1,
-    );
-    final bobSession = await OmemoSessionManager.generateNewIdentity(
-      bobJid,
-      AlwaysTrustingTrustManager(),
-      opkAmount: 2,
-    );
-
-    // Alice sends Bob a message
-    final msg1 = await aliceSession.encryptToJid(
-      bobJid,
-      'Hallo Welt',
-      newSessions: [
-        await bobSession.getDeviceBundle(),
-      ],
-    );
-    await bobSession.decryptMessage(
-      msg1.ciphertext,
-      aliceJid,
-      await aliceSession.getDeviceId(),
-      msg1.encryptedKeys,
-    );
-
-    // Bob responds
-    final msg2 = await bobSession.encryptToJid(
-      aliceJid,
-      'Hello!',
-    );
-    await aliceSession.decryptMessage(
-      msg2.ciphertext,
-      bobJid,
-      await bobSession.getDeviceId(),
-      msg2.encryptedKeys,
-    );
-
-    // Due to some issue with the transport protocol, the first message Alice received is
-    // received again.
-    try {
-      await aliceSession.decryptMessage(
-        msg2.ciphertext,
-        bobJid,
-        await bobSession.getDeviceId(),
-        msg2.encryptedKeys,
-      );
-      expect(true, false);
-    } catch (_) {
-      // NOOP
+
+      expect(result, 'Hello $i');
     }
 
+    // Due to some issue with the transport protocol, the messages to Bob are received
+    // again.
+    final ratchetPreError = bobSession
+      .getRatchet(aliceJid, await aliceSession.getDeviceId())
+      .clone();
+    var invalidKex = 0;
+    var errorCounter = 0;
+    for (var i = 0; i < bobsReceivedMessages.length; i++) {
+      final msg = bobsReceivedMessages[i];
+      try {
+        await bobSession.decryptMessage(
+          msg.ciphertext,
+          aliceJid,
+          await aliceSession.getDeviceId(),
+          msg.encryptedKeys,
+          bobsReceivedMessagesTimestamps[i],
+        );
+        expect(true, false);
+      } on InvalidMessageHMACException catch (_) {
+        errorCounter++;
+      } on InvalidKeyExchangeException catch (_) {
+        invalidKex++;
+      }
+    }
+    final ratchetPostError = bobSession
+      .getRatchet(aliceJid, await aliceSession.getDeviceId())
+      .clone();
+
+    // The 100 messages including the initial KEX message
+    expect(invalidKex, 1);
+    expect(errorCounter, 100);
+    expect(await ratchetPreError.equals(ratchetPostError), true);
+
+    
     final msg3 = await aliceSession.encryptToJid(
       bobJid,
       'Are you okay?',
@@ -789,6 +780,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       msg3.encryptedKeys,
+      104,
     );
 
     expect(result, 'Are you okay?');

test/serialisation_test.dart

@@ -62,6 +62,7 @@ void main() {
       aliceJid,
       await aliceSession.getDeviceId(),
       aliceMessage.encryptedKeys,
+      getTimestamp(),
     );
     final aliceOld = aliceSession.getRatchet(bobJid, await bobSession.getDeviceId());
     final aliceSerialised = jsonify(await aliceOld.toJson());