fix: Restore the ratchets in case of an error

This means that if the ratchet fails to decrypt a message, from the
outside it will be as if that one message had never been received.
Thus, the ratchet can be used normally. This is to guard against
messages that are received again.

test/omemo_test.dart

@@ -614,4 +614,144 @@ void main() {
     expect(await aliceRatchet1.equals(aliceRatchet2), false);
     expect(await bobRatchet1.equals(bobRatchet2), false);
   });
+
+  test('Test receiving an old message that contains 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 Bob received is
+    // received again
+    try {
+      await bobSession.decryptMessage(
+        msg1.ciphertext,
+        aliceJid,
+        await aliceSession.getDeviceId(),
+        msg1.encryptedKeys,
+      );
+      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
+    }
+
+    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?');
+  });
 }