feat: Each OPK now gets it's own unique id

lib/src/omemo/decryption_result.dart

@@ -3,7 +3,15 @@ import 'package:omemo_dart/src/errors.dart';
 
 @immutable
 class DecryptionResult {
-  const DecryptionResult(this.payload, this.error);
+  const DecryptionResult(this.payload, this.usedOpkId, this.error);
+
+  /// The decrypted payload or null, if it was an empty OMEMO message.
   final String? payload;
+
+  /// In case a key exchange has been performed: The id of the used OPK. Useful for
+  /// replacing the OPK after a message catch-up.
+  final int? usedOpkId;
+
+  /// The error that occurred during decryption or null, if no error occurred.
   final OmemoError? error;
 }

lib/src/omemo/device.dart

@@ -35,7 +35,16 @@ class OmemoDevice {
 
     final opks = <int, OmemoKeyPair>{};
     for (var i = 0; i < opkAmount; i++) {
-      opks[i] = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+      // Generate unique ids for each key
+      while (true) {
+        final opkId = generateRandom32BitNumber();
+        if (opks.containsKey(opkId)) {
+          continue;
+        }
+
+        opks[opkId] = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+        break;
+      }
     }
 
     return OmemoDevice(jid, id, ik, spk, spkId, signature, null, null, opks);
@@ -72,7 +81,18 @@ class OmemoDevice {
   /// a new Device object that copies over everything but replaces said key.
   @internal
   Future<OmemoDevice> replaceOnetimePrekey(int id) async {
-    opks[id] = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+    opks.remove(id);
+
+    // Generate a new unique id for the OPK.
+    while (true) {
+      final newId = generateRandom32BitNumber();
+      if (opks.containsKey(newId)) {
+        continue;
+      }
+
+      opks[newId] = await OmemoKeyPair.generateNewPair(KeyPairType.x25519);
+      break;
+    }
 
     return OmemoDevice(
       jid,

lib/src/omemo/omemo.dart

@@ -351,6 +351,7 @@ class OmemoManager {
     final key = stanza.keys.firstWhereOrNull((key) => key.rid == deviceId);
     if (key == null) {
       return DecryptionResult(
+        null,
         null,
         NotEncryptedForDeviceError(),
       );
@@ -359,6 +360,7 @@ class OmemoManager {
     // Protobuf will happily parse this and return bogus data.
     if (key.value.isEmpty) {
       return DecryptionResult(
+        null,
         null,
         MalformedEncryptedKeyError(),
       );
@@ -396,6 +398,7 @@ class OmemoManager {
         spk = device.oldSpk!;
       } else {
         return DecryptionResult(
+          null,
           null,
           UnknownSignedPrekeyError(),
         );
@@ -438,7 +441,7 @@ class OmemoManager {
         final error = keyAndHmac.get<OmemoError>();
         _log.warning('Failed to decrypt symmetric key: $error');
 
-        return DecryptionResult(null, error);
+        return DecryptionResult(null, null, error);
       }
 
       Result<OmemoError, String?> result;
@@ -452,6 +455,7 @@ class OmemoManager {
           _log.warning('Decrypting payload failed: $error');
 
           return DecryptionResult(
+            null,
             null,
             error,
           );
@@ -503,6 +507,7 @@ class OmemoManager {
 
       return DecryptionResult(
         result.get<String?>(),
+        kexMessage.pkId,
         null,
       );
     } else {
@@ -517,6 +522,7 @@ class OmemoManager {
         await _sendOmemoHeartbeat(stanza.bareSenderJid);
 
         return DecryptionResult(
+          null,
           null,
           NoSessionWithDeviceError(),
         );
@@ -540,7 +546,7 @@ class OmemoManager {
       if (keyAndHmac.isType<OmemoError>()) {
         final error = keyAndHmac.get<OmemoError>();
         _log.warning('Failed to decrypt symmetric key: $error');
-        return DecryptionResult(null, error);
+        return DecryptionResult(null, null, error);
       }
 
       Result<OmemoError, String?> result;
@@ -553,6 +559,7 @@ class OmemoManager {
           final error = result.get<OmemoError>();
           _log.warning('Failed to decrypt message: $error');
           return DecryptionResult(
+            null,
             null,
             error,
           );
@@ -586,6 +593,7 @@ class OmemoManager {
       return DecryptionResult(
         result.get<String?>(),
         null,
+        null,
       );
     }
   }
@@ -958,7 +966,33 @@ class OmemoManager {
   @visibleForTesting
   OmemoDoubleRatchet? getRatchet(RatchetMapKey key) => _ratchetMap[key];
 
-  /// Trust management functions
+  /// Replaces the OPK with id [opkId] and commits the new device to storage. This
+  /// function should not be called. It's only useful for rotating OPKs after message
+  /// catch-up, because in that case the OPKs are not rotated automatically.
+  Future<void> replaceOnetimePrekey(int opkId) async {
+    await _deviceLock.synchronized(() async {
+      // Replace OPK
+      await _device.replaceOnetimePrekey(opkId);
+
+      // Commit the device
+      await commitDevice(_device);
+    });
+  }
+
+  /// Replaces the SPK of our device and commits it to storage.
+  Future<void> replaceSignedPrekey() async {
+    await _deviceLock.synchronized(() async {
+      // Replace SPK
+      await _device.replaceSignedPrekey();
+
+      // Commit the device
+      await commitDevice(_device);
+    });
+  }
+
+  /// Acquire a lock for interacting with the trust manager for modifying the trust
+  /// state of [jid]. [callback] is called from within the critical section with the
+  /// trust manager as its parameter.
   Future<void> withTrustManager(
     String jid,
     Future<void> Function(TrustManager) callback,