feat: Guard against malformed ciphertext

lib/src/common/result.dart

@@ -3,7 +3,7 @@ class Result<T, V> {
   const Result(this._data)
       : assert(
           _data is T || _data is V,
-          'Invalid data type: Must be either $T or $V',
+          'Invalid data type $_data: Must be either $T or $V',
         );
   final dynamic _data;
 

lib/src/crypto.dart

@@ -1,5 +1,7 @@
 import 'dart:convert';
 import 'package:cryptography/cryptography.dart';
+import 'package:omemo_dart/src/common/result.dart';
+import 'package:omemo_dart/src/errors.dart';
 import 'package:omemo_dart/src/keys.dart';
 
 /// Performs X25519 with [kp] and [pk]. If [identityKey] is set, then
@@ -92,7 +94,7 @@ Future<List<int>> aes256CbcEncrypt(
 
 /// A small helper function to make AES-256-CBC easier. Decrypt [ciphertext] using [key] as
 /// the encryption key and [iv] as the IV. Returns the ciphertext.
-Future<List<int>> aes256CbcDecrypt(
+Future<Result<MalformedCiphertextError, List<int>>> aes256CbcDecrypt(
   List<int> ciphertext,
   List<int> key,
   List<int> iv,
@@ -100,13 +102,19 @@ Future<List<int>> aes256CbcDecrypt(
   final algorithm = AesCbc.with256bits(
     macAlgorithm: MacAlgorithm.empty,
   );
-  return algorithm.decrypt(
+  try {
+    return Result(
+      await algorithm.decrypt(
         NoMacSecretBox(
           ciphertext,
           nonce: iv,
         ),
         secretKey: SecretKey(key),
+      ),
     );
+  } catch (ex) {
+    return Result(MalformedCiphertextError(ex)); 
+  }
 }
 
 /// OMEMO often uses the output of a HMAC-SHA-256 truncated to its first 16 bytes.

lib/src/double_ratchet/double_ratchet.dart

@@ -261,7 +261,11 @@ class OmemoDoubleRatchet {
     }
 
     final plaintext = await aes256CbcDecrypt(ciphertext, keys.encryptionKey, keys.iv);
-    return Result(plaintext);
+    if (plaintext.isType<MalformedCiphertextError>()) {
+      return Result(plaintext.get<MalformedCiphertextError>());
+    }
+
+    return Result(plaintext.get<List<int>>());
   }
 
   /// Checks whether we could decrypt the payload in [header] with a skipped key. If yes,

lib/src/errors.dart

@@ -13,22 +13,10 @@ class SkippingTooManyKeysError extends OmemoError {}
 /// Triggered by the Session Manager if the message key is not encrypted for the device.
 class NotEncryptedForDeviceError extends OmemoError {}
 
-/// Triggered by the Session Manager when there is no key for decrypting the message.
-class NoDecryptionKeyException extends OmemoError implements Exception {
-  String errMsg() => 'No key available for decrypting the message';
-}
-
 /// Triggered by the Session Manager when the identifier of the used Signed Prekey
 /// is neither the current SPK's identifier nor the old one's.
 class UnknownSignedPrekeyError extends OmemoError {}
 
-/// 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 extends OmemoError implements Exception {
-  String errMsg() => 'The key exchange was sent before the last kex finished';
-}
-
 /// Triggered by the OmemoManager when we could not encrypt a message as we have
 /// no key material available. That happens, for example, when we want to create a
 /// ratchet session with a JID we had no session with but fetching the device bundle
@@ -38,3 +26,11 @@ class NoKeyMaterialAvailableError extends OmemoError {}
 /// A non-key-exchange message was received that was encrypted for our device, but we have no ratchet with
 /// the device that sent the message.
 class NoSessionWithDeviceError extends OmemoError {}
+
+/// Caused when the AES-256 CBC decryption failed.
+class MalformedCiphertextError extends OmemoError {
+  MalformedCiphertextError(this.ex);
+
+  /// The exception that was raised while decryption.
+  final Object ex;
+}

lib/src/omemo/omemo.dart

@@ -152,14 +152,20 @@ class OmemoManager {
       return Result(InvalidMessageHMACError());
     }
 
-    // TODO: Handle an exception from the crypto implementation
+    final result = await aes256CbcDecrypt(
-    return Result(
-      utf8.decode(
-        await aes256CbcDecrypt(
       ciphertext,
       derivedKeys.encryptionKey,
       derivedKeys.iv,
-        ),
+    );
+    if (result.isType<MalformedCiphertextError>()) {
+      return Result(
+        result.get<MalformedCiphertextError>(),
+      );
+    }
+
+    return Result(
+      utf8.decode(
+        result.get<List<int>>(),
       ),
     );
   }
@@ -316,8 +322,6 @@ class OmemoManager {
         kexMessage.ek,
         KeyPairType.x25519,
       );
-
-      // TODO: Guard against invalid signatures
       final kex = await x3dhFromInitialMessage(
         X3DHMessage(
           kexIk,