feat: Guard against malformed ciphertext

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;
+}