feat: Rework the double ratchet

lib/src/errors.dart

@@ -1,61 +1,47 @@
-abstract class OmemoException {}
+abstract class OmemoError {}
 
 /// Triggered during X3DH if the signature if the SPK does verify to the actual SPK.
-class InvalidSignatureException extends OmemoException implements Exception {
+class InvalidSignatureException extends OmemoError implements Exception {
   String errMsg() =>
       'The signature of the SPK does not match the provided signature';
 }
 
 /// Triggered by the Double Ratchet if the computed HMAC does not match the attached HMAC.
-/// Triggered by the Session Manager if the computed HMAC does not match the attached HMAC.
-class InvalidMessageHMACException extends OmemoException implements Exception {
-  String errMsg() => 'The computed HMAC does not match the provided HMAC';
-}
+class InvalidMessageHMACError extends OmemoError {}
 
 /// Triggered by the Double Ratchet if skipping messages would cause skipping more than
 /// MAXSKIP messages
-class SkippingTooManyMessagesException extends OmemoException
-    implements Exception {
-  String errMsg() => 'Skipping messages would cause a skip bigger than MAXSKIP';
-}
+class SkippingTooManyKeysError extends OmemoError {}
 
 /// Triggered by the Session Manager if the message key is not encrypted for the device.
-class NotEncryptedForDeviceException extends OmemoException
-    implements Exception {
-  String errMsg() => 'Not encrypted for this device';
-}
+class NotEncryptedForDeviceError extends OmemoError {}
 
 /// Triggered by the Session Manager when there is no key for decrypting the message.
-class NoDecryptionKeyException extends OmemoException implements Exception {
+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 UnknownSignedPrekeyException extends OmemoException implements Exception {
-  String errMsg() => 'Unknown Signed Prekey used.';
-}
+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 OmemoException implements Exception {
+class InvalidKeyExchangeException extends OmemoError 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 extends OmemoException
-    implements Exception {
-  String errMsg() => 'The message has already been decrypted';
-}
-
 /// 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
 /// failed.
-class NoKeyMaterialAvailableException extends OmemoException
+class NoKeyMaterialAvailableException extends OmemoError
     implements Exception {
   String errMsg() =>
       'No key material available to create a ratchet session with';
 }
+
+/// 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 {}