[omemo] Add back 12 byte IV read support for aesgcm links

omemo/file_crypto.py

@@ -169,10 +169,19 @@ class FileDecryption:
             raise ValueError('Invalid fragment')
 
         fragment = binascii.unhexlify(fragment)
-        key = fragment[16:]
+        size = len(fragment)
-        iv = fragment[:16]
+        # Clients started out with using a 16 byte IV but long term
-        if len(key) != 32 or len(iv) != 16:
+        # want to swtich to the more performant 12 byte IV
-            raise ValueError('Invalid fragment')
+        # We have to support both
+        if size == 48:
+            key = fragment[16:]
+            iv = fragment[:16]
+        elif size == 44:
+            key = fragment[12:]
+            iv = fragment[:12]
+        else:
+            raise ValueError('Invalid fragment size: %s' % size)
+
         return key, iv
 
     @staticmethod