diff --git a/telethon/network/mtproto_sender.py b/telethon/network/mtproto_sender.py
index b4483903..400ea6cf 100644
--- a/telethon/network/mtproto_sender.py
+++ b/telethon/network/mtproto_sender.py
@@ -6,7 +6,7 @@ from .. import helpers as utils
 from ..crypto import AES
 from ..errors import (
     BadMessageError, InvalidChecksumError, BrokenAuthKeyError,
-    rpc_message_to_error
+    rpc_message_to_error, SecurityError
 )
 from ..extensions import BinaryReader
 from ..tl import TLMessage, MessageContainer, GzipPacked
@@ -158,6 +158,8 @@ class MtProtoSender:
             msg_key = reader.read(16)
 
             key, iv = utils.calc_key(self.session.auth_key.key, msg_key, False)
+            if reader.read(len(body) - reader.tell_position()) % 16 != 0:
+                raise SecurityError('AES block size missmatch')
             plain_text = AES.decrypt_ige(
                 reader.read(len(body) - reader.tell_position()), key, iv)