diff --git a/telethon/crypto/authkey.py b/telethon/crypto/authkey.py
index 8475ec17..37f65017 100644
--- a/telethon/crypto/authkey.py
+++ b/telethon/crypto/authkey.py
@@ -39,7 +39,7 @@ class AuthKey:
         with BinaryReader(sha1(self._key).digest()) as reader:
             self.aux_hash = reader.read_long(signed=False)
             reader.read(4)
-            self.key_id = reader.read_long(signed=False)
+            self.key_id = reader.read_long(signed=True)
 
     # TODO This doesn't really fit here, it's only used in authentication
     def calc_new_nonce_hash(self, new_nonce, number):
diff --git a/telethon/network/mtprotostate.py b/telethon/network/mtprotostate.py
index 49fb7478..0ceaa513 100644
--- a/telethon/network/mtprotostate.py
+++ b/telethon/network/mtprotostate.py
@@ -144,7 +144,7 @@ class MTProtoState:
         msg_key = msg_key_large[8:24]
         aes_key, aes_iv = self._calc_key(self.auth_key.key, msg_key, True)
 
-        key_id = struct.pack('<Q', self.auth_key.key_id)
+        key_id = struct.pack('<q', self.auth_key.key_id)
         return (key_id + msg_key +
                 AES.encrypt_ige(data + padding, aes_key, aes_iv))
 
@@ -158,7 +158,7 @@ class MTProtoState:
             raise InvalidBufferError(body)
 
         # TODO Check salt, session_id and sequence_number
-        key_id = struct.unpack('<Q', body[:8])[0]
+        key_id = struct.unpack('<q', body[:8])[0]
         if key_id != self.auth_key.key_id:
             raise SecurityError('Server replied with an invalid auth key')