Inverse of `encrypt_message_data` for incoming server messages.

def decrypt_message_data(self, body):
        """
        Inverse of `encrypt_message_data` for incoming server messages.
        """
        if len(body) < 8:
            raise InvalidBufferError(body)

        # TODO Check salt, session_id and sequence_number
        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')

        msg_key = body[8:24]
        aes_key, aes_iv = self._calc_key(self.auth_key.key, msg_key, False)
        body = AES.decrypt_ige(body[24:], aes_key, aes_iv)

        # https://core.telegram.org/mtproto/security_guidelines
        # Sections "checking sha256 hash" and "message length"
        our_key = sha256(self.auth_key.key[96:96 + 32] + body)
        if msg_key != our_key.digest()[8:24]:
            raise SecurityError(
                "Received msg_key doesn't match with expected one")

        reader = BinaryReader(body)
        reader.read_long()  # remote_salt
        if reader.read_long() != self.id:
            raise SecurityError('Server replied with a wrong session ID')

        remote_msg_id = reader.read_long()
        remote_sequence = reader.read_int()
        reader.read_int()  # msg_len for the inner object, padding ignored

        # We could read msg_len bytes and use those in a new reader to read
        # the next TLObject without including the padding, but since the
        # reader isn't used for anything else after this, it's unnecessary.
        obj = reader.tgread_object()

        return TLMessage(remote_msg_id, remote_sequence, obj)