    def encrypt_message(self, signed_message, message, key, iv):
        raw = signed_message + message
        block_size = AES.block_size
        pad = lambda s: s + (block_size - len(s) % block_size) * chr(block_size - len(s) % block_size).encode('utf-8')
        message_to_encrypt = pad(raw)
        cipher = AES.new(key, AES.MODE_CBC, iv)
        return cipher.encrypt(message_to_encrypt)