def _getCipher(self, cip, iv, key):
        """
        Creates an initialized cipher object.

        @param cip: the name of the cipher, maps into cipherMap
        @param iv: the initialzation vector
        @param key: the encryption key

        @return: the cipher object.
        """
        algorithmClass, keySize, modeClass = self.cipherMap[cip]
        if algorithmClass is None:
            return _DummyCipher()

        return Cipher(
            algorithmClass(key[:keySize]),
            modeClass(iv[:algorithmClass.block_size // 8]),
            backend=default_backend(),
        )

        Creates an initialized cipher object.

        @param cip: the name of the cipher, maps into cipherMap
        @param iv: the initialzation vector
        @param key: the encryption key

        @return: the cipher object.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/ssh/transport.py