'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.'

def _getCipher(self, cip, iv, key):

    (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())
