'Creates a crypto context for encrypting
:param key: 256-bit key
:param iv: 128-bit iv or nonce used for encryption
:raises ValueError: on invalid key or iv
:returns: an instance of an encryptor'

def create_encryption_ctxt(self, key, iv):

    self.check_key(key)
    engine = Cipher(algorithms.AES(key), modes.CTR(iv), backend=self.backend)
    return engine.encryptor()
