Generate and hmac signature for this encrypted data
        :param key:
        :param iv:
        :param value:
        :return string:

def _hash(self, iv, value):
        """
        Generate and hmac signature for this encrypted data
        :param key:
        :param iv:
        :param value:
        :return string:
        """
        return hmac.new(self.key, msg=iv+value, digestmod=hashlib.sha256).hexdigest()