def __sec_key(self, data):
        """ returns secret key and block id

            Args
                data: string
        """
        h = SHA256d(data)
        if not self.__convergence_secret:
            self.__warn_convergence()
        else:
            h.update(self.__convergence_secret)
        key = h.digest()
        del h
        id = SHA256d(key).digest()
        return key, id