def issuer_serial(self):
        """
        :return:
            A byte string of the SHA-256 hash of the issuer concatenated with
            the ascii character ":", concatenated with the serial number as
            an ascii string
        """

        if self._issuer_serial is None:
            self._issuer_serial = self.issuer.sha256 + b':' + str_cls(self.serial_number).encode('ascii')
        return self._issuer_serial