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