code: def authority_issuer_serial(self):
          
  
          if self._authority_issuer_serial is False:
              akiv = self.authority_key_identifier_value
              if akiv and akiv['authority_cert_issuer'].native:
                  issuer = self.authority_key_identifier_value['authority_cert_issuer'][0].chosen
                  # We untag the element since it is tagged via being a choice from GeneralName
                  issuer = issuer.untag()
                  authority_serial = self.authority_key_identifier_value['authority_cert_serial_number'].native
                  self._authority_issuer_serial = issuer.sha256 + b':' + str_cls(authority_serial).encode('ascii')
              else:
                  self._authority_issuer_serial = None
          return self._authority_issuer_serial
docs: :return:
              None or a byte string of the SHA-256 hash of the isser from the
              authority key identifier extension concatenated with the ascii
              character ":", concatenated with the serial number from the
              authority key identifier extension as an ascii string