':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'

@property
def authority_issuer_serial(self):

    if (self._authority_issuer_serial is False):
        akiv = self.authority_key_identifier_value
        if (akiv and akiv[u'authority_cert_issuer'].native):
            issuer = self.authority_key_identifier_value[u'authority_cert_issuer'][0].chosen
            issuer = issuer.untag()
            authority_serial = self.authority_key_identifier_value[u'authority_cert_serial_number'].native
            self._authority_issuer_serial = ((issuer.sha256 + ':') + str_cls(authority_serial).encode(u'ascii'))
        else:
            self._authority_issuer_serial = None
    return self._authority_issuer_serial
