':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'

@property
def issuer_serial(self):

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