def auth(self):
        """ HTTP authentication data as a (user, password) tuple. This
            implementation currently supports basic (not digest) authentication
            only. If the authentication happened at a higher level (e.g. in the
            front web-server or a middleware), the password field is None, but
            the user field is looked up from the ``REMOTE_USER`` environ
            variable. On any errors, None is returned. """
        basic = parse_auth(self.environ.get('HTTP_AUTHORIZATION',''))
        if basic: return basic
        ruser = self.environ.get('REMOTE_USER')
        if ruser: return (ruser, None)
        return None