public static function create($id,$guid,$context_id,$debug=false) {
        global $CFG;
        $pt = $CFG->cookiepad.'::'.$id.'::'.$guid.'::'.$context_id;
        if ( $debug ) echo("PT1: $pt\n");
        $ct = \Tsugi\Crypt\AesCtr::encrypt($pt, $CFG->cookiesecret, 256) ;
        return $ct;
    }