public static function getCompositeKey($post, $session_secret) {
        $comp = $session_secret .'::'. $post['key'] .'::'. $post['context_id'] .'::'.
            U::get($post,'link_id')  .'::'. $post['user_id'] .'::'. intval(time() / 1800) .
            $_SERVER['HTTP_USER_AGENT'] . '::' . __FILE__;
        return md5($comp);
    }