public function isExpired()
    {
        $this->validatePrepared();

        if (!$this->sessionBag->has(self::CSRF_TKN_TIME)) {
            return true;
        } else {
            $age = time() - $this->sessionBag->get(self::CSRF_TKN_TIME);
            return $age >= $this->ttl;
        }
    }