protected function verifyTimestamp($timestamp, $throw = false) {
        if (!is_numeric($timestamp)) {
            return (bool)$this->exception($throw, "Invalid timestamp.", 403);
        }
        $intTimestamp = (int)$timestamp;
        $now = time();
        if ($intTimestamp + $this->timestampExpiry <= $now) {
            return (bool)$this->exception($throw, "The timestamp has expired.", 403);
        }

        return true;
    }