public function getSignHeaderValue()
    {
        $signature = '';
        if (isset($_SERVER['HTTP_X_VIBER_CONTENT_SIGNATURE'])) {
            $signature = $_SERVER['HTTP_X_VIBER_CONTENT_SIGNATURE'];
        } elseif (isset($_GET['sig'])) {
            $signature = $_GET['sig'];
        }
        if (empty($signature)) {
            throw new \RuntimeException('Signature header not found', 1);
        }

        return $signature;
    }