function getSignature(array $params, $method = null)
    {
        ksort($params);
        unset($params['sign']);
        unset($params['signature']);
        array_push($params, $this->secretKey);

        if ($method) {
            array_unshift($params, $method);
        }

        return hash('sha256', join('{up}', $params));
    }