private function _generateSignatureKey()
    {
        $date_h = hash_hmac('sha256', $this->_date, 'AWS4' . $this->_aws_secret, true);
        $region_h = hash_hmac('sha256', $this->_region, $date_h, true);
        $service_h = hash_hmac('sha256', self::SERVICE, $region_h, true);
        $signing_h = hash_hmac('sha256', 'aws4_request', $service_h, true);

        return $signing_h;
    }