protected function buildSignatureKey()
    {
        $signatures = [
            'a' => $this->appId,
            'b' => $this->bucket,
            'k' => $this->secretId,
            'e' => time() + 2592000,
            't' => time(),
            'r' => rand(),
            'u' => '0',
            'f' => ''
        ];

        return http_build_query($signatures);
    }