特约商户进件 / 查询申请单状态

string mchId="AAAA" //微信支付商户号

string serialNo="xxxx" //API证书序列号

 HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://api.mch.weixin.qq.com/v3/applyment4sub/applyment/business_code/" + businessCode);

            string uri = req2.RequestUri.PathAndQuery;

            string message = $"GET\n{uri}\n{timestamp}\n{nonceStr}\n";

            string signature;

            string privateKey = "MII="; //这里是根据apiclient_key.pem文件里面的信息，去除头尾后

            byte[] keyData = Convert.FromBase64String(privateKey);

            using (CngKey cngKey = CngKey.Import(keyData, CngKeyBlobFormat.Pkcs8PrivateBlob))

            using (RSACng rsa = new RSACng(cngKey))

            {

                byte[] data = Encoding.UTF8.GetBytes(message);

                signature = Convert.ToBase64String(rsa.SignData(data, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));

            }

            req2.Headers.Add("Authorization", string.Format("WECHATPAY2-SHA256-RSA2048 mchid=\"{0}\",serial_no=\"{1}\",nonce_str=\"{2}\",timestamp=\"{3}\",signature=\"{4}\"", mchId, serialNo, nonceStr, timestamp, signature));

            req2.Accept = "application/json";

            req2.ContentType = "application/json";

            req2.Headers.Add("Wechatpay-Serial", serialNo);

            var resp = req2.GetResponse();

为什么总是提示：错误的签名，验签失败

请帮助一下！