HttpWebRequest报错：用戶端和伺服器無法溝通，因為它們沒有公用的演算法

string strFormValues;
string strNewValues;
WedaData1.ReturnInput re = data.Web_GetPaypalConfig(ConfigurationManager.ConnectionStrings["wedaweb"].ConnectionString);
string serverURL = re.entity.GetType().GetProperty("serverURL").GetValue(re.entity).ToString();
string pdtToken = re.entity.GetType().GetProperty("pdtToken").GetValue(re.entity).ToString();
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(serverURL);
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";
string txToken = Request.QueryString["tx"].ToString();
string query = "cmd=_notify-synch&tx=" + txToken + "&at=" + pdtToken;
byte[] param = HttpContext.Current.Request.BinaryRead(HttpContext.Current.Request.ContentLength);
strFormValues = Encoding.ASCII.GetString(param);
strNewValues = strFormValues + query;
req.ContentLength = strNewValues.Length;
StreamWriter stout = new StreamWriter(req.GetRequestStream(), Encoding.ASCII);
stout.Write(strNewValues);
stout.Close();
StreamReader sr = new StreamReader(req.GetResponse().GetResponseStream());
string strResponse = sr.ReadToEnd();
sr.Close();
return strResponse;