I am writing a code to authenticate the username and password through the POST request but I am getting an error which is saying that "The underlying connection was closed".

I am trying to convert my old code with GET request to new code with POST request.

My GET code which is working fine (Old Code):

string url = "https://www.example.com/?username=" + username + "&password=" + password;



XmlDocument xmldoc = new XmlDocument();

ServicePointManager.ServerCertificateValidationCallback += new 

RemoteCertificateValidationCallback(customXertificateValidation);

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;



xmldoc.Load(url);

XmlNode name = xmldoc.SelectSingleNode("/Node/Name");

My POST request Code which is throwing an error (New Code):

var request = (HttpWebRequest)WebRequest.Create("https://www.example.com/authenticate.aspx");

ServicePointManager.ServerCertificateValidationCallback += new 

RemoteCertificateValidationCallback(customXertificateValidation);    

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;

var postData = "user=username";

postData += "&pass=password";

var data = Encoding.ASCII.GetBytes(postData);



request.Method = "POST";

request.ContentType = "application/x-www-form-urlencoded";

request.ContentLength = data.Length;



using (var stream = request.GetRequestStream())

{

    stream.Write(data, 0, data.Length);

}



var response = (HttpWebResponse)request.GetResponse();

var responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

Error:

System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a receive. ---> System.ComponentModel.Win32Exception: The client and server cannot communicate, because they do not possess a common algorithm

at System.Net.SSPIWrapper.AcquireCredentialsHandle(SSPIInterface SecModule, String package, CredentialUse intent, SecureCredential scc)

at System.Net.Security.SecureChannel.AcquireCredentialsHandle(CredentialUse credUsage, SecureCredential& secureCredential)

at System.Net.Security.SecureChannel.AcquireClientCredentials(Byte& thumbPrint)

at System.Net.Security.SecureChannel.GenerateToken(Byte input, Int32 offset, Int32 count, Byte& output)

at System.Net.Security.SecureChannel.NextMessage(Byte incoming, Int32 offset, Int32 count)

at System.Net.Security.SslState.StartSendBlob(Byte incoming, Int32 count, AsyncProtocolRequest asyncRequest)

at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte buffer, AsyncProtocolRequest asyncRequest)

at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)

at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

at System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result)

at System.Net.TlsStream.Write(Byte buffer, Int32 offset, Int32 size)

at System.Net.ConnectStream.WriteHeaders(Boolean async)

--- End of inner exception stack trace ---

at System.Net.HttpWebRequest.GetRequestStream(TransportContext& context)

at System.Net.HttpWebRequest.GetRequestStream()

at OdlumBrown.Members.AuthenticateUser(String username, String password) in c:Websiteexample.comApp_CodeMembership.cs:line 148