.Net SslStream with Client Certificate

Question

I'm having no luck getting client certificates working with my SslStream project. No matter what I do, I can't get it to actually use the client certificate, despite the fact that all certificates are valid and trusted, and I have imported the CA certificate for the ones I generated myself, and it just doesn't work. I must be missing something, but I've been over it dozens of times, reviewed documentation, examples, and hours of google searching, and I just can't get it to work. What am I doing wrong?

The Client:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Security;
using System.Net.Sockets;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using System.Text;

namespace SslClient
{
    class SslClientProgram
    {
        static void Main(string[] args)
        {
            TcpClient client = new TcpClient("localhost", 443);

            SslStream stream = new SslStream(client.GetStream(), false, VerifyServerCertificate, null);

            Assembly assembly = System.Reflection.Assembly.GetExecutingAssembly();
            string location = assembly.Location;
            int pos = location.LastIndexOf('\\');
            location = location.Substring(0, pos);
            X509Certificate2 certificate = new X509Certificate2(location + "\\my.client.certificate.pfx", "password");

            stream.AuthenticateAsClient("my.host.name", new X509Certificate2Collection(certificate), System.Security.Authentication.SslProtocols.Tls, false);

            StreamReader reader = new StreamReader(stream);
            StreamWriter writer = new StreamWriter(stream);

            while (true)
            {
                string line = System.Console.ReadLine();
                writer.WriteLine(line);
                writer.Flush();
                if (line == "close") break;
                line = reader.ReadLine();
                System.Console.WriteLine("Received: {0}", line);
            }

            stream.Close();
        }

        private static bool VerifyServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            return true;
        }
    }
}

The Server:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Security;
using System.Net.Sockets;
using System.Reflection;
using System.Security.Cryptography.X509Certificates;
using System.Text;

namespace SslServer
{
    class SslServerProgram
    {
        static void Main(string[] args)
        {
            TcpListener server = new TcpListener(System.Net.IPAddress.Loopback, 443);

            server.Start();

            TcpClient client = server.AcceptTcpClient();

            SslStream stream = new SslStream(client.GetStream(), false, VerifyClientCertificate, null);

            Assembly assembly = System.Reflection.Assembly.GetExecutingAssembly();
            string location = assembly.Location;
            int pos = location.LastIndexOf('\\');
            location = location.Substring(0, pos);
            X509Certificate2 certificate = new X509Certificate2(location + "\\my.server.certificate.pfx", "password");

            stream.AuthenticateAsServer(certificate, false, System.Security.Authentication.SslProtocols.Tls, false);

            if (stream.RemoteCertificate != null)
            {
                System.Console.WriteLine(stream.RemoteCertificate.Subject);
            }
            else
            {
                System.Console.WriteLine("No client certificate.");
            }

            StreamReader reader = new StreamReader(stream);
            StreamWriter writer = new StreamWriter(stream);

            bool clientClose = false;
            while (!System.Console.KeyAvailable)
            {
                System.Console.WriteLine("Waiting for data...");
                string line = reader.ReadLine();
                System.Console.WriteLine("Received: {0}", line);

                if (line == "close")
                {
                    clientClose = true;
                    break;
                }

                writer.WriteLine(line);
                writer.Flush();
            }

            if (!clientClose) System.Console.ReadKey();

            stream.Close();
            server.Stop();
        }

        private static bool VerifyClientCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            return true;
        }
    }
}

No matter what I try, the server always says "No client certificate."

Answer 1

As it turns out, AuthenticateAsServer is the key here - more specifically, the second parameter.

If clientCertificateRequired is false, it will completely ignore client certificates, even if one is specified by the client, but if it is true, it will allow them, but does not throw an exception if no client certificate is specified.

Silly me - I thought clientCertificateRequired set to true meant that it would be actually required, because the .Net documentation describes it as:

"A Boolean value that specifies whether the client must supply a certificate for authentication."* (emphasis mine)

My expectation was that if it was true, and I did not send a client certificate, then it would fail. This is a clear case of less than completely accurate documentation on the part of Microsoft.

Update: The latest documentation for the clientCertificateRequired parameter includes the phrase "Note that this is only a request -- if no certificate is provided, the server still accepts the connection request."