How to use Client SSL Certificates with .Net Core

Question

I recently wanted to configure a .net core website to use client ssl certificate authentication

I couldn't find a good example so I did a bit of research and decided to post the results here for others.

Answer 1

In .net core 2.2 you can configure client certificates as an option inside the .UseHttps method while configuring Kestrel in Program.cs

With this configuration when a user pulls up the site in the browser the browser will present a dialog asking the user to select a client certificate for authentication. If the certificate is invalid, the server will return a HTTP 495 SSL Certificate Error

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
        WebHost.CreateDefaultBuilder(args)
            .UseStartup<Startup>()
            .ConfigureKestrel((context, options) =>
            {
                options.Listen(IPAddress.Loopback, 5022);
                options.Listen(IPAddress.Loopback, 5023, listenOptions =>
                {
                    listenOptions.UseHttps((httpsOptions) =>
                    {
                        var certFileName = "server_cert.pfx";
                        var contentRoot = context.HostingEnvironment.ContentRootPath;
                        X509Certificate2 serverCert;
                        var path = Path.Combine(contentRoot, certFileName);
                        serverCert = new X509Certificate2(path, "<server cert password>");

                        httpsOptions.ServerCertificate = serverCert;
                        // this is what will make the browser display the client certificate dialog
                        httpsOptions.ClientCertificateMode = ClientCertificateMode.RequireCertificate;
                        httpsOptions.CheckCertificateRevocation = false;
                        httpsOptions.ClientCertificateValidation = (certificate2, validationChain, policyErrors) =>
                        {
                            // this is for testing non production certificates, do not use these settings in production
                            validationChain.ChainPolicy.RevocationMode = X509RevocationMode.NoCheck;
                            validationChain.ChainPolicy.RevocationFlag = X509RevocationFlag.ExcludeRoot;
                            validationChain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllowUnknownCertificateAuthority;
                            validationChain.ChainPolicy.VerificationTime = DateTime.Now;
                            validationChain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan(0, 0, 0);
                            validationChain.ChainPolicy.ExtraStore.Add(serverCert);

                            var valid = validationChain.Build(certificate2);
                            if (!valid)
                                return false;

                            // only trust certs that are signed by our CA cert
                            valid = validationChain.ChainElements
                                .Cast<X509ChainElement>()
                                .Any(x => x.Certificate.Thumbprint == serverCert.Thumbprint);

                            return valid;
                        };
                    });
                });
            });
}