SSL Client Authentication with certificate using c#

Question

I need to create a c# application that has to send API request to a server using SSL. I need to create the Client authentication. I already have the server CA certificate, the client certificate (cer), the client private key (pem) and passphrase. I can't find an example on how to create the client connection. Can someone suggest me where to start with a small code well explained? In my hand I have the client certificate (PEM), the Client Provate Key and the Passphrase for the Client Key. I have no idea where to start to write the code to send a request to the server

Answer 1

Some time ago I've created this POC for client authentication with certificate in .Net Core. It uses idunno.Authentication package that is now build-in in .Net Core. My POC probably is bit outdated now, but it can be a good starting point for you.

First create an extension method to add certificate to HttpClientHandler:

public static class HttpClientHandlerExtensions
{
    public static HttpClientHandler AddClientCertificate(this HttpClientHandler handler, X509Certificate2 certificate)
    {
        handler.ClientCertificateOptions = ClientCertificateOption.Manual;
        handler.ClientCertificates.Add(certificate);

        return handler;
    }
}

Then another extension method to add certificate to IHttpClientBuilder

    public static IHttpClientBuilder AddClientCertificate(this IHttpClientBuilder httpClientBuilder, X509Certificate2 certificate)
    {
        httpClientBuilder.ConfigureHttpMessageHandlerBuilder(builder =>
        {
            if (builder.PrimaryHandler is HttpClientHandler handler)
            {
                handler.AddClientCertificate(certificate);
            }
            else
            {
                throw new InvalidOperationException($"Only {typeof(HttpClientHandler).FullName} handler type is supported. Actual type: {builder.PrimaryHandler.GetType().FullName}");
            }
        });

        return httpClientBuilder;
    }

Then load the certificate and register HttpClient in HttpClientFactory

        var cert = CertificateFinder.FindBySubject("your-subject");
        services
            .AddHttpClient("ClientWithCertificate", client => { client.BaseAddress = new Uri(ServerUrl); })
            .AddClientCertificate(cert);

Now when you will use client created by factory it will automatically send your certificate with the request;

public async Task SendRequest()
{
    var client = _httpClientFactory.CreateClient("ClientWithCertificate");
    ....
}