How to implement Certificate Authentication in ASP.NET Core

Certificate-based Authentication is the use of a Digital Certificate to identify a client request before granting it the access to a resource, network, application, etc. Certificate Authentication provides added security to web applications. You can easily implement it in ASP.NET Core 3.0. Let us understand how to do it.

Implementing Certificate Based Authentication

In your ASP.NET Core app, first add the package called Microsoft.AspNetCore.Authentication.Certificate from NuGet.

Check the below code which does the exact thing:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews();
    
    services.AddAuthentication(
        CertificateAuthenticationDefaults.AuthenticationScheme)
        .AddCertificate(options =>
        {
            options.Events = new CertificateAuthenticationEvents
            {
                OnCertificateValidated = context =>
                {
                    var validationService = context.HttpContext.RequestServices.GetService<MyCertificateValidationService>();

                    if (validationService.ValidateCertificate(context.ClientCertificate))
                    {
                        context.Success();
                    }
                    else
                    {
                        context.Fail("invalid cert");
                    }

                    return Task.CompletedTask;
                },
                OnAuthenticationFailed = context =>
                {
                    context.Fail("invalid cert");
                    return Task.CompletedTask;
                }
            };
        });
}

Note that inside the OnAuthenticationFailed event you show ‘invalid cert’ message.

I have also specified MyCertificateValidationService class where the certificate will be validated. It’s full code is given below:

public class MyCertificateValidationService
{
    public bool ValidateCertificate(X509Certificate2 clientCertificate)
    {
        var cert = new X509Certificate2(Path.Combine("localhost_root_l1.pfx"), "1234");
        if (clientCertificate.Thumbprint == cert.Thumbprint)
        {
            return true;
        }

        return false;
    }
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseHttpsRedirection();
    app.UseStaticFiles();

    app.UseRouting();

    app.UseCertificateForwarding();
    app.UseAuthentication();

    app.UseAuthorization();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });
}

Configure your app to require certificates

Finally, you have to configure Kestrel so that the application always require certificates. This is done by adding the below code to the Program.cs file:

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .ConfigureKestrel(options =>
        {
            options.ConfigureHttpsDefaults(opt => 
                opt.ClientCertificateMode = 
                    ClientCertificateMode.RequireCertificate);
        })
        .Build();

Download full source codes:

Download Congrats you have successfully implemented the Certificate Authentication system in your app. You app now becomes more secure than before.