TLS Client Certificates

Clients can use an X.509 client certificate as an authentication mechanism to endpoints in your IdentityServer.

For this you need to associate a client certificate with a client in your IdentityServer and enable MTLS support on the options.

var builder = service.AddIdentityServer(options =>
{
    options.MutualTls.Enabled = true;
})

Use the DI extensions methods to add the services to DI which contain a default implementation to do that either thumbprint or common-name based:

builder.AddMutualTlsSecretValidators();

Then add client secret of type SecretTypes.X509CertificateName (for PKI-based scenarios) or SecretTypes.X509CertificateThumbprint (for self-issued certificates) to the client you want to authenticate.

For example::

new Client
{
    ClientId = "mtls.client",
    AllowedGrantTypes = GrantTypes.ClientCredentials,
    AllowedScopes = { "api1" },

    ClientSecrets = 
    {
        // name based
        new Secret(@"CN=client, OU=production, O=company", "client.dn")
        {
            Type = SecretTypes.X509CertificateName
        },

        // or thumbprint based
        new Secret("bca0d040847f843c5ee0fa6eb494837470155868", "mtls.tb")
        {
            Type = SecretTypes.X509CertificateThumbprint
        },
    }
}

.NET client library

When writing a client to connect to IdentityServer, the SocketsHttpHandler (or HttpClientHandler depending on you .NET version) class provides a convenient mechanism to add a client certificate to outgoing requests.

Use such a handler with HttpClient to perform the client certificate authentication handshake at the TLS channel. The following snippet is using IdentityModel to read the discovery document and request a token:

static async Task<TokenResponse> RequestTokenAsync()
{
    var handler = new SocketsHttpHandler();
    var cert = new X509Certificate2("client.p12", "password");
    handler.SslOptions.ClientCertificates = new X509CertificateCollection { cert };

    var client = new HttpClient(handler);

    var disco = await client.GetDiscoveryDocumentAsync(Constants.Authority);
    if (disco.IsError) throw new Exception(disco.Error);

    var response = await client.RequestClientCredentialsTokenAsync(new ClientCredentialsTokenRequest
    {
        Address = disco.MtlEndpointAliases.TokenEndpoint
                        
        ClientId = "mtls.client",
        Scope = "api1"
    });

    if (response.IsError) throw new Exception(response.Error);
    return response;
}

TODO: add ASP.NET Core client sample