Negotiating SAML tokens for REST clients with the HttpClient class

Continuing my post “Brokered authentication for REST active clients”, I will show today how the client code can be simplified using the new HttpClient (WCF REST Starter kit 2) and some custom http processing stages attached to its pipeline.

The first thing we have to do is to implement a custom processing stage (a class that derives from HttpStage) to centralize all the logic needed to negotiate a SAML token from an existing STS.

The pipeline contains basically two kinds of stage, a regular http stage that can be injected through the HttpClient.Stages collection, and a more specialized implementation HttpWebRequestTransportStage, which runs last in the pipeline and has access to all the transport settings. This last one can only be replaced with a custom version of the HttpClient that overrides the protected method “CreateTransportStage”,

public class HttpClient : IDisposable

{

  protected virtual HttpStage CreateTransportStage();

}

Having said this, two possible options for implementing the token negotiation in a pipeline stage could be,

1. A regular http stage that can be initialized with the STS address and the user credentials through the class constructor or a property setter.

2. A custom HttpWebRequestTransportStage and the corresponding HttpClient (FederatedHttpClient) implementation to return that stage.

From my point of view, the second approach seems to work better because the HttpClient instance does not get tied to the user credentials. This is the approach I will use for this example.

public class NegociateTokenStage : HttpWebRequestTransportStage

{

private string stsUri = "";

public NegociateTokenStage(string stsUri) : base()

{

    this.stsUri = stsUri;

}

protected override void ProcessRequestAndTryGetResponse(HttpRequestMessage request, out HttpResponseMessage response, out object state)

{

    string token = GetToken(stsUri, request.Uri.AbsoluteUri, this.Settings.Credentials);

    request.Headers.Add("Authorization", token);

    base.ProcessRequestAndTryGetResponse(request, out response, out state);

}

The custom transport stage derives from the built-in transport stage “HttpWebRequestTransportStage” and adds some custom code in the ProcessRequestAndTryGetResponse to negotiate the SAML token from the STS before the final service gets called (This is being done in the GetToken method). After that, the SAML token get passed to the final service through the authorization html header.

The custom implementation of the HttpClient application is quite simple, only returns our custom transport stage in the CreateTransportStage method,

public class FederatedHttpClient : HttpClient

{

    public string StsUri

    {

        get; set;

    }

    protected override HttpStage CreateTransportStage()

    {

        NegociateTokenStage stage = new NegociateTokenStage(this.StsUri);

        stage.Settings = this.TransportSettings;

        return stage;

    }

}

Now, the client application can use our custom version of the HttpClient for consuming the final service, only a few lines are required.

FederatedHttpClient client = new FederatedHttpClient { StsUri = "http://localhost:7481/STS/Service.svc/Tokens" };

client.TransportSettings.Credentials = new NetworkCredential("cibrax", "foo");

string response = client.Get("http://localhost:7397/RestServices/Service.svc/Claims").Content.ReadAsString();

The SAML negotiation is totally transparent to the client application, it does not even know that a SAML token exists, sweet :).

The code is available to download at this location.

UPDATE: As John Lambert from the WCF team pointed out, a custom transport stage also needs to override the BeginProcessRequestAndTryGetResponse and EndProcessRequestAndTryGetResponse to support async scenarios. I will try to update the example to override these methods any time soon. Thanks John for the feedback!!!.

Comments

No Comments