Implementing Facebook authentication in a Silverlight out-of-browser application

July 11, 2010

In my previous post about the GraphLight library, I mentioned that the first step in using the library is to supply it with an access token. What I conveniently skimmed over is the explanation of what an access token is and how does one get a hold of it. I did this because answering those questions isn’t simple, and would benefit from dedicated elaboration.

According to Facebook: “An access token allows an application to perform authorized requests on behalf of the user by including the access token in Graph API requests.” You can read about the process of obtaining an access token on the Facebook Developers Site. Although Facebook documentation provides a good starting point, it does not account for peculiarities of Silverlight. In this post, I will go over issues I encountered while implementing Facebook authentication in a Silverlight OOB application, and offer my solutions.

Facebook documentation outlines two main flows of authentication available to applications: The first method utilizes sequence of redirects to Facebook site and back; the second method uses single sign-on with the JavaScript SDK. Of the two, only the former method can be used in a Silverlight OOB application due to restrictions on access to the hosting page’s DOM. Upon closer examination, the Desktop Application Authentication process emerges as the most obvious choice for an OOB application.

Implementing a desktop authentication flow in accordance with Facebook documentation is quite straightforward:

  1. Create WebBrowser control in code or in XAML
  2. Navigate to https://graph.facebook.com/oauth/authorize…
  3. Intercept the redirect to login_succes.html using LoadCompleted event and read the access token out of the URL

Problem #1

WebBrowser control needs Full-Trust to show pages from sites other then application’s origin. Because the Facebook authentication process requires redirect to facebook.com, the OOB application must be in full-trust mode! After many different attempts to find an acceptable workaround for standard OOB applications, I came to the conclusion that although it can be done, the resulting user experience is so poor that it is simply not worth it.

Solution: Configure application to require Full-Trust.

Problem #2

Using the LoadCompleted event to intercept the redirect to login_succes.html doesn’t work because the Uri property of NavigationEventArgs is always null. If one is unable to read the access token out of the URL, using the Desktop Application Authentication process is no longer an option.

Solution: An alternative approach comes in the form of an authentication flow recommended for Web Applications and Mobile Applications. Web application authentication uses a two-step process, with the first step similar to that of desktop authentication, but instead of an access token, it returns code that can be exchanged for an access token in a separate step.

In order to detect redirects and pass code to the Silverlight application, following steps need to be taken:

  1. Create success.html page
	<html>
	  <head>
	   <title>Facebook Login Callback</title>
	   <script type="text/javascript">
	      window.external.notify(window.location.href);
	   </script>
	  </head>
	  <body />
	</html>

  1. Success page should be hosted on the same domain as the application and its url should start with Connect URL (see Facebook connect settings). Both conditions are required for authentication process to work.

  2. Point redirect_uri parameter to the location of the success page

  3. Add ScriptNotify handler to WebBrowser control:

	browser.ScriptNotify += (a, b) =>
	{
	    int n = b.Value.IndexOf("?code=");
	    if (n > 0)
	    {
	        //extract code from b.Value
	        //exchange code for access token as described in Facebook documentation
	        WebClient wc = new WebClient();
	        wc.DownloadStringAsync(new Uri(
	             string.Format(token_xchange, app_id, redirect_url, secret, code)));
	        wc.DownloadStringCompleted += (c, d) =>
	        {
	            //extract access token from d.Result
	            LoginSuccess(access_token);
	        };
	    }
	    else
	    {
	        if (b.Value.IndexOf("user_denied") > 0)
	        {
	            LoginFailed();
	        }
	    }
	};

At last a bit of good news: The method described above actually works, but…

Problem #3

Doing a token exchange from a Silverlight application requires a secret key to be stored inside of the package redistributed to the end users, which is inherently not secure.

Solution: Replace success.html with success.aspx or success.php or any other technology that allows one to execute code server-side. Perform token exchange on the server and simply pass the access token to ScriptNotify handler.

Postmortem

Implementing Facebook authentication in an OOB silverlight application turned out to be a challenge. Most of the issues I encountered were related to limitations of the WebBrowser control. Some security restrictions were expected and quite reasonable, while others appear to be to overreaching. It would be nice to see Microsoft considering use cases for Silverlight applications needing to login to OAuth based services (Facebook, Twitter, Flicker…).

Bonus feature: Logout or “What goes up must come down“

After all the trouble with authentication, it would be nice if logging out was simple… and it mostly is. Presently the only way to properly log a user out of Facebook is by redirecting the user to logout.php with app id and session as parameters. One little problem is how do we get a hold of the session? Well, as it happens, session is part of the access token and can be extracted from it with little effort.

private string logout_format = 
    "http://www.facebook.com/logout.php?app_key={0}&session_key={1}&next={2}";

public void Logout(string access_token)
{
    access_token = HttpUtility.UrlDecode(access_token);
    int s = access_token.IndexOf("|");
    int e = access_token.LastIndexOf("|");
    string session = access_token.Substring(s + 1, e - s - 1);
    browser.Navigate(new Uri(string.Format(logout_format, app_id,
        session, HttpUtility.UrlEncode(OAuthUrl))));
}