Read OAuth2.0 Signed_Request Facebook Registration C# MVC

Question

My question is very similar this but I guess I need to take it one step further.

Facebook says "The data is passed to your application as a signed request. The signed_request parameter is a simple way to make sure that the data you're receiving is the actual data sent by Facebook."

After a user has logged into my asp c# MVC site and clicked "Register", the redirect-url is http://site/account/register. At that point (the post to the account/register control), I would like to gather the user's information using the signed request so that I can register them with my site locally. I cannot figure out how to access the data facebook makes available.

$data = json_decode(base64_url_decode($payload), true);

What is the equivalent in C#? What type of variable/data is facebook passing in the post? And how do I access "$payload"?

[HttpPost]
    public ActionResult RegisterFacebook(RegisterFacebookModel model)
    {
        Facebook.FacebookSignedRequest sr = Facebook.FacebookSignedRequest.Parse("secret", model.signed_request);

        return View(model);
    }

Answer 1

Here is the code we used in the Facebook C# SDK. You don't need to do this manually if you use our sdk, but if you need to do it yourself here it is:

/// <summary>
/// Parses the signed request string.
/// </summary>
/// <param name="signedRequestValue">The encoded signed request value.</param>
/// <returns>The valid signed request.</returns>
internal protected FacebookSignedRequest ParseSignedRequest(string signedRequestValue)
{
    Contract.Requires(!String.IsNullOrEmpty(signedRequestValue));
    Contract.Requires(signedRequestValue.Contains("."), Properties.Resources.InvalidSignedRequest);

    string[] parts = signedRequestValue.Split('.');
    var encodedValue = parts[0];
    if (String.IsNullOrEmpty(encodedValue))
    {
        throw new InvalidOperationException(Properties.Resources.InvalidSignedRequest);
    }

    var sig = Base64UrlDecode(encodedValue);
    var payload = parts[1];

    using (var cryto = new System.Security.Cryptography.HMACSHA256(Encoding.UTF8.GetBytes(this.AppSecret)))
    {
        var hash = Convert.ToBase64String(cryto.ComputeHash(Encoding.UTF8.GetBytes(payload)));
        var hashDecoded = Base64UrlDecode(hash);
        if (hashDecoded != sig)
        {
            return null;
        }
    }

    var payloadJson = Encoding.UTF8.GetString(Convert.FromBase64String(Base64UrlDecode(payload)));
    var data = (IDictionary<string, object>)JsonSerializer.DeserializeObject(payloadJson);
    var signedRequest = new FacebookSignedRequest();
    foreach (var keyValue in data)
    {
        signedRequest.Dictionary.Add(keyValue.Key, keyValue.Value.ToString());
    }

    return signedRequest;
}

/// <summary>
/// Converts the base 64 url encoded string to standard base 64 encoding.
/// </summary>
/// <param name="encodedValue">The encoded value.</param>
/// <returns>The base 64 string.</returns>
private static string Base64UrlDecode(string encodedValue)
{
    Contract.Requires(!String.IsNullOrEmpty(encodedValue));

    encodedValue = encodedValue.Replace('+', '-').Replace('/', '_').Trim();
    int pad = encodedValue.Length % 4;
    if (pad > 0)
    {
        pad = 4 - pad;
    }

    encodedValue = encodedValue.PadRight(encodedValue.Length + pad, '=');
    return encodedValue;
}

You can find the full source code here: http://facebooksdk.codeplex.com/SourceControl/changeset/view/f8109846cba5#Source%2fFacebook%2fFacebookApp.cs