Facebook Registration C# handling

// In Default.aspx:

<iframe src="http://www.facebook.com/plugins/registration.php?
             client_id=your_client_id&
             redirect_uri=http%3A%2F%2Fmydomain.com%2FCallback.aspx&
             fields=name,birthday,gender,location,email"
        scrolling="auto"
        frameborder="no"
        style="border:none"
        allowTransparency="true"
        width="100%"
        height="330">
</iframe>

// In Callback.aspx.cs:

using System;
using Newtonsoft.Json.Linq;
using System.Text;
using System.Security.Cryptography;

public partial class Callback: System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        var signed_request = Request.Form["signed_request"];

        var obj = ParseSignedRequest(signed_request, "your_secret_key");

        var name = obj["name"].Value<string>();
        var birthday = obj["birthday"].Value<DateTime>();
        var location = obj["location"]["name"].Value<string>();
        var gender = obj["gender"].Value<string>();
        var email = obj["gender"].Value<string>();

        // Save values to database...

    }

    private JToken ParseSignedRequest(string signedRequestValue, string secret_key)
    {
        string[] signedRequestSplit = signedRequestValue.Split('.');
        string expectedSignature = signedRequestSplit[0];
        string payload = signedRequestSplit[1];

        // Attempt to get same hash
        var encoding = new UTF8Encoding();
        var hmac = SignWithHmac(encoding.GetBytes(payload), encoding.GetBytes(secret_key));
        var hmacBase64 = ToUrlBase64String(hmac);

        if (hmacBase64 != ReplaceSpecialCharactersInSignedRequest(expectedSignature))
            return null;

        var decodedJson = ReplaceSpecialCharactersInSignedRequest(payload);
        var base64JsonArray = Convert.FromBase64String(decodedJson.PadRight(decodedJson.Length + (4 - decodedJson.Length % 4) % 4, '='));


        var data = JObject.Parse(encoding.GetString(base64JsonArray));

        if (data.HasValues && data["registration"] != null)
        {
            return data["registration"];
        }
        else
            return null;

    }

    private static string ReplaceSpecialCharactersInSignedRequest(string str)
    {
        return str.Replace("=", string.Empty).Replace('-', '+').Replace('_', '/');
    }

    private static byte[] SignWithHmac(byte[] dataToSign, byte[] keyBody)
    {
        using (var hmacAlgorithm = new HMACSHA256(keyBody))
        {
            hmacAlgorithm.ComputeHash(dataToSign);
            return hmacAlgorithm.Hash;
        }
    }
    private static string ToUrlBase64String(byte[] Input)
    {
        return ReplaceSpecialCharactersInSignedRequest(Convert.ToBase64String(Input));
    }
}