I'm trying to implement authorization through Last.fm. I'm submitting my arguments as a Dictionary to make the signing easier. This is the code I'm using to sign my calls:

public static string SignCall(Dictionary<string, string> args)
{
    IOrderedEnumerable<KeyValuePair<string, string>> sortedArgs = args.OrderBy(arg => arg.Key);
    string signature = 
        sortedArgs.Select(pair => pair.Key + pair.Value).
        Aggregate((first, second) => first + second);
    return MD5(signature + SecretKey);
}

I've checked the output in the debugger, it's exactly how it should be, however, I'm still getting WebExceptions every time I try.

Here's my code I use to generate the URL in case it'll help:

public static string GetSignedURI(Dictionary<string, string> args, bool get)
{
    var stringBuilder = new StringBuilder();
    if (get)
        stringBuilder.Append("http://ws.audioscrobbler.com/2.0/?");
    foreach (var kvp in args)
        stringBuilder.AppendFormat("{0}={1}&", kvp.Key, kvp.Value);
    stringBuilder.Append("api_sig="+SignCall(args));
    return stringBuilder.ToString();
}

And sample usage to get a SessionKey:

var args = new Dictionary<string, string>
                       {
                           {"method", "auth.getSession"},
                           {"api_key", ApiKey},
                           {"token", token}
                       };
string url = GetSignedURI(args, true);

EDIT:

Oh, and the code references an MD5 function implemented like this:

public static string MD5(string toHash)
{
    byte[] textBytes = Encoding.UTF8.GetBytes(toHash);
    var cryptHandler = new System.Security.Cryptography.MD5CryptoServiceProvider();
    byte[] hash = cryptHandler.ComputeHash(textBytes);
    return hash.Aggregate("", (current, a) => current + a.ToString("x2"));
}