AWS signature for signed URL

I came across a .NET example on how to get a signature needed for signing aws request api calls.

I am working on a windows phone 8 app and got stuck with the line

KeyedHashAlgorithm kha = KeyedHashAlgorithm.Create(algorithm);

It appears that windows phone 8 does not have the Create method, the error is below:

'System.Security.Cryptography.KeyedHashAlgorithm' does not contain a definition for Create

Is there an alternative way around this?

Here is the complete code snippet

public static byte[] HmacSHA256(String data, byte[] key)
 {
     String algorithm = "HmacSHA256";
     KeyedHashAlgorithm kha = KeyedHashAlgorithm.Create(algorithm);
     kha.Key = key;

     return kha.ComputeHash(Encoding.UTF8.GetBytes(data));
 }



static byte[] getSignatureKey(String key, String dateStamp, String      regionName, String serviceName)
    {
        byte[] kSecret = Encoding.UTF8.GetBytes(("AWS4" + key).ToCharArray());
        byte[] kDate = HmacSHA256(dateStamp, kSecret);
        byte[] kRegion = HmacSHA256(regionName, kDate);
        byte[] kService = HmacSHA256(serviceName, kRegion);
        byte[] kSigning = HmacSHA256("aws4_request", kService);

        return kSigning;
    }