I'm doing some work with a server side service I don't control. The following works fine in standard C#, but fails to compile (due to missing crypto classes) in Silverlight 2 and 3.

    static String DecryptString()
    {
        s = "<cipherTextHere>";

        byte[] toDecryptArray = Convert.FromBase64String(s);
        string key = "<key here>";

        byte[] keyArray = new byte[key.Length / 2];
        for (int i = 0; i < keyArray.Length; i++)
        {
            keyArray[i] = Convert.ToByte(Convert.ToInt32(key.Substring(i * 2, 2), 16));
        }

        using (var algo = new System.Security.Cryptography.AesManaged())
        {

            algo.Padding = PaddingMode.PKCS7;
            algo.Mode = CipherMode.ECB;

            algo.Key = keyArray;

            ICryptoTransform cTransform = algo.CreateDecryptor();
            byte[] resultArray = cTransform.TransformFinalBlock(
                                 toDecryptArray, 0, toDecryptArray.Length);

            return (UTF8Encoding.UTF8.GetString(resultArray, 0, resultArray.Length));

        }
    }

What are my options?