Creating A Service Bus SAS Token and Consuming Relay in WinRT

private static string CreateSasToken()
{
    TimeSpan sinceEpoch = DateTime.UtcNow - new DateTime(1970,1, 1);
    var expiry = Convert.ToString((int)sinceEpoch.TotalSeconds + 3600);
    string stringToSign = webUtility.UrlEncode(ServiceUri.AbsoluteUri) + "\n" + expiry;

    string hashKey = Encoding.UTF8.GetBytes(Secret).ToString();

    MacAlgorithmProvider macAlgorithmProvider = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha256);
    BinaryStringEncoding encoding = BinaryStringEncoding.Utf8;

    var messageBuffer = CryptographicBuffer.ConvertStringToBinary(stringToSign,encoding);
    IBuffer keyBuffer = CryptographicBuffer.ConvertStringToBinary(hashKey,encoding);

    CryptographicKey hmacKey = macAlgorithmProvider.CreateKey(keyBuffer);
    IBuffer signedMessage = CryptographicEngine.Sign(hmacKey, messageBuffer);

    string signature = CryptographicBuffer.EncodeToBase64String(signedMessage);

    var sasToken = String.Format(CultureInfo.InvariantCulture,
        "SharedAccessSignature sr={0}&sig={1}&se={2}&skn={3}",
        WebUtility.UrlEncode(ServiceUri.AbsoluteUri),
        WebUtility.UrlEncode(signature), expiry, Issuer);

    return sasToken;
}

My Client class:

   public partial class ServiceClient
    {
        public async Task<string> GetDataUsingDataContract(string item, string sasToken)
        {

            HttpClient client = new HttpClient();

            client.DefaultRequestHeaders.Add("ServiceBusAuthorization",sasToken);
            client.DefaultRequestHeaders.Add("SOAPAction",".../GetDataUsingDataContract");
            client.DefaultRequestHeaders.Add("Host", "xxxxxxxxxxx.servicebus.windows.net");

            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post,ServiceUri);

            var content =new StringContent(@"<s:Envelope
                xmlns:s=""http://schemas.xmlsoap.org/soap/envelope/""><s:Header></s:Header><s:Body>"+ item +@"</s:Body></s:Envelope>",System.Text.Encoding.UTF8,"application/xml");
            request.Content = content;

            HttpResponseMessage wcfResponse = client.SendAsync(request).Result;
            HttpContent stream = wcfResponse.Content;

            var response = stream.ReadAsStringAsync();
            var returnPacket = response.Result;

            return returnPacket;
        }
    }