Estoy tratando de usar Google GCM para enviar notificaciones push. Pero obtenga una WebException que dice que el servidor remoto devuelve 401 sin autorización. No puedo entender por qué no funciona.

¿Alguien que sepa por qué no funciona?

Aquí está mi código:

            ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateServerCertificate);

           HttpWebRequest Request = (HttpWebRequest)WebRequest.Create("https://android.googleapis.com/gcm/send");


            Request.Method = "POST";
            Request.KeepAlive = false;

            string postData = "{ 'registration_ids': [ '"+registrationId+"' ], 'data': {'message': '"+message+"'}}";

            byte[] byteArray = Encoding.UTF8.GetBytes(postData);

            Request.ContentType = "application/json";
            //Request.ContentLength = byteArray.Length;


            //Request.Headers.Add(HttpRequestHeader.Authorization, "GoogleLogin auth=" + AuthString);
            Request.Headers.Add(HttpRequestHeader.Authorization, "Authorization: key=AIzaSyCEygavdzrNM3pWNPtvaJXpvW66CKnjH_Y");
            //-- Delegate Modeling to Validate Server Certificate --//


            //-- Create Stream to Write Byte Array --// 
            Stream dataStream = Request.GetRequestStream();
            dataStream.Write(byteArray, 0, byteArray.Length);
            dataStream.Close();

            //-- Post a Message --//
            WebResponse Response = Request.GetResponse();
            HttpStatusCode ResponseCode = ((HttpWebResponse)Response).StatusCode;
            if (ResponseCode.Equals(HttpStatusCode.Unauthorized) || ResponseCode.Equals(HttpStatusCode.Forbidden))
            {
                var text = "Unauthorized - need new token";

            }
            else if (!ResponseCode.Equals(HttpStatusCode.OK))
            {
                var text =  "Response from web service isn't OK";
            }

            StreamReader Reader = new StreamReader(Response.GetResponseStream());
            string responseLine = Reader.ReadLine();
            Reader.Close();