/// <summary>
        /// Authenticates the server response by reading the Server-Authorization header and creates 
        /// the the HTTP Authorization header in hawk scheme.
        /// </summary>
        /// <param name="options">Hawk authentication options</param>
        public HawkClient(ClientOptions options)
        {
            if (options == null || options.CredentialsCallback == null)
                throw new ArgumentNullException("Invalid Hawk authentication options. Credentials callback cannot be null.");

            this.options = options;
        }
        static void Main(string[] args)
        {
            string uri = "http://localhost:12345/values";

            var credential = new Credential()
            {
                Id = "dh37fgj492je",
                Algorithm = SupportedAlgorithms.SHA256,
                User = "******",
                Key = "werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn"
            };

            // GET and POST using the Authorization header
            var options = new ClientOptions()
            {
                CredentialsCallback = () => credential,
                RequestPayloadHashabilityCallback = (r) => true,
                NormalizationCallback = (req) =>
                {
                    string name = "X-Request-Header-To-Protect";
                    return req.Headers.ContainsKey(name) ? 
                                name + ":" + req.Headers[name].First() : null;
                }
            };

            var handler = new HawkValidationHandler(options);

            HttpClient client = HttpClientFactory.Create(handler);
            client.DefaultRequestHeaders.Add("X-Request-Header-To-Protect", "secret");

            var response = client.GetAsync(uri).Result;
            Console.WriteLine(response.Content.ReadAsStringAsync().Result);

            response = client.PostAsJsonAsync(uri, credential.User).Result;
            Console.WriteLine(response.Content.ReadAsStringAsync().Result);

            // GET using Bewit
            var hawkClient = new HawkClient(options);
            var request = new HttpRequestMessage() { RequestUri = new Uri(uri) };

            string bewit = hawkClient.CreateBewit(new WebApiRequestMessage(request),
                                                        lifeSeconds: 60);

            // Bewit is handed off to a client needing temporary access to the resource.
            var clientNeedingTempAccess = new WebClient();
            var resource = clientNeedingTempAccess.DownloadString(uri + "?bewit=" + bewit);
            Console.WriteLine(resource);

            Console.Read();
        }
        internal static HawkClient Create(Func<Credential> callback = null,
                                            Func<IRequestMessage, string> normalizationCallback = null,
                                                Func<IRequestMessage, bool> requestPayloadHashabilityCallback = null,
                                                    Func<IResponseMessage, string, bool> verificationCallback = null)
        {
            var options = new ClientOptions()
            {
                LocalTimeOffsetMillis = 0,
                EnableResponseValidation = true,
                EnableAutoCompensationForClockSkew = true,
                CredentialsCallback = callback ?? (() => ServerFactory.DefaultCredential),
                NormalizationCallback = normalizationCallback,
                RequestPayloadHashabilityCallback = requestPayloadHashabilityCallback,
                VerificationCallback = verificationCallback
            };

            return new HawkClient(options);
        }