gistfile1.cs

                    // Ask the server for a password challenge string
                    var requestId = CryptographicBuffer.EncodeToHexString(CryptographicBuffer.GenerateRandom(4));
                    var challengeResponse = await _httpClient.GetAsync(_clientBaseUrl + "GetPasswordChallenge?requestId=" + requestId);
                    challengeResponse.EnsureSuccessStatusCode();
                    var challengeEncoded = await challengeResponse.Content.ReadAsAsync<string>();
                    var challengeBuffer = CryptographicBuffer.DecodeFromHexString(challengeEncoded);

                    // Use HMAC_SHA512 hash to encode the challenge string using the password being authenticated as the key.
                    var provider = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha512);
                    var passwordBuffer = CryptographicBuffer.ConvertStringToBinary(password, BinaryStringEncoding.Utf8);
                    var hmacKey = provider.CreateKey(passwordBuffer);
                    var buffHmac = CryptographicEngine.Sign(hmacKey, challengeBuffer);
                    var hmacString = CryptographicBuffer.EncodeToHexString(buffHmac);