gistfile1.cs

using RestSharp;
using System;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace Hitbtc.Api.Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            const string apiKey = "xxx";
            const string secretKey = "yyy";

            var client = new RestClient("https://api.hitbtc.com");

            var request = new RestRequest("/api/1/trading/balance", Method.GET);
            request.AddParameter("nonce", GetNonce());
            request.AddParameter("apikey", apiKey);

            string sign = CalculateSignature(client.BuildUri(request).PathAndQuery, secretKey);
            request.AddHeader("X-Signature", sign);

            var response = client.Execute(request);

            Console.WriteLine(response.Content);
        }

        private static long GetNonce()
        {
            return DateTime.Now.Ticks * 10 / TimeSpan.TicksPerMillisecond; // use millisecond timestamp or whatever you want
        }

        public static string CalculateSignature(string text, string secretKey)
        {
            using (var hmacsha512 = new HMACSHA512(Encoding.UTF8.GetBytes(secretKey)))
            {
                hmacsha512.ComputeHash(Encoding.UTF8.GetBytes(text));
                return string.Concat(hmacsha512.Hash.Select(b => b.ToString("x2")).ToArray()); // minimalistic hex-encoding and lower case
            }
        }
    }
}