using System;

using System.Net;

using System.Text;

using System.Security.Cryptography;

public class Program

{

    public static void Main()

    {

        Encoding ascii = Encoding.ASCII;

​

                String fullURL = "https://www.somedomain.com/?parameter1=bob&parameter2=fred&now=2019-12-13T09:50:22Z&sig=vcdf/8wJo4vvLDwdbmxlK4cyLoTyG4sruVp24bzOxEM=";

                String fullURLDecode = WebUtility.HtmlDecode(fullURL);

                Console.WriteLine(fullURLDecode);

                //https://www.somedomain.com/?parameter1=bob&parameter2=fred&now=2019-12-13T09:50:22Z&sig=vcdf/8wJo4vvLDwdbmxlK4cyLoTyG4sruVp24bzOxEM=

                String sentHASH64 = fullURLDecode.Substring(fullURLDecode.LastIndexOf("sig=")+4);

                Console.WriteLine(sentHASH64);

                //bdc75fffcc09a38bef2c3c1d6e6c652b87322e84f21b8b2bb95a76e1bccec443

                //String fullQS = new Uri(fullURLDecode).Query;

                String fullQS = fullURLDecode.Substring(fullURLDecode.LastIndexOf("?")+1);

                Console.WriteLine(fullQS);

                //parameter1=bob&parameter2=fred&now=2019-12-13T09:50:22Z&sig=vcdf/8wJo4vvLDwdbmxlK4cyLoTyG4sruVp24bzOxEM=

                String justparmasQS = fullQS.Substring(0, fullQS.LastIndexOf("sig=")-1);

                Console.WriteLine(justparmasQS);

                //parameter1=bob&parameter2=fred&now=2019-12-13T09:50:22Z

               String client_secret = "aSampleSecret";

​

               HMACSHA256 hmac = new HMACSHA256(Encoding.UTF8.GetBytes(client_secret));

​

               String calc_sig = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(justparmasQS)));

                Console.WriteLine(sentHASH64);

                Console.WriteLine(calc_sig);

​

               if (sentHASH64 == calc_sig) 

                    Console.WriteLine("Matches");

                else 

                    Console.WriteLine("Does not match");

​

    }

}