using System;

using System.Security.Cryptography;

using System.Text;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var datetime = DateTime.Now.ToString("u"); //yyyy-MM-dd hh:mm:ssZ

        Console.WriteLine(datetime);

        var msisdn = "97400000000";

        var operatorCode = "42702";

        var message = msisdn + operatorCode + datetime;

        var publicKey = "KLxY3gJtDkXvnclpGa4h";

        var privateKey = "76Ie2lIJMypTBJyA5Wby";

        var theDigest = CalculateDigest(publicKey, privateKey, message);

        Console.WriteLine(theDigest);

    }

​

    public static string CalculateDigest(string publicKey, string privateKey, string message)

    {

        var digest = "";

        var hash = new System.Security.Cryptography.HMACSHA256(System.Text.Encoding.UTF8.GetBytes(privateKey));

        var correctHash = string.Join(string.Empty, hash.ComputeHash(System.Text.Encoding.UTF8.GetBytes(message)).Select(b => b.ToString("x2")));

        digest = publicKey + ":" + correctHash;

        return digest;

    }

}