using System;

using System.Security.Cryptography;

using System.Collections.Generic;

using System.Text.Json;

using System.Text;

using System.Threading;

public class Program

{

    public static void Main()

    {

        //while(true)

        //{

            var epochSecondsIncrement = DateTimeOffset.UtcNow.ToUnixTimeSeconds() / 30; //Production code should divide by 30

            var list = new List<long>{3, 2, epochSecondsIncrement};

            HashAlgorithm md5 = MD5.Create();

            var json = JsonSerializer.Serialize(list);

            var dataString = $"{(int)list[0]}-{(int)list[1]}-{epochSecondsIncrement}";

            var dataArray = Encoding.Unicode.GetBytes(dataString);

            byte[] md5out = md5.ComputeHash(dataArray); //Yields 128 bits or 16 bytes

            UInt32 result = BitConverter.ToUInt32(md5out, 12); //Start on byte 13 so result will be limited to four bytes

            result &= 0x000fffff; //Clear top 12 bits to limit number to 2^20

            result %= 1000000; //If number is over six digits then reduce it to six digits

            if(result == 0) result++; //Never use zero as a valid code

            var isValid = IsTwoFactorAuthCodeValid((int)list[0], (int)list[1], result);

            Console.WriteLine($"Time: {DateTime.Now} code: {result:D6} isValid: {isValid} dataStr: {dataString}");

            //Thread.Sleep(1000);

        //}

    }

​

    private static bool IsTwoFactorAuthCodeValid(int clientID, int trackerID, uint code)

    {

        var matchingCodes = new List<uint>();

        var epochSecondsIncrement = DateTimeOffset.UtcNow.ToUnixTimeSeconds() / 30;

        for (int i = -2; i <= 2; i++)

        {

            //var list = new List<long> { 100004, 12, epochSecondsIncrement };

            HashAlgorithm md5 = MD5.Create(); //Good enough since only using 20 bits of the 128 bit hash

            var dataString = $"{clientID}-{trackerID}-{epochSecondsIncrement+i}";

            var dataArray = Encoding.Unicode.GetBytes(dataString);

            byte[] md5out = md5.ComputeHash(dataArray); //Yields 128 bits or 16 bytes (way more than enough since only need 20 bits)

            var result = BitConverter.ToUInt32(md5out, 12); //Start on byte 12 (zero index) so result will be limited to four bytes

            result &= 0x000fffff; //Clear top 12 bits to limit number to 2^20

            result %= 1000000; //Limit number to six digits

            if(result == 0) result++; //Never use zero as a valid code

            matchingCodes.Add(result);

            Console.WriteLine($"Validator: dataStr: {dataString} i: {i}");

        }

        return matchingCodes.Contains(code);

    }

}