using System;

using System.Text;

public class Program

{

    public static void Main()

    {

         string base64 = "MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEYYMMoOaFYAhMO/steotfZyavr6p11SSlwsK9azmsLY7b1b+FLhqMArhB2dqHKboxqKNfvkKDePhpqjui5hcn0Q==";

​

         string sallerName = GetHexString(1, Encoding.UTF8.GetBytes("Bobs Records"));            //Tag1

         string vatReg = GetHexString(2, Encoding.UTF8.GetBytes("310122393500003"));             //Tag2

         string dateTimeStr = GetHexString(3, Encoding.UTF8.GetBytes("2022-04-25 15:30:00"));    //Tag3

         string totalAmt = GetHexString(4, Encoding.UTF8.GetBytes("10000.00"));                  //Tag4

         string vatAmt = GetHexString(5, Encoding.UTF8.GetBytes("150.00"));                      //Tag5

         string pk = GetHexString(6, Convert.FromBase64String(base64));                          //Tag6

​

         string decString = sallerName + vatReg + dateTimeStr + totalAmt + vatAmt + pk;

         Console.WriteLine(HexToBase64(decString));

         Console.WriteLine(Convert.ToBase64String(Convert.FromHexString(decString))); // as of .NET5

    }

    static string GetHexString(int tagNo, byte[] tagValue)

    {

        string strTagNo = string.Format("0{0:X}", tagNo);

        string tagNoVal = strTagNo.Substring(strTagNo.Length - 2, 2);

​

        string strTagValueLength = string.Format("0{0:X}", tagValue.Length);

        string tagValueLengthVal = strTagValueLength.Substring(strTagValueLength.Length - 2, 2);

​

        return tagNoVal + tagValueLengthVal + BitConverter.ToString(tagValue).Replace("-", "");

    }

    public static string HexToBase64(string strInput)

    {

        try

        {

            var bytes = new byte[strInput.Length / 2];

            for (var i = 0; i < bytes.Length; i++)

            {

                bytes[i] = Convert.ToByte(strInput.Substring(i * 2, 2), 16);

            }

            return Convert.ToBase64String(bytes);

        }

        catch (Exception)

        {

            return "-1";

        }

    }

}