using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Dictionary<string, string> map = new Dictionary<string, string>{{"A", "K"}, {"B", "L"}, {"C", "M"}, {"D", "N"}, {"E", "O"}, 

                                                                        {"F", "P"}, {"0", "A"}, {"1", "B"}, {"2", "C"}, {"3", "D"}, 

                                                                        {"4", "E"}, {"5", "F"}, {"6", "G"}, {"7", "H"}, {"8", "I"}, 

                                                                        {"9", "J"}};

​

        byte[] r = Convert.FromBase64String("XFEWtnopccImhpHTzGeoeXBg4ws=");

        string c = BitConverter.ToString(r).Replace("-", string.Empty);

​

        var regex = new Regex(String.Join("|",map.Keys));

        var newStr = regex.Replace(c, m => map[m.Value]);

        Console.WriteLine(newStr.ToLower());

    }

}

​