using System;

using System.Linq;

using System.Text;

​

    public class Program

    {

        public static void Main(string[] args)

        {

            var raw = Convert.FromBase64String(

                "MTUtMDktMDItMTItNTItMGYtMWItMGItNTMtMTEtMGItMWMtMDEtNDktMDAtMTgtMDAtMGUtNDMtMWMtMDEtNDktNTYtMTAtNTMtMDktMGEtMWUtMTctNDktMGMtMTYtMDEtNDc=");

            var data = Encoding.ASCII.GetString(raw)

                .Split('-')

                .Select((v, i) => Convert.ToByte(v, 16))

                .ToArray();

            var key = Encoding.ASCII.GetBytes("security");

            var l = data.Length;

            var kl = key.Length;

            for (var i = 0; i < l; i++)

            {

               data[i] = (byte)(data[i] ^ key[i % kl]);

            }

            Console.WriteLine(Encoding.ASCII.GetString(data));

        }

    }