Sneazzy C# KeygenME

using System;
using System.Diagnostics;


namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            var cipherBuffer = "";
            sbyte i;

            Console.Write("YOUR ID :");
            var buffer = Console.ReadLine();

            Debug.Assert(buffer != null, "buffer != null");
            for (i = 0; i < buffer.Length; i++)
            {
                cipherBuffer = StringFunctions.ChangeCharacter(cipherBuffer, i, (char)((buffer[i] + 1 & 0x1337)));
            }

            Console.Write("Your Key : " + cipherBuffer);

            Console.ReadLine();
        }
    }

    internal static class DefineConstants
    {
        public const int BuffSize = 20;
    }

    internal static class StringFunctions
    {

        internal static string ChangeCharacter(string sourcestring, int charindex, char changechar)
        {
            return (charindex > 0 ? sourcestring.Substring(0, charindex) : "")
                   + changechar +
                   (charindex < sourcestring.Length - 1 ? sourcestring.Substring(charindex + 1) : "");
        }

        internal static bool IsXDigit(char character)
        {
            if (char.IsDigit(character))
                return true;
            return "ABCDEFabcdef".IndexOf(character) > -1;
        }

        internal static string StrChr(string stringtosearch, char chartofind)
        {
            var index = stringtosearch.IndexOf(chartofind);
            return index > -1 ? stringtosearch.Substring(index) : null;
        }

        internal static string StrRChr(string stringtosearch, char chartofind)
        {
            var index = stringtosearch.LastIndexOf(chartofind);
            return index > -1 ? stringtosearch.Substring(index) : null;
        }

        internal static string StrStr(string stringtosearch, string stringtofind)
        {
            var index = stringtosearch.IndexOf(stringtofind, StringComparison.Ordinal);
            return index > -1 ? stringtosearch.Substring(index) : null;
        }

        private static string _activestring;
        private static int _activeposition;

        internal static string StrTok(string stringtotokenize, string delimiters)
        {
            if (stringtotokenize != null)
            {
                _activestring = stringtotokenize;
                _activeposition = -1;
            }

            if (_activestring == null)
                return null;

            if (_activeposition == _activestring.Length)
                return null;

            _activeposition++;
            while (_activeposition < _activestring.Length && delimiters.IndexOf(_activestring[_activeposition]) > -1)
            {
                _activeposition++;
            }

            if (_activeposition == _activestring.Length)
                return null;

            int startingposition = _activeposition;

            do
            {
                _activeposition++;
            } while (_activeposition < _activestring.Length && delimiters.IndexOf(_activestring[_activeposition]) == -1);

            return _activestring.Substring(startingposition, _activeposition - startingposition);
        }
    }
}