public static string Encode(string txt)
        {
            string result = string.Empty;
            foreach (char ch in txt)
            {
                result += Convert.ToString((int)ch, 10);
            }
            result = result.Replace("0", "dos");
            result = result.Replace("1", "vso");
            result = result.Replace("2", "otw");
            result = result.Replace("3", "foa");
            result = result.Replace("4", "bae");
            result = result.Replace("5", "xgd");
            result = result.Replace("6", "ymt");
            result = result.Replace("7", "ksx");
            result = result.Replace("8", "wte");
            result = result.Replace("9", "rom");
            return result;
        }

        public static string Decode(string txt)
        {
            string result = string.Empty;
            result = result.Replace("dos", "0");
            result = result.Replace("vso", "1");
            result = result.Replace("otw", "2");
            result = result.Replace("foa", "3");
            result = result.Replace("bae", "4");
            result = result.Replace("xgd", "5");
            result = result.Replace("ymt", "6");
            result = result.Replace("ksx", "7");
            result = result.Replace("wte", "8");
            result = result.Replace("rom", "9");
            return result;
        }