string pwd = "";

            //字符串 ToBase64

            byte[] bytes = Encoding.Default.GetBytes(pwd);

            pwd = Convert.ToBase64String(bytes);

            //字符串分割反转。

            var strs = pwd.ToCharArray().Reverse<char>();

            pwd = string.Join("",strs ); //合并字符串

            strs = pwd.ToCharArray().Reverse<char>();

            pwd = string.Join("", strs);

            //字符串 FromBase64

            byte[] outputb = Convert.FromBase64String(pwd);

            pwd = Encoding.Default.GetString(outputb);