base64

 public static string EncodeToBase64(string value)
        {
            byte[] bytes = System.Text.Encoding.ASCII.GetBytes(value);

            return Convert.ToBase64String(bytes);
        }

        public static string DecodeFromBase64(string value)
        {
            byte[] bytes = Convert.FromBase64String(value)
            return System.Text.Encoding.ASCII.GetString(bytes);
        }

http://stackoverflow.com/questions/3355407/validate-string-is-base64-format-using-regex