方式一：效率較快！

        /// <summary>
        /// 加密
        /// </summary>        
        /// <param name="original">字串</param>
        /// <returns>回傳加密結果</returns>
        public string Encrypt(string Str)
        {
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            byte[] bytes = md5.ComputeHash(Encoding.Default.GetBytes(Str));
            return BitConverter.ToString(bytes).Replace("-", string.Empty);
        }

方式二：效率較慢，程式碼簡單！

/// <summary>
/// 加密
/// </summary>
/// <param name="Str">字串</param>
/// <returns>回傳加密結果</returns>
public string Encrypt(string Str)
{
    return FormsAuthentication.HashPasswordForStoringInConfigFile(Str, "md5");
}