Xml 加密

/// <summary>
        /// 保存用户
        /// </summary>
        public void SetUser()
        {
            XmlDocument xmlDoc = new XmlDocument();          // 创建一个XmlDocument 对象
            xmlDoc.Load(Application.StartupPath + "//user.xml");  // 加载Xml
            XmlNodeList userNodes = xmlDoc.SelectSingleNode("root").ChildNodes; //声明XmlNodeList对象  
            foreach (XmlNode userNode in userNodes) // 循环
            {
                XmlNodeList idandpwd = userNode.ChildNodes; 
                foreach (XmlNode idnode in idandpwd) // 查找密码
                {
                    if (idnode.InnerText == cmbUserName.Text) //  用户名
                    {
                        idnode.NextSibling.InnerText = jnzp.DESEncrypt.Encrypt(txtPwd.Text, "duowanSG");
                        xmlDoc.Save(Application.StartupPath + "//user.xml");
                        return;
                    }
                }
            }

            XmlNode root = xmlDoc.SelectSingleNode("root");//查找<root>
            XmlElement user = xmlDoc.CreateElement("user");
            XmlElement id = xmlDoc.CreateElement("id");
            XmlElement pwd = xmlDoc.CreateElement("pwd");
            id.InnerText = cmbUserName.Text;
            user.AppendChild(id);
            if (checkBox1.Checked)
            {
                pwd.InnerText = jnzp.DESEncrypt.Encrypt(txtPwd.Text, "duowanSG");
            }
            else
            {
                pwd.InnerText = "";
            }
            user.AppendChild(pwd);
            root.AppendChild(user);//添加到<bookstore>节点中
            xmlDoc.Save(Application.StartupPath + "//user.xml");

        }
        /// <summary>
        /// 提取密码
        /// </summary>
        /// <param name="userId">账号</param>
        /// <returns></returns>
        public string GetPwd(string userId)
        {
            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load(Application.StartupPath + "//user.xml");
            XmlNodeList userNodes = xmlDoc.SelectSingleNode("root").ChildNodes;
            foreach (XmlNode user in userNodes)
            {
                XmlNodeList idandpwd = user.ChildNodes;
                foreach (XmlNode idnode in idandpwd)
                {
                    if (idnode.InnerText == cmbUserName.Text)
                    {
                        string pwd=idnode.NextSibling.InnerText;
                        if (pwd == "")
                        {
                            return "";
                        }
                        return jnzp.DESEncrypt.Decrypt(pwd, "duowanSG");
                    }
                }
            }
            return "";
        }