用winform做的HttpWebRequest 模拟登陆器，post参数要如何暗文发送？

用了Encrypt()不是会改变原值吗？发送的账号密码就会提示不正确，怎么加密呢？
            Uri uri = new Uri(web_url);
            NameValueCollection values = new NameValueCollection();
            values.Add("username", this.Encrypt(username, key));
            values.Add("password", this.Encrypt(password, key));
            string data = this.PostData(uri, values, true);private string PostData(Uri uri, NameValueCollection values, [Optional, DefaultParameterValue(true)] bool status)
        {
            try
            {
                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(uri);
                request.Method = "POST";
                request.ContentType = "application/x-www-form-urlencoded";
                request.Accept = "*/*";
                request.Headers.Add("Accept-Encoding: gzip,deflate");
                request.Headers.Add("Accept-Language: zh-cn");
                request.ServicePoint.Expect100Continue = false;
                request.KeepAlive = true;
                request.UserAgent = "Mozilla/5.0 (iPad; CPU OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A403 Safari/8536.25";
                if (status)
                {
                    CookieContainer container = new CookieContainer();
                    container.Add(uri, new Cookie("cookie", this.SessionID));
                    //container.Add(uri, new Cookie("Version", this.Version));
                    request.CookieContainer = container;
                }
                string s = string.Empty;
                if (values != null)
                {
                    foreach (string str2 in values.Keys)
                    {
                        s = s + string.Format("{0}={1}&", str2, values[str2]);
                    }
                    s = s.Remove(s.Length - 1);
                }
                byte[] bytes = Encoding.UTF8.GetBytes(s);
                Stream requestStream = request.GetRequestStream();
                requestStream.Write(bytes, 0, bytes.Length);
                requestStream.Close();
                StreamReader reader = new StreamReader(request.GetResponse().GetResponseStream(), Encoding.UTF8);
                return reader.ReadToEnd();
            }
            catch (Exception exception)
            {
                this.AddLog("网络错误", 500, exception.Message);
            }
            return string.Empty;
        }public string Encrypt(string toEstr, string key)
        {
            byte[] bytes = Encoding.UTF8.GetBytes(toEstr);
            byte[] buffer2 = Encoding.UTF8.GetBytes(key);
            AesManaged managed2 = new AesManaged
            {
                Padding = PaddingMode.PKCS7,
                Mode = CipherMode.ECB,
                Key = buffer2
            };
            ICryptoTransform transform = managed2.CreateEncryptor();
            try
            {
                byte[] inArray = transform.TransformFinalBlock(bytes, 0, bytes.Length);
                return Convert.ToBase64String(inArray, 0, inArray.Length);
            }
            catch (CryptographicException exception)
            {
                return exception.Message;
            }
        }