ASP.NET C# URL加密解密

using System.Security.Cryptography;
using System.IO;
using System.text;
/// <summary>
        /// 加密
        /// </summary>
        /// <param name="str"></param>
        /// <param name="key"></param>
        /// <returns></returns>
        public static string Encode(string str, string key)
         {
             DESCryptoServiceProvider provider = new DESCryptoServiceProvider();
             provider.Key = Encoding.ASCII.GetBytes(key.Substring(0, 8));
             provider.IV = Encoding.ASCII.GetBytes(key.Substring(0, 8));
            byte[] bytes = Encoding.GetEncoding("GB2312").GetBytes(str);
             MemoryStream stream = new MemoryStream();
             CryptoStream stream2 = new CryptoStream(stream, provider.CreateEncryptor(), CryptoStreamMode.Write);
             stream2.Write(bytes, 0, bytes.Length);
             stream2.FlushFinalBlock();
             StringBuilder builder = new StringBuilder();
            foreach (byte num in stream.ToArray())