ASP.NET C# MD5加密 解密

    readonly static string key = "DA39A3EE5E6B4B0D3255BFEF95601890AFD80709";

        string MD5_EncodeStr = MD5_Encode(“468498148418”);
        Response.Write("[RandomNumMD5Encode]" + MD5_EncodeStr + "[/RandomNumMD5Encode]<br/>");
        string MD5_DecodeStr = MD5_Decode(MD5_EncodeStr);
        Response.Write("[RandomNumMD5Decode]" + MD5_DecodeStr + "[/RandomNumMD5Decode]<br/>");

   /// <summary>
    /// 加密
    /// </summary>
    /// <param name="data"></param>
    /// <returns></returns>
    public static string MD5_Encode(string data)
    {
        byte[] byKey = System.Text.Encoding.Default.GetBytes(key.Substring(5, 8));
        byte[] byIV = System.Text.Encoding.Default.GetBytes(key.Substring(5, 8));
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        int i = cryptoProvider.KeySize;
        MemoryStream ms = new MemoryStream();
        CryptoStream cst = new CryptoStream(ms, cryptoProvider.CreateEncryptor(byKey, byIV), CryptoStreamMode.Write);
        StreamWriter sw = new StreamWriter(cst);
        cst.Write(System.Text.Encoding.Default.GetBytes(data), 0, System.Text.Encoding.Default.GetByteCount(data));
        //sw.Write(data);   
        sw.Flush();
        cst.FlushFinalBlock();
        sw.Flush();
        return Convert.ToBase64String(ms.GetBuffer(), 0, (int)ms.Length);
    }
    /// <summary>
    /// 解密
    /// </summary>
    /// <param name="data"></param>
    /// <returns></returns>
    public static string MD5_Decode(string data)
    {
        //把密鑰轉成二進制數組
        byte[] byKey = System.Text.Encoding.Default.GetBytes(key.Substring(5, 8));
        byte[] byIV = System.Text.Encoding.Default.GetBytes(key.Substring(5, 8));
        byte[] byEnc;
        try
        {
            //base64解碼
            byEnc = Convert.FromBase64String(data);
        }
        catch
        {
            return null;
        }
        DESCryptoServiceProvider cryptoProvider = new DESCryptoServiceProvider();
        MemoryStream ms = new MemoryStream(byEnc);
        CryptoStream cst = new CryptoStream(ms, cryptoProvider.CreateDecryptor(byKey, byIV), CryptoStreamMode.Read);
        StreamReader sr = new StreamReader(cst);
        byte[] tmp = new byte[ms.Length];
        cst.Read(tmp, 0, tmp.Length);
        string result = System.Text.Encoding.Default.GetString(tmp);
        return result.Replace("\0", "");
    }