需要用到的命名空间：

using System.IO;
using System.Security.Cryptography;

加密解密类定义：

public class AuthcodeHelper
{
    const string KEY_64 = "HuidTeac";//注意了，是8个字符，64位
    const string IV_64 = "HuidTeac";

    public static string Encode(string data)
    {
        byte[] byKey = System.Text.ASCIIEncoding.ASCII.GetBytes(KEY_64);
        byte[] byIV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV_64);

        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);
        sw.Write(data);
        sw.Flush();
        cst.FlushFinalBlock();
        sw.Flush();
        return Convert.ToBase64String(ms.GetBuffer(), 0, (int)ms.Length);

    }

    public static string Decode(string data)
    {
        byte[] byKey = System.Text.ASCIIEncoding.ASCII.GetBytes(KEY_64);
        byte[] byIV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV_64);

        byte[] byEnc;
        try
        {
            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);
        return sr.ReadToEnd();
    }

}

使用方法：

//加密
string userdata = "{\"userid\":\"123\",\"username\":\"zhangsan\",\"roleid\":\"3\",\"truename\":\"张三\"}";
userdata = AuthcodeHelper.Encode(userdata);
//保存到客户端
StreamWriter streamw = new StreamWriter("filename.txt", false);
streamw.Write(userdata);
streamw.Close();
//解密
string userdata = File.ReadAllText("filename.txt");
userdata = AuthcodeHelper.Decode(userdata);