const string KEY_64 = "HuidTeac";//注意了，是8个字符
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();
}