具体请百度，下面为封装好的类（代码细节请参考msdn）:

/// <summary>
///DES加密算法
/// </summary>
public class DES
{
public DES()
{
    //
    //TODO: 在此处添加构造函数逻辑
    //
}

/// <summary>
/// 对字符进行加密
/// </summary>
/// <param name="pToEncrypt">输入字符串</param>
/// <param name="sKey">密钥，要求为8个字节共64位</param>
/// <returns>返回加密结果</returns>
public static string Encrypt(string pToEncrypt, string sKey)
{
    DESCryptoServiceProvider des = new DESCryptoServiceProvider();

    byte[] inputByteArray = Encoding.Default.GetBytes(pToEncrypt);
    des.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
    des.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
    MemoryStream ms = new MemoryStream();
    CryptoStream cs = new CryptoStream(ms, des.CreateEncryptor(), CryptoStreamMode.Write);
    cs.Write(inputByteArray, 0, inputByteArray.Length);
    cs.FlushFinalBlock();
    StringBuilder ret = new StringBuilder();
    foreach (byte b in ms.ToArray())
    {
        ret.AppendFormat("{0:X2} ", b);
    }
    ret.ToString();
    return ret.ToString();
}

/// <summary>
/// 对已加密字符密文解密
/// </summary>
/// <param name="pToDecrypt">已加密字符</param>
/// <param name="sKey">密钥，要求为8个字节共64位</param>
/// <returns>解密结果</returns>
public static string Decrypt(string pToDecrypt, string sKey)
{
    pToDecrypt = pToDecrypt.Replace(" ", "");//加密后的密文两位一组，组与组之间以空格隔开
                                             //这里要先去除所有空格

    DESCryptoServiceProvider des = new DESCryptoServiceProvider();
    byte[] inputByteArray = new byte[pToDecrypt.Length / 2];
    for (int x = 0; x < pToDecrypt.Length / 2; x++)
    {
        int i = (Convert.ToInt32(pToDecrypt.Substring(x * 2, 2), 16));
        inputByteArray[x] = (byte)i;
    }
    des.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
    des.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
    MemoryStream ms = new MemoryStream();
    CryptoStream cs = new CryptoStream(ms, des.CreateDecryptor(), CryptoStreamMode.Write);
    cs.Write(inputByteArray, 0, inputByteArray.Length);
    cs.FlushFinalBlock();
    StringBuilder ret = new StringBuilder();
    return System.Text.Encoding.Default.GetString(ms.ToArray());
}

/// <summary>
/// 加密文件
/// </summary>
/// <param name="sInputFile">源文件</param>
/// <param name="sOutputFile">加密过后的文件</param>
/// <param name="sKey">密钥，要求为8个字节共64位</param>
/// <param name="sMsg">返回信息</param>
/// <returns>是否成功</returns>
public static bool EncryptFile(string sInputFilename, string sOutputFilename, string sKey, out string sMsg)
{
    bool IsSuccess = true;
    FileStream fsInput = null;
    FileStream fsEncrypted = null;
    CryptoStream cryptostream = null;
    try
    {
        fsInput = new FileStream(sInputFilename, FileMode.Open, FileAccess.Read);
        fsEncrypted = new FileStream(sOutputFilename, FileMode.Create,FileAccess.Write);
        DESCryptoServiceProvider des = new DESCryptoServiceProvider();
        des.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
        des.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
        ICryptoTransform desencrypt = des.CreateEncryptor();
        cryptostream = new CryptoStream(fsEncrypted,
                                        desencrypt,
                                        CryptoStreamMode.Write);
        byte[] bytearrayinput = new byte[fsInput.Length];
        fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
        cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
        cryptostream.Close();
        fsInput.Close();
        fsEncrypted.Close();
        sMsg = "成功";
        return true;
    }
    catch (Exception ex)
    {
        sMsg = ex.Message;
        IsSuccess = true;
    }
    finally
    {
        try
        {
            if (cryptostream != null) cryptostream.Close();
            if (fsInput != null) fsInput.Close();
            if (fsEncrypted != null) fsEncrypted.Close();
        }
        catch
        {
        }
    }
    return IsSuccess;
}

/// <summary>
/// 解密文件
/// </summary>
/// <param name="sInputFilename">输入文件（已加密文件）路径</param>
/// <param name="sOutputFilename">解密后文件路径</param>
/// <param name="sKey">密钥，要求为8个字节共64位</param>
/// <param name="sMsg">返回信息</param>
/// <returns>是否成功</returns>
public static bool DecryptFile(string sInputFilename, string sOutputFilename, string sKey, out string sMsg)
{
    bool IsSuccess = true;
    FileStream fsInput = null;
    FileStream fsDecrypted = null;
    CryptoStream cryptostream = null;
    try
    {
        fsInput = new FileStream(sInputFilename, FileMode.Open, FileAccess.Read);
        fsDecrypted = new FileStream(sOutputFilename, FileMode.Create, FileAccess.Write);
        DESCryptoServiceProvider des = new DESCryptoServiceProvider();
        des.Key = ASCIIEncoding.ASCII.GetBytes(sKey);
        des.IV = ASCIIEncoding.ASCII.GetBytes(sKey);
        ICryptoTransform desdecrypt = des.CreateDecryptor();
        cryptostream = new CryptoStream(fsDecrypted,
                                        desdecrypt,
                                        CryptoStreamMode.Write);
        byte[] bytearrayinput = new byte[fsInput.Length];
        fsInput.Read(bytearrayinput, 0, bytearrayinput.Length);
        cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length);
        cryptostream.Close();
        fsInput.Close();
        fsDecrypted.Close();
        sMsg = "成功";
        return true;
    }
    catch (Exception ex)
    {
        sMsg = ex.Message;
        IsSuccess = true;
    }
    finally
    {
        try
        {
            if (cryptostream != null) cryptostream.Close();
            if (fsInput != null) fsInput.Close();
            if (fsDecrypted != null) fsDecrypted.Close();
        }
        catch
        {
        }
    }
    return IsSuccess;
}
}