主要用到C#提供的以下三个类：
MemoryStream 内存流
DESCryptoServiceProvider 加密服务提供者类
CryptoStream 讲数据流连接到加密转换的流

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

namespace Common
{
    /// 

    /// DESEncrypt加密解密算法。     /// 

    public sealed class DESEncrypt
    {
        private DESEncrypt()
        {
            //
            // TODO: 在此处添加构造函数逻辑
            //
        }

        private static string key = "zhoufoxcn";

        /// 

        /// 对称加密解密的密钥         /// 

        public static string Key
        {
            get
            {
                return key;
            }
            set
            {
                key = value;
            }
        }

        /// 

        /// DES加密         /// 

        /// 
        /// 
        public static string DesEncrypt(string encryptString)
        {
            byte[] keyBytes = Encoding.UTF8.GetBytes(key.Substring(0, 8));
            byte[] keyIV = keyBytes;
            byte[] inputByteArray = Encoding.UTF8.GetBytes(encryptString);
            DESCryptoServiceProvider provider = new DESCryptoServiceProvider();
            MemoryStream mStream = new MemoryStream();
            CryptoStream cStream = new CryptoStream(mStream, provider.CreateEncryptor(keyBytes, keyIV), CryptoStreamMode.Write);
            cStream.Write(inputByteArray, 0, inputByteArray.Length);
            cStream.FlushFinalBlock();
            return Convert.ToBase64String(mStream.ToArray());
        }

        /// 

        /// DES解密         /// 

        /// 
        /// 
        public static string DesDecrypt(string decryptString)
        {
            byte[] keyBytes = Encoding.UTF8.GetBytes(key.Substring(0, 8));
            byte[] keyIV = keyBytes;
            byte[] inputByteArray = Convert.FromBase64String(decryptString);
            DESCryptoServiceProvider provider = new DESCryptoServiceProvider();
            MemoryStream mStream = new MemoryStream();
            CryptoStream cStream = new CryptoStream(mStream, provider.CreateDecryptor(keyBytes, keyIV), CryptoStreamMode.Write);
            cStream.Write(inputByteArray, 0, inputByteArray.Length);
            cStream.FlushFinalBlock();
            return Encoding.UTF8.GetString(mStream.ToArray());
        }
    }
}