沒什麼好說的，直接上類。

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

namespace Secret
{
    public class MD5
    {
        /// <summary>
        /// MD5加密
        /// </summary>
        /// <param name="s">需要加密的字串</param>
        /// <returns></returns>
        public static string EncryptMD5(string s)
        {
            var md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
            var result = "";
            if (!string.IsNullOrWhiteSpace(s))
            {
                result = BitConverter.ToString(md5.ComputeHash(UnicodeEncoding.UTF8.GetBytes(s.Trim())));
            }
            return result;
        }
    }

    public class DES
    {
        //DES加密祕鑰，要求為8位
        private const string desKey = "xianglk1";
        //預設金鑰向量
        private static byte[] Keys = { 0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF };

        /// <summary>
        /// DES加密
        /// </summary>
        /// <param name="encryptString">待加密的字串，未加密成功返回原串</param>
        /// <returns></returns>
        public static string EncryptDES(string encryptString)
        {
            try
            {
                byte[] rgbKey = Encoding.UTF8.GetBytes(desKey);
                byte[] rgbIV = Keys;
                byte[] inputByteArray = Encoding.UTF8.GetBytes(encryptString);
                DESCryptoServiceProvider dCSP = new DESCryptoServiceProvider();
                MemoryStream mStream = new MemoryStream();
                CryptoStream cStream = new CryptoStream(mStream, dCSP.CreateEncryptor(rgbKey, rgbIV), CryptoStreamMode.Write);
                cStream.Write(inputByteArray, 0, inputByteArray.Length);
                cStream.FlushFinalBlock();
                return Convert.ToBase64String(mStream.ToArray());
            }
            catch
            {
                return encryptString;
            }
        }

        /// <summary>
        /// DES解密
        /// </summary>
        /// <param name="decryptString">待解密的字串，未解密成功返回原串</param>
        /// <returns></returns>
        public static string DecryptDES(string decryptString)
        {
            try
            {
                byte[] rgbKey = Encoding.UTF8.GetBytes(desKey);
                byte[] rgbIV = Keys;
                byte[] inputByteArray = Convert.FromBase64String(decryptString);
                DESCryptoServiceProvider DCSP = new DESCryptoServiceProvider();
                MemoryStream mStream = new MemoryStream();
                CryptoStream cStream = new CryptoStream(mStream, DCSP.CreateDecryptor(rgbKey, rgbIV), CryptoStreamMode.Write);
                cStream.Write(inputByteArray, 0, inputByteArray.Length);
                cStream.FlushFinalBlock();
                return Encoding.UTF8.GetString(mStream.ToArray());
            }
            catch
            {
                return decryptString;
            }
        }

    }
}