注：由于加密、解密字符串时，需要用到加密类和内存流，所以首先需要在命名空间区域添加System.Security.Cryptography和System.IO命名空间。

//自定义的加密字符串 

static string encryptKey = "1234"; 

//字符串加密
        static string Encrypt(string str)
        {
            DESCryptoServiceProvider descsp = new DESCryptoServiceProvider();
            byte[] key = Encoding.Unicode.GetBytes(encryptKey);
            byte[] data = Encoding.Unicode.GetBytes(str);
            MemoryStream MStream = new MemoryStream();
            CryptoStream CStream = new CryptoStream(MStream, descsp.CreateEncryptor(key, key), CryptoStreamMode.Write);
            CStream.Write(data, 0, data.Length);
            CStream.FlushFinalBlock();
            return Convert.ToBase64String(MStream.ToArray());
        }

 //字符串解密
        static string Decrypt(string str)
        {
            DESCryptoServiceProvider descsp = new DESCryptoServiceProvider();
            byte[] key = Encoding.Unicode.GetBytes(encryptKey);
            byte[] data = Convert.FromBase64String(str);
            MemoryStream MStream = new MemoryStream();
            CryptoStream CStram = new CryptoStream(MStream, descsp.CreateDecryptor(key, key), CryptoStreamMode.Write);
            CStram.Write(data, 0, data.Length);
            CStram.FlushFinalBlock();
            return Encoding.Unicode.GetString(MStream.ToArray());
        }

//调用方法

static void Main(string[] args)
        {

           Console.Write("请输入要加密的字符串：");
            Console.WriteLine();
            string strOld = Console.ReadLine();
            string strNew = Encrypt(strOld);
            Console.WriteLine("加密后的字符串:" + strNew);
            Console.WriteLine("解密后的字符串:" + Decrypt(strNew));
            Console.ReadLine();

       }