编写高质量代码改善C#程序的157个建议——建议117：使用SSL确保通信中的数据安全

建议117：使用SSL确保通信中的数据安全

SSL（Secure Socket Layer）最初是由NetScape公司设计的，用于Web安全的网络协议。目前它已经广泛应用到各类网络传输通信中了。SSL利用数字证书技术（非对称加密），保证了通信过程中的唯一性、不可篡改性、不可抵赖性。SSL通道原理图：

非对称加密中：

• 秘钥分为两部分：公钥PK和私钥SK。
• 公钥用于加密数据用，私钥用于解密。
• 公钥可公开而且应该公开，私钥只属于创建者。

经过公钥加密的数据只有证书创建者才能解密。这是构成SSL通道所有理论的依据。

在传统的网络传输过程中，我们将通信双方定义为：服务器端和客户端。假定服务器端是数字证书的创建者，它保存好自己的私钥，同时公布了自己的公钥给所有的客户端。满足了这个条件，我们来构建SSL通道。

首先，客户端随机生成一个字符串作为密钥K，然后用公钥PK对这个密钥加密，并将加密后密钥发送给服务器端。如果客户端曾经在服务器端注册过自己的信息，则还可以在这个密钥上加上自己的身份信息，从而向服务器端汇报自己的唯一性，但在本例中略去这一步。

服务器端用私钥解密消息，获取了客户端的K，并确认了客户端的身份（不可抵赖性），SSL通道建立。

服务器端和客户端现在可以进行安全通信。过程是：发送方使用密钥K对要传输的消息进行对称加密，接受方则使用K进行解密。这就是传输过程中的不可篡改性。

我们来模拟SSL的通信，服务器部分的代码：

        #region server

        //用于保存非对称加密（数字证书）的公钥
        string publicKey = string.Empty;
        //用于保存非对称加密（数字证书）的私钥
        string pfxKey = string.Empty;

        ///======================
        ///服务器端代码
        ///======================

        ///用于跟客户端通信的socket
        Socket serverCommunicateSocket;
        ///定义接受缓存块的大小
        static int serverBufferSize = ;
        ///缓存块
        byte[] bytesReceivedFromClient = new byte[serverBufferSize];
        ///密钥K
        string key = string.Empty;
        StringBuilder messageFromClient = new StringBuilder();

        ///开启服务器
        private void buttonStartServer_Click(object sender, EventArgs e)
        {
            //先生成数字证书（模拟，即非对称密钥对）
            RSAKeyInit();
            //负责侦听
            StartListen();
        }

        private void RSAKeyInit()
        {
            RSAProcessor.CreateRSAKey(ref publicKey, ref pfxKey);
        }

        private void StartListen()
        {
            IPEndPoint iep = new IPEndPoint(IPAddress.Parse("192.168.1.100"), );
            //负责侦听的socket
            Socket listenSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            listenSocket.Bind(iep);
            listenSocket.Listen();
            listenSocket.BeginAccept(new AsyncCallback(this.Accepted), listenSocket);
            ListBoxServerShow("开始侦听。。。");
            buttonStartServer.Enabled = false;
        }

        ///负责客户端的连接，并开始将自己置于接收状态
        void Accepted(IAsyncResult result)
        {
            Socket listenSocket = result.AsyncState as Socket;
            //初始化和客户端进行通信的socket
            serverCommunicateSocket = listenSocket.EndAccept(result);
            ListBoxServerShow("有客户端连接到。。。");
            serverCommunicateSocket.BeginReceive(bytesReceivedFromClient, , serverBufferSize, SocketFlags.None, new AsyncCallback(this.ReceivedFromClient), null);
        }

        ///负责处理接受自客户端的数据
        void ReceivedFromClient(IAsyncResult result)
        {
            int read = serverCommunicateSocket.EndReceive(result);
            if (read > )
            {
                messageFromClient.Append(UTF32Encoding.Default.GetString(bytesReceivedFromClient, , read));
                //处理并显示数据
                ProcessAndShowInServer();
                serverCommunicateSocket.BeginReceive(bytesReceivedFromClient, , serverBufferSize, , new AsyncCallback(ReceivedFromClient), null);
            }
        }

        private void ProcessAndShowInServer()
        {
            string msg = messageFromClient.ToString();
            //如果接收到<EOF>则表示完成完成一次，否则继续将自己置于接收状态
            if (msg.IndexOf("<EOF>") > -)
            {
                //如果客户端发送key，则负责初始化key
                if (msg.IndexOf("<KEY>") > -)
                {
                    //用私钥解密发送过来的Key信息
                    key = RSAProcessor.RSADecrypt(pfxKey, msg.Substring(, msg.Length - ));
                    ListBoxServerShow(string.Format("接收到客户端密钥：{0}", key));
                }
                else
                {
                    //解密SSL通道中发送过来的密文并显式
                    ListBoxServerShow(string.Format("接收到客户端消息：{0}", RijndaelProcessor.DencryptString(msg.Substring(, msg.Length - ), key)));
                }
                messageFromClient.Clear();
            }
        }

        ///负责向客户端发送数据
        private void buttonStartSendToClient_Click(object sender, EventArgs e)
        {
            //加密消息体
            string msg = string.Format("{0}{1}", RijndaelProcessor.EncryptString(DateTime.Now.ToString(), key), "<EOF>");
            RijndaelProcessor.DencryptString(msg.Substring(, msg.Length - ), key);
            byte[] msgBytes = UTF32Encoding.Default.GetBytes(msg);
            serverCommunicateSocket.BeginSend(msgBytes, , msgBytes.Length, SocketFlags.None, null, null);
            ListBoxServerShow(string.Format("发送：{0}", msg));
        }

        private void ListBoxServerShow(string msg)
        {
            listBoxServer.BeginInvoke(new Action(() =>
            {
                listBoxServer.Items.Add(msg);
            }));
        }
        #endregion server

RSAProcessor工具类，用于封装非对称加密算法：

    public class RSAProcessor
    {
        public static void CreateRSAKey(ref string publicKey, ref string pfxKey)
        {
            RSACryptoServiceProvider provider = new RSACryptoServiceProvider();
            pfxKey = provider.ToXmlString(true);
            publicKey = provider.ToXmlString(false);
        }

        public static string RSAEncrypt(string xmlPublicKey, string m_strEncryptString)
        {
            byte[] btEncryptedSecret = Encoding.UTF8.GetBytes(m_strEncryptString);
            btEncryptedSecret = CRSAWrap.EncryptBuffer(xmlPublicKey, btEncryptedSecret);
            return Convert.ToBase64String(btEncryptedSecret);
        }

        public static string RSADecrypt(string xmlPrivateKey, string m_strDecryptString)
        {
            byte[] btDecryptedSecred = Convert.FromBase64String(m_strDecryptString);
            btDecryptedSecred = CRSAWrap.DecryptBuffer(xmlPrivateKey, btDecryptedSecred);
            return Encoding.UTF8.GetString(btDecryptedSecred);
        }

        class CRSAWrap
        {
            public static byte[] EncryptBuffer(string rsaKeyString, byte[] btSecret)
            {
                int keySize = ;
                int blockSize = ;
                int lastblockSize = ;
                int counter = ;
                int iterations = ;
                int index = ;
                byte[] btPlaintextToken;
                byte[] btEncryptedToken;
                byte[] btEncryptedSecret;
                RSACryptoServiceProvider rsaSender = new RSACryptoServiceProvider();
                rsaSender.FromXmlString(rsaKeyString);
                keySize = rsaSender.KeySize / ;
                blockSize = keySize - ;

                if ((btSecret.Length % blockSize) != )
                {
                    iterations = btSecret.Length / blockSize + ;
                }
                else
                {
                    iterations = btSecret.Length / blockSize;
                }
                btPlaintextToken = new byte[blockSize];
                btEncryptedSecret = new byte[iterations * keySize];
                for (index = , counter = ; counter < iterations; counter++, index += blockSize)
                {
                    if (counter == (iterations - ))
                    {
                        lastblockSize = btSecret.Length % blockSize;
                        btPlaintextToken = new byte[lastblockSize];
                        Array.Copy(btSecret, index, btPlaintextToken, , lastblockSize);
                    }
                    else
                    {
                        Array.Copy(btSecret, index, btPlaintextToken, , blockSize);
                    }
                    btEncryptedToken = rsaSender.Encrypt(btPlaintextToken, false);
                    Array.Copy(btEncryptedToken, , btEncryptedSecret, counter * keySize, keySize);
                }
                return btEncryptedSecret;
            }

            public static byte[] DecryptBuffer(string rsaKeyString, byte[] btEncryptedSecret)
            {
                int keySize = ;
                int blockSize = ;
                int counter = ;
                int iterations = ;
                int index = ;
                int byteCount = ;
                byte[] btPlaintextToken;
                byte[] btEncryptedToken;
                byte[] btDecryptedSecret;
                RSACryptoServiceProvider rsaReceiver = new RSACryptoServiceProvider();
                rsaReceiver.FromXmlString(rsaKeyString);
                keySize = rsaReceiver.KeySize / ;
                blockSize = keySize - ;
                if ((btEncryptedSecret.Length % keySize) != )
                {
                    return null;
                }
                iterations = btEncryptedSecret.Length / keySize;
                btEncryptedToken = new byte[keySize];
                Queue<byte[]> tokenQueue = new Queue<byte[]>();
                for (index = , counter = ; counter < iterations; index += blockSize, counter++)
                {
                    Array.Copy(btEncryptedSecret, counter * keySize, btEncryptedToken, , keySize);
                    btPlaintextToken = rsaReceiver.Decrypt(btEncryptedToken, false);
                    tokenQueue.Enqueue(btPlaintextToken);
                }
                byteCount = ;
                foreach (var PlaintextToken in tokenQueue)
                {
                    byteCount += PlaintextToken.Length;
                }
                counter = ;
                btDecryptedSecret = new byte[byteCount];
                foreach (var PlaintextToken in tokenQueue)
                {
                    if (counter == (iterations - ))
                    {
                        Array.Copy(PlaintextToken, , btDecryptedSecret, btDecryptedSecret.Length - PlaintextToken.Length, PlaintextToken.Length);
                    }
                    else
                    {
                        Array.Copy(PlaintextToken, , btDecryptedSecret, counter * blockSize, blockSize);
                    }
                    counter++;
                }
                return btDecryptedSecret;
            }

        }
    }

RijndaelProcessor工具类，用于封装对称加密算法：

    public class RijndaelProcessor
    {
        static int bufferSize =  * ;
        static byte[] salt = { , , , , , , , , , , , , , , ,  };
        static byte[] iv = { , , , , , , , , , , , , , , ,  };

        static SymmetricAlgorithm CreateRijndael(string password, byte[] salt)
        {
            PasswordDeriveBytes pdb = new PasswordDeriveBytes(password, salt, "SHA256", );
            SymmetricAlgorithm sma = Rijndael.Create();
            sma.KeySize = ;
            sma.Key = pdb.GetBytes();
            sma.Padding = PaddingMode.PKCS7;
            return sma;
        }

        public static string EncryptString(string input, string password)
        {
            using (MemoryStream memoryStream = new MemoryStream())
            using (SymmetricAlgorithm algorithm = CreateRijndael(password, salt))
            {
                algorithm.IV = iv;
                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, algorithm.CreateEncryptor(), CryptoStreamMode.Write))
                {
                    byte[] bytes = UTF32Encoding.Default.GetBytes(input);
                    cryptoStream.Write(bytes, , bytes.Length);
                    cryptoStream.Flush();
                }
                return Convert.ToBase64String(memoryStream.ToArray());
            }
        }

        public static string DencryptString(string input, string password)
        {
            using (MemoryStream inputMemoryStream = new MemoryStream(Convert.FromBase64String(input)))
            using (SymmetricAlgorithm algorithm = CreateRijndael(password, salt))
            {
                algorithm.IV = iv;
                using (CryptoStream cryptoStream = new CryptoStream(inputMemoryStream, algorithm.CreateDecryptor(), CryptoStreamMode.Read))
                {
                    StreamReader sr = new StreamReader(cryptoStream);
                    return sr.ReadToEnd();
                }
            }
        }
    }

这是一WinForm窗体程序，模拟的是服务器端部分，其中有两个按钮。按钮事件方法buttonStartServe_Click负责让服务器处理侦听状态。当然，为了模拟需要，在方法中还初始化了非对称加密密钥对。记住，公钥要公开给客户端。注意，也可以使用数字证书，但是为了演示方便，本例仅使用公钥-私钥对。

通信部分直接使用了FCL中的Socket类型，并采用异步的方式处理发送和接收任务。关于通信部分，本建议不再赘述。唯一要注意的是，在发送和接收过程中，要调用RijndaelProcessor.EncryptString方法加密，然后用RijndaelProcessor.DencryptString方法解密。RijndaelProcessor类型是用来封装对称加密、解密算法的工具类。

客户端部分代码：

        #region client
        ///======================
        ///客户端代码
        ///======================

        ///用于跟服务器通信的socket
        Socket clientCommunicateSocket;
        ///用于暂存接收到的字符串
        StringBuilder messageFromServer = new StringBuilder();
        ///定义接受缓存块的大小
        static int clientBufferSize = ;
        ///缓存块
        byte[] bytesReceivedFromServer = new byte[clientBufferSize];
        //随机生成的key，在这里硬编码为key123
        string keyCreateRandom = "key123";

        private void buttonConnectAndReceiveMsg_Click(object sender, EventArgs e)
        {
            IPEndPoint iep = new IPEndPoint(IPAddress.Parse("192.168.1.100"), );
            Socket connectSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            connectSocket.BeginConnect(iep, new AsyncCallback(this.Connected), connectSocket);
            buttonConnectAndReceiveMsg.Enabled = false;
        }

        void Connected(IAsyncResult result)
        {
            clientCommunicateSocket = result.AsyncState as Socket;
            clientCommunicateSocket.EndConnect(result);
            clientCommunicateSocket.BeginReceive(bytesReceivedFromServer, , clientBufferSize, SocketFlags.None, new AsyncCallback(this.ReceivedFromServer), null);
            ListBoxClientShow("客户端连接上服务器。。。");
            //连接成功便发送密钥K给服务器
            SendKey();
        }

        void ReceivedFromServer(IAsyncResult result)
        {
            int read = clientCommunicateSocket.EndReceive(result);
            if (read > )
            {
                messageFromServer.Append(UTF32Encoding.Default.GetString(bytesReceivedFromServer, , read));
                //处理并显示客户端数据
                ProcessAndShowInClient();
                clientCommunicateSocket.BeginReceive(bytesReceivedFromServer, , clientBufferSize, , new AsyncCallback(ReceivedFromServer), null);
            }
        }

        private void ProcessAndShowInClient()
        {
            //如果接收到<EOF>则表示完成一次接收，否则继续将自己置于接收状态
            if (messageFromServer.ToString().IndexOf("<EOF>") > -)
            {
                //解密消息体并呈现出来
                ListBoxClientShow(string.Format("接收到服务器消息：{0}", RijndaelProcessor.DencryptString(messageFromServer.ToString().Substring(, messageFromServer.ToString().Length - ), keyCreateRandom)));
                messageFromServer.Clear();
            }
        }

        private void buttonStartSendToServer_Click(object sender, EventArgs e)
        {
            //加密消息体
            string msg = string.Format("{0}{1}", RijndaelProcessor.EncryptString(DateTime.Now.ToString(), keyCreateRandom), "<EOF>");
            byte[] msgBytes = UTF32Encoding.Default.GetBytes(msg);
            clientCommunicateSocket.BeginSend(msgBytes, , msgBytes.Length, SocketFlags.None, null, null);
            ListBoxClientShow(string.Format("发送：{0}", msg));
        }

        private void SendKey()
        {
            string msg = RSAProcessor.RSAEncrypt(publicKey, keyCreateRandom) + "<KEY><EOF>";
            byte[] msgBytes = UTF32Encoding.Default.GetBytes(msg);
            clientCommunicateSocket.BeginSend(msgBytes, , msgBytes.Length, SocketFlags.None, null, null);
            ListBoxClientShow(string.Format("发送：{0}", keyCreateRandom));
        }

        private void ListBoxClientShow(string msg)
        {
            listBoxClient.BeginInvoke(new Action(() =>
            {
                listBoxClient.Items.Add(msg);
            }));
        }
        #endregion client

客户端部分也包含两个按钮，在服务器部分按下“侦听”按钮后，客户端可以按下“链接”按钮。这个过程，程序主要完成两件事情。首先，程序会根据服务器IP地址连接上服务器；其次，一旦连接上服务器，客户端会立即将自己用于加密的密钥发送给服务器。

完成这个步骤后，SSL通道已经建立起来的，这个时候就可以随意发送加密数据而不担心被盗走了。我们可以看到，客户端的代码与服务器端一样，在发送之前，消息要加密，而在接收到消息体之后，首先会解密。

转自：《编写高质量代码改善C#程序的157个建议》陆敏技