证书的应用之一 —— TCP&SSL通信实例及协议分析(下)
前面两部分分别讲解了如何在.net程序中使用SSL实现安全通信以及SSL的通信过程，并通过抓包工具具体分析了ssl的握手过程，本文通过一个demo来模拟ssl协议，在TCP之上实现自己的安全通信。
创建证书
为了实现安全的通信，必须使用证书对传输的数据进行加密，有两种方法可以得到证书，一是安装CA服务器，使用CA来发放证书，二是通过makecert.exe工具自己创建。
创建服务器证书：
cmd>makecert.exe -r -pe -n "CN=MySslServer" -sky Exchange -ss My
创建客户端证书
cmd>makecert.exe -r -pe -n "CN=MySslClient" -sky Exchange -ss My
关于makecert.exe的更多信息可以操作这里。
建立TCP连接
使用普通的Socket类来建立TCP连接即可。后续的握手，传输数据都是在TCP之上的，在这里，定义每条信息的第一个字节是类型，0x01是整数，0x02是session key，0x03是应用层数据，0x04是错误信息，未使用。
我们知道TCP通信是stream的，消息没有边界，有可能发送方发送的数据，在接收方两次才能接收完，或者发送方连续发送两次数据，接收方一次就全部接受，这就要求应用层自己对消息的边界进行区分，常用的有3种方法，一是固定长度，所有消息长度一样，不够就填充，显然不够实用而且浪费带宽；二是定义消息边界，每条消息以固定字节流结尾如<EOF>，如果传输的数据刚好含有这样的字节流，就必须进行转义；三是消息的起始处使用固定字节来描述本消息的长度。本文为方便起见，没有进行消息边界处理，每两次发送有个间隔。
交换证书
在TCP连接建立之后，开始进行握手过程，主要是交换证书，交换session key。
server与client各自将自己的证书发送给对方，证书被export成.cer格式，只含有public key。证书的作用一是交换session key，二是发送数据时签名。
1 byte[] certBytes = so.localCert.Export(X509ContentType.Cert);
2 byte[] sentBytes = AddType(certBytes, type);
3 so.workSocket.Send(sentBytes, 0, sentBytes.Length, SocketFlags.None);
4 Console.WriteLine("Send>>>>>>>>>>>>>>>>>>>>>");
5 PrintCert(so.localCert);
6 。。。。。。
7 byte[] data = RemoveType(so.buffer, read);
8 so.remoteCert = new X509Certificate2(data);
9 Console.WriteLine("Recv<<<<<<<<<<<<<<<<<<<<<<");
10 PrintCert(so.remoteCert);
交换session key
本demo使用DES对称算法来加密，必须在发送应用层数据之前交换算法的key，使用双方约定固定的IV。
其中一方生成随机的key，用对方的证书加密，调用RSA的Encrypt方法，会使用Public key，将加密后的数据发送给对方，由于只有对方才有证书的private key，所以也只有对方才能解密出key，第三方即使中途截获了数据也不能破解出key。
生成session key
1 RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)so.remoteCert.PublicKey.Key;
2 byte[] encrypedSessionKey = rsa.Encrypt(so.localSessionKey, false);
3 byte[] sentBytes = AddType(encrypedSessionKey, type);
4 so.workSocket.Send(sentBytes, 0, sentBytes.Length, SocketFlags.None);
5 Console.WriteLine("Send>>>>>>>>>>>>>>>>>>>>>");
6 PrintSessionKey(so.localSessionKey);
解析session key
1 byte[] data = RemoveType(so.buffer, read);
2 RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)so.localCert.PrivateKey;
3 so.remoteSessionKey = rsa.Decrypt(data, false);
4 Console.WriteLine("Recv<<<<<<<<<<<<<<<<<<<<<<");
5 PrintSessionKey(so.remoteSessionKey);
发送加密和签名数据
发送数据使用DES算法加密，防止被第三方截获；同时对发送的数据签名，让对方确认数据没有被篡改并且确认数据是自己发送的。
数据格式:
|
0x03 |
密文长度 |
密文 |
签名长度 |
签名 |
对原文使用hash算法，并使用自己的证明对hash进行签名，签名是使用的private key，接收方收到数据后，用DES解密出原文，对原文计算hash，然后对收到的签名使用对方的证书的public key还原，比较与hash是否一致。
加密数据并签名
1 MemoryStream memoryStream = new MemoryStream();
2
3 // encrypt
4 DESCryptoServiceProvider des = new DESCryptoServiceProvider();
5 ICryptoTransform cTransform = des.CreateEncryptor(so.remoteSessionKey, new byte[8]);
6 MemoryStream ms = new MemoryStream();
7 CryptoStream encStream = new CryptoStream(ms, cTransform, CryptoStreamMode.Write);
8 encStream.Write(so.sentData, 0, so.sentData.Length);
9 encStream.FlushFinalBlock();
10
11 byte[] temp = ms.ToArray();
12 memoryStream.Write(BitConverter.GetBytes(temp.Length), 0, 4);
13 memoryStream.Write(temp, 0, temp.Length);
14
15 // hash
16 MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
17 byte[] hash = md5.ComputeHash(so.sentData);
18
19 // sign
20 RSACryptoServiceProvider rsa = (RSACryptoServiceProvider) so.localCert.PrivateKey;
21 byte[] sign = rsa.SignHash(hash, CryptoConfig.MapNameToOID("MD5"));
22
23 memoryStream.Write(BitConverter.GetBytes(sign.Length), 0, 4);
24 memoryStream.Write(sign, 0, sign.Length);
25
26 byte[] sentBytes = AddType(memoryStream.ToArray(), type);
27 so.workSocket.Send(sentBytes, 0, sentBytes.Length, SocketFlags.None);
28 Console.WriteLine("Send>>>>>>>>>>>>>>>>>>>>>");
29 PrintData(so.sentData);
解密数据并验证
1 byte[] data = RemoveType(so.buffer, read);
2
3 // Decrypt
4 DESCryptoServiceProvider des = new DESCryptoServiceProvider();
5 ICryptoTransform cTransform = des.CreateDecryptor(so.localSessionKey, new byte[8]);
6 int len = BitConverter.ToInt32(data, 0);
7 byte[] cipherText = new byte[len];
8 Array.Copy(data, 4, cipherText, 0, len);
9 MemoryStream ms2 = new MemoryStream(cipherText);
10 CryptoStream decStream = new CryptoStream(ms2, cTransform, CryptoStreamMode.Read);
11
12 Byte[] tempBuffer = new Byte[1024];
13 Byte[] outputBytes = new byte[0];
14 int nRead = decStream.Read(tempBuffer, 0, tempBuffer.Length);
15 int nLength = 0;
16 while (0 != nRead)
17 {
18 nLength = outputBytes.Length;
19 Array.Resize(ref outputBytes, nLength + nRead);
20 Array.Copy(tempBuffer, 0, outputBytes, nLength, nRead);
21 nRead = decStream.Read(tempBuffer, 0, tempBuffer.Length);
22 }
23 so.recvData = outputBytes;
24 Console.WriteLine("Recv<<<<<<<<<<<<<<<<<<<<<<");
25 PrintData(so.recvData);
26
27 // hash
28 MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
29 byte[] hash = md5.ComputeHash(so.recvData);
30
31 // verify
32 int signlen = BitConverter.ToInt32(data, len + 4);
33 byte[] sign = new byte[signlen];
34 Array.Copy(data, 4+len+4,sign, 0, signlen);
35 RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)so.remoteCert.PublicKey.Key;
36 try
37 {
38 bool verify = rsa.VerifyHash(hash, CryptoConfig.MapNameToOID("MD5"), sign);
39 if (verify)
40 {
41 Console.WriteLine("verify correct!");
42 }
43 else
44 {
45 Console.WriteLine("verify error!");
46 }
47 }
48 catch
49 {
50 Console.WriteLine("verify error!");
51 return PackageType.Error;
52 }
运行结果
服务器端输出
1 C:\D\BACKUP\Projects\TestProject\MySSLServer\bin\Debug>MySSLServer.exe
2 Recv<<<<<<<<<<<<<<<<<<<<<<
3 ============================================
4 cert
5 ============================================
6 Subject: CN=MySslClient
7 Issuer: CN=MySslClient
8 Version: 3
9 Valid Date: 7/7/2010 11:25:28 AM
10 Expiry Date: 1/1/2040 7:59:59 AM
11 Thumbprint: D9AE7CF97A254FCA05EB7B704DDC90878FC56B0C
12 Serial Number: 2500552056343D874C0B714A71CB047B
13 Friendly Name: RSA
14 Public Key Format: 30 81 89 02 81 81 00 9f 6f 17 84 0f 03 a0 8c b7 86 45 49 ba 0c c5 ab 37 c9 30 24 32 b5 dc 90 31 e4 d6
15 ea 83 45 1f c8 2d dd 2f 81 3e da 09 44 7f 2a cf bb 18 9a 92 28 d3 79 f0 33 b8 04 f2 1a 1d f8 e0 b6 f9 cc da 36 16 2b e3
16 8b 8d 82 5f 9c 88 6a 79 54 53 4f 08 10 dc b9 53 f1 c8 d4 bd 3d c4 94 71 1d db a2 76 65 8c 37 a3 cd 88 0d a0 72 0e 93 3a
17 64 62 dc 90 cf 19 3f f7 58 59 d6 bd 4e 3f a4 15 30 5b 36 75 2f 02 03 01 00 01
18 Send>>>>>>>>>>>>>>>>>>>>>
19 ============================================
20 cert
21 ============================================
22 Subject: CN=MySslServer
23 Issuer: CN=MySslServer
24 Version: 3
25 Valid Date: 7/7/2010 11:25:32 AM
26 Expiry Date: 1/1/2040 7:59:59 AM
27 Thumbprint: 5B1CEC828E92A64950284E557477FDD7E2C05300
28 Serial Number: 907E95210C949144FD68342B32FD07
29 Friendly Name: RSA
30 Public Key Format: 30 81 89 02 81 81 00 a4 97 38 0d dd c7 fa 6b b3 e6 23 17 df cd bf 06 c7 40 e1 5e 9a 04 81 02 62 00 b5
31 52 82 c9 6d b0 fc 16 ce e8 72 be ae 31 80 85 44 75 6c 1d 6a 2c 33 55 ee 04 6a fd f8 f4 ff 3a e0 22 28 a7 4f f8 c4 ba 96
32 c7 e6 f0 27 aa 43 09 11 ce eb cb 5e fe 63 84 ef e0 74 11 99 91 5d a0 39 5b 54 e6 74 8d db c8 ce d2 bd 08 53 63 19 3d d6
33 4c 37 99 b1 28 6a 07 2b 0c 70 49 fa df 9e ed 8b 2d e0 ed bf 03 02 03 01 00 01
34 Send>>>>>>>>>>>>>>>>>>>>>
35 ============================================
36 session key
37 ============================================
38 Session Key: 44 97 AC 07 5E 41 27 0B[8]
39 Recv<<<<<<<<<<<<<<<<<<<<<<
40 ============================================
41 session key
42 ============================================
43 Session Key: BD 28 62 18 04 90 C9 57[8]
44 Recv<<<<<<<<<<<<<<<<<<<<<<
45 ============================================
46 data
47 ============================================
48 It's just a text.
49 verify correct!
50 Send>>>>>>>>>>>>>>>>>>>>>
51 ============================================
52 data
53 ============================================
54 Response to client.
客户端输出
1 C:\D\BACKUP\Projects\TestProject\MySSLClient\bin\Debug>MySSLClient.exe 10.175.13.77
2 Send>>>>>>>>>>>>>>>>>>>>>
3 ============================================
4 cert
5 ============================================
6 Subject: CN=MySslClient
7 Issuer: CN=MySslClient
8 Version: 3
9 Valid Date: 7/7/2010 11:25:28 AM
10 Expiry Date: 1/1/2040 7:59:59 AM
11 Thumbprint: D9AE7CF97A254FCA05EB7B704DDC90878FC56B0C
12 Serial Number: 2500552056343D874C0B714A71CB047B
13 Friendly Name: RSA
14 Public Key Format: 30 81 89 02 81 81 00 9f 6f 17 84 0f 03 a0 8c b7 86 45 49 ba 0c c5 ab 37 c9 30 24 32 b5 dc 90 31 e4 d6
15 ea 83 45 1f c8 2d dd 2f 81 3e da 09 44 7f 2a cf bb 18 9a 92 28 d3 79 f0 33 b8 04 f2 1a 1d f8 e0 b6 f9 cc da 36 16 2b e3
16 8b 8d 82 5f 9c 88 6a 79 54 53 4f 08 10 dc b9 53 f1 c8 d4 bd 3d c4 94 71 1d db a2 76 65 8c 37 a3 cd 88 0d a0 72 0e 93 3a
17 64 62 dc 90 cf 19 3f f7 58 59 d6 bd 4e 3f a4 15 30 5b 36 75 2f 02 03 01 00 01
18 Recv<<<<<<<<<<<<<<<<<<<<<<
19 ============================================
20 cert
21 ============================================
22 Subject: CN=MySslServer
23 Issuer: CN=MySslServer
24 Version: 3
25 Valid Date: 7/7/2010 11:25:32 AM
26 Expiry Date: 1/1/2040 7:59:59 AM
27 Thumbprint: 5B1CEC828E92A64950284E557477FDD7E2C05300
28 Serial Number: 907E95210C949144FD68342B32FD07
29 Friendly Name: RSA
30 Public Key Format: 30 81 89 02 81 81 00 a4 97 38 0d dd c7 fa 6b b3 e6 23 17 df cd bf 06 c7 40 e1 5e 9a 04 81 02 62 00 b5
31 52 82 c9 6d b0 fc 16 ce e8 72 be ae 31 80 85 44 75 6c 1d 6a 2c 33 55 ee 04 6a fd f8 f4 ff 3a e0 22 28 a7 4f f8 c4 ba 96
32 c7 e6 f0 27 aa 43 09 11 ce eb cb 5e fe 63 84 ef e0 74 11 99 91 5d a0 39 5b 54 e6 74 8d db c8 ce d2 bd 08 53 63 19 3d d6
33 4c 37 99 b1 28 6a 07 2b 0c 70 49 fa df 9e ed 8b 2d e0 ed bf 03 02 03 01 00 01
34 Send>>>>>>>>>>>>>>>>>>>>>
35 ============================================
36 session key
37 ============================================
38 Session Key: BD 28 62 18 04 90 C9 57[8]
39 Recv<<<<<<<<<<<<<<<<<<<<<<
40 ============================================
41 session key
42 ============================================
43 Session Key: 44 97 AC 07 5E 41 27 0B[8]
44 Send>>>>>>>>>>>>>>>>>>>>>
45 ============================================
46 data
47 ============================================
48 It's just a text.
49 Recv<<<<<<<<<<<<<<<<<<<<<<
50 ============================================
51 data
52 ============================================
53 Response to client.
54 verify correct!
