Java SSL 证书细节

关于SSL这块，网上很多，但很多都是讲原理或怎么生成证书实现简单通信，没有讲到实践时的诸多细节。
SSL, 即Secure Sockets Layer 安全套接层。本文介绍SSL的一些常见问题，用的语言主要是Java。

首先明确SSL的目的，就是加密。就是在Socket的基础上增加安全性。一般来讲，就是避免客户端向服务端传输数据时，被人拦截篡改，避免所谓的中间人攻击，防范钓鱼网站。
关于它的基础概念，可以看
【上】安全HTTPS-全面详解对称加密，非对称加密，数字签名，数字证书和HTTPS
【下】安全HTTPS-全面详解对称加密，非对称加密，数字签名，数字证书和HTTPS

问题1：证书生成和格式

网上搜索SSL，有很多生成证书的教程。可是会发现有的是用Java的Keytool生成的cer，有的是用openssl来生成的pem。这是为什么呢？其实这些证书本质是一样的，只是文件格式不一样。所以如果想在Java里使用pem格式的证书，就要转化。转化的方式在这个问题里已有回答：
Convert a PEM-formatted String to a java.security.cert.X509Certificate
另外，公钥和私钥分别以什么形式存在？公钥存在于证书文件中，私钥存在于Keystore文件中。注意Java里的Keystore类可以是有私钥的Keystore，也可以是只有公钥的Keystore。
还有，为什么使用Java keytool生成的证书要设置密码？这个密码是JKS文件的密码，注意不是公钥或私钥哦，是用于防范别人随便乱拿的。

问题2：既然是加密，那么可不可以脱离Socket存在？

可以。可以直接使用公钥来加密数据，再用私钥解密数据。具体方法参考：
Java加密技术（八）——数字证书

问题3：单向认证，即只确认服务端是否真实可靠的话，要做什么？以SSLSocket举例。

如果只需要信任自己生成的证书
代码：

    public class TestSSLSocketClient {  
        private static String path = "e:\\keytool\\sslclient.keystore";  
        private static char[] password = "aaaaaaa".toCharArray();  

        /** 
         * @param args 
         */  
        public static void main(String[] args) {  
            SSLContext context = null;  
            try {  
                KeyStore ts = KeyStore.getInstance("JKS");  
                ts.load(new FileInputStream(path), password);  
                TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509");  
                tmf.init(ts);  
                TrustManager [] tm = tmf.getTrustManagers();  
                context = SSLContext.getInstance("SSL");  
                context.init(null, tm, null);  
            } catch (...... e) {         //省略捕获的异常信息  
                e.printStackTrace();  
            }   
            SSLSocketFactory ssf = context.getSocketFactory();  
            try {  
                SSLSocket ss = (SSLSocket) ssf.createSocket("localhost", 8000);  
                System.out.println("客户端就绪。");  
                ObjectInputStream br = new ObjectInputStream(ss.getInputStream());  
                try {  
                    System.out.println(br.readObject());  
                } catch (ClassNotFoundException e) {  
                    e.printStackTrace();  
                }  
                br.close();  
                ss.close();  
                System.out.println("客户端测试ok");  
            } catch (UnknownHostException e) {  
                e.printStackTrace();  
            } catch (IOException e) {  
                e.printStackTrace();  
            }  
        }  
    }  

可以从代码看到，对于SSLContext，在做单向认证时，只需要TrustManagerFactory即可，
context.init(null, tm, null);
TrustManagerFactory是使用了服务端的证书的，即只使用了公钥来加密数据。

如果只需要信任系统自带的证书

            SSLContext sslContext = SSLContext.getDefault();

如果需要信任自己的以及系统的证书
引用：http://stackoverflow.com/questions/24555890/using-a-custom-truststore-in-java-as-well-as-the-default-one?noredirect=1

TrustManagerFactory tmf = TrustManagerFactory
    .getInstance(TrustManagerFactory.getDefaultAlgorithm());
// Using null here initialises the TMF with the default trust store.
tmf.init((KeyStore) null);

// Get hold of the default trust manager
X509TrustManager defaultTm = null;
for (TrustManager tm : tmf.getTrustManagers()) {
    if (tm instanceof X509TrustManager) {
        defaultTm = (X509TrustManager) tm;
        break;
    }
}

FileInputStream myKeys = new FileInputStream("truststore.jks");

// Do the same with your trust store this time
// Adapt how you load the keystore to your needs
KeyStore myTrustStore = KeyStore.getInstance(KeyStore.getDefaultType());
myTrustStore.load(myKeys, "password".toCharArray());

myKeys.close();

tmf = TrustManagerFactory
    .getInstance(TrustManagerFactory.getDefaultAlgorithm());
tmf.init(myTrustStore);

// Get hold of the default trust manager
X509TrustManager myTm = null;
for (TrustManager tm : tmf.getTrustManagers()) {
    if (tm instanceof X509TrustManager) {
        myTm = (X509TrustManager) tm;
        break;
    }
}

// Wrap it in your own class.
final X509TrustManager finalDefaultTm = defaultTm;
final X509TrustManager finalMyTm = myTm;
X509TrustManager customTm = new X509TrustManager() {
    @Override
    public X509Certificate[] getAcceptedIssuers() {
        // If you're planning to use client-cert auth,
        // merge results from "defaultTm" and "myTm".
        return finalDefaultTm.getAcceptedIssuers();
    }

    @Override
    public void checkServerTrusted(X509Certificate[] chain,
            String authType) throws CertificateException {
        try {
            finalMyTm.checkServerTrusted(chain, authType);
        } catch (CertificateException e) {
            // This will throw another CertificateException if this fails too.
            finalDefaultTm.checkServerTrusted(chain, authType);
        }
    }

    @Override
    public void checkClientTrusted(X509Certificate[] chain,
            String authType) throws CertificateException {
        // If you're planning to use client-cert auth,
        // do the same as checking the server.
        finalDefaultTm.checkClientTrusted(chain, authType);
    }
};


SSLContext sslContext = SSLContext.getInstance("TLS");
sslContext.init(null, new TrustManager[] { customTm }, null);

问题4：可以看到SSLContext.init()的参数有KeyManager和TrustManager，它俩的差异是？

KeyManager是服务端用的，用于在客户端请求时发送证书及其公钥。
可以看这个链接
http://stackoverflow.com/questions/13997419/difference-between-keystore-and-keymanager-trustmanager

问题5：服务器端会在连接时向客户端发送证书，那么如何从代码上获取？

引用：http://www.xinotes.net/notes/note/1088/

import java.io.InputStream;
import java.io.OutputStream;

import java.security.cert.X509Certificate;

import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSession;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;

public class RetrieveSSLCert {
    public static void main(String[] args) throws Exception {
    if (args.length < 2) {
        System.out.println("Usage: java RetrieveSSLCert <host> <port>");
        return;
    }

    String host = args[0];
    int port = Integer.parseInt(args[1]);

    // create custom trust manager to ignore trust paths
    TrustManager trm = new X509TrustManager() {
        public X509Certificate[] getAcceptedIssuers() {
        return null;
        }

        public void checkClientTrusted(X509Certificate[] certs, String authType) {
        }

            public void checkServerTrusted(X509Certificate[] certs, String authType) {
            }
    };

        SSLContext sc = SSLContext.getInstance("SSL");
        sc.init(null, new TrustManager[] { trm }, null);
        SSLSocketFactory factory =sc.getSocketFactory();
        SSLSocket socket =(SSLSocket)factory.createSocket(host, port);
        socket.startHandshake();
    SSLSession session = socket.getSession();
    java.security.cert.Certificate[] servercerts = session.getPeerCertificates();
    for (int i = 0; i < servercerts.length; i++) {
            System.out.print("-----BEGIN CERTIFICATE-----\n");
            System.out.print(new sun.misc.BASE64Encoder().encode(servercerts[i].getEncoded()));
            System.out.print("\n-----END CERTIFICATE-----\n");
    }

    socket.close();
    }
}