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Android 根证书管理与证书验证（4）
达芬奇密码2018-08-17 13:46私有 CA 签名证书的应用
自签名证书是无需别的证书为其签名来证明其合法性的证书，根证书都是自签名证书。私有 CA 签名证书则是指，为域名证书签名的 CA，其合法有效性没有得到广泛的认可，该 CA 的根证书没有被内置到系统中。
在实际的开发过程中，有时为了节省昂贵的购买证书的费用，而想要自己给自己的服务器的域名签发域名证书，这即是私有 CA 签名的证书。为了能够使用这种证书，需要在客户端预埋根证书，并对客户端证书合法性验证的过程进行干预，通过我们预埋的根证书为服务端的证书做合法性验证，而不依赖系统的根证书库。
自定义 javax.net.ssl.SSLSocket 的代价太高，通常不会通过自定义 javax.net.ssl.SSLSocket 来修改服务端证书的合法性验证过程。以此为基础，从上面的分析中不难看出，要想定制 OpenSSLSocketImpl 的证书验证过程，则必然要改变 SSLParametersImpl，要改变 OpenSSLSocketImpl 的 SSLParametersImpl，则必然需要修改 SSLSocketFactory。修改 SSLSocketFactory 常常是一个不错的方法。
在 Java 中，SSLContext 正是被设计用于这一目的。创建定制了 SSLParametersImpl，即定制了 TrustManager 的 SSLSocketFactory 的方法如下：
TrustManager[] trustManagers = new TrustManager[] { new HelloX509TrustManager() };;
SSLContext context = null;
try {
context = SSLContext.getInstance("TLS");
context.init(null, trustManagers, new SecureRandom());
} catch (NoSuchAlgorithmException e) {
Log.i(TAG,"NoSuchAlgorithmException INFO:"+e.getMessage());
} catch (KeyManagementException e) {
Log.i(TAG, "KeyManagementException INFO:" + e.getMessage());
}
HttpsURLConnection.setDefaultSSLSocketFactory(context.getSocketFactory());
SSLContext 的相关方法实现（位于libcore/ojluni/src/main/java/javax/net/ssl/SSLContext.java）如下：
private final SSLContextSpi contextSpi;
. . . . . .
public static SSLContext getInstance(String protocol)
throws NoSuchAlgorithmException {
GetInstance.Instance instance = GetInstance.getInstance
("SSLContext", SSLContextSpi.class, protocol);
return new SSLContext((SSLContextSpi)instance.impl, instance.provider,
protocol);
}
. . . . . .
public final void init(KeyManager[] km, TrustManager[] tm,
SecureRandom random)
throws KeyManagementException {
contextSpi.engineInit(km, tm, random);
}
/**
* Returns a <code>SocketFactory</code> object for this
* context.
*
* @return the <code>SocketFactory</code> object
* @throws IllegalStateException if the SSLContextImpl requires
* initialization and the <code>init()</code> has not been called
*/
public final SSLSocketFactory getSocketFactory() {
return contextSpi.engineGetSocketFactory();
}
其中 SSLContextSpi 为 OpenSSLContextImpl，该类的实现（位于external/conscrypt/src/main/java/org/conscrypt/OpenSSLContextImpl.java）如下：
package org.conscrypt;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.security.KeyManagementException;
import java.security.SecureRandom;
import javax.net.ssl.KeyManager;
import javax.net.ssl.SSLContextSpi;
import javax.net.ssl.SSLEngine;
import javax.net.ssl.SSLServerSocketFactory;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManager;
/**
* OpenSSL-backed SSLContext service provider interface.
*/
public class OpenSSLContextImpl extends SSLContextSpi {
/**
* The default SSLContextImpl for use with
* SSLContext.getInstance("Default"). Protected by the
* DefaultSSLContextImpl.class monitor.
*/
private static DefaultSSLContextImpl DEFAULT_SSL_CONTEXT_IMPL;
/** TLS algorithm to initialize all sockets. */
private final String[] algorithms;
/** Client session cache. */
private final ClientSessionContext clientSessionContext;
/** Server session cache. */
private final ServerSessionContext serverSessionContext;
protected SSLParametersImpl sslParameters;
/** Allows outside callers to get the preferred SSLContext. */
public static OpenSSLContextImpl getPreferred() {
return new TLSv12();
}
protected OpenSSLContextImpl(String[] algorithms) {
this.algorithms = algorithms;
clientSessionContext = new ClientSessionContext();
serverSessionContext = new ServerSessionContext();
}
/**
* Constuctor for the DefaultSSLContextImpl.
*
* @param dummy is null, used to distinguish this case from the public
* OpenSSLContextImpl() constructor.
*/
protected OpenSSLContextImpl() throws GeneralSecurityException, IOException {
synchronized (DefaultSSLContextImpl.class) {
this.algorithms = null;
if (DEFAULT_SSL_CONTEXT_IMPL == null) {
clientSessionContext = new ClientSessionContext();
serverSessionContext = new ServerSessionContext();
DEFAULT_SSL_CONTEXT_IMPL = (DefaultSSLContextImpl) this;
} else {
clientSessionContext = DEFAULT_SSL_CONTEXT_IMPL.engineGetClientSessionContext();
serverSessionContext = DEFAULT_SSL_CONTEXT_IMPL.engineGetServerSessionContext();
}
sslParameters = new SSLParametersImpl(DEFAULT_SSL_CONTEXT_IMPL.getKeyManagers(),
DEFAULT_SSL_CONTEXT_IMPL.getTrustManagers(), null, clientSessionContext,
serverSessionContext, algorithms);
}
}
/**
* Initializes this {@code SSLContext} instance. All of the arguments are
* optional, and the security providers will be searched for the required
* implementations of the needed algorithms.
*
* @param kms the key sources or {@code null}
* @param tms the trust decision sources or {@code null}
* @param sr the randomness source or {@code null}
* @throws KeyManagementException if initializing this instance fails
*/
@Override
public void engineInit(KeyManager[] kms, TrustManager[] tms, SecureRandom sr)
throws KeyManagementException {
sslParameters = new SSLParametersImpl(kms, tms, sr, clientSessionContext,
serverSessionContext, algorithms);
}
@Override
public SSLSocketFactory engineGetSocketFactory() {
if (sslParameters == null) {
throw new IllegalStateException("SSLContext is not initialized.");
}
return Platform.wrapSocketFactoryIfNeeded(new OpenSSLSocketFactoryImpl(sslParameters));
}
如我们前面讨论，验证服务端证书合法性是 PKI 体系中，保障系统安全极为关键的环节。如果不验证服务端证书的合法性，则即使部署了 HTTPS，HTTPS 也将形同虚设，毫无价值。因而在我们自己实现的 X509TrustManager 中，加载预埋的根证书，并据此验证服务端证书的合法性必不可少，这一检查在 checkServerTrusted() 中完成。然而为了使我们实现的 X509TrustManager 功能更完备，在根据我们预埋的根证书验证失败后，我们再使用系统默认的 X509TrustManager 做验证，像下面这样：
private final class HelloX509TrustManager implements X509TrustManager {
private X509TrustManager mSystemDefaultTrustManager;
private X509Certificate mCertificate;
private HelloX509TrustManager() {
mCertificate = loadRootCertificate();
mSystemDefaultTrustManager = systemDefaultTrustManager();
}
private X509Certificate loadRootCertificate() {
String certName = "netease.crt";
X509Certificate certificate = null;
InputStream certInput = null;
try {
certInput = new BufferedInputStream(MainActivity.this.getAssets().open(certName));
CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
certificate = (X509Certificate) certificateFactory.generateCertPath(certInput).getCertificates().get(0);
} catch (IOException e) {
e.printStackTrace();
} catch (CertificateException e) {
e.printStackTrace();
} finally {
if (certInput != null) {
try {
certInput.close();
} catch (IOException e) {
}
}
}
return certificate;
}
private X509TrustManager systemDefaultTrustManager() {
try {
TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(
TrustManagerFactory.getDefaultAlgorithm());
trustManagerFactory.init((KeyStore) null);
TrustManager[] trustManagers = trustManagerFactory.getTrustManagers();
if (trustManagers.length != 1 || !(trustManagers[0] instanceof X509TrustManager)) {
throw new IllegalStateException("Unexpected default trust managers:"
+ Arrays.toString(trustManagers));
}
return (X509TrustManager) trustManagers[0];
} catch (GeneralSecurityException e) {
throw new AssertionError(); // The system has no TLS. Just give up.
}
}
@Override
public void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException {
mSystemDefaultTrustManager.checkClientTrusted(chain, authType);
}
@Override
public void checkServerTrusted(X509Certificate[] chain, String authType) throws CertificateException {
for (X509Certificate certificate : chain) {
try {
certificate.verify(mCertificate.getPublicKey());
return;
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (InvalidKeyException e) {
e.printStackTrace();
} catch (NoSuchProviderException e) {
e.printStackTrace();
} catch (SignatureException e) {
e.printStackTrace();
}
}
mSystemDefaultTrustManager.checkServerTrusted(chain, authType);
}
@Override
public X509Certificate[] getAcceptedIssuers() {
return mSystemDefaultTrustManager.getAcceptedIssuers();
}
}
此外，也可以不自己实现 X509TrustManager，而仅仅修改 X509TrustManager 所用的根证书库，就像下面这样：
private TrustManager[] createX509TrustManager() {
CertificateFactory cf = null;
InputStream in = null;
TrustManager[] trustManagers = null
try {
cf = CertificateFactory.getInstance("X.509");
in = getAssets().open("ca.crt");
Certificate ca = cf.generateCertificate(in);
KeyStore keystore = KeyStore.getInstance(KeyStore.getDefaultType());
keystore.load(null, null);
keystore.setCertificateEntry("ca", ca);
String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
tmf.init(keystore);
trustManagers = tmf.getTrustManagers();
} catch (CertificateException e) {
e.printStackTrace();
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (KeyStoreException e) {
e.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
} finally {
if (in != null) {
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return trustManagers;
}
自己实现 X509TrustManager 接口和通过 TrustManagerFactory，仅定制 KeyStore 这两种创建 X509TrustManager 对象的方式，当然是后一种方式更好一些了。如我们前面看到的，系统的 X509TrustManager 实现 RootTrustManager 集成自 X509ExtendedTrustManager，而不是直接实现的 X509TrustManager 接口 。JCA 的接口层也在随着新的安全协议和 SSL 库的发展在不断扩展，在具体的 Java 加密服务实现中，可能会实现并依赖这些扩展的功能，如上面看到的 X509TrustManager，而且加密服务的实现中常常通过反射，来动态依赖一些扩展的接口。因而，自己实现 X509TrustManager 接口时，以及其它加密相关的接口时，如 SSLSocket 等，可能会破坏一些功能。
很多时候可以看到，为了使用私有 CA 签名的证书，而定制域名匹配验证的逻辑，即自己实现 HostnameVerifier。不过通常情况下，网络库都会按照规范对域名与证书的匹配性做严格的检查，因而不是那么地有必要，除非域名证书有什么不那么规范的地方。
关于证书钉扎，在使用私有 CA 签名的证书时，通常似乎也没有那么必要。
参考文章：
Android实现https网络通信之添加指定信任证书/信任所有证书
Java https请求 HttpsURLConnection
Done。
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