PKI 体系依赖证书执行极为关键的身份验证，以此确认服务端的可信任性。证书验证在 SSL/TLS 握手过程中完成，验证过程通常包含三个步骤：

验证证书的合法性：这一步主要是验证证书是由合法有效的 CA 签发的。在客户端预先保存一个可靠的 CA 的根证书库，比如 FiexFox、Chrome、Android、Microsoft 等都有维护自己的根证书库，并据此验证服务端证书链的合法性。PKI 体系借助于可靠的中心化身份验证系统，即 CA，为服务端的身份合法性背书。根证书库的安全是 PKI 系统正常工作非常关键的部分。
验证证书域名的匹配性：服务端的证书都是为特定域名签发的，证书就像是网站的身份证一样。通过验证域名匹配性，可以有效的防止身份的仿冒，比如经营着 A 网站的经营者，拦截用户请求，并冒充 B 网站的身份，盗取信息。如果客户端不对域名的匹配性做检查，则将造成极大的攻击面，拿到任何一个域名的合法证书的人都将可以仿冒目标服务器。
证书钉扎验证：这是 PKI 体系中比较新的一种增强安全性的机制。目前的证书签发机构 CA 非常多，总数大概有几百个上千个，每个 CA 都可以为任何域名签发合法有效的证书，因而众多的 CA 就造成了非常大的攻击面。比如某个 CA 被攻破，或者犯了其它什么错误，为攻击者签发了 www.google.com 等域名的证书，则攻击者将可以仿冒这些网站。证书钉扎机制正是为了解决这一问题而产生——证书钉扎机制中，在客户端将特定域名的证书与特定的签发者绑定，即客户端只承认特定签发者签发的某个域名的证书，而不承认其它 CA 为该域名签发的证书。通过这种方式，来解除大量 CA 这个攻击面的威胁。

在 Android 系统的 Java 应用程序中，证书验证通常由不同层面的多个组件完成。第一步的证书合法性验证，主要由 Java 标准库的 javax.net.ssl.SSLSocket 在 startHandshake() 方法中完成，后面两个步骤由更上层的组件完成，比如 HTTPS 库 OkHttp 等。

本文主要讨论 Android 中根证书库的管理和证书的合法性验证。（本文分析说明主要依据 android-7.1.1/android-7.1.2 系统的行为，可以通过 Google 的 OpenGrok 服务器阅读 Android 系统的源码。）

Android 的根证书管理

在 AOSP 源码库中，CA 根证书主要存放在 system/ca-certificates 目录下，而在 Android 系统中，则存放在 /system/etc/security/ 目录下，以 Android 7.1.1 系统的 Pixel 设备为例：

sailfish:/ # ls -l  /system/etc/security/
total 40
drwxr-xr-x 2 root root 4096 2017-07-18 16:37 cacerts
drwxr-xr-x 2 root root 4096 2017-07-18 16:36 cacerts_google
-rw-r--r-- 1 root root 4995 2017-07-18 16:03 mac_permissions.xml
-rw-r--r-- 1 root root 1073 2017-07-18 16:59 otacerts.zip

其中 cacerts_google 目录下的根证书，主要用于 system/update_engine、external/libbrillo 和 system/core/crash_reporter 等模块，cacerts 目录下的根证书则用于所有的应用。cacerts 目录下的根证书，即 Android 系统的根证书库，像下面这样：

sailfish:/ # ls -l /system/etc/security/cacerts
total 2408
-rw-r--r-- 1 root root 4767 2017-07-18 16:37 00673b5b.0
-rw-r--r-- 1 root root 7195 2017-07-18 16:37 02756ea4.0
-rw-r--r-- 1 root root 4919 2017-07-18 16:37 02b73561.0
-rw-r--r-- 1 root root 7142 2017-07-18 16:37 03f2b8cf.0
-rw-r--r-- 1 root root 2877 2017-07-18 16:37 04f60c28.0
-rw-r--r-- 1 root root 4836 2017-07-18 16:37 052e396b.0
-rw-r--r-- 1 root root 5322 2017-07-18 16:37 08aef7bb.0
-rw-r--r-- 1 root root 4922 2017-07-18 16:37 0d5a4e1c.0
-rw-r--r-- 1 root root 2308 2017-07-18 16:37 0d69c7e1.0
-rw-r--r-- 1 root root 4614 2017-07-18 16:37 10531352.0
-rw-r--r-- 1 root root 4716 2017-07-18 16:37 111e6273.0
-rw-r--r-- 1 root root 5375 2017-07-18 16:37 119afc2e.0
-rw-r--r-- 1 root root 4927 2017-07-18 16:37 124bbd54.0
. . . . . .

它们都是 PEM 格式的 X.509 证书。Android 系统通过 SystemCertificateSource、DirectoryCertificateSource 和 CertificateSource 等类管理系统根证书库。CertificateSource定义（位于frameworks/base/core/java/android/security/net/config/CertificateSource.java）了可以对根证书库执行的操作，主要是对根证书的获取和查找：

package android.security.net.config;import java.security.cert.X509Certificate;
import java.util.Set;/** @hide */
public interface CertificateSource {Set<X509Certificate> getCertificates();X509Certificate findBySubjectAndPublicKey(X509Certificate cert);X509Certificate findByIssuerAndSignature(X509Certificate cert);Set<X509Certificate> findAllByIssuerAndSignature(X509Certificate cert);void handleTrustStorageUpdate();
}

DirectoryCertificateSource 类则基于文件系统上分开存放的根证书文件的形式保存的根证书库，提供证书的创建、获取和查找操作，这个类的定义（位于frameworks/base/core/java/android/security/net/config/DirectoryCertificateSource.java）如下：

package android.security.net.config;import android.os.Environment;
import android.os.UserHandle;
import android.util.ArraySet;
import android.util.Log;
import android.util.Pair;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.IOException;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.util.Collections;
import java.util.Set;
import libcore.io.IoUtils;import com.android.org.conscrypt.Hex;
import com.android.org.conscrypt.NativeCrypto;import javax.security.auth.x500.X500Principal;/*** {@link CertificateSource} based on a directory where certificates are stored as individual files* named after a hash of their SubjectName for more efficient lookups.* @hide*/
abstract class DirectoryCertificateSource implements CertificateSource {private static final String LOG_TAG = "DirectoryCertificateSrc";private final File mDir;private final Object mLock = new Object();private final CertificateFactory mCertFactory;private Set<X509Certificate> mCertificates;protected DirectoryCertificateSource(File caDir) {mDir = caDir;try {mCertFactory = CertificateFactory.getInstance("X.509");} catch (CertificateException e) {throw new RuntimeException("Failed to obtain X.509 CertificateFactory", e);}}protected abstract boolean isCertMarkedAsRemoved(String caFile);@Overridepublic Set<X509Certificate> getCertificates() {// TODO: loading all of these is wasteful, we should instead use a keystore style API.synchronized (mLock) {if (mCertificates != null) {return mCertificates;}Set<X509Certificate> certs = new ArraySet<X509Certificate>();if (mDir.isDirectory()) {for (String caFile : mDir.list()) {if (isCertMarkedAsRemoved(caFile)) {continue;}X509Certificate cert = readCertificate(caFile);if (cert != null) {certs.add(cert);}}}mCertificates = certs;return mCertificates;}}@Overridepublic X509Certificate findBySubjectAndPublicKey(final X509Certificate cert) {return findCert(cert.getSubjectX500Principal(), new CertSelector() {@Overridepublic boolean match(X509Certificate ca) {return ca.getPublicKey().equals(cert.getPublicKey());}});}@Overridepublic X509Certificate findByIssuerAndSignature(final X509Certificate cert) {return findCert(cert.getIssuerX500Principal(), new CertSelector() {@Overridepublic boolean match(X509Certificate ca) {try {cert.verify(ca.getPublicKey());return true;} catch (Exception e) {return false;}}});}@Overridepublic Set<X509Certificate> findAllByIssuerAndSignature(final X509Certificate cert) {return findCerts(cert.getIssuerX500Principal(), new CertSelector() {@Overridepublic boolean match(X509Certificate ca) {try {cert.verify(ca.getPublicKey());return true;} catch (Exception e) {return false;}}});}@Overridepublic void handleTrustStorageUpdate() {synchronized (mLock) {mCertificates = null;}}private static interface CertSelector {boolean match(X509Certificate cert);}private Set<X509Certificate> findCerts(X500Principal subj, CertSelector selector) {String hash = getHash(subj);Set<X509Certificate> certs = null;for (int index = 0; index >= 0; index++) {String fileName = hash + "." + index;if (!new File(mDir, fileName).exists()) {break;}if (isCertMarkedAsRemoved(fileName)) {continue;}X509Certificate cert = readCertificate(fileName);if (cert == null) {continue;}if (!subj.equals(cert.getSubjectX500Principal())) {continue;}if (selector.match(cert)) {if (certs == null) {certs = new ArraySet<X509Certificate>();}certs.add(cert);}}return certs != null ? certs : Collections.<X509Certificate>emptySet();}private X509Certificate findCert(X500Principal subj, CertSelector selector) {String hash = getHash(subj);for (int index = 0; index >= 0; index++) {String fileName = hash + "." + index;if (!new File(mDir, fileName).exists()) {break;}if (isCertMarkedAsRemoved(fileName)) {continue;}X509Certificate cert = readCertificate(fileName);if (cert == null) {continue;}if (!subj.equals(cert.getSubjectX500Principal())) {continue;}if (selector.match(cert)) {return cert;}}return null;}private String getHash(X500Principal name) {int hash = NativeCrypto.X509_NAME_hash_old(name);return Hex.intToHexString(hash, 8);}private X509Certificate readCertificate(String file) {InputStream is = null;try {is = new BufferedInputStream(new FileInputStream(new File(mDir, file)));return (X509Certificate) mCertFactory.generateCertificate(is);} catch (CertificateException | IOException e) {Log.e(LOG_TAG, "Failed to read certificate from " + file, e);return null;} finally {IoUtils.closeQuietly(is);}}
}

获取根证书库的 getCertificates() 操作在第一次被调用时，遍历文件系统，并加载系统所有的根证书文件，并缓存起来，以备后面访问。根证书的查找操作，主要依据证书文件的文件名进行，证书文件被要求以 [SubjectName 的哈希值].[Index] 的形式命名。

SystemCertificateSource 类主要定义（位于frameworks/base/core/java/android/security/net/config/SystemCertificateSource.java）了系统根证书库的路径，以及无效一个根证书的机制：

package android.security.net.config;import android.os.Environment;
import android.os.UserHandle;
import java.io.File;/*** {@link CertificateSource} based on the system trusted CA store.* @hide*/
public final class SystemCertificateSource extends DirectoryCertificateSource {private static class NoPreloadHolder {private static final SystemCertificateSource INSTANCE = new SystemCertificateSource();}private final File mUserRemovedCaDir;private SystemCertificateSource() {super(new File(System.getenv("ANDROID_ROOT") + "/etc/security/cacerts"));File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());mUserRemovedCaDir = new File(configDir, "cacerts-removed");}public static SystemCertificateSource getInstance() {return NoPreloadHolder.INSTANCE;}@Overrideprotected boolean isCertMarkedAsRemoved(String caFile) {return new File(mUserRemovedCaDir, caFile).exists();}
}

Android 系统的根证书位于 /system/etc/security/cacerts/ 目录下。用户可以通过将特定根证书复制到用户配置目录的 cacerts-removed 目录下来无效一个根证书。

Android framework 还提供了另外一个用于加载并访问用户根证书库的组件 UserCertificateSource，这个类的定义（位于 frameworks/base/core/java/android/security/net/config/UserCertificateSource.java）如下：

package android.security.net.config;import android.os.Environment;
import android.os.UserHandle;
import java.io.File;/*** {@link CertificateSource} based on the user-installed trusted CA store.* @hide*/
public final class UserCertificateSource extends DirectoryCertificateSource {private static class NoPreloadHolder {private static final UserCertificateSource INSTANCE = new UserCertificateSource();}private UserCertificateSource() {super(new File(Environment.getUserConfigDirectory(UserHandle.myUserId()), "cacerts-added"));}public static UserCertificateSource getInstance() {return NoPreloadHolder.INSTANCE;}@Overrideprotected boolean isCertMarkedAsRemoved(String caFile) {return false;}
}

这个组件与 SystemCertificateSource 类似，只是它定义了用户根证书库的路径。

证书链合法性验证

有了根证书库之后，根证书库又是如何被用于 SSL/TLS 握手的证书验证过程的呢？

证书的合法性由 Java 标准库的 javax.net.ssl.SSLSocket 在 startHandshake() 方法中完成。对于 Android 系统而言，SSLSocket 基于 OpenSSL 库实现，这一实现由 external/conscrypt 模块提供，SSLSocket 的实现为 OpenSSLSocketImpl 类（位于external/conscrypt/src/main/java/org/conscrypt/OpenSSLSocketImpl.java）。

OpenSSLSocketImpl.startHandshake() 中的 SSL/TLS 握手是一个极为精巧的过程，我们略过详细的握手过程，主要关注证书验证的部分。

OpenSSLSocketImpl.startHandshake() 通过 NativeCrypto 类（位于external/conscrypt/src/main/java/org/conscrypt/NativeCrypto.java）中的静态本地层方法 SSL_do_handshake() 方法执行握手操作：

    /*** Returns the sslSessionNativePointer of the negotiated session. If this is* a server negotiation, supplying the {@code alpnProtocols} will enable* ALPN negotiation.*/public static native long SSL_do_handshake(long sslNativePointer,FileDescriptor fd,SSLHandshakeCallbacks shc,int timeoutMillis,boolean client_mode,byte[] npnProtocols,byte[] alpnProtocols)throws SSLException, SocketTimeoutException, CertificateException;

NativeCrypto 类内部定义了一组将会在本地层由与 SSL 握手相关的 OpenSSL C/C++ 代码调用的回调 SSLHandshakeCallbacks，在上面的 SSL_do_handshake() 方法中，这组回调作为参数传入本地层。

SSLHandshakeCallbacks 定义如下：

    /*** A collection of callbacks from the native OpenSSL code that are* related to the SSL handshake initiated by SSL_do_handshake.*/public interface SSLHandshakeCallbacks {/*** Verify that we trust the certificate chain is trusted.** @param sslSessionNativePtr pointer to a reference of the SSL_SESSION* @param certificateChainRefs chain of X.509 certificate references* @param authMethod auth algorithm name** @throws CertificateException if the certificate is untrusted*/public void verifyCertificateChain(long sslSessionNativePtr, long[] certificateChainRefs,String authMethod) throws CertificateException;/*** Called on an SSL client when the server requests (or* requires a certificate). The client can respond by using* SSL_use_certificate and SSL_use_PrivateKey to set a* certificate if has an appropriate one available, similar to* how the server provides its certificate.** @param keyTypes key types supported by the server,* convertible to strings with #keyType* @param asn1DerEncodedX500Principals CAs known to the server*/public void clientCertificateRequested(byte[] keyTypes,byte[][] asn1DerEncodedX500Principals)throws CertificateEncodingException, SSLException;/*** Gets the key to be used in client mode for this connection in Pre-Shared Key (PSK) key* exchange.** @param identityHint PSK identity hint provided by the server or {@code null} if no hint*        provided.* @param identity buffer to be populated with PSK identity (NULL-terminated modified UTF-8)*        by this method. This identity will be provided to the server.* @param key buffer to be populated with key material by this method.** @return number of bytes this method stored in the {@code key} buffer or {@code 0} if an*         error occurred in which case the handshake will be aborted.*/public int clientPSKKeyRequested(String identityHint, byte[] identity, byte[] key);/*** Gets the key to be used in server mode for this connection in Pre-Shared Key (PSK) key* exchange.** @param identityHint PSK identity hint provided by this server to the client or*        {@code null} if no hint was provided.* @param identity PSK identity provided by the client.* @param key buffer to be populated with key material by this method.** @return number of bytes this method stored in the {@code key} buffer or {@code 0} if an*         error occurred in which case the handshake will be aborted.*/public int serverPSKKeyRequested(String identityHint, String identity, byte[] key);/*** Called when SSL state changes. This could be handshake completion.*/public void onSSLStateChange(long sslSessionNativePtr, int type, int val);}

其中 verifyCertificateChain() 回调用于服务端证书的验证。Android 系统通过这一回调，将根证书库的管理模块和底层 OpenSSL 的 SSL/TLS 握手及身份验证连接起来。

SSLHandshakeCallbacks 回调由 OpenSSLSocketImpl 实现，verifyCertificateChain() 的实现如下：

    @SuppressWarnings("unused") // used by NativeCrypto.SSLHandshakeCallbacks@Overridepublic void verifyCertificateChain(long sslSessionNativePtr, long[] certRefs, String authMethod)throws CertificateException {try {X509TrustManager x509tm = sslParameters.getX509TrustManager();if (x509tm == null) {throw new CertificateException("No X.509 TrustManager");}if (certRefs == null || certRefs.length == 0) {throw new SSLException("Peer sent no certificate");}OpenSSLX509Certificate[] peerCertChain = new OpenSSLX509Certificate[certRefs.length];for (int i = 0; i < certRefs.length; i++) {peerCertChain[i] = new OpenSSLX509Certificate(certRefs[i]);}// Used for verifyCertificateChain callbackhandshakeSession = new OpenSSLSessionImpl(sslSessionNativePtr, null, peerCertChain,getHostnameOrIP(), getPort(), null);boolean client = sslParameters.getUseClientMode();if (client) {Platform.checkServerTrusted(x509tm, peerCertChain, authMethod, this);if (sslParameters.isCTVerificationEnabled(getHostname())) {byte[] tlsData = NativeCrypto.SSL_get_signed_cert_timestamp_list(sslNativePointer);byte[] ocspData = NativeCrypto.SSL_get_ocsp_response(sslNativePointer);CTVerifier ctVerifier = sslParameters.getCTVerifier();CTVerificationResult result =ctVerifier.verifySignedCertificateTimestamps(peerCertChain, tlsData, ocspData);if (result.getValidSCTs().size() == 0) {throw new CertificateException("No valid SCT found");}}} else {String authType = peerCertChain[0].getPublicKey().getAlgorithm();Platform.checkClientTrusted(x509tm, peerCertChain, authType, this);}} catch (CertificateException e) {throw e;} catch (Exception e) {throw new CertificateException(e);} finally {// Clear this before notifying handshake completed listenershandshakeSession = null;}}

OpenSSLSocketImpl 的 verifyCertificateChain() 从 sslParameters 获得 X509TrustManager，然后在 Platform.checkServerTrusted() （com.android.org.conscrypt.Platform，位于 external/conscrypt/src/compat/java/org/conscrypt/Platform.java）中执行服务端证书合法有效性的检查：

    public static void checkServerTrusted(X509TrustManager tm, X509Certificate[] chain,String authType, OpenSSLSocketImpl socket) throws CertificateException {if (!checkTrusted("checkServerTrusted", tm, chain, authType, Socket.class, socket)&& !checkTrusted("checkServerTrusted", tm, chain, authType, String.class,socket.getHandshakeSession().getPeerHost())) {tm.checkServerTrusted(chain, authType);}}

Platform.checkServerTrusted() 通过执行 X509TrustManager 的 checkServerTrusted() 方法执行证书有合法性检查。

X509TrustManager 来自于 OpenSSLSocketImpl 的 sslParameters，那 sslParameters 又来自于哪里呢？OpenSSLSocketImpl 的 sslParameters 由对象的创建者传入：

public class OpenSSLSocketImplextends javax.net.ssl.SSLSocketimplements NativeCrypto.SSLHandshakeCallbacks, SSLParametersImpl.AliasChooser,SSLParametersImpl.PSKCallbacks {. . . . . .private final SSLParametersImpl sslParameters;
. . . . . .protected OpenSSLSocketImpl(SSLParametersImpl sslParameters) throws IOException {this.socket = this;this.peerHostname = null;this.peerPort = -1;this.autoClose = false;this.sslParameters = sslParameters;}protected OpenSSLSocketImpl(String hostname, int port, SSLParametersImpl sslParameters)throws IOException {super(hostname, port);this.socket = this;this.peerHostname = hostname;this.peerPort = port;this.autoClose = false;this.sslParameters = sslParameters;}protected OpenSSLSocketImpl(InetAddress address, int port, SSLParametersImpl sslParameters)throws IOException {super(address, port);this.socket = this;this.peerHostname = null;this.peerPort = -1;this.autoClose = false;this.sslParameters = sslParameters;}protected OpenSSLSocketImpl(String hostname, int port,InetAddress clientAddress, int clientPort,SSLParametersImpl sslParameters) throws IOException {super(hostname, port, clientAddress, clientPort);this.socket = this;this.peerHostname = hostname;this.peerPort = port;this.autoClose = false;this.sslParameters = sslParameters;}protected OpenSSLSocketImpl(InetAddress address, int port,InetAddress clientAddress, int clientPort,SSLParametersImpl sslParameters) throws IOException {super(address, port, clientAddress, clientPort);this.socket = this;this.peerHostname = null;this.peerPort = -1;this.autoClose = false;this.sslParameters = sslParameters;}/*** Create an SSL socket that wraps another socket. Invoked by* OpenSSLSocketImplWrapper constructor.*/protected OpenSSLSocketImpl(Socket socket, String hostname, int port,boolean autoClose, SSLParametersImpl sslParameters) throws IOException {this.socket = socket;this.peerHostname = hostname;this.peerPort = port;this.autoClose = autoClose;this.sslParameters = sslParameters;// this.timeout is not set intentionally.// OpenSSLSocketImplWrapper.getSoTimeout will delegate timeout// to wrapped socket}

也就是说，OpenSSLSocketImpl 的 sslParameters 来自于 javax.net.ssl.SSLSocketFactory，即 OpenSSLSocketFactoryImpl。OpenSSLSocketFactoryImpl 定义（位于 external/conscrypt/src/main/java/org/conscrypt/OpenSSLSocketFactoryImpl.java）如下：

package org.conscrypt;import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;
import java.security.KeyManagementException;public class OpenSSLSocketFactoryImpl extends javax.net.ssl.SSLSocketFactory {private final SSLParametersImpl sslParameters;private final IOException instantiationException;public OpenSSLSocketFactoryImpl() {SSLParametersImpl sslParametersLocal = null;IOException instantiationExceptionLocal = null;try {sslParametersLocal = SSLParametersImpl.getDefault();} catch (KeyManagementException e) {instantiationExceptionLocal = new IOException("Delayed instantiation exception:");instantiationExceptionLocal.initCause(e);}this.sslParameters = sslParametersLocal;this.instantiationException = instantiationExceptionLocal;}public OpenSSLSocketFactoryImpl(SSLParametersImpl sslParameters) {this.sslParameters = sslParameters;this.instantiationException = null;}@Overridepublic String[] getDefaultCipherSuites() {return sslParameters.getEnabledCipherSuites();}@Overridepublic String[] getSupportedCipherSuites() {return NativeCrypto.getSupportedCipherSuites();}@Overridepublic Socket createSocket() throws IOException {if (instantiationException != null) {throw instantiationException;}return new OpenSSLSocketImpl((SSLParametersImpl) sslParameters.clone());}@Overridepublic Socket createSocket(String hostname, int port) throws IOException, UnknownHostException {return new OpenSSLSocketImpl(hostname, port, (SSLParametersImpl) sslParameters.clone());}@Overridepublic Socket createSocket(String hostname, int port, InetAddress localHost, int localPort)throws IOException, UnknownHostException {return new OpenSSLSocketImpl(hostname,port,localHost,localPort,(SSLParametersImpl) sslParameters.clone());}@Overridepublic Socket createSocket(InetAddress address, int port) throws IOException {return new OpenSSLSocketImpl(address, port, (SSLParametersImpl) sslParameters.clone());}@Overridepublic Socket createSocket(InetAddress address,int port,InetAddress localAddress,int localPort)throws IOException {return new OpenSSLSocketImpl(address,port,localAddress,localPort,(SSLParametersImpl) sslParameters.clone());}@Overridepublic Socket createSocket(Socket s, String hostname, int port, boolean autoClose)throws IOException {return new OpenSSLSocketImplWrapper(s,hostname,port,autoClose,(SSLParametersImpl) sslParameters.clone());}
}

OpenSSLSocketImpl 最主要的职责，即是将 SSL/TLS 参数 SSLParametersImpl 与 SSLSocket 粘起来。主要来看默认情况下 SSLParametersImpl 的 X509TrustManager 是什么（位于external/conscrypt/src/main/java/org/conscrypt/SSLParametersImpl.java ）：

    /*** Initializes the parameters. Naturally this constructor is used* in SSLContextImpl.engineInit method which directly passes its* parameters. In other words this constructor holds all* the functionality provided by SSLContext.init method.* See {@link javax.net.ssl.SSLContext#init(KeyManager[],TrustManager[],* SecureRandom)} for more information*/protected SSLParametersImpl(KeyManager[] kms, TrustManager[] tms,SecureRandom sr, ClientSessionContext clientSessionContext,ServerSessionContext serverSessionContext, String[] protocols)throws KeyManagementException {this.serverSessionContext = serverSessionContext;this.clientSessionContext = clientSessionContext;// initialize key managersif (kms == null) {x509KeyManager = getDefaultX509KeyManager();// There's no default PSK key managerpskKeyManager = null;} else {x509KeyManager = findFirstX509KeyManager(kms);pskKeyManager = findFirstPSKKeyManager(kms);}// initialize x509TrustManagerif (tms == null) {x509TrustManager = getDefaultX509TrustManager();} else {x509TrustManager = findFirstX509TrustManager(tms);}// initialize secure random// We simply use the SecureRandom passed in by the caller. If it's// null, we don't replace it by a new instance. The native code below// then directly accesses /dev/urandom. Not the most elegant solution,// but faster than going through the SecureRandom object.secureRandom = sr;// initialize the list of cipher suites and protocols enabled by defaultenabledProtocols = NativeCrypto.checkEnabledProtocols(protocols == null ? NativeCrypto.DEFAULT_PROTOCOLS : protocols).clone();boolean x509CipherSuitesNeeded = (x509KeyManager != null) || (x509TrustManager != null);boolean pskCipherSuitesNeeded = pskKeyManager != null;enabledCipherSuites = getDefaultCipherSuites(x509CipherSuitesNeeded, pskCipherSuitesNeeded);}protected static SSLParametersImpl getDefault() throws KeyManagementException {SSLParametersImpl result = defaultParameters;if (result == null) {// single-check idiomdefaultParameters = result = new SSLParametersImpl(null,null,null,new ClientSessionContext(),new ServerSessionContext(),null);}return (SSLParametersImpl) result.clone();}. . . . . . /*** @return X.509 trust manager or {@code null} for none.*/protected X509TrustManager getX509TrustManager() {return x509TrustManager;}. . . . . . /*** Gets the default X.509 trust manager.* <p>* TODO: Move this to a published API under dalvik.system.*/public static X509TrustManager getDefaultX509TrustManager()throws KeyManagementException {X509TrustManager result = defaultX509TrustManager;if (result == null) {// single-check idiomdefaultX509TrustManager = result = createDefaultX509TrustManager();}return result;}private static X509TrustManager createDefaultX509TrustManager()throws KeyManagementException {try {String algorithm = TrustManagerFactory.getDefaultAlgorithm();TrustManagerFactory tmf = TrustManagerFactory.getInstance(algorithm);tmf.init((KeyStore) null);TrustManager[] tms = tmf.getTrustManagers();X509TrustManager trustManager = findFirstX509TrustManager(tms);if (trustManager == null) {throw new KeyManagementException("No X509TrustManager in among default TrustManagers: "+ Arrays.toString(tms));}return trustManager;} catch (NoSuchAlgorithmException e) {throw new KeyManagementException(e);} catch (KeyStoreException e) {throw new KeyManagementException(e);}}

将 createDefaultX509TrustManager() 的代码复制到我们的应用程序中，就像下面这样：

        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.}}

在应用程序执行时打断点，借助于 Android Studio 确认系统默认的 X509TrustManager 是什么，不难确认，它是 android.security.net.config.RootTrustManager。android.security.net.config.RootTrustManager 的 checkServerTrusted() 定义（位于 frameworks/base/core/java/android/security/net/config/RootTrustManager.java）如下：

    @Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType, Socket socket)throws CertificateException {if (socket instanceof SSLSocket) {SSLSocket sslSocket = (SSLSocket) socket;SSLSession session = sslSocket.getHandshakeSession();if (session == null) {throw new CertificateException("Not in handshake; no session available");}String host = session.getPeerHost();NetworkSecurityConfig config = mConfig.getConfigForHostname(host);config.getTrustManager().checkServerTrusted(certs, authType, socket);} else {// Not an SSLSocket, use the hostname unaware checkServerTrusted.checkServerTrusted(certs, authType);}}@Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType, SSLEngine engine)throws CertificateException {SSLSession session = engine.getHandshakeSession();if (session == null) {throw new CertificateException("Not in handshake; no session available");}String host = session.getPeerHost();NetworkSecurityConfig config = mConfig.getConfigForHostname(host);config.getTrustManager().checkServerTrusted(certs, authType, engine);}@Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType)throws CertificateException {if (mConfig.hasPerDomainConfigs()) {throw new CertificateException("Domain specific configurations require that hostname aware"+ " checkServerTrusted(X509Certificate[], String, String) is used");}NetworkSecurityConfig config = mConfig.getConfigForHostname("");config.getTrustManager().checkServerTrusted(certs, authType);}/*** Hostname aware version of {@link #checkServerTrusted(X509Certificate[], String)}.* This interface is used by conscrypt and android.net.http.X509TrustManagerExtensions do not* modify without modifying those callers.*/public List<X509Certificate> checkServerTrusted(X509Certificate[] certs, String authType,String hostname) throws CertificateException {if (hostname == null && mConfig.hasPerDomainConfigs()) {throw new CertificateException("Domain specific configurations require that the hostname be provided");}NetworkSecurityConfig config = mConfig.getConfigForHostname(hostname);return config.getTrustManager().checkServerTrusted(certs, authType, hostname);}

NetworkSecurityConfig 的 getTrustManager() 定义（位于 frameworks/base/core/java/android/security/net/config/NetworkSecurityConfig.java）如下：

    public NetworkSecurityTrustManager getTrustManager() {synchronized(mTrustManagerLock) {if (mTrustManager == null) {mTrustManager = new NetworkSecurityTrustManager(this);}return mTrustManager;}}

NetworkSecurityConfig 将管根证书库的组件 SystemCertificateSource 、 UserCertificateSource 和执行证书合法性验证的 NetworkSecurityTrustManager 粘起来：

    public static final Builder getDefaultBuilder(int targetSdkVersion) {Builder builder = new Builder().setCleartextTrafficPermitted(DEFAULT_CLEARTEXT_TRAFFIC_PERMITTED).setHstsEnforced(DEFAULT_HSTS_ENFORCED)// System certificate store, does not bypass static pins..addCertificatesEntryRef(new CertificatesEntryRef(SystemCertificateSource.getInstance(), false));// Applications targeting N and above must opt in into trusting the user added certificate// store.if (targetSdkVersion <= Build.VERSION_CODES.M) {// User certificate store, does not bypass static pins.builder.addCertificatesEntryRef(new CertificatesEntryRef(UserCertificateSource.getInstance(), false));}return builder;}

同时 NetworkSecurityConfig 还提供了一些根据特定条件查找根证书的操作：

    public Set<TrustAnchor> getTrustAnchors() {synchronized (mAnchorsLock) {if (mAnchors != null) {return mAnchors;}// Merge trust anchors based on the X509Certificate.// If we see the same certificate in two TrustAnchors, one with overridesPins and one// without, the one with overridesPins wins.// Because mCertificatesEntryRefs is sorted with all overridesPins anchors coming first// this can be simplified to just using the first occurrence of a certificate.Map<X509Certificate, TrustAnchor> anchorMap = new ArrayMap<>();for (CertificatesEntryRef ref : mCertificatesEntryRefs) {Set<TrustAnchor> anchors = ref.getTrustAnchors();for (TrustAnchor anchor : anchors) {X509Certificate cert = anchor.certificate;if (!anchorMap.containsKey(cert)) {anchorMap.put(cert, anchor);}}}ArraySet<TrustAnchor> anchors = new ArraySet<TrustAnchor>(anchorMap.size());anchors.addAll(anchorMap.values());mAnchors = anchors;return mAnchors;}}
. . . . . .public NetworkSecurityTrustManager getTrustManager() {synchronized(mTrustManagerLock) {if (mTrustManager == null) {mTrustManager = new NetworkSecurityTrustManager(this);}return mTrustManager;}}/** @hide */public TrustAnchor findTrustAnchorBySubjectAndPublicKey(X509Certificate cert) {for (CertificatesEntryRef ref : mCertificatesEntryRefs) {TrustAnchor anchor = ref.findBySubjectAndPublicKey(cert);if (anchor != null) {return anchor;}}return null;}/** @hide */public TrustAnchor findTrustAnchorByIssuerAndSignature(X509Certificate cert) {for (CertificatesEntryRef ref : mCertificatesEntryRefs) {TrustAnchor anchor = ref.findByIssuerAndSignature(cert);if (anchor != null) {return anchor;}}return null;}/** @hide */public Set<X509Certificate> findAllCertificatesByIssuerAndSignature(X509Certificate cert) {Set<X509Certificate> certs = new ArraySet<X509Certificate>();for (CertificatesEntryRef ref : mCertificatesEntryRefs) {certs.addAll(ref.findAllCertificatesByIssuerAndSignature(cert));}return certs;}

真正执行证书合法性验证的还不是 NetworkSecurityTrustManager，而是 TrustManagerImpl（位于 external/conscrypt/src/platform/java/org/conscrypt/TrustManagerImpl.java），由 NetworkSecurityTrustManager 的定义（位于frameworks/base/core/java/android/security/net/config/NetworkSecurityTrustManager.java）不难看出这一点：

    public NetworkSecurityTrustManager(NetworkSecurityConfig config) {if (config == null) {throw new NullPointerException("config must not be null");}mNetworkSecurityConfig = config;try {TrustedCertificateStoreAdapter certStore = new TrustedCertificateStoreAdapter(config);// Provide an empty KeyStore since TrustManagerImpl doesn't support null KeyStores.// TrustManagerImpl will use certStore to lookup certificates.KeyStore store = KeyStore.getInstance(KeyStore.getDefaultType());store.load(null);mDelegate = new TrustManagerImpl(store, null, certStore);} catch (GeneralSecurityException | IOException e) {throw new RuntimeException(e);}}
. . . . . .@Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType)throws CertificateException {checkServerTrusted(certs, authType, (String) null);}@Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType, Socket socket)throws CertificateException {List<X509Certificate> trustedChain =mDelegate.getTrustedChainForServer(certs, authType, socket);checkPins(trustedChain);}@Overridepublic void checkServerTrusted(X509Certificate[] certs, String authType, SSLEngine engine)throws CertificateException {List<X509Certificate> trustedChain =mDelegate.getTrustedChainForServer(certs, authType, engine);checkPins(trustedChain);}/*** Hostname aware version of {@link #checkServerTrusted(X509Certificate[], String)}.* This interface is used by conscrypt and android.net.http.X509TrustManagerExtensions do not* modify without modifying those callers.*/public List<X509Certificate> checkServerTrusted(X509Certificate[] certs, String authType,String host) throws CertificateException {List<X509Certificate> trustedChain = mDelegate.checkServerTrusted(certs, authType, host);checkPins(trustedChain);return trustedChain;}private void checkPins(List<X509Certificate> chain) throws CertificateException {PinSet pinSet = mNetworkSecurityConfig.getPins();if (pinSet.pins.isEmpty()|| System.currentTimeMillis() > pinSet.expirationTime|| !isPinningEnforced(chain)) {return;}Set<String> pinAlgorithms = pinSet.getPinAlgorithms();Map<String, MessageDigest> digestMap = new ArrayMap<String, MessageDigest>(pinAlgorithms.size());for (int i = chain.size() - 1; i >= 0 ; i--) {X509Certificate cert = chain.get(i);byte[] encodedSPKI = cert.getPublicKey().getEncoded();for (String algorithm : pinAlgorithms) {MessageDigest md = digestMap.get(algorithm);if (md == null) {try {md = MessageDigest.getInstance(algorithm);} catch (GeneralSecurityException e) {throw new RuntimeException(e);}digestMap.put(algorithm, md);}if (pinSet.pins.contains(new Pin(algorithm, md.digest(encodedSPKI)))) {return;}}}// TODO: Throw a subclass of CertificateException which indicates a pinning failure.throw new CertificateException("Pin verification failed");}

TrustedCertificateStoreAdapter 为根证书库提供了 TrustedCertificateStore 接口的查找操作，以方便 TrustManagerImpl 使用（位于frameworks/base/core/java/android/security/net/config/TrustedCertificateStoreAdapter.java）：

public class TrustedCertificateStoreAdapter extends TrustedCertificateStore {private final NetworkSecurityConfig mConfig;public TrustedCertificateStoreAdapter(NetworkSecurityConfig config) {mConfig = config;}@Overridepublic X509Certificate findIssuer(X509Certificate cert) {TrustAnchor anchor = mConfig.findTrustAnchorByIssuerAndSignature(cert);if (anchor == null) {return null;}return anchor.certificate;}@Overridepublic Set<X509Certificate> findAllIssuers(X509Certificate cert) {return mConfig.findAllCertificatesByIssuerAndSignature(cert);}@Overridepublic X509Certificate getTrustAnchor(X509Certificate cert) {TrustAnchor anchor = mConfig.findTrustAnchorBySubjectAndPublicKey(cert);if (anchor == null) {return null;}return anchor.certificate;}@Overridepublic boolean isUserAddedCertificate(X509Certificate cert) {// isUserAddedCertificate is used only for pinning overrides, so use overridesPins here.TrustAnchor anchor = mConfig.findTrustAnchorBySubjectAndPublicKey(cert);if (anchor == null) {return false;}return anchor.overridesPins;}

不难看出 Android 中 Java 层证书验证的过程如下图所示：

OpenSSLSocketImpl.startHandshake() 和 NativeCrypto.SSL_do_handshake() 执行完整的 SSL/TLS 握手过程。证书合法性验证作为 SSL/TLS 握手的一个重要步骤，通过本地层调用的 Java 层的回调方法 SSLHandshakeCallbacks.verifyCertificateChain() 完成，OpenSSLSocketImpl 实现这一回调。OpenSSLSocketImpl.verifyCertificateChain()、Platform.checkServerTrusted()、RootTrustManager.checkServerTrusted() 和NetworkSecurityTrustManager.checkServerTrusted() 用于将真正的根据系统根证书库执行证书合法性验证的 TrustManagerImpl 和 SSL/TLS 握手过程粘起来。OpenSSLSocketFactoryImpl 将 OpenSSLSocketImpl 和 SSLParametersImpl 粘起来。SSLParametersImpl 将 OpenSSLSocketImpl 和 RootTrustManager 粘起来。

NetworkSecurityConfig 将 RootTrustManager 和 NetworkSecurityTrustManager 粘起来。NetworkSecurityConfig、NetworkSecurityTrustManager 和 TrustedCertificateStoreAdapter 将 TrustManagerImpl 和管理系统根证书库的 SystemCertificateSource 粘起来。

TrustManagerImpl 是证书合法性验证的核心，它会查找系统根证书库，并对服务端证书的合法性做验证。

这个过程的调用栈如下：

com.android.org.conscrypt.TrustManagerImpl.checkServerTrusted()
android.security.net.config.NetworkSecurityTrustManager.checkServerTrusted()
android.security.net.config.NetworkSecurityTrustManager.checkServerTrusted()
android.security.net.config.RootTrustManager.checkServerTrusted()
com.android.org.conscrypt.Platform.checkServerTrusted()
com.android.org.conscrypt.OpenSSLSocketImpl.verifyCertificateChain()
com.android.org.conscrypt.NativeCrypto.SSL_do_handshake()
com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake()
com.android.okhttp.Connection.connectTls()

还有两个问题，一是 SSLParametersImpl 是如何找到的 RootTrustManager；二是如何定制或者影响证书合法性的验证过程。

TrustManager 的查找

Java 加密体系架构（JCA）是一个非常灵活的架构，它的整体结构如下图：

Java 应用程序通过接口层访问加密服务，接口层的组成包括 JAAS（Java Authentication Authorization Service，Java验证和授权API）、JSSE（Java Secure Socket Extension，Java 安全套接字扩展）、JGSS（Java Generic Security Service ）和 CertPath等。具体的组件如我们前面看到的 CertificateFactory、TrustManagerFactory 和 SSLSocketFactory 等。

JCA 还定义了一组加密服务 Provider 接口，如 javax.net.ssl.SSLContextSpi 和 javax.net.ssl.TrustManagerFactorySpi 等。加密服务的实现者实现这些接口，并通过 java.security.Security 提供的接口注册进 JCA 框架。

对于 Android 系统来说，TrustManagerFactory 加密服务的注册是在 ActivityThread 的 handleBindApplication() 中做的，相关代码（位于 frameworks/base/core/java/android/app/ActivityThread.java）如下：

        // Install the Network Security Config Provider. This must happen before the application// code is loaded to prevent issues with instances of TLS objects being created before// the provider is installed.Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "NetworkSecurityConfigProvider.install");NetworkSecurityConfigProvider.install(appContext);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

NetworkSecurityConfigProvider 类的定义（位于 frameworks/base/core/java/android/security/net/config/NetworkSecurityConfigProvider.java）如下：

package android.security.net.config;import android.content.Context;
import java.security.Security;
import java.security.Provider;/** @hide */
public final class NetworkSecurityConfigProvider extends Provider {private static final String PREFIX =NetworkSecurityConfigProvider.class.getPackage().getName() + ".";public NetworkSecurityConfigProvider() {// TODO: More clever name than thissuper("AndroidNSSP", 1.0, "Android Network Security Policy Provider");put("TrustManagerFactory.PKIX", PREFIX + "RootTrustManagerFactorySpi");put("Alg.Alias.TrustManagerFactory.X509", "PKIX");}public static void install(Context context) {ApplicationConfig config = new ApplicationConfig(new ManifestConfigSource(context));ApplicationConfig.setDefaultInstance(config);int pos = Security.insertProviderAt(new NetworkSecurityConfigProvider(), 1);if (pos != 1) {throw new RuntimeException("Failed to install provider as highest priority provider."+ " Provider was installed at position " + pos);}libcore.net.NetworkSecurityPolicy.setInstance(new ConfigNetworkSecurityPolicy(config));}
}

在 NetworkSecurityConfigProvider.install() 方法中，通过 Security.insertProviderAt() 将 NetworkSecurityConfigProvider 注册进 JCA 框架中。从 NetworkSecurityConfigProvider 的构造函数可以看到，它将 android.security.net.config.RootTrustManagerFactorySpi 带进 JCA 框架。

android.security.net.config.RootTrustManagerFactorySpi 的定义（位于 frameworks/base/core/java/android/security/net/config/RootTrustManagerFactorySpi.java）如下：

package android.security.net.config;import android.util.Pair;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidParameterException;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.Provider;
import java.security.Security;
import java.util.Set;
import javax.net.ssl.ManagerFactoryParameters;
import javax.net.ssl.TrustManager;
import javax.net.ssl.TrustManagerFactory;
import javax.net.ssl.TrustManagerFactorySpi;import com.android.internal.annotations.VisibleForTesting;/** @hide */
public class RootTrustManagerFactorySpi extends TrustManagerFactorySpi {private ApplicationConfig mApplicationConfig;private NetworkSecurityConfig mConfig;@Overridepublic void engineInit(ManagerFactoryParameters spec)throws InvalidAlgorithmParameterException {if (!(spec instanceof ApplicationConfigParameters)) {throw new InvalidAlgorithmParameterException("Unsupported spec: " +  spec + ". Only "+ ApplicationConfigParameters.class.getName() + " supported");}mApplicationConfig = ((ApplicationConfigParameters) spec).config;}@Overridepublic void engineInit(KeyStore ks) throws KeyStoreException {if (ks != null) {mApplicationConfig = new ApplicationConfig(new KeyStoreConfigSource(ks));} else {mApplicationConfig = ApplicationConfig.getDefaultInstance();}}@Overridepublic TrustManager[] engineGetTrustManagers() {if (mApplicationConfig == null) {throw new IllegalStateException("TrustManagerFactory not initialized");}return new TrustManager[] { mApplicationConfig.getTrustManager() };}@VisibleForTestingpublic static final class ApplicationConfigParameters implements ManagerFactoryParameters {public final ApplicationConfig config;public ApplicationConfigParameters(ApplicationConfig config) {this.config = config;}}
}

RootTrustManagerFactorySpi 的 TrustManager 来自于 ApplicationConfig，ApplicationConfig 中 TrustManager 相关的代码（位于 frameworks/base/core/java/android/security/net/config/ApplicationConfig.java）如下：

public final class ApplicationConfig {private static ApplicationConfig sInstance;private static Object sLock = new Object();private Set<Pair<Domain, NetworkSecurityConfig>> mConfigs;private NetworkSecurityConfig mDefaultConfig;private X509TrustManager mTrustManager;
. . . . . ./*** Returns the {@link X509TrustManager} that implements the checking of trust anchors and* certificate pinning based on this configuration.*/public X509TrustManager getTrustManager() {ensureInitialized();return mTrustManager;}
. . . . . .private void ensureInitialized() {synchronized(mLock) {if (mInitialized) {return;}mConfigs = mConfigSource.getPerDomainConfigs();mDefaultConfig = mConfigSource.getDefaultConfig();mConfigSource = null;mTrustManager = new RootTrustManager(this);mInitialized = true;}}

ApplicationConfig 的 TrustManager 是 RootTrustManager。

再来看 JCA 接口层的 javax.net.ssl.TrustManagerFactory 的定义：

public class TrustManagerFactory {// The providerprivate Provider provider;// The provider implementation (delegate)private TrustManagerFactorySpi factorySpi;// The name of the trust management algorithm.private String algorithm;
. . . . . .public final static String getDefaultAlgorithm() {String type;type = AccessController.doPrivileged(new PrivilegedAction<String>() {public String run() {return Security.getProperty("ssl.TrustManagerFactory.algorithm");}});if (type == null) {type = "SunX509";}return type;}
. . . . . ./*** Creates a TrustManagerFactory object.** @param factorySpi the delegate* @param provider the provider* @param algorithm the algorithm*/protected TrustManagerFactory(TrustManagerFactorySpi factorySpi,Provider provider, String algorithm) {this.factorySpi = factorySpi;this.provider = provider;this.algorithm = algorithm;}
. . . . . .public static final TrustManagerFactory getInstance(String algorithm)throws NoSuchAlgorithmException {GetInstance.Instance instance = GetInstance.getInstance("TrustManagerFactory", TrustManagerFactorySpi.class,algorithm);return new TrustManagerFactory((TrustManagerFactorySpi)instance.impl,instance.provider, algorithm);}
. . . . . .public final void init(KeyStore ks) throws KeyStoreException {factorySpi.engineInit(ks);}/*** Initializes this factory with a source of provider-specific* trust material.* <P>* In some cases, initialization parameters other than a keystore* may be needed by a provider.  Users of that particular provider* are expected to pass an implementation of the appropriate* <CODE>ManagerFactoryParameters</CODE> as defined by the* provider.  The provider can then call the specified methods in* the <CODE>ManagerFactoryParameters</CODE> implementation to obtain the* needed information.** @param spec an implementation of a provider-specific parameter*          specification* @throws InvalidAlgorithmParameterException if an error is*          encountered*/public final void init(ManagerFactoryParameters spec) throwsInvalidAlgorithmParameterException {factorySpi.engineInit(spec);}/*** Returns one trust manager for each type of trust material.** @throws IllegalStateException if the factory is not initialized.** @return the trust managers*/public final TrustManager[] getTrustManagers() {return factorySpi.engineGetTrustManagers();}

TrustManagerFactory 通过 JCA 框架提供的 sun.security.jca.GetInstance 找到注册的 javax.net.ssl.TrustManagerFactorySpi。应用程序通过 javax.net.ssl.TrustManagerFactory -> android.security.net.config.RootTrustManagerFactorySpi -> android.security.net.config.ApplicationConfig 得到 android.security.net.config.RootTrustManager，即 X509TrustManager。

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*/@Overridepublic void engineInit(KeyManager[] kms, TrustManager[] tms, SecureRandom sr)throws KeyManagementException {sslParameters = new SSLParametersImpl(kms, tms, sr, clientSessionContext,serverSessionContext, algorithms);}@Overridepublic 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.}}@Overridepublic void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException {mSystemDefaultTrustManager.checkClientTrusted(chain, authType);}@Overridepublic 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);}@Overridepublic 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 = nulltry {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 https 自定义证书问题
Android实现https网络通信之添加指定信任证书/信任所有证书
HTTPS（含SNI）业务场景“IP直连”方案说明
HTTP Public Key Pinning 介绍
Java https请求 HttpsURLConnection

Done。

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Android 根证书管理与证书验证

Android 的根证书管理

证书链合法性验证

TrustManager 的查找

私有 CA 签名证书的应用

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