Android 根证书管理与证书验证

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：

1. 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。

2. 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。

3. 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);

    @Override
    public 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;
        }
    }

    @Override
    public X509Certificate findBySubjectAndPublicKey(final X509Certificate cert) {
        return findCert(cert.getSubjectX500Principal(), new CertSelector() {
            @Override
            public boolean match(X509Certificate ca) {
                return ca.getPublicKey().equals(cert.getPublicKey());
            }
        });
    }

    @Override
    public X509Certificate findByIssuerAndSignature(final X509Certificate cert) {
        return findCert(cert.getIssuerX500Principal(), new CertSelector() {
            @Override
            public boolean match(X509Certificate ca) {
                try {
                    cert.verify(ca.getPublicKey());
                    return true;
                } catch (Exception e) {
                    return false;
                }
            }
        });
    }

    @Override
    public Set<X509Certificate> findAllByIssuerAndSignature(final X509Certificate cert) {
        return findCerts(cert.getIssuerX500Principal(), new CertSelector() {
            @Override
            public boolean match(X509Certificate ca) {
                try {
                    cert.verify(ca.getPublicKey());
                    return true;
                } catch (Exception e) {
                    return false;
                }
            }
        });
    }

    @Override
    public 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;
    }

    @Override
    protected 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;
    }

    @Override
    protected 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。

相关的几个组件结构如下图：

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
@Override
public 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 callback
        handshakeSession = 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 listeners
        handshakeSession = 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 OpenSSLSocketImpl
        extends javax.net.ssl.SSLSocket
        implements 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;
    }

    @Override
    public String[] getDefaultCipherSuites() {
        return sslParameters.getEnabledCipherSuites();
    }

    @Override
    public String[] getSupportedCipherSuites() {
        return NativeCrypto.getSupportedCipherSuites();
    }

    @Override
    public Socket createSocket() throws IOException {
        if (instantiationException != null) {
            throw instantiationException;
        }
        return new OpenSSLSocketImpl((SSLParametersImpl) sslParameters.clone());
    }

    @Override
    public Socket createSocket(String hostname, int port) throws IOException, UnknownHostException {
        return new OpenSSLSocketImpl(hostname, port, (SSLParametersImpl) sslParameters.clone());
    }

    @Override
    public Socket createSocket(String hostname, int port, InetAddress localHost, int localPort)
            throws IOException, UnknownHostException {
        return new OpenSSLSocketImpl(hostname,
                                     port,
                                     localHost,
                                     localPort,
                                     (SSLParametersImpl) sslParameters.clone());
    }

    @Override
    public Socket createSocket(InetAddress address, int port) throws IOException {
        return new OpenSSLSocketImpl(address, port, (SSLParametersImpl) sslParameters.clone());
    }

    @Override
    public Socket createSocket(InetAddress address,
                               int port,
                               InetAddress localAddress,
                               int localPort)
            throws IOException {
        return new OpenSSLSocketImpl(address,
                                     port,
                                     localAddress,
                                     localPort,
                                     (SSLParametersImpl) sslParameters.clone());
    }

    @Override
    public 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 managers
       if (kms == null) {
           x509KeyManager = getDefaultX509KeyManager();
           // There's no default PSK key manager
           pskKeyManager = null;
       } else {
           x509KeyManager = findFirstX509KeyManager(kms);
           pskKeyManager = findFirstPSKKeyManager(kms);
       }

       // initialize x509TrustManager
       if (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 default
       enabledProtocols = 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 idiom
           defaultParameters = 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 idiom
           defaultX509TrustManager = 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：

@Override
public 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);
    }
}

@Override
public 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);
}

@Override
public 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);
        }
    }
. . . . . .
    @Override
    public void checkServerTrusted(X509Certificate[] certs, String authType)
            throws CertificateException {
        checkServerTrusted(certs, authType, (String) null);
    }

    @Override
    public void checkServerTrusted(X509Certificate[] certs, String authType, Socket socket)
            throws CertificateException {
        List<X509Certificate> trustedChain =
                mDelegate.getTrustedChainForServer(certs, authType, socket);
        checkPins(trustedChain);
    }

    @Override
    public 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;
    }

    @Override
    public X509Certificate findIssuer(X509Certificate cert) {
        TrustAnchor anchor = mConfig.findTrustAnchorByIssuerAndSignature(cert);
        if (anchor == null) {
            return null;
        }
        return anchor.certificate;
    }

    @Override
    public Set<X509Certificate> findAllIssuers(X509Certificate cert) {
        return mConfig.findAllCertificatesByIssuerAndSignature(cert);
    }

    @Override
    public X509Certificate getTrustAnchor(X509Certificate cert) {
        TrustAnchor anchor = mConfig.findTrustAnchorBySubjectAndPublicKey(cert);
        if (anchor == null) {
            return null;
        }
        return anchor.certificate;
    }

    @Override
    public 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 this
        super("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;

    @Override
    public 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;
    }

    @Override
    public void engineInit(KeyStore ks) throws KeyStoreException {
        if (ks != null) {
            mApplicationConfig = new ApplicationConfig(new KeyStoreConfigSource(ks));
        } else {
            mApplicationConfig = ApplicationConfig.getDefaultInstance();
        }
    }

    @Override
    public TrustManager[] engineGetTrustManagers() {
        if (mApplicationConfig == null) {
            throw new IllegalStateException("TrustManagerFactory not initialized");
        }
        return new TrustManager[] { mApplicationConfig.getTrustManager() };
    }

    @VisibleForTesting
    public 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 provider
    private 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) throws
            InvalidAlgorithmParameterException {
        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
     */
    @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 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。