Android App签名(证书)校验过程源码分析_app 程序启动时是否校验签名证书

  Android App安装是需要证书支持的，我们在Eclipse或者Android Studio中开发App时，并没有注意关于证书的事，也能正确安装App。这是因为使用了默认的debug证书。在Android App升级的时候，证书发挥的作用就尤为明显了。只有证书相同时，才能对App进行升级。证书也是为了防止App伪造的，属于Android安全策略的一部分。另外，Android沙箱机制中，也和证书有关。两个App如要共享文件，代码，或者资源时，需要使用shareUid属性，只有证书相同的App的才能shareUid。才外，如果一个App中申明了signature级别的权限，也是只有和那个App签名相同的App才能申请到对应的权限。

  虽然之前也了解过Android App的签名校验过程，但都是根据别人总结的结果，没有自己动手分析Android源码。所以本篇Blog将从源码出发分析Android App的签名校验过程，分析完源码之后，也会和网上大多数的资料一样给出总结。

  注意：由于签名校验过程是在App安装时进行的，所以源码分析的起始点是上篇Blog：PackageInstaller源码分析。不过不想了解PackageInstall源码也没有关系，只要不纠结程序的起点，分析过程就是App 签名校验模块。

一、 源码分析

  上篇BlogPackageInstaller源码分析中，程序安装过程调用了installPackageLI()方法。而在installPackageLI()方法内部，调用了collectCertificates()方法，从而进入了App的签名检验过程。下面我们查看collectCertificates()的源码实现，源码路径：/frameworks/base/core/java/android/content/pm/PackageParser.java

public void collectCertificates(Package pkg, int flags) throws PackageParserException {
    pkg.mCertificates = null;
    pkg.mSignatures = null;
    pkg.mSigningKeys = null;

    collectCertificates(pkg, new File(pkg.baseCodePath), flags);

    if (!ArrayUtils.isEmpty(pkg.splitCodePaths)) {
        for (String splitCodePath : pkg.splitCodePaths) {
            collectCertificates(pkg, new File(splitCodePath), flags);
        }
    }
}
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private static void collectCertificates(Package pkg, File apkFile, int flags)
        throws PackageParserException {
    final String apkPath = apkFile.getAbsolutePath();

    StrictJarFile jarFile = null;
    try {
        jarFile = new StrictJarFile(apkPath);

        // Always verify manifest, regardless of source
        final ZipEntry manifestEntry = jarFile.findEntry(ANDROID_MANIFEST_FILENAME);
        if (manifestEntry == null) {
            throw new PackageParserException(INSTALL_PARSE_FAILED_BAD_MANIFEST,
                    "Package " + apkPath + " has no manifest");
        }

        final List<ZipEntry> toVerify = new ArrayList<>();
        toVerify.add(manifestEntry);

        // If we're parsing an untrusted package, verify all contents
        if ((flags & PARSE_IS_SYSTEM) == 0) {
            final Iterator<ZipEntry> i = jarFile.iterator();
            while (i.hasNext()) {
                final ZipEntry entry = i.next();

                if (entry.isDirectory()) continue;
                if (entry.getName().startsWith("META-INF/")) continue;
                if (entry.getName().equals(ANDROID_MANIFEST_FILENAME)) continue;

                toVerify.add(entry);
            }
        }

        // Verify that entries are signed consistently with the first entry
        // we encountered. Note that for splits, certificates may have
        // already been populated during an earlier parse of a base APK.
        for (ZipEntry entry : toVerify) {
            final Certificate[][] entryCerts = loadCertificates(jarFile, entry);
            if (ArrayUtils.isEmpty(entryCerts)) {
                throw new PackageParserException(INSTALL_PARSE_FAILED_NO_CERTIFICATES,
                        "Package " + apkPath + " has no certificates at entry "
                        + entry.getName());
            }
            final Signature[] entrySignatures = convertToSignatures(entryCerts);

            if (pkg.mCertificates == null) {
                pkg.mCertificates = entryCerts;
                pkg.mSignatures = entrySignatures;
                pkg.mSigningKeys = new ArraySet<PublicKey>();
                for (int i=0; i < entryCerts.length; i++) {
                    pkg.mSigningKeys.add(entryCerts[i][0].getPublicKey());
                }
            } else {
                if (!Signature.areExactMatch(pkg.mSignatures, entrySignatures)) {
                    throw new PackageParserException(
                            INSTALL_PARSE_FAILED_INCONSISTENT_CERTIFICATES, "Package " + apkPath
                                    + " has mismatched certificates at entry "
                                    + entry.getName());
                }
            }
        }
    } catch (GeneralSecurityException e) {
        throw new PackageParserException(INSTALL_PARSE_FAILED_CERTIFICATE_ENCODING,
                "Failed to collect certificates from " + apkPath, e);
    } catch (IOException | RuntimeException e) {
        throw new PackageParserException(INSTALL_PARSE_FAILED_NO_CERTIFICATES,
                "Failed to collect certificates from " + apkPath, e);
    } finally {
        closeQuietly(jarFile);
    }
}
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  在collectCertificates(Package pkg, File apkFile, int flags)函数里面，首先提取apk的manifest.xml文件。

final ZipEntry manifestEntry = jarFile.findEntry(ANDROID_MANIFEST_FILENAME);
if (manifestEntry == null) {
   throw new PackageParserException(INSTALL_PARSE_FAILED_BAD_MANIFEST,
           "Package " + apkPath + " has no manifest");
}
final List<ZipEntry> toVerify = new ArrayList<>();
toVerify.add(manifestEntry);
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  然后，程序遍历apk文件的所有文件节点，把除了META-INF/文件夹里面的文外外的所以文件加入待检验List。

// If we're parsing an untrusted package, verify all contents
if ((flags & PARSE_IS_SYSTEM) == 0) {
    final Iterator<ZipEntry> i = jarFile.iterator();
    while (i.hasNext()) {
        final ZipEntry entry = i.next();

        if (entry.isDirectory()) continue;
        if (entry.getName().startsWith("META-INF/")) continue;
        if (entry.getName().equals(ANDROID_MANIFEST_FILENAME)) continue;

        toVerify.add(entry);
    }
}
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  紧接着把所以节点传入loadCertificates()方法，

for (ZipEntry entry : toVerify) {
   final Certificate[][] entryCerts = loadCertificates(jarFile, entry);
   if (ArrayUtils.isEmpty(entryCerts)) {
       throw new PackageParserException(INSTALL_PARSE_FAILED_NO_CERTIFICATES,
               "Package " + apkPath + " has no certificates at entry "
               + entry.getName());
   }
   final Signature[] entrySignatures = convertToSignatures(entryCerts);

   if (pkg.mCertificates == null) {
       pkg.mCertificates = entryCerts;
       pkg.mSignatures = entrySignatures;
       pkg.mSigningKeys = new ArraySet<PublicKey>();
       for (int i=0; i < entryCerts.length; i++) {
           pkg.mSigningKeys.add(entryCerts[i][0].getPublicKey());
       }
   } else {
       if (!Signature.areExactMatch(pkg.mSignatures, entrySignatures)) {
           throw new PackageParserException(
                   INSTALL_PARSE_FAILED_INCONSISTENT_CERTIFICATES, "Package " + apkPath
                           + " has mismatched certificates at entry "
                           + entry.getName());
       }
   }
}
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  要知道loadCertificates()的作用需要分析其方法实现原型。在PackageParser.java中实现了loadCertificates()方法。

private static Certificate[][] loadCertificates(StrictJarFile jarFile, ZipEntry entry)throws PackageParserException {
   InputStream is = null;
   try {
       // We must read the stream for the JarEntry to retrieve
       // its certificates.
       is = jarFile.getInputStream(entry);
       readFullyIgnoringContents(is);
       return jarFile.getCertificateChains(entry);
   } catch (IOException | RuntimeException e) {
       throw new PackageParserException(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,
               "Failed reading " + entry.getName() + " in " + jarFile, e);
   } finally {
       IoUtils.closeQuietly(is);
   }
}
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  在StrictJarFile.java中,实现了getCertificateChains()方法，代码路径/libcore/luni/src/main/java/java/util/jar/StrictJarFile.java。

public Certificate[][] getCertificateChains(ZipEntry ze) {
if (isSigned) {
   return verifier.getCertificateChains(ze.getName());
}

return null;
}
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  StrictJarFile.java中的getCertificateChains()继续调用JarVerifier中的getCertificateChains()方法,代码路径：/libcore/luni/src/main/java/java/util/jar/JarVerifier.java。

Certificate[][] getCertificateChains(String name) {
    return verifiedEntries.get(name);
}
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private final Hashtable<String, Certificate[][]> verifiedEntries=new Hashtable<String, 
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