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一、前言

今天是元旦，也是Single Dog的嚎叫之日，只能写博客来祛除寂寞了，今天我们继续来看一下Android中的签名机制的姊妹篇：Android中是如何验证一个Apk的签名。在前一篇文章中我们介绍了，Android中是如何对程序进行签名的，不了解的同学可以转战：

http://blog.csdn.net/jiangwei0910410003/article/details/50402000

当然在了解我们今天说到的知识点，这篇文章也是需要了解的，不然会有些知识点有些困惑的。

二、知识摘要

在我们没有开始这篇文章之前，我们回顾一下之前说到的签名机制流程：

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1、对Apk中的每个文件做一次算法(数据摘要+Base64编码)，保存到MANIFEST.MF文件中

2、对MANIFEST.MF整个文件做一次算法(数据摘要+Base64编码)，存放到CERT.SF文件的头属性中，在对MANIFEST.MF文件中各个属性块做一次算法(数据摘要+Base64编码)，存到到一个属性块中。

3、对CERT.SF文件做签名，内容存档到CERT.RSA中

所以通过上面的流程可以知道，我们今天来验证签名流程也是这三个步骤

三、代码分析

我们既然要了解Android中的应用程序的签名验证过程的话，那么我们肯定需要从一个类来开始看起，那就是PackageManagerService.java，因为这个类是Apk在安装的过程中核心类：frameworks\base\services\core\java\com\android\server\pm\PackageManagerService.java

private void installPackageLI(InstallArgs args, PackageInstalledInfo res) {    ……    PackageParser pp = new PackageParser();    ……    try {        pp.collectCertificates(pkg, parseFlags);        pp.collectManifestDigest(pkg);    } catch (PackageParserException e) {        res.setError("Failed collect during installPackageLI", e);        return;    }    ……

我们可以看到，有一个核心类：PackageParser

frameworks\base\core\java\android\content\pm\PackageParser.java

这个类也是见名知意，就是需要解析Apk包，那么就会涉及到签名信息了，下面我们就从这个类开始入手：

import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_BAD_MANIFEST;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_BAD_PACKAGE_NAME;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_CERTIFICATE_ENCODING;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_INCONSISTENT_CERTIFICATES;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_NOT_APK;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_NO_CERTIFICATES;import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION;

我们看到了几个我们很熟悉的信息：

import static android.content.pm.PackageManager.INSTALL_PARSE_FAILED_NO_CERTIFICATES;

这个是在安装apk包的时候出现的错误，没有证书：

那么我们就先来查找一下这个字段：

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); }}

这里看到了，当有异常的时候就会提示这个信息，我们在跟进去看看：

// 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());  } }}

这里有一个重要的方法：loadCertificates

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); }}

这个方法是加载证书内容的

1、验证Apk中的每个文件的算法(数据摘要+Base64编码)和MANIFEST.MF文件中的对应属性块内容是否配对

首先获取StrictJarFile文件中的InputStream对象

StrictJarFile这个类：libcore\luni\src\main\java\java\util\jar\StrictJarFile.java

public InputStream getInputStream(ZipEntry ze) { final InputStream is = getZipInputStream(ze); if (isSigned) {  JarVerifier.VerifierEntry entry = verifier.initEntry(ze.getName());  if (entry == null) {   return is;  }  return new JarFile.JarFileInputStream(is, ze.getSize(), entry); } return is;}

1》获取到VerifierEntry对象entry

在JarVerifier.java：libcore\luni\src\main\java\java\util\jar\JarVerifier.java

VerifierEntry initEntry(String name) { // If no manifest is present by the time an entry is found, // verification cannot occur. If no signature files have // been found, do not verify. if (manifest == null || signatures.isEmpty()) {  return null; } Attributes attributes = manifest.getAttributes(name); // entry has no digest if (attributes == null) {  return null; } ArrayList<Certificate[]> certChains = new ArrayList<Certificate[]>(); Iterator<Map.Entry<String, HashMap<String, Attributes>>> it = signatures.entrySet().iterator(); while (it.hasNext()) {  Map.Entry<String, HashMap<String, Attributes>> entry = it.next();  HashMap<String, Attributes> hm = entry.getValue();  if (hm.get(name) != null) {   // Found an entry for entry name in .SF file   String signatureFile = entry.getKey();   Certificate[] certChain = certificates.get(signatureFile);   if (certChain != null) {    certChains.add(certChain);   }  } } // entry is not signed if (certChains.isEmpty()) {  return null; } Certificate[][] certChainsArray = certChains.toArray(new Certificate[certChains.size()][]); for (int i = 0; i < DIGEST_ALGORITHMS.length; i++) {  final String algorithm = DIGEST_ALGORITHMS[i];  final String hash = attributes.getValue(algorithm + "-Digest");  if (hash == null) {   continue;  }  byte[] hashBytes = hash.getBytes(StandardCharsets.ISO_8859_1);  try {   return new VerifierEntry(name, MessageDigest.getInstance(algorithm), hashBytes,     certChainsArray, verifiedEntries);  } catch (NoSuchAlgorithmException ignored) {  } } return null;}

就是构造一个VerifierEntry对象：

/** * Stores and a hash and a message digest and verifies that massage digest * matches the hash. */static class VerifierEntry extends OutputStream { private final String name; private final MessageDigest digest; private final byte[] hash; private final Certificate[][] certChains; private final Hashtable<String, Certificate[][]> verifiedEntries; VerifierEntry(String name, MessageDigest digest, byte[] hash,   Certificate[][] certChains, Hashtable<String, Certificate[][]> verifedEntries) {  this.name = name;  this.digest = digest;  this.hash = hash;  this.certChains = certChains;  this.verifiedEntries = verifedEntries; } /**  * Updates a digest with one byte.  */  @Override  public void write(int value) {  digest.update((byte) value);  }  /**   * Updates a digest with byte array.   */  @Override  public void write(byte[] buf, int off, int nbytes) {   digest.update(buf, off, nbytes);  }  /**   * Verifies that the digests stored in the manifest match the decrypted   * digests from the .SF file. This indicates the validity of the   * signing, not the integrity of the file, as its digest must be   * calculated and verified when its contents are read.   *   * @throws SecurityException   *             if the digest value stored in the manifest does <i>not</i>   *             agree with the decrypted digest as recovered from the   *             <code>.SF</code> file.   */  void verify() {   byte[] d = digest.digest();   if (!MessageDigest.isEqual(d, Base64.decode(hash))) {    throw invalidDigest(JarFile.MANIFEST_NAME, name, name);   }   verifiedEntries.put(name, certChains);  }}

要构造这个对象，必须事先准备好参数。第一个参数很简单，就是要验证的文件名，直接将name传进来就好了。第二个参数是计算摘要的对象，可以通过MessageDigest.getInstance获得，不过要先告知到底要用哪个摘要算法，同样也是通过查看MANIFEST.MF文件中对应名字的属性值来决定的：

所以可以知道所用的摘要算法是SHA1。第三个参数是对应文件的摘要值，这是通过读取MANIFEST.MF文件获得的：

第四个参数是证书链，即对该apk文件签名的所有证书链信息。为什么是二维数组呢？这是因为Android允许用多个证书对apk进行签名，但是它们的证书文件名必须不同，这个知识点，我在之前的一篇文章中：签名过程详解中有提到。

最后一个参数是已经验证过的文件列表，VerifierEntry在完成了对指定文件的摘要验证之后会将该文件的信息加到其中。

2》再去JarFile的JarFileInputStream类中看看：

static final class JarFileInputStream extends FilterInputStream { private long count; private ZipEntry zipEntry; private JarVerifier.VerifierEntry entry; private boolean done = false; JarFileInputStream(InputStream is, ZipEntry ze,   JarVerifier.VerifierEntry e) {  super(is);  zipEntry = ze;  count = zipEntry.getSize();  entry = e; } @Override public int read() throws IOException {  if (done) {   return -1;  }  if (count > 0) {   int r = super.read();   if (r != -1) {    entry.write(r);    count--;   } else {    count = 0;   }   if (count == 0) {    done = true;    entry.verify();   }   return r;  } else {   done = true;   entry.verify();   return -1;  } } @Override public int read(byte[] buf, int off, int nbytes) throws IOException {  if (done) {   return -1;  }  if (count > 0) {   int r = super.read(buf, off, nbytes);   if (r != -1) {    int size = r;    if (count < size) {     size = (int) count;    }    entry.write(buf, off, size);    count -= size;   } else {    count = 0;   }   if (count == 0) {    done = true;    entry.verify();   }   return r;  } else {   done = true;   entry.verify();   return -1;  } } @Override public int available() throws IOException {  if (done) {   return 0;  }  return super.available(); } @Override public long skip(long byteCount) throws IOException {  return Streams.skipByReading(this, byteCount); }}

3》PackageParser的readFullyIgnoringContents方法：

public static long readFullyIgnoringContents(InputStream in) throws IOException { byte[] buffer = sBuffer.getAndSet(null); if (buffer == null) {  buffer = new byte[4096]; } int n = 0; int count = 0; while ((n = in.read(buffer, 0, buffer.length)) != -1) {  count += n; } sBuffer.set(buffer); return count;}

得到第二步之后的一个InputStream对象，然后就开始read操作，这里我没发现什么猫腻，但是我们从第一件事做完之后可以发现，这里的InputStream对象其实是JarInputStream，所以我们可以去看一下他的read方法的实现：

玄机原来在这里，这里的JarFileInputStream.read确实会调用其父类的read读取指定的apk内文件的内容，并且将其传给JarVerifier.VerifierEntry.write函数。当文件读完后，会接着调用JarVerifier.VerifierEntry.verify函数对其进行验证。JarVerifier.VerifierEntry.write函数非常简单：

就是将读到的文件的内容传给digest，这个digest就是前面在构造JarVerifier.VerifierEntry传进来的，对应于在MANIFEST.MF文件中指定的摘要算法。万事具备，接下来想要验证就很简单了：

通过digest就可以算出apk内指定文件的真实摘要值。而记录在MANIFEST.MF文件中对应该文件的摘要值，也在构造JarVerifier.VerifierEntry时传递给了hash变量。不过这个hash值是经过Base64编码的。所以在比较之前，必须通过Base64解码。如果不一致的话，会抛出SecurityException异常：

private static SecurityException invalidDigest(String signatureFile, String name,  String jarName) { throw new SecurityException(signatureFile + " has invalid digest for " + name +   " in " + jarName);}

到这里我们就分析了，Android中是如何验证MANIFEST.MF文件中的内容的，我们这里再来看一下，这里抛出异常出去：

这里捕获到异常之后，会在抛异常出去：

在这里就会抛出异常信息，所以如果我们修改了一个Apk中的一个文件内容的话，这里肯定是安装不上的。

2、验证CERT.SF文件的签名信息和CERT.RSA中的内容是否一致

1》我们就来看看StrictJarFile中的getCertificateChains方法：

/** * Return all certificate chains for a given {@link ZipEntry} belonging to this jar. * This method MUST be called only after fully exhausting the InputStream belonging * to this entry. * * Returns {@code null} if this jar file isn't signed or if this method is * called before the stream is processed. */public Certificate[][] getCertificateChains(ZipEntry ze) { if (isSigned) {  return verifier.getCertificateChains(ze.getName()); } return null;}

这里有一个变量判断：isSigned，他是在构造方法中赋值的：

public StrictJarFile(String fileName) throws IOException { this.nativeHandle = nativeOpenJarFile(fileName); this.raf = new RandomAccessFile(fileName, "r"); try {  // Read the MANIFEST and signature files up front and try to  // parse them. We never want to accept a JAR File with broken signatures  // or manifests, so it's best to throw as early as possible.  HashMap<String, byte[]> metaEntries = getMetaEntries();  this.manifest = new Manifest(metaEntries.get(JarFile.MANIFEST_NAME), true);  this.verifier = new JarVerifier(fileName, manifest, metaEntries);  isSigned = verifier.readCertificates() && verifier.isSignedJar(); } catch (IOException ioe) {  nativeClose(this.nativeHandle);  throw ioe; } guard.open("close");}

去verifier中看看这两个方法：

/** * If the associated JAR file is signed, check on the validity of all of the * known signatures. * * @return {@code true} if the associated JAR is signed and an internal *         check verifies the validity of the signature(s). {@code false} if *         the associated JAR file has no entries at all in its {@code *         META-INF} directory. This situation is indicative of an invalid *         JAR file. *         <p> *         Will also return {@code true} if the JAR file is <i>not</i> *         signed. * @throws SecurityException *             if the JAR file is signed and it is determined that a *             signature block file contains an invalid signature for the *             corresponding signature file. */synchronized boolean readCertificates() { if (metaEntries.isEmpty()) {  return false; } Iterator<String> it = metaEntries.keySet().iterator(); while (it.hasNext()) {  String key = it.next();  if (key.endsWith(".DSA") || key.endsWith(".RSA") || key.endsWith(".EC")) {   verifyCertificate(key);   it.remove();  } } return true;}

这个方法其实很简单，就是判断metaEntries中是否为空，说白了，就是判断Apk中的META-INF文件夹中是否为空，只有文件就返回true。再来看看isSignedJar方法：

/** * Returns a <code>boolean</code> indication of whether or not the * associated jar file is signed. * * @return {@code true} if the JAR is signed, {@code false} *         otherwise. */boolean isSignedJar() { return certificates.size() > 0;}

这个方法直接判断certificates这个集合是否为空。我们全局搜索一下这个集合在哪里存入的数据的地方，找到了verifyCertificate方法，同时我们发现，在上面的readCertificates方法中，就调用了这个方法，其实这个方法就是读取证书信息的。

下面来看一下verifyCertificate方法：

/** * @param certFile */private void verifyCertificate(String certFile) { // Found Digital Sig, .SF should already have been read String signatureFile = certFile.substring(0, certFile.lastIndexOf('.')) + ".SF"; byte[] sfBytes = metaEntries.get(signatureFile); if (sfBytes == null) {  return; } byte[] manifestBytes = metaEntries.get(JarFile.MANIFEST_NAME); // Manifest entry is required for any verifications. if (manifestBytes == null) {  return; } byte[] sBlockBytes = metaEntries.get(certFile); try {  Certificate[] signerCertChain = JarUtils.verifySignature(    new ByteArrayInputStream(sfBytes),    new ByteArrayInputStream(sBlockBytes));  if (signerCertChain != null) {   certificates.put(signatureFile, signerCertChain);  } } catch (IOException e) {  return; } catch (GeneralSecurityException e) {  throw failedVerification(jarName, signatureFile); } // Verify manifest hash in .sf file Attributes attributes = new Attributes(); HashMap<String, Attributes> entries = new HashMap<String, Attributes>(); try {  ManifestReader im = new ManifestReader(sfBytes, attributes);  im.readEntries(entries, null); } catch (IOException e) {  return; } // Do we actually have any signatures to look at? if (attributes.get(Attributes.Name.SIGNATURE_VERSION) == null) {  return; } boolean createdBySigntool = false; String createdBy = attributes.getValue("Created-By"); if (createdBy != null) {  createdBySigntool = createdBy.indexOf("signtool") != -1; } // Use .SF to verify the mainAttributes of the manifest // If there is no -Digest-Manifest-Main-Attributes entry in .SF // file, such as those created before java 1.5, then we ignore // such verification. if (mainAttributesEnd > 0 && !createdBySigntool) {  String digestAttribute = "-Digest-Manifest-Main-Attributes";  if (!verify(attributes, digestAttribute, manifestBytes, 0, mainAttributesEnd, false, true)) {   throw failedVerification(jarName, signatureFile);  } } // Use .SF to verify the whole manifest. String digestAttribute = createdBySigntool ? "-Digest" : "-Digest-Manifest"; if (!verify(attributes, digestAttribute, manifestBytes, 0, manifestBytes.length, false, false)) {  Iterator<Map.Entry<String, Attributes>> it = entries.entrySet().iterator();  while (it.hasNext()) {   Map.Entry<String, Attributes> entry = it.next();   Manifest.Chunk chunk = manifest.getChunk(entry.getKey());   if (chunk == null) {    return;   }   if (!verify(entry.getValue(), "-Digest", manifestBytes,     chunk.start, chunk.end, createdBySigntool, false)) {    throw invalidDigest(signatureFile, entry.getKey(), jarName);   }  } } metaEntries.put(signatureFile, null); signatures.put(signatureFile, entries);}

2》获取证书信息，并且验证CERT.SF文件的签名信息和CERT.RSA中的内容是否一致。

// Found Digital Sig, .SF should already have been readString signatureFile = certFile.substring(0, certFile.lastIndexOf('.')) + ".SF";byte[] sfBytes = metaEntries.get(signatureFile);if (sfBytes == null) { return;}byte[] manifestBytes = metaEntries.get(JarFile.MANIFEST_NAME);// Manifest entry is required for any verifications.if (manifestBytes == null) { return;}byte[] sBlockBytes = metaEntries.get(certFile);try { Certificate[] signerCertChain = JarUtils.verifySignature(   new ByteArrayInputStream(sfBytes),   new ByteArrayInputStream(sBlockBytes)); if (signerCertChain != null) {  certificates.put(signatureFile, signerCertChain); }} catch (IOException e) { return;} catch (GeneralSecurityException e) { throw failedVerification(jarName, signatureFile);}

这里首先获取到，签名文件。我们在之前的一篇文章中说到了，签名文件和证书文件的名字是一样的。

同时这里还调用了JarUtils类：libcore\luni\src\main\java\org\apache\harmony\security\utils\JarUtils.java

中的verifySignature方法来获取证书，这里就不做太多的解释了，如何从一个RSA文件中获取证书，这样的代码网上也是有的，而且后面我会演示一下，如何获取。

/** * This method handle all the work with  PKCS7, ASN1 encoding, signature verifying, * and certification path building. * See also PKCS #7: Cryptographic Message Syntax Standard: * http://www.ietf.org/rfc/rfc2315.txt * @param signature - the input stream of signature file to be verified * @param signatureBlock - the input stream of corresponding signature block file * @return array of certificates used to verify the signature file * @throws IOException - if some errors occurs during reading from the stream * @throws GeneralSecurityException - if signature verification process fails */public static Certificate[] verifySignature(InputStream signature, InputStream  signatureBlock) throws IOException, GeneralSecurityException { BerInputStream bis = new BerInputStream(signatureBlock); ContentInfo info = (ContentInfo)ContentInfo.ASN1.decode(bis); SignedData signedData = info.getSignedData(); if (signedData == null) {  throw new IOException("No SignedData found"); } Collection<org.apache.harmony.security.x509.Certificate> encCerts = signedData.getCertificates(); if (encCerts.isEmpty()) {  return null; } X509Certificate[] certs = new X509Certificate[encCerts.size()]; int i = 0; for (org.apache.harmony.security.x509.Certificate encCert : encCerts) {  certs[i++] = new X509CertImpl(encCert); } List<SignerInfo> sigInfos = signedData.getSignerInfos(); SignerInfo sigInfo; if (!sigInfos.isEmpty()) {  sigInfo = sigInfos.get(0); } else {  return null; } // Issuer X500Principal issuer = sigInfo.getIssuer(); // Certificate serial number BigInteger snum = sigInfo.getSerialNumber(); // Locate the certificate int issuerSertIndex = 0; for (i = 0; i < certs.length; i++) {  if (issuer.equals(certs[i].getIssuerDN()) &&    snum.equals(certs[i].getSerialNumber())) {   issuerSertIndex = i;   break;  } } if (i == certs.length) { // No issuer certificate found  return null; } if (certs[issuerSertIndex].hasUnsupportedCriticalExtension()) {  throw new SecurityException("Can not recognize a critical extension"); } // Get Signature instance Signature sig = null; String da = sigInfo.getDigestAlgorithm(); String dea = sigInfo.getDigestEncryptionAlgorithm(); String alg = null; if (da != null && dea != null) {  alg = da + "with" +  dea;  try {   sig = Signature.getInstance(alg, OpenSSLProvider.PROVIDER_NAME);  } catch (NoSuchAlgorithmException e) {} } if (sig == null) {  alg = da;  if (alg == null) {   return null;  }  try {   sig = Signature.getInstance(alg, OpenSSLProvider.PROVIDER_NAME);  } catch (NoSuchAlgorithmException e) {   return null;  } } sig.initVerify(certs[issuerSertIndex]); ......

这里返回的是一个证书的数组。

3、MANIFEST.MF整个文件签名在CERT.SF文件中头属性中的值是否匹配以及验证MANIFEST.MF文件中的各个属性块的签名在CERT.SF文件中是否匹配

1》第一件事是：验证MANIFEST.MF整个文件签名在CERT.SF文件中头属性中的值是否匹配

// Use .SF to verify the mainAttributes of the manifest// If there is no -Digest-Manifest-Main-Attributes entry in .SF// file, such as those created before java 1.5, then we ignore// such verification.if (mainAttributesEnd > 0 && !createdBySigntool) { String digestAttribute = "-Digest-Manifest-Main-Attributes"; if (!verify(attributes, digestAttribute, manifestBytes, 0, mainAttributesEnd, false, true)) {  throw failedVerification(jarName, signatureFile); }}

这里的manifestBytes：

byte[] manifestBytes = metaEntries.get(JarFile.MANIFEST_NAME);

就是MANIFEST.MF文件内容。继续看一下verify方法：

private boolean verify(Attributes attributes, String entry, byte[] data,  int start, int end, boolean ignoreSecondEndline, boolean ignorable) { for (int i = 0; i < DIGEST_ALGORITHMS.length; i++) {  String algorithm = DIGEST_ALGORITHMS[i];  String hash = attributes.getValue(algorithm + entry);  if (hash == null) {   continue;  }  MessageDigest md;  try {   md = MessageDigest.getInstance(algorithm);  } catch (NoSuchAlgorithmException e) {   continue;  }  if (ignoreSecondEndline && data[end - 1] == '\n' && data[end - 2] == '\n') {   md.update(data, start, end - 1 - start);  } else {   md.update(data, start, end - start);  }  byte[] b = md.digest();  byte[] hashBytes = hash.getBytes(StandardCharsets.ISO_8859_1);  return MessageDigest.isEqual(b, Base64.decode(hashBytes)); } return ignorable;}

这个方法其实很简单，就是验证传入的data数据块的数据摘要算法和传入的attributes中的算法块的值是否匹配，比如这里：

String algorithm = DIGEST_ALGORITHMS[i];String hash = attributes.getValue(algorithm + entry);

这里的algorithm是算法：

private static final String[] DIGEST_ALGORITHMS = new String[] { "SHA-512", "SHA-384", "SHA-256", "SHA1",};

这里的entry也是传入的，我们看到传入的是：-Digest

这样就是CERT.SF文件中的一个条目：

2》第二件事是：验证MANIFEST.MF文件中的各个属性块的签名在CERT.SF文件中是否匹配

// Use .SF to verify the whole manifest.String digestAttribute = createdBySigntool ? "-Digest" : "-Digest-Manifest";if (!verify(attributes, digestAttribute, manifestBytes, 0, manifestBytes.length, false, false)) { Iterator<Map.Entry<String, Attributes>> it = entries.entrySet().iterator(); while (it.hasNext()) {  Map.Entry<String, Attributes> entry = it.next();  Manifest.Chunk chunk = manifest.getChunk(entry.getKey());  if (chunk == null) {   return;  }  if (!verify(entry.getValue(), "-Digest", manifestBytes,    chunk.start, chunk.end, createdBySigntool, false)) {   throw invalidDigest(signatureFile, entry.getKey(), jarName);  } }}

这里我们可以看到也是同样调用verify方法来验证CERT.SF中的条目信息的。

最后我们再看一下是如何配对签名信息的，在PackageParser中的collectCertificates方法：

这里会比对已经安装的apk的签名和准备要安装的apk的签名是否一致，如果不一致的话，就会报错：

这个错，也是我们经常会遇到的，就是同样的apk，签名不一致导致的问题。

我们从上面的分析代码中可以看到，这里的Signature比对签名，其实就是比对证书中的公钥信息：

上面我们就看完了Android中验证签名信息的流程，下面我们再来梳理一下流程吧：

所有有关apk文件的签名验证工作都是在JarVerifier里面做的，一共分成三步：

1、JarVerifier.VerifierEntry.verify做了验证，即保证apk文件中包含的所有文件，对应的摘要值与MANIFEST.MF文件中记录的一致。

2、JarVeirifer.verifyCertificate使用证书文件(在META-INF目录下，以.DSA、.RSA或者.EC结尾的文件)检验签名文件(在META-INF目录下，和证书文件同名，但扩展名为.SF的文件)是没有被修改过的。这里我们可以注意到，Android中在验证的过程中对SF喝RSA文件的名字并不关心，这个在之前的签名过程文章中介绍到了。

3、JarVeirifer.verifyCertificate中使用签名文件CERT.SF，检验MANIFEST.MF文件中的内容也没有被篡改过

综上所述：

首先，如果你改变了apk包中的任何文件，那么在apk安装校验时，改变后的文件摘要信息与MANIFEST.MF的检验信息不同，于是验证失败，程序就不能成功安装。
其次，如果你对更改的过的文件相应的算出新的摘要值，然后更改MANIFEST.MF文件里面对应的属性值，那么必定与CERT.SF文件中算出的摘要值不一样，照样验证失败。
这里都会提示安装失败信息：

如果你还不死心，继续计算MANIFEST.MF的摘要值，相应的更改CERT.SF里面的值.

那么数字签名值必定与CERT.RSA文件中记录的不一样，还是失败。

这里的失败信息：

那么能不能继续伪造数字签名呢？不可能，因为没有数字证书对应的私钥。
所以，如果要重新打包后的应用程序能再Android设备上安装，必须对其进行重签名。
从上面的分析可以得出，只要修改了Apk中的任何内容，就必须重新签名，不然会提示安装失败，当然这里不会分析，后面一篇文章会注重分析为何会提示安装失败。

总结

到这里我们就介绍完了Android中的apk的签名验证过程，再结合之前的一篇文章，我们可以了解到了Android中的签名机制了。这个也是对Android中的安全机制的一个深入了解吧，新年快乐~~