Android签名机制之---签名验证过程详解

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：

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

         // If we're parsing an untrusted package, verify all contents
         if ((flags & PARSE_IS_SYSTEM) == 0) {
             final Iterator 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();
                 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();
         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 certChains = new ArrayList();
     Iterator>> it = signatures.entrySet().iterator();
     while (it.hasNext()) {
         Map.Entry> entry = it.next();
         HashMap 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 verifiedEntries;
     VerifierEntry(String name, MessageDigest digest, byte[] hash,
             Certificate[][] certChains, Hashtable 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 not
       *             agree with the decrypted digest as recovered from the
       *             .SF 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.
         HashMapbyte[]> 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.
  *
  *         Will also return {@code true} if the JAR file is not
  *         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 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 boolean 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 entries = new HashMap();
     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> it = entries.entrySet().iterator();
         while (it.hasNext()) {
             Map.Entry 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 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);
 }

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 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 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> it = entries.entrySet().iterator();
     while (it.hasNext()) {
         Map.Entry 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~~