首页 > 代码库 > [转载] Android签名机制之—签名验证过程详解
[转载] Android签名机制之—签名验证过程详解
本文转载自: http://www.wjdiankong.cn/android%E7%AD%BE%E5%90%8D%E6%9C%BA%E5%88%B6%E4%B9%8B-%E7%AD%BE%E5%90%8D%E9%AA%8C%E8%AF%81%E8%BF%87%E7%A8%8B%E8%AF%A6%E8%A7%A3/
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:点击进入
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<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 = http://www.mamicode.com/info.getSignedData();"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~~
[转载] Android签名机制之—签名验证过程详解