Android签名机制之—签名验证过程详解

Android技术篇 尼古拉斯.赵四 14844℃ 0评论

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 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<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~~

 

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转载请注明:尼古拉斯.赵四 » Android签名机制之—签名验证过程详解

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