android安全学习之6—signapk分析
Android签名的时候使用signapk进行签名，是个java的应用程序，代码为build/tools/signapk.java，下面简要分析下签名的流程。
java -jar out/host/linux-x86/framework/signapk.jar build/target/product/security/platform.x509.pem build/target/product/security/platform.pk8 before_sign.apk signed.apk
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String inputFilename = args[args.length-2];
String outputFilename = args[args.length-1];inputFilename 即为before_sign.apk，而outputFilename为signed.apk。
int numKeys = ((args.length - argstart) / 2) - 1;argstart为0，numKeys 为1 ，有一对密钥，即一个数字证书，一个私钥
File firstPublicKeyFile = new File(args[argstart+0]);数字证书文件，即为platform.x509.pem
X509Certificate[] publicKey = new X509Certificate[numKeys];
try {
for (int i = 0; i < numKeys; ++i) {
int argNum = argstart + i*2;
publicKey[i] = readPublicKey(new File(args[argNum]));
hashes |= getDigestAlgorithm(publicKey[i]);
}
} catch (IllegalArgumentException e) {
System.err.println(e);
System.exit(1);
}上面创建了一个X509Certificate数组，用来保存数字证书，看来可以有多组密钥对。
调用–>readPublicKey()
private static X509Certificate readPublicKey(File file)
throws IOException, GeneralSecurityException {
FileInputStream input = new FileInputStream(file);
try {
CertificateFactory cf = CertificateFactory.getInstance("X.509");
return (X509Certificate) cf.generateCertificate(input);
} finally {
input.close();
}
}输入是platform.x509.pem，很经典的读取数字证书代码。
调用–>getDigestAlgorithm()
// bitmasks for which hash algorithms we need the manifest to include.
private static final int USE_SHA1 = 1;
private static final int USE_SHA256 = 2;
private static int getDigestAlgorithm(X509Certificate cert) {
String sigAlg = cert.getSigAlgName().toUpperCase(Locale.US);
if ("SHA1WITHRSA".equals(sigAlg) ||
"MD5WITHRSA".equals(sigAlg)) { // see "HISTORICAL NOTE" above.
return USE_SHA1;
} else if (sigAlg.startsWith("SHA256WITH")) {
return USE_SHA256;
} else {
throw new IllegalArgumentException("unsupported signature algorithm \"" + sigAlg +
"\" in cert [" + cert.getSubjectDN());
}
}利用cert.getSigAlgName()获取签名的算法，为md5WithRSAEncryption，则返回USE_SHA1。
读取私钥
PrivateKey[] privateKey = new PrivateKey[numKeys];
for (int i = 0; i < numKeys; ++i) {
int argNum = argstart + i*2 + 1;
privateKey[i] = readPrivateKey(new File(args[argNum]));
}调用–>readPrivateKey()
/** Read a PKCS#8 format private key. */
private static PrivateKey readPrivateKey(File file)
throws IOException, GeneralSecurityException {
DataInputStream input = new DataInputStream(new FileInputStream(file));
try {
byte[] bytes = new byte[(int) file.length()];
input.read(bytes);
/* Check to see if this is in an EncryptedPrivateKeyInfo structure. */
PKCS8EncodedKeySpec spec = decryptPrivateKey(bytes, file);
if (spec == null) {
spec = new PKCS8EncodedKeySpec(bytes);
}
/* * Now it's in a PKCS#8 PrivateKeyInfo structure. Read its Algorithm * OID and use that to construct a KeyFactory. */
ASN1InputStream bIn = new ASN1InputStream(new ByteArrayInputStream(spec.getEncoded()));
PrivateKeyInfo pki = PrivateKeyInfo.getInstance(bIn.readObject());
String algOid = pki.getPrivateKeyAlgorithm().getAlgorithm().getId();
return KeyFactory.getInstance(algOid).generatePrivate(spec);
} finally {
input.close();
}
}接着，
inputJar = new JarFile(new File(inputFilename), false); // Don't verify.
outputFile = new FileOutputStream(outputFilename);
JarOutputStream outputJar = new JarOutputStream(outputFile);
Manifest manifest = addDigestsToManifest(inputJar, hashes);
调用–>addDigestsToManifest()
/** * Add the hash(es) of every file to the manifest, creating it if * necessary. */
private static Manifest addDigestsToManifest(JarFile jar, int hashes)
throws IOException, GeneralSecurityException {
Manifest input = jar.getManifest();
Manifest output = new Manifest();
Attributes main = output.getMainAttributes();
if (input != null) {
main.putAll(input.getMainAttributes());
} else {
main.putValue("Manifest-Version", "1.0");
main.putValue("Created-By", "1.0 (Android SignApk)");
}
MessageDigest md_sha1 = null;
MessageDigest md_sha256 = null;
//上文是用的USE_SHA1
if ((hashes & USE_SHA1) != 0) {
md_sha1 = MessageDigest.getInstance("SHA1");
}
if ((hashes & USE_SHA256) != 0) {
md_sha256 = MessageDigest.getInstance("SHA256");
}
byte[] buffer = new byte[4096];
int num;
// We sort the input entries by name, and add them to the
// output manifest in sorted order. We expect that the output
// map will be deterministic.
TreeMap<String, JarEntry> byName = new TreeMap<String, JarEntry>();
for (Enumeration<JarEntry> e = jar.entries(); e.hasMoreElements(); ) {
JarEntry entry = e.nextElement();
byName.put(entry.getName(), entry);
}
for (JarEntry entry: byName.values()) {
String name = entry.getName();
//不是目录
if (!entry.isDirectory() &&
(stripPattern == null || !stripPattern.matcher(name).matches())) {
InputStream data = jar.getInputStream(entry);
while ((num = data.read(buffer)) > 0) {
if (md_sha1 != null) md_sha1.update(buffer, 0, num);
if (md_sha256 != null) md_sha256.update(buffer, 0, num);
}
Attributes attr = null;
if (input != null) attr = input.getAttributes(name);
attr = attr != null ? new Attributes(attr) : new Attributes();
if (md_sha1 != null) {
attr.putValue("SHA1-Digest",
new String(Base64.encode(md_sha1.digest()), "ASCII"));
}
if (md_sha256 != null) {
attr.putValue("SHA-256-Digest",
new String(Base64.encode(md_sha256.digest()), "ASCII"));
}
output.getEntries().put(name, attr);
}
}
return output;
}上述代码就是将所有非目录文件都进行hash计算，保存到一个Manifest 中，
调用–>signFile(),生成MANIFEST.MF，CERT.SF，CERT.RSA
signFile(manifest, inputJar, publicKey, privateKey, outputJar);
signFile是最重要的函数，
private static void signFile(Manifest manifest, JarFile inputJar,
X509Certificate[] publicKey, PrivateKey[] privateKey,
JarOutputStream outputJar)
throws Exception {
// Assume the certificate is valid for at least an hour.
long timestamp = publicKey[0].getNotBefore().getTime() + 3600L * 1000;
// MANIFEST.MF
//将manifest写入到MANIFEST.MF文件
JarEntry je = new JarEntry(JarFile.MANIFEST_NAME);
je.setTime(timestamp);
outputJar.putNextEntry(je);
manifest.write(outputJar);
int numKeys = publicKey.length;
for (int k = 0; k < numKeys; ++k) {
// CERT.SF / CERT#.SF
je = new JarEntry(numKeys == 1 ? CERT_SF_NAME :
(String.format(CERT_SF_MULTI_NAME, k)));
je.setTime(timestamp);
outputJar.putNextEntry(je);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
writeSignatureFile(manifest, baos, getDigestAlgorithm(publicKey[k]));
byte[] signedData = baos.toByteArray();
outputJar.write(signedData);
// CERT.{EC,RSA} / CERT#.{EC,RSA}
final String keyType = publicKey[k].getPublicKey().getAlgorithm();
je = new JarEntry(numKeys == 1 ?
(String.format(CERT_SIG_NAME, keyType)) :
(String.format(CERT_SIG_MULTI_NAME, k, keyType)));
je.setTime(timestamp);
outputJar.putNextEntry(je);
writeSignatureBlock(new CMSProcessableByteArray(signedData),
publicKey[k], privateKey[k], outputJar);
}
}调用writeSignatureFile（）写CERT.SF，
/** Write a .SF file with a digest of the specified manifest. */
private static void writeSignatureFile(Manifest manifest, OutputStream out,
int hash)
throws IOException, GeneralSecurityException {
Manifest sf = new Manifest();
Attributes main = sf.getMainAttributes();
main.putValue("Signature-Version", "1.0");
main.putValue("Created-By", "1.0 (Android SignApk)");
MessageDigest md = MessageDigest.getInstance(
hash == USE_SHA256 ? "SHA256" : "SHA1");
PrintStream print = new PrintStream(
new DigestOutputStream(new ByteArrayOutputStream(), md),
true, "UTF-8");
// Digest of the entire manifest
manifest.write(print);
print.flush();
main.putValue(hash == USE_SHA256 ? "SHA-256-Digest-Manifest" : "SHA1-Digest-Manifest",
new String(Base64.encode(md.digest()), "ASCII"));
Map<String, Attributes> entries = manifest.getEntries();
for (Map.Entry<String, Attributes> entry : entries.entrySet()) {
// Digest of the manifest stanza for this entry.
/*取MANIFEST.MF文件中的 Name: res/drawable-hdpi/option_file_select_icon.png SHA1-Digest: x089gM3+Hdujo12U0gnmf973qVA= */
print.print("Name: " + entry.getKey() + "\r\n");
for (Map.Entry<Object, Object> att : entry.getValue().entrySet()) {
print.print(att.getKey() + ": " + att.getValue() + "\r\n");
}
//同时添加一个空行\r\n
print.print("\r\n");
print.flush();
//计算上面三行的hash值
Attributes sfAttr = new Attributes();
sfAttr.putValue(hash == USE_SHA256 ? "SHA-256-Digest" : "SHA1-Digest-Manifest",
new String(Base64.encode(md.digest()), "ASCII"));
sf.getEntries().put(entry.getKey(), sfAttr);
}
CountOutputStream cout = new CountOutputStream(out);
sf.write(cout);
// A bug in the java.util.jar implementation of Android platforms
// up to version 1.6 will cause a spurious IOException to be thrown
// if the length of the signature file is a multiple of 1024 bytes.
// As a workaround, add an extra CRLF in this case.
if ((cout.size() % 1024) == 0) {
cout.write('\r');
cout.write('\n');
}
}调用writeSignatureBlock()对CERT.SF文件的hash进行签名，同时添加数字证书。
//输入CMSTypedData data就是CERT.SF中的文件内容
/** Sign data and write the digital signature to 'out'. */
private static void writeSignatureBlock(
CMSTypedData data, X509Certificate publicKey, PrivateKey privateKey,
OutputStream out)
throws IOException,
CertificateEncodingException,
OperatorCreationException,
CMSException {
//创建一个X509证书实例
ArrayList<X509Certificate> certList = new ArrayList<X509Certificate>(1);
certList.add(publicKey);
JcaCertStore certs = new JcaCertStore(certList);
CMSSignedDataGenerator gen = new CMSSignedDataGenerator();
ContentSigner signer = new JcaContentSignerBuilder(getSignatureAlgorithm(publicKey))
.setProvider(sBouncyCastleProvider)
.build(privateKey);
gen.addSignerInfoGenerator(
new JcaSignerInfoGeneratorBuilder(
new JcaDigestCalculatorProviderBuilder()
.setProvider(sBouncyCastleProvider)
.build())
.setDirectSignature(true)
.build(signer, publicKey));
//CERT.RSA中将证书添加进去
gen.addCertificates(certs);
//对CERT.SF文件中的内容进行签名
CMSSignedData sigData = gen.generate(data, false);
ASN1InputStream asn1 = new ASN1InputStream(sigData.getEncoded());
DEROutputStream dos = new DEROutputStream(out);
dos.writeObject(asn1.readObject());
}