本文章由Jack_Jia编写，转载请注明出处。

文章链接：http://blog.csdn.net/jiazhijun/article/details/8746917

作者：Jack_Jia    邮箱： 309zhijun@163.com

一、序言

在上篇“Android APK加壳技术方案”（http://blog.csdn.net/jiazhijun/article/details/8678399）博文中，根据加壳数据在解壳程序Dex文件所处的位置，我提出了两种Android Dex加壳技术实现方案，本片博文将对方案1代码实现进行讲解。博友可以根据方案1的代码实现原理对方案2自行实现。

在方案1的代码实现过程中，各种不同的问题接踵出现，最初的方案也在不同问题的出现、解决过程中不断的得到调整、优化。

本文的代码实现了对整个APK包的加壳处理。加壳程序不会对源程序有任何的影响。

二、代码实现

本程序基于Android2.3代码实现，因为牵扯到系统代码的反射修改，本程序不保证在其它android版本正常工作，博友可以根据实现原理，自行实现对其它Android版本的兼容性开发。

1、 加壳程序流程及代码实现

1、加密源程序APK为解壳数据

2、把解壳数据写入解壳程序DEX文件末尾，并在文件尾部添加解壳数据的大小。

3、修改解壳程序DEX头中checksum、signature 和file_size头信息。

代码实现如下：

package com.android.dexshell;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.zip.Adler32;public class DexShellTool {/*** @param args*/public static void main(String[] args) {// TODO Auto-generated method stubtry {File payloadSrcFile = new File("g:/payload.apk");File unShellDexFile = new File("g:/unshell.dex");byte[] payloadArray = encrpt(readFileBytes(payloadSrcFile));byte[] unShellDexArray = readFileBytes(unShellDexFile);int payloadLen = payloadArray.length;int unShellDexLen = unShellDexArray.length;int totalLen = payloadLen + unShellDexLen +4;byte[] newdex = new byte[totalLen];//添加解壳代码System.arraycopy(unShellDexArray, 0, newdex, 0, unShellDexLen);//添加加密后的解壳数据System.arraycopy(payloadArray, 0, newdex, unShellDexLen,payloadLen);//添加解壳数据长度System.arraycopy(intToByte(payloadLen), 0, newdex, totalLen-4, 4);//修改DEX file size文件头fixFileSizeHeader(newdex);//修改DEX SHA1 文件头fixSHA1Header(newdex);//修改DEX CheckSum文件头fixCheckSumHeader(newdex);String str = "g:/classes.dex";File file = new File(str);if (!file.exists()) {file.createNewFile();}FileOutputStream localFileOutputStream = new FileOutputStream(str);localFileOutputStream.write(newdex);localFileOutputStream.flush();localFileOutputStream.close();} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}}//直接返回数据，读者可以添加自己加密方法private static byte[] encrpt(byte[] srcdata){return srcdata;}private static void fixCheckSumHeader(byte[] dexBytes) {Adler32 adler = new Adler32();adler.update(dexBytes, 12, dexBytes.length - 12);long value = adler.getValue();int va = (int) value;byte[] newcs = intToByte(va);byte[] recs = new byte[4];for (int i = 0; i < 4; i++) {recs[i] = newcs[newcs.length - 1 - i];System.out.println(Integer.toHexString(newcs[i]));}System.arraycopy(recs, 0, dexBytes, 8, 4);System.out.println(Long.toHexString(value));System.out.println();}public static byte[] intToByte(int number) {byte[] b = new byte[4];for (int i = 3; i >= 0; i--) {b[i] = (byte) (number % 256);number >>= 8;}return b;}private static void fixSHA1Header(byte[] dexBytes)throws NoSuchAlgorithmException {MessageDigest md = MessageDigest.getInstance("SHA-1");md.update(dexBytes, 32, dexBytes.length - 32);byte[] newdt = md.digest();System.arraycopy(newdt, 0, dexBytes, 12, 20);String hexstr = "";for (int i = 0; i < newdt.length; i++) {hexstr += Integer.toString((newdt[i] & 0xff) + 0x100, 16).substring(1);}System.out.println(hexstr);}private static void fixFileSizeHeader(byte[] dexBytes) {byte[] newfs = intToByte(dexBytes.length);System.out.println(Integer.toHexString(dexBytes.length));byte[] refs = new byte[4];for (int i = 0; i < 4; i++) {refs[i] = newfs[newfs.length - 1 - i];System.out.println(Integer.toHexString(newfs[i]));}System.arraycopy(refs, 0, dexBytes, 32, 4);}private static byte[] readFileBytes(File file) throws IOException {byte[] arrayOfByte = new byte[1024];ByteArrayOutputStream localByteArrayOutputStream = new ByteArrayOutputStream();FileInputStream fis = new FileInputStream(file);while (true) {int i = fis.read(arrayOfByte);if (i != -1) {localByteArrayOutputStream.write(arrayOfByte, 0, i);} else {return localByteArrayOutputStream.toByteArray();}}}}

2、 解壳程序流程及代码实现

在解壳程序的开发过程中需要解决如下几个关键的技术问题：

（1）解壳代码如何能够第一时间执行？

Android程序由不同的组件构成，系统在有需要的时候启动程序组件。因此解壳程序必须在Android系统启动组件之前运行，完成对解壳数                据的解壳及APK文件的动态加载，否则会使程序出现加载类失败的异常。

Android开发者都知道Applicaiton做为整个应用的上下文，会被系统第一时间调用，这也是应用开发者程序代码的第一执行点。因此通过对              AndroidMainfest.xml的application的配置可以实现解壳代码第一时间运行。

    <applicationandroid:icon="@drawable/ic_launcher"android:label="@string/app_name"android:theme="@style/AppTheme" android:name="com.android.dexunshell.ProxyApplication" ></application>

（2）如何替换回源程序原有的Application?

当在AndroidMainfest.xml文件配置为解壳代码的Application时。源程序原有的Applicaiton将被替换，为了不影响源程序代码逻辑，我们需要              在解壳代码运行完成后，替换回源程序原有的Application对象。我们通过在AndroidMainfest.xml文件中配置原有Applicaiton类信息来达到我们              的目的。解壳程序要在运行完毕后通过创建配置的Application对象，并通过反射修改回原Application。

    <applicationandroid:icon="@drawable/ic_launcher"android:label="@string/app_name"android:theme="@style/AppTheme" android:name="com.android.dexunshell.ProxyApplication" ><meta-data android:name="APPLICATION_CLASS_NAME" android:value="com.***.Application"/></application>

（3）如何通过DexClassLoader实现对apk代码的动态加载。

我们知道DexClassLoader加载的类是没有组件生命周期的，也就是说即使DexClassLoader通过对APK的动态加载完成了对组件类的加载，              当系统启动该组件时，还会出现加载类失败的异常。为什么组件类被动态加载入虚拟机，但系统却出现加载类失败呢？

通过查看Android源代码我们知道组件类的加载是由另一个ClassLoader来完成的，DexClassLoader和系统组件ClassLoader并不存在关                  系，系统组件ClassLoader当然找不到由DexClassLoader加载的类，如果把系统组件ClassLoader的parent修改成DexClassLoader，我们就可              以实现对apk代码的动态加载。

（4）如何使解壳后的APK资源文件被代码动态引用。

代码默认引用的资源文件在最外层的解壳程序中，因此我们要增加系统的资源加载路径来实现对借壳后APK文件资源的加载。

解壳实现代码：

package com.android.dexunshell;import java.io.BufferedInputStream;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.zip.ZipEntry;
import java.util.zip.ZipInputStream;import dalvik.system.DexClassLoader;
import android.app.Application;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.content.pm.PackageManager.NameNotFoundException;
import android.os.Bundle;public class ProxyApplication extends Application {private static final String appkey = "APPLICATION_CLASS_NAME";private String apkFileName;private String odexPath;private String libPath;@Overridepublic void onCreate() {// TODO Auto-generated method stubsuper.onCreate();try {File odex = this.getDir("payload_odex", MODE_PRIVATE);File libs = this.getDir("payload_lib", MODE_PRIVATE);odexPath = odex.getAbsolutePath();libPath = libs.getAbsolutePath();apkFileName = odex.getAbsolutePath()+"/payload.apk";File dexFile = new File(apkFileName);if(!dexFile.exists())dexFile.createNewFile();//读取程序classes.dex文件byte[] dexdata = this.readDexFileFromApk();//分离出解壳后的apk文件已用于动态加载this.splitPayLoadFromDex(dexdata);//配置动态加载环境this.configApplicationEnv();} catch (Exception e) {// TODO Auto-generated catch blocke.printStackTrace();}}private void configApplicationEnv() throws NameNotFoundException, IllegalAccessException, InstantiationException, ClassNotFoundException, IOException{ Object currentActivityThread = RefInvoke.invokeStaticMethod("android.app.ActivityThread", "currentActivityThread", new Class[]{}, new Object[]{});HashMap mPackages = (HashMap)RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread, "mPackages");//替换组件类加载器为DexClassLoader，已使动态加载代码具有组件生命周期WeakReference wr = (WeakReference) mPackages.get(this.getPackageName());DexClassLoader dLoader = new DexClassLoader(apkFileName,odexPath, libPath, (ClassLoader) RefInvoke.getFieldOjbect("android.app.LoadedApk", wr.get(), "mClassLoader"));RefInvoke.setFieldOjbect("android.app.LoadedApk", "mClassLoader", wr.get(), dLoader);//如果源应用配置有Appliction对象，则替换为源应用Applicaiton，以便不影响源程序逻辑。ApplicationInfo appInfo = this.getPackageManager().getApplicationInfo(this.getPackageName(),PackageManager.GET_META_DATA);Bundle bundle =  appInfo.metaData;if(bundle != null && bundle.containsKey(appkey)){String appClassName = bundle.getString(appkey);Application app = (Application)Class.forName(appClassName).newInstance();RefInvoke.setFieldOjbect("android.app.ContextImpl", "mOuterContext", this.getBaseContext(), app);RefInvoke.setFieldOjbect("android.content.ContextWrapper", "mBase", app, this.getBaseContext());Object mBoundApplication = RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread, "mBoundApplication");Object info = RefInvoke.getFieldOjbect("android.app.ActivityThread$AppBindData", mBoundApplication, "info");RefInvoke.setFieldOjbect("android.app.LoadedApk", "mApplication", info, app);Object oldApplication = RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread, "mInitialApplication");RefInvoke.setFieldOjbect("android.app.ActivityThread", "mInitialApplication", currentActivityThread, app);ArrayList<Application> mAllApplications = (ArrayList<Application>)RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread, "mAllApplications");mAllApplications.remove(oldApplication);mAllApplications.add(app);HashMap mProviderMap = (HashMap) RefInvoke.getFieldOjbect("android.app.ActivityThread", currentActivityThread, "mProviderMap");Iterator it = mProviderMap.values().iterator();while(it.hasNext()){Object providerClientRecord = it.next();Object localProvider = RefInvoke.getFieldOjbect("android.app.ProviderClientRecord", providerClientRecord, "mLocalProvider");RefInvoke.setFieldOjbect("android.content.ContentProvider", "mContext", localProvider, app);}RefInvoke.invokeMethod(appClassName, "onCreate", app, new Class[]{}, new Object[]{});}}private void splitPayLoadFromDex(byte[] data) throws IOException{byte[] apkdata = decrypt(data);int ablen = apkdata.length;byte[] dexlen = new byte[4];System.arraycopy(apkdata, ablen - 4, dexlen, 0, 4);ByteArrayInputStream bais = new ByteArrayInputStream(dexlen);DataInputStream in = new DataInputStream(bais);int readInt = in.readInt();System.out.println(Integer.toHexString(readInt));byte[] newdex = new byte[readInt];System.arraycopy(apkdata, ablen - 4 - readInt, newdex, 0, readInt);File file = new File(apkFileName);try {FileOutputStream localFileOutputStream = new FileOutputStream(file);localFileOutputStream.write(newdex);localFileOutputStream.close();} catch (IOException localIOException) {throw new RuntimeException(localIOException);}ZipInputStream localZipInputStream = new ZipInputStream(new BufferedInputStream(new FileInputStream(file)));while (true) {ZipEntry localZipEntry = localZipInputStream.getNextEntry();if (localZipEntry == null) {localZipInputStream.close();break;}String name = localZipEntry.getName();if (name.startsWith("lib/") && name.endsWith(".so")) {File storeFile = new File(libPath+"/"+name.substring(name.lastIndexOf('/')));storeFile.createNewFile();FileOutputStream fos = new FileOutputStream(storeFile);byte[] arrayOfByte = new byte[1024];while (true) {int i = localZipInputStream.read(arrayOfByte);if (i == -1)break;fos.write(arrayOfByte, 0, i);}fos.flush();fos.close(); }localZipInputStream.closeEntry();}localZipInputStream.close(); }private byte[] readDexFileFromApk() throws IOException {ByteArrayOutputStream dexByteArrayOutputStream = new ByteArrayOutputStream();ZipInputStream localZipInputStream = new ZipInputStream(new BufferedInputStream(new FileInputStream(this.getApplicationInfo().sourceDir)));while (true) {ZipEntry localZipEntry = localZipInputStream.getNextEntry();if (localZipEntry == null) {localZipInputStream.close();break;}if (localZipEntry.getName().equals("classes.dex")) {byte[] arrayOfByte = new byte[1024];while (true) {int i = localZipInputStream.read(arrayOfByte);if (i == -1)break;dexByteArrayOutputStream.write(arrayOfByte, 0, i);}}localZipInputStream.closeEntry();}localZipInputStream.close();return dexByteArrayOutputStream.toByteArray();}直接返回数据，读者可以添加自己解密方法private byte[] decrypt(byte[] data){return data;}
}

RefInvoke为反射调用工具类：

package com.android.dexunshell;import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;public class RefInvoke {public static  Object invokeStaticMethod(String class_name, String method_name, Class[] pareTyple, Object[] pareVaules){try {Class obj_class = Class.forName(class_name);Method method = obj_class.getMethod(method_name,pareTyple);return method.invoke(null, pareVaules);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();}  catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchMethodException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (InvocationTargetException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}return null;}public static  Object invokeMethod(String class_name, String method_name, Object obj ,Class[] pareTyple, Object[] pareVaules){try {Class obj_class = Class.forName(class_name);Method method = obj_class.getMethod(method_name,pareTyple);return method.invoke(obj, pareVaules);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();}  catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchMethodException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (InvocationTargetException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}return null;}public static Object getFieldOjbect(String class_name,Object obj, String filedName){try {Class obj_class = Class.forName(class_name);Field field = obj_class.getDeclaredField(filedName);field.setAccessible(true);return field.get(obj);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchFieldException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}return null;}public static Object getStaticFieldOjbect(String class_name, String filedName){try {Class obj_class = Class.forName(class_name);Field field = obj_class.getDeclaredField(filedName);field.setAccessible(true);return field.get(null);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchFieldException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}return null;}public static void setFieldOjbect(String classname, String filedName, Object obj, Object filedVaule){try {Class obj_class = Class.forName(classname);Field field = obj_class.getDeclaredField(filedName);field.setAccessible(true);field.set(obj, filedVaule);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchFieldException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}   }public static void setStaticOjbect(String class_name, String filedName, Object filedVaule){try {Class obj_class = Class.forName(class_name);Field field = obj_class.getDeclaredField(filedName);field.setAccessible(true);field.set(null, filedVaule);} catch (SecurityException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (NoSuchFieldException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalArgumentException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}     }}

三、总结

本文代码基本实现了APK文件的加壳及脱壳原理，该代码作为实验代码还有诸多地方需要改进。比如：

1、加壳数据的加密算法的添加。

2、脱壳代码由java语言实现，可通过C代码的实现对脱壳逻辑进行保护，以达到更好的反逆向分析效果。