一、什么是加壳？加壳是在二进制的程序中植入一段代码，在运行的时候优先取得程序的控制权，做一些额外的工作。大多数病毒就是基于此原理。
二、加壳作用加壳的程序可以有效阻止对程序的反汇编分析，以达到它不可告人的目的。这种技术也常用来保护软件版权，防止被软件破解。
三、Android Dex文件加壳原理PC平台现在已存在大量的标准的加壳和解壳工具，但是Android作为新兴平台还未出现APK加壳工具。Android Dex文件大量使用引用给加壳带来了一定的难度，但是从理论上讲，Android APK加壳也是可行的。在这个过程中，牵扯到三个角色：1、加壳程序：加密源程序为解壳数据、组装解壳程序和解壳数据2、解壳程序：解密解壳数据，并运行时通过DexClassLoader动态加载3、源程序：需要加壳处理的被保护代码根据解壳数据在解壳程序DEX文件中的不同分布，本文将提出两种Android Dex加壳的实现方案。
解壳数据位于解壳程序文件尾部：该种方式简单实用，合并后的DEX文件结构如下。
四、加壳程序工作流程：1、加密源程序APK文件为解壳数据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。4、修改源程序AndroidMainfest.xml文件并覆盖解壳程序AndroidMainfest.xml文件。
五、解壳DEX程序工作流程：1、读取DEX文件末尾数据获取借壳数据长度。2、从DEX文件读取解壳数据，解密解壳数据。以文件形式保存解密数据到a.APK文件3、通过DexClassLoader动态加载a.apk。
解壳数据位于解壳程序文件头该种方式相对比较复杂， 合并后DEX文件结构如下：
六、加壳程序工作流程：1、加密源程序APK文件为解壳数据2、计算解壳数据长度，并添加该长度到解壳DEX文件头末尾，并继续解壳数据到文件头末尾。（插入数据的位置为0x70处）3、修改解壳程序DEX头中checksum、signature、file_size、header_size、string_ids_off、type_ids_off、proto_ids_off、field_ids_off、method_ids_off、class_defs_off和data_off相关项。 分析map_off 数据，修改相关的数据偏移量。 4、修改源程序AndroidMainfest.xml文件并覆盖解壳程序AndroidMainfest.xml文件。
七、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 stub try { 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 block e.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(); } } } }
八、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。
<application android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" android:name=" </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。
<application android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" android:name=" </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; @Override public void onCreate() { // TODO Auto-generated method stub super.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 block e.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)dLoader.loadClass(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 block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (NoSuchMethodException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (InvocationTargetException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (NoSuchMethodException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (InvocationTargetException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (NoSuchFieldException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (NoSuchFieldException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (NoSuchFieldException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.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 block e.printStackTrace(); } catch (NoSuchFieldException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalArgumentException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (IllegalAccessException e) { // TODO Auto-generated catch block e.printStackTrace(); } catch (ClassNotFoundException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
九、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。
父母养我不容易，我在学校争口气。
