为Android的apk应用程序文件加壳以防止反编译的教程
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。