89 lines
6.6 KiB
Typst
89 lines
6.6 KiB
Typst
#import "../lib.typ": todo, APK, IDE, SDK, DEX, ADB, ART, eg, XML, AXML, API, jfl-note
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== Reverse Engineering Tools <sec:bg-tools>
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Due to the specificities of Android, reverse engineers need tools adapted to Android.
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The developement tools provided by Google can be used for basic operations.
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Apktool and Jadx are common tools used to read the content of an application, meanwhile Androguard and Soot can be used as librairy to automate analysis.
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For a more dynamic approach, Frida is a toolkit that can be use to intercept method call and execute custom while an application is running.
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=== Android Studio <sec:bg-android-studio>
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The whole Android developement ecosystem is packaged by Google in the #IDE Android Studio#footnote[https://developer.android.com/studio].
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In practice, Android Studio is a source-code editor that wrap arround the different tools of the android #SDK.
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The #SDK tools and packages can be installed manually with the `sdkmanager` tool.
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Among the notable tools in the #SDK, they are:
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- `emulator`: an Android emulator.
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This tools allow to run an emulated Android phone on a computer.
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Although very usefull, Android emulator has several limitation.
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For once, it cannot emulate another achitecture.
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An x86_64 computer cannot emulate an ARM smartphone.
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This can be an issue because a majority of smartphone run on ARM processor.
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Also, for certain version of Android, the proprietary GooglePlay libraries are not available on rooted emulators.
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Lastly, emulators are not designed to be stealthy and can easily be detected by an application.
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Malware will avoid detection by not running their payload on emulators.
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- #ADB: a tool to send commands to Android smartphone or emulator.
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It can be used to install applications, send instructions, events, and generally perform debuging operations.
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- Platform Packages: Those packages contains data associated to a version of android needed to compile an application.
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Especially, they contains the so call `android.jar` files, that contains the list of #API for a version of Android.
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- `d8`: The main use of `d8` is to convert java bytecode files (`.class`) to Android #DEX format.
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It can also be used to perform different level of optimization of the bytecode generated.
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- `aapt`/`aapt2` (Android Asset Packaging Tool): This tools is used to build the #APK file.
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It is commonly used by other tools that repackage applications like Apktool.
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Behind the scene, it we convert #XML to binary #AXML and ensure the right files have the right compression and alignment. (#eg some ressource files are mapped in memory by the #ART, and thus need to be aligned and not compressed).
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- `apksigner`: the tool used to sign an #APK file.
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When repackaging an application, for example with Apktool, the new application need to be signed.
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=== Apktool <sec:bg-apktool>
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Apktool#footnote[https://apktool.org/] is a _reengineering tool_ for Android #APK files.
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It can be used to disassemble an application: it will extract the files from the #APK file, convert the binary #AXML to text #XML, and use smali/backsmali#footnote[https://github.com/JesusFreke/smali] to convert the #DEX files to smali, an assembler-like langage that match the Dalvik bytecode instructions.
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The main strenght of Apktool is that after having disassemble an application, the content of the application can be edited and reassemble into a new #APK. #jfl-note[limites? ca marche toujours?]
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=== Androguard <sec:bg-androguard>
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Androguard#footnote[https://github.com/androguard/androguard]~@desnos:adnroguard:2011 is a python library for parsing and analysing #APK files.
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#jfl-note[Its main feature is disassembling #APK files.][backend #sym.eq.not apktool?]
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It can be used to automatically read Android manifests, ressources, and bytecode.
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Contrary to Apktool, it can be used programatically, whithout parsing text files, to analyse the application, but it cannot repackage a modified application.
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In addition, it can perform additionnal analysis, like computing a call graph or control flow graph.
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=== Jadx <sec:bg-jadx>
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Jadx#footnote[https://github.com/skylot/jadx] is an application decompiler.
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It convert #DEX files to Java source code.
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It is not always capable of decompiling all classes of an application, so it cannot be used to recompile a new application, but the code generated can be verry helpfull to reverse an application.
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In addition to decompilling #DEX files, Jadx can also decode Android manifests and application ressources.
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=== Soot <sec:bg-soot>
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Soot#footnote[https://github.com/soot-oss/soot]~@Arzt2013 is a Java optimization framework.
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It can leaft java bytecode to other intermediate representations that can be used to perform optimization then converted back to bytecode.
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Because Dalvik bytecode and Java bytecode are equivalent, support for Android was added to Soot, and Soot features are now leveraged to analyse Android applications.
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One of the best known example of Soot usage for Android analysis is Flowdroid~@Arzt2014a, a tool that compute data flow in an application.
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A new version of Soot, SootUp#footnote[https://github.com/soot-oss/SootUp], is currently beeing worked on.
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Compared to Soot, it has a modernize interface and architecture, but it is not yet feature complete and some tools like Flowdroid are still using Soot.
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=== Frida <sec:bg-frida>
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Fidra#footnote[https://frida.re/] is a dynamic intrumentation toolkit.
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It allows the reverse engineer to inject and run javascript code inside a running application.
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To instrument an application, the frida server must be running as root on the phone, or the frida librairy must be injected inside the #APK file before installing it.
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Frida defines a javascript wrapper arround the Java Native Interface (JNI) used by native code to interact with Java classes and the Android #API.
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In addition to allowing interaction with Java objects from the application and the Android API, this wrapper provides the option to replace a method implementation by a javascript function (that itself can call the original method implementation if needed).
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This make Frida a powerful tool capable of collecting runtime informations or modifying the behavior of an application as needed.
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The main drawback of using Frida is that it is a known tools easily detected by applications.
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Malware might implement countermeasures that avoid running malicious payload in presence of Frida.
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#v(2em)
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Those tools are quite usefull for manual operations.
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However, considering the complexity of modern Android applications, it might take a lot of work for a reverse engineer to analyse one application.
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In the next section, we will see more advance techniques that have been developped to analyse Android applications.
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