Tag: RE

 

During this article we’re going to take a whistle stop tour of Android reverse engineering to get you to the stage of being able to take apart and review a simple Android game, as soon as possible. 

What  you’ll need to get started

For this walkthrough the only tooling you’ll need is an APK (Android PacKage) decompiler, called Jadx. Jadx can be installed onto Windows, Linux, and MacOS by following the instructions on their website. Using Jadx is as simple as running:

jadx-gui <path to the APK file>

How to reverse engineer an application

Android application’s are commonly written in either Java or Kotlin. When a software engineer wants to create an APK (the Android pacKage), that contains the code and materials that are run on an Android device, they will need to compile that Java or Kotlin source code to a Dalvik executable/ bytecode. This Dalvik executable is a binary that is run on the Android device. This works where each process on the device uses its own virtual machine (VM) which segregates applications. Prior to Android API level 21 (Android 5), this would have been a Dalvik Virtual Machine, and in later versions will instead use the Android Runtime (ART). Both operate in similar fashions, where they simulate a device’s CPUs, registers, and other features while running an application’s compiled Dalvik bytecode.

While it is the Dalvik bytecode that needs to be run on a device, this is not human readable and so if we are to reverse engineer an application or game we’ll need to decompile it back into a human readable form. This is where Jadx comes in. Using Jadx we can decompile the Dalvik bytecode back into Java. This is often called pseudo Java, as it is not a one for one representation of what the original source code would have been, and instead is the decompiler’s best guess. 

Reverse Engineering Android Games

There are several reasons why you might want to start taking apart Android games and applications – the obvious being curiosity and the ability to alter key traits in games such as health, score, and more. For the purpose of today we’re going to download and reverse engineer an MIT licensed game, ‘Original 2048’. You can download the APK of this game from APKPure or APKMirror (However, please be warned that these are external websites and I do not control the content on them – ensure to download the APK safely). 

Now that we’ve downloaded the 2048 game APK we can open it in Jadx, with the command above. In the panel on the left hand side of Jadx-gui you will be able to see all of the resources bundled into the APK, including decompiled source code, assets, images, and more. The source code is most probably obfuscated, this means that at compile time the name of the classes, functions, variables, and so on were all altered and modified to obscure the program if a malicious actor was to reverse engineer it (i.e. the class called ‘update’ could be renamed to ‘ca’). While this is the case we can find look at the entry point for the decompiled source code – this can be seen in the source code drop down at the below path:

com.androbaby.original2048

The next steps are up to you, look at what images are used in the game, review the configuration via the AndroidManifest.xml file, or take it a step further and reverse engineer the .dex file located in the applications assets folder. 

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