How to Reverse Engineer an Android APK Easily | Step by Step Guide

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<p> Today, reverse engineering is commonly used in order to access the source codes of Android applications (.apk files). It involves the use of various tools and processes which may not be familiar to many developers and engineers. In this blog, we will explore the process of reverse engineering an apk file; how to gain access to its source code and the best tools available to effectively do so.</p>

What is Reverse Engineering an APK?

Reverse engineering an Android app (.apk file) is the process of taking the source code from an existing Android application, and in the process, getting access to its source code for further study and analysis. In this way, developers and engineers can effectively change and modify the existing application or even create their own applications from it. The whole process of reverse engineering an APK is known as reverse engineering.

When performing reverse engineering on an APK, a developer can gain insight into how certain aspects of the application such as user interface, graphics, business logic, and beyond are built. Additionally, one can also view the source code in order to understand its structure better, uncover any potential security issues associated with it, or make minor alterations if necessary. Furthermore, the underlying API calls that are made to interact with the server, services calls, data flow between different layers of application can also be studied by reverse engineering.

How to Reverse Engineer an APK File?

The process of reverse engineering involves several steps: obtaining the app's .apk file; unpacking the apk file; analyzing code with an Integrated Development Environment (IDE); extracting resources from the application package; modifying source code as required; recompiling modified source into a new apk file; signing the newly created apk with a keystore signature; installing the signed apk onto a device; testing the modified version of app on the device and finally, publishing the updated version in Google Play Store.

Tools and software needed for the task

To reverse engineer an APK, there are several essential tools and software needed. Some of them include:

• APK Tool: A tool designed to help users quickly extract and decode resources from an Android application package file. It supports both decoding and recompilation processes.
• dex2jar: This tool provides a convenient way to convert Android Dalvik executable (.dx) files to Java class files.
• JADX: An open source command-line tool for decompiling and analyzing applications written in Java language.
• Android Studio or Eclipse IDE: Allows developers to debug, compile and deploy applications developed in Java.
• Smali: An assembler/disassembler for the Dalvik bytecode format used by Android. It can decompile most methods and classes of DEX files.
• Apktool: A tool used to decompile Android application packages (apk files) into their original source code form.

Step-by-Step Guide on Reversing an APK File

By following the step-by-step guide outlined below, engineers and developers will be able to reverse engineer an apk file with relative ease:

1. Decompiling the .apk file using Apktool or dex2jar tool

The first step in the process is to use either a tool like Apktool or dex2jar to decompile the application's .apk file. This will cause the file to be converted from a binary version into its readable source code version.

2. Analyzing code with Android Studio or Eclipse IDE

After decompiling the file, the next step is to open the freshly extracted source code in an IDE such as Android Studio or Eclipse. This will allow developers to get a better view of the application's source code and structure, enabling them to analyze and understand its logic.

3. Extracting resources from the application package (images, strings etc.)

The next step is to extract all the resources from the apk file such as images, strings, layout files etc. which can be done simply by navigating to the “res” folder. Once these files are retrieved, they can be copied and pasted into their own folders for further analysis.

4. Modifying source code as per requirement

At this point, developers can go ahead and make any modifications they want to the existing source code. Doing this requires knowledgeable use of programming languages such as Java and XML (Extensible Markup Language).

5. Recompiling modified source into new .apk file

Once all modifications are made, engineers and developers should use Apktool to compile the newly modified source into a new apk file. Once the apk is built, it can be tested to make sure everything works as intended.

6. Signing newly created apk with keystore signature

To test an application on a device or emulator, it must first be signed with a keystore signature. This involves generating a unique code-signing certificate and then using it to sign the application package.

7. Installing signed apk onto device/emulator

This step requires the developer to install the signed apk onto their device or emulator in order to test that the modifications made have worked correctly.

8. Testing modified version of app on device/emulator

Once the .apk file has been successfully installed, developers and engineers can run their modified version of the app on their device or emulator in order to confirm that all their changes have taken effect.

9. Publishing updated version in Google Play Store

Finally, once the modified application has passed all tests and is ready for release, engineers and developers can publish the updated version in the Google Play Store or other app marketplaces.

10. Troubleshooting errors encountered during any step

The entire process of reverse engineering an APK can be quite complicated and involved. Therefore, it is possible for errors to be encountered in any step of the process. In such cases, developers should refer to their IDE’s customization manual, search online for debugging resources, or contact a professional if need be.

Conclusion

Reverse engineering an APK is a complex yet rewarding task that can allow developers to gain valuable insights into existing apps, create new ones based on existing code structure, fix existing app bugs and vulnerabilities, and overall enhance the user experience by making necessary modifications.

It's important to remember that reverse engineering an APK is not without its risks — especially when it comes to security. Therefore, when performing reverse engineering, it is essential to be aware of these risks and plan accordingly. Moreover, it is best practice to always track changes made and back up original source code before making any modifications.

References

• https://www.forbes.com/sites/forbestechcouncil/2018/04/19/the-fundamentals-of-reverse-engineering-android-apps/?sh=3f25a1a7dbf2
• https://developer.android.com/studio
• https://www.bignerdranch.com/blog/reverse-engineering-an-apk-with-apktool/
• https://source.android.com/devices/tech/dalvik
• https://www.javatpoint.com/java_smali_code_generation

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