The document provides steps for reverse engineering an Android APK file, including decompiling the APK to obtain Smali code and manifest files, modifying files, recompiling, and signing the APK. It also describes using dex2jar and Java Decompiler tools to decompile DEX files to Java code for modifying higher level code.

1. The Basic Reverse
Engineering Steps About
APK
Carl Adler
IDSL - Dep. IM - NTUST

2. What’s in the .apk file?

3. What’s in the META-INF?

4. What’s in the res?

5. Before We Start…
First, download all the yellow files shown in figure below.

6. Put the tools together
Extract all the files you downloaded, and choose aapt.exe, apktool.bat and apktool.jar, just copy them
and move those files into the same file(it’s optional, but I think it can make your work more convenient).

7. Does it work?
Moving to the folder where the files you post them, then, open the cmd window, type command
“apktool -version” to ensure the tool really works.

8. Install framework
Ensure that you’ve already prepared an app.apk file, then type in the command
“apktool if BMICalculator.apk” and you may see something like below.

9. Decompile your .apk file
Type in command “apktool d xxx.apk path_you_want_to_put_the_output_file”.

10. Decompile your .apk file
Let’s check the result…

11. Decompile your .apk file
In this way, you’ll get smali file and AndroidManifest.xml.

12. Recompile your .apk file
After you modify the code, use the following command to recompile.
“apktool b path_your_decompiled_source path_you_want_to_put_the_recompile_apk”

13. Recompile your .apk file
And you’ll see a folder named “build” in the decompiled path.

14. Recompile your .apk file
And the new .apk file is also located at the path you specify.

15. Wait, not yet.
You MUST follow this step. Or else the app won’t work and when you try to push it into system,
the rom will be stuck in bootloop XD(Please use the signapk you downloaded before to execute
the following command).
java –jar signapk.jar certificate.pem key.pk8 your_repackaged_apk your_final_apk_after_sign

16. It’s done, but…
You might not want to edit the smali code, maybe you want to edit some higher-level code,
extract the following two yellow file you downloaded before and put to the location you want.

17. Using dex2jar
Type in the following command:
jar xvf your_apk xxx.dex

18. Using dex2jar
Type in the following command:
d2j-dex2jar xxx.dex
And you’ll see a xxx-dex2jar.jar file.

19. Using dex2jar
Type in the following command:
jar xvf xxx-dex2jar.jar

20. Using dex2jar
And you may see two folder was generated, one is named with the top-level package name,
and the other one is android.

21. Using JD
Open Java Decompiler, choose File -> Open File…

22. Using JD
Choose the .class file we extracted with dex2jar tool.

23. Using JD
It’s the original java code.

24. END