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  5. Android
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Android

Introduction

Android applications are distributed as either APK (Android Package) or AAB (Android App Bundle) files. In CTF reverse engineering, analyzing these files can expose hardcoded keys, credential checks, or base64‑encoded secrets.

Quick Reference

  • Decompile APK: jadx -d output/ sample.apk
  • Decode resources & smali: apktool d sample.apk -o output/
  • Build smali back to APK: apktool b output/ -o rebuilt.apk
  • Extract APKs from AAB:
bundletool build-apks --bundle=app.aab --output=output.apks
unzip output.apks -d extracted/

Tools

Tool Purpose
JADX Decompile APK/Dex files to Java source
APKTool Decode resources, manifests, and rebuild APKs
bundletool Unpack AAB bundles into individual APKs

Tips

  • Focus on code under smali/com/.../MainActivity.smali or equivalent Java packages for flag logic.
  • Inspect AndroidManifest.xml for exported components or permissions leakage.
  • Search compiled code (grep) for known variable names like flag, api_key, or secret.
  • Use JADX and APKTool together, JADX for readable code, APKTool for precise resource mapping.