baby-android-2 - ByuCTF Writeup

Challenge Info

Field	Details
CTF	ByuCTF
Challenge	baby-android-2
Category	Android / Reverse Engineering

Description

If you’ve never reverse engineered an Android application, now is the time!! Get to it, already!! Learn more about how they work!!

Solution

We can start by installing and opening the app in our emulator

The app has a textview and button for a flag sanity check, we can enter the flag and check if its correct We can open the APK in Jadx,

We see that the MainActivity calls a method check() from FlagChecker and the class FlagChecker loads a native library and has a native method check We can proceed to inspect the native library in Ghidra

Our input is being checked against an encrypted string bycnu)_aacGly~}tt+?=<_ML?f^i_vETkG+b{nDJrVp6=)=, the encryption is very basic and we can use python to reverse it.

print(''.join('bycnu)_aacGly~}tt+?=<_ML?f^i_vETkG+b{nDJrVp6=)='[(i*i)%47] for i in range(23)))

We can run this script to get the flag, \

\

Flag

FLAG:byuctf{c++_in_an_apk??}