Gate Keeper - Payatu Hiring CTF Writeup

Challenge Info

Field	Details
CTF	Payatu Hiring CTF
Challenge	Gate Keeper
Category	Android / Reverse Engineering

Solution

We can by opening the app in our emulator

The app shows a hint 🔐There is a phrase I expect, only the right people can C, and if we input anything random it shows "Wrong key. Try again." Now, we can proceed to inspect it in Jadx to understand the logic it implements

A native library libnative-lib.so is loaded, our input is passed to a native function sumbitKey and if we enter the correct key, the flag is returned and displayed in the textview We can inspect libnative-lib.so in Ghidra, to understand what the KEY could be

Our input is checked using strcmp() against "undefined", and if it matches the flag is returned So, now can we submit undefined in the app to get the flag

Flag

FLAG:PATATU{NOW_U_C_M3}