Brain Calc - l3ak CTF Writeup

Challenge Info

Field	Details
CTF	l3ak CTF
Challenge	Brain Calc
Category	Android / Reverse Engineering

Description

Quick and fun math quiz app for practicing.

Solution

We can start by inspecting the app in Jadx

From the decompiled Main Activity, we understand that the whole app run on python The python source code is usually stored in assets/chaquopy

We can see app.imy in assets/chaquopy, we can export that file and analyze it

 file app.imy
app.imy: Zip archive data, made by v2.0, extract using at least v2.0, last modified Feb 01 1980 00:00:00, uncompressed size 0, method=deflate

It is a Zip archive data, we can extract it using 7z x app.imy After extracting it, we can navigate to /BrainCalc under which app.pyc is present which is compiled python file We can convert it to .py using a tool - pycdc
pycdc app.pyc We see the flag’s logic,

#code above
def get_secret_reward():
Warning: Stack history is not empty!
Warning: block stack is not empty!
    compressed_flag = 'eJzzMXb0rvYqLS6JN4kPNynKjQ8tiHfOMMnJqQUAeHcJQA=='

    try:
        decoded = base64.b64decode(compressed_flag)
        flag = zlib.decompress(decoded).decode('utf-8')
        return flag
    except:
        return 'Error: Could not decode secret'
#code below

We can just reconstruct the flag logic in a python file and run it to get the flag

import zlib
import base64
compressed_flag = 'eJzzMXb0rvYqLS6JN4kPNynKjQ8tiHfOMMnJqQUAeHcJQA=='
decoded = base64.b64decode(compressed_flag)
flag = zlib.decompress(decoded).decode('utf-8')
print(flag)

Flag

Flag:L3AK{Just_4_W4rm_Up_Ch4ll}