Interference

TJCTF 2018 - Miscellaneous (15 pts).

TJCTF 2018: Interference

Challenge details

Event Challenge Category Points Solves
TJCTF 2018 Interference Miscellaneous 15 270 solves

Download: interference.zip - md5: ab1e7054649da290274a02f0e01b3d40

Description

Author: jfrucht25 > I was looking at some images but I couldn’t see them clearly. I think there’s some interference.

TL;DR

We had 2 images composed by white and black pixels. We used Stegsolve to apply operations: NOT(IMG1 ^ SUB(IMG1,IMG2)).

Methology

Looking at the files

The first thing we did was opening the files contained in interference.zip.
The archive was containing 2 images named v1.png and v2.png :

v1.png v2.png

By seeing the two files, I decided to do some operations with them such as xor.

Operations

To test differents opetations, I used the tool named “Stegsolve” by Caesum. The tool is written in Java (jar file) and allow us to combine images (“Image Combiner”).
I saw a different image while using the SUB operation, I decided to save it to solved.bmp.

solved.bmp

Then I started to do operations between v1 and v3 and here it is (I added an extra XOR then a NOT)!

final.png

FLAG

By decoding qrcode we got

tjctf{m1x1ing_and_m4tchIng_1m4g3s_15_fun}

Zeecka