CM01

CM01

250pts

Briefing

Download the file and find a way to get the flag.

Solution

The provided file can be found here.

Scanning the first QR code, frame.png, gives us the output Hey, I've put the flag into the other file using the same trick we always use. You know what to do. :).

This means we need to focus on the QR code code.png.

Solution 1

Opening the image in Adobe Photoshop and subtract the black one from the white one - also known as changing your layer type to Difference:

Now this is good enough to get the flag, however if you happen to have no life you can do a touch of post-processing. For all you nerds, this is done by leveling out a lot of noise followed by scaling it down then back up using nearest neighbour:

The QR code now outputs FLAG: A_Code_For_A_Code.

Solution 2

Try to XOR pixels of two images by PIL to a image, logical_xor.png

from PIL import Image, ImageChops

img1 = Image.open('code.png').convert('1')
img2 = Image.open('frame.png').convert('1')

result = ImageChops.logical_xor(img1, img2)
result.save('logical_xor.png')

And then qr decode it by zbar

zbarimg logical_xor.png
> QR-Code:FLAG: A_Code_For_A_Code

Flag

Flag: A_Code_For_A_Code