MatryoshkaQR (Huntress CTF 2024)

Author: @JohnHammond Wow! This is a big QR code! I wonder what it says…?

Huge QR code! Using zbarimg I first decoded the image to reveal another PNG file - except it is an ascii text string with the starting magic number header 0x89 P N G. If you run file on this, you get ASCII text, with very long lines (943). This is an example of the outputted string:

x89PNGrnx1anx00x00x00rIHDRx00x00x00

It was irritating but a little bit of python can solve this. I imported the result into a string in python. Then with the codecs library, escape the string with codecs.escape_decode(x) and then write back the byte string using the wb flag.

This gives a proper PNG file: PNG image data, 39 x 39, 1-bit grayscale, non-interlaced. Yes. I view it and it is another QR code! This time much much smaller. Running zbarimg on this gives another ascii text! Thankfully this is the actual flag. yay.