Tags: qr 


*This is a short summary of the full write-up at

After analyzing the problem one can see that we need to provide the
web service with a single image that produces at least two valid QR
codes with different results under the resizing sequence

Since the service is using the `NEAREST` operation, it is enough to
find the pixels from the original image that remain after resizing.

A simple test produces the pixels with indices `(10i + 7, 10j +
7)`. So in order to create our image we need to overlay every pixel
from one image into the above indices of the second image:

im1[7::10, 7::10] = im2

Uploading the resulting image to the service gives us back the flag.

Original writeup (https://smyghalloumi.se/posts/2018-11-01-seccon2018-qr.html).