Hey all! This was a fun weekend and involved a ton of CTFing between Thotcon and ASIS quals. I want to do a super quick writeup of an ASIS challenge I was able to solve fairly easily. So lets get right into it! The challenge simple, we were given a
PNG file, which seemed complete however it had errors when it would load. The name suggests parts of the file need to be reordered, and upon looking closer it became obvious the internal
IDAT sections were out of order. I was able to use a tool called
TweakPNG on Windows to easily reorder the IDAT sections. After some playing around with this, you can see the correct order of IDAT sections below. First you can see the original, where the sections are all out of order. Compare the checksums of IDAT sections in the first, to the checksums of the IDAT sections in the second to see where you have to move each IDAT section to properly reassemble the flag. And that's it, easy!!
