Reverse Engineering Unknown File Formats with ImHex

Damn a video game spoiler warning, guess I can't read the rest of the article.

I may feed it into an LLM to strip out the spoilers actually

I’ve had a lot of luck with ImHex looking at various binary formats and even using it to aid in writing some file magic.

It would be really nice to be able to roundtrip C headers with it. The syntax is close, but often I older formats there is a 1:1 mapping with C structs that doesn’t quite match the imhex syntax.

Story time: I worked the night shift at a motel during college. "Bored to tears" was an understatement. I was poking around the front desk computer system one night, as one does, and found its data file. Lacking anything else to do, I wrote a little hex dumper in BASIC so I could explore the file. The first thing I noted was that customer names were spaced exactly N bytes apart. Oooh, fixed records! Then I spent the next week or so reverse engineering the DB file format. I'd twiddle a field in a random customer record then look at their record in the hex dump to see what changed, then update my notes.

Eventually I wrote another little BASIC program to run analytics, like which customers spent the most with us, and auto-fix mistyped names ("This says 'Bb Jones'. Did you mean 'Bob Jones'?") by writing directly to the file.

I got a pretty sweet little bonus for my hackery.

I've always wondered how something like an Autodesk Inventor sldprt file was formatted so that I could write a program to manipulate those files.

I assumed it was proprietary and left it at that.

A few years later, I saw that Onshape allows importing sldprt files into their system and I wonder how they were able to provide that ability without some proprietary knowledge transferred

I've been reverse engineering some old Minecraft Legacy Console Edition save files with ImHex and the pattern templates work pretty well, though debugging is sometimes painful (e.g. reading runtime-sized arrays can just overflow the file and error without showing previously decoded structs). Great tool though. LLMs are, of course, scarily good at reverse engineering and spotting patterns that take me way too much effort to spot.