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PostPosted: Thu Nov 04, 2021 1:56 am 

Joined: Thu Nov 04, 2021 12:41 am
Posts: 2
I'm looking for curious people interested in helping me reverse a file format for my 3d printer at work (EOS M100 LPBF metal printer). I highly suspect the files are encrypted with a baked-in key in the software, so the task starts off with however one would go about finding that (key and algo). I have basically 0 practical experience with hooking up a debugger to a program or sifting through DLL's. Older software from the same company used .ini - like files for the same purpose, so I'm hoping once the data is decrypted / deobfuscated it will be a simple format. I can provide files, as well as the program itself, but actually running the program is dependent on a usb token I obviously can't send.

I just started in earnest this afternoon, and forgot to copy any files to take home with me, but here's what I found so far:
- there's a small header and footer without much info (file size, and possibly version?)(33 byte header 16 byte footer)
- crafted input: files created to be exactly the same (exact same edits within software then saved) are exactly the same, so suspected encryption key is static (no nonce, no timestamp included)
- crafted input: files with a change to a single parameter in the editor result in entirely different data (though exactly the same size)(hence why I'm convinced the data section is encrypted in some way).
- data size seems to be multiple 16 bytes (probably more. I didn't think to check greatest common denominator between many files before I left work.)
- The software included a 3rd party dll for zip files that I briefly looked up to also have encryption tools, so best case scenario we find a simple call into that library with a static password (fingers crossed :? ).

I'm a bit new to zenhax, but you can also @ me on discord: Aaron#8011
I'd be super grateful for any help!

PostPosted: Wed Dec 01, 2021 1:52 pm 
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Joined: Wed Jul 30, 2014 9:32 pm
Posts: 12984
Maybe the key is derived from the 16 byte footer which may act as MD5 hash too.
The 16 bytes padding may be related to AES.
I hardly doubt you would be so lucky to successfully decrypt the file using the last 16 bytes as key and AES 128 ECB as algorithm :D
Anyway just my quick toughts (I can't help further).

PostPosted: Thu Dec 02, 2021 7:43 pm 

Joined: Thu Nov 04, 2021 12:41 am
Posts: 2
I've started going through the header and footer, and changing one byte at a time to see if the file is still open-able by the software, and if not, how it fails / errors. Some bytes can be changed with no impact, but will be changed back when re-saving the file. Some bytes show that behavior for some values but not all. Others just result in various error messages resulting in an un-openable file, or causing the process to exit. I've tried various combinations of the header and footer as the key / IV with CBC and ECB (not all permutations I guess....) with no success yet. I have to assume there's something like options / protocol version information somewhere in there. The only concrete data I really have right now is that the data size is contained within the footer (bytes 4-7).

I'm willing to dip my toes in trying to learn how attach a debugger to the software as it's running, but IDK where to start with that. Anyone got some good tutorials / etc for disassembling / debugging software you didn't write yourself? I would imagine doing that I could drop a breakpoint on any call to write to a file, then try to back up from there and see if any calls are made to an encryption library, and see if I can find where it gets the key.

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