dc2019q - ASRybaB
This was a jeopardy challenge, part of dc2019q. (CTFtime)
Was solved by: 14 teams
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This challenge requires very minor bruteforcing, which if properly parallelized shouldn't take more than 15 minutes.
That's less than $1 on Amazon (for instance) -- in general, an amount we considered fair.
If it looks like you have to bruteforce for many hours... you're missing some step :)
Hint 1Could you decompile the python code? What's stopping you?
Hint 2Try adding the correct header and using a decompiler like uncompyle6 -- there's some opcode stopping it, can you remove it?
Hint 3All STOP_CODE occurrences are in dead code. Just remove them and decompile.
Hint 4What properties does the key have? Is it close to a known attack?
Hint 5Read up on the Boneh-Durfee attack (https://github.com/mimoo/RSA-and-LLL-attacks/raw/master/survey_final.pdf) -- it's not immediately applicable but it's close
Hint 6Try rewriting the paper equations' using what you know of the key's factors.
Hint 7It's a different modulus than the plain paper...
Hint 8If you've done things right, "375 computations" will make sense to you.
If you wish, you can contribute more.
There's an author-recommended writeup.