dc2019q - ASRybaB crypto hinted reversing

by anton00b

This was a jeopardy challenge, part of dc2019q. (CTFtime)

Points: 201
Was solved by: 14 teams

On your laptop

Install docker
docker pull archiveooo/pub:ASRybaB (docker-saved download)
docker run -d --name asrybab archiveooo/pub:ASRybaB
Local IP: docker inspect asrybab -f '{{ .NetworkSettings.Networks.bridge.IPAddress }}'
Connect to that IP:1280 — Remember to docker rm -f when done.

Hosted by OOO

Currently unavailable.

Just RSA

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 1

Could you decompile the python code? What's stopping you?

Hint 2

Try adding the correct header and using a decompiler like uncompyle6 -- there's some opcode stopping it, can you remove it?

Hint 3

All STOP_CODE occurrences are in dead code. Just remove them and decompile.

Hint 4

What properties does the key have? Is it close to a known attack?

Hint 5

Read 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 6

Try rewriting the paper equations' using what you know of the key's factors.

Hint 7

It's a different modulus than the plain paper...

Hint 8

If you've done things right, "375 computations" will make sense to you.

If you wish, you can contribute more.