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dc2019q - chainedrsa crypto reversing

by tiffanyb

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

Points: 240
Was solved by: 10 teams

Description

Read carefully. Messages speak.

line_weight service

Hints

Hint 1Could publishing the upper half of an RSA decryption exponent be bad? Maybe there's a paper on that...
Hint 2Check the public key header -- what format is it?
Hint 3It's PKCS#1.2
Hint 4You need to make some changes and some optimizations, but you can skip some. On a regular laptop, it should be possible to complete in 3 minutes.

If you wish, you can contribute more.

Source

Spoilers ahead! Code for this challenge is publicly available.