dc2019q - chainedrsa crypto reversing

by tiffanyb

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

Points: 240
Was solved by: 10 teams

Read carefully. Messages speak.

line_weight service

Hint 1

Could publishing the upper half of an RSA decryption exponent be bad? Maybe there's a paper on that...

Hint 2

Check the public key header -- what format is it?

Hint 3

It's PKCS#1.2

Hint 4

You 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.

Spoilers ahead! Code for this challenge is publicly available.