Amsterdam Writeup

Crypto CTF 2020 - Crypto 55 - 96 solves

Is it normal to have such encoding?

Encryption logic

Flag was encoded to bit contained list, and encoded again by adding binomai coefficients.

Exploit

Write decode() function which simply reverses encryption logic. Used sage for faster calculation of binomials.

I get flag:

CCTF{With_Re3p3ct_for_Sch4lkwijk_dec3nt_Encoding!}

Exploit code: solve.sage with config.py