Writeup by challenge author.

tl;dr Malbolge + Fcrackzip

Link to the write-up : https://volatilevirus.home.blog/2018/10/27/bsides-delhi-ctf18-recursive-cracker-write-up/

Original writeup (https://volatilevirus.home.blog/2018/10/27/bsides-delhi-ctf18-recursive-cracker-write-up/).