TL;DR:1. Mount grub.iso -> Find a grub.cfg file that contains the password checking logic.2. Analyze the checking logic -> Learn that it uses regexp to make alot of regular expression assignments/checks.3. Simplify the regex checks into normal logical equations -> use z3 to solve them.
Full: https://lkmidas.github.io/posts/20210517-omhctf2021-writeups/#grub