// owlspace

I like CTFs. Admittedly, I’m not particularly good at them. I try my best and learn from other people’s solutions to unique challenges after the end of a competition. I also like my roommate who I’ve known for several years now. And even though he’s a magnificent problem solver, at least in my opinion, I’m yet to convince him to team up and play some CTFs together. It was his birthday very recently and I prefer experiences as gifts as opposed to materialistic things. So I cobbled together a relatively simple CTF-style challenge for him. In this article, I want to outline a couple interesting bits here and there that I learned in the process of creating a custom challenge. Maybe you’ll find yourself inspired to create a simple challenge of your own.

FEAL-4 provides as gentle of an introduction to differential cryptanalysis as there could possibly be. Hence I found myself a little disappointed at the lack of freely available documentation on that subject. You'll find research papers which (rightfully so) appear very abstract and complex. And if you're a crypto noob as much as I am, then these resources won't help you much or, even worse, discourage you. In this article, I want to sacrifice scientific accuracy for comprehensibility and share my own view on one of the most intriguing aspects — at least in my opinion — of the differential cryptanalysis of FEAL-4.