ksi

KSI $(\Xi)$ - Elementary Cryptanalysis Challenges

A small set of challenges based on a home-made symmetric cipher called Ksi $(\Xi)$ that I came across in the late 90s. The password for the all challenge archives is infected. Successfully solving a challenge should yield a flag in the format flag{????????}. I hope you enjoy solving them and they help you practice and improve your skills. Twitter DMs are open if you have any constructive feedback.

1. ksi1997 - This is the original $\Xi$ cipher as it was first sent to me. It was very easy to crack even with the hardware available at the time.
2. ksi2002 - A slightly updated version of the $\Xi$ cipher with a 128-bit key, a key schedule generator and much better diffusion properties than the original (plus a fix for a bug in the original cipher). Appears a lot more challenging, but its bark is much worse than its bite - you can solve it quite efficiently even on 1997 hardware. May contain a bonus flag...

Enjoy,

@eleemosynator