Signature is a crypto challenge from TSJ CTF 2022, which ended up having two solves. From this challenge, we can see how ECDSA private keys can be recovered by having a (weak) deterministic ephemeral key, $k$.

@blackb6a played TSJ CTF and CODEGATE CTF this weekend. Both of the CTFs had a bunch of epic (and hard-ish) crypto challenges, which made our head scratched for days.

We were two points behind @balsnctf few minutes before the game ends and I found one more flag. It is proud to tell that we won TSJ CTF 🎉!

Anyway, I will compile writeups for (at least) three challenges for TSJ CTF, namely Cipher Switching Service, Signature and Genie. I will go through Cipher Switching Service as the first part of the series.