powmethod. @grhkm2023 and I spent a lot of time working on this challenge. Although we did not solve it, we actually learned a lot from the challenge and lost a pile of hair (maybe it is just me).
Black Bauhinia coorganized HKCERT CTF 2021 and helped 95% of the technical stuffs, including challenge setting, platform development, infrastructure and etc. I will be writing a series of blog posts talking about the contest, and the first four would be the writeups of the challenges those I wrote.
In the first blog post, we will be going through four easier crypto challenges: A Joke Cipher, Cipher Mode Picker, Key Backup Service 1 and Key Backup Service 2.