**TL;DR**: Block cipher is easily reversible. We can bruteforce the initial key since we know what the first block of plaintext will be, and from there, we just do the same steps to encrypt, but we reverse the addition of bytes modulo 256 to be a subtraction of bytes modulo 256.

Original writeup (https://an00brektn.github.io/htb-cyber-apocalypse-crypto-short-and-sweet/#jenny-from-the-block).