tldr;- weird mode of operation that uses two IVs has been used to encrypt some (known) plaintext- the flag is split across the two IVs- we are given part of the key and some of the ciphertext (a majority has been "corrupted")- bruteforce a few bits of the key, then work backwards to recover the IVs
[DUCTF GitHub](https://github.com/DownUnderCTF/Challenges_2020_public/tree/master/crypto/cosmic-rays)
[writeup](https://jsur.in/posts/2020-09-20-downunderctf-2020-writeups#cosmic-rays)