Rating:

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://www.josephsurin.me/posts/2020-09-20-downunderctf-2020-writeups#cosmic-rays)

Original writeup (https://www.josephsurin.me/posts/2020-09-20-downunderctf-2020-writeups#cosmic-rays).