@blackb6a played Dragon CTF 2021 last weekend and I spent most of the time solving the CRC duo. They are very fun but unfortunately that we were close enough while unable to get the second flag. Warmup edition only. If you are interested to learn the harder edition that involves a 128-bit CRC, you may want to look at hellman’s writeup. I would only cover a solution of the easier edition with some unsuccessful attempts of the harder one.
