TL;DR

* Shanks's giantstep-babystep algorithm (or just bruteforce) to find the Diffie-Hellman shared secret
* use shared secret as key for hmac-md5. Message = (sequence number || bit)
* the resulting hash will match for either 0 or 1. Remember this bit.
* once you have all the right bits, binary -> hex gives you the flag

Original writeup (https://gitlab.com/Serenest/writeups/-/blob/master/2021/UTCTF/farmers_only/README.md).