(solution was a more complex sol)

This challenge is about forging a AES CTR and a custom MAC(GHASH)

AES-CTR:

It is pretty trivial to forge this, it is basically a one time pad

GHASH:

Since the hash is basically a polynomial, we can easily predict the output of it given some inputs, so its easily forged

Original writeup (https://github.com/Ariana1729/CTF-Writeups/blob/master/2019/nullcon/GenuineCounterMode/README.md).