TL;DR: do horrible things to a one-way compression function by leveraging weak keys in the DES block cipher.

https://fortenf.org/e/ctfs/crypto/2018/06/26/google-ctf-2018-dm-collision.html

Original writeup (https://fortenf.org/e/ctfs/crypto/2018/06/26/google-ctf-2018-dm-collision.html).