tl;dr
1. Notice short capacity
2. Find collision on 6 capacity bytes using birthday paradox
3. Force rest of the state of 2 blocks to the same value

https://github.com/p4-team/ctf/tree/master/2019-03-23-0ctf-quals/crypto_keccak

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-03-23-0ctf-quals/crypto_keccak).