TL;DR By choosing a well-crafted sequence of block numbers on which to trigger the seed update, we made sure that the seed would become 12345 and won the lottery.

[Read More](https://github.com/pietroferretti/ctf/tree/master/secconquals2018/gacha)

Original writeup (https://github.com/pietroferretti/ctf/tree/master/secconquals2018/gacha).