Rating: 5.0

tl;dr

1. Run Z3
2. If you don't want to wait, notice that '+' can cause carry if bit key is 1 and this carry can be visible after odd number of rounds

Full writeup: https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/simple_logic

Original writeup (https://github.com/p4-team/ctf/tree/master/2019-09-02-tokyowesterns/simple_logic).