Using pySym to solve the cipher by symbolically executing the python source: https://bannsecurity.com/index.php/home/10-ctf-writeups/44-tokyo-westerns-ctf-2017-my-simple-cipher

Original writeup (https://bannsecurity.com/index.php/home/10-ctf-writeups/44-tokyo-westerns-ctf-2017-my-simple-cipher).