tl;dr:

1. Reverse the assembly code to extract the serial.
2. Inject the shellcode on a stack to reveal the flag.

Full writeup: [https://github.com/ZenLulz/CTF-writeups/tree/master/blackalps19/foo-fighter](https://github.com/ZenLulz/CTF-writeups/tree/master/blackalps19/foo-fighter)

Original writeup (https://github.com/ZenLulz/CTF-writeups/tree/master/blackalps19/foo-fighter).