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)