Some kind of xor cipher. Knowing the flag format, it is easy to recover the key and decrypt the flag.
Solver code: [https://github.com/CTF-STeam/ctf-writeups/blob/master/2020/COMPFEST/lost-my-source/solve.py](https://github.com/CTF-STeam/ctf-writeups/blob/master/2020/COMPFEST/lost-my-source/solve.py)
Flag: `COMPFEST12{Th1s_15_y0ur5_abcdef}`