Down the Reversing Hole
Misc (50 pts)
Don’t forget. This is a MISC challenge.
Of course I disregarded the warning about the “MISC challenge” and went whole hog into reversing the algorithm… only to discover that, based on the code I saw, bruteforcing the flag would likely take longer than the length of the CTF. 
One of the CTF IRC channel OPs reiterated that this was not a reversing challenge. So, back to the drawing board I went… Played with different input values, looked for interesting artifacts inside the binary - nothing seemed to hint at a flag.
Finally, the MS-DOS stub caught my eye, it had a non-standard message - This program cannot be run in WIN mode. The regular message is This program cannot be run in DOS mode. The stub also looked larger than usual.
Running the program in DOSBox revealed the reason why:
The flag is PCTF{at_l3a5t_th3r3s_d00m_p0rts_0n_d0s}.