Factorie

I recall that the description given on the flag format was something similar to this:

KCTF{small number_big number}

First, we were given a text file.

The content inside the text file was value n.

There’s a website called factordb where it could help in calculating the prime numbers of value n.

Final flag: KCTF{39434538531451803895327_55131777675015246472249}

Easy peasy :)