a fact of CTF

The very first challenge ever completed for AlpacaHack was never released because we adjusted the difficulty for the first Crypto round. (Organizers' note)

Beginner Hint

• This challenge is in the Crypto (Cryptography) category.
• AlpacaHack currently features four main categories: Crypto, Pwn, Rev, and Web; everything else goes into Misc.
• Start by downloading the attached a-fact-of-CTF.tar.gz and extracting it.
• In chall.py, the environment variable FLAG is read. This is the flag.
• The flag is then transformed using prime numbers.
• The output of chall.py is output.txt.
• The goal is to recover the flag from the value in output.txt.