Description

45
15
31
23
31
4, 1, 3, 5, 2

Solution

This challenge was very random because the way to solve it was to basically guess. It looks like a Polybius Square cipher – two-digit numbers, each digit is from 1-5. The name of the challenge also hints at this: Polygon -> Polybius.

I found a Polybius square decoder online on cryptii.com. It gives you a default alphabet a..z. You give it the first 5 numbers of the challenge text, you get plaintext

uelhl

I figured that uelhl looks enough like hello. So the solution is FLAG{hello}.

Flag

FLAG{hello}