1. Check first 3-4 n value

2. Google it

3. It's perfect numbers (wiki)

4. Find first 14 perfect numbers in the internet (perfect numbers list)

5. plain_text[i] == cipher[i] ^ perfect_number[i]

solution script: solution.py

flag is flag{tHE_br0kEN_Xor}

Original writeup (https://github.com/wetox-team/writeup/tree/master/hacktivitycon/internet_cattos).