We didn’t solve this puzzle and submit the flag in time, but when we did… well it was a huge reminder not to overthink things.
Of course, we assumed the organisers were giving us a clue with the arrow pointing at the word ‘Schneier’ and we tried multiple transposition and substitution ciphers with that key.
However, the key was to simply read it from the top right corner down to the left.
[text] TTHOH FEEUA FSBIC TTESK WCSTY [/text]
So the flag was: HACKYOUISTHEBESTESTCTFFTW.
Damn it lol.