Tsryb Fydyw Nwdz Ktkwtt Msryt Shrynyt... Repack -

A quick check shows that if you apply a (or +21 in the other direction), it may decode to something readable.

So not ROT-13.

Let’s try (common in puzzles): tsryb → gfel o ? No. Let’s do systematically: tsryb fydyw nwdz ktkwtt msryt shrynyt... REPACK