is RNG calculated locally or on server? (even if you play online)

saw that thread discussing the

Key drop rate, I'm always wondering how RNG is carried out in game, I reckon even if you play online, RNG is still calculated locally otherwise the back and forth net communication would be too much? if RNG is purely local, wouldn't some factors kick in differently from the RNG itself (hence the system calls and all that) thus you would have vastly different drop experience from day to day?

Im sure it would be on the server otherwise people could and would hack it.