What a commitment actually locks
Four steps, and one honest limit at the end.
Before you play, the casino publishes SHA-512 of a server seed it has already chosen. The hash is a lock on that value: it cannot be changed afterwards without the hash changing too.
You supply a client seed. Because it enters the same HMAC, the operator cannot know the outcome of a round until you have set it.
Each round runs HMAC-SHA512 over the server seed as key and client seed plus nonce as message. The nonce increases by one per round, so every round has its own digest.
When you rotate seeds, the server seed is revealed. Hash it yourself: if it matches the commitment published in step one, no roll was altered after the fact.
Provably fair confirms that results follow from the seeds. It says nothing about the house edge, the payout table or whether the operator will pay you. A verified roll can still come from a game with a 10 percent margin.