How Provably-Fair Slots Work (and How to Verify Every Spin)
“Provably fair” means you don’t have to trust the operator — you can prove each spin was decided before you played and was not altered. Here’s exactly how it works, and how to verify it yourself.
What does “provably fair” mean?
A provably-fair slot is one where every outcome is generated by a published algorithm from a set of seeds, and the operator commits to its secret seed in advance by publishing a hash of it. Because a hash can’t be reversed or changed without changing the hash, the operator can’t pick a different result after you bet. Once the seed is revealed, anyone can re-run the algorithm and confirm the outcome.
Are online slots rigged?
Traditional online slots use a server-side random number generator you simply have to trust (audited at best). Provably-fair slots remove that trust: the math is open and every spin is independently reproducible. If a result can be reproduced from the revealed seed, it could not have been rigged after your bet.
Commit-reveal seeds, step by step
- Commit: the server creates a secret server seed and shows you only its
SHA-256hash before you play. - Play: you provide (or are given) a client seed you control. Each spin uses an increasing nonce (spin counter).
- Generate: the outcome comes from
HMAC-SHA256(serverSeed, clientSeed:nonce)— deterministic, so the same inputs always give the same spin. - Reveal: when you rotate your seed, the server discloses the old server seed.
- Verify: you check that
SHA-256(serverSeed)equals the hash you were shown, then re-run the algorithm for any nonce to reproduce the exact grid and payout.
How to verify a slot spin yourself
On Ultimate Ark Link it takes about a minute:
- Open the 🛡 FAIR panel and note your committed server-seed hash and client seed.
- Play some spins, then click Reveal & Rotate Seed to disclose the server seed.
- Open the provably-fair verifier, paste the server seed, client seed, a nonce and the bet, and click verify.
- Confirm the hash matches and the recomputed win equals what you were paid.
The verifier runs the same open code the server uses, in your own browser — so you’re not trusting us, you’re checking the math.
Why provably fair matters for free/social slots too
Even in a free, play-money game, provability builds trust and makes leaderboards meaningful: a big win on the wins feed isn’t just a screenshot — it’s a result anyone can reproduce.