In a provably fair system the nonce is a counter showing which bet is being made with the same seeds. The first bet usually uses nonce 0 or 1, the next one a number one higher, and so on. When you change the seed pair, the counter starts again.
Why the nonce is needed
A provably fair result is calculated from three inputs: the server seed, the client seed and the nonce. The seeds stay the same for the duration of a pair. Without a nonce the same seeds would give the same result every time. The nonce makes each bet's input unique, so the results differ but are still predetermined and verifiable afterwards.
At many casinos the calculation uses HMAC-SHA256: the server seed is the key and the message consists of the client seed and the nonce. The exact formula differs by casino and by game — it should be written on the casino's fairness or verification page. Technical background: HMAC on Wikipedia.
What to check with the nonce
- Continuity. In the bet history the nonce should rise one at a time. A gap or a repeat needs an explanation.
- Verification. Once the server seed has been revealed, enter the seeds and nonce into an independent verifier and compare the result with the one in the history.
- Seed change. Verification is only possible after you have changed the seed pair and the old server seed has been revealed. Before that you only see its hash.
Common misconceptions
- "You can predict the next result from the nonce." You cannot. Without the server seed the result is unpredictable for you; that is exactly why the seed stays hidden until the change.
- "A high nonce means a win is near." It does not. Each result is a separate calculation and earlier ones do not affect the next — see the gambler's fallacy.
- "Provably fair means the casino does not win." It does not. Verification proves the result was not changed. The house edge is in the game rules themselves.
A step-by-step guide to verifying: provably fair: how to check it yourself. The maths of original games: house edge and expected value in originals.