People who ask "Why would a casino call me?" can check the answer on their own, round by round. Provably fair games let a player confirm that a result was fixed before the bet and stayed untouched afterwards, using nothing but a published hash and a short calculation. The casino commits to its part of the result in advance, the player adds a part of their own, and the two combine into the outcome. Anyone can repeat the calculation once the round has ended.
Each result comes from combining the server seed, the client seed and the nonce into a new hash, then turning part of that hash into a number the game can use: a dice roll, a card or a multiplier. The formula is published by the casino, often with sample code. Different games use different conversions, so a verification tool has to match the game in question, and the published formula is what makes an independent check possible.
Before any bet, the casino generates a secret value called the server seed and shows only its hash, a fingerprint that cannot be reversed into the seed itself. Publishing the hash first locks the casino in: any later change to the seed would produce a different fingerprint. The seed stays hidden while it is in use, because knowing it would let a player predict every result. Once it is retired, the casino reveals it so it can be checked.
The client seed is the player's contribution, a value set by the browser or typed in by hand, and changing it at any time is part of the design. A counter called the nonce rises by one with every bet made under the same pair of seeds, so each round gets a fresh input. Because the player controls the client seed, the casino cannot pick a server seed that favours the house for a value it does not know.
Common questions
Does provably fair prove the house edge is fair?
No. It only proves a result was not altered after the bet; the edge built into the payouts is published separately and varies by game.
When is the server seed revealed?
Once it is retired through seed rotation; while in use it stays hidden, because knowing it would let a player predict every result.
What does the nonce do?
A counter that rises by one with every bet made under the same pair of seeds, giving each round a fresh input.
How does the hash become a game result?
The server seed, client seed and nonce combine into a new hash, and a published formula turns part of it into a dice roll, card or multiplier.
Can a player rotate seeds on request?
Yes. Rotating ends the current pair, reveals the old server seed for checking and starts a fresh hashed seed for the next rounds.