The idea in one paragraph
In a provably fair game the casino commits to a secret before you play, in a form it cannot change later. After you have played, it reveals the secret, and you can confirm that it matches the commitment and recompute your results from it. If the numbers match, the casino cannot have swapped the outcomes after seeing your bets.
How it works
Three values are used. The server seed is a secret string chosen by the casino. Before you play, the casino shows you a hash of it - typically a SHA-256 fingerprint - which locks the casino into that seed without revealing it. The client seed is a string you can set yourself. A nonce is a counter that goes up by one with every bet.
Each result is generated by combining the server seed, client seed and nonce, usually with an HMAC-SHA256 function, and turning the output into a game outcome such as a dice roll or a crash multiplier. When you change your client seed or ask for a new server seed, the casino reveals the old server seed so you can check it against the hash you were shown earlier.
A worked example: a dice roll
Implementations differ, so a casino's own published algorithm is the one to follow, but a common approach for a dice game looks like this. The casino computes an HMAC-SHA256 of a message made from your client seed and the nonce, using the server seed as the key. That produces 64 hexadecimal characters. The first few characters are read as a number, divided by the largest value they could take to give a fraction between 0 and 1, and scaled to the game's range, for example 0.00 to 99.99.
Because every step is deterministic, anyone with the three inputs gets the same roll. That is the whole point: the casino cannot have "chosen" your roll after seeing your bet, because the server seed was fixed, and its hash shown to you, before you placed it.
What provably fair proves
It proves the integrity of results: that the outcome you got is the one the committed seed produces, so the casino did not alter it after you bet. Because the seeds and algorithm are published, you can check this with the casino's own verifier or an independent tool.
What it does not prove
- That the odds are favourable. A provably fair game still has a house edge, and the edge is set by the game's rules.
- That the published odds are the ones actually used. Checking that requires analysing many rounds, which a single verification does not do.
- That the operator is trustworthy. A casino can run fair dice and still be slow to pay or freeze accounts.
- That every game on the site works this way. Provably fair is mostly used for in-house "originals" such as dice, crash, plinko and mines. Slots from third-party studios use certified random number generators tested by independent laboratories instead, and you cannot verify an individual spin yourself.
How to verify a result
- Before you play, note the hashed server seed shown for your current session.
- Set your own client seed if the game allows it.
- After you have played, rotate the seed so the casino reveals the old server seed.
- Hash the revealed seed and check it matches the hash you noted.
- Recompute a few outcomes using the casino's verifier, or an independent open-source tool, and compare them with what you were shown.
Common mistakes when verifying
- Trying to verify while the seed is still active. The casino only reveals the server seed after you rotate it, so you will not be able to check bets from the current session until then.
- Changing the client seed part-way through a session and then verifying with the wrong one. Each result depends on the client seed in force when the bet was placed.
- Using the wrong nonce. The count starts at zero or one depending on the casino, and it resets when the seeds are rotated.
- Applying one game's algorithm to another. Dice, crash and mines turn the same output into outcomes in different ways.
- Treating a single matching result as proof of the odds. Verification checks that a result is genuine, not that the game's published odds are what they claim.
Provably fair and certified random number generators
Traditional online casino games take a different route. A certified random number generator is tested by an independent laboratory, which checks that its output is statistically random and that the game pays out as its published return to player says. You trust the lab and the regulator, and you cannot replay a single spin yourself.
Provably fair gives you a check you can run on any single bet, but nobody audits the game's odds unless the operator also has the game tested. Neither approach replaces the other, and the strongest position is an operator that offers verifiable results and independent testing, and is open about both.