The mechanism

Three inputs produce each result. The casino generates a server seed and shows you its hash before you play, which commits the casino to that seed without revealing it. You hold a client seed, which you can change at any time. A nonce counts your bets, so the same seeds produce a different result each round. The three are combined through a hashing function to produce the number that decides the outcome. When the server seed is later revealed, you can rerun the calculation and confirm that each result came from the committed seed and your own input.

Craze's provably fair page describes exactly this arrangement, and its verification page lets you enter the seeds and nonce to check a past bet. Shuffle publishes the same design for its originals, with the implementation code and an on-site verifier. Stake does too, and adds that its originals can be checked with third-party open-source verifiers, which is the stronger form of the promise because the calculation does not depend on the casino's own page. Gamdom uses an older design: for each original it pre-generates a chain of ten million SHA-256 hashes from a server secret and plays them in reverse, publishing the final hash of the chain in advance and each round's hash afterwards. That proves each round belonged to the committed chain, but it has no client seed, so you cannot contribute your own randomness to the result.

What the check proves

  • That the casino could not have changed the result after seeing your bet, because the server seed was committed before you played.
  • That your input influenced the result, because the client seed is yours.
  • That the published results are the ones the algorithm produced, once you rerun the calculation.

What it does not prove

  • The odds. A provably fair game can have any house edge its operator chooses. Fairness here means "not tampered with", not "favourable". The edge is in the game's published rules, if the operator publishes them.
  • Third-party games. Slots and live dealer tables from studios such as Pragmatic Play or Hacksaw Gaming run on the studio's own random number generator, which is certified by a testing laboratory rather than verifiable by you. When a casino says every game is provably fair, ask which verification method covers the studio games; usually the answer is the laboratory certificate, not a seed you can check.
  • That you will be paid. Fairness of the result and the operator's willingness to process a withdrawal are separate questions, and the second one is decided by the terms, not the algorithm.
  • Anything about a result you did not verify. The tool exists; most players never use it. It protects you only if you run the check, or at least rotate your client seed so the operator cannot predict your input.

How to actually use it

  1. Before a session, set a new client seed of your own choosing in the fairness settings.
  2. Note the hash of the current server seed.
  3. After the session, rotate the server seed. The casino reveals the old one.
  4. Use the verification page, or any independent calculator that implements the same algorithm, to check a sample of bets against the revealed seed, your client seed and the nonce.

A verification page that only accepts the casino's own inputs is still useful, but an operator that publishes the algorithm so that a third-party calculator can check it is giving you more.

How the reviewed casinos describe it

Each review on this site records the operator's provably fair statement under the Provably fair entry: whether a verification method is published and which games the operator says it covers. Where an operator claims coverage of every game, the review says which games that can and cannot mean.