Provably Fair Explained

Most crypto casino in-house games, such as dice, crash, plinko, mines and limbo, use the same basic structure.

• Server seed: a random value generated by the casino and kept secret while it is in use. Before any bets, the casino shows you its hash, typically SHA-256. A hash is a one-way fingerprint: it reveals nothing about the seed, but the casino cannot later swap in a different seed without the hash changing.
• Client seed: a value on your side. Good implementations let you change it at any time. Because the casino has already committed to its server seed, it cannot choose a server seed that works against a client seed it has not yet seen.
• Nonce: a counter that increases by one with each bet under the same seed pair, so every bet produces a different result from the same seeds.

A common pattern is to compute HMAC-SHA256 with the server seed as the key and the client seed and nonce as the message. The output is a long string of bytes. The game converts part of that output into a number, for example a dice roll between 0 and 100, or a crash multiplier, using a published formula.

Because the calculation is deterministic, anyone with the same three inputs and the formula gets the same result. That is what makes the result reproducible. Exact formulas differ between casinos and between games, so a verifier has to use the casino's published method for that specific game.

You do not need to trust the casino's own verifier page. Independent open-source verifiers exist, or you can run the calculation in a few lines of code.

1. 1Before playing, open the fairness or seed settings and record the hashed server seed shown to you. Take a dated screenshot.
2. 2Set your own client seed, ideally a random string you generate yourself.
3. 3Play, and note the nonce and result of the bets you want to check.
4. 4Rotate the seed pair. The casino then reveals the previous server seed in plain text.
5. 5Hash the revealed server seed with SHA-256 and confirm it exactly matches the hash you recorded before playing.
6. 6Recompute each result from the server seed, client seed and nonce using the casino's published formula, and confirm it matches what the game showed.

The security comes from ordering. The casino must commit to its randomness before it knows your input, and your input must be something the casino cannot predict or control. If either condition fails, the word 'provably' no longer applies.

• The server seed hash is shown before the first bet it applies to.
• You can set or change the client seed, and the casino cannot override it.
• The nonce increments with every bet.
• The server seed is revealed after rotation, and it hashes to the published commitment.
• The result formula is published in enough detail to reproduce.

The following is an unconfirmed allegation, included to show what to look for, not as a finding against the operator. A post on the Bitcointalk forum, accompanied by a public code repository, alleged that a private game build on Winna was not provably fair. According to the allegation, the value meant to be unpredictable was fixed to a Bitcoin block hash from 2023 chosen by the casino rather than by the player, and the per-bet counter was pinned to zero. If true, the outcome would have depended on a single value the operator controlled, which it could in principle have generated and inspected before publishing.

We have not independently verified the claim and found no operator response. In our data it is marked as unconfirmed and does not affect Winna's score. The lesson is general: a 'provably fair' label and a verifier page are not enough. Check that you control the client seed, that the nonce changes, and that the commitment is shown before you bet.

Provably fair is useful and narrow. It is often presented as a general trust signal, which it is not.

• In-house games only. Slots, live dealer tables and most games from studios such as Pragmatic Play, Evolution or Hacksaw Gaming do not use the casino's seed system. For those, fairness depends on the provider and on independent testing labs such as GLI, BMM, iTech Labs or eCOGRA, whose certificates should be checked on the lab's own website.
• Not a statement of solvency. A casino can run perfectly fair dice and still be unable or unwilling to process withdrawals. We are not aware of any major crypto casino that publishes a verifiable proof of reserves covering player liabilities.
• Not a statement about terms. Win caps, withdrawal limits, KYC demands at withdrawal and 'irregular play' clauses operate entirely outside the game maths.
• No protection against hacks. Stake's September 2023 hot-wallet theft of about $41 million, which the FBI attributed to the Lazarus Group, had nothing to do with game fairness.
• House edge still applies. A fair game still has a built-in house edge, so the expected result over time is a loss.

In our Trust Score, working provably fair that we can verify ourselves counts towards the Verifiability component, alongside certified third-party providers and proof of reserves. That component is worth 10 of 100 points. Licence status, complaints and the fairness of terms carry far more weight, because those are what decide whether players actually get paid.

Gambling is for adults 18+ or the legal age where you live. Verifiable fairness does not change the odds, and offshore sites are not covered by national self-exclusion schemes like GamStop or CRUKS. Free help is available from BeGambleAware, GamCare and Gambling Therapy.