A casino-design pattern that lets a player cryptographically verify, after the bet, that the outcome was decided before the bet was placed — and that the casino had no way to change it.
Casino publishes the SHA-256 hash of a secret seed before play. You bet. Casino later reveals the seed. You re-derive the outcome from seed + your client seed + nonce. If it matches what they paid out, the game was fair. If it doesn't, you have on-chain proof of cheating.
Traditional online casinos run an internal RNG that decides every outcome. You have to trust they aren't rigging it. There's no way to know — a third party would need access to the server. Audits help but they're snapshots. The casino could behave one way during the audit and another way the rest of the time.
Crypto-native players want a stronger guarantee. They want to verify each individual bet, not just trust an annual report. That's the niche provably fair fills.
Provably fair gambling uses a three-input deterministic function for every bet result:
The bet outcome is computed as SHA-256(server_seed:client_seed:nonce), with specific bits of the hash mapped to game outputs — e.g., the first 13 hex digits become a 0-99.99 dice roll, or the first 12 bits become a 12-row Plinko path.
Three security properties combine:
Provably fair guarantees randomness, not pricing. The casino can still set a high house edge, manipulate the payout table, or refuse withdrawals. Provably fair specifically does not prevent these. What it prevents is rigging the RNG itself.
Sanity checks: 1) Verify the published server-seed hash matches the SHA-256 of the revealed seed. 2) Verify a few of your bet outcomes by re-computing locally. 3) Check that the published house edge matches the empirical edge over a large number of bets. If all three check out, the math is sound. If any fail, you have crypto-grade evidence to publish.
The crypto-native casinos all use some variant: Stake, Rollbit, BC.Game, Sportsbet.io, Roobet, AgentBet, and many others. Each has small implementation differences (which hash bits map to which outputs, how nonces are encoded) but the core scheme is the same. AgentBet's exact implementation is open-source-readable in /includes/fair.php and you can verify any bet in your browser.
Some newer protocols (Polymarket-style) push verification entirely on chain — the seed commit and reveal are recorded as transactions, making them tamper-evident from the start. Off-chain provably fair (like AgentBet) is still cryptographically sound; the difference is that the proof is logged in our database rather than on a public ledger. For most players, off-chain is functionally identical and much cheaper to operate.
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