Provably fair roulette: how a crypto casino lets you check a spin
The game
Provably fair roulette is the one kind of roulette where you do not have to trust the casino about a single spin. You can recompute the result yourself. Here is how the seeds, the hash and the nonce fit together, and what the check does not prove.
Provably fair roulette grew out of the first bitcoin dice sites, where players had no licence to rely on and wanted another way to know a result was not chosen after the bet. The answer was cryptography: the casino fixes the outcome in advance, hides it behind a hash, and reveals it later so you can check. It is simple enough to verify by hand with a free online tool, and it is the one guarantee in online roulette that does not depend on trusting anybody.
The three ingredients
Server seed. A long random string chosen by the casino. Before you play, you see only its hash, a fingerprint produced by a function such as SHA-256. The hash reveals nothing about the seed, but once published, the casino cannot switch to a different seed without the hash changing.
Client seed. A string you choose, or one the casino suggests that you can edit. It means the casino cannot pick a server seed that is bad for you in particular, because it does not know what you will mix in.
Nonce. A counter that rises by one with every bet: 0, 1, 2 and so on. It makes each spin under the same pair of seeds different.
From seeds to a pocket
For each bet, the game combines the server seed, the client seed and the nonce with a keyed hash function, usually HMAC-SHA256 or HMAC-SHA512. The output is a long hexadecimal string. The game reads the first few characters as a number, turns it into a fraction between 0 and 1, and multiplies by 37 for a European wheel. The whole-number part is the pocket: 0 to 36. The exact recipe varies by casino, and a proper provably fair game publishes it, often with sample code.
Checking a spin yourself
- Before you play, open the fairness panel. Copy the hashed server seed and note your client seed.
- Play. Each bet in your history shows the nonce used.
- Rotate the seed. Changing your client seed, or asking for a new server seed, makes the casino reveal the old server seed.
- Check the hash. Put the revealed server seed through SHA-256 and compare it with the hash you copied at the start. They must match exactly.
- Recompute the result with the casino's published formula, or with an independent verifier, using the server seed, your client seed and the nonce. The pocket must match the one in your bet history.
If step 4 fails, the casino changed the seed after you bet. If step 5 fails, the result was not produced by the formula it published. Either is a reason to stop playing there.
What it proves, and what it does not
Provably fair proves that a spin's result was fixed before your bet and was not adjusted to it. It does not prove:
- that the payouts are fair. A game can be provably fair and still pay 34:1 on a number. Check the payout table, and see roulette odds and payouts for the standard ones.
- that you will be paid. Withdrawals, caps and identity checks are the casino's business, not the game's; see our page on bitcoin roulette.
- anything about live roulette. A dealer's wheel has no seed.
Where to find it
Provably fair roulette sits among a casino's “originals” or in-house games, not in the third-party slots and live sections. On our list of crypto roulette casinos, DuckDice, BetFury and Bitsler are the ones built around in-house provably fair games. Most casinos also carry third-party software roulette from studios; those rely on the studio's certified random number generator instead, which you cannot check yourself.
Speed is the catch
A provably fair wheel spins as fast as you click. At the same stake and the same 2.70% edge, a hundred spins cost a hundred times what one spin costs. Set a stake and a limit before you start, and read roulette strategy on why no system changes that cost.