Provably Fair Casinos

A provably fair game lets you check, after the round is over, that the result was fixed before you placed the bet. Not promised. Not certified by a lab you have never heard of. Checked, by you, with a hash function you can run on your own machine.

That is a genuinely different trust model from the rest of the industry, and it is also narrower than the marketing suggests. It says nothing about the house edge, it does not cover the slots that make up most of any casino library, and a badge in a footer proves nothing at all. This page covers how the system actually works, how to verify a bet yourself in about two minutes, what the proof does not cover, and which of the casinos we track offer the real thing.

Which casinos here run provably fair games

There are two different things hiding behind the same phrase, and separating them matters more than any other distinction on this page.

A casino running its own engine controls the seeds and publishes the algorithm, so the whole chain from commitment to result sits with the operator you are betting against. A casino hosting a studio’s provably fair game is passing the guarantee through from someone else. Both are verifiable. The second gives you a smaller catalogue and puts the proof in the hands of a third party, which is not worse, just different. Aviator is the same game with the same guarantee wherever you find it.

How the system actually works

The whole thing rests on one idea from cryptography: a commitment. I can prove I knew a number in advance without telling you the number, by giving you its hash. Later I reveal the number, you hash it yourself, and if it matches the hash I gave you at the start, I could not have changed my mind in between.

Applied to a casino, that becomes four inputs.

Those four go into a keyed hash. The server seed is the key. The message is the client seed, the nonce and the round joined by colons. The output is 32 bytes of noise that is completely reproducible if you know the inputs, and impossible to predict if you do not.

HMAC_SHA256( key: serverSeed, message: "clientSeed:nonce:round" )

The first four bytes of that output are turned into a number between 0 and 1, and that number is mapped onto the game. On a crash game it becomes the multiplier the round busts at. On Mines it picks the tiles. On Dice it is the roll. The mapping is published, so there is nothing left to take on trust.

Why neither side can cheat

The casino cannot change the result, because the server seed was fixed and published in hashed form before you bet, and it cannot find a different seed that produces the same hash. You cannot predict the result, because you never see the server seed until the session is closed. The nonce stops replaying an old outcome. Change your client seed and the entire sequence changes with it.

Verify a bet yourself

This is the part almost nobody does, and it takes two minutes.

1. Before you play, copy the server seed hash. It is in the fairness or provably fair panel, usually behind a shield icon in the game or in account settings. Save it somewhere outside the casino.
2. Set your own client seed. Do not leave the default. Type something only you would pick. If a casino will not let you change it, stop there and read the red flags section below.
3. Play, and note the nonce of the bet you want to check. Every bet in your history shows one.
4. Rotate the seed. Rotating retires the current server seed and issues a new one, and only then does the casino reveal the old seed in plain text. You cannot verify a session while it is still running, by design.
5. Hash the revealed seed. Run SHA-256 over it and compare the result, character by character, with the hash you saved in step one. If they match, the seed was fixed the whole time.
6. Recompute the outcome. Feed the revealed server seed, your client seed and the nonce into HMAC-SHA256 and map the bytes the way the game does. Compare with what your bet history says happened.

Steps five and six run in about ten lines. This is Node, and it needs nothing installed.

const { createHash, createHmac } = require('crypto');

const serverSeed = 'the revealed 64 character seed';
const clientSeed = 'whatever you set';
const nonce      = 42;

// Step 5: does the revealed seed match the hash you were given?
console.log(createHash('sha256').update(serverSeed).digest('hex'));

// Step 6: rebuild the raw result of that bet
const bytes = createHmac('sha256', serverSeed)
  .update(`${clientSeed}:${nonce}:0`)
  .digest();

const roll = bytes[0] / 256
           + bytes[1] / 256 ** 2
           + bytes[2] / 256 ** 3
           + bytes[3] / 256 ** 4;

console.log(roll); // a number from 0 to 1, which the game maps to a result

Use the casino’s own verifier too, but never only that. A verifier hosted by the casino that sends your seeds back to the casino to be checked proves nothing. The point of the exercise is that you do not have to trust the party holding your money.

What the proof does not cover

This is where most pages on this subject stop, and it is the half that costs people money.

It does not remove the house edge. A provably fair Dice game with a 1 percent edge is provably a game you lose 1 percent of your turnover on. The proof covers whether the number was rigged, not whether the maths favours you. It does not.

It does not cover slots. A Pragmatic Play or Hacksaw slot runs on the studio’s own certified random number generator, on the studio’s servers. The casino has no access to it and cannot make it provably fair. Every casino in the table above runs the vast majority of its library this way, so on most of the games you play, the provably fair badge is not in force.

It does not cover the live casino. A dealer, a wheel and a camera cannot be hashed. Live games are governed by physical audit and licence conditions instead.

It does not prove the casino will pay you. Fairness and solvency are separate problems. A perfectly verifiable game at an operator that stalls withdrawals is still a bad account. That is why fairness is one input into our score rather than the whole of it, and why payout speed is measured separately on the instant withdrawal list.

It does not stop selective generosity. Because the casino knows the server seed and can see your client seed, it can compute the outcomes waiting under the current pair. It cannot change them. It can, in theory, decide when to nudge you toward rotating, or which promotions to show. This is a narrow gap and there is no evidence of it at the operators here, but honest is honest.

Provably fair against certified RNG

These are two different answers to the same question, and a good casino has both.

Certification answers a question provably fair cannot: is the advertised return to player the real one. Only a lab running millions of simulated rounds can establish that. Provably fair answers a question certification cannot: was this specific bet, the one you just lost, generated honestly. You want both, which is why our scoring treats a licence and third party auditing as part of the trust factor rather than accepting a fairness badge on its own. The full weighting is on the how we test and rate page.

Which games can be provably fair

The technique needs a game whose outcome is a small number of random values with published rules. That rules in the originals catalogue and rules out most of everything else.

Red flags in a fairness page

Most casinos claiming to be provably fair are describing a feature they do not have. These are the tells, in the order they appear.

• No hash before you play. If the server seed hash appears only after the round, there is no commitment and the whole thing is theatre.
• You cannot set your own client seed. Without player entropy, the casino controls every input. This is the single biggest red flag.
• No seed rotation. If there is no way to retire the current server seed and see it revealed, nothing can ever be verified.
• Verification only on their site. A verify button that posts your seeds to the casino and returns a green tick proves only that their server says yes. The algorithm should be published so you can run it anywhere.
• The algorithm is not documented. No description of how bytes map to outcomes means the maths cannot be reproduced.
• The badge covers the whole site. A casino claiming every game is provably fair, slots included, does not understand its own claim or is counting on you not to.
• Seeds change without you asking. Rotation should be yours to trigger. Silent changes break the audit trail across your session.

How we check it

We do not take the badge at face value. On every casino here we open the fairness panel, set a client seed of our own, place bets, rotate the seed, and confirm that the revealed server seed hashes to the value published beforehand. Then we recompute at least one outcome outside the casino, with the code above, and compare it against the bet history.

A casino that passes gets credit under the trust part of our rating. A casino that shows a badge but fails any step of that sequence does not, and we say so in the review. The reviewer’s name and the date of the last check sit at the top of every review page, so you can see when the test was actually run.

Which of these should you use

Stake has the deepest originals catalogue and the clearest published implementation, and it is the operator most third party verifier tools are built around. It also scores 97 on our Trust Index, the highest we have recorded, and its welcome offer is one of only two here that carries no wagering. See the no wagering list for that comparison.

Duel is the tightest package: four originals, payouts in 5 to 15 minutes, no wagering on any bonus it runs, and no document check to open an account, which puts it on the no KYC list as well.

BC.Game has the widest originals range of the three, including Hash Dice and Wheel, no stated withdrawal cap and a one dollar minimum deposit. The welcome package carries 30x wagering, so claim it deliberately rather than by default.

If you mainly play slots, none of this applies to the games you are actually playing, and you should be choosing on payout speed, licence and bonus terms instead. Bitcoin casinos and the currency pages are a better starting point in that case.

Gambling is for adults only. A verifiable result is still a result you can lose.

Provably fair: common questions