Provably Fair Explained: How Duel.com Verifies Game Results

Provably fair is a way for a casino to show that it fixed the outcome of your bet before you placed it and did not touch it afterwards. It is cryptography, not a promise: you can check it yourself with a few lines of code. On Duel.com it applies to the in-house Duel Originals, and Duel publishes the exact method for nine of them.

It is also narrower than the marketing around it suggests. The proof covers one thing, the integrity of a result. It says nothing about whether the operator can pay, how your withdrawal is handled, or how third-party games behave.

The problem it solves#

A normal online casino runs a random number generator on its own server. You see the result and have to trust that it was not adjusted for your stake, your balance or your recent wins. A testing lab may have certified the generator, but you cannot check any single spin.

Provably fair removes that trust for one specific question: was this result decided before I bet, and was it left alone? It does so with a commit-reveal scheme.

How commit-reveal works#

The inputs are covered one by one in server seed, client seed and nonce.

Duel's documented scheme#

These mechanics are shared by the games that use your seed pair: Dice, Blackjack, Plinko, Mines, Keno, Beef and Video Poker.

The last row deserves a sentence. 4,294,967,296 possible values do not divide evenly into 52 cards or 10,001 dice results, so a plain remainder would favour the lowest results by a tiny margin. Rejecting the few leftover values makes every result exactly equally likely. For Dice the accepted range is under 4,294,959,453, which leaves 429,453 values for each of the 10,001 results.

How each game turns the number into a result#

On Blackjack, Duel states that every card has a 1 in 52 chance of appearing, with each card produced by its own HMAC. Its documentation does not describe a shoe from which dealt cards are removed.

Check one result yourself#

This reproduces Duel's published Dice example. It runs with Node.js and no extra packages.

// Duel Dice, using Duel's published worked example
const crypto = require("node:crypto");

const serverSeed = "e8c83961a3961baee67cc08da34ada1c00a4515450a3cdbd2b1db81a35ca7f72"; // revealed after rotation
const clientSeed = "UzR5CIqog4DXox5H";
const nonce = 21;

const digest = crypto
  .createHmac("sha256", Buffer.from(serverSeed, "hex")) // key: the seed as 32 bytes, not as text
  .update(`${clientSeed}:${nonce}`)
  .digest("hex");

const value = parseInt(digest.slice(0, 8), 16); // first 4 bytes as a number
console.log(digest.slice(0, 8), value, (value % 10001) / 100);

It prints 24081264 604508772 83.28, the same roll Duel publishes for this example. The comment on the key matters: the server seed has to be decoded from hex. Used as text, the same inputs give 73.32, which is wrong.

Scripts for every documented game, the commitment check and a Python version are in how to verify a bet.

Why the scheme holds#

Duel names the same warning signs itself: a hashed server seed that was not shown before play, one shown only after you committed to a client seed, a changed algorithm, or a revealed seed that does not match its hash.

The scheme only protects you if you do your part: note the hash before you play, use your own client seed, and run the check after rotating. A commitment nobody checks deters nothing.

Crash and Castle Roulette: drand instead of a client seed#

A per-player client seed does not fit a game where everyone watches the same round. Many crypto casinos solve this with a pre-generated hash chain. Duel documents a different design: drand.

Drand is a public, distributed randomness beacon. A group of independent organisations jointly produce a new random value every few seconds, each with a round number, and publish it for anyone to read. No single party, Duel included, can predict a future value or choose it.

For Crash and Castle Roulette, Duel documents this sequence:

1. Before the round, Duel shows its hashed server seed and a drand round ID. The ID points to a value on drand's quicknet beacon that has not been published yet.
2. When drand publishes that round, its value takes the place of the client seed. The nonce is fixed at 0.
3. The result is the HMAC-SHA256 of that value with Duel's server seed as the key.

Duel cannot steer the result, because it committed to its seed before the drand value existed. Players cannot predict it, because the server seed is hidden. The published values are at api.drand.sh/v2/beacons/quicknet/rounds/ followed by the round ID.

The Crash formula shows the house edge in plain sight:

crash point = (2^32 / (number + 1)) x (1 - 0.001)

The 0.001 is the 0.1% base edge of the Originals. The crash guide covers the game and the roulette guide has the Castle Roulette payout table. The verification script for both is in how to verify a bet.

What it proves and what it does not#

A provably fair loss is still a loss. For the questions the proof cannot reach, the checks are different: is Duel.com legit covers the licence and operator, and a small test withdrawal covers payouts.

What is covered on Duel.com#

Duel's sitemap lists twelve Originals with their own pages: Dice, Blackjack, Plinko, Crash, Mines, Beef, Keno, Castle Roulette, Video Poker, Rock Paper Scissors, Limbo and Groomers Van.

Duel publishes the derivation for the nine in the table above. It has not done so for Rock Paper Scissors, Limbo and Groomers Van as of October 2026, so they are not documented here; check duel.com/fairness/verify or ask live chat for those.

Third-party slots and live dealer games are not provably fair. Their outcomes come from the studio's certified random number generator or from a physical dealer, and you cannot recompute them from seeds. That includes the Duel-branded live blackjack table, which is a live dealer product and not the Blackjack Original.

The house edge is a separate matter from fairness. Originals run at a 0.1% base edge, which Duel refunds as instant rakeback inside a daily allowance of $50,000 wagered and $1,000 per bet, with Beef at about 99.2% RTP by design. See house edge and RTP explained and Duel's Zero Edge page. The refund is pricing; no hash proves it.

Where the seed controls sit inside a Duel account is not something we can document reliably. Look for a fairness link in the game window or the bet detail, and ask the 24/7 live chat if you cannot find it.

Common misconceptions#

• Provably fair means the game can be beaten. It cannot; the payout formula is fixed
• A long losing streak proves rigging. Streaks are normal variance; verify the bets if in doubt
• It covers every game on the site. Only the in-house Originals
• Rotating seeds changes your luck. A new pair has the same odds
• Seeing the hash is enough. The proof only exists once you compare it to the revealed seed

When to verify#

• Once, early. Rotate after your first session and check the commitment, so you know the process works on your account.
• When a result looks wrong. Recomputing it settles the question in a minute.
• Before you rely on the site for larger stakes. One verified pair is worth more than any review.

The rotation steps are in how to rotate your seed.

Summary#

Provably fair is commit-reveal: Duel shows the SHA-256 hash of a server seed, derives each result with HMAC-SHA256 from that seed plus your client seed, nonce and cursor, then reveals the seed on rotation so you can check both the hash and the results. Crash and Castle Roulette use a public drand value in place of a client seed. The method is published by Duel and reproducible with your own code. It proves the result was not altered. Solvency, withdrawals and third-party games need separate checks.