Provably fair explained

“Provably fair” is a promise you can check yourself. The game commits to a hidden random seed before you bet by showing you its hash. After the round it reveals the seed, so anyone can recompute the result and confirm nothing was changed. It’s covered briefly in How Crash Games Work. This guide goes deeper.

The ingredients

• Server seed: a secret random string chosen by the game. You see only its hash until it’s revealed.
• Server seed hash: the SHA-256 (or similar) hash of the server seed. Hashes are one-way: you can’t work out the seed from the hash, but any change to the seed changes the hash completely.
• Client seed: a string you, or your browser, supply. Because the server picked its seed before seeing yours, it can’t pick a seed that’s bad for you.
• Nonce: a counter, 1, 2, 3…, so each round with the same seeds gives a new result.

How a result is produced

A common pattern is:

1. Compute HMAC-SHA256(key = server seed, message = client seed:nonce).
2. Take the first 52 bits of the output as a number, and divide by 2⁵² to get U between 0 and 1.
3. Apply the crash formula: crash = max(1, floor(100 × (1 − edge) ÷ (1 − U)) ÷ 100).

Real games differ. Some use SHA-512, some combine several players’ seeds, and some use a chain of hashes generated far in advance. Always use the game’s own documented method. Our verifier offers a generic mode and adds provider presets only where the algorithm is published and we’ve confirmed it.

Try it

The verifier below runs entirely in your browser. Paste a revealed server seed, your client seed and the nonce.

What provably fair doesn’t do

• It doesn’t make the game profitable: the edge is in the formula.
• It doesn’t prove the casino pays out. That’s the regulator’s job.
• It doesn’t help predict future rounds. The next server seed stays hidden until it’s used, which is exactly why “predictor” tools can’t work. See Crash Game Hacks and Bots.