Provably fair, explained simply

"Provably fair" means fair in a way you can prove. A provably fair game gives you what you need to check for yourself that a result was fixed before your bet and wasn't changed afterwards. No trust needed: you recompute it.

The problem it solves

In an online game the draw happens on the server. Nothing stops you from suspecting the site looks at your bet and then picks a crash just before your cash-out. Provably fair answers that suspicion with a simple cryptography tool: the fingerprint.

Seed and hash: the two ingredients

• The seed is a random string of characters. The whole round's outcome follows from it through a fixed calculation.
• The hash (fingerprint) is the output of the SHA-256 function applied to the seed. It has two useful properties: the same seed always gives the same fingerprint, and you can't recover the seed from the fingerprint, nor find another seed with the same one.

How the proof works

1. Before the round, the server draws the seed and shows you only its fingerprint. It's now committed: it can't change the seed without the fingerprint changing.
2. You play. The fingerprint tells you nothing about the outcome.
3. At the end, the server reveals the seed. You compute its SHA-256: if it matches the fingerprint from the start, the seed didn't move. Then you redo the outcome calculation and compare.

Many casinos add a client seed (chosen by the player) and a round counter, combined with the server seed, so the player also contributes to the randomness.

Pothole as an example

In Pothole, the seed is a 32-character hexadecimal string drawn at the start. For each km i, the game computes SHA-256 of "seed:i", takes the first 8 characters, turns them into a number and divides by 2³², giving a number u between 0 and 1. If u is greater than or equal to that km's threshold (q, set by the multiplier table), the pothole is there.

A real example on the City street, with the seed 7c1e4b9a2f0d8e63a5b1c4d7e9f20a38, whose fingerprint shown at the start would be 3d93cd37bec7c8c519374e9c789f308f30fc4bf13e42e7109e5f5e224331328a:

The pothole was at km 2 from the start. A player who cashes out at km 1 wins; a player who drives on falls in. Either way, the outcome was written before the first tap. To recompute a real round, use the round verification tool.

What the proof shows, and what it doesn't

It proves the result didn't change during your round. On its own it doesn't prove the seeds are truly random, especially without a client seed. That part is checked over time: across many rounds, results should follow the stated probabilities. A provably fair game can also have a poor RTP: an honest draw and a generous game are two different things. More in are crash games rigged?, and it's also why a predictor can't see anything coming.