What Is Provably Fair?
Provably fair is a cryptographic method that lets you verify a crypto casino bet's outcome was locked in before it was placed and could not be changed afterward. It combines a server seed, a client seed and a nonce through a hash function — you can recompute the same hash yourself and compare it to the published result. It proves the math wasn't rigged; it doesn't prove the casino is licensed, pays out reliably, or has fair terms.
What is provably fair gambling?
A cryptographic system letting players independently verify that each game outcome was fixed in advance and not altered by the casino after a bet was placed.
Traditional online casinos rely on a Random Number Generator (RNG) that's audited periodically by a third party like eCOGRA or iTech Labs — you have to trust that audit. Provably fair removes the need for third-party trust: the math is open, and you can check every single bet result yourself using free tools, without waiting for anyone's certification.
What are the server seed, client seed and nonce?
The server seed is the casino's secret input, the client seed is yours, and the nonce is a counter that increases with each bet — together they're hashed to produce the result.
- Server seed: A random string generated by the casino. Before you play, the casino shows you only its hash (a scrambled fingerprint) — not the seed itself, so it can't quietly swap it mid-session.
- Client seed: A string you provide (or the casino generates one you can change). Because you control it, the casino can't predict the final outcome ahead of time even though it holds the server seed.
- Nonce: A number that increments by one with every bet you place using the same seed pair, so identical seeds never produce identical results twice in a row.
How the pieces combine
The casino's software runs the server seed, your client seed, and the current nonce through a hash algorithm — commonly HMAC-SHA256. The hash output is converted into the game result (a dice roll, a card, a crash multiplier). Because hash functions are one-directional, nobody can work backward from the published hash to predict future results, but once the server seed is revealed, anyone can work forward and confirm the math.
How do I actually verify a provably fair bet?
After the casino reveals the unhashed server seed, run it plus your client seed and nonce through the stated hash algorithm yourself and compare the output to the published result.
Note the seed hash before you play
Before placing bets, the casino should display a hash of the current server seed — save it or screenshot it.
Play your session as normal
Every bet uses the same server seed and client seed, with the nonce increasing by one each time.
Rotate the seed to reveal it
When you change your client seed or request a new server seed, the casino reveals the previous (now-retired) server seed in full.
Confirm the hash matches
Run the revealed server seed through the same hash function the casino uses (SHA-256) and check it matches the hash shown in step 1 — this proves the seed wasn't swapped.
Recompute the bet result
Using a free HMAC-SHA256 calculator (many are available online), feed in the revealed server seed, your client seed, and each bet's nonce, and confirm the output matches the game result you were shown.
| Step | What you check | What it proves |
|---|---|---|
| Before playing | Casino-published server seed hash | Server seed exists and is fixed |
| During play | Client seed & nonce for each bet | Your inputs are recorded correctly |
| After seed rotation | Revealed (unhashed) server seed | Casino didn't change the seed mid-session |
| Independent recompute | Hash of seed + client seed + nonce | Each bet's result wasn't altered after the fact |
Provably fair verifies the game math only. It says nothing about whether a casino holds a valid gambling licence, pays out withdrawals reliably, or has fair bonus terms — those need separate checks, which is exactly what we cover in our review methodology.
Does provably fair guarantee a casino is trustworthy?
No. It only proves individual game outcomes weren't tampered with — it says nothing about licensing, withdrawal reliability, or terms, which need separate checks.
Treat provably fair as one input among several. Combine it with the licensing and terms checks described in our methodology page, and with basics like reading a glossary of terms such as RTP and house edge before you judge a casino's overall trustworthiness. For a ranked comparison of operators that publish working provably-fair tools, see our best crypto casinos page.