Provably fair verifier

How provably fair works

Provably fair games, common at crypto casinos and in crash games, replace “trust us” with maths. Each round uses three inputs:

• Server seed: a random string the casino generates. Before you play, it shows you only the hash of this seed (a SHA-256 fingerprint), which commits it without revealing it.
• Client seed: a string from your side, which you can usually change.
• Nonce: a counter that goes up by one with every bet.

The game combines them, usually with HMAC-SHA256, into a long hexadecimal hash and turns part of that hash into the result. When you rotate the seed, the casino reveals the old server seed. If hashing it gives the exact hash you were shown earlier, the casino could not have changed the seed after seeing your bets.

How to use the verifier

1. Choose the method the game uses. HMAC-SHA256 (key = server seed, message = client seed:nonce) is the most common; plain SHA-256 of server:client:nonce is used by some older games.
2. Paste the revealed server seed and, optionally, the server seed hash you saw before playing.
3. Enter your client seed and the nonce of the bet you want to check.
4. Read the results: the seed check (match or no match), the full result hash, a roll between 0 and 1 from its first eight hex characters, the same roll as a 0–100 dice number, and an example crash point.

All of this uses your browser’s built-in Web Crypto; no seed is sent anywhere.

A worked example

With the sample inputs (server seed b7e1c0d4a9f25e8b3c6d, client seed casinoslots, nonce 42) and HMAC-SHA256, the result hash begins f8fcfd90…. The first eight hex characters, f8fcfd90, divided by 2³² give a roll of 0.972610, which is 97.27 on a 0–100 dice. The example crash formula, 0.97 ÷ (1 − roll) rounded down to two decimals, gives 35.41×.

Put real money on it to see what that means: if you had bet R100 on that round with an auto cash-out at 2×, you would have collected R200, and the hash proves the 35.41× point existed before you bet. Change the nonce to 43 and you get a completely different hash, as you should.

What the check proves, and what it doesn’t

A matching hash proves the server seed wasn’t swapped. It doesn’t prove the game’s formula is fair, that its RTP is what the casino claims, or that the operator will pay out. Provably fair casinos in our data are offshore-licensed, so check the operator as well as the maths. Our provably fair casinos list and the provably fair games section cover both.

To see what the house edge means for cash-out targets, try the crash odds visualiser. If you’re playing to win back losses, take a break and call the NRGP free on 0800 006 008.