How to verify a provably fair bet, step by step

Before you start: get the data

1. 1Find the fairness or verification page of the site and note the formula it publishes.
2. 2Note your active client seed, the nonce of the bet and the hashed server seed shown before you played.
3. 3Rotate the seed pair, so the site reveals the old server seed.
4. 4Copy the revealed server seed and the old client seed.

Step 1: check the commitment

Hash the revealed server seed with SHA-256. You can use our SHA-256 generator. Compare the result with the hash you were shown before you played. They must be identical. If not, the site did not use the seed it committed to.

Step 2: compute the HMAC

Using the site's formula, compute HMAC-SHA256 with the server seed as the key and a message built from your client seed, the nonce and sometimes a round number, typically "clientSeed:nonce:round". Our provably fair calculator does this and shows each round, so you can compare it with your bet.

Step 3: convert to the game result

The site's documentation says how to turn the hash into an outcome. A common approach for a dice roll is to read the first four bytes as a fraction between 0 and 1, multiply by 10,001, round down and divide by 100. For other games, such as crash or cards, the mapping differs and may use more bytes or a formula with a house edge built in. Follow the documentation exactly.

Step 4: compare

If the number matches the result shown for your bet, that bet was computed honestly. Repeat for a few bets, especially ones with large wagers or surprising outcomes. If the number does not match, double-check the formula, the nonce and whether you used the right seed pair. If everything is correct and it still differs, contact the site and keep screenshots.

What a match does and does not tell you

• It tells you the result followed the committed inputs.
• It does not tell you the odds are good. Check the house edge separately.
• It does not guarantee you will be paid.

For lottery draws

The same idea applies to a lottery draw, with a public random value instead of a secret seed. Follow our step-by-step guide to verify a CryptoDrawz draw, or try the verify button on a published result on the Draws page.

A full example to follow

Use the demo values from our provably fair guide: server seed "cd-demo-server-seed", client seed "my-client-seed", nonce 0. Hash the seed with SHA-256 and compare with the commitment 3f51770c...681e4c. Compute HMAC-SHA256 with the seed as key and "my-client-seed:0:0" as the message, giving 5fbcfc92...cb3628. The first four bytes (5f bc fc 92) convert to 0.37397746, and the dice formula gives 37.40. Enter the same values in the provably fair calculator to see each step, then change the nonce and watch the result change.

When verification fails

• Check you are using the seeds from the right seed pair. After a rotation, new bets use new seeds.
• Check the nonce starts at 0 or 1 according to the site's documentation.
• Check the message format, including colons and the order of fields.
• Check the site's mapping from hash to game result.
• If everything is right and the result differs, screenshot the evidence and contact support.

Make verification a habit

Verify a few bets early on to see how the system works, and again after large wins or losses. A site that makes this easy and documents it clearly is showing confidence. A site that hides the formula or makes verification hard has told you something.