How to Generate Provably Fair Random Numbers

How do you generate provably fair random numbers?

Concise answer: Commit to a secret server seed by publishing its SHA-256 hash, accept an independently chosen client seed, and derive the random number from both seeds plus a nonce. Reveal the server seed afterward so anyone can confirm the commitment and reproduce the exact result.

To generate one now, use the live provably fair random number generator or call the random integer API. Keep the returned clientSeed, serverHash, cursor, nonce, and outcome as the verification receipt. Then use the provably fair outcome verifier to check it.

Generate and verify in four steps

1. Record the server hash published before the request.
2. Supply a client seed and request the required float or integer range.
3. Save the result, seed coordinates, and public outcome permalink.
4. Reveal the server seed, check its hash, and reproduce the same result.

The trust problem

If a server tells you "here is a random number," how do you know it wasn't picked on purpose? Provably fair randomness solves this by letting you mathematically prove, after the fact, that the result was not chosen to favour anyone.

The three ingredients

Every provably fair outcome relies on three pieces:

• Server seed — a secret random string the server generates and commits to before you ask for an outcome.
• Server hash — a cryptographic hash (SHA-256) of the server seed, published up front. Once it's out, the server seed cannot be changed without the hash also changing.
• Client seed — a value you choose. Because the server has already committed to its seed, your contribution makes the final result unpredictable to either party.

The verification flow

1. The server gives you a serverHash before you ever request a number.
2. You request an outcome, supplying your clientSeed. The API combines server seed + client seed + a nonce and hashes the result to derive the random value.
3. When you're ready, the server reveals the original serverSeed. You hash it yourself and confirm it matches the serverHash you were given earlier.
4. You re-run the same hashing math locally with the revealed seeds. If you get the same outcome, the result was provably fair.

Try it now

Head to the verification page, paste a clientSeed and serverHash from any past response, and confirm the result. The page calls GET /api/verifyServerHash and tells you instantly whether the values match our records.

Further reading

• Quickstart with cURL
• Quickstart with Node.js
• Audit the open-source core library on GitHub