Provably fair RNG API

Cryptographically verifiable random numbers.

A free, open-source API for provably fair random floats and integers. Every outcome includes a server hash you can independently verify.

API Documentation View on GitHub

How it works

Three steps. Anyone can re-run the math and prove the outcome wasn't cherry-picked.

  1. 1

    Server commits

    Before you call, the server publishes SHA-256(serverSeed). The seed itself stays secret.

  2. 2

    You request

    Send a clientSeed the server can't predict. The outcome is derived from both seeds — neither party can rig it.

  3. 3

    Anyone verifies

    The revealed serverSeed hashes back to the original commitment. Hit /verify or replay it offline.

Try It Live

Generate random numbers right now with our API.

cURL
Live Response
{
  "outcome": [],
  "message": "Click 'Generate' to try the API"
}
Your random numbers will appear here

Built For

Sharply-targeted landing pages for the most common verifiable-RNG workloads.

Dice Roller API

Standard NdM notation, verifiable per-roll outcomes.

Raffle Picker API

Draw a winner from a list, publish proof anyone can verify.

Card Shuffler API

A verifiable Fisher-Yates shuffle in one HTTP call.

Lottery API

Pre-committed lottery draws anyone can replay.

Sweepstakes API

Compliance-ready winner selection with a paper trail.

NFT Mint Randomizer API

Gas-free, off-chain trait and reveal-order randomness.

See all use cases →

Compare To

How Provable.io stacks up against the usual suspects.

vs Random.org

Hardware entropy vs publicly verifiable randomness.

vs Chainlink VRF

The off-chain alternative — no gas, no LINK, same proof.

vs crypto.randomUUID()

When the local CSPRNG isn't enough.

See all comparisons →

Frequently Asked Questions

Got questions? We've got answers.

What is a provably fair random number?

A provably fair random number is an outcome that anyone can independently verify was not chosen on purpose. The server publishes a cryptographic hash of its secret seed before you ask for a number; once your client seed is mixed in and the server seed is revealed, anyone can recompute the result and prove it matches the original hash.

Is the API free to use?

Yes. Anonymous calls are free and rate-limited by IP. Signing up gives you an API key for usage attribution and a daily quota of 10,000 requests at no cost.

How do I verify a past outcome?

Go to the /verify page or call GET /api/verifyServerHash with the original clientSeed and serverHash. The endpoint returns true if the values match our records, confirming the outcome was generated as claimed.

Can I use this for online gaming or a casino?

Provable.io is a general-purpose verifiable RNG suitable for game mechanics, raffles, NFT trait reveals, and similar use cases. It is not itself a gaming licence — operators in regulated jurisdictions are responsible for their own compliance, but the cryptographic transparency the API provides is exactly the kind of evidence regulators and players ask for.

How does the commit-reveal flow work?

Call POST /api/commit to receive a commitment id and a hash of an outcome that has been generated but not yet shown to you. Later, call POST /api/reveal with the commitment id to receive the outcome itself. Because the hash was issued before reveal, neither party could have changed the result after the fact.