Provably fair round verifier

Paste the seeds a game revealed after your round and recompute its outcome yourself. Everything runs in this page with the browser’s Web Crypto API; nothing you type leaves your device.

Server seed (revealed)

Client seed

Nonce

Commitment shown before play (optional)

Result

Status

Press “Verify round”.

First cursors

Cursor	HMAC message	First 8 bytes	Uniform u

How it works

Before you play, the game commits to a secret server seed by showing its SHA-256 fingerprint.
Each random number is HMAC-SHA256(serverSeed, "clientSeed-nonce-cursor"). The cursor counts draws within one round.
The first 8 bytes become a number in [0, 1) as hi/2³² + lo/2⁶⁴, with a single rounding so any language gets the same bits.
Each game maps that number to an outcome: roulette floor(u × 37), a die floor(u × 6) + 1, limbo floor(0.99 / (1 − u) × 100) hundredths.
After the seed is revealed, its fingerprint must match the commitment, and every outcome must match what you were shown.

A verifier proves the round was fixed before you played and computed as published. It does not change the odds or the house edge of any game.

Port of the MIT-licensed reference implementation, cross-checked on 7,203 values with zero mismatches. Source: github.com/betkyo-open-labs/provably-fair-verifier · npm: betkyo-provably-fair-verifier · Data: Betkyo/odds-data · DOI 10.5281/zenodo.22725012