Cryptographic verification · runs in your browser

Provably Fair Verifier.

Reproduce any settled prediction-game's outcome from its committed seeds and confirm the revealed server seed matches the hash that was published before play began. This page never talks to FiftyFifty's servers — paste the audit data, press verify, and watch the math resolve in your own machine.

01

Inputs

From a settled game in the FiftyFifty app, open the Provably Fair panel and copy each value here. Or follow a "Verify this game" link to prefill these fields automatically.

Game-specific configuration defaults match FiftyFifty's launch ruleset
03

Specification

Every FiftyFifty prediction game commits a 32-byte server seed at creation. The SHA-256 of that seed is published immediately so the server can't change the seed afterwards. Both players also contribute a 64-character hex client seed. The game outcome is then derived deterministically from these inputs:

HMAC-SHA256 byte stream
1HMAC_SHA256(
2  key   = serverSeed,
3  data  = `${fairnessVersion}|${gameId}|${creatorSeed}|${opponentSeed}|${purposeTag}:${counter}`
4)

The counter starts at 0 and increments per 32-byte block. Bytes from the stream are read big-endian and reduced to uniform integers via mask-and-reject sampling — no modulo bias. Each game type uses a distinct purpose tag so streams never collide:

At settlement, the server seed is revealed — the same 32 bytes whose hash was published before play began. Anyone can then re-run the algorithm and confirm the outcome wasn't altered after the fact.

04

Trust Model

Don't trust this page. The whole point is that you don't have to.

  1. A
    Read the source. View Source on this page, or open verify.js in the repo. Around 100 lines wrap crypto.subtle — the rest is form handling.
  2. B
    Re-implement the algorithm in your favorite language. HMAC-SHA256 plus mask-and-reject sampling is small enough to write in an afternoon. Run a settled game's seeds through your version.
  3. C
    Compare. If your independent derivation disagrees with what FiftyFifty stored for the game, that's a critical bug — please report with the Game ID.