Provably Fair

Verify Your Bets

Don't trust — verify. Every game on FairWin uses cryptographic proofs so you can independently confirm any outcome.

New to provably fair? Read the full explainer →

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Verify a Bet🪙Coin Flip
How It Works— Coin Flip
1

Before your bet

A server seed is hashed (SHA-256) and shown to you before you flip. This proves the outcome was locked before your bet.

2

During the bet

The server seed, your client seed, and your bet nonce are combined using HMAC-SHA256. The result determines heads or tails.

3

After rotation

When you rotate seeds, the old server seed is revealed. Re-compute the HMAC yourself to confirm the flip was fair.

Algorithm— Coin Flip
HMAC Generation
hmac = HMAC-SHA256(serverSeed, clientSeed:nonce)
Outcome Formula
hmac[0..3] % 2
How It's Calculated

The first 4 bytes of the HMAC are read as an unsigned 32-bit integer, then taken modulo 2.

Possible Outcomes

0 (Heads) or 1 (Tails)

A simple even/odd check on the HMAC bytes determines the coin side. This gives an exactly 50/50 probability.