Provably fair is the system that lets you check a game wasn't rigged, without having to take anyone's word for it. It applies to the BC Originals (Crash, Dice, Limbo and the rest), and it's what makes the platform trustworthy in the first place.
The three ingredients
Server seed: a secret string the site generates and commits to by publishing its hash before you play.
Client seed: a string tied to your browser that you can change at any time.
Nonce: a counter that increments each round.
Why it cannot be faked
Since the server seed is hashed and shown to you up front, the site is committed to that value before the round runs. The outcome is worked out from all three inputs. After the round the server seed is revealed, and you can re-run the public algorithm to confirm the result matches. If the site had changed anything, the revealed seed wouldn't match its earlier hash, and you'd catch it straight away.
It means a game round's outcome comes from combining a server seed with your own client seed, hashed in advance. Because you can change your seed and check the result afterwards, the operator can't change an outcome without you being able to spot it.
Can I verify a game round myself?
Yes. After a round you can reveal the server seed and, with your client seed and the nonce, reproduce the exact result using a public algorithm. Third-party verifiers let you check it independently.
