Server seed, client seed and nonce

The server seed is a secret random value chosen by the game; only its hash is shown before play, which commits the game to it. The client seed is a value on the player side that the server cannot choose alone. The nonce is a counter that goes up by one every round, so the same pair of seeds never produces the same result twice.

After the server seed is revealed, a player can recompute any round from these three values. In Diver and Neon Rush, for example, the result comes from an HMAC-SHA256 of the client seed and nonce, keyed with the server seed.

Keeping the server seed secret until after play is what stops anyone from predicting results, while publishing its hash first is what stops the game from changing them.