A block hash looks random, but the party that produces the block can influence it, and on some chains, anyone can read it before the spin settles, so a roulette contract that draws its pocket from a block hash alone is drawing from a number that is neither secret nor beyond control. The people who matter, block producers and fast bettors, have ways to shape or read it that ordinary players do not.
Four reasons are enough to rule out the method on its own. Sites in guides to the best crypto roulette sites that run on-chain have moved to a VRF or a commit and reveal, with the block hash as at most one input among several.
- The block producer can discard a losing block – A miner or validator who has a bet on the table can compute the hash of a candidate block before publishing it. If the hash would produce a losing pocket, they can withhold the block and try another arrangement of transactions, at the cost of the block reward. When the maximum payout exceeds a block reward, that trade is worth making.
- The hash is readable before settlement on many designs – Contracts that settle a bet in the same transaction as the bet, using the current block’s hash, give a bettor who can see the pending block the chance to bet only when the hash favours them. Using a future block’s hash closes this only if the bet is locked first and settlement cannot be delayed.
- The hash expires – Ethereum contracts can only read the hashes of the last 256 blocks. A design that draws from a future block and lets the player trigger settlement later allows a player to wait more than 256 blocks, at which point the contract reads a zero hash. Whatever pocket zero maps to is then the result of every expired bet, and players who know this can farm it.
- The hash has few bits of entropy that matter – A pocket is one of 37 values, so only the last few bits of the hash decide the outcome. A block producer does not need to control the whole hash. Changing the order of two transactions, or the timestamp by a second, is enough to flip those bits, and either change is free.
Tables that still read a block hash limit the damage in four ways, and the contract’s code shows which are in place:
- Combined with a committed secret – The hash is mixed with a house secret committed before the bet and revealed after, so the producer cannot compute the pocket without the secret.
- Future block with a tight window – The bet locks, the hash is taken from a block a few positions ahead, and settlement must happen within a short window well inside the 256-block limit.
- Small maximum payout – The most a single spin can win is kept below the block reward, so discarding a block is never worth it.
- Producer exclusion – Some designs delay settlement past a point where the producer of the source block cannot also be the producer of the settling block.
A block hash is a fine ingredient and a poor sole source. When you read a table’s fairness page or its contract, the question is what else feeds the draw. If the answer is nothing, the table is drawing from a number that its block producers can shape.