Commit Reveal Scheme on Ethereum

Hiding Actions and Generating Random Numbers

The Ethereum blockchain is public.

Since all transactions are public we have to use extra tricks to keep some things temporarily hidden. Let’s say we need input like an answer to a quiz or a move in a game from a group of players. We don’t want these players to be able to just watch the blockchain for their competitors’ answers. What we’ll do is have everyone hash their answer and submit that first (the commit). Next, everyone will submit their real answer (the reveal) and we can prove on-chain that it hashes to the committed value.

The Ethereum blockchain is public and deterministic.

That means random number generation is a hard problem. Miners can’t just generate randomness right now. One trick many have used in the past is to use the previous blockhash as a source or randomness. This has a few flaws including its public nature and susceptibility to miner tampering.

What we’ll do is have a player generate a random number and then hash it and send it on-chain (the commit). Next, on a future block, we’ll have them submit their original random number. Finally, we’ll hash their random number (that the miner shouldn’t know about) with the blockhash on the commit block (that the player couldn’t know about). This final hash is a pretty good source of randomness on-chain because it gives the player an assurance that the the miner didn’t manipulate it.

However, we probably shouldn’t use this randomness for something that is worth more than the block reward. Players and miners could collude and by sharing information they would have the opportunity to withhold mined blocks that aren’t winners for them.

This isn’t really a worry if we are just trying to generate a good random number for a game. We just want players to be confident that if they don’t share their reveal, they will get a good random number that isn’t manipulated by the miners. Let’s build out an example project to show how this works.


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