how do we use `commit/reveal` to count votes?

Question

Let's say we are doing an election and it's easy to note that we need commit/reveal scheme to hide votes.

So, we store hashes while users commit their votes. Now, let's say the election is over. How do we count the votes ?

Solution: all users call reveal and pass their actual votes with the corresponding hash which means we get the actual vote from each one of users and we count the votes directly in reveal function. like this:

function reveal(vote) {
   require(keccak256(vote) == data[msg.sender]);
   if(vote == true) {
     trueVotes+=1;
   }else{
     falseVotes+=1;
   }
}

The problem with this solution is that all the users have to call reveal and some might forget about it at all to call it, which means we lose votes.

Are there any other solutions or how does it happen at all ?

Answer 1

Are there any other solutions

Can't say definitively that there isn't another solution but the core problem boils down to this. If it is possible for the contract to use any form of deduction whatsoever to deduce the result then the result is calculable by everyone which clearly defeats the secrecy goal. This is the generalized problem that commit/reveal solves.

In terms of "other" solutions to rising gas costs, off-chain but cryptographically verifiable processes could possibly help. That could be decentralized given a non-trivial approach that uses incentives. I can't give you a complete solution because it would be non-trivial.

If something is as simple as true or false, the hash for this is not hidden

I thought this comment deserves a remark. He is quite right. Two possibilities means it's not even brute-force to try both and one of them will work, so nothing is really hidden. That's easily solved by adding "salt" - a secret word known only to the voter. As a further buttress, make sure salt words are never used again.

Clients can easily generate a secret and retain it until reveal-time so it doesn't have to be a burden on the users. Something like:

function vote(bool vote, bytes32 salt) ...

Hope it helps.