MACI

What is MACI?

tl;dr: bribers are ngmi (not going to make it)

Clr.fund uses MACI, Minimum Anti-Collusion Infrastructure, to help prevent vote manipulation like bribery or collusion. To understand why this is important let's recap quadratic funding...

Quadratic funding

This is the mechanism clr.fund uses in our funding rounds. When you contribute to a project you're not just sending them money, you're voting for them. At the end, the matching pool funds are distributed to projects based on number of votes at the end of the round.

Background

Vote Manipulation

Vote manipulation involves maliciously changing the incentives of a voting system, and it comes in many flavors, including:

Bribery — when a person or entity offers to pay others to vote for an outcome they otherwise might not have, e.g. paying a football team to throw a match
Coercion — when a person or entity threatens to punish others in some way if they don't vote for a particular outcome, e.g. blackmail

These kinds of manipulation break voting systems by diverting them from their intended goal. Voting systems are generally designed to discover a group's preference between a number of specified outcomes. Each voter is expected to choose how to vote based on which outcomes they prefer and how their vote can effect the result.

In an election where the outcomes are 1) Alice wins and 2) Bob wins, if I prefer Alice winning and believe my vote has a high enough chance of influencing the result to be worth the effort, I'll vote for Alice. Now, let's say Charlie offers me $1,000,000 to vote for Bob instead. For me, the possible outcomes of the election now shift to 1) Alice wins and 2) Bob wins and I get $1,000,000. In that scenario, I might vote for Bob even though I prefer Alice, and the election result will no longer reflect my preference between Bob and Alice, as was intended. If enough people are successfully bribed this way, the election will be a sham.

Vulnerability of Public Voting Systems

In order to manipulate the outcomes for a particular voter, the manipulator needs to know that voter's eventual choices: If Charlie offers to pay me $1,000,000 to vote for Bob, he needs to know that I'll actually vote for Bob before paying me. Otherwise, I can vote for Alice, tell Charlie I voted for Bob to get the $1,000,000, and the bribe has failed.

In traditional elections, votes are secret, making it difficult to manipulate voters. But there aren't any secrets on public blockchains like Ethereum! In most existing voting systems on Ethereum, votes are public, which means manipulation is a serious threat.

Why MACI?

Ethereum is a core part of clr.fund and other quadratic funding solutions like Gitcoin Grants, because:

Ethereum has a great combination of low infrastructure costs and high security (i.e. hard to hack) compared to other electronic voting solutions
Ethereum offers seamless interoperability with a large and growing ecosystem of communities and tools that both enhance and benefit from quadratic funding, such as DAOs and identity systems like BrightID
clr.fund was built by and for the Ethereum community

While moving away from the public-ness of Ethereum isn't an attractive option, building Ethereum-based tools to resist vote manipulation is, which brings us to MACI.

How MACI helps

Bribery only works if the briber can confirm that the person they're paying actually goes through with the conditions of the bribe. This is where MACI comes in. With MACI, it's impossible to prove how you voted, so bribers will never know if you did what they paid you to do.

MACI — Minimum Anti-Collusion Infrastructure

MACI is a set of Ethereum smart contracts and supplementary scripts that effectively transforms public voting systems into private ones, in which 1) participation in a vote and correct vote tabulation are publicly verifiable, but 2) specific voting information is not public, making vote manipulation much more difficult.

How it works

When you contribute to a project on clr.fund you register an identity with the round coordinator. But at any time before the end of the round, you can invalidate your identity by secretly changing the public key associated with your identity before casting the vote. You can then use a different key pair to sign the vote (which renders the vote invalid, but it's impossible to see if the vote is invalid).

MACI does all this using zero-knowledge proofs. These mathematically prove the authenticity of votes without providing any of the details. So bribers can’t tell what actions the people they bribed took.

From MACI's introduction documentation:

Whitelisted voters named Alice, Bob, and Charlie register to vote by sending their public key to a smart contract. Additionally, there is a central coordinator Dave, whose public key is known to all.
When Alice casts her vote, she signs her vote with her private key, encrypts her signature with Dave's public key, and submits the result to the smart contract.
Each voter may change her keypair at any time. To do this, she creates and signs a key-change command, encrypts it, and sends it to the smart contract. This makes it impossible for a briber to ever be sure that their bribe has any effect on the bribee's vote.
If Bob, for instance, bribes Alice to vote a certain way, she can simply use the first public key she had registered ⁠— which is now void ⁠— to cast a vote. Since said vote is encrypted, as was the key-changing message which Alice had previously sent to Dave, Bob has no way to tell if Alice had indeed voted the way he wanted her to.
Even if Alice reveals the cleartext of her vote to Bob, she just needs to not show him the updated key command that she previously used to invalidate that key. In short, as long as she had submitted a single encrypted command before her vote, there is no way to tell if said vote is valid or not.

MACI's constraints

This is cutting-edge technology and comes with a few constraints right now.

There are limits on the number of projects, contributors, and contributions. This means you may miss out if a round hits capacity but know that it's for the integrity of the round.
You can only contribute once per round. Once you have contributed, you can add/remove projects and reallocate your funds but you can't increase your total contribution amount.