Minimum Anti-Collusion Infrastructure (MACI) is a base layer for bribery-resistant, secure, and private digital voting.

Applications like build atop MACI to increase privacy and discourage bribery for public goods funding.

MACI offers the following guarantees:

  • Collusion resistance: no-one except a trusted coordinator should be certain of the validity of a vote, reducing the effectiveness of bribery.
  • Receipt-freeness: no-one voter prove (besides to the coordinator) which way they voted.
  • Privacy: no-one except a trusted coordinator should be able to decrypt a vote.
  • Uncensorability: no-one — not even the trusted coordinator — should be able to censor a vote.
  • Unforgeability: only the owner of a user's private key may cast a vote tied to its corresponding public key.
  • Non-repudiation: no-one may modify or delete a vote after it is cast, although a user may cast another vote to nullify it.
  • Correct execution: no-one — not even the trusted coordinator — should be able to produce a false tally of votes.

Under the hood, MACI uses Ethereum smart contracts and zero-knowledge proofs. It inherits security and uncensorability from the underlying Ethereum blockchain, ensures unforgeability via asymmetric encryption, and achieves collusion resistance, privacy, and correct execution via zk-SNARK proofs.

Although MACI can provide collusion resistance only if the coordinator is honest, a dishonest coordinator can neither censor nor tamper with its execution.

Note that MACI presumes an identity system where each legitimate member controls a unique Ethereum private key.

MACI was originally proposed by Vitalik Buterin in this post.