ark-anon-vote

Experimental implementation of onchain anonymous voting using arkworks, following a similar design done in vocdoni/zk-franchise-proof and aragonzkresearch/oav (which are done in Circom).

Scheme

The main idea is that users send their vote + zk-proof to the smart contract, proving that they belong to the census (without revealing who they are) and that their vote has not been already casted.

Each user generates a random SecretKey, and computes the corresponding VotingKey by hashing it. The census creator adds all the VotingKeys as leafs in the census tree, obtaining the CensusRoot:

Each user computes a zk-proof which will be verified onchain, proving that they know a SecretKey corresponding to a VotingKey (without revealing any of both), which is placed in some CensusTree leaf (without revealing which one) under the CensusRoot. Also proves that their SecretKey hashed together with the ProcessId leads to the given Nullifier, in order to prevent proof reusability.

Constraints system (grey background indicates public inputs):

Usage

Import this lib:

ark-anon-vote = { git = "https://github.com/aragonzkresearch/ark-anon-vote" }

use ark_anon_vote::*;

let parameters = Parameters::init(&mut rng);
let av = AnonVote::<Groth16<Bls12_381>>::new(parameters.clone());

// set the process_id
let process_id: ProcessId = av.new_process_id(1 as u16);
// set number of max voters
let n_voters = 10;

// each voter generate voter's data
let voter = av.new_voter(&mut rng);
let nullifier = voter.nullifier(process_id);
let vote: Vote = av.new_vote(1 as u8);
// do this for each voter

// create the CensusTree
let mut census = av.new_censustree(n_voters).unwrap();

// add each voter VotingKey to the CensusTree
tree.update(0, &voter.to_bytes_le()).unwrap();
// [...]

// get Census Root
let root = tree.root();
// get user's census proof (merkletree membership proof)
let censusproof = census.generate_proof(0).unwrap();

// prepare Voter's inputs
let circuit =
   av.new_circuit_instance(root, process_id, nullifier, vote, voter.sk, censusproof);
   
// generate circuit setup
let (pk, vk) =
   AnonVote::<Groth16<Bls12_381>>::circuit_setup(&mut rng, circuit.clone()).unwrap();

// generate the zk-proof
let proof = AnonVote::<Groth16<Bls12_381>>::prove(&mut rng, pk, circuit.clone()).unwrap();

// prepare the public inputs
let pub_inputs = circuit.public_inputs();

// verify the zk-proof for the given VerificationKey and public inputs
let valid_proof = AnonVote::<Groth16<Bls12_381>>::verify(vk, proof, pub_inputs).unwrap();
assert!(valid_proof);

Check src/lib.rs for more details.