MACI Circuits setup

Compile circuits

Prerequisites

Before building the project, make sure you have the installed the dependencies as explained in the installation page.

Building MACI circuits

To build the main circuits of MACI, run the following command (-c postfix for c++ witness gen, and -wasm postfix for WASM witness gen only):

pnpm build-test-circuits-c -- --out-path $OUT_PATH
pnpm build-test-circuits-wasm -- --out-path $OUT_PATH

Please note that the circuits are configured with testing purpose parameters, which means it can only handle a limited amount of messages (up to 25 messages). For more information on the parameters and how to configure them, please refer to the individual circuit documentation within this page. Also, within the configure-circomkit section of the installation page, you'll see how you can update the config file with new params.

To compile a single circuit, you can run:

pnpm circom:build $CIRCUIT_NAME

Please note that the name should match one of the circuit names inside the circom.json file.

### Parameters

MACI's circuits are parameterized, and thus can be configured to support different number of users, messages (votes), and vote options. Please ensure you understand what each of these parameter means and correctly configure them when building circuits from scratch, and when deploying the smart contracts.

• STATE_TREE_DEPTH = how many users the system supports
• MESSAGE_TREE_DEPTH = how many messages (votes) the system supports
• VOTE_OPTIONS_TREE_DEPTH = how many vote options the system supports
• MESSAGE_BATCH_TREE_DEPTH = how many messages in a batch can the circuit process
• INT_STATE_TREE_DEPTH = how many ballots can be processed per batch when tallying the results

For instance, given a binary tree for signups, if you set STATE_TREE_DEPTH to 2, the system will support 4 users, as 2 ** 2 = 4.

For messages, we use a quinary (five leaves) merkle tree, so if you set MESSAGE_TREE_DEPTH to 2, the system will support 32 messages, as 5 ** 2 = 32.

Please refer to the individual circuit documentation for more details on the inner working of each circuit and where parameters fit.

Generating zKeys

Run from the root directory to save to the cli/zkeys folder:

pnpm setup:zkeys -- --outPath ../cli/zkeys

Run from the circuits folder with --outPath to save to a custom folder:

cd circuits && pnpm gen-zkeys --outPath ./CUSTOM_FOLDER_NAME

The larger the trees, the more time this process may take. You may also need a machine with a very large amount of memory.

Note that you will have to modify the parameters inside the ./circuits/circom/circuits.json file to match your use case. For example, if you want to support up to 3125 messages, the message tree depth parameter should be set to 5 (as $5^5 = 3125$).

Measure the circuit sizes

The size of a circuit is denoted by its number of constraints. The larger this number, the more time it takes to compile it, generate its .zkey file, and perform phase 2 contributions.

Run this command to measure a circuit:

pnpm exec snarkjs r1cs info CIRCUIT_NAME.circom

Download the .ptau file

This file should be the result of the Perpetual Powers of Tau trusted setup contribution which Hermez Network selected.

When running the setup:zkeys command, the .ptau file will be downloaded automatically.

Generating and Validating ZK Proofs

To generate and validate ZK proofs from the artifacts produced by circom, you will need snarkjs.

Testing

To test the circuits package, please use pnpm run test. This will run all of the tests inside the tests folder.

To run individual tests, you can use the following commands (for all other circuits please refer to the package.json scripts section):

• pnpm run test:processMessages to run the tests for the processMessages circuit.
• pnpm run test:tallyVotes to run the tests for the tallyVotes circuit.

More details on testing are provided in the testing section of the documentation.