hardhat-circom

Hardhat plugin to integrate Circom and SnarkJS into your build process.

What

This combines the multiple steps of the Circom & SnarkJS workflow into your Hardhat workflow.

By providing configuration containing your Phase 1 Powers of Tau and circuits, this plugin will:

Compile the circuits
Apply the final beacon
Output your wasm and zkey files
Generate and output Verifier contracts

Installation

npm install hardhat-circom

Import the plugin in your hardhat.config.js:

require("hardhat-circom");

Or if you are using TypeScript, in your hardhat.config.ts:

import "hardhat-circom";

Tasks

This plugin adds the circom task to build circuit(s) into wasm and zkey file and template them to seperate Verifier contracts saved to the Hardhat sources directory (usually contracts/).

Usage: hardhat [GLOBAL OPTIONS] circom --circuit <STRING> [--debug] [--deterministic]

OPTIONS:

  --circuit      	limit your circom task to a single circuit name
  --debug        	output intermediate files to artifacts directory, generally for debug
  --deterministic	enable deterministic builds for groth16 protocol circuits (except for .wasm)

circom: compile circom circuits and template Verifier

For global options help run: hardhat help

You must run hardhat circom at least once to build the assets before compiling or deploying your contracts. Additionally, you can hook Hardhat's compile task to build your circuits before every compile, see Hooking compile below.

Basic configuration

Set up your project (we'll use best_dapp_ever/) with the following minimal hardhat.config.js at the root. The two required properties are ptau (see Powers of Tau) and circuits.

module.exports = {
  solidity: "0.6.7",
  circom: {
    // (optional) Base path for input files, defaults to `./circuits/`
    inputBasePath: "./circuits",
    // (required) The final ptau file, relative to inputBasePath, from a Phase 1 ceremony
    ptau: "pot15_final.ptau",
    // (required) Each object in this array refers to a separate circuit
    circuits: [{ name: "init" }],
  },
};

Your project structure should look like this:

j:~/best_dapp_ever/ $ tree
└── circuits
    ├── init.circom
    ├── init.json
    └── pot15_final.ptau

Now, you can use npx hardhat circom --verbose to compile the circuits and output InitVerifier.sol, init.zkey, and init.wasm files into their respective directories:

j:~/best_dapp_ever/ $ tree
├── circuits
│   ├── init.circom
│   ├── init.json
│   ├── init.wasm
│   ├── init.zkey
│   └── pot15_final.ptau
└── contracts
    └── InitVerifier.sol

Advanced configuration

If you'd like to adjust details about the circuit compilation or input/output locations, you can adjust any of these settings:

module.exports = {
  circom: {
    // (optional) Base path for files being read, defaults to `./circuits/`
    inputBasePath: "./mycircuits/",
    // (optional) Base path for files being output, defaults to `./circuits/`
    outputBasePath: "./client/",
    // (required) The final ptau file, relative to inputBasePath, from a Phase 1 ceremony
    ptau: "pot15_final.ptau",
    // (required) Each object in this array refers to a separate circuit
    circuits: [
      {
        // (required) The name of the circuit
        name: "init",
        // (optional) The circom version used to compile circuits (1 or 2), defaults to 2
        version: 2,
        // (optional) Protocol used to build circuits ("groth16" or "plonk"), defaults to "groth16"
        protocol: "groth16",
        // (optional) Input path for circuit file, inferred from `name` if unspecified
        circuit: "init/circuit.circom",
        // (optional) Input path for witness input file, inferred from `name` if unspecified
        input: "init/input.json",
        // (optional) Output path for wasm file, inferred from `name` if unspecified
        wasm: "circuits/init/circuit.wasm",
        // (optional) Output path for zkey file, inferred from `name` if unspecified
        zkey: "init.zkey",
        // Used when specifying `--deterministic` instead of the default of all 0s
        beacon: "0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f",
      },
      {
        name: "play",
        version: 1,
        protocol: "plonk",
        circuit: "play/circuit.circom",
        input: "play/input.json",
        wasm: "circuits/play/circuit.wasm",
        zkey: "play.zkey",
        // beacon isn't used for plonk protocol
      },
    ],
  },
};

Using the above file structure, you'd get this resulting tree after compile:

j:~/best_dapp_ever/ $ tree
├── client
│   ├── circuits
│   │   ├── init
│   │   │   └── circuit.wasm
│   │   └── play
│   │       └── circuit.wasm
│   ├── init.zkey
│   └── play.zkey
├── contracts
│   ├── InitVerifier.sol
│   └── PlayVerifier.sol
└── mycircuits
    ├── init
    │   ├── circuit.circom
    │   └── input.json
    ├── play
    │   ├── circuit.circom
    │   └── input.json
    └── pot15_final.ptau

Powers of Tau

You must provide a Powers of Tau from a Phase 1 ceremony. We recommend using one of the .ptau files from the Hermez Protocol's ceremony, available from their Dropbox folder.

These are all named powersOfTau28_hez_final_*.ptau where the * is some number. This number indicates the number of constraints (2^x) that can exist in your circuits.

Verifier contracts and templating

This plugin defers to the Solidity templates provided by SnarkJS, which generates a Verifier contract for each circuit.

However, there are no guarantees these templates are audited or up to date. It would be best to override it by hooking the templating task yourself (exported as TASK_CIRCOM_TEMPLATE).

You can hook the TASK_CIRCOM_TEMPLATE to output your own Verifier.sol contract.

For example, if you wanted to output a single Verifier for all your circuits:

import * as path from "path";
import * as fs from "fs/promises";
import { TASK_CIRCOM_TEMPLATE } from "hardhat-circom";
import { subtask } from "hardhat/config";

subtask(TASK_CIRCOM_TEMPLATE, "generate Verifier template shipped by SnarkjS").setAction(circomTemplate);

async function circomTemplate({ zkeys }, hre) {
  const myGroth16Template = await fs.readSync(path.resolve("./my_verifier_groth16.sol"), "utf8");
  const myPlonkTemplate = await fs.readSync(path.resolve("./my_verifier_plonk.sol"), "utf8");

  let combinedVerifier = "";
  for (const zkey of zkeys) {
    const verifierSol = await hre.snarkjs.zKey.exportSolidityVerifier(zkey, {
      groth16: myGroth16Template,
      plonk: myPlonkTemplate,
    });

    combinedVerifier += verifierSol;
  }

  const verifierPath = path.join(hre.config.paths.sources, "Verifier.sol");
  await fs.writeFile(verifierPath, combinedVerifier);
}

Determinism

Note: Determinism only applies to compiling with the groth16 protocol because it requires a trusted ceremony. The plonk protocol only relies on the universal powers of tau ceremony provided as your ptau configuration.

When you recompile the same circuit using the groth16 protocol, even with no changes, this plugin will apply a new final beacon, changing all the zkey output files. This also causes your Verifier contracts to be updated.

This causes lots of churn on large binary files in git, and makes it hard to know if you've actually made fundamental changes between commits.

For development builds of groth16 circuits, we provide the --deterministic flag in order to use a NON-RANDOM and UNSECURE hardcoded entropy (0x000000 by default) which will allow you to more easily inspect and catch changes in your circuits. You can adjust this default beacon by setting the beacon property on a circuit's config in your hardhat.config.js file.

Note: The wasm files currently have hardcoded system paths, so they will be deterministic on the same machine, but not between machines. If the .zkey files haven't changed you may disregard changes in the wasm files.

Debugging

When making circuit changes, it may be necessary to inspect the intermediate assets built between circom steps. You can output all intermediate files with the --debug flag. All the files from the circom build process will be saved to a circom/ directory in Hardhat's artifacts directory (./artifacts/circom/ by default).

j:~/best_dapp_ever/ $ tree artifacts
└── circom
    ├── init-contribution.zkey
    ├── init.r1cs
    ├── init.wasm
    ├── init.wtns
    └── init.zkey

Hooking compile

Some users might want their circuits compiled each time they run the Hardhat compile task. Hardhat's compile task isn't hooked by default because it imposes ordering restrictions on tasks you import and Circom compiles can take quite a long time to generate.

To opt into this behavior, you can hook the Hardhat compile task like so:

import { TASK_CIRCOM } from "hardhat-circom";
import { TASK_COMPILE } from "hardhat/builtin-tasks/task-names";

task(TASK_COMPILE, "hook compile task to include circuit compile and template").setAction(circuitsCompile);

async function circuitsCompile(args, hre, runSuper) {
  await hre.run(TASK_CIRCOM, args);
  await runSuper();
}

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