Halo2 Guide

In this guide we'll talk about how to run Halo2 (PLONK) Circuits on your iOS/Android devices. We won't go in details about how to setup and build your Halo2 Circuits or how to make an Expo App.

Quickstart

Prepping the Project

Let's get started with a blank expo project, you can also use any existing expo app, adding zk-proofs requires only a couple of lines really.

bun create expo --template blank-typescript

and then add our expo module and relevant libs.

bunx expo install zk-expo expo-asset

We won't go in details about how to setup and build your Halo2 Circuits or how to make an Expo App. You can find a tutorial on that here and here.

We already have some sample files ready for you that you'll need to get your circuits running, create a directory /public in the project root and put these files in it.

• halo2circuit.wasm

Next, create a file in the root called metro.config.js so that expo can pick up the wasm extension.

const { getDefaultConfig } = require("expo/metro-config");
const path = require("path");
 
const config = getDefaultConfig(__dirname);
 
config.resolver.assetExts = ["wasm"];
 
module.exports = config;

Running the circuits

import { useAssets } from "expo-asset";  
import * as ZkExpo from "zk-expo";  
 
export default function App() {
 
  const [assets, error] = useAssets([ 
    require("./public/halo2circuit.wasm"),  
  ]);  
 
  return (
    <View style={styles.container}>
      <Button title="Make Halo2 Proof" />
    </View>
  );
 
}
 
const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: "#fff",
    alignItems: "center",
    justifyContent: "center",
  },
});

import { useAssets } from "expo-asset";
import * as ZkExpo from "zk-expo";
 
export default function App() {
 
  const [assets, error] = useAssets([
    require("./public/halo2circuit.wasm"),
  ]);
 
  async function make() { 
    let inputs = { 
      secret: "1362766633228928585266883205500641602962979188179006392651332184307221268928", 
      message: "246923712567881323126559150739123310486883838966133236273155052809857112023", 
      scope: "187035976211163640032000461805755068405187575174480755232212391996596767257", 
    }; 

    if (assets) { 
      let proof = await ZkExpo.halo2Prove( 
        assets[0].localUri as string, 
        [1, 1, 55], 
      ); 
      console.log({proof}) 
    } else { 
      alert("Assets not loaded"); 
    } 
  } 
 
  return (
    <View style={styles.container}>
      <Button title="Make Halo2 Proof" onPress={make} /> // [!code highlight]
    </View>
  );
 
}
 
const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: "#fff",
    alignItems: "center",
    justifyContent: "center",
  },
});

On-chain Verification

• Tutorial on On-chain Verification

API Reference

ZkExpo.halo2Prove

async function halo2Prove(
  wasmPath: string,         // Local path to the wasm file.
  publicInputs: number[],   // An array of numerical inputs to the circuit.
): Promise<string>          // Returns the proof in hexadecimal.

Some Gotchas

1. Make sure to update your dependencies, also use npx expo install --fix.
2. Running Halo2 circuits in a WASM runtime using zk-expo requires some simple C glue, read about this in the Halo2 tutorial here.
3. We are following the PSE's fork of Halo2 v3 which has excellent support for on-chain proof verification.
4. Use Development Client Builds, this won't work on Expo Go as it requires native code.
5. iOS has memory restrictions of around 2-3GB imposed on each app. If your circuits are really large you might run into issues, consider splitting the circuits.
6. If your iOS app runs into any linking issues on developement when using eas build, do an bun expo prebuild and open the ios folder in XCode. XCode GUI uses certain Apple internals that perform better than eas builds that use xcodebuild under the hood.