index.js

const express = require('express');
const snarkjs = require("snarkjs");
const ffjavascript = require('ffjavascript');

const app = express();

app.use(express.static('public'));

app.use(express.json());

const {stringifyBigInts: stringifyBigInts$3, unstringifyBigInts: unstringifyBigInts$1} = ffjavascript.utils;

// copy pasta p256 from snarkjs cli.cjs line 6726
function p256(n) {
  let nstr = n.toString(16);
  while (nstr.length < 64) nstr = "0"+nstr;
  nstr = `0x${nstr}`;
  return nstr;
}

async function genSolidityCalldata(publicName, proofName) {
  const pub = unstringifyBigInts$1(publicName);
  const proof = unstringifyBigInts$1(proofName);

  let inputs = [];
  for (let i=0; i<pub.length; i++) {
      inputs.push(p256(pub[i]));
  }

  if ((typeof proof.protocol === "undefined") || (proof.protocol != "groth16")) {
    throw new Error("InvalidProof");
  }

  let S = {
    'pi_a': [p256(proof.pi_a[0]), p256(proof.pi_a[1])],
    'pi_b': [[p256(proof.pi_b[0][1]), p256(proof.pi_b[0][0])], [p256(proof.pi_b[1][1]), p256(proof.pi_b[1][0])]],
    'pi_c': [p256(proof.pi_c[0]), p256(proof.pi_c[1])],
    'inputs': inputs,
  };

  return S;
}

app.get('/', (req, res) => res.send('Im a teapot'));

app.post('/generate_proof', async function (req, res) {
  const input = req.body;

  const { proof, publicSignals } = await snarkjs.groth16.fullProve(
    input,
    "./circuit.wasm",
    "./circuit.zkey"
  );

  const genCalldata = await genSolidityCalldata(publicSignals, proof);

  res.json(genCalldata);
})

const port = process.env.PORT || 3000;

app.listen(port, () => console.log(`Server running on ${port}, http://localhost:${port}`));