Express verify signed message from metamask

this gist show you how you can verify in express a message signature generated by web3 with the metamask injected provider using the etherjs library

initial setup

cd /tmp && mkdir test-evm-sign && cd test-evm-sign
yarn init -y
yarn add express ethers
touch index.js # copy paste content of below's index.js in this file
touch index.html # copy paste content of below's index.html in this file
node index.js # browse http://localhost:3000/ metamask popup show at page load to sign message 'hello'

index.js

const path = require('path')
const express = require('express')
const ethers = require('ethers')

const app = express()
const port = 3001

app.use(express.json())

app.get('/', (req,res) => {
	res.sendFile(path.join(__dirname+'/index.html'))
})

app.post('/verify', (req, res) => {

  const { signer, signature, message } = req.body

  const msgHash = ethers.utils.hashMessage(message)
  const msgHashBytes = ethers.utils.arrayify(msgHash)

  const recoveredPubKey = ethers.utils.recoverAddress(msgHashBytes, signature)

  res.json(signer === recoveredPubKey)
})

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

index.html

<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title></title>
</head>
<body>

<script src="https://cdnjs.cloudflare.com/ajax/libs/ethers/5.5.4/ethers.umd.min.js" integrity="sha512-xmbPx0riylir51GhTZCFd20yS7NYZNpfDTbEWBjDRzs+UaGb2RyjtASTVtF2ydQWp3xkso9j4sJj39PdSH8/EA==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
<script type="text/javascript" type="module">
window.addEventListener('DOMContentLoaded',async (event) => {

	const provider = new ethers.providers.Web3Provider(window.ethereum)
	const signer = provider.getSigner()
	const sig = await signer.signMessage('hello')

	const reqBody = {
		signer: await signer.getAddress(),
		signature: sig,
		message: 'hello'
	};

	const rawResponse = await fetch('http://localhost:3000/verify', {
	    method: 'POST',
	    headers: {
	      'Accept': 'application/json',
	      'Content-Type': 'application/json'
	    },
	    body: JSON.stringify(reqBody)
	  });
	  const isVerified = await rawResponse.json();
	  console.log(`isVerified: ${isVerified}`)
});
</script>
</body>
</html>

minute-med/verify-metamask-signed-message-express.md

Express verify signed message from metamask

initial setup

index.js

index.html