An example of how to verify a signed message on Ethereum

example-verify.js

const util = require('ethereumjs-util');
const Web3 = require('web3');

// msg is the plaintext message (preimage)
// sig is the signed message
// walletAddress is the address that you want to verify the message comes from

const verifySignature = async (msg, sig, walletAddress) => {
  const web3 = new Web3(null);
  const res = util.fromRpcSig(sig);
  const prefix = Buffer.from("\x19Ethereum Signed Message:\n");
  const prefixedMsg = web3.utils.sha3(Buffer.concat([prefix, Buffer.from(String(msg.length)), Buffer.from(msg)]));
  const pubKey = util.ecrecover(util.toBuffer(prefixedMsg), res.v, res.r, res.s);
  const addrBuf = util.pubToAddress(pubKey);
  const calcAddr = await util.bufferToHex(addrBuf);

  return (walletAddress.toLowerCase() === calcAddr);
}