web3.eth.sign sometimes gives V parameter of 54 or 55, ecrecover then fails. What's going on?

Question

I've been using ecrecover and web3.eth.sign to authorize transactions, but when I run web3.eth.sign a second time on my server, the V part of the signature is wrong(or, at least, ecrecover thinks it's wrong). I read that it should be 27 or 28, but sign() is giving me 54 and 55. However, if I changes these to 27 and 28, respectively, ecrecover works again. What's going on????

exports.sign = async(signer, nonce, amount, forUser) => {
  // pack data into the message
  let ret = [];
  let data = (nonce * 2 ** 32) + amount;
  let hexData = web3.utils.toHex(data).slice(2);
  for(let i = hexData.length; i < 16; i++) {
    hexData = "0" + hexData
  }
  let message = "0x1337beef" + forUser.slice(2) + hexData;
  // sign the message
  let sig = (await web3.eth.sign(message, signer)).slice(2);
  let r = "0x" + sig.slice(0, 64);
  let s = "0x" + sig.slice(64, 128);
  let v = web3.utils.toDecimal('0x' + sig.slice(128, 130)) + 27;
  // THESE NEXT TWO LINES MAKE THINGS WORK BUT THEY
  // ARE STINKY AND I MUST KNOW WHY THEY ARE NEEDED!!
  if(v == 54) { v = 27;}
  if(v == 55) { v = 28;}
  ret.push(message);
  ret.push(v);
  ret.push(r);
  ret.push(s);
  return ret;
}

Thanks in advance!!

Answer 1

You have not provided many details of your setup. What backend node are you using? What signature is returned when you sign the message? Web3.js sign method just calls through to the nodes implementation.

As outlined in this answer, Geth previously did not return the v value in the appropriate format. You had to add 27.

I suspect from brief consideration that you are using a Parity node which returns the correct v value. You are then adding 27 unnecessarily.

Hence why your values are too high.. by 27.