Sign message with web3 and verify with openzeppelin-solidity ECDSA.sol

Question

I'm trying to get a little example working with ECDSA.sol here: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/cryptography/ECDSA.sol

This contract:

1. Generate a random(ish) bytes32 (stub for a future message digest).
2. Morph it into an EthSignedHash with keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
3. Use ecrecover to work out who signed a message.

pragma solidity 0.5.8;

import "../../node_modules/openzeppelin-solidity/contracts/cryptography/ECDSA.sol";

contract Sigs {

    using ECDSA for bytes32;

    function rndHash() public view returns(bytes32) {
        return keccak256(abi.encodePacked(block.number));
    }

    function ethSignedHash(bytes32 messageHash) public pure returns(bytes32) {
        return messageHash.toEthSignedMessageHash();
    }

    function recover(bytes32 hash, bytes memory signature) public pure returns(address) {
        return hash.recover(signature);
    }
}

On the client side:

1. Conjure up a random(ish) message.
2. Make an ethHash of the message.
3. Sign it.
4. Send ethHash and the signature to ecRecover and get the signer address.

I'm not sure what I'm missing. I think the original message hash should be signed and EIP-712 does the "Ethereum Signing", so I should send the "Ethereum Hash" to ecrecover (as implemented in ECDSA.sol). In any case, no combination brings any joy.

var Sigs = artifacts.require("./Junk/Sigs");

contract("Sigs", accounts => {

  var signer;

  beforeEach(async () => {
    signer = accounts[0];
    sigs = await Sigs.new({from: signer}); 
  });

  it("should recover signer address.", async () =>  {

    console.log("SIGNER: ", signer);

    var random  = await sigs.rndHash({from: signer});
    var ethHash = await sigs.ethSignedHash(random);

    // four possible combinations tried for the next step :/
    var signature = await web3.eth.sign(random, signer);    // sign the random hash or the ethHash
    var recovered = await sigs.recover(ethHash, signature); // recover from the random hash or the ethHash

    console.log("random1: ", random);
    console.log("signature: ", signature);
    console.log("recovered: ", recovered);

    assert.strictEqual(recovered, signer, "The recovered signature does not match the signer.");

  });    
});

I'd be very grateful if someone can sort me out.

Truffle v5.0.41 (core: 5.0.41)
Solidity - 0.5.8 (solc-js)
Node v8.10.0
Web3.js v1.2.1

Thanks!

Update

For anyone else who happens across this, this is the revised, passing test.

const Sigs = artifacts.require("./Junk/Sigs");
const EthCrypto = require("eth-crypto");

contract("Sigs", accounts => {

  var signer;

  beforeEach(async () => {
    signer = accounts[0];
    sigs = await Sigs.new({from: signer}); 
  });

  it("should recover signer address.", async () =>  {

    console.log("SIGNER: ", signer);

    var message = "0x1234";
    console.log("message: ", message);
    var msgHash    = await sigs.messageHash(message);
    var ethHash = await sigs.ethSignedHash(msgHash);

    var signature = await web3.eth.sign(msgHash, signer);    // sign the mesage hash
    signature = signature.substr(0, 130) + (signature.substr(130) == "00" ? "1b" : "1c"); // v: 0,1 => 27,28
    var recovered = await sigs.recover(ethHash, signature); // recover from the ethHash

    console.log("msgHash: ", msgHash);
    console.log("ethHash: ", ethHash);
    console.log("signature: ", signature);
    console.log("recovered: ", recovered);

    assert.strictEqual(recovered, signer, "The recovered signature does not match the signer.");

  });    
});

Answer 1

The problem is that eth.sign returns a signature where v is 0 or 1 and ecrecover expect it to be 27 or 28.

A note in the documentation for web3 v0.20 is clear:

Note that if you are using ecrecover, v will be either "00" or "01". As a result, in order to use this value, you will have to parse it to an integer and then add 27. This will result in either a 27 or a 28.

You have to do something like this

var signature = await web3.eth.sign(random, signer);    // sign the random hash or the ethHash
signature = signature.substr(0, 130) + (signature.substr(130) == "00" ? "1b" : "1c");
var recovered = await sigs.recover(ethHash, signature); // recover from the random hash or the ethHash

Answer 2

Rob, I see that you have already resolved (which is great).

For future readers:

For OpenZeppelin Contracts you can check the documentation for more information, in this case the Cryptography section: https://docs.openzeppelin.com/contracts/2.x/utilities#cryptography

The tests for the contracts can be helpful to look at, in this case ECDSA.test.js: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/test/cryptography/ECDSA.test.js

There is a tutorial on signing in the forum: https://forum.openzeppelin.com/t/sign-it-like-you-mean-it-creating-and-verifying-ethereum-signatures/697

If you have questions on using OpenZeppelin you can also ask in the Community Forum: https://forum.openzeppelin.com/

Disclosure: I am the Community Manager at OpenZeppelin