As in web3.js get the same signature as the signature from the signTypedData_v4 method?

Question

I have a standard typed data object that uses Metamask, according to this scheme(without JSON.stringify):

If you sign such an object like this:

let sig = web3.eth.accounts.sign(JSON.stringify(order), secret_key, true)

then the resulting signature will differ from the signature obtained when signing a similar object in the Metamask by the signTypedData_v4 method. Therefore, I want to get a signature in web3js similar to a Metamask. How can I do this? Thank you in advance!

Answer 1

let sig = web3.eth.accounts.sign(JSON.stringify(order), secret_key, true)

You are signing a JSON string, which is not correct as per EIP712 specs

You need to sign:

keccak256(
  abi.encodePacked(
    '0x1901', // EIP-712 prefix
    DOMAIN_SEPERATOR // a.k.a hash of domain
    hashStruct // hash of data to sign, definition - https://eips.ethereum.org/EIPS/eip-712#definition-of-hashstruct
  )
)

Following are a few of the examples:

• openzeppelin
• eth-sig-util (metamask)
• ethers.js

Answer 2

From MetaMask Docs, we find the following JavaScript example of signTypedData_v4. I think this is the only way to implement signTypedData_v4 in web3. Otherwise, you'll need to use MetaMasks's signTypedData(...) method from eth-sig-util.

const msgParams = JSON.stringify({
    types: {
      EIP712Domain: [
        { name: 'name', type: 'string' },
        { name: 'version', type: 'string' },
        { name: 'chainId', type: 'uint256' },
        { name: 'verifyingContract', type: 'address' },
      ],
      // Not an EIP712Domain definition
      Bid: [
        { name: 'name', type: 'string' },

      ],
      // Refer to PrimaryType
      Mail: [
        { name: 'from', type: 'Person' },
        { name: 'to', type: 'Person[]' },
        { name: 'contents', type: 'string' },
      ],
      // Not an EIP712Domain definition
      Person: [
        { name: 'name', type: 'string' },
        { name: 'wallets', type: 'address[]' },
      ],
    },
});

var from = web3.eth.accounts[0];

var params = [from, msgParams];
var method = 'eth_signTypedData_v4';

web3.currentProvider.sendAsync(
    {
      method,
      params,
      from,
    },
    function (err, result) {
      if (err) return console.dir(err);
      if (result.error) {
        alert(result.error.message);
      }
      if (result.error) return console.error('ERROR', result);
      console.log('TYPED SIGNED:' + JSON.stringify(result.result));
    }
)