openzeppelin-contracts/test/helpers/sign.js

const utils = require('ethereumjs-util');

/**
 * Hash and add same prefix to the hash that ganache use.
 * @param {string} message the plaintext/ascii/original message
 * @return {string} the hash of the message, prefixed, and then hashed again
 */
function hashMessage (message) {
  const messageHex = Buffer.from(utils.sha3(message).toString('hex'), 'hex');
  const prefix = utils.toBuffer('\u0019Ethereum Signed Message:\n' + messageHex.length.toString());
  return utils.bufferToHex(utils.sha3(Buffer.concat([prefix, messageHex])));
}

// signs message using web3 (auto-applies prefix)
function signMessage (signer, message = '', options = {}) {
  return web3.eth.sign(signer, web3.sha3(message, options));
}

// signs hex string using web3 (auto-applies prefix)
function signHex (signer, message = '') {
  return signMessage(signer, message, { encoding: 'hex' });
}

module.exports = {
  hashMessage,
  signMessage,
  signHex,
};