hook-provider-sign.js

const Web3 = require('web3');
const SignerProvider = require('ethjs-provider-signer');
const EthereumTx = require('ethereumjs-tx');
const keystore = require('../lib/keystore');

const provider = new SignerProvider('http://localhost:8545', {
    signTransaction(rawTx, cb) {
        const { from } = rawTx;
        const privateKey = keystore.getPrivateKeyByAddress(from); // Load user's private key from your own keystore
        const privateKeyHex = Buffer.from(privateKey, 'hex');
        const tx = new EthereumTx(rawTx);

        tx.sign(privateKeyHex);
        cb(null, '0x' + tx.serialize().toString('hex'));
    }
});

const web3 = new Web3(provider); // Use this instance of web3 to sign with your own logic above