users.js

// Get user nonce
router.get('/:wallet_address/nonce', (req, res) => {
    // Check if user exists
    // ... search in database for user and returns its current nonce
});

// Process signed message
router.post('/:user/signature', (req, res) => {
    // Get user from db
    User.findOne({wallet_address: req.params.user}, (err, user) => {
        if (err) {
            res.send(err);
        }
        if (user) {
            const msg = `Nonce: ${user.nonce}`;
            // Convert msg to hex string
            const msgHex = ethUtil.bufferToHex(Buffer.from(msg));

            // Check if signature is valid
            const msgBuffer = ethUtil.toBuffer(msgHex);
            const msgHash = ethUtil.hashPersonalMessage(msgBuffer);
            const signatureBuffer = ethUtil.toBuffer(req.body.signature);
            const signatureParams = ethUtil.fromRpcSig(signatureBuffer);
            const publicKey = ethUtil.ecrecover(
                msgHash,
                signatureParams.v,
                signatureParams.r,
                signatureParams.s
            );
            const addresBuffer = ethUtil.publicToAddress(publicKey);
            const address = ethUtil.bufferToHex(addresBuffer);

            // Check if address matches
            if (address.toLowerCase() === req.params.user.toLowerCase()) {
                // Change user nonce
                user.nonce = Math.floor(Math.random() * 1000000);
                user.save((err) => {
                    if (err) {
                        res.send(err);
                    }
                });
                // Set jwt token
                const token = jwt.sign({
                    _id: user._id,
                    address: user.address
                }, process.env.JWT_SECRET, {expiresIn: '6h'});
                res.status(200).json({
                    success: true,
                    token: `Bearer ${token}`,
                    user: user,
                    msg: "You are now logged in."
                });
            } else {
                // User is not authenticated
                res.status(401).send('Invalid credentials');
            }
        } else {
            res.send('User does not exist');
        }
    });
});