Webhook: Compute the hash property

API
2

Assuming express, we can throw a little bodyParser middleware at it

var express = require('express');
var bodyParser = require('body-parser');

var app = express();

app.use(bodyParser.json({
    verify: function(req, res, buf, encoding) {
        // is there a hub to verify against
        req.twitch_hub = false;
        if (req.headers && req.headers['x-hub-signature']) {
            req.twitch_hub = true;

            var xHub = req.headers['x-hub-signature'].split('=');

            req.twitch_hex = crypto.createHmac(xHub[0], mysecret)
                .update(buf)
                .digest('hex');
            req.twitch_signature = xHub[1];
        }
    }
}));
app.route('/webhook/').post(function(req, res) {
    res.send('OK');
    if (req.twitch_hub && req.twitch_hex == req.twitch_signature) {
        // it's good
    } else {
        // it's bad
    }
})

Additional example/previous forum post on the same subject: