Receive Completion Events

import express from 'express';
import crypto from 'node:crypto';

const app = express();
const MAX_SKEW_SECONDS = 300; // 5 minutes

// Fail fast at boot rather than inside createHmac on the first delivery.
const SIGNING_SECRET = process.env.ENCORE_WEBHOOK_SECRET; // whsec_...
if (!SIGNING_SECRET) {
  throw new Error(
    'ENCORE_WEBHOOK_SECRET is not set: copy the signing secret from the dashboard under Settings.',
  );
}

// Mount the RAW body parser on this route: the signature covers exact bytes.
app.post(
  '/webhooks/encore',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const signature = req.get('X-Webhook-Signature');
    const timestamp = req.get('X-Webhook-Timestamp');
    const rawBody = req.body.toString('utf8');

    if (!signature || !timestamp) {
      return res.status(400).send('Missing signature headers');
    }

    // 1. Replay protection: reject stale (or future-dated) timestamps.
    const age = Math.abs(Math.floor(Date.now() / 1000) - Number(timestamp));
    if (!Number.isFinite(age) || age > MAX_SKEW_SECONDS) {
      return res.status(401).send('Timestamp outside tolerance');
    }

    // 2. Recompute the HMAC over `${timestamp}.${rawBody}`.
    const expected = crypto
      .createHmac('sha256', SIGNING_SECRET)
      .update(`${timestamp}.${rawBody}`)
      .digest('hex');

    // 3. Constant-time compare: never `===`, which leaks timing information.
    const a = Buffer.from(signature, 'utf8');
    const b = Buffer.from(expected, 'utf8');
    if (a.length !== b.length || !crypto.timingSafeEqual(a, b)) {
      return res.status(401).send('Invalid signature');
    }

    // 4. Verified. Acknowledge FIRST, then work asynchronously: there are no
    //    retries, so a slow handler risks the 5s timeout even though we
    //    already have the event.
    res.status(200).send('OK');

    const payload = JSON.parse(rawBody);
    if (payload.event === 'offer_completed') {
      // Idempotent on transactionId (see "Design for the guarantees").
      void grantReward({
        transactionId: payload.transactionId,
        userId: payload.userId,
        campaignName: payload.campaignName,
        payout: payload.payout, // may be null
      });
    }
  },
);

app.listen(3000);