Signature verification

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

const app = express();
const SECRET = process.env.ANCHOR_WEBHOOK_SECRET;
const REPLAY_WINDOW_SECONDS = 2 * 60; // 2 minutes — reject anything older

// ─── Idempotency store (in-memory; swap for Redis/DB in production) ──
// Maps event id (e.g. evt_01HXJ4...) -> expiry timestamp in ms.
// We keep entries for at least the retry budget so a late retry is still
// recognised as a duplicate. 24 h covers Anchor's worst-case retry window.
const DEDUPE_TTL_MS = 24 * 60 * 60 * 1000;
const seenEventIds = new Map();
setInterval(() => {
  const now = Date.now();
  for (const [id, expiresAt] of seenEventIds) {
    if (expiresAt < now) seenEventIds.delete(id);
  }
}, 60_000).unref();

// IMPORTANT: capture the raw bytes BEFORE JSON parsing.
app.post(
  '/anchor/webhooks',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const signatureHeader = req.header('Anchor-Signature') || '';
    const timestamp = req.header('Anchor-Timestamp') || '';
    if (!signatureHeader || !timestamp) return res.status(400).send('missing signature');

    // 1. Freshness / replay protection (compare the request's `Anchor-Timestamp`
    //    against the local clock at the moment the request arrived).
    const ageSeconds = Math.abs(Date.now() / 1000 - Number(timestamp));
    if (Number.isNaN(Number(timestamp)) || ageSeconds > REPLAY_WINDOW_SECONDS) {
      return res.status(400).send('stale request');
    }

    // 2. Parse the v1=<hex> value out of "t=...,v1=..."
    const parts = Object.fromEntries(
      signatureHeader.split(',').map((p) => p.split('=')),
    );
    const provided = parts.v1;
    if (!provided) return res.status(400).send('missing v1 signature');

    // 3. Constant-time HMAC compare
    const baseString = `v0:${timestamp}:${req.body.toString('utf8')}`;
    const expected = crypto
      .createHmac('sha256', SECRET)
      .update(baseString)
      .digest('hex');
    if (
      expected.length !== provided.length ||
      !crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(provided))
    ) {
      return res.status(401).send('invalid signature');
    }

    const event = JSON.parse(req.body.toString('utf8'));

    // 4. Idempotency — short-circuit duplicates with 200 so Anchor stops retrying.
    if (seenEventIds.has(event.id)) {
      console.log('Duplicate event, acknowledging without re-processing:', event.id);
      return res.status(200).send('ok');
    }
    seenEventIds.set(event.id, Date.now() + DEDUPE_TTL_MS);

    console.log('Verified event:', event.type, event.id);
    // ... business logic here ...
    res.status(200).send('ok');
  },
);

app.listen(3000);