Verifying signatures

import crypto from 'node:crypto';

/**
 * Throws if the signature is invalid or the timestamp is stale.
 * @param {string} rawBody   - The raw request body as a string (do NOT parse JSON first)
 * @param {string} sigHeader - The value of the X-Kwill-Signature header
 * @param {string} secret    - Your endpoint's signing secret (whsec_…)
 */
function verifyKwillSignature(rawBody, sigHeader, secret) {
  const TOLERANCE_SECONDS = 300;

  // Parse t=… and v1=… from the header
  const parts = Object.fromEntries(
    sigHeader.split(',').map((p) => { const i = p.indexOf('='); return [p.slice(0, i), p.slice(i + 1)]; })
  );
  const timestamp = parseInt(parts.t, 10);
  const signature = parts.v1;

  if (!timestamp || !signature) {
    throw new Error('Invalid X-Kwill-Signature header format.');
  }

  // Reject stale timestamps
  const now = Math.floor(Date.now() / 1000);
  if (Math.abs(now - timestamp) > TOLERANCE_SECONDS) {
    throw new Error('Webhook timestamp is too old — possible replay attack.');
  }

  // Compute expected signature
  const payload = `${timestamp}.${rawBody}`;
  const expected = crypto
    .createHmac('sha256', secret)
    .update(payload, 'utf8')
    .digest('hex');

  // Constant-time comparison
  const expectedBuf = Buffer.from(expected, 'hex');
  const receivedBuf = Buffer.from(signature, 'hex');
  if (
    expectedBuf.length !== receivedBuf.length ||
    !crypto.timingSafeEqual(expectedBuf, receivedBuf)
  ) {
    throw new Error('Webhook signature mismatch.');
  }
}

// Express example
app.post('/webhooks/kwill', express.raw({ type: 'application/json' }), (req, res) => {
  try {
    verifyKwillSignature(
      req.body.toString('utf8'),
      req.headers['x-kwill-signature'],
      process.env.KWILL_WEBHOOK_SECRET
    );
  } catch (err) {
    return res.status(400).send(err.message);
  }

  const event = JSON.parse(req.body);
  // handle event…
  res.sendStatus(200);
});