const crypto = require('crypto');

// IMPORTANTE: capture o corpo BRUTO. Ex.:
// app.use('/webhook', express.raw({ type: 'application/json' }));

function verificarWebhook(req, apiSecret) {
  const header = req.headers['x-webhook-signature']; // "sha256=<hex>"
  const timestamp = req.headers['x-webhook-timestamp'];
  if (!header || !timestamp) return false;

  // 1. Anti-replay: rejeita timestamps fora de ±5 min
  const agora = Math.floor(Date.now() / 1000);
  if (Math.abs(agora - Number(timestamp)) > 300) return false;

  // 2. Recalcula o HMAC sobre `${timestamp}.${rawBody}`
  const rawBody = req.body.toString('utf8'); // Buffer -> string crua
  const signedPayload = `${timestamp}.${rawBody}`;
  const esperado = crypto
    .createHmac('sha256', apiSecret)
    .update(signedPayload)
    .digest('hex');

  // 3. Comparação em tempo constante
  const recebido = header.replace(/^sha256=/i, '');
  const a = Buffer.from(esperado, 'hex');
  const b = Buffer.from(recebido, 'hex');
  return a.length === b.length && crypto.timingSafeEqual(a, b);
}

app.post('/webhook', (req, res) => {
  if (!verificarWebhook(req, process.env.API_SECRET)) {
    return res.status(401).send('assinatura inválida');
  }
  const evento = JSON.parse(req.body.toString('utf8'));
  // ... idempotência via req.headers['x-webhook-event-id'] e processamento
  res.status(200).send('OK');
});