Verificación de firmas de webhooks

X-Tokeflow-Signature: t=1737030609,v1=5257a869e7ecebeda32affa62cdca3fa51cad7e77a0e56ff536d0ce8e108d8bd

import express from 'express';
import crypto from 'crypto';

const app = express();
const SIGNING_SECRET = process.env.TOKEFLOW_WEBHOOK_SECRET; // "whsec_..."
const TOLERANCE_SECONDS = 5 * 60;

// Use the raw body parser on the webhook route so the bytes are untouched.
app.post(
  '/webhooks/tokeflow',
  express.raw({ type: 'application/json' }),
  (req, res) => {
    const header = req.get('X-Tokeflow-Signature') || '';
    const rawBody = req.body; // a Buffer, thanks to express.raw

    if (!verify(rawBody, header, SIGNING_SECRET)) {
      return res.status(400).send('Invalid signature');
    }

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

    // Acknowledge immediately, then process asynchronously.
    res.status(200).send('ok');
    handleEvent(event); // dedupe on event.id, reconcile via the API
  },
);

function verify(rawBody, signatureHeader, secret) {
  // Parse "t=...,v1=..."
  const parts = Object.fromEntries(
    signatureHeader.split(',').map((kv) => kv.split('=')),
  );
  const timestamp = Number(parts.t);
  const provided = parts.v1;
  if (!timestamp || !provided) return false;

  // Reject stale timestamps (replay protection).
  const now = Math.floor(Date.now() / 1000);
  if (Math.abs(now - timestamp) > TOLERANCE_SECONDS) return false;

  // Recompute the HMAC over `${t}.${rawBody}`.
  const signedPayload = `${timestamp}.${rawBody.toString('utf8')}`;
  const expected = crypto
    .createHmac('sha256', secret)
    .update(signedPayload)
    .digest('hex');

  // Constant-time compare.
  const a = Buffer.from(expected, 'hex');
  const b = Buffer.from(provided, 'hex');
  return a.length === b.length && crypto.timingSafeEqual(a, b);
}