Webhooks

import { createHmac, timingSafeEqual } from 'crypto'

function verifyWebhook(body, signatureHeader, secret, tolerance = 300) {
  // Parse signature header
  const parts = signatureHeader.split(',')
  const timestamp = parts.find(p => p.startsWith('t='))?.slice(2)
  const signature = parts.find(p => p.startsWith('v1='))?.slice(3)

  if (!timestamp || !signature) {
    throw new Error('Invalid signature format')
  }

  // Check timestamp freshness (5-minute tolerance)
  const now = Math.floor(Date.now() / 1000)
  if (Math.abs(now - parseInt(timestamp)) > tolerance) {
    throw new Error('Timestamp too old')
  }

  // Compute expected signature
  const signedPayload = `${timestamp}.${body}`
  const hmac = createHmac('sha256', secret)
  hmac.update(signedPayload)
  const expected = hmac.digest('hex')

  // Constant-time comparison
  const sigBuffer = Buffer.from(signature, 'hex')
  const expectedBuffer = Buffer.from(expected, 'hex')

  if (sigBuffer.length !== expectedBuffer.length) {
    return false
  }

  return timingSafeEqual(sigBuffer, expectedBuffer)
}

// Express example
app.post('/webhook', express.raw({ type: 'application/json' }), (req, res) => {
  const signature = req.headers['x-pymstr-signature']
  const body = req.body.toString()

  try {
    if (!verifyWebhook(body, signature, process.env.WEBHOOK_SECRET)) {
      return res.status(401).send('Invalid signature')
    }

    const payload = JSON.parse(body)
    // Process the event...

    res.status(200).send('OK')
  } catch (error) {
    res.status(400).send(error.message)
  }
})