Webhook subscriptions

import { createHmac, timingSafeEqual } from "node:crypto"

// IMPORTANT: capture the raw body BEFORE any JSON middleware parses it.
// app.use("/webhooks/chert", express.raw({ type: "application/json" }))

export function verifyChertWebhook(rawBody, headers, subscriptionSecret) {
  // Prefer the modern header; fall back to the legacy one.
  const header =
    headers["x-webhook-signature"] || headers["x-chert-signature"] || ""

  const modern = /^t=(\d+),v1=([a-f0-9]+)$/i.exec(header)
  const legacy = /^v1,(\d+),([a-f0-9]+)$/i.exec(header)
  const m = modern || legacy
  if (!m) return false

  const ts = Number(m[1])
  const provided = m[2].toLowerCase()

  // 5-minute replay window.
  if (Math.abs(Date.now() / 1000 - ts) > 300) return false

  const expected = createHmac("sha256", subscriptionSecret)
    .update(`${ts}.${rawBody}`)        // <-- period, not colon
    .digest("hex")

  const a = Buffer.from(provided, "hex")
  const b = Buffer.from(expected, "hex")
  return a.length === b.length && timingSafeEqual(a, b)
}