Webhook verifiers,and the wire format.

import crypto from "node:crypto";

// Your stored secret from the Webhooks page (whsec_...). We hash it
// to match the key the server signs with — the server never stores
// the raw secret, so verifiers must derive the HMAC key the same way.
const SIGNING_KEY = crypto
  .createHash("sha256")
  .update(process.env.NARA_WEBHOOK_SECRET)
  .digest("hex");

export function verifyNaraWebhook(req) {
  const sig = req.headers["x-nara-signature"] || "";
  const ts = req.headers["x-nara-timestamp"] || "";
  const body = req.rawBody; // raw bytes, NOT the parsed JSON

  // Replay-window guard: reject anything older than 5 minutes.
  const now = Math.floor(Date.now() / 1000);
  if (Math.abs(now - Number(ts)) > 300) {
    return { ok: false, reason: "stale_timestamp" };
  }

  const expected = crypto
    .createHmac("sha256", SIGNING_KEY)
    .update(`${ts}.${body.toString("utf8")}`)
    .digest("hex");

  // Constant-time compare to defeat timing attacks.
  const got = sig.replace(/^sha256=/, "");
  if (got.length !== expected.length) return { ok: false, reason: "bad_sig" };
  if (!crypto.timingSafeEqual(Buffer.from(got), Buffer.from(expected))) {
    return { ok: false, reason: "bad_sig" };
  }
  return { ok: true };
}