Delivery and signature

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

const TOLERANCE_SECONDS = 300;
const SIGNATURE_PATTERN = /^t=(\d{1,12}),v1=([0-9a-f]{64})$/;

/**
 * Verifies an `x-bbrands-signature` header against the raw request body.
 *
 * @param {string} secret   The `whsec_…` value returned once by create or rotate-secret.
 * @param {string} header   The `x-bbrands-signature` header.
 * @param {string} rawBody  The request body exactly as received (no re-serialisation).
 * @param {number} [now]    Current time in milliseconds (injectable for tests).
 */
export function verifyBBrandsSignature(secret, header, rawBody, now = Date.now()) {
  const match = SIGNATURE_PATTERN.exec(header ?? "");
  if (!match) return { valid: false, reason: "malformed" };

  const timestamp = Number(match[1]);
  if (Math.abs(Math.floor(now / 1000) - timestamp) > TOLERANCE_SECONDS) {
    return { valid: false, reason: "expired" };
  }

  const expected = createHmac("sha256", secret)
    .update(`${timestamp}.${rawBody}`)
    .digest("hex");
  const a = Buffer.from(expected, "hex");
  const b = Buffer.from(match[2], "hex");
  if (a.length !== b.length || !timingSafeEqual(a, b)) {
    return { valid: false, reason: "mismatch" };
  }
  return { valid: true, timestamp };
}

// Express: keep the raw body so the HMAC is computed over the original bytes.
app.post(
  "/hooks/bbrands",
  express.raw({ type: "application/json" }),
  (req, res) => {
    const result = verifyBBrandsSignature(
      process.env.BBRANDS_WEBHOOK_SECRET,
      req.get("x-bbrands-signature"),
      req.body.toString("utf8"),
    );
    if (!result.valid) return res.status(401).end();

    const delivery = JSON.parse(req.body.toString("utf8"));
    // Deduplicate on delivery.id, then hand the work to a queue.
    res.status(202).end();
  },
);