Sécurité & signature

import express from "express";
import crypto from "node:crypto";

const SIGNING_SECRET = process.env.LECOMMIS_SIGNING_SECRET;
const TOLERANCE = 5 * 60; // secondes

const app = express();

// Important : récupérer le corps BRUT, pas du JSON re-sérialisé.
app.post(
  "/webhooks/lecommis",
  express.raw({ type: "application/json" }),
  (req, res) => {
    const rawBody = req.body; // Buffer
    const signature = req.get("X-LeCommis-Signature") || "";
    const timestamp = req.get("X-LeCommis-Timestamp") || "";

    // Anti-rejeu.
    if (Math.abs(Date.now() / 1000 - Number(timestamp)) > TOLERANCE) {
      return res.status(401).send("stale timestamp");
    }

    const signedPayload = `${timestamp}.${rawBody.toString("utf8")}`;
    const digest = crypto
      .createHmac("sha256", SIGNING_SECRET)
      .update(signedPayload)
      .digest("hex");
    const expected = `sha256=${digest}`;

    const ok =
      expected.length === signature.length &&
      crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(signature));

    if (!ok) return res.status(401).send("invalid signature");

    // Signature valide -> répondre vite, traiter en async.
    res.status(200).send("ok");
  }
);