Webhooks

webhook-id:        msg_5f0c…            # unique per delivery — your idempotency key
webhook-timestamp: 1757721600           # unix seconds, and part of what is signed
webhook-signature: v1,K3m…= v1,9Qa…=    # one or more signatures, space-separated

{webhook-id}.{webhook-timestamp}.{raw request body}

import crypto from 'node:crypto';

const TOLERANCE_SECONDS = 5 * 60;

/**
 * Verify an Esy webhook. `rawBody` must be the exact bytes received —
 * parse JSON only after this returns true.
 */
export function verifyEsyWebhook(rawBody, headers, secret) {
  const id = headers['webhook-id'];
  const timestamp = headers['webhook-timestamp'];
  const signatures = headers['webhook-signature'];
  if (!id || !timestamp || !signatures) return false;

  // Reject stale deliveries so a captured request cannot be replayed later.
  const age = Math.abs(Date.now() / 1000 - Number(timestamp));
  if (!Number.isFinite(age) || age > TOLERANCE_SECONDS) return false;

  const expected = crypto
    .createHmac('sha256', secret)
    .update(`${id}.${timestamp}.${rawBody}`)
    .digest('base64');

  // During a rotation Esy signs with both secrets; any match is valid.
  return signatures.split(' ').some((sig) => {
    const [version, value] = sig.split(',');
    if (version !== 'v1' || !value) return false;
    const a = Buffer.from(value);
    const b = Buffer.from(expected);
    return a.length === b.length && crypto.timingSafeEqual(a, b);
  });
}

// app/api/esy/route.ts
import { verifyEsyWebhook } from '@/lib/verify-esy-webhook';

export async function POST(request: Request) {
  const raw = await request.text(); // BEFORE parsing — the signature covers bytes
  const ok = verifyEsyWebhook(raw, Object.fromEntries(request.headers), process.env.ESY_WEBHOOK_SECRET!);
  if (!ok) return new Response('invalid signature', { status: 401 });

  const event = JSON.parse(raw);
  // …handle it, keyed on the webhook-id header so a retry is a no-op
  return Response.json({ received: true });
}