Verifying Webhook Signatures

Each webhook request includes a signature in the X-Premier-Letters-Signature header. Verify it using HMAC-SHA256:

const crypto = require('crypto');

function verifySignature(payload, signature, secret) {
  const expected = crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');
  return signature === expected;
}