Github push event signature don't match

Question

I'm coding a Webhook for GitHub, and implemented secure verification in KOA.js as:

function sign(tok, blob) {
  var hmac;

  hmac = crypto
    .createHmac('sha1', tok)
    .update(blob)
    .digest('hex');

  return 'sha1=' + hmac;
}

...

key = this.request.headers['x-hub-signature'];
blob = JSON.stringify(this.request.body);

if (!key || !blob) {
  this.status = 400;
  this.body = 'Bad Request';
}

lock = sign(settings.api_secret, blob);

if (lock !== key) {
  console.log(symbols.warning, 'Unauthorized');
  this.status = 403;
  this.body = 'Unauthorized';
  return;
}

...

for pull_requests and create events this works ok, even pushing new branches works, but for push commits events the x-hub-signature and the computed hash from the payload don't match, so it always get 403 unauthorized.

Update

I've noticed that for this kind of push payloads the commits and head_commit are added to the payload. I've tried removing the commits and the head_commit from the body but it didn't work.

Update

For more information please review these example payloads. I've also included url for the test repo and token info: https://gist.github.com/marcoslhc/ec581f1a5ccdd80f8b33

Answer 1

The default encoding of Crypto hash.update() is binary as detailed in the answer to Node JS crypto, cannot create hmac on chars with accents. This causes a problem in your push-event payload, which contains the character U+00E1 LATIN SMALL LETTER A WITH ACUTE in Hernández four times, and GitHub services is hashing the payload as utf-8 encoded. Note that your Gist shows these incorrectly-encoded in ISO-8859-1, so also make sure that you are handling the incoming request character-encoding properly (but this should happen by-default).

To fix this you need to either use a Buffer:

hmac = crypto.createHmac('sha1', tok).update(new Buffer(blob, 'utf-8')).digest('hex');

... or pass the encoding directly to update:

hmac = crypto.createHmac('sha1', tok).update(blob, 'utf-8').digest('hex');

The correct hash of 7f9e6014b7bddf5533494eff6a2c71c4ec7c042d will then be calculated.