code: function getGoogleCerts(kid, callback) {
    request({uri: 'https://www.googleapis.com/oauth2/v1/certs'}, function(err, res, body) {
      if (err || !res || res.statusCode != 200) {
        err = err || new Error('error while retrieving Google certs');
        callback(err);
      } else {
        try {
          var keys = JSON.parse(body);
        } catch (e) {
          return callback(new Error('could not parse certs'));
        }
  
        callback(null, keys[kid]);
      }
    });
  }
docs: Return the Google certificate that will be used for signature validation.
  
  A custom function can be used instead when passed as an option in the Strategy
  constructor. It can be interesting e.g. if caching is needed.
  
  @param {String} kid The key id specified in the token
  @param {Function} callback
  @api protected