authHelper.js

var request = require("request");
var readJSON = require("read-json");
var writeFile = require("write");
var googleIdToken = require("google-id-token");

function getGoogleCerts(kid, callback) {
  console.log("Checking cache for google certs");
  readJSON("/tmp/certs.json", function(error, cert){
    if (error) {
      console.log("Cert not found, downloading from google");
      request({uri: "https://www.googleapis.com/oauth2/v1/certs"},
          function (err, response, body) {
            if (err && response.statusCode !== 200) {
              console.log(err || "error while retrieving the google certs");
              callback(err, {});
            } else {
              console.log("Writing certs to disk");
              writeFile("/tmp/certs.json", body, function(err) {
                if (err) {
                  console.log("Error writing cert to disk", err);
                }
              });
              var keys = JSON.parse(body);
              callback(null, keys[kid]);
            }
          });
    } else {
      console.log("Using cert found on disk");
      callback(null, cert[kid]);
    }
  });
}

exports.getUserByGoogleToken = function (token, accountId, callback) {
  var parser = new googleIdToken({getKeys: getGoogleCerts});
  parser.decode(token, function(err, token) {
    if (err) {
      console.log("error while parsing the google token: " + err);
      callback("Can't parse googleToken");
    } else {
      console.log("Checking for account with email", token.data.email);
      //check if email and accountId match in dynamoDB
      callback(null, /* user object from db */);
    }
  });
}