How can I decode a google OAuth 2.0 JWT (OpenID Connect) in a node app?

Question

I'm having a heck of a time here trying to use google OAuth to authenticate users in my node express app. I can successfully do the OAuth, which returns a response like so:

{
  access_token: 'token string',
  id_token: 'id.string',
  expires_in: 3599,
  token_type: "Bearer"
}

This all makes sense, but I can't for the life of me figure out how to decode the JWT. I am a bit inexperienced in all this, so this is all a bit foreign to me.

Following the instructions listed here: https://developers.google.com/accounts/docs/OAuth2Login#validatinganidtoken I am attempting to decode the JWT locally in my node app.

I installed https://github.com/hokaccha/node-jwt-simple in my node environment.

And I'm pretty certain I need to use this certificate (https://www.googleapis.com/oauth2/v1/certs) in all this somehow to decode it, but I am at a bit of a loss here. I don't really understand how I get the certificate into my node app, and after that how to use it with node-jwt-simple. And I also don't really understand how I know when I need to pull a fresh certificate, vs using a cached one.

Anyone out there with some experience in this that can help me out?

Thanks for any help. I'm totally at a loss at this point.

** Update **

So I have made some progress... Kind of. By calling jwt.decode(id_token, certificate, true); I am able to successfully decode the token. Even if the certificate var is an empty object {}. This leaves me with 3 questions still. 1: What is the best way to get the certificate into my express app using the url from google? 2: How will I know when I need to pull in a fresh version of it? 3: It seems like passing in true for noVerify (3rd arg in jwt.decode) is a terrible idea. How can I get that to work without passing that in? It looks like perhaps jwt-simple is expecting hs256 and the token is using rs256.

Again, I'm super inexperienced in this, so I may be way off base here.

* UPDATE * Thanks to the help from Nat, I was able to get this working! I think I tried every single JWT and JWS node module out there. What I finally landed on is as follows: I found that none of the modules that I looked at did quite what I wanted out of the box. I created the following jwt decoding helper methods that I am using to decode the id_token, so I can get the kid from the header.

module.exports = {
  decodeJwt: function (token) {
    var segments = token.split('.');

    if (segments.length !== 3) {
      throw new Error('Not enough or too many segments');
    }

    // All segment should be base64
    var headerSeg = segments[0];
    var payloadSeg = segments[1];
    var signatureSeg = segments[2];

    // base64 decode and parse JSON
    var header = JSON.parse(base64urlDecode(headerSeg));
    var payload = JSON.parse(base64urlDecode(payloadSeg));

    return {
      header: header,
      payload: payload,
      signature: signatureSeg
    }

  }
}

function base64urlDecode(str) {
  return new Buffer(base64urlUnescape(str), 'base64').toString();
};

function base64urlUnescape(str) {
  str += Array(5 - str.length % 4).join('=');
  return str.replace(/\-/g, '+').replace(/_/g, '/');
}

I am using this decoding to determine if I need to pull in a new public cert from: https://www.googleapis.com/oauth2/v1/certs

Then I am using that public cert and node-jws (https://github.com/brianloveswords/node-jws) jws.verify(id_token, cert) to verify the signature!

Hooray! Thanks again for the extra explanation you gave in your response. That went a long way in helping me understand what I was even trying to do. Hope this might help others too.

Answer 1

From the specification point of view, what you are encountering is [OpenID Connect].

id_token is a [JWS] signed [JWT]. In this case, it is a "." separated string with three components. The first portion is the header. The second is the payload. The third is the signature. Each of them are Base64url encoded string.

When you decode the header, you will get something like:

{"alg":"RS256","kid":"43ebb53b0397e7aaf3087d6844e37d55c5fb1b67"}

The "alg" indicates that the signature algorithm is RS256, which is defined in [JWA]. The "kid" indicates the key id of the public key that corresponds to the key used to sign.

Now I am ready to answer some of your questions:

2: How will I know when I need to pull in a fresh version of it?

When the kid of the cached cert file (a [JWK] file) does not match the kid in the header, fetch a new cert file. (BTW, the URL from which you pull the certs are called x5u.)

3: It seems like passing in true for noVerify (3rd arg in jwt.decode) is a terrible idea. How can I get that to work without passing that in?

Indeed. Perhaps you might want to look at another library such as kjur.github.io/jsjws/ .

References

• [OpenID Connect] openid.bitbucket.org/openid-connect-core-1_0.html
• [JWS] tools.ietf.org/html/draft-ietf-jose-json-web-signature
• [JWT] tools.ietf.org/html/draft-ietf-oauth-json-web-token‎
• [JWK] tools.ietf.org/html/draft-ietf-oauth-json-web-keys
• [JWA] tools.ietf.org/html/draft-ietf-jose-json-web-algorithms