Building a valid JWT Claims Set

Intro

I needed to review a Pull Request that was fixing a reported issue with a sample and while tests were passing, it felt like there must be something more going on with the sample that needed to change.

I've known about JSON Web Tokens (JWTs pronounced "jots") for a while, I don't know them. Seeing this PR gave me a reason to dig into them more.

What are JWTs

JWTs are an open standard method for securely transmitting information between parties. They are often used to authenticate users and authorize access to resources. They consist of 3 parts, a header, payload, and signature.

Why do you need JWTs

The IAM Service Account Credentials API creates short-lived credentials for impersonating IAM service accounts. The signJwtmethod will sign the JWT using a service account's system-managed private key. Within the request body of the signJwt method, the payload field should contain a serialized JSON object that contains a JWT claims set.

What are JWT Claims

Claims are the core information that the JWT transmits.

Here is an example of a valid claims set:

{
  "iss": "https://cloud.google.com/iam",
  "sub": "projects/-/serviceAccounts/my-service-account@my-project.iam.gserviceaccount.com",
  "aud": "https://my-iap-protected-app.example.com",
  "iat": 1694003600,
  "exp": 1694007200
}

This claims set includes the following fields:

• iss: The issuer of the JWT, the service account that will be authenticating, in this case should be the service account email.
• sub: The user subject of the JWT, which is the service account email.
• aud: The audience of the JWT, which is the URL of the IAP-protected resource.
• iat: The issued at time, which is the time at which the JWT was generated. This needs to be an integer in UTC.
• exp: The expiration time, which is the time at which the JWT will no longer be valid. There are additional limitations that Cloud IAM documents: that it can not be in the past and no more than 12 hours in the future.

By including these claims in the payload of your JWT, you can ensure that it is valid and can be used to access IAP-protected resources.

So about that PR

The original code looked like

    iat = datetime.datetime.now(tz=datetime.timezone.utc)
    exp = iat + 3600
    return json.dumps(
        {
            "iss": service_account_email,
            "sub": service_account_email,
            "aud": resource_url,
            "iat": iat,
            "exp": exp,
        }
    )

I know something is up with the testing but I don't want to block getting the underlying issue with this code resolved.

The PR author submitted the change

iat = datetime.datetime.now(tz=datetime.timezone.utc).timestamp()

This seemed like it would be an incomplete fix. Based on the documentation for the API, I realized that the submitted fix still wouldn't set iat to type int and would still fail. I proposed a slight change that would resolve the PR's issue of

now_utc = datetime.datetime.now(tz=datetime.timezone.utc)  
iat = int(now_utc.timestamp())

The more I thought about it, I realized that Datetime isn't useful for this sample. The Datetime module supplies classes for manipulating dates and times which are useful when you want dates. We literally need an integer in UTC so the Time module is more useful.

Insetad, we can do this

  now = int(time.time())

    return json.dumps(
        {
            "iss": service_account_email,
            "sub": service_account_email,
            "aud": resource_url,
            "iat": now,
            "exp": now + 3600,
        }
    )

Find the complete (updated!) code for this sample here.

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