Getting Google IdToken for service account? - google-cloud-platform

Question

I have a backend that is serving android clients, authenticating them with IdToken sent from the android app.
Now, I need to authenticate a service running on aws that is using my apis. So I figured a service account would do the trick, using the private pem file to create a IdToken and send it along just as the android clients do. But I find no way of obtaining an IdToken with these credentials. Is this possible (preferrably in nodejs).
Or am I on the wrong path here?

Answer 1

I know this is older, but I found this question and it didn't lead me to the answer I ended up with.
I followed the guide in https://cloud.google.com/endpoints/docs/openapi/service-account-authentication#using_a_google_id_token with some mix of https://cloud.google.com/iap/docs/authentication-howto, which mentioned that the key to this was to include a target_audience claim in the generated JWT.
So, essentially I made a JWT that looked like:
{
"exp": 1547576771,
"iat": 1547575906,
"aud":"https://www.googleapis.com/oauth2/v4/token",
"target_audience": "https://example.com/",
"iss": EMAIL OF SERVICE ACCOUNT
}
and posted that to https://www.googleapis.com/oauth2/v4/token with params grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer and assertion=<THE JWT>
Without target_audience the endpoint gave me an access token, but with it I got an id_token instead.

Answer 2

Grettings since 2020
I had problems in Java for take ID_TOKEN of a Google Service Account. My project had two years and i were using GoogleCredentials, fromStream method and a JSON credential, but this class didn't gave me ID_TOKEN, only access_token on a not JWT format.
I solved because on this years Google updated here java code for authentication, for take ID_TOKEN you must use this library https://github.com/googleapis/google-auth-library-java
<dependency>
<groupId>com.google.auth</groupId>
<artifactId>google-auth-library-credentials</artifactId>
<version>0.20.0</version>
</dependency>
And then use ServiceAccountCredential
String credPath = "/path/to/svc_account.json";
ServiceAccountCredentials sourceCredentials = ServiceAccountCredentials
.fromStream(new FileInputStream(credPath));
When you create this class, itself will authenticate with google and have a access_token,refreshToken...
For extract ID_TOKEN you must use this function:
String audience = "http://localhost"; //Your server domain
IdToken idToken = credential.idTokenWithAudience(audience, new ArrayList<IdTokenProvider.Option>());
String id_token = idToken.getTokenValue();
And with this you have a JWT token.
I hope this help people like me,that are trying get ID_TOKEN.

Answer 3

You cannot use service accounts generated for Google Cloud APIs to directly authenticate against your own APIs. How will you know which service account private keys are valid and which have been revoked? Google does not expose this information.
Service accounts are rather meant for delegation of credentials. When you access Google Cloud platform service, you will be authenticating with your google account credentials. You will not want to provision the very same credentials everywhere your running code needs to access any of the Google cloud services (i.e. Cloud APIs). Instead you create service accounts whose scope can be reduced to a subset of the scope of your google account credentials. This way a particular piece of code can be limited to only a few set of APIs.
Service Accounts
A service account is a special account that can be used by services
and applications running on your Google Compute Engine instance to
interact with other Google Cloud Platform APIs. Applications can use
service account credentials to authorize themselves to a set of APIs
and perform actions within the permissions granted to the service
account and virtual machine instance.
What are service accounts?
Service accounts authenticate applications running on your virtual
machine instances to other Google Cloud Platform services. For
example, if you write an application that reads and writes files on
Google Cloud Storage, it must first authenticate to the Google Cloud
Storage API. You can create a service account and grant the service
account access to the Cloud Storage API. Then, you would update your
application code to pass the service account credentials to the Cloud
Storage API. In this way, your application authenticates seamlessly to
the API without embedding any secret keys or user credentials in your
instance, image, or application code.
I know where your confusion stems, it is because service account also have the same OAuth model you are used to.
You can use service accounts to get access tokens and refresh them as needed, but the scope of authentication is at the very maximum limited to the surface of the Google Cloud APIs. You will not be able to mix and match your APIs with that.
Alternative is to either build your own authentication model (which is not so clear from your question when you say authenticating them with IdToken sent from the android app) or rely on something like Cloud endpoints which you create and manage APIs along with API keys for authentication.
As you already mentioned in one of your comments, you can follow the Service-to-Service authentication guide which describes how you can use Google Cloud Service accounts to authenticate with your APIs running on Google Cloud Endpoint.
It supports using Google ID JWT tokens. The caller will have to send the JWT to Google Token endpoints to obtain a Google ID token and then use this Google ID token in all of your requests. This approach also has the advantage that you only have to whitelist the Google ID token server in your API configuration.