Google Oauth - TokenVerifier How to USE? - oauth

Question

I'm trying to use Google OAuth with Sign in & Sign Up for my Web Server Application.
This is the page : https://developers.google.com/identity/sign-in/web/backend-auth that I have referenced, but I am stuck in using the Google Client API, the TokenVerifier that is mentioned below in the document. I tried to find some examples, but I couldn't find one, as I am not sure how to handle the parameters in the methods that the sample shows.
import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken.Payload;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdTokenVerifier;
...
GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(transport, jsonFactory)
.setAudience(Arrays.asList(CLIENT_ID))
.build();
// (Receive idTokenString by HTTPS POST)
GoogleIdToken idToken = verifier.verify(idTokenString);
if (idToken != null) {
Payload payload = idToken.getPayload();
if (payload.getHostedDomain().equals(APPS_DOMAIN_NAME)
// If multiple clients access the backend server:
&& Arrays.asList(ANDROID_CLIENT_ID, IOS_CLIENT_ID).contains(payload.getAuthorizedParty())) {
System.out.println("User ID: " + payload.getSubject());
} else {
System.out.println("Invalid ID token.");
}
} else {
System.out.println("Invalid ID token.");
}
For example, I know what these CLIENT_ID, ANDROID_CLIENT_ID, IOS_CLIENT_ID parameters mean in the sample code(in the reference page), but the server only receives id_token, which is basically a String Text. (That was made by the google api in the client-side, the javascript)
So, I do not have these parameter values passed to the server from the client. I know that google shows another way: the tokeninfo endpoint, but they mentioned that it is for only 100user/month only. (If I translated it correctly) However, for the tokeninfo endpoint way, they return the JSON file of containing client ids, which I think that would be the values for the parameters that I mentioned before, but I do not want to use the token info endpoint method.
So, my question is, how do I get the right parameter values for the sample code that is showed in the google document page? I only receive id_token value from the client.

Answer 1

ANDROID_CLIENT_ID or IOS_CLIENT_ID should be hard coded (in a config file) in your server's code.
Essentially your server is getting an id_token in a request and you need to make sure if it is meant for your app or server by checking the audience in there and making sure it matches one of the values you are expecting.