How to implement oauth in a Chrome app to access appengine app service

Question

I am working on a chrome app that uses the appengine app as a backend. Typically, you can use Google OAuth or the legacy Google ClientLogin to get the ACSID token. The Chrome API provides an identity to support oauth2 in chrome apps.

I can get the auth token but cannot convert it to a valid ACSID token. I have successfully done something similar on Android. The key point was that when I requested the authentication token, I had to specify a specific service ("ah" for the application). Can someone tell me what the scope should be and how to configure outh2 parameters in the manifest file ?!

Thanks in advance for your help.