App Identity
The Application Identity service provides the ability for an application to identify itself, by retrieving its App ID or the hostname part of its URL. The app's identity may be used to generate a URL or email address, or to make some run-time decision.
Many Google APIs support OAuth assertions to identify the source of a request. The App Identity API can create tokens that can be used to assert that the source of a request is the application itself. This token can be included in the HTTP headers of a call to identify the calling application. The OAuth token only works against Google systems. However you can use the underlying signing technology in the API to assert the identity of your application to other systems.