Google APIs use the OAuth 2.0 protocol for authentication and authorization. Google supports common OAuth 2.0 scenarios such as those for web server, installed, and client-side applications. When posting questions, it is most helpful if you could explain who owns the resource that is to be accessed.
Google OAuth 2.0 is a relatively simple protocol. To begin, you obtain OAuth 2.0 credentials from the Google Developers Console. Then your client application requests an access token from the Google Authorization Server, extracts a token from the response, and sends the token to the Google API that you want to access.
References
- OAuth 2.0 for Web Server Applications
- OAuth 2.0 for Installed Applications
- OAuth 2.0 for Client-side Applications
- OAuth 2.0 for Devices
- OAuth 2.0 for Server to Server Applications
- Cross-client Identity
Useful links
Unofficial support forums:
Google OAuth2 Google+ community