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How do I start?
There are three sets of libraries available for the Authentication services:
Authentication for Installed Applications: The ClientLogin API lets you incorporate programmatic login into your desktop or mobile applications. This API supports CAPTCHA for greater security against password trollers.
Authentication for Web Applications: These APIs let your web application access a user's Google data without handling the user's login information. Google provides a library based on the OAuth open standard. As an alternative, developers can continue to use Google's AuthSub library, which offers several options related to security requirements.
Federated Login with OpenID: The Federated Login API lets you hand off login authentication to Google.
* CAPTCHA is a trademark of Carnegie Mellon University
What is the Account Authentication Service?
We welcome the development of third-party applications that communicate and exchange information with Google services for their users. Many Google services are protected by Google accounts, and users must explicitly authorize third-party access. This means your application needs a way to manage user authorization. The Google Accounts Authentication Service helps streamline this task by processing requests for access and issuing authentication tokens. Many Google services support the Authentication Service, including those based on the Google Data API.
The Authentication Service can be used to access both regular Google accounts and hosted Google accounts (Google Apps). Although the user experience varies slightly, the process of managing authentication programmatically is the same for both types of accounts; no special steps need to be taken.
Google now supports federated login; your users can now log into your website or application using their Google account. The goal of federated login is improve the online login experience for everyone by (1) minimizing the number of different names and passwords people need for the sites they visit, and (2) letting developers deploy web sites without setting up a login system. Google's login authentication uses the OpenID 2.0 protocol, which has open source implementations available for a number of different development platforms used by Google Data API developers. (Currently enables for regular Google accounts only.)