From their site: Google Accounts authentication for web-based applications allows the application to access a Google service protected by a user's Google account. To maintain a high level of security, the Authentication Proxy interface, AuthSub, enables the application to get an authentication token without ever handling the user's account login information. Using the proxy, the user of the web application logs into their account through a Google-supplied login page and consents to grant limited access to the web application.
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