问题:

How does Stack Overflow allow new users to login into Stack Overflow using their Gmail or Facebook or Yahoo accounts? I am working on a little project website and I was curious how Stack Overflow does this. Do I need to get permission from these companies to do this or can I just do it?

回答1:

StackOverflow (and all other StackExchange sites) use OpenID.

OpenID is a decentralized authentication protocol that makes it easy for people to sign up and access web accounts.

Yahoo, Google and facebook are all OpenID providers, so simply implementing it on your site will be enough for your users to be able to login using them (and any other OpenID provider).

There is no need to get permission - the permission will be gotten by the users of your application.

回答2:

You can "just do it" using OpenId. That's what StackOverflow uses.

回答3:

There are libraries available for Java Python, PHP, and others. Google authentication has been tested against the OpenID for Java library; see also libraries at OpenID Enabled. A comprehensive list of libraries is available on the OpenID site. For developers using OpenID for Java, this quick-start guide covers how to set up as an OpenID relying party. You may also use an implementation provided by a third-party such as JanRain