OpenID is a decentralized single sign-on authentication system for the Internet. The goal of the OpenID initiative is to allow users to log in at websites around the Internet with one ID, instead of having to create multiple unique accounts.
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To use OpenID, a user must initially register with a Web site that supports OpenID. AOL users, for instance, can simply use their screen name because AOL supports OpenID. When visiting other sites that support OpenID, the user logs in with a URL, which in this case would be http://openid.aol.com/screenname.
Previous attempts at universal authentication methods, like Microsoft's Passport or the Liberty Alliance, failed in part because of proprietary implementations. OpenID was developed using the open source software model to be an interoperable protocol independent from any single organization.
As OpenID matures and evolves into a commercially viable, trusted identity provider service, enterprises will be able to consider outsourcing their federated identity management needs to the framework. Key issues to enterprise adoption will be resolving DNS spoofing vulnerabilities and tighter integration with the Security Assertion Markup Language ( SAML ). Today there are over a quarter of a billion OpenIDs and well over 10,000 Web sites that accept them.