o you have a cool personalized application that you want to offer to over 400 million users? Do you want to light it up with Live controls or create a mashup with Live resources? Windows Live ID now offers a simple way for third parties to get Live ID authentication in their web and rich client applications, letting these applications reach millions of Live ID users, integrate with Live Controls, and access Live services.

This capability is increasingly attractive. Windows Live ID currently supports more than 400 million active users, performing over a billion authentications per day, with a consistent availability of over 99.9 percent. This foundation identity service is also available to third-party developers, letting them build identity-aware applications that take advantage of Windows Live services, user data, and social relationships.

This article focuses on the Windows Live ID web and client authentication and explains how you can use it in your application.

What Is Windows Live ID?
Windows Live ID is the identity and authentication platform for all Microsoft online services, including Windows Live, Xbox Live, Office Live, and so forth. Many third-party sites also use Live ID authentication. Formerly known as Microsoft Passport, the service was rebranded to reflect the "Windows Live" name, as well as new scenarios and features developed to support Windows Live.

For users, Live ID provides a single identity that lets them log into numerous Microsoft online services. They can use that same ID to send/receive e-mail, chat on messenger, set up a new Office Live account, or gain access anywhere they see the Live ID logo. Because the same sign-on is valid for all Live ID-enabled services, once a user has logged in at one site or rich application, that user is also seamlessly signed into other Windows Live sites. For example, a signed-in user can open Live Mail or Windows Live Spaces from Windows Live Messenger—without the need to provide credentials again. Also, signed-in users get a consistent experience across Microsoft online sites, because all these sites have access to the user's profile.

You can include Windows Live controls in your web application to seamlessly combine their features with your content.

Windows Live ID authenticates users using credentials such as a username and password or Cardspace information cards, which provide higher security against phishing. The Live ID service supports a variety of authentication protocols, including WS-*, allowing Windows Live federation partner sites to use Windows Live and other Live ID-enabled services. Live ID supports a variety of browsers and Windows PCs, as well as phones, MSN TV, and XBox.

Windows Live ID Web Authentication
Windows Live ID Web Authentication helps you build identity-aware Web applications by allowing you to:

• Verify the identity of visitors to your site.
• Offer personalized access to your site's content to millions of Live ID users, who don't need to create yet another identity.
• Provide single sign-on from your application to Windows Live services.
• Seamlessly integrate the functionality of Windows Live controls in a non-programmatic way.
• Access a user's Windows Live data with their explicit consent.

Why Does Your Web Application Need Live ID?
Wondering what kind of cool functionality Live ID can help you deliver?

The Party Planner
You have a Web application that helps users host that perfect party. With the user's explicit consent, you get access to their Live Calendar and add tasks to help them get everything done well in time for the big day.

Discovering the Lonely Planet
You have a mashup Web application that helps users with their travel plans. Your site is a repository of information on cool locations all over the world. You help users from picking their vacation spot to booking their flights, hotels, and attractions to sharing memories from their holiday with their friends. You incorporate the Live Contacts Control, letting the user choose what contact information they want to share with your site. The user then shares vacation plans or even plans a vacation with selected contacts. You incorporate the Live Spaces Photo Control and let users upload photos from their vacation from their Live space. By using Windows Live ID, users will be authenticated automatically by these rich controls.

Editor's Note: This article was first published in the "Windows Live" edition of CoDe Focus Magazine (2008, Vol. 5, Issue 2), and is reprinted here by permission.