Google quietly introduced a new feature for some AdSense registered members, allowing them to place adverts on the private websites protected with passwords. Although there is no official
statement concerning the authentication feature, the function is now available to some publishers without any beta inclusion approval. The "Site Authentication" function was first introduced in November 2006 when some of the AdSense clients received an email to inform them about the starting of a new beta testing for the advertising platform.
"You are one of a select group of publishers that has been chosen to try out our new Site Authentication feature. Site Authentication allows publishers to give our crawler access to login-protected content so that users can receive targeted ads. While there is no obligation for you to try out this new feature, you may find it helpful if you have pages of your site protected behind a login screen," the email sustained according to ProBlogger.
As there is no message in my email, I think the feature has become public and available to all the users, allowing them to include adverts distributed through AdSense even on the password-protected websites. Basically, the configuration process is based on three simple steps: the set up of an authentication rule, the verification of the site ownership using Google Webmaster Tools and the viewing of the new ad targeting.
You might think that this feature is somehow useless because the private websites are not interested in AdSense adverts. But think a little bit of an online newspaper that provides content for free but also for premium members who are paying for the news. The paying subscribers must login into the account using a special username and password so it's almost impossible to include AdSense and to earn money with the premium pages without this new feature.