Today morning when I login to my Google Adsense account to check any update I found a new feature call Site Authentication was added.
This new feature will allow Google crawler to access page that require authentication, means password protected page. This feature allows Google crawler to access authenticated page and help to display more relevant ads based on content of particular web page.
Here are steps those are listed on Google Adsense page to getting started with Google Adsense Authentication
1. Enter an authentication rule
Specify an authentication URL and a set of parameters that will allow Google's web crawlers to access your content.
2. Verify your site ownership using Google Webmaster Tools
We will direct you to Google Webmaster Tools, where you can verify that you own the site by uploading a file to it or changing the META tag of a file.
3. View your improved ad targeting
When your authentication rule is verified, you should see better ad targeting on your pages. You can check the status of your authentication rules at any time.
Above text directly copied from Google.com