Authentification Active Directory Azure on an old Asp.net WebSite

I've some issues finding information on how I can use Windows Azure Active Directory authentification in an existing, old, Asp.net WebSite.

The framework is .net 3.5 and the website use Asp Webforms. I just want the user to authentificate through a simple aspx page... The Windows Azure Active Directory already exists. Do you know if it's possible?

Thanks in advance for your answers.