Convert ASP .Net Site to 'Log in Only'

We have an AbleCommerce 7.0.3 ASP/C# .Net that needs to be converted in to a private, log-in only website. Users of the site will be required to enter a secret pass code that will allow access to the products and features of the site. Once the user is logged in, portions of the site content will load dynamically depending on the pass code entered by the user.