Is there a way to have a user log in to an asp page then pass the authentication to a DNN module?

My company has several websites and one is in DNN while another is written in ASP connecting to a SQL db for login, etc. I want to have the user login to the ASP site and once successfully logged in, have the user be able to click a link that will take them to a hidden page on another site in DNN to a Documents Management module. I am not sure how to tell the module to only display the content if the user came from the other site and was logged in.

In other words, how do I make sure that only users who've logged into another site (written in asp and authenticating users in a SQL db) can see the module? A user would login to the asp page and then see a link that would link out to another site page that is DNN (containing a module on a hidden page) and if the user was not logged into the asp site then the DNN page needs to pass the user back to the asp login page.

Thank you anyone for your help with this!