User access to pages

I’m in the process of writing a plugin, and I am trying to figure out how to limit select pages from being displayed to users on the site. Furthermore, a logged in user will have a given credential that will match a given page/post in WP, allowing them to view a “protected” page.

So to make sure I’m clear:

#1: Given a list of page/post id’s, how do I make sure that certain page links are not shown publicly
#2: When a page is viewed, how do I hook in so that I can retrieve the page/post id before the user gets actual page content?

Thanks!