As someone who's done classified work, the general guidance we got is that we can speak in generalities.

So I can't say what the code specifically does or who it's for, but I can talk about how I worked on a c++ engine wrapped in a Java server that was responsible for coordinating a large group of non-homogeneous assets, as well as various architectural details (what libraries did we use, database, messaging setup, etc). So it's not like I have nothing to talk about in an interview, plus the fact that I can't get too specific adds mystique. It's kinda fun, because in my experience there's an assumption most engineers/engineering managers make about what I've worked on from that first statement, particularly if they know where I work, and it's actually not that. :)