A few years ago, I once met someone who worked on imaging devices for satellites for a military contractor, which was about all he would say about the specific work he did. He indicated that the building they did their work in had _no_ internet access. Generally, if they wanted to refer to things on the internet, they had to go to some internet-connected computers in another building, print out whatever they wanted and bring it back to the building they worked in.

Perhaps it's unrealistic to expect security without tradeoffs of convenience.