Bruce Schneier explains why you’d want to do this … and how much of a pain it can be to set up and work with:
Since I started working with Snowden’s documents, I have been using a number of tools to try to stay secure from the NSA. The advice I shared included using Tor, preferring certain cryptography over others, and using public-domain encryption wherever possible.
I also recommended using an air gap, which physically isolates a computer or local network of computers from the Internet. (The name comes from the literal gap of air between the computer and the Internet; the word predates wireless networks.)
But this is more complicated than it sounds, and requires explanation.
Since we know that computers connected to the Internet are vulnerable to outside hacking, an air gap should protect against those attacks. There are a lot of systems that use — or should use — air gaps: classified military networks, nuclear power plant controls, medical equipment, avionics, and so on.
Osama Bin Laden used one. I hope human rights organizations in repressive countries are doing the same.
Air gaps might be conceptually simple, but they’re hard to maintain in practice. The truth is that nobody wants a computer that never receives files from the Internet and never sends files out into the Internet. What they want is a computer that’s not directly connected to the Internet, albeit with some secure way of moving files on and off.
He also provides a list of ten rules (or recommendations, I guess) you should follow if you want to set up an air-gapped machine of your own.
