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[liberationtech] Bruce Schneier on the good, old air gap

Eugen Leitl eugen at
Mon Oct 7 08:16:29 PDT 2013

Want to Evade NSA Spying? Don’t Connect to the Internet

BY BRUCE SCHNEIER 10.07.13 6:30 AM

Photo: Ariel Zambelich / WIRED; Illustration: Ross Patton / WIRED

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

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.

But every time a file moves back or forth, there’s the potential for attack.

And air gaps have been breached. Stuxnet was a U.S. and Israeli
military-grade piece of malware that attacked the Natanz nuclear plant in
Iran. It successfully jumped the air gap and penetrated the Natanz network.
Another piece of malware named agent.btz, probably Chinese in origin,
successfully jumped the air gap protecting U.S. military networks.

These attacks work by exploiting security vulnerabilities in the removable
media used to transfer files on and off the air gapped computers.

Bruce Schneier is a security technologist and author. His latest book is
Liars and Outliers: Enabling the Trust Society Needs to Survive.

Since working with Snowden’s NSA files, I have tried to maintain a single
air-gapped computer. It turned out to be harder than I expected, and I have
ten rules for anyone trying to do the same:

1. When you set up your computer, connect it to the internet as little as
possible. It’s impossible to completely avoid connecting the computer to the
internet, but try to configure it all at once and as anonymously as possible.
I purchased my computer off-the-shelf in a big box store, then went to a
friend’s network and downloaded everything I needed in a single session. (The
ultra-paranoid way to do this is to buy two identical computers, configure
one using the above method, upload the results to a cloud-based anti-virus
checker, and transfer the results of that to the air gap machine using a
one-way process.)

2. Install the minimum software set you need to do your job, and disable all
operating system services that you won’t need. The less software you install,
the less an attacker has available to exploit. I downloaded and installed
OpenOffice, a PDF reader, a text editor, TrueCrypt, and BleachBit. That’s
all. (No, I don’t have any inside knowledge about TrueCrypt, and there’s a
lot about it that makes me suspicious. But for Windows full-disk encryption
it’s that, Microsoft’s BitLocker, or Symantec’s PGPDisk — and I am more
worried about large U.S. corporations being pressured by the NSA than I am
about TrueCrypt.)

3. Once you have your computer configured, never directly connect it to the
internet again. Consider physically disabling the wireless capability, so it
doesn’t get turned on by accident.

4. If you need to install new software, download it anonymously from a random
network, put it on some removable media, and then manually transfer it to the
air gapped computer. This is by no means perfect, but it’s an attempt to make
it harder for the attacker to target your computer.

5. Turn off all auto-run features. This should be standard practice for all
the computers you own, but it’s especially important for an air-gapped
computer. Agent.btz used autorun to infect U.S. military computers.

6. Minimize the amount of executable code you move onto the air-gapped
computer. Text files are best. Microsoft Office files and PDFs are more
dangerous, since they might have embedded macros. Turn off all macro
capabilities you can on the air-gapped computer. Don’t worry too much about
patching your system; in general, the risk of the executable code is worse
than the risk of not having your patches up to date. You’re not on the
internet, after all.

7. Only use trusted media to move files on and off air-gapped computers. A
USB stick you purchase from a store is safer than one given to you by someone
you don’t know — or one you find in a parking lot.

8. For file transfer, a writable optical disk (CD or DVD) is safer than a USB
stick. Malware can silently write data to a USB stick, but it can’t spin the
CD-R up to 1000 rpm without your noticing. This means that the malware can
only write to the disk when you write to the disk. You can also verify how
much data has been written to the CD by physically checking the back of it.
If you’ve only written one file, but it looks like three-quarters of the CD
was burned, you have a problem. Note: the first company to market a USB stick
with a light that indicates a write operation — not read or write; I’ve got
one of those — wins a prize.

9. When moving files on and off your air-gapped computer, use the absolute
smallest storage device you can. And fill up the entire device with random
files. If an air-gapped computer is compromised, the malware is going to try
to sneak data off it using that media. While malware can easily hide stolen
files from you, it can’t break the laws of physics. So if you use a tiny
transfer device, it can only steal a very small amount of data at a time. If
you use a large device, it can take that much more. Business-card-sized
mini-CDs can have capacity as low as 30 MB. I still see 1-GB USB sticks for

10. Consider encrypting everything you move on and off the air-gapped
computer. Sometimes you’ll be moving public files and it won’t matter, but
sometimes you won’t be, and it will. And if you’re using optical media, those
disks will be impossible to erase. Strong encryption solves these problems.
And don’t forget to encrypt the computer as well; whole-disk encryption is
the best.

One thing I didn’t do, although it’s worth considering, is use a stateless
operating system like Tails. You can configure Tails with a persistent volume
to save your data, but no operating system changes are ever saved. Booting
Tails from a read-only DVD — you can keep your data on an encrypted USB stick
— is even more secure. Of course, this is not foolproof, but it greatly
reduces the potential avenues for attack.

Yes, all this is advice for the paranoid. And it’s probably impossible to
enforce for any network more complicated than a single computer with a single
user. But if you’re thinking about setting up an air-gapped computer, you
already believe that some very powerful attackers are after you personally.
If you’re going to use an air gap, use it properly.

Of course you can take things further. I have met people who have physically
removed the camera, microphone, and wireless capability altogether. But
that’s too much paranoia for me right now.

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