Life in a faraday cage

There was an internal discussion about an unrelated topic recently, and it reminded me of an early experience in my career at Microsoft.

When I started, my 2nd computer was a pre-production PC/AT (the first was an XT). The AT had been announced by IBM about a week before I started, so our pre-production units were allowed to be given to other MS employees (since I had to write the disk drivers for that machine, it made sense for me to own one of them).

Before I got the machine, however, it was kept in a room that we semi-affectionately called "the fishtank" (it was the room where we kept the Salmons (the code name for the PC/AT)).

IBM insisted that we keep all the pre-production computers we received from them in this room - why?

Two reasons.  The first was that there was a separate lock on the door that would limit access to the room.

The other reason was that IBM had insisted that we build a faraday cage around the room.  They were concerned that some n'er-do-well would use the RF emissions from the computer (and monitor) to read the contents of the screen and RAM.  I was told that they had technology that would allow them to read the contents of an individual screen from across the street, and they were worried about others being able to do the same thing.

Someone at work passed this link along to a research paper by Wim van Eyk that discusses the technical details behind the technology.


Comments (30)

  1. Anonymous says:

    Cryptonomicon delves heavily into van Eck phreaking (a little too heavily for my tastes) and the average teen geek that read it probably believes it’s commonly done by every 3-letter agency in the world.

    IIRC, the best way to defeat it is simply to use aliased fonts, and with the advent of near-ubiquitious LCDs I’d wager that it’s harder than ever to actually mount a successful van Eck attack.

  2. Anonymous says:

    It’s possible (although very difficult) to reconstruct the image on a monitor by measuring variations in reflected light from the surrounding room, so even a Faraday cage or tinfoil hat isn’t enough to save you…

  3. Anonymous says:

    There is a paper on this web site (below) that describes a technique for reading the contents of a screen using just the light reflected of a nearby wall so I hope the room didn’t have any windows (except for the Microsoft kind)!

    (skip to page 11 if you just want to see the demo pictures)

  4. Jonathan, you can’t build a faraday cage and have windows…

    THe only opening in the cage was the door (which was wooden). There was chicken wire inside the walls surrounding the room, and the ceiling tiles were constructed to ensure that people couldn’t come over the top of the wall.

  5. Anonymous says:

    "you can’t build a faraday cage and have windows"

    Why not? You can build a free-standing Faraday in a room with windows, or if you’re embedding the cage in the walls of the room you can also embed the mesh inside a sheet of glass (or between two sheets, which is probably easier)…

  6. Anonymous says:

    Oh, and can I be picky and ask you to give Faraday a capital F? (Hey, don’t blame me, you’re the one who started complaining about poor English yesterday. That’s the trouble with pedantry — it comes back to haunt you sooner or later… :D)

  7. Anonymous says:

    The lunatics on MythBusters built a Faraday cage, which successfully made a cell phone go dead. Pretty impressive to see one of these things actually work.

  8. Anonymous says:

    The use of these kinds of rooms are common practice for contractors working with classified military projects, at least here in Sweden.

  9. Anonymous says:

    Have any of you ever seen "Enemy of the state", a Faraday cage was featured as a defence from the CIA or whoever.

    In reality I’m sure our security services and other countries have technology to trap keyboard taps or monitor displays.. But if you ask me you should be MORE concerned about that wire going into the back of the computer, you know that thing that connects you to a global network of other computers.

  10. Anonymous says:

    There is a rumour (or a rumor, I suppose) here in the UK that says that the TV licence enforcers can use a similar technology to detect not only that you have a television but detect what you are watching on it. While I don’t doubt that it’s possible, I’ve always doubted that they actually make use of it, as it would probably be a lot more productive to simply visit the few residences left which don’t have a television licence and accuse them of not having a licence. Most houses have a television, so you’d get more hits than not.

    I digress, though. The technology almost certainly exists, but must cost a lot to actually implement. I doubt it’s something that your average Joe (or average TV licence enforcement agency) can actually afford given that they’d have to create small, portable devices for doing it in unideal environments. Consequently, I think the idea that someone might be able to apply it to find out trade secrets about the IBM AT quite ridiculous, especially considering that we’re talking about quite a few years ago.

  11. Anonymous says:


    TVs have a small RF generator (the local oscillator) used to mix with the incoming RF signal and produce the IF signal (which is further amplified & downmixed to generate the audio and video).

    A sensitive RF receiver can detect the local oscillator; hence, the UK TV detector vans.

    Since the frequency of the local oscillator depends on the channel (the IF frequency is fixed, obviously), you can determine which channel the TV is tuned to.

    With a decent parabolic antenna, the hardware is fairly cheap – easily doable in a 50s hardware environment.

    Today, with current low-noise amplifiers, you could probably build one in a box the size of a briefcase – and most of that would be antenna.

  12. Anonymous says:

    So the solution for unlicensed UK residents is to put their TVs into a Faraday cage?

  13. Anonymous says:

    You’d really have to put the TV aerial in the Faraday cage as well, or it’ll weakly transmit the tell-tale signal. Of course if you put the aerial in the cage, you won’t receive anything!

    I’ve seen great demonstrations of people standing inside Faraday cages that got zapped with tens of kilovolts.

  14. Anonymous says:

    Sorry for offtopic, but as the highest profile MSFT multimedia developer I know, I just want to tell you how much you (MSFT, not you unless you’re responsible) suck.

    I just read on /. about how you’re routing OpenGL API calls through DirectX. What happened to "we love backcompat", stable APIs, and the Raymond Chen camp?

    There’s a reason why Microsoft is hated, and I hope you get slapped silly with anticompetition lawsuits around the world.

  15. Anonymous says:

    Yes I’ve also those documentaries with the open air cage and zap demonstration.


    What’s the UK TV detector going to do when you have IPTV with LCD ? Or worse, don’t even have that and download your TV from the net!

    Actually we have IPTV bundled with phone/broadband service here, however the price still has some premium if you consider that it gives you 1 box to watch the TV with, and when the IPTV is being used, the DSL connection is limited to 3 Mbit/s instead of 8 Mbit/s. So a slower service, on only 1 TV and higher price. I would SO get it if it allowed atleast 2 TV’s and used atleast ADSL2+ (24 Mbit/s instead of 8).

  16. Jerry,

    DirectX is a part of WGGT (Windows Graphics and Gaming Technologies). They’re a customer of our group (Windows Media Technologies) because they use our APIs to play audio.

  17. Anonymous says:


    I just read on /.


    Never a good way to start…

  18. Anonymous says:

    Where I work we us Faraday cages on a daily basis, Ours are capable of 100db isolation from the real world, we use the cages to isolate equipment from equipment operating just outside the room, and allows us to test the product inside. They range in size from 20 by 30 feet down to a 10 by 10 and TV’s, radios, and cellphones do not work inside the rooms. we do have specially designed 24 X 24 vents made of a honeycomb of small pipes. They almost look like a window when you look directly through. We use special power filters to supply electricity inside the room and special water fittings to supply water. An almost perfect world inside.

Skip to main content