Optical Tempest!

Optical Emission Security:
"An image is created on the CRT surface by varying the electron beam intensity for each pixel. The room in which the CRT is located is partially illuminated by the pixels. As a result, the light in the room becomes a measure for the electron beam current. In particular, there is a little invisible ultrafast flash each time the electron beam refreshes a bright pixel that is surrounded by dark pixels on its left and right.

"So if you measure the brightness of a wall in this room with a very fast photosensor, and feed the result in another monitor that receives the exact same synchronization signals for steering its electron beam, you get to see an image like this."

"Tempest" refers to the ability to evesdrop on the contents of a monitor by listening to the radio frequency noise emitted by its electronics.

Here's another fun one from a few years back: Tempest for Eliza:

"A program that uses your computer monitor to send out AM radio signals. You can then hear computer generated music in your radio. [...] the song will be on the radio at multiple frequencies, but the best transmission is at the frequency you gave on the commandline."
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13 Responses:

  1. granting says:

    The NSA has been dealing with Tempest shit for years and years.

  2. ivan_ghandhi says:

    A while ago, in the middle of Siberia, where you could not get any tv signal at all (couple hundred miles of empty space in each direction), our tv received the image off the monitor from our computer in the next room. That was all it could get. Unfortunately, no sound - but we helped to fill the silence with well-known Russian words.

  3. retardotron says:

    I've been away from LJ so I don't know if you already caught this, but it seemed like one of your "I for one..." posts waiting to happen.


  4. mackys says:

    You're falling down here, Zawinski. How come you haven't welcomed our fast-selling, nursing home tending, machine-gun toting, slide guitar playing robot masters yet?

    (On preview it looks like <lj user=retardotron> beat me to it. Damn you!)

  5. ghewgill says:

    I read about this technique some time ago, and the article I read said that introducing a noisy source of light into the room pretty much foils this technique. An ordinary candle was found to be sufficient to obscure the CRT emissions.

    That's an impressive demo though.

  6. And to be pedantic, I think "Tempest" is correctly the government-standard set of *countermeasures* against CRT-RF eavesdropping, not the eavesdropping itself. Though the erroneous usage is getting more and more widespread, to the point where we may just have to surrender.

  7. scosol says:

    ahhhh tempest-eliza was the shit-

  8. Clearly you need to mount strobe lights to your monitor in sync with the refresh rate to fox this optical emission thing, and wrap it in tinfoil to stop Tempest :)

  9. Did this come up in Cryptonomicon? Or was that just regular Van Eck Phreaking?