Monitoring Your Monitor

bje2 writes "Rememeber this story from a couple months ago about reconstructing data from the blinking LEDs of modems...well, CNet is running a story about reconstructing the display of a computer by using special hardware and the reflected glow of the monitor." Kuhn's paper (400k PDF) is available.

Comments

Mr. Peabody's Slashback Machine

[realgone]

Same article appeared on /. back in March, dinnit?

Old news

[ozbird]

CRT Eavesdropping: Optical Tempest by michael with 219 comments on 10:57 10 March 2002

LCD is the answer

[Lxy]

Looks like it doesn't apply to flat panels. It relies on the CRT electrons coming out of the monitor and striking a photosensitive component. Not to mention, what if you have a large person using a small monitor? It would seem to me that you'd have to have an unobstructed view for this to work.

This could be detrimental to geeks though. Quoting the article: the safest solution is to compute with the lights on. Dangit.

Re:LCD is the answer

[nochops]

First of all, they're not electrons, they're photons, the quantegy of light. Your CRT has an electron gun that directs a narrow beam of electrons onto a phosphorus coated glass (the 'screen'). The phosphorus then glows, and radiates photons.

While LCD panels don't have an electron beam to radiate phosphorus, they still radiate photons. Otherwise you wouldn't be able to see them.

Basically, if your monitor is radiating photons (read: turned on) someone can intercept those photons and reconstruct an image, given the right equipment and circumstances.

I suppose given the right equipment and circumstances, they can read your mind as well, so we're screwed anyway.

Re:LCD is the answer

[markmoss]

Basically, if your monitor is radiating photons (read: turned on) someone can intercept those photons and reconstruct an image, given the right equipment and circumstances.

If by "right equipment and circumstances" you mean direct vision or a mirror-like reflection, then that's true. However, this article is about a technique for reconstructing CRT images when the monitor is facing away from the window and the only reflections are off of rough surfaces, which thoroughly scramble the pixels. You cannot directly determine what part of the screen a photon came from, but you can determine when it was emitted. Since the CRT scans one dot at a time, that creates the possibility of turning a recording of brightness & color vs time back into a picture.

However, most flat-panel displays will set a number of pixels at the same time (for example, writing to an entire row at a time). This makes it impossible to separate out one pixel or even one small area of the screen by the time when the light arrives. Also, LCD's don't create light, it is created by the backlights, generally flourescent lights running on high voltage, high frequency AC -- so the only thing time analysis gets you is the high frequency flicker of the backlights. The liquid crystals retain most of their "set" between scans through the display, so the light passed through a pixel doesn't vary much depending on how long it's been since the pixel was scanned.

OTOH, unless your video cable and electronics is all shielded very well, you are probably transmitting radio waves that could be turned back into the picture. This might be even more difficult than reconstructing a CRT image from the visible light, but certain three-letter government agencies can do it when they really want to. One limitation to the radio ("Tempest) method is that you've got to be able to isolate the target computer's signal from all the others; with optical methods this probably requires just pointing the scope in the right direction (if you are lucky enough to get a strong enough reflection in any direction), but radio waves bend around corners, reflect, and merge more so it's pretty unlikely that Tempest could find the one computer bringing up atomic bomb diagrams in a college dorm (say) among the hundreds downloading MP3's, playing Quake, or whatever.