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Saturday, 21 April 2007 |
Have you considered that someone could be reading what's on your
monitor from a few rooms away? It's unlikely, but possible, as work by
Cambridge University computer security researcher Markus Kuhn shows.
A
radio antenna and radio receiver - equipment totalling less than £1000
- is all you need. Kuhn managed to grab the image to the left through
two intermediate offices and three plasterboard walls.
Back in
1985, Wim Van Eck proved it was possible to tune into the radio
emissions produced by electromagentic coils in a CRT display and then
reconstruct the image. The practice became known as Van Eck Phreaking,
and NATO spent a fortune making its systems invulnerable to it. It was
a major part of Neal Stephenson's novel Cryptonomicon.
CRTs
are now well on the way to being history. But Kuhn has shown that
eavesdropping is possible on flat panel displays too. It works slightly
differently. With a flat panel display the aim is to tune into the
radio emissions produced by the cables sending a signal to the monitor.
The on-screen image is fed through the cable one pixel at a time.
Because they come through in order you just have to stack them up. And
Kuhn has worked out how to decode the colour of each pixel from its
particular wave form.
If everything is just right, you can pick
up signals from some distance. "I was able to eavesdrop certain laptops
through three walls," says Kuhn. "At the CEBIT conference, in 2006, I
was able to see the Powerpoint presentation from a stand 25 metres
away." Here's the image he managed to get: 
Kuhn also mentioned that one laptop was vulnerable because it had metal
hinges that carried the signal of the display cable. I asked if you
could alter a device to make it easier to spy on. "There are a lot of
innocuous modifications you can make to maximise the chance of getting
a good signal," he told me. For example, adding small pieces of wire or
cable to a display could make a big difference.
As for defending
against this kind of attack, Kuhn says using well-shielded cables,
certain combinations of colours and making everything a little fuzzy
all work. Source: New Scientist |
