Monitors may give away their secrets

Security forces and snoopers may be able to read what is happening on a suspect’s monitor even when they can’t see the screen after a researcher was able to reconstruct a screen image from the reflected glow of a monitor.

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By  Neil Denslow Published  May 15, 2002

Security forces, spies and snoopers may be able to read what is happening on a suspect’s monitor even when they can’t see the screen. Using a special light detector and some lab hardware, a researcher has been able to reconstruct a screen image from the reflected glow of a monitor.

Marcus Kuhn, an associate professor at Cambridge University in England, presented his research at the Institute of Electrical and Electronics Engineers’ (IEEE) Symposium on Security and Privacy in Berkley, California. Many security experts were amazed as they had considered it to be impossible to get a detailed picture in this way.

“No one even thought about the optical issues of computer information leakage”, Fred Cohen, security practitioner in residence for the University of New Haven told CNET.com. “This guy didn’t just publish, he blew (the assumptions) apart.”

The eavesdropping exploits how the cathode-ray tubes behind the screen of most monitors work. An image is created by a beam of electrons that is shot at the inside of the screen. The screen is covered in various phosphors, which cause each pixel to glow red, green or blue when it is hit by the beam.

The beam moves from side to side to “light” each pixel, scanning the screen 60 to 100 times a second. Light from the pixel then causes a pulsating signal that can light up a darkened room. Kuhn was able to reconstruct the image on the screen by averaging the signal reflected off a wall.

Kuhn was quick to point out the limitations of the system outside of a test environment, however, such as interference from other light sources.

“At this point, this is a curiosity. It’s not a revolution,” he told CNET.com.

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