Reading your Screen through a Wall

Technique for reading a computer screen from a distance was recently developed by a German computer scientist working in Cambridge University. The technique allows eavesdropping on flat panels from a distance of dozens of meters away using fairly cheap electronic equipment.

Dr. Markus Kuhn from Cambridge University developed a technique for eavesdropping on flat panels using radio antennas and a special software. This technique was demonstrated back in 2006 when Kuhn demonstrated how he could read a computer screen located 25 meters away behind three walls using only 1000 pound worth of equipment.  

The technique basically enhances the radio emissions produced by the computer cables sending a signal to the monitor. The image on the computer screen is fed through a cable one pixel at a time. Because the image is built in a certain order Kuhn was able to work out how to decode the color of each pixel from its particular wave form. 

Electronic eavesdropping goes a long way back. Back in 1914 the German army used valve amplifiers for listening to ground return signals of distant British, French and Russian field telephones. In the mid 1980s a different technique was developed by Wim Van Eck called the Van Eck Phreaking for eavesdropping on CRT monitors using electromagnetic emissions. Efforts have been made by intelligence agencies across the world to defend against this kind of espionage in the past using shielding and other methods.  

Kuhn himself admitted that it is possible to make eavesdropping more difficult on his system using well-shielded cables and other methods. However without awareness sensitive systems might be compromised.

More information on Kuhn’s technique could be found on this (fairly technical) presentation.    
Image: R1250 wideband receiver used by Kuhn.