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Van Eck phreaking is a form of digital espionage whereby an eavesdropper detects and analyzes digital signals using equipment that can pick up the electromagnetic emissions put out by a cathode ray tube (CRT) or LCD display. This makes it possible for someone to read the images on another person's display from a distance.
There are ways to obscure or reduce display emission in order to thwart Van Eck phreaking, but they are not commercially viable because the security risk that Van Eck phreaking poses to the general public is considered to be very small. This is because Van Eck phreaking requires specialized knowledge and decoding equipment that is not widely available. Highly sensitive computer networks do, however, use Faraday cages to prevent any possibility of a breach via Van Eck phreaking.
The ability to read a display using the electromagnetic radiation it emits was first discovered by a Dutch researcher named Wim van Eck. In his 1985 paper, "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?", Eck provided the theoretical explanation that makes Van Eck phreaking possible.
The emissions from the display are collected and then demodulated into an understandable facsimile of the display, so a CRT display that is not electromagnetically shielded can be viewed from nearby using special equipment. The range is limited so the equipment has to be within a short distance of the target computer screen.
Modern LCD flat panels are thought to also be susceptible to electromagnetic eavesdropping using a similar method to Van Eck phreaking