Van Eck phreaking
From Free net encyclopedia
van Eck phreaking is the process of eavesdropping on the contents of a CRT or LCD display by detecting its electromagnetic emissions. It is named after Dutch computer researcher Wim van Eck, who in 1985 published the first paper on it, including proof of concept. (An unknown government researcher had discovered it long before, see TEMPEST.) Phreaking is the process of tapping into phone lines, used here because of its connection to eavesdropping.
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Basic principle
Information that drives the video display takes the form of high frequency electrical signals. These oscillating electric currents create electromagnetic radiation in the radio frequency range. The radio emissions are correlated to the video image being displayed, so in theory they can be used to recover the displayed image.
CRTs
In a cathode ray tube (CRT) the image is generated by an electron beam that sweeps back and forth along the screen. The electron beam excites the phosphor coating on the monitor glass and causes it to glow. The beam intensity determines the intensity of the pixel. See the article on CRTs for a detailed description. The electron beam is deflected by several electromagnetic lenses. The strength of the lenses are modulated at a high frequency and contain information correlated to the video image. These high frequency, high voltage signals create electromagnetic radiation that has, according to van Eck, "a remarkable resemblance to a broadcast TV signal". What these radiated signals lack is the synchronizing portion of the TV signal. By supplying external synchronization, the radiated signal can be displayed on another television.
In the paper, van Eck reports that in February 1985 a successful test of this concept was carried out with the cooperation of the BBC. Using a van with a VHF antenna array, they were able to eavesdrop from a "large distance".
Countermeasures
Countermeasures are detailed in the article on TEMPEST, the NSA's standard on spy-proofing digital equipment. Countermeasures involve either shielding the equipment to minimize electromagnetic emissions, or, specifically for video information, scrambling the signals such that the image is perceptually undisturbed, but the emissions are harder to reverse engineer into images. Examples of this include low-pass filtering fonts and randomizing the LSB of the video data information.
Limited use
While van Eck phreaking potentially offers eavesdropping options without the interested person having to compromise the affected system in any hardware- or software-related way, in fact without even having to be in the same room, the practical applications are likely limited, unless unknown and substantial breakthroughs in ease of use have been made.
Trivia
In Neal Stephenson's 1999 novel Cryptonomicon, van Eck phreaking serves as an important plot device.