Van Eck phreaking
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. Phreaking is the process of exploiting telephone networks, used here because of its connection to eavesdropping.
Van Eck phreaking might also be used to compromise the secrecy of the votes in an election using electronic voting. This caused the Dutch government to ban the use of NewVote computer voting machines manufactured by SDU in the 2006 national elections, under the belief that ballot information might not be kept secret. In a 2009 test of electronic voting systems in Brazil, Van Eck phreaking was used to successfully compromise ballot secrecy as a proof of concept.
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 RF range. These radio emissions are correlated to the video image being displayed, so, in theory, they can be used to recover the displayed image.
In a CRT the image is generated by an electron beam that sweeps back and forth across the screen. The electron beam excites the phosphor coating on the glass and causes it to glow. The strength of the beam determines the brightness of individual pixels (see CRT for a detailed description). The electric signal which drives the electron beam is amplified to hundreds of volts from TTL circuitry. This high frequency, high voltage signal creates electromagnetic radiation that has, according to Van Eck, "a remarkable resemblance to a broadcast TV signal". The signal leaks out from displays and may be captured by an antenna, and once synchronization pulses are recreated and mixed in, an ordinary analog television receiver can display the result. The sync can be recreated either through manual adjustment or by processing the signals emitted by electromagnetic coils as they deflect the CRT's electron beam back and forth.
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 filled with electronic equipment and equipped with a VHF antenna array, they were able to eavesdrop from a "large distance".
In April 2004, academic research revealed that flat panel and laptop displays are also vulnerable to electromagnetic eavesdropping. The required equipment for espionage was constructed in a university lab for less than US$2000.
Countermeasures are detailed in the article on TEMPEST, the NSA's standard on spy-proofing digital equipment. One countermeasure involves shielding the equipment to minimize electromagnetic emissions. Another method, specifically for video information, scrambles 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 least significant bit of the video data information.
See also 
- TEMPEST, a US government standard for limiting electric or electromagnetic radiation emanations from electronic equipment
- RINT, the acronym for Radiation INTelligence, military application
- Election fraud
- Van Eck, Wim (1985). "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?". Computers & Security 4 (4): 269–286. doi:10.1016/0167-4048(85)90046-X.
- Dutch government scraps plans to use voting computers in 35 cities including Amsterdam (Herald tribune, 30. October 2006)
- Use of SDU voting computers banned during Dutch general elections (Heise, October 31. 2006)
- "Brazilian Breaks Secrecy of Brazil's E-Voting Machines With Van Eck Phreaking". Slashdot. November 21, 2009.
- Van Eck Phreaking
- The Screen Savers: Dark Tip - Van Eck Phreaking
- Kuhn, M.G. (2004). "Electromagnetic Eavesdropping Risks of Flat-Panel Displays". 4th Workshop on Privacy Enhancing Technologies: 23–25.
- Van Eck phreaking
- Van Eck phreaking Demonstration
- Tempest for Eliza is a program that uses a computer monitor to send out AM radio signals, making it possible to hear computer-generated music in a radio.
- Video eavesdropping demo at CeBIT 2006 by a Cambridge University security researcher
- eckbox – unsuccessful or abandoned attempt in spring 2004 to build an open-source Van Eck phreaking implementation