Van Eck phreaking is a form of eavesdropping in which special equipment is used to pick up telecommunication signals or data within a computer device by monitoring and picking up the electromagnetic fields (EM fields) that are produced by the signals or movement of the data. This electromagnetic radiation is present in, and with the proper equipment, can be captured from computer displays that use cathode ray tubes (CRTs), from printers, and from other devices.
Here is an example: The image on a CRT is created by electron beams that scan across the screen in a series of horizontal lines from left-to-right and top-to-bottom, in the same way you read a page of text (except much faster). This occurs at a specific frequency for each individual monitor; there are only a few standard frequencies in existence, and every monitor uses one of them. The intensity of the electron beams determines the relative red, blue, and green brightness for each pixel (picture element) on the screen. As a result, the CRT produces a modulated EM field that contains all the information in the image displayed on the screen at any moment. This information looks like a meaningless, irregular waveform if viewed directly on an oscilloscope. But, like a television (TV) signal, it can be demodulated with special equipment, and the image on the screen thereby retrieved, from some distance away.
This term combines the name of Wim van Eck, who in 1985 authored an academic paper that described this form of electronic eavesdropping, with the term phreaking, the earlier practice of using special equipment to make phone calls without paying. Van Eck phreaking is identified in the U.S. government project known as Tempest and, although some information remains classified, has probably been used to spy on suspected criminals and in espionage. The Tempest project has also led to advice and some standards development for how to shield devices so that eavesdropping is not possible. However, the cost of shielding means that many commercial devices are still vulnerable and, for this and other reasons, some of the details about what equipment is required to do van Eck phreaking remains classified. Susceptibility to eavesdropping can also be minimized by designing equipment that generates little EM energy.
Depending on the type of CRT used, the sensitivity of the detection equipment, and the general level of EM energy in the area, Van Eck phreaking can be done over distances ranging from a few meters up to several hundred meters.
Also see ELF.