van Eck phreaking

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.