15-06-2013, 03:48 PM
Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk?
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ABSTRACT
This paper describes the results of research into the possibility of
"eavesdropping' on video display units. by picking up and
decoding the electromagnetic interference produced by this type
of equipment. During the research project. which started in
January. 1983. it became more and more clear that this type of
information theft can be committed very easily using a normal
TV receiver.
Introduction
PTT Dr. Neher Laboratories, St. Paulusstroat 4. 2264 XZ
Leidschendam, The Netherlands It is well known that electronic equipment produces
electromagnetic fields which may cause interference
to radio and television reception. The phenomena
underlying this have been thoroughly studied over the
past few decades. These studies have resulted in
internationally agreed methods for measuring the
interference produced by equipment. These are needed
because the maximum interference levels which
equipment may generate have been laid down by law
in most countries.
However, interference is not the only problem caused
by electromagnetic radiation. It is possible in some
cases to obtain information on the signals used inside
the equipment when the radiation is picked up and the
received signals are decoded. Especially in the case of
digital equipment this possibility constitutes a
problem, because remote reconstruction of signals
inside the equipment may enable reconstruction of the
data the equipment is processing
Cause and Ef fects in Brief by means of a normal television receiver.
Phenomenon
The application of square wave signals and high
switching frequencies in digital equipment leads to the
radiation of electromagnetic fields containing
frequency components up into the UHF region.
Although the power spectral density of these signals
decreases with increasing frequency, this is
compensated for in the radiated field, because the
radiation effectiveness of the electronic circuits inside
the equipment increases with frequency. This means
that the radiation level produced by digital equipment
may be constant up to several hundred MHz.
In some cases, resonances in circuits may lead to
higher radiation levels at some frequencies in the
radiated spectrum. Even circuits not designed to carry a
certain signal may radiate part of this signal due to
cross-talk and because the circuits are resonant for
some of the signal's frequency components. A striking
example of such a radiating circuit is the main power
cable of a piece of equipment.
Electromagnetic Eavesdropping
Many video display units or terminals are based on
the same principles as black-and-white television.
The free-running synchronization oscillators in a TV
receiver can therefore sometimes generate nearly the
same frequency as the one used in the VDU. If this
happens the displayed information can easily be
reproduced on the TV screen, and this can even occur
accidentally.
Experimental Eavesdropping
To prove that eavesdropping is feasible in a practical
situation using this simple set-up, the following
experiment was carried out. The equipment (dipole
antenna, TV receiver, and synchronization oscillators)
was put in a car, which was placed in the car park of a
building in which a word processor was being used.
An attempt was then made to copy the information
from this word processor's video unit by taking
photographs of the screen of the receiving television
set. The photographs convinced even the most
skeptical people in our organization of the threat of
this possibility to information security.
Conclusions
1. Video display units or terminals generate
electromagnetic fields with frequency components
up into the UHF region which contain the
harmonics of the video signal.
2. A normal TV receiver made suitable for this
purpose will in some cases be able to restore the
information displayed on a video display unit or
terminal on its own screen, when this field is
picked up. Depending on the type of video display
unit or terminal, this reconstruction may under
optimum conditions be feasible from distances of
up to 1 km.
3. The information in video display units or terminals
will not be detectable at such distances if an
electromagnetic shield is applied. Adequate
shielding of the electromagnetic fields generated
may double or even triple the price of a video
display unit or terminal.