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Phrack Inc. Volume 04 Issue 44 File 10
==Phrack Magazine==
Volume Four, Issue Forty-Four, File 10 of 27
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[Editor's Note: This file is reprinted with permission of InterPact
Press. The actual document contains many pictures, charts, and tables
that due to our format, we were unable to reproduce. We encourage the
reader to contact InterPact Press at 813-393-6600 and order a hard copy
of the document for $25.00]
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Protective Measures Against Compromising Electro Magnetic Radiation
Emitted by Video Display Terminals
by Professor Erhart Moller
University of Aachen, Aachen, Germany
0. Introduction
Compromising electromagnetic radiation emitted by machinery or
instruments used in data processing or communication engineering can be
received, decoded and recorded even across large distances. It is also
possible to recognize the data or information which was processed and
transmitted by the emitting instrument as text in clear. Compromising
emitted electromagnetic radiation thus jeopardizes the protection and
security of data.
The Laboratory for Communication Engineering at the Fachhochschule
Aachen is developing protective measures against compromising emission
of radiation. However, these protective measures can only be effective
if they are derived from the characteristics, the effects, and risks of
compromising emitted electromagnetic radiation. Therefore we first
consider only the forms of appearance and the characteristics of
compromising emitted electromagnetic radiation.
1. Compromising Emitted Electromagnetic Radiation
In this context one often refers only to the so-called computer
radiation. But this is only one form of compromising emitted
electromagnetic radiation. There are three types of such emissions.
1.1. Types of Compromising Emitted Electromagnetic Radiation
Figure 1.1 shows an n example of an arbitrary electric device with various
electric connections: a power supply line, a high frequency coaxial
transmission line, and a coolant line with in- and outflux. This device
emits three types of compromising electromagnetic radiation:
1. electromagnetic radiation in form of electric and magnetic fields
and electromagnetic waves;
2. electromagnetic waves on the outer surface of all coaxial metallic
connections (shell waves);
3. electric interference currents and interference voltages in power
lines connected to the device.
Each of the three types can be transformed into the other two. For
instance, shell waves can be emitted as fields or waves. On the other
hand, electromagnetic waves can be caught by a nearby conductor and can
propagate on it as shell waves. These phenomena are the reason for the
difficult control of compromising electromagnetic radiation, and they
imply that one must deal with all and not just one form of compromising
electromagnetic radiation. Also, electromagnetic protection against
compromising emitted radiation must deal with all forms of it.
1.2. Examples of Compromising Emitted Electromagnetic Radiation
To exemplify the three types of compromising electromagnetic radiation
we consider the monitor depicted in figure 1.2.
1.2.1. Compromising Electromagnetic Radiation
Figure 1.3. shows the experimental set-up. The video display
terminal is connected via the power line to the power supply. The
power line is surrounded by absorbers so that the terminal can only emit
electromagnetic radiation. The absorbers prevent the generation of
shell waves on the power line. The dipole antenna of the television
receiver is 10 m from the video terminal. Figure 1.4. shows the screen
of the television receiver after it received and decoded the signal.
Not only is the large FH=AC well readable but also the smaller letters.
This demonstration yields the following results:
* The video display terminal emits electromagnetic radiation;
* Despite being within (standards committee) norms the emitted
electromagnetic radiation can be received and decoded across a certain
distance;
* The electromagnetic radiation emitted by the terminal can be decoded
into readable information and symbols on a television screen.
Therefore, this emitted radiation is compromising.
1.2.2. Compromising Surface or Shell Waves
The video display terminal and the television receiver are positioned as
in figure 1.5. The power line of the terminal is surrounded by a
current transformer clamp which absorbs the shell waves. The television
screen shows again the picture seen in figure 1.4. The quality of the
picture is often better than in the previous case. Another experiment
would demonstrate that secondary shell waves can form on a nearby
conductor. The emitted radiation is then caught by nearby conductors
and continues to propagate as shell waves. These emissions also give
good receptions but are almost uncontrollable along their path of
propagation.
1.2.3. Demonstration of Compromising Emitted Radiation Through the
Power Line
Figure 1.6 shows the experimental set-up for the proof of compromising
power supply voltages. The video display terminal acts as a generator
whose current and voltage is entered into the power supply. Using a
capacitive line probe, the entered signal can be retrieved and fed into
the television receiver.
This form of transmission is the known basis for intercom systems or
so-called babysitter monitors where the signals are transmitted from
room to room via the energy supply lines in a home. As in the case
of electromagnetic radiation or shell waves, one obtains the same
picture quality as in figure 1.4.
2. Facts About Compromising Emitted Radiation
Protective measures against compromising emitted radiation are not only
determined by the above-mentions\ed three types of compromising
emissions but also by taking into account the following data:
# level of intensity and spectral distribution;
# frequency (emission frequency) and frequency range;
# directional characteristics of the radiation.
These data can then be used to derive the damping and the
amplitude-frequency response for the protective measure and its
location.
2.1. Emission Spectrum and Level of Intensity
The spectral distribution of compromising emitted radiation depends on
the frequencies used to generate the picture on a screen. The regular
repetition of dots and lines gives rise to the video and line frequency
which is found in the spectrum. However, the emission of video or line
frequencies is not compromising since their knowledge does not yet give
access to processed data. If the lines are covered regularly by
symbols, a symbol frequency is obtained which is also detectable in the
spectrum. A single symbol consists of a dot or pixel matrix.
The dot matrix of the symbol @ is also known in figure 2.1 The electron
beam scans the individual dots or pixels line-by-line and keys them
bright or dark. This keying is done using the so-called dot or pixel
frequency. For instance, the highest keying frequency is obtained by
scanning the center of the @ symbol since there one has a long sequence
of successive bright and dark pixels. It also follows from figure 2.1
that the keying is slower, i.e., the keying frequency is lower, along
the upper part of the @ symbol because of a long sequence of only dark
or bright pixels. It follows that the emissions due to the keying
frequency are highly compromising since they give direct information
about the structure of the picture.
Until recently, the frequencies in the following table were used:
video frequency 45 Hz - 55 Hz
line frequency 10 kHz - 20 kHz
symbol frequency 2 MHz - 5 MHz
dot or pixel frequency 15 MHz - 20 MHz.
The pulses for the electron beam are formed in the video part, i.e., the
video amplifier, of the monitor. Therefore, the cathode-grid of the
picture tube and the video amplifier are the main emitters of radiation.
The upper diagram in figure 2.2 shows the calculated spectrum for the
cathode-keying. It represents a sequence of dots from the center of the
@ symbol using a dot-sequential frequency of 18 MHz. The diagram in the
center of figure 2.2 shows the measured spectrum at the keyed cathode of
the picture tube. The agreement between the calculated and measured
spectrum for the frequency is clearly visible. However, the calculated
and measured spectral representation differ in the form of the envelopes.
In the measured spectrum one finds an amplitude increase between 175 MHz
and 225 MHz. This increase is usually found in the same or similar form
in monitors. The reasons for this amplitude increase are design,
construction parts, and dimensions of the video display terminal. In
the lower part of figure 2.2 we see the compromising radiation emitted by
the terminal as measured at a distance of 10 m. The spectrum of the
radiation emitted by the terminal is superimposed by broadcast, radio
and interference spectra since the measurement took place on open
ground. Despite this interference one can recognize the typical form of
the cathode spectrum. The increase in the amplitude between 175 MHz
and 225 MHz presents a particular risk since the television transmitters
for Band III operate within this frequency range and all television sets are
tuned to it (see figure 2.2).
A comparison of the intensity level of the television transmitter with the
level of the compromising radiation in figure 2.2 shows their agreement.
It is therefore not very difficult to receive the compromising radiation in
proximity of the emitter using only a regular television set with normal
sensitivity.
Figure 2.3 shows the spectral distribution of compromising shell waves
emitted by the video display terminal. Here again one recognizes the
particular form of the dot or pixel frequency. The height of the shell wave
spectrum is much lower at higher frequencies than the height of the
radiation spectrum. The shell waves have lower intensity in the range of
broadcast television but higher intensity in the range of cable television.
To receive the shell waves a television set must be cable-ready.
Figure 2.4 shows the spectrum for the third type of emission: the
compromising currents and voltages entering the power supply lines. It
is very similar to the shell wave spectrum. The height of this spectrum at
higher frequencies is even smaller than the shell wave spectrum. In
order to receive any signal a cable-ready television set must be used.
The intensity of the currents and voltages is so high that they can
easily be received using a regular television set with normal
sensitivity.
2.2. Frequency and Frequency Range
It follows from figures 2.2, 2.3, and 2.4 that the best reception for
the three types of emissions is for the following frequencies:
compromising radiation approx. 200 MHz;
compromising shell waves approx. 60 MHz;
compromising voltages approx. 20 MHz.
The video information of the picture on the monitor has a frequency
range of half a spectral arc. The frequency range of the receiver must
therefore be 10 MHz for all three types of emission.
2.3. Directional Characteristics of the Radiation
Figure 2.5 shows the directional characteristics for compromising
radiation emitted by a video display terminal inside a plastic casing.
According to this diagram the lateral radiation dominates. The field
intensity along the front and back direction is about 30% of the lateral
intensity. The power of the emitted radiation along these directions is
only about 10% of the power emitted laterally. The range for the
emitted radiation along the front and back direction is therefore also
reduced to 30%. This phenomenon suggests for the first time a
protection against compromising radiation, namely proper positioning of
the device.
The compromising shell waves and power line voltages propagate according
to the configuration of the lines. There is no preferred direction.
2.4. Range
The range of compromising radiation emitted from a video display
terminal is defined as the maximum distance between the emitting
terminal and a television receiver and readable picture.
The range can be very different for the three types of emitted
radiation. It depends on the type of emitter and the path of
propagation.
The spectacular ranges for emitted ranges are often quoted - some of
which do not always come from the technical literature - give in general
no indication just under which conditions they were obtained. It is
therefore meaningful to verify these spectacular ranges before using
them.
2.4.1. The Range of Compromising Emitted Radiation
The dependence of the field intensity on distance is illustrated in
figure 2.6.
The dependence of the range on the receiver used is shown at 25 m, 40 m,
and 80 m. The field intensity at 25 m is just strong enough to receive
a picture with an ordinary television receiver using the set-up in figure
1.3. If one uses a narrow-band television antenna or a noiseless antenna
amplifier than the field intensities at 40 m and 80 m, respectively, are
still strong enough to receive a legible picture.
The flattening out of the curve at large distances suggests that the
range can be increased to several hundred meters by using more sensitive
antenna or better receivers. The range can also be increased through a
high altitude connection, for instance, if both emitter and receiver are
in or on a high rise. This was verified by an experiment involving two
high rises separated by over 150 m. A very clear picture was received
using a relatively simple antenna with G = 6 db.
2.4.2. Range of Compromising Shell Waves
Measurements have shown that shell waves can propagate across a large
area without any noticeable damping if only the surrounding metallic
conductors extend also across the entire area.
The propagation is reduced considerably by a metallic conductor that
crosses metallic surfaces such as metal walls or metallic grids such as
reinforcements in concrete walls.
Dissipative building materials also damp shell waves. Lightweight
construction such as the use of dry walls or plastic walls in large
buildings increases the range of shell waves to about 100 m without the
picture becoming illegible.
2.4.3 Range of Emissions Through Power Supply Lines
In this case the conditions are even less clear than in the previous
cases. It must be assumed that inside a building the compromising
currents and voltages can be received through the phase of the power
supply lines feeding the video display terminal . The possibility of
receiving the signal through other phase lines by coupling across phases
in the power supply line cannot be excluded.
The range depends very much on the type of set-up and the instruments
used. It is conceivable that a range of about 100 m can be obtained.
3. Protective Measures
Protective measures fall into three categories:
- modification of devices and instruments by changing procedures
and circuitry;
- heterodyning by noise or signals from external sources;
- shielding, interlocking, and filtering.
3.1. Instrument Modification
The instrument modifications consist of changing the signal processing
method and the circuitry of the instrument. It is the objective of
these measures to alter the spectral distribution and intensity of the
emitted radiation in such a way that the reception by television sets or
slightly modified television sets is no longer possible.
For instance, a change of procedure could consist of a considerable
increase in the dot or pixel frequency, the symbol and line frequencies.
A reduction in the impulse amplitude and impulse slope also changes the
reduction in the impulse slope also changes emission spectrum so that
reception is rendered more difficult. However, the subsequent
modification of the video display terminal has serious disadvantages of
its own: First of all, the user of video display terminals does in
general not possess the personal and apparative equipment to perform the
modifications. To complicate things further, the so-modified
instruments loose their manufacturer's warranty and also their permit of
operation issued by governmental telecommunication offices. A subsequent
instrument modification by the user is for these reasons in general out
of question.
3.2. HETERODYNING STRATEGY
We refer to a protective measure as a heterodyning strategy whenever the
compromising emitted radiation is superimposed by electromagnetic noise
of specific electromagnetic signals.
The television set receives the compromising emitted radiation together
with the superimposed noise of spurious signal. The noise or the
spurious signal are such that a filtering out or decoding of the
compromising emitted radiation by simple means is impossible.
Since the noise and the spurious signal not only interfere with the
television receiver of the listener but also with other television sets
in the vicinity the heterodyning strategy is by all means in violation
with the laws and regulations governing telecommunications. As far as
is known, this is a protective measure only used under extremely
important circumstances involving high government officials.
3.3 Shielding
In contrast to the previously considered protective measures, shielding
has two important advantages:
* shielding protects not only against compromising emitted radiation
but also against electromagnetic emissions which can enter data
processing devices from the outside and cause interference;
* furthermore, shielding neither violates the laws governing the use
of telecommunications nor does it jeopardize the manufacturer's
warranty.
The term shielding is used here to describe, shielding, interlocking,
and filtering.
3.3.1. Shielding Data
The requirements on a shield are described by the shield damping. The
shield damping is twenty times the logarithm of the ratio between the
electric or magnetic field intensity inside the shield and outside the
shield.
Actual applications and individual situations may require different
values for the shield. The shield data are derived from the so-called
zone model. In the zone model one considers the type and intensity of
the emitted radiation, the composition of the path of propagation, and
the local accessibility for the receiver.
The shield data not only influence the shield damping but also the
frequency range of the shield's effectiveness. Figure 3.1 shows a
diagram listing different types if shields according to regulations MIL
STD 285 and 461B, NSA 656, and VG norms 95 375.
3.3.2. Applicability of Shielding
Electromagnetic shielding can be used on emitting or interfered with
instruments, on building and rooms, and on mobile cabins.
3.3.2.1. Shielding of Instruments
The shielding of instruments though it can often be done very quickly
and effortlessly is not without problems.
In general but especially after subsequent installation, it can lead to
a loss in design and styling of the shielded device. Openings in the
shield, for instance for ventilation or control and operating elements,
cannot always be sealed off completely. In this case they are emission
openings with particularly high emission rates.
Trying to maintain ergonometric conditions - good viewing conditions for
the users - renders the shielding of screens especially difficult. If
the casing of the instruments is not made of metal but of plastic, the
following shielding materials are considered: metal foils, metal cloth,
metal-coated plastics, electrolytical layers and coats of metallic paint
or paste. Recently, the plastics industry is also offering metallized
plies of fabric. Such glasses are for instance offered by VEGLA,
Aachen. Ventilation openings are sealed off with metallic fabric of
honey-comb wirings.
Interlocking systems and filters on all leads coming out of the
instrument prevent the emission of compromising shell waves and power
supply voltages.
3.3.2.2. Building and Room Shielding
There are some advantages in shielding buildings and rooms. The
building and room shielding lies solely in the competence of the user.
Minor restrictions dealing with the static of the building and local
building regulations only occur with external shielding. Building and
room shielding offers a protection that is independent of the instrument
or its type. It is a lasting and effective protection. Maintenance is
minimal, and subsequent costs hardly exist. Interior design and room
lay-out are not changed.
If one requires better shielding values or a building and room design
which emphasizes better comfort than greater expenses and thus higher
costs will occur.
3.3.2.3. Cabin Shielding
Cabin shielding has all the advantages of building and room shielding.
In addition, cabin shielding is not affected by the static of the
building or local building regulations. Furthermore, cabin shielding
requires less expenses and costs than building or room shielding.
However, shielded cabins do not offer the same comfort or interior
design as shielded buildings or rooms.
3.3.3. Shielding Components
Electromagnetic shielding consists of three components:
# the actual shield together with various structural elements as a
protection against emitted radiation;
# the interlocking of all non-electric and electric supply lines to
protect against shell waves;
# electric filters at all supply lines to protect against compromising
power supply voltages.
3.3.3.1. The Electromagnetic Shield
The shield consists of the hull and the shielding structural elements.
3.3.3.1.1. Shield Hull - Method and Construction
In general, one uses metal sheets or metal foil to construct
electromagnetic shields for buildings and rooms. If one lowers the
requirements on the shield damping and the upper limit frequency then
screen wire, metallic nets, and - if properly constructed - even the
reinforced wire net in concrete can be used; the obvious disadvantage
is that the settlements or movements of the building can cause cracks
that will render the shield ineffective.
Therefore, only metal shields or strong wire netting is used for the
construction of electromagnetically shielded cabins.
The building or room shield can be built using several construction
principles. Figure 3.2 above shows the essential construction principles.
For the Sandwich construction, the shield is between the outer and inner
layer of the wall. A new type of construction uses the Principle of
the Lost Form. The shield itself which consists of 3 to 5 mm thick
sheet iron is used as an inner layer in the manufacturing of concrete
walls. The sheets touch one another and have to be welded together at
the contact points. If the building or room shields he\ave to satisfy a
special purpose then they have to be grounded at only one point; they
have to be assembled in such a way that they electrically insulate
against the building or room walls. The so-called inner shields offer
this protection. In simple cases, the inners shield is placed on top of
the walls maintaining insulation by using a special underneath
construction. However, this space-saving and simple construction has a
disadvantage; the part of the shield that faces the wall such as
corrosion, settling or moving of the building, or damages due to work on
the exterior of the building can no longer be detected. The use of
non-corrosive shield material or sufficient back ventilation of the
shield protects against corrosion in these cases. The self-supporting
inner shield is suspended from a supporting grid construction. This
construction can be similar to a cabin construction. In the case
of large rooms, such as halls, one should use a truss for statistical
reasons. The self-supporting inner shield has the advantage of
accessibility, although the usable room volume has been decreased.
In rooms where the shield is exposed to only slight mechanical wear and
tear and not required to shield completely, shielding metal foil is
glued directly to the wall and welded at the contact points.
The floor construction is almost the same for all four construction
principles. It is important that the floor onto which the shield is
placed is protected from humidity and is even. In the case of
electrically insulating layers of, for instance, laminated paper or PVC
are first put on the floor. The ceiling construction depends on the
specific requirements and necessities. The ceiling shield can be a
suspended metallic ceiling or a self-supporting ceiling construction.
3.3.3.1.2. Shield Construction Elements
Construction elements which seal off viewing openings or access openings
are called shield construction elements. Access openings are doors, gates,
and hatches. Viewing openings are windows.
The shielded doors, gates, and hatches serve two purposes: first to
close off the room, and second to shield the room.
The door, gate, or hatch shield is in general made of sheet iron.
Passing from the door or gate shield to the room shield causes
shield-technical problems. A construction which is due to the company
of TRUBE & KINGS has proven to be especially effective for this kind of
problem (see figure 3.3).
The set-on-edge door shield, the so-called knife, is moved into a
U-shape which contains spring contacts. The difference between this and
other available constructions is that the knife is not moved into the
spring upward. This construction reduces the wear and tear of the
transition point between door and room shield and thus increases the
durability of the construction which implies a better protection and
higher reliability. This construction by TRUBE & KINGS satisfies the
highest requirements on shield damping.
Windows in shielded room are sealed off with the shielding glass or
so-called honey-comb chimneys. It si understood that these windows are
not to be opened. Figure 3.4 shows the cross-section of a glass
especially developed by VEGLA for data processing rooms. The glass
consists of multiple layers which are worked into a very fine metallic
net and an evaporated metallic layer. The thickness of the wire is in
the range of a few micrometers so that the net is hardly visible. This
glass can also be manufactured so that it is rupture- and fire-resistant
and bullet-proofed.
Using glass one can reach shield dampings in the medium range (refer to
figure 3.1). Specially manufactured glass reaches even higher shield
dampings.
Figure 3.4 also shows the so-called honey-comb chimneys as manufactured
by SIEMENS. Visibility and the comfort of light are highly restricted.
But the advantage is that this type of shielding satisfies the
requirements for highest shield damping.
3.3.3.2. Interlocking
All non-electric supply lines leaving a shielded room must be
interlocked in order to protect against the propagation of shell and
surface waves. Water pipes, heating pipes, pneumatic and hydraulic
pipes are connected via rings to the metallic shield. Depending on the
required frequency range, the pipe diameter is also subdivided by filter
pieces. At high frequencies on can achieve dampings of up to 100dB
using such interlocking devices.
The ventilation of shielded rooms may cause problems. Problems will
occur if shield dampings up to the highest frequencies are required. In
this case one has to use two-step ventilation filters. The first step
consist of adding concave conductor filters which work for the
frequencies up to 200 GHz, the second step of adding absorber filters
which protect against compromising emitted frequencies above 200 GHz.
Figure 3.5 shows the set-up for the above-described ventilation lock
which is due to the SCHORCH.
3.3.3.3. Electric Filters
Filters must be put on electric power supply lines, telephone wires, and
data processing supply lines at the room exit point. The filters have
to be installed at the shield.
The filters used here are the same as the ones shown in the area of
electromagnetic compatibility.
4. Summary
Electric devices used in data processing, data transmission and data
handling emit electromagnetic radiation, electromagnetic shell and
surface waves, and currents and voltages in power supply lines,
telephone wires, and data supply lines.
If this emitted radiation carries actual data or information from the
data processing device then it is compromising.
Using a television receiver, it is very easy to receive, decode and make
these compromising emissions legibly. Several possibilities present
themselves as protective measures against compromising emissions from
data processing and data transmitting equipment. The use of shielding
in the form of room shields, interlocking of supply lines, and filters
for electric lines is the best protection for the user of data
processing, data transmitting, and data handling equipment.
