The Hacker's Encyclopedia 1998/hackers_encyclopedia.iso/zines/a_m/lod5_10.txt

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The LOD Technical Journal: File 10 of 12

                      Tempest in a Teapot
                      -------------------


Do-it-yourself techniques to inhibit electromagnetic eavesdropping
of personal computers.

Grady Ward   <[e-mail]>

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Version 1.0   22 March 93

TEMPEST is the code name for technology related to limiting unwanted
electromagnetic emissions from data processing and related equipment. Its
goal is to limit an opponent's capability to collect information about the
internal data flow of computer equipment. Most information concerning TEMPEST
specifications is classified by the United States Government and is not
available for use by its citizens.

The reason why TEMPEST technology is particularly important for
computers and other data processing equipment is the kinds of signals
components in a computer use to talk to each other ("square waves") and their
clock speeds (measured in megahertz) produce a particularly rich set of
unintentional signals in a wide portion of the electromagnetic spectrum.
Because the spurious emissions occupy so wide a portion of that spectrum,
technologies used to block one portion of the spectrum (as pulling the shades
closed on a window to stop the visible light portion) are not necessarily
effective in another portion.

Unintentional emissions from a computer system can be captured and
processed to reveal information about the target systems from simple levels
of activity to even remotely copying keystrokes or capturing
monitor information. It is speculated that poorly protected systems can be
effectively monitored up to the order of one kilometer from the target
equipment.

This note will examine some practical aspects of reducing the
susceptibility of your personal computer equipment to remote monitoring using
easily-installed, widely available after-market components.


   I

One way of looking at TEMPEST from the lay person's point-of-view is that it
is virtually identical to the problem of preventing electromagnetic
interference ("EMI") by your computer system to others' radios, televisions,
or other consumer electronics. That is, preventing the emission of wide-band
radio "hash" from your computers, cabling, and peripherals both prevents
interference to you and your neighbours television set and limits the useful
signal available to a person surreptitiously monitoring.

Viewing the problem in this light, there are quite a few useful documents
available form the government and elsewhere attacking this problem and
providing a wealth of practical solutions and resources. Very useful for the
lay person are:

Radio Frequency Interference: How to Find It and Fix It. Ed Hare, KA1CV and
Robert Schetgen, KU7G, editors
The American Radio Relay League, Newington , CT
ISBN 0-87259-375-4  (c) 1991, second printing 1992

Federal Communications Commission Interference Handbook
(1991)
FCC Consumers Assistance Branch
Gettysburg, PA  17326
717-337-1212
and
MIL-STD-188-124B in preparation
(includes information on military shielding of tactical
communications systems)
Superintendent of Documents
US Government Printing Office
Washington, DC  20402
202-783-3238

Information on shielding a particular piece of consumer
electronic equipment may be available from the:

Electronic Industries Association (EIA)
2001 Pennsylvania Ave NW
Washington, DC  20006


Preventing unintended electromagnetic emissions is a relative term.
It is not feasible to reduce to zero all unintended emissions. My personal
goal, for example, might be to reduce the amount and quality of spurious
emission until the monitoring van a kilometer away would have to be in my
front yard before it could effectively eavesdrop on my computer. Apartment
dwellers with unknown neighbours only inches away (through a wall) might want
to even more carefully adopt as many of the following suggestions as possible
since signal available for detection decreases as approximately the inverse
square of the distance from the monitoring equipment to your computer.


   II
   Start with computer equipment that meets modern standards for emission.

In the United States, the "quietest" standard for computers and peripherals
is known as the "class B" level. (Class A level is a less stringent standard
for computers to be use in a business environment.).

You want to verify that all computers and peripherals you use meet the class
B standard which permits only one-tenth the power of spurious emissions than
the class A standard. If you already own computer equipment with an FCC ID,
you can find out which standard applies. Contact the FCC Consumers Assistance
Branch at 1-717-337-1212 for details in accessing their database.

Once you own good equipment, follow the manufacturer's recommendations for
preserving the shielding integrity of the system. Don't operated the system
with the cover off and keep "slot covers" in the back of the computer in
place.


   III
   Use only shielded cable for all system interconnections.

A shielded cable surrounds the core of control wires with a metal braid or
foil to keep signals confined to that core. In the late seventies it was
common to use unshielded cable such as "ribbon" cable to connect the computer
with,  say, a diskette drive. Unshielded cable acts just like an antenna for
signals generated by your computer and peripherals. Most computer
manufacturer supply shielded cable for use with their computers in order to
meet FCC standards. Cables bought from third-parties are an unknown and
should be avoided (unless you are willing to take one apart to see for
yourself!)
Try to avoid a "rat's nest" of wire and cabling behind your equipment and by
keeping all cables as short as possible. You want to reduced the length of
unintended antennas and to more easily predict the likely paths of electric
and magnetic coupling from cable to cable so that it can be more effectively
filtered.


   IV
   Block radiation from the power cord(s) into the house wiring.

Most computers have an EMI filter built into their body where the AC line
cord enters the power supply. This filter is generally insufficient to
prevent substantial re-radiation of EMI voltages back into the power wiring
of your house and neighbourhood. To reduce the power retransmitted down the
AC power cords of your equipment, plug them in to special EMI filters that
are in turn plugged into the wall socket. I use a model 475-3
overvoltage and EMI filter manufactured by

Industrial Communication Engineers, Ltd.
P.O. Box 18495
Indianapolis, IN  46218-0495
1-800-ICE-COMM
ask for their package of free information sheets

(AC and other filters mentioned in this note are available from a wide
variety of sources including, for example, Radio Shack. I am enthusiastic
about ICE because of the "over-designed" quality of their equipment. Standard
disclaimers apply.)

This particular filter from ICE is specified to reduce retransmission of EMI
by a factor of at least 1000 in its high-frequency design range. Although
ideally every computer component using an AC line cord ought to be filtered,
it is especially important for the monitor and computer CPU to be filtered in
this manner as the most useful information available to opponents is believed
to come from these sources.


   V
   Block retransmitted information from entering your fax/modem or
   telephone line.

Telephone line is generally very poorly shielded. EMI  from your computer can
be retransmitted directly into the phone line through your modem or can be
unintentionally picked up by the magnetic portion of the EMI spectrum through
magnetic induction from power supplies or the yoke of your cathode ray tube
"CRT" monitor.

To prevent direct retransmission, EMI filters are specifically designed for
modular telephone jacks to mount at the telephone or modem, and for
mounting directly at the service entrance to the house.

Sources of well-designed telephone-line filter products include ICE
(address above) and

K-COM
Box 82
Randolph, OH  44265
216-325-2110

Your phone company or telephone manufacturer may be able to supply
you with free modular filters, although the design frequencies of these
filters may not be high enough to be effective through much of the EMI
spectrum of interest. Keep telephone lines away from power supplies of
computers or peripherals and the rear of CRTs:  the magnetic field often
associated with those device can inductively transfer to unshielded lines
just as if the telephone line were directly electrically connected to them.
Since this kind of coupling decreases rapidly with distance, this kind of
magnetic induction can be virtually eliminated by keeping as much distance
(several feet or more) as possible between the power supply/monitor yoke and
cabling.


   VI
   Use ferrite toroids and split beads to prevent EMI from escaping on    the
surface of your cables.

Ferrites are magnetic materials that,  for certain ranges of EMI
frequencies, attenuate the EMI by causing it to spend itself in heat in the
material rather than continuing down the cable. They can be applied without
cutting the cable by snapping together a "split bead" form over a thick cable
such as a power cord or by threading  thinner cable such as telephone several
times around the donut-shaped ferrite form. Every cable leaving your monitor,
computer, mouse, keyboard, and other computer
peripherals should have at least one ferrite core attentuator. Don't forget
the telephone lines from your fax, modem, telephone or the unshielded DC
power cord to your modem. Ferrites are applied as close to the EMI emitting
device as possible so as to afford the least amount of cable that can act as
an antenna for the EMI.


Good sources for ferrite split beads and toroids include

Amidon Associates, Inc.
P.O. Box 956
Torrance, CA  90508
310-763-5770
(ask for their free information sheet)

Palomar Engineers
P.O. Box 462222
Escondido, CA  92046
619-747-3343
(ask for their free RFI information sheet)

and Radio Shack.


   VII
   Other practical remedies.

Other remedies that are somewhat more difficult to correctly apply
include providing a good EMI "ground" shield for your computer equipment and
other more intrusive filters such as bypass capacitor filters.

You probably ought not to think about adding bypass capacitors unless you are
familiar with electronic circuits and digital design. While quite effective,
added improperly to the motherboard or cabling of a computer they can "smooth
out" the square wave digital waveform -- perhaps to the extent that signals
are interpreted erroneously causing mysterious "crashes" of your system. In
other cases, bypass capacitors can cause unwanted parasitic oscillation on
the transistorized output drivers of certain circuits which could damage or
destroy those circuits in the computer or peripherals. Also, unlike ferrite
toroids, adding capacitors requires actually physically splicing them in or
soldering them into circuits. This opens up the
possibility of electric shock, damage to other electronic components or
voiding the warranty on the computer equipment.

A good EMI ground is difficult to achieve. Unlike an electrical safety
ground, such as the third wire in a three-wire AC power system, the EMI
ground must operate effectively over a much wider part of the EMI spectrum.
This effectiveness is related to a quality known as electrical impedance. You
desire to reduce the impedance to as low a value as possible over the entire
range of EMI frequencies.

Unlike the AC safety ground, important factors in achieving low impedance
include having as short a lead from the equipment to a good EMI earth ground
as possible (must be just a few feet); the gauge of the connecting lead (the
best EMI ground lead is not wire but woven grounding "strap" or wide copper
flashing sheets; and the physical coupling of the EMI into the actual earth
ground. An 8 ft. copper-plated ground may be fine for AC safety ground, but
may present appreciable impedance resistance to an EMI voltage. Much better
would be to connect a network of six to eight copper pipes arranged in a six-
foot diameter circle driven in a foot or two into the ground,
electrically bonded together with heavy ground strap and connected to the
equipment to be grounded via a short (at most, several feet), heavy (at least
3/4-1" wide) ground strap.

If you can achieve a good EMI ground, then further shielding possibilities
open up for you such as surrounding your monitor and computer equipment in a
wire-screen Faraday cage. You want to use mesh rather than solid sheet
because you must preserve the free flow of cooling air to your equipment. Buy
aluminum (not nylon) screen netting at your local hardware store. This
netting typically comes in rolls 36" wide by several feet long. Completely
surround your equipment you want to reduce the EMI being careful to make good
electrical bonds between the different panels of netting and your good earth
ground. I use stainless steel nuts, bolts, and lock washers along with
special non-oxidizing electrical paste (available from Electrical
contractors supply houses or from ICE) to secure my ground strapping to my
net "cages".  A good Faraday cage will add several orders of magnitude of EMI
attenuation to your system.


   VIII
   Checking the effectiveness of your work.

It is easy to get a general feeling about the effectiveness of your EMI
shielding work with an ordinary portable AM radio. Bring it very close to the
body of your computer and its cables in turn. Ideally, you should not hear an
increased level of static. If you do hear relatively more at one cable than
at another, apply more ferrite split beads or obtain better shielded cable
for this component. The practice of determining what kind of operating system
code is executing by listening to a nearby AM radio is definitely obsolete
for an well-shielded EMI-proof system!

To get an idea of the power and scope of your magnetic field emissions, an
ordinary compass is quite sensitive in detecting fields. Bring a compass
within a few inches of the back of your monitor and see whether it is
deflected. Notice that the amount of deflection decreases rapidly with
distance. You want to keep cables away from magnetic sources about as far as
required not to see an appreciable deflection on the compass.


   VIIII
   Summary

If you start with good, shielded equipment that has passed the FCC level B
emission standard then you are off to a great start. You may even be able to
do even better with stock OEM equipment by specifying "low-emission" monitors
that have recently come on the market in response to consumer fears of
extremely low frequency ("ELF") and other electromagnetic radiation.
Consistently use shielded cables, apply filtering and ferrite toroids to all
cabling entering or leaving your computer equipment. Finally, consider a good
EMI ground and Faraday cages. Beyond this there are even more effective means
of confining the electrical and magnetic components of your system through
the use of copper foil adhesive tapes, conductive paint sprays, "mu metal"
and other less common components.
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