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     | |      c   o   m   m   u   n   i   c   a   t   i   o   n   s     | |
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  ...presents...              TEMPEST in a Teapot
                                                         by Grady Ward

             __///////\ -cDc- CULT OF THE DEAD COW -cDc- /\\\\\\\__
               \\\\\\\/  Everything You Need Since 1986  \///////
  ___    _   _    ___     _   _    ___       _   _      ___    _   _      ___

     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

     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.


     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 neighbors 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 from 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

Federal Communications Commission Interference Handbook 
FCC Consumers Assistance Branch
Gettysburg, PA  17326


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

     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
neighbors 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 at approximately the inverse square of the distance
from the monitoring equipment to your computer.


     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 717-337-1212 for details on accessing their database.

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


     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 disk drive.
Unshielded cable acts just like an antenna for signals generated by your
computer and peripherals.  Most computer manufacturers 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 by
keeping all cables as short as possible.  You want to reduce the length of
unintended antennas and more easily predict the likely paths of electric and
magnetic coupling from cable to cable so that it can be better filtered.


     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 neighborhood.  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.  AC and other filters mentioned in this note are
available from a wide variety of sources including, for example, Radio Shack. 
Some filters reduce retransmission of EMI by a factor of at least 1000 in the
high-frequency 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 usually comes from these sources.


     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.  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.


     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 attenuator.  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.


     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 want
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) 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.


     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.


     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 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, and 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.  The details of these I leave for you to discover.
     .-.                             _   _                             .-.
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      Oooo                                                            / )   __
 /)(\ (   \  Copyright (c)1996 Grady Ward and Cult of the Dead Cow   /  (/ \
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       (_/     is published by cDc communications, P.O. Box 53011,    oooO  _
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