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Eavesdropping On the Electromagnetic Emanations of Digital Equipment
by Christopher J. Seline
Eavesdropping On the Electromagnetic Emanations of Digital Equipment: The Laws of Canada, England and the United States
© 1990 Christopher J. Seline
This document is a rough draft. The Legal Sections are overviews. T
h e y w i l l b e significantly expanded in the next version.
We in this country, in this generation, are, by destiny rather than choice, the watchmen on the walls of world freedom.
[1] -President John F. Kennedy
1. Undelivered speech of President John F. Kennedy, Dallas Citizens Council (Nov. 22, 1963)
In the novel 1984, George Orwell foretold a future where
individuals had no expectation of privacy because the state
monopolized the technology of spying. The government watched the
actions of its subjects from birth to death. No one could protect
himself because surveillance and counter- surveillance technology
was controlled by the government. This note explores the legal status
of a surveillance technology ruefully known as TEMPEST[2]. Using
TEMPEST technology the information in any digital device may be
intercepted and reconstructed into useful intelligence without the
operative ever having to come near his target. The technology is
especially useful in the interception of information stored in digital
computers or displayed on computer terminals. The use of
TEMPEST is not illegal under the laws of the United States[3], or
England. Canada has specific laws criminalizing TEMPEST
eavesdropping but the laws do more to hinder surveillance
countermeasures than to prevent TEMPEST surveillance. In the
United States it is illegal for an individual to take effective counter-
measures against TEMPEST surveillance. This leads to the
conundrum that it is legal for individuals and the government to
invade the privacy of others but illegal for individuals to take steps
to protect their privacy. The author would like to suggest that the
solution to this conundrum is straightforward. Information on
2. TEMPEST is an acronym for Transient
Electromagnetic Pulse Emanation Standard. This standard sets forth
the official views of the United States on the amount of
electromagnetic radiation that a device may emit without
compromising the information it is processing. TEMPEST is a
defensive standard; a device which conforms to this standard is
referred to as TEMPEST Certified. The United States government
has refused to declassify the acronym for devices used to intercept
the electromagnetic information of non-TEMPEST Certified
devices. For this note, these devices and the technology behind them
will also be referred to as TEMPEST; in which case, TEMPEST
stands for Transient Electromagnetic Pulse Surveillance Technology.
The United States government refuses to release details regarding
TEMPEST and continues an organized effort to censor the
dissemination of information about it. For example the NSA
succeeded in shutting down a Wang Laboratories presentation on
TEMPEST Certified equipment by classifying the contents of the
speech and threatening to prosecute the speaker with revealing
classified information. [cite coming].
3. This Note will not discuses
how TEMPEST relates to the Warrant Requirement under the
United States Constitution. Nor will it discuss the Constitutional
exclusion of foreign nationals from the Warrant Requirement.
protecting privacy under TEMPEST should be made freely available;
TEMPEST Certified equipment should be legally available; and
organizations possessing private information should be required by
law to protect that information through good computer security
practices and the use of TEMPEST Certified equipment. I.
INTELLIGENCE GATHERING Spying is divided by professionals
into two main types: human intelligence gathering (HUMINT) and
electronic intelligence gathering (ELINT). As the names imply,
HUMINT relies on human operatives, and ELINT relies on
technological operatives. In the past HUMINT was the sole method
for collecting intelligence.[4] The HUMINT operative would steal
important papers, observe troop and weapon movements[5], lure
people into his confidences to extract secrets, and stand under the
eavesdrip[6] of houses, eavesdropping on the occupants. As
technology has progressed, tasks that once could only be performed
by humans have been taken over by machines. So it has been with
spying. Modern satellite technology allows troop and weapons
movements to be observed with greater precision and from greater
distances than a human spy could ever hope to accomplish. The theft
of documents and eavesdropping on conversations may now be
performed electronically. This means greater safety for the human
operative, whose only involvement may be the placing of the initial
ELINT devices. This has led to the ascendancy of ELINT over
HUMINT because the placement and _____________________
4.
HUMINT has been used by the United States since the Revolution.
"The necessity of procuring good intelligence is apparent & need not
be further urged -- All that remains for me to add is, that you keep
the whole matter as secret as possible. For upon Secrecy, Success
depends in Most Enterprises of the kind, and for want of it, they are
generally defeated, however well planned & promising a favorable
issue." Letter of George Washington (Jul. 26, 1777).
5. "... I wish
you to take every possible pains in your powers, by sending trusty
persons to Staten Island in whom you can confide, to obtain
Intelligence of the Enemy's situation & numbers -- what kind of
Troops they are, and what Guards they have -- their strength &
where posted." Id.
6. Eavesdrip is an Anglo-Saxon word, and refers
to the wide overhanging eaves used to prevent rain from falling close
to a house's foundation. The eavesdrip provided "a sheltered place
where one could hide to listen clandestinely to conversation within
the house." W. MORRIS & M. MORRIS, MORRIS DICTIONARY
OF WORD AND PHRASE ORIGINS, 198 (1977). monitoring of
ELINT devices may be performed by a technician who has no
training in the art of spying. The gathered intelligence may be
processed by an intelligence expert, perhaps thousands of miles
away, with no need of field experience. ELINT has a number of
other advantages over HUMINT. If a spy is caught his existence
could embarrass his employing state and he could be forced into
giving up the identities of his compatriots or other important
information. By its very nature, a discovered ELINT device (bug)
cannot give up any information; and the ubiquitous nature of bugs
provides the principle state with the ability to plausibly deny
ownership or involvement. ELINT devices fall into two broad
categories: trespassatory and non-trespassatory. Trespassatory bugs
require some type of trespass in order for them to function. A
transmitter might require the physical invasion of the target premises
for placement, or a microphone might be surreptitiously attached to
the outside of a window. A telephone transmitter can be placed
anywhere on the phone line, including at the central switch. The
trespass comes either when it is physically attached to the phone
line, or if it is inductive, when placed in close proximity to the
phone line. Even microwave bugs require the placement of the
resonator cone within the target premises.
[7] Non-trespassatory
ELINT devices work by receiving electromagnetic radiation (EMR)
as it radiates through the aether, and do not require the placement of
bugs. Methods include intercepting[8] information transmitted by
satellite, microwave, and radio, including mobile and cellular phone
transmissions. This information was purposely transmitted with the
intent that some intended person or persons would receive it. Non-
trespassatory ELINT also includes the interception of information
that was never intended to be transmitted. All electronic devices emit
electromagnetic radiation. Some of the radiation, as with radio
waves, is intended to transmit information. Much of this radiation is
not intended to transmit information and is merely incidental to
_____________________
7. Pursglove, How Russian Spy Radios Work, RADIO ELECTRONICS, 89-91 (Jan 1962).
8. Interception is
an espionage term of art and should be differentiated from its more
common usage. When information is intercepted, the interceptor as
well as the intended recipient receive the information. Interception
when not used as a term of art refers to one person receiving
something intended for someone else; the intended recipient never
receives what he was intended to receive. whatever work the target
device is performing.[9] This information can be intercepted and
reconstructed into a coherent form. With current TEMPEST
technology it is possible to reconstruct the contents of computer
video display terminal (VDU) screens from up to a kilometer
distant[10]; reconstructing the contents of a computer's
_____________________
9. There are two types of emissions,
conducted and radiated. Radiated emissions are formed when
components or cables act as antennas for transmit the EMR; when
radiation is conducted along cables or other connections but not
radiated it is referred to as "conducted". Sources include cables, the
ground loop, printed circuit boards, internal wires, the power supply
to power line coupling, the cable to cable coupling, switching
transistors, and high-power amplifiers. WHITE & M.
MARDIGUIAN, EMI CONTROL METHODOLOGY AND
PROCEDURES,
10.1 (1985). "[C]ables may act as an antenna to
transmit the signals directly or even both receive the signals and re-
emit them further away from the source equipment. It is possible that
cables acting as an antenna in such a manner could transmit the
signals much more efficiently than the equipment itself...A similar
effect may occur with metal pipes such as those for domestic water
supplies. ... If an earthing [(grounding)] system is not installed
correctly such that there is a path in the circuit with a very high
resistance (for example where paint prevents conduction and is
acting as an insulator), then the whole earthing system could well act
in a similar fashion to an antenna. ... [For a VDU] the strongest
signals, or harmonics thereof, are usually between 60-250 MHz
approximately. There have however been noticeable exception of
extremely strong emissions in the television bands and at higher
frequencies between 450-800 MHz. Potts, Emission Security, 3
COMPUTER LAW AND SECURITY REPORT 27 (1988). 10. The
TEMPEST ELINT operator can distinguish between different VDUs
in the same room because of the different EMR characteristics of
both homo and heterogeneous units. "[T]here is little comparison
between EMR characteristics from otherwise comparable equipment.
Only if the [VDU] was made with exactly the same components is
there any similarity. If some of the components have come from a
different batch, have been updated in some way, and especially if
they are from a different manufacturer, then completely different
results are obtained. In this way a different mark or version of the
same [VDU] will emit different signals. Additionally because of the
variation of manufacturing standards between counties, two [VDUs]
made by the same company but sourced from different counties will
have entirely different EMR signal characteristics...From this it way
be thought that there is such a jumble of emissions around, that it
would not be possible to isolate those from any one particular
source. Again, this is not the case. Most received signals have
memory or the contents of its mass storage devices is more
complicated and must be performed from a closer distance.[11] The
reconstruction of information via EMR, a process for which the
United States government refuses to declassify either the exact
technique or even its name[12], is not limited to computers and
digital devices but is applicable to all devices that generate
electromagnetic radiation.[13] TEMPEST is especially effective
against VDUs because they produce a very high level of EMR.[14]
_____________________ a different line synchronization, due to
design, reflection, interference or variation of component tolerances.
So that if for instance there are three different signals on the same
frequency ... by fine tuning of the RF receiver, antenna manipulation
and modification of line synchronization, it is possible to lock onto
each of the three signals separately and so read the screen
information. By similar techniques, it is entirely possible to
discriminate between individual items of equipment in the same
room." Potts, supra note 9. For a discussion of the TEMPEST
ELINT threat See e.g., Memory Bank, AMERICAN BANKER 20
(Apr 1 1985); Emissions from Bank Computer Systems Make
Eavesdropping Easy, Expert Says, AMERICAN BANKER 1 (Mar
26 1985); CRT spying: a threat to corporate security, PC WEEK
(Mar 10 1987). 11. TEMPEST is concerned with the transient
electromagnetic pulses formed by digital equipment. All electronic
equipment radiates EMR which may be reconstructed. Digital
equipment processes information as 1's and 0's--on's or off's.
Because of this, digital equipment gives off pulses of EMR. These
pulses are easier to reconstruct at a distance than the non-pulse
EMR given off by analog equipment. For a thorough discussion the
radiation problems of broadband digital information see e.g. military
standard MIL-STD-461 REO2; White supra note 9, 10.2. 12. See
supra note 2. 13. Of special interest to ELINT collectors are EMR
from computers, communications centers and avionics. Schultz,
Defeating Ivan with TEMPEST, DEFENSE ELECTRONICS 64
(June 1983). 14. The picture on a CRT screen is built up of picture
elements (pixels) organized in lines across the screen. The pixels are
made of material that fluoresces when struck with energy. The
energy is produced by a beam of electrons fired from an electron gun
in the back of the picture tube. The electron beam scans the screen
of the CRT in a regular repetitive manner. When the voltage of the
beam is high then the pixel it is focused upon emits photons and
appears as a dot on the screen. By selectively firing the gun as it
scans across the face of the CRT, the pixels form characters on the
CRT screen. ELINT is not limited to governments. It is routinely
used by individuals for their own purposes. Almost all forms of
ELINT are available to the individual with either the technological
expertise or the money to hire someone with the expertise.
Governments have attempted to criminalize all use of ELINT by their
subjects--to protect the privacy of both the government and the
population. II. UNITED STATES LAW In the United States, Title III
of the Omnibus Streets and Crimes Act of 1968[15] criminalizes
trespassatory ELINT as the intentional interception of wire
communications.[16] As originally passed, Title III did not prohibit
non- _____________________ The pixels glow for only a very
short time and must be routinely struck by the electron beam to stay
lit. To maintain the light output of all the pixels that are supposed to
be lit, the electron beam traverses the entire CRT screen sixty times
a second. Every time the beam fires it causes a high voltage EMR
emission. This EMR can be used to reconstruct the contents of the
target CRT screen. TEMPEST ELINT equipment designed to
reconstruct the information synchronizes its CRT with the target
CRT. First, it uses the EMR to synchronize its electron gun with the
electron gun in the target CRT. Then, when the TEMPEST ELINT
unit detects EMR indicating that the target CRT fired on a pixel, the
TEMPEST ELINT unit fires the electron gun of its CRT. The
ELINT CRT is in perfect synchronism with the target CRT; when
the target lights a pixel, a corresponding pixel on the TEMPEST
ELINT CRT is lit. The exact picture on the target CRT will appear
on the TEMPEST ELINT CRT. Any changes on the target screen
will be instantly reflected in the TEMPEST ELINT screen.
TEMPEST Certified equipment gives off emissions levels that are
too faint to be readily detected. Certification levels are set out in
National Communications Security Information Memorandum
5100A (NACSIM 5100A). "[E]mission levels are expressed in the
time and frequency domain, broadband or narrow band in terms of
the frequency domain, and in terms of conducted or radiated
emissions." White, supra, note 9, 10.1. For a thorough though
purposely misleading discussion of TEMPEST ELINT see Van Eck,
Electromagnetic Radiation from Video Display units: An
Eavesdropping Risk?, 4 Computers & Security 269 (1985). 15. Pub.
L. No. 90-351, 82 Stat. 197. The Act criminalizes trespassatory
ELINT by individuals as well as governmental agents. cf. Katz v.
United States, 389 U.S. 347 (1967) (Fourth Amendment prohibits
surveillance by government not individuals.) 16. 18 U.S.C.
2511(1)(a). trespassatory ELINT,[17] because courts found that non-
wire communication lacked any expectation of p2IIIrivacy.[18] The
Electronic Communications Privacy Act of 1986[19] amended Title
III to include non-wire communication. ECPA was specifically
designed to include electronic mail, inter- computer
communications, and cellular telephones. To accomplish this, the
expectation of privacy test was eliminated.[20] As amended, Title III
still outlaws the electronic interception of communications. The
word "communications" indicates that someone is attempting to
communicate something to someone; it does not refer to the
inadvertent transmission of information. The reception and
reconstruction of emanated transient electromagnetic pulses (ETEP),
however, is based on obtaining information that the target does not
mean to transmit. If the ETEP is not intended as communication, and
is therefore not transmitted in a form approaching current
communications protocols, then it can not be considered
communications as contemplated by Congress when it amended Title
III. Reception, or interception, of emanated transient electromagnetic
pulses is not criminalized by Title III as amended. III. ENGLISH
LAW In England the Interception of Communications Act 1985[21]
criminalizes the tapping of communications sent over
_____________________ 17. United States v. Hall, 488 F.2d 193
(9th Cir. 1973) (found no legislative history indicating Congress
intended the act to include radio-telephone conversations). Further,
Title III only criminalized the interception of "aural"
communications which excluded all forms of computer
communications. 18. Willamette Subscription Television v. Cawood,
580 F.Supp 1164 (D. Or. 1984) (non-wire communications lacks
any expectation of privacy). 19. Pub. L. No. 99-508, 100 Stat. 1848
(codified at 18 U.S.C. 2510-710) [hereinafter ECPA]. 20. 18 U.S.C.
2511(1)(a) criminalizes the interception of "any wire, oral or
electronic communication" without regard to an expectation of
privacy. 21. Interception of Communications Act 1985, Long Title,
An Act to make new provision for and in connection with the
interception of communications sent by post or by means of public
telecommunications systems and to amend section 45 of the
Telecommunications Act 1984. public telecommunications lines.[22]
The interception of communications on a telecommunication line
can take place with a physical tap on the line, or the passive
interception of microwave or satellite links.[23] These forms of
passive interception differ from TEMPEST ELINT because they are
intercepting intended communication; TEMPEST ELINT intercepts
unintended communication. Eavesdropping on the emanations of
computers does not in any way comport to tapping a
telecommunication line and therefore falls outside the scope of the
statute.[24] IV. CANADIAN LAW Canada has taken direct steps to
limit eavesdropping on computers. The Canadian Criminal
Amendment Act of 1985 _____________________ 22. Interception
of Communications Act 1985 1, Prohibition on Interception: (1)
Subject to the following provisions of this section, a person who
intentionally intercepts a communication in the course of its
transmission by post or by means of a public telecommunications
system shall be guilty of an offence and liable-- (a) on summary
conviction, to a fine not exceeding the statutory maximum; (b) on
conviction on indictment, to imprisonment for a term not exceeding
two years or to a fine or to both. *** 23. Tapping (aka trespassatory
eavesdropping) is patently in violation of the statute. "The offense
created by section 1 of the Interception of Communications Act
1985 covers those forms of eavesdropping on computer
communications which involve "tapping" the wires along which
messages are being passed. One problem which may arise, however,
is the question of whether the communication in question was
intercepted in the course of its transmission by means of a public
telecommunications system. It is technically possible to intercept a
communication at several stages in its transmission, and it may be a
question of fact to decide the stage at which it enters the "public"
realm. THE LAW COMMISSION,WORKING PAPER NO. 110:
COMPUTER MISUSE, 3.30 (1988). 24. "There are also forms of
eavesdropping which the Act does not cover. For example.
eavesdropping on a V.D.U. [referred to in this text as a CRT] screen
by monitoring the radiation field which surrounds it in order to
display whatever appears on the legitimate user's screen on the
eavesdropper's screen. This activity would not seem to constitute any
criminal offence..." THE LAW COMMISSION, WORKING PAPER
NO. 110: COMPUTER MISUSE, 3.31 (1988). criminalized indirect
access to a computer service.[25] The specific reference to an
"electromagnetic device" clearly shows the intent of the legislature to
include the use of TEMPEST ELINT equipment within the ambit of
the legislation. The limitation of obtaining "any computer service"
does lead to some confusion. The Canadian legislature has not made
it clear whether "computer service" refers to a computer service
bureau or merely the services of a computer. If the Canadians had
meant access to any computer, why did they refer to any "computer
service". This is especially confusing considering the al-
encompassing language of (b) 'any function of a computer system'.
Even if the Canadian legislation criminalizes eavesdropping on all
computers, it does not solve the problem of protecting the privacy of
information. The purpose of criminal law is to control crime.[26]
Merely making TEMPEST ELINT illegal will not control its use.
First, because it is an inherently passive crime it is impossible to
detect and hence punish. Second, making this form of eavesdropping
illegal without taking a proactive stance in controlling compromising
emanations gives the public a false sense of security. Third,
criminalizing the possession of a TEMPEST ELINT device prevents
public sector research into countermeasures. Finally, the law will not
prevent eavesdropping on private information held in company
computers unless disincentives are given for companies that do not
take sufficient precautions against eavesdropping and simple, more
common, information crimes.[27] _____________________ 25.
301.2(1) of the Canadian criminal code states that anyone who: ...
without color of right, (a) obtains, directly or indirectly, any
computer service, (b) by means of an electromagnetic ... or other
device, intercepts or causes to be intercepted, either directly or
indirectly, any function of a computer system ... [is guilty of an
indictable offence]. 26. UNITED STATES SENTENCING
COMM'N, FEDERAL SENTENCING GUIDELINES MANUAL
(1988) (Principles Governing the Redrafting of the Preliminary
Guidelines "g." (at an unknown page)) 27. There has been great
debate over what exactly is a computer crime. There are several
schools of thought. The more articulate school, and the one to which
the author adheres holds that the category computer crime should be
limited to crimes directed against computers; for example, a terrorist
destroying a computer with explosives would fall into this category.
Crimes such as putting ghost employees on a payroll computer and
V. SOLUTIONS TEMPEST ELINT is passive. The computer or
terminal emanates compromising radiation which is intercepted by
the TEMPEST device and reconstructed into useful information.
Unlike conventional ELINT there is no need to physically trespass
or even come near the target. Eavesdropping can be performed from a
nearby office or even a van parked within a reasonable distance. This
means that there is no classic scene of the crime; and little or no
chance of the criminal being discovered in the act.[28] If the crime is
discovered it will be ancillary to some other investigation. For
example, if an individual is investigated for insider trading a search
of his residence may yield a TEMPEST ELINT device. The device
would explain how the defendant was obtaining insider information;
but it was the insider trading, not the device, that gave away the
crime. This is especially true for illegal TEMPEST ELINT
performed by the state. Unless the perpetrators are caught in the act
there is little evidence of their spying. A trespassatory bug can be
detected and located; further, once found it provides tangible
evidence that a crime took place. A TEMPEST ELINT device by its
inherent passive nature leaves nothing to detect. Since the
government is less likely to commit an ancillary crime which might
be detected there is a very small chance that the spying will ever be
discovered. The only way to prevent eavesdropping is to encourage
the use of countermeasures: TEMPEST Certified[29] computers and
_____________________ collecting their pay are merely age-old
accounting frauds; today the fraud involves a computer because the
records are kept on a computer. The computer is merely ancillary to
the crime. This has been mislabeled computer crime and should
merely be referred to as a fraud perpetrated with the aid of a
computer. Finally, there are information crimes. These are crimes
related to the purloining or alteration of information. These crimes
are more common and more profitable due to the computer's ability
to hold and access great amounts of information. TEMPEST ELINT
can best be categorized as a information crime. 28. Compare, for
example, the Watergate breakin in which the burglars were
discovered when they returned to move a poorly placed spread
spectrum bug. 29. TEMPEST Certified refers to the equipment
having passed a testing and emanations regime specified in
NACSIM 5100A. This classified document sets forth the emanations
levels that the NSA believes digital equipment can give off without
compromising the information it is processing. TEMPEST Certified
equipment is theoretically secure against TEMPEST eavesdropping.
terminals. In merely making TEMPEST ELINT illegal the public is
given the false impression of security; they lulled into believing the
problem has been solved. Making certain actions illegal does not
prevent them from occurring. This is especially true for a TEMPEST
ELINT because it is undetectable. Punishment is an empty threat if
there is no chance of being detected; without detection there can be
no apprehension and conviction. The only way to prevent some
entity from eavesdropping on one's computer or computer terminal is
for the equipment not to give off compromising emanation; it must
be TEMPEST Certified. The United States can solve this problem by
taking a proactive stance on compromising emanations. The National
Institute of Standards and Technology (NIST[30]) is in charge of
setting forth standards of computer security for the private sector.
NIST is also charged with doing basic research to advance the art of
computer security. Currently NIST does not discuss TEMPEST with
the private sector. For privacy's sake, this policy must be changed to
a proactive one. The NIST should publicize the TEMPEST ELINT
threat to computer security and should set up a rating system for
level of emanations produced by computer equipment.[31] Further,
legislation should be enacted to require the labeling of all computer
equipment with its level of emanations and whether it is TEMPEST
Certified. Only if the public knows of the problem can it begin to
take steps to solve it. Title III makes possession of a surveillance
device a crime, unless it is produced under contract to the
government. This means that research into surveillance and counter-
surveillance equipment is monopolized by the government and a few
companies working under contract with _____________________
NACSIM 5100A is classified, as are all details of TEMPEST. To
obtain access to it, contractor must prove that there is demand within
the government for the specific type of equipment that intend to
certify. Since the standard is classified, the contractors can not sell
the equipment to non-secure governmental agencies or the public.
This prevents reverse engineering of the standard for its physical
embodiment, the Certified equipment. By preventing the private
sector from owning this anti- eavesdropping equipment, the NSA has
effectively prevented the them from protecting the information in
their computers. 30. Previously the Bureau of Standards. The NIST
is a division of the Commerce Department. 31. In this case computer
equipment would include all peripheral computer equipment. There
is no use is using a TEMPEST Certified computer if the printer or
the modem are not Certified. the government. If TEMPEST
eavesdropping is criminalized, then possession of TEMPEST ELINT
equipment will be criminal. Unfortunately,this does not solve the
problem. Simple TEMPEST ELINT equipment is easy to make. For
just a few dollars many older television sets can be modified to
receive and reconstruct EMR. For less than a hundred dollars a more
sophisticated TEMPEST ELINT receiver can be produced[32]. The
problem with criminalizing the possession of TEMPEST ELINT
equipment is not just that the law will have little effect on the use of
such equipment, but that it will have a negative effect on counter-
measures research. To successfully design counter-measures to a
particular surveillance technique it is vital to have a complete
empirical understanding of how that technique works. Without the
right to legally manufacture a surveillance device there is no possible
way for a researcher to have the knowledge to produce an effective
counter-measures device. It is axiomatic: without a surveillance
device, it is impossible to test a counter-measures device. A number
of companies produce devices to measure the emanations from
electrical equipment. Some of these devices are specifically designed
for bench marking TEMPEST Certified equipment. This does not
solve the problem. The question arises: how much radiation at a
particular frequency is compromising? The current answer is to refer
_____________________ 32. The NSA has tried to limit the
availability of TEMPEST information to prevent the spread of the
devices. For a discussion of the First Amendment and prior restraint
See, e.g. The United States of America v. Progressive, Inc. 467
F.Supp 990 (1979, WD Wis.)(magazine intended to publish plans
for nuclear weapon; prior restraint injunction issued), reh. den.
United States v. Progressive Inc. 486 F.Supp 5 (1979, WD Wis.),
motion den Morland v. Sprecher 443 US 709 (1979)(mandamus),
motion denied United States v. Progressive, Inc. 5 Media L R (1979,
7th Cir.), dismd. without op. U.S. v. Progressive, Inc 610 F.2d 819
(1979, 7th Cir.); New York Times, Co. v. United States, 403 U.S.
713 (1971)(per curium)(Pentagon Papers case: setting forth prior
restraint standard which government was unable to meet); T.
EMERSON, THE SYSTEM OF FREEDOM OF EXPRESSION
(1970); Balance Between Scientific Freedom and NAtional Security,
23 JURIMETRICS J. 1 (1982)(current laws and regulations limiting
scientific and technical expression exceed the legitimate needs of
national security); Hon. M. Feldman, Why the First Amendment is
not Incompatible with National Security, HERITAGE
FOUNDATION REPORTS (Jan. 14, 1987). Compare Bork, Neutral
Principles and Some First Amendment Problems, 47 IND. L. J. 1
(First Amendment applies only to political speech); G. Lewy, Can
Democracy Keep Secrets, 26 POLICY REVIEW 17
(1983)(endorsing draconian secrecy laws mirroring the English
system). to NACSIM 5100A. This document specifies the
emanations levels suitable for Certification. The document is only
available to United States contractors having sufficient security
clearance and an ongoing contract to produce TEMPEST Certified
computers for the government. Further, the correct levels are
specified by the NSA and there is no assurance that, while these
levels are sufficient to prevent eavesdropping by unfriendly
operatives, equipment certified under NACSIM 5100A will have
levels low enough to prevent eavesdropping by the NSA itself. The
accessibility of supposedly correct emanations levels does not solve
the problem of preventing TEMPEST eavesdropping. Access to
NACSIM 5100A limits the manufacturer to selling the equipment
only to United States governmental agencies with the need to
process secret information.[33] Without the right to possess
TEMPEST ELINT equipment manufacturers who wish to sell to the
public sector cannot determine what a safe level of emanations is.
Further those manufacturers with access to NACSIM 5100A should
want to verify that the levels set out in the document are, in fact, low
enough to prevent interception. Without an actual eavesdropping
device with which to test, no manufacturer will be able to produce
genuinely uncompromising equipment. Even if the laws allow
ownership of TEMPEST Certified equipment by the public, and
even if the public is informed of TEMPEST's threat to privacy,
individuals' private information will not necessarily by protected.
Individuals may choose to protect their own information on their
own computers. Companies may choose whether to protect their own
private information. But companies that hold the private information
of individuals must be forced to take steps to protect that
information. In England the Data Protection Act 1984[34] imposes
sanctions against anyone who stores the personal information[35] on
a computer and fails to take reasonable _____________________
33. For example, the NSA has just recently allowed the Drug
Enforcement Agency (DEA) to purchase TEMPEST Certified
computer equipment. The DEA wanted secure computer equipment
because wealthy drug lords had were using TEMPEST
eavesdropping equipment. 34. An Act to regulate the use of
automatically processed information relating to individuals and the
provision of services in respect of such information. -Data
Protection Act 1984, Long Title. 35. "Personal data" means data
consisting of information which relates to a living individual who
can be identified from that measures to prevent disclosure of that
information. The act mandates that personal data may not be stored
in any computer unless the computer bureau or data user[36] has
registered under the act.[37] This provides for a central registry and
the tracking of which companies or persons maintain databases of
personal information. Data users and bureaux must demonstrate a
need and purpose behind their possession of personal data. The act
provides tort remedies to any person who is damaged by disclosure
of the personal data.[38] Reasonable care to prevent the disclosure is
a defense.[39] English _____________________ information (or
from that and other information in the possession of the data user),
including any expression of opinion about the individual but not any
indication of the intentions of the data user in respect of that
individual. -Data Protection Act 1984 1(3) 36. "Data user" means a
person who holds data, and a persons "Holds" data if -- (a) the data
form part of a collection of data processed or intended to be
processed by or on behalf of that person as mentioned in subsection
(2) above; [subsection (2) defines "data"] and (b) that person (either
alone or jointly or in common with other persons) controls the
contents and use of the data comprised in the collection; and © the
data are in the form in which they have been or are intended to be
processed as mentioned in paragraph (a) above or (though not for the
time being in that form) in a form into which they have been
converted after being so processed and with a view to being further
so processed on a subsequent occasion. - Data Protection Act 1(5).
37. Data Protection Act 1984, 4,5. 38. An individual who is the
subject of personal data held by a data user... and who suffers
damage by reason of (1)© ... the disclosure of the data, or access
having been obtained to the data without such authority as aforesaid
shall be entitled to compensation from the data user... for any
distress which the individual has suffered by reason of the ...
disclosure or access. - Data Protection Act 1984 23. 39. ... it shall be
a defense to prove that ... the data user ... had taken such care as in
all the circumstances was reasonably required to prevent the...
disclosure or access in question. Data Protection Act 1984 23(3)
courts have not yet ruled what level of computer security measures
constitute reasonable care. Considering the magnitude of invasion
possible with TEMPEST ELINT it should be clear by now that
failure to use TEMPEST Certified equipment is prima facie
unreasonable care. The Remedies section of the act provides
incentive for these entities to provide successful protection of person
data from disclosure or illicit access. Failure to protect the data will
result in monetary loss. This may be looked at from the economic
efficiency viewpoint as allocating the cost of disclosure the persons
most able to bear those costs, and also most able to prevent
disclosure. Data users that store personal data would use TEMPEST
Certified equipment as part of their computer security plan,
thwarting would-be eavesdroppers. The Data Protection Act 1984
allocates risk to those who can bear it best and provides an incentive
for them to keep other individuals' data private. This act should be
adopted by the United States as part of a full-spectrum plan to
combat TEMPEST eavesdropping. Data users are in the best
position to prevent disclosure through proper computer security.
Only by making them liable for failures in security can we begin to
rein in TEMPEST ELINT. VII Recommendations Do not criminalize
TEMPEST ELINT. Most crimes that TEMPEST ELINT would aid,
such a insider trading, are already illegal; the current laws are
adequate. The National Institute of Standards and Technology
should immediately begin a program to educate the private sector
about TEMPEST. Only if individuals are aware of the threat can
they take appropriate precautions or decide whether any precautions
are necessary. Legislation should be enacted to require all electronic
equipment to prominently display its level of emanations and
whether it is TEMPEST Certified. If individuals are to choose to
protect themselves they must be able to make a informed decision
regarding how much protection is enough. TEMPEST Certified
equipment should be available to the private sector. The current ban
on selling to non- governmental agencies prevents individuals who
need to protect information from having the technology to do so.
Possession of TEMPEST ELINT equipment should not be made
illegal. The inherently passive nature and simple design of
TEMPEST ELINT equipment means that making its possession
illegal will not deter crime; the units can be easily manufactured and
are impossible to detect. Limiting their availability serves only to
monopolize the countermeasures research, information, and
equipment for the government; this prevents the testing, design and
manufacture of counter-measures by the private sector. Legislation
mirroring England's Data Protection Act 1984 should be enacted.
Preventing disclosure of personal data can only be accomplished by
giving those companies holding the data a reason to protect it. If data
users are held liable for their failure to take reasonable security
precautions they will begin to take reasonable security precautions,
including the use of TEMPEST Certified equipment. ust be forced
to take steps to protect that information. In England the Data
Protection Act 1984[34] imposes sanctions against anyone who
stores the personal information[35] on a computer and fails to take
reasonable _____________________ 33. FTEMPEST-shielding
644 2066 1274 37713 5561131303 7241 Preventing
Electromagnetic Eavesdropping --------- NOTE: Even though the
author of this document uses the term "TEMPEST" at various points,
he certifies that his text consists of general concepts to reduce
electromagnetic emissions, and was not based on any actual
TEMPEST specifications or data. Further questions on this topic
should be directed to the author, "[email protected]". --
PRIVACY Forum Moderator --------- Abstract Eavesdropping on
personal computers is not limited to looking over the shoulder of the
operator or physically tapping in to an Ethernet cable. U.S.
Government standards relating to the prevention of information
capture via the emission of electromagnetic radiation from computers
and peripherals are known as TEMPEST. However, actual
TEMPEST specifications are classified. TEMPEST aside, there are
inexpensive and easily applied means for individuals to minimize
unintentional emissions from equipment. My document "Preventing
Electromagnetic Eavesdropping," discusses these techniques. Grady
Ward --------- Preventing Electromagnetic Eavesdropping A note
discussing the prevention of electromagnetic eavesdropping of
personal computers. Grady Ward public key verification by PK
server, finger, or by request 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 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 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 © 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 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 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
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. 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. Copyright
© 1993 by Grady Ward. All Rights Reserved. Permission is granted
for free electronic distribution. ought to be filtered, it is especially
important for more-on-TEMPEST 600 2066 1274 11221
5603104744 6627 | [email protected] (Kingsley G. Morse
Jr.) said: | In [email protected]. | u (William
VanHorne) writes: | >TEMPEST is nothing more than a way to |
>force manufacturers to do some rudimentary shielding of their
devices | >so as to cut down on RF emissions, and concerns stuff
like making sure | >that the cables are coax-like shielded and real
high-tech stuff like | >that. | >Now, you, as Mr./Ms. Consumer, do
not buy electronic items that have | >*any* shielding in them. Not
correct: all consumer electronics that produce RF have *some*
shielding. There are many "leaky" spots, however, and most of these
involve cabling and ground loops. | PGP is popular, and perhaps
TEMPEST equipment would sell well also. | Does anyone know of a
distributor or catalog of TEMPEST rated shielding | or equipment?
Shielding is not difficult or complex; anyone can do it. Pop down to
your local magazine rack and get a mag on Amateur radio, look up a
mail order ham book place, and order a book on the subject. As for
personal computers, here's a few starting things to consider: o Get a
case that's made of steel, on all 6 sides. o Get the highest quality
shielded cables you can find. The GOOD commercial ones will have
metal hoods on the connectors and little grounding pigtails. (If you
pay $4.95 for a serial cable you have probably bought a good
antenna). o Ground the case and cables with SHORT ground wires,
directly to a 10' copper ground rod just outside the wall. o In a well-
shielded system, the keyboard will emit the most RF, and it may be
impossible to shield it adequately, tho the heavier, steel-base
keyboards like Focus and Northgate are quieter than cheap plastic
ones. The coily-cords radiate a lot, too. Replace your keyboard cable
with a quality, straight, shielded one and ground it. o Use a portable
shortwave radio to check for leaks. Tune it first to your buss speed,
then to processor speed. Then scan around for leaks on other
frequencies. If you're *really* serious, drop 2 grand+ on an RF
spectrum analyzer. That's what any snoop would use to locate your
emission frequencies. Grounding and Shielding -- Misconceptions
& things to know: o A DC ground and an RF ground are not the
same thing. A ground wire that is longer than 1/4 wavelength is an
antenna, not a ground (regardless of what you ground it to). For
example, a ground wire longer than 4.6 feet is an antenna at 50MHz.
At 8 MHz, wires approaching 29 feet become antennas. Use 234/f
(in mhz), then keep it quite a bit shorter than that. Obviously, you
can't use your house grounding system, or cold water pipes, as RF
grounds -- they're too long. o Shielded multiconductor cables, and
coax, are not really designed to stop all emissions of RF. They are
designed to stop electrical interference from getting *into* the cable.
If the shield of a cable is carrying RF (either by direct feed or
induction from the wires within), it will radiate it at certain lengths,
just like any other wire. o Twists: The most effective way to stop
cable RF radiation is to use cables that contain tightly twisted pairs
of wires. This results in near-total phase cancellation. But it doesn't
stop "common-mode" radiation. This is where RF at some frequency
treats all wires in the cable as one. For example, 8 MHz buss noise
may travel in-phase down your printer cable, to your printer chassis,
to house ground, and back to your computer chassis. If this whole
loop is => 29 feet, you'll radiate buss noise. The shield radiates
either because it is carrying RF, or because the pairs inside are
radiating and inducing RF on the shield. o Ferrite toroids are
available in a variety of sizes, and don't cost much. You probably
already have some small ones in your power supply, on the input
leads. These doughnuts will "eat" common-mode RF induction. You
wrap several turns of the wire in question thru the doughnut. You
can add these to your power supply (AC side), and all other wires
leaving the computer case, including ground wires, keyboard, video,
serial, parallel, LAN, and even speaker wires. Notwithstanding all of
the above, it is probably impossible to pick up any *useful*
information off of keyboards and monitors. Networks, with their
loooong data cables, are another matter. Users needing tight security
on networks should use fiber optics, then apply all the stuff above to
all pieces of hardware in the network. (Continued to next message) *
QMPro 1.02 41-7176 * This is an egg. This is an egg in a skillet. e
to locate your emission frequencies. Grounding and Shielding --
Misconceptions & things to know: o A DC ground and an RF
ground are not the same thing. A ground wire that is longer than 1/4
wavelength is an antenna, not a ground (regardless of what you
ground it to). For example, a ground wire longer than 4.6 feet is an
antenna at 50MHz. At 8 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. Copyright © 1993 by Grady Ward. All Rights
Reserved. Permission is granted for free electronic distribution.
ought to be filtered, it is especially important for more-on-
TEMPEST 600 2066 1274 11221 5603104744 6627 |
[email protected] (Kingsley G. Morse Jr.) said: | In
[email protected]. | u (William VanHorne) writes: |
>TEMPEST is nothing more than a way to | >force manufacturers to
do some rudimentary shielding of their devices | >so as to cut down
on RF emissions, and concerns stuff like making sure | >that the
cables are coax-like shielded and real high-tech stuff like | >that. |
>Now, you, as Mr./Ms. Consumer, do not buy electronic items that
have | >*any* shielding in them. Not correct: all consumer
electronics that produce RF have *some* shielding. There are many
"leaky" spots, however, and most of these involve cabling and
ground loops. | PGP is popular, and perhaps TEMPEST equipment
would sell well also. | Does anyone know of a distributor or catalog
of TEMPEST rated shielding | or equipment? Shielding is not
difficult or complex; anyone can do it. Pop down to your local
magazine rack and get a mag on Amateur radio, look up a mail order
ham book place, and order a book on the subject. As for personal
computers, here's a few starting things to consider: o Get a case that's
made of steel, on all 6 sides. o Get the highest quality shielded
cables you can find. The GOOD commercial ones will have metal
hoods on the connectors and little grounding pigtails. (If you pay
$4.95 for a serial cable you have probably bought a good antenna). o
Ground the case and cables with SHORT ground wires, directly to a
10' copper ground rod just outside the wall. o In a well-shielded
system, the keyboard will emit the most RF, and it may be
impossible to shield it adequately, tho the heavier, steel-base
keyboards like Focus and Northgate are quieter than cheap plastic
ones. The coily-cords radiate a lot, too. Replace your keyboard cable
with a quality, straight, shielded one and ground it. o Use a portable
shortwave radio to check for leaks. Tune it first to your buss speed,
then to processor speed. Then scan around for leaks on other
frequencies. If you're *really* serious, drop 2 grand+ on an RF
spectrum analyzer. That's what any snoop would use to locate your
emission frequencies. Grounding and Shielding -- Misconceptions
& things to know: o A DC ground and an RF ground are not the
same thing. A ground wire that is longer than 1/4 wavelength is an
antenna, not a ground (regardless of what you ground it to). For
example, a ground wire longer than 4.6 feet is an antenna at 50MHz.
|
|