21 April 2004
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| United States Patent |
6,721,423 |
| Anderson , et al. |
April 13, 2004 |
Lost cost countermeasures against compromising electromagnetic
computer emanations
Abstract
A set of methods is specified whereby software reduces compromising
electromagnetic emanations of computers that could otherwise allow eavesdroppers
to reconstruct sensitive processed data using periodic averaging techniques.
Fonts for screen display of text are low-pass filtered to attenuate those
spectral components that radiate most strongly, without significantly affecting
the readability of the text, while the character glyphs displayed are chosen
at random from sets that are visually equivalent but that radiate differently.
Keyboard microcontroller scan loops are also furnished with random variations
that hinder reconstruction of the signal emanated by a keyboard. Drivers
for hard disks and other mass-storage devices ensure that the read head is
never parked over confidential data longer than necessary.
| Inventors: |
Anderson; Ross J. (10 Water End,
Wrestlingworth, Sandy, Bedfordshire, GB SG29 2HA); Kuhn; Markus Guenther
(Schlehenweg 9, Uttenreuth, DE D-91080) |
| Appl. No.: |
238560 |
| Filed: |
January 28, 1999 |
| Current U.S. Class: |
380/252; 380/268; 380/210;
380/54 |
| Intern'l Class: |
H04L 009/00 |
| Field of Search: |
380/205,210,268,287,22,1,252,54
713/190,189 |
References Cited
[Referenced
By]
U.S. Patent Documents
| 3770269 |
Nov., 1973 |
Elder |
463/18. |
| 4203102 |
May., 1980 |
Hydes |
345/467. |
| 4695904 |
Sep., 1987 |
Shinyagaito et al. |
|
| 5379343 |
Jan., 1995 |
Grube et al. |
|
| 5530390 |
Jun., 1996 |
Russell |
327/164. |
| 5726538 |
Mar., 1998 |
Jackson et al. |
315/370. |
| 5894517 |
Apr., 1999 |
Hutchison et al. |
380/268. |
Primary Examiner: Barron; Gilberto
Assistant Examiner: Gurshman; G
Claims
What is claimed is:
1. A method of obstructing the reconstruction of information shown on a
video-display system from electromagnetic emissions generated by that system,
in which the display is altered using character fonts that compose each displayed
graphic character using more than two pixel amplitudes in order to reduce
the electromagnetic emissions in video-signal frequencies that are radiated
or conducted to potential eavesdropper receiver positions particularly well.
2. A method of obstructing the reconstruction of information shown on a
video-display system from electromagnetic emissions generated by said
video-display system comprising: generating several character fonts consisting
of pixel images of glyphs; each of said fonts providing a glyph image for
each graphic character of a supported character set, said character set being
common across all generated fonts; each of said glyph images differing slightly
in style, size, position and quantization noise from glyph images that represent
the same character in the other generated fonts responsive to monitored emission
measurements and subject to a trade-off that keeps the differences in visual
appearance at a minimum and that maximizes the differences in electromagnetic
emissions in video-signal frequencies that are radiated or conducted to a
potential eavesdropper receiver, and a mechanism to alter said video display
by randomly choosing among said fonts for each newly displayed instance of
a character.
3. A method of obstructing the reconstruction of information shown on a
video-display system from electromagnetic emission generated by said
video-display system comprising: generating character fonts consisting of
grey-level pixel images of glyphs; filtering said generated character fonts
in a horizontal direction responsive to monitored emission measurements and
a signal-energy to display-quality trade-off, and altering said video display
by using character fonts that compose displayed characters using more than
two pixel amplitudes for reducing the electromagnetic emissions in video-signal
frequencies that are radiated or conducted to a potential eavesdropper receiver.
Description
TECHNICAL FIELD
This invention is related to the protection of confidential computer data
against eavesdroppers who try to reconstruct it from the electromagnetic
emanations generated by computers.
BACKGROUND OF THE INVENTION
It has been known to military organizations since at least the early 1960s
that computers generate electromagnetic radiation which not only interferes
with radio reception, but which also makes information about the processed
data available to a remote radio receiver (see for example Peter Wright:
Spycatcher--The Candid Autobiography of a Senior Intelligence Officer. William
Heinemann Australia, 1987, ISBN 0-85561-098-0). Known as compromising emanation
or Tempest radiation, this electromagnetic broadcast of data has been a
significant concern in security-sensitive computer applications.
Compromising emanations of video display units (see for example
Wim van Eck: Electromagnetic Radiation from Video Display Units: An Eavesdropping
Risk? Computers & Security vol 4 (1985) 269-286; Erhard Moller, Lutz
Bernstein, Ferdinand Kolberg: Schutzma.beta. nahmen gegen kompromittierende
elektromagnetische Emissionen von Bildschirmsichtgeraten [Protective measures
against compromising electromagnetic emissions from video display terminals].
Labor fur Nachrichtentechnik, Fachhochschule Aachen, Aachen, Germany) and
serial data cables (see Peter Smulders: The Threat of Information Theft by
Reception of Electromagnetic Radiation from RS-232 Cables. Computers &
Security vol 9 (1990) 53-58) have been described in the open literature.
One common and expensive countermeasure is to fit metallic shielding to the
device, the room, or the entire building (see Electromagnetic Pulse (EMP)
and Tempest Protection for Facilities. Engineer Pamphlet EP 1110-3-2, 469
pages, U.S. Army Corps of Engineers, Publications Depot, Hyattsville, Dec.
31, 1990; and Deborah Russell, G. T. Gangemi Sr.: Computer Security Basics.
O'Reilly & Associates, 1991, ISBN 0-937175-71-4). Cross-correlation test
methods suitable for verifying the effectiveness of such shielding have been
described in Wolfgang Bitzer, Joachim Opfer: Schaltungsanordnung zum Messen
der Korrelationsfunktion zwischen zwei vorgegebenen Signalen [Circuit arrangement
for measuring the correlation function between two given signals]. German
Patent DE.sup..about. 3911155.sup..about. C2, Deutsches Patentamt, Nov. 11,
1993, and Joachim Opfer, Reinhart Engelbart: Verfahren zum Nachweis von
verzerrten und stark gestorten Digitalsignalen und Schaltungsanordnung zur
Durchfuhrung des Verfahrens [Method for the detection of distorted and strongly
interfered digital signals and circuit arrangement for implementing this
method]. German Patent DE.sup..about. 4301701.sup..about. C1, Deutsches
Patentamt, May 5, 1994. Devices that generate a correlated jamming signal
in order to make eavesdropping more difficult have been described in John
H. Dunlavy: System for Preventing Remote Detection of Computer Data from
TEMPEST Signal Emissions. U.S. Pat. No. 5,297,201, Mar. 22, 1994, and Lars
Hoivik: System for Protecting Digital Equipment Against Remote Access. U.S.
Pat. No. 5,165,098, Nov. 17, 1992.
The electromagnetic data-dependent signals generated by computers and emanated
over the air, or via power supply and communication cables, are rather weak
and distorted. In addition, if several computers are located in close proximity,
their signals will be overlaid. The eavesdropper will therefore use various
techniques to separate the signals of interest from the background noise
before attempting further decoding (see Markus G. Kuhn, Ross J. Anderson:
Soft Tempest: Hidden Data Transmission Using Electromagnetic Emanations,
in David Aucsmith (Ed.): Information Hiding, Second International Workshop,
IH'98, Portland, Oreg., USA, Apr. 15-17, 1998, Proceedings, LNCS 1525,
Springer-Verlag, ISBN 3-540-65386-4, pp. 126-143). Periodic averaging is
a very powerful noise elimination technique and can be applied to many signals
of particular interest from computer systems that process confidential data.
If the signal of interest s(t) has a known period T such that s(t)=s(t+T)
most of the time, then the eavesdropper can reconstruct from the received
noisy signal r(t)=s(t)+n(t), where n(t) is uncorrelated background noise,
a noise-reduced estimate of the signal from a moving average: ##EQU1##
which has a significantly better signal-to-noise ratio than s(t).
Three periodic signals found in a typical computer may contain confidential
information and are thus of particular interest to an eavesdropper:
1. The video display signal is generated by writing the content of the display
frame buffer to the display with a period equivalent to the vertical refresh
frequency of the cathode-ray tube, liquid crystal panel, or other display
device.
2. A microcontroller or a specialized circuit in the keyboard applies voltages
in succession to each row of a matrix circuit to which the keys are connected.
Scanning the column lines for this voltage allows the microcontroller or
specialized circuit to determine which key is currently pressed in order
to report the appropriate key code word to the main processor (see Ed L.
Sonderman, Walter Z. Davis: Scan-controlled keyboard, U.S. Pat. No. 4,277,780,
Jul. 7, 1981). This scan cycle is repeated with high frequency to ensure
that no key-press events are missed. The sequence of instructions executed
in the scan loop often depends on which key is currently pressed. Therefore
the precise shape of the emanations reveals information about key presses,
and manually entered text may be reconstructed by an eavesdropper.
3. In most mass storage devices such as magnetic or magneto-optical discs,
data is organized into storage tracks and a motor moves the head between
them. After data has been read from or written to a track, the head usually
remains located on that track until a request to access another track is
received. During this time, the readout amplifier receives, amplifies and
emits the data content of the storage track periodically, where the period
is identical to the rotation time of the disk.
SUMMARY OF THE INVENTION
The present invention is a low-cost means of making it more difficult for
an eavesdropper to gain knowledge about the data processed on a normal computer
system that features standard components such as a video display, a keyboard
and a hard disk. In its most general terms the presents invention proposes
that instead of, or in addition to, physical screening of an electronic system,
the system should be designed or modified to reduce (or substantially eliminate)
the generation of electromagnetic signals which are periodic or otherwise
predictable.
Accordingly, the invention may be expressed as a method of obstructing the
reconstruction of information contained in an electronic apparatus from
electromagnetic emissions, by reducing the energy of certain periodic signals
in electromagnetic emissions generated by the system and destroying the
periodicity of residual signals or other signals.
These methods may involve only software or firmware changes in the computer
system and can therefore be implemented at a much lower cost than the
conventional techniques described above, in which electromagnetic radiation
is reabsorbed after it has been generated (i.e. physical shielding). They
may also be implemented using low-cost hardware devices. Whether they are
implemented in software, firmware or hardware, these techniques can also
be combined with traditional physical shields in order to provide an independent
layer of protection against shield failure.
The general means of protection is to render signals more difficult for an
attacker to recover using periodic averaging and cross-correlation techniques.
Three specific methods are filtering out from periodic signals those spectral
components that cause the highest levels of compromising radiation, spreading
the spectrum of the residual information-bearing radiation using a sequence
unknown to the attacker, and removing periodic signals directly. We will
describe examples of these three techniques in turn.
An example of the first method consists of displaying text on the video display
device using a special font that employs a plurality of pixel luminosities
in order to represent character glyphs. The use of more than two pixel
luminosities to display anti-aliased characters and thus avoid staircase
effects in slanted lines and italic characters has been described in Richard
B. Preiss, John C. Dalrymple: System and method for smoothing the lines and
edges of an image on a raster-scan display, U.S. Pat. No. 4,672,369, Jun.
9, 1987, and Bradley J. Beitel, Robert D. Gordon, Joseph B. Witherspoon III:
Anti-alias font generation, U.S. Pat. No. 5,390,289, Feb. 14, 1995}. The
innovation in the present invention is to use a font specially designed so
that the horizontal spatial frequency spectrum of the glyphs is adapted to
the emission spectrum of the video display device so as to reduce the broadcast
energy and thus minimize the range within which eavesdroppers can identify
the displayed characters.
An example of the second method consists, firstly, of using a random number
generator to select one of a number of character glyphs which are visually
similar but which are generated by different video signals, in order to make
it more difficult to reconstruct the signal using signal processing techniques;
and secondly, introducing a variable delay into the keyboard matrix scan
cycle, which makes it harder for eavesdroppers to reconstruct the compromising
emissions of the keyboard. The innovation in the present invention is to
randomise the inadvertently emitted signal and thus make its reconstruction
by an attacker more difficult.
An example of the third method is to modify the device driver software or
controller firmware responsible for the control of disk drives, or in general
any mass storage device that uses moveable read/write heads to access a plurality
of storage tracks on the surface of a storage medium. The innovation in the
present invention is to park inactive read/write heads on a storage track
that does not contain confidential data.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 shows a pixel field containing normal raster text.
FIG. 2 shows a pixel field containing horizontally low-pass filtered raster
text, illustrating the application of the second emanation protection method
described in this invention.
FIG. 3 shows a magnified photograph of the pixel field in FIG. 1 as it is
displayed on a cathode-ray computer monitor.
FIG. 4 shows a magnified photograph of the pixel field in FIG. 2 as it is
displayed on a cathode-ray computer monitor.
FIG. 5 shows an excerpt from the video signal generated by the pixel field
shown in FIG. 1.
FIG. 6 shows an excerpt from the video signal generated by the pixel field
shown in FIG. 2, taken from the same pixel coordinates as those used in FIG.
5.
FIG. 7 shows the video signal from FIG. 6 after it has passed a simple analog
low-pass filter that has been installed on the computer video adapter output
in order to attenuate the aliasing frequencies generated by the discrete
nature of the video signal and by the shape of a single pixel pulse.
FIG. 8 shows a photograph of the screen of a Tempest eavesdropping receiver
when the computer screen under surveillance contains normal raster text fonts
as shown in FIG. 1.
FIG. 9 shows a photograph of the screen of a Tempest eavesdropping receiver
when the computer screen under surveillance contains horizontally low-pass
filtered content as shown in FIG. 2, demonstrating the protective effect
of this invention.
DETAILED DESCRIPTION
In the case of the video display unit, we shape the spectrum of the periodic
video signal by using digital filtering or by combining digital filtering
and anti-aliasing techniques to generate a character font with little spectral
energy in those frequency ranges in which the computer monitor radiates
particularly well. The spectral characteristics of the monitor are first
determined by using the graphics adapter of the computer to display test
images such as a zoneplate pattern. The emanations are then measured in an
electromagnetic compatibility laboratory using a spectrum analyzer or a Tempest
monitoring receiver. In one test system described in Markus G. Kuhn, Ross
J. Anderson "Soft Tempest: Hidden Data Transmission Using Electromagnetic
Emanations" (in David Aucsmith (Ed.): Information Hiding, Second International
Workshop, IH'98, Portland, Oreg., USA, Apr. 15-17, 1998, Proceedings, LNCS
1525, Springer-Verlag, ISBN 3-540-65386-4, pp. 126-143) these measurements
showed that for a video mode with 95 MHz pixel frequency, most of the emitted
energy came from parts of the test image with frequencies in the range 33-47.5
MHZ. The emitted energy was not only present in this frequency range but
also as higher harmonics of frequencies in this band.
Preferably, the present invention reduces the amount of emitted information
bearing radiation by at least 10 dB, or more preferably by at least 20 dB
or even 30 dB. This is because in the zoning model used by many governments
to decide which classification of information may be processed on which type
of apparatus in which zone of a building, a signal attenuation of 10 dB
corresponds to a single zone (see Deborah Russell, G. T. Gangemi Sr.: Computer
Security Basics. O'Reilly & Associates, 1991, ISBN 0-937175-71-4). Text
displayed with a font in which all horizontal pixel lines have been processed
with a digital filter to attenuate frequency components in this range by
about 20 dB becomes practically invisible on a Tempest monitor while the
display quality and readability of the text by persons in front of the authorised
display device is only marginally affected. This processing can be achieved
by passing the video signal through a suitable hardware filter, or more
conveniently by software graphic processing.
In our typical embodiment, we start out with a high-resolution version of
a character font and generate grey-level pixel images of the glyphs, selecting
for the background and foreground luminosity 85% and 15% of the available
maximal white luminosity in order to prevent overflow or underflow during
subsequent filtering. We then apply a normal subsampling filter in both
horizontal and vertical directions in order to prevent aliasing by removing
all frequency components that are above the Nyquist limit of the final pixel
spacing. Our innovation over existing anti-aliasing technology is to apply
in the horizontal direction a further filter that attenuates those frequencies
at which the video display device radiates compromising RF emanations
efficiently. The spectral shape of the anti-emission filter depends on the
results of the monitor emission measurements and on a signal energy versus
display quality tradeoff.
After these filtering steps, the filtered high-resolution font is subsampled
and stored for use by display routines. The resulting filtered glyphs may
be significantly wider than the underlying original glyphs and thus the display
routine must superpose them using addition, with the background (85%) luminosity
treated as zero for the purpose of this addition. An example text that has
been generated this way is shown in FIG. 2 as a pixel field and in FIG. 4
as a CRT screen photograph. FIG. 6 shows a typical video signal generated
this way, from which further harmonics can be removed by an analog filter
at the video adapter output, resulting in a smoother signal such as that
shown in FIG. 7. For best performance, a 30 MHz low-pass hardware filter
is used; if the application admits only software countermeasures, then the
filters installed in monitor cables for EMC and RFI compliance purposes together
with the natural inductance of the cables and the limitations of the video
amplifier circuitry have a similar if less controlled effect.
FIG. 9 shows the signal received by the eavesdropping receiver described
in Markus G. Kuhn, Ross J. Anderson "Soft Tempest: Hidden Data Transmission
Using Electromagnetic Emanations" (in David Aucsmith (Ed.): Information Hiding,
Second International Workshop, IH'98, Portland, Oreg., USA, Apr. 15-17, 1998,
Proceedings, LNCS 1525, Springer-Verlag, ISBN 3-540-65386-4, pp. 126-143),
when the screen content has been low-pass filtered using software only as
described by this invention. FIG. 1, FIG. 3, FIG. 5, and FIG. 8 illustrate
the corresponding situation found with normal video display units if no
protective filtering takes place; this gives a considerably better received
signal as shown in FIG. 8.
To further complicate automated radio frequency character recognition of
displayed text using a digital eavesdropping receiver and pattern matching
techniques, one typical embodiment utilizes a plurality of fonts that differ
slightly in character style, size, and position and it randomly selects for
every character of the displayed text one of these font variations.
In the case of the keyboard scan cycle, we adapt the same idea and spread
the spectrum of the emanations by adding a variation and a random delay into
the scan sequence. Transforming the scan cycle into a non-periodic process
spreads the harmonics of the sample cycle frequency in the spectrum such
that they cannot be extracted easily by periodic averaging. The random repetition
delay between the application of voltages to the rows of the keyboard matrix
is accomplished both by varying the order in which rows are scanned and by
using delay loops to vary slightly the time that passes between the scan
of one row and the next.
The choice of row order and delays depends on the output of a cryptographically
strong random number generator that is periodically reseeded by combining
its old internal state with keyboard input so as to make its output unpredictable
to an eavesdropper. Cryptographic random number generators are described
in Bruce Schneier: Applied Cryptography (John Wiley & Sons Inc, 1996,
ISBN 0-471-11709-9). The emitted spectrum of the keyboard scan microcontroller
and other processors in general can also be spread by slightly frequency
modulating the clock signal of this processor using a random noise source,
which creates an additional difficulty for eavesdropping receivers. Finally,
the scan codes are encrypted for transmission along the keyboard cable to
the computer in order to prevent direct eavesdropping of the serial cable
emanations as described in Peter Smulders: The Threat of Information Theft
by Reception of Electromagnetic Radiation from RS-232 Cables (Computers &
Security vol 9 (1990) 53-58).
In the case of the mass storage device, we could also reduce the readability
of confidential data in the unavoidable periodic signal that the read amplifiers
generate as the device turns, by moving the disk head in a random or pseudorandom
manner when it is not in use. However in this case there is available a simpler
and deterministic remedy which imposes less mechanical wear on the device.
We simply move the read head as soon as possible away from a sensitive track
if no further read requests are pending. In our preferred implementation,
the head is always moved to safe tracks--tracks that contain either no data
at all or non-sensitive data--during disk idle times. The disk driver maintains
a list of safe tracks to which the writing of sensitive data is prevented,
and where there are a number of mechanically coupled heads to access stacked
or otherwise juxtaposed media, there will be allocated a number of sets of
safe tracks corresponding to disk head positions at which the writing of
sensitive data is similarly not permitted.
Whenever the request queue for a device is empty and the last access was
to a sector other than on a safe track, the driver will determine the closest
safe track and either move the read head there directly or issue a read
instruction to one of the sectors in this track depending on the disk interface.
This way, the sensitive data content of the hard disk will only be amplified
for the minimal necessary time and the probability that an eavesdropper can
successfully reconstruct any of it by periodic averaging is significantly
reduced.
* * * * *