United States Patent: 6,721,423

[R. A. Hettinga]

<http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=6721423.WKU.&OS=PN/6721423&RS=PN/6721423>





 
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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.


 Other References


van Eck, "Electromagnetic Radiation for Video Display Units: An
Eavesdropping Risk?" Computers and Technology 4 (1985) 269-286.

 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.

* * * * *


-- 
-----------------
R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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