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Soft Tempest Demonstration Fonts 1.0
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Copyright ｩ 1998 Markus G. Kuhn <mkuhn@acm.org>
This is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
The GNU General Public License is available from
<ftp://prep.ai.mit.edu/pub/gnu/COPYING> or from Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307,
USA
This package contains Soft Tempest filtered and anti-aliased versions
of the Courier font. These fonts were produced using the public domain
X11 pixel font -adobe-courier-*-r-normal--40-386-75-75-m-0-iso8859-1.
The two available fixed glyph cell sizes are 13x24 pixels and 8x13
pixels, in both medium (m) and bold (b) weight.
The enclosed files are
cur-13x24b-a.gif cur-13x24m-a.gif cur-8x14b-a.gif cur-8x14m-a.gif
cur-13x24b-b.gif cur-13x24m-b.gif cur-8x14b-b.gif cur-8x14m-b.gif
cur-13x24b-c.gif cur-13x24m-c.gif cur-8x14b-c.gif cur-8x14m-c.gif
cur-13x24b-d.gif cur-13x24m-d.gif cur-8x14b-d.gif cur-8x14m-d.gif
To use the fonts, just cut the provided images into 16x12 blocks of
the respective cell size and supply these 8-bit greyscale bitmaps to
your text-output routine. The provided glyps cover the entire ISO/IEC
8859-1 character set.
Every font is provided in four variations (a/b/c/d), which differ
slightly in their sampling offset and filter parameters. We recommend
that display routines switch randomly between the four variations, in
order to make automatic character recognition in eavesdropping
receivers more challenging. It is also possible to add some amount of
random noise when placing characters on the screen. This random noise
should be added individually for every character in the displayed text
and not commonly for all characters in the glyph table (otherwise the
noise would become characteristic for a character and would just aid
pattern matching algorithms that try to distinguish between
characters). If the characters are displayed in video modes with less
than 256 luminosity levels, then the noise should be added to the
8-bit glyph-pixel values before the least significant bits are
stripped off for the display buffer (dithering).
Some Background Information
Computer monitors (both CRT and LCD) emit the video signal that they
display as electromagnetic waves over the air and via the power supply
lines. Eavesdroppers can pick up this signal at a distance and can
reconstruct the screen content [1]. Some signals can easily be picked
up in neighbor rooms even using cheap modified household equipment.
With professional eavesdropping receivers and antennas, this works
even outside the building in which the computer is located, under good
conditions even hundreds of meters away.
Professional monitoring receivers for compromising radiation use
automatic character recognition algorithms to transcribe received
display signals into text files. These can then be indexed, searched
for relevant information, and archived for further reference in
intelligence gathering databases. Experiments indicate that the
reception and especially the automatic recognition of characters can
be made significantly more difficult with a number of software
countermeasures [2,3]. Greyscale fonts in which high-frequency
vertical edges have been carefully smoothed by low-pass filters emit
significantly less energy and the emitted remaining signal is more
difficult to separate from the background noise. Minor random
variations of the glyph shapes increase the number of patterns that
the matching algorithms in eavesdropping receivers have to cope with.
These variations help to increase significantly the error rate of the
pattern recognition process, thus reducing the value of the gathered
data for automatic processing.
Filtered fonts do not guarantee the same level of protection as fully
shielded Tempest-certified hardware, but they can be implemented at a
significantly lower cost and can often achieve the required level of
protection if they are used together with a reasonable security
policy. Filtered fonts do for instance not provide protection against
malicious software that modifies the screen content at a pixel level
in order to transmit data covertly [2]. Protection against malicious
software requires secure operating systems--preferably with mandatory
access control--and strict adherence to adequate operational
procedures. Since filtered fonts require successful eavesdroppers to
come much closer to the target machine, they reduce the probability of
a successful interception of confidential text considerably. They are
therefore a valuable additional precaution that can be applied easily
to maintain a reasonable level of communication and computer security.
Tempest protection by filtered fonts and related techniques are in the
process of being patented internationally. This demonstration font can
be copied and used freely in products for which the source code is
made freely available (see the GNU General Public License for
details). Contact the author for further information if you want to
use this technology in commercial or military products.
This package is available from
http://www.cl.cam.ac.uk/~mgk25/st-fonts.zip
References:
[1] Wim van Eck. Electromagnetic Radiation from Video Display Units:
An Eavesdropping Risk? Computers & Security, vol 4, no 4,
pp 269--286, December 1985
[2] Markus G. Kuhn and Ross J. Anderson. Soft Tempest: Hidden Data
Transmission Using Electromagnetic Emanations, in Proceedings of
the Second International Workshop on Information Hiding, Portland,
Oregon, USA, April 15-17, 1998, Springer-Verlag, Berlin, Heidelberg,
LNCS 1525, pp 126-143
[3] Markus G. Kuhn, Ross J. Anderson: Low Cost Countermeasures
Against Compromising Electromagnetic Computer Emanations. UK
patent application no 9801745.2, January 28, 1998
--
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org, home page: <http://www.cl.cam.ac.uk/~mgk25/>

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