Soft Tempest Demonstration Fonts 1.0 ------------------------------------ Copyright © 1998 Markus G. Kuhn 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 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: