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LED printers and safe fonts as effective protection against the formation of unwanted emission

Authors:
  • Military Communication Institute - State Research Institute

Abstract and Figures

Due to the widespread use of computer equipment, electromagnetic protection of processed data is still an issue. Structurally modified commercial equipment is used to protect devices against this phenomenon. The acquisition costs of such modified devices are enormous. However, the market offers information devices with very low susceptibility to electromagnetic infiltration. Safe fonts are a new solution in the protection of sensitive information against electromagnetic infiltration processes. The use of safe fonts not only increases resistance to electromagnetic eavesdropping but also makes it impossible. These devices are computer printers that use a slat with hundreds of LEDs arranged in several rows during the process of photoconductor exposure. The solution in the form of safe fonts is a universal method that protects process information against electromagnetic penetration. Safe fonts are effective not only for printers with slat LED. The solution can also be used for the protection of analog standard VGA, digital standard DVI, and printers with one diode and two diode laser systems.
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Turk J Elec Eng & Comp Sci
()
c
T¨
UB˙
ITAK
doi:10.3906/elk-1701-128
Turkish Journal of Electrical Engineering & Computer Sciences
http://journals.tubitak.gov.tr/elektrik/
Researc h Article
LED printers and safe fonts as effective protection against the formation of
unwanted emission
Ireneusz KUBIAK
Military Communication Institute, Zegrze, Poland
Received: 13.01.2017 Accepted/Published Online: 18.05.2017 Final Version: ..201
Abstract: Due to the widespread use of computer equipment, electromagnetic protection of processed data is still
an issue. Structurally modified commercial equipment is used to protect devices against this phenomenon. The
acquisition costs of such modified devices are enormous. However, the market offers information devices with very
low susceptibility to electromagnetic infiltration. Safe fonts are a new solution in the protection of sensitive information
against electromagnetic infiltration processes. The use of safe fonts not only increases resistance to electromagnetic
eavesdropping but also makes it impossible. These devices are computer printers that use a slat with hundreds of LEDs
arranged in several rows during the process of photoconductor exposure. The solution in the form of safe fonts is a
universal method that protects process information against electromagnetic penetration. Safe fonts are effective not only
for printers with slat LED. The solution can also be used for the protection of analog standard VGA, digital standard
DVI, and printers with one diode and two diode laser systems.
Key words: Slat light-emitting diode, printer, unwanted emission, electromagnetic infiltration
1. Introduction
Computer printers of various types are an essential part of computer sets and take part in almost every step of
information processing. Many believe that the first reading of created documents must be based on their paper
versions. Therefore, most often, every document, even an electronically distributed one, is printed. However, is
this element of the entire process of information processing safe [1]?
We often try to adequately protect files stored on a drive by protecting them against the possibility of
opening. If there is no such need, we do not connect the computer to the Internet network [2]. State and private
institutions pay increasing attention to the need for special computing solutions for the processing of especially
important information [3]. We begin to realize, due to the appearance of so much information, the dangers
related to revealing emission and the possibility of using it in noninvasive information acquiring [4,5]. This
particularly refers to sources of video signals in the form of computer graphic tracks such as the graphic card,
video cable [6], and monitor [7,8]. It is extremely disturbing that information can be so easily recreated and
presented in a form that is understandable to humans. Earlier emerging information about the electromagnetic
protection of LCD monitors or the DVI standard proved to be untrue [8,9].
Another no less dangerous source of electromagnetic emission, formidable from the point of view of
the possibility of conducting electromagnetic “watch”, is the computer printer [10–12]. It should be noted
that studies on various technical and software solutions that would allow for adequate protection of processed
Correspondence: i.kubiak@wil.waw.pl
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information without undue financial outlays have still not been carried out. It should also be noted that the
cost of acquiring a single computer set, referred to as belonging to the TEMPEST class, is an expense of several
thousands of dollars. This is why “software solutions”, based on the use of “safe fonts”, are mentioned more
and more often. As shown by the results of conducted studies, classified information processed with their use
becomes safe for sources in both the form of video track standard VGA and DVI and the video track of laser
printers (Figure 1).
Figure 1. Photoconductor exposure in laser printers in a one diode laser system.
Nevertheless, the simultaneous use of additional solutions, enhancing the protection of electromagnetically
processed information without incurring additional costs, is not redundant. This only increases the reliability
of the system in protecting our information. The said solution is realized by commercial computer printers,
commonly available on the market, with photoconductor exposure technology based on a slat of LEDs (Figure
2) controlled by a parallel-serial signal [13].
Figure 2. Photoconductor exposure in printers with slat LEDs.
Due to such a solution, the registered revealing emission signal does not have as clearly distinctive features
as a signal that comes from a single laser diode controlled by serial signal or the VGA or DVI video standard
[14].
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2. Sources of revealing emission of computer printers
The most commonly used office printers are laser printers. We can distinguish two basic types of such printers,
classified according to the photoconductor exposure technology used. The first type is a printer based on the
use of a dual laser diode that is controlled by a serial signal (Figure 3). Here, the source of revealing emission is
simple and may be easily subject to electromagnetic infiltration. In such a case, a single line of text is exposed
point by point until its end is reached. The process is then repeated for subsequent lines [15].
Figure 3. Example of a photoconductor exposure system used in standard laser printers (HP LaserJet P2035).
The other type of printer is based on a solution using a slat of LEDs (Figure 4), which is controlled via
a parallel-serial signal (partial analogy to line printers). The LEDs are arranged in a few lines with several
hundreds of diodes per line. As it may be seen, the source of emission is more complex; therefore, the recreation
of information requires more attention.
Figure 4. Example of photoconductor exposure system in the form of slat LEDs (OKI B401a).
The slat is controlled in a parallel manner; however, the signal supplied to the array is a serial signal.
This means that from the point of view of electromagnetic infiltration such a solution may be safer than the
conventional single or dual laser diode exposure systems. Moreover, it results in a lower failure frequency from
the device and, due to the smaller number of mechanical components, it is more compact.
Additional use of safe fonts (symmetrical safe, asymmetrical safe, or simple safe [16,17]) in text infor-
mation processing, with an electric form devoid of distinctive features as compared to typical computer fonts,
reduces the degree of susceptibility to electromagnetic eavesdropping of printers.
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3. Results of revealing emission testing
3.1. Testing conditions
Due to the photoconductor exposure technology used, tests for the viability of electromagnetic eavesdropping
were conducted on two types of printers: a dual laser diode and a slat (array) of LEDs. The printed document
contained a text written in the Arial font (Figure 5). In order to establish the suitability of safe fonts in
the protection of processed electromagnetic information, research was also conducted for a text written in the
symmetrical safe, asymmetrical safe, and simple safe fonts, which are similar to the Arial font.
Figure 5. Fragment of a text printed during the testing of electromagnetic infiltration susceptibility of computer printers.
The measurement of revealing emission, in the system shown in Figure 6, was made over a distance
of 1 m from the source, in accordance with MIL-STD-461F “Requirements for the control of electromagnetic
interference. Characteristics of subsystems and equipment”, in an anechoic chamber. In this way, the source of
emission was separated from additional unwanted sources of electromagnetic disturbances [18]. The computer
used was in the TEMPEST class. The controlled measuring bands were 5 MHz, 10 MHz, 20 MHz, and 50 MHz,
and the signal sampling frequencies were 15 Ms/s, 62.5 MHz, and 125 Ms/s. This allowed for the observation
of the impact of technological solutions of the printers and the fonts used in the form of recreated images.
At the same time, it also allowed for classification of the suitability of design and software solutions in the
electromagnetic protection of printing devices.
Figure 6. Actual measuring system in an anechoic chamber.
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Moreover, a series of tests related to printout quality was conducted for the printer with an LED array.
The tested printer produced a printout of the following quality:
ProQ 1200 dpi ×120 dpi
High quality 1200 dpi ×600 dpi
Normal 600 dpi ×600 dpi
Draft 300 dpi ×300 dpi
Each of the options may be additionally supported by the possibility to select a toner save option.
According to the data obtained from the manufacturer, the printing quality, e.g., 300 dpi ×300 dpi,
means that there are 300 LEDs for each 2.54 cm of a line’s length. For text with a width of 160 mm (between
210 mm (the width of the A4 paper format) and 50 mm (total left and right margin)), this results in about 1890
diodes. However, for a quality of 1200 dpi ×1200 dpi, the number of diodes is 7560. Therefore, does printing
quality impact the change of electromagnetic infiltration capability? Does it directly translate into the quality
of recreated images? The obtained data are presented later in the article.
3.2. Results of analyses
In typical laser printers (single laser diode), the laser is controlled by a serial signal. An important element,
in terms of commercial device susceptibility to infiltration, is the form of the signal that controls the operation
of the laser. The printout (photoconductor exposure) of a single horizontal line is related to the control signal
with a pulse structure. The number of pulses is related to the number of exposed points, which also depends
on printing resolution. In this case, we are not dealing with a single pulse of duration corresponding to the
length of the exposed line (analogous to the analogue video standard VGA [19]). As a result of photoconductor
exposure, graphic elements included in the recreated images are filled with contours in a color that is different
from the background (Figure 7).
Figure 7. Fragment of a recreated image for revealing emission source in the form of a typical laser printer (image
inversion, original character size of 26 points); reception frequency fo= 145 MHz, BW = 10 MHz; sampling frequency
fs= 62.5 Ms/s.
This impacts the high level of the efficiency of electromagnetic infiltration and can be compared to the
situation that occurs when trying to eavesdrop on electromagnetic digital standard DVI (Figure 8) [1]. The
essence of this research, however, is studying the possibility of infiltration of computer printers, based on the
solution of exposing a photoconductor to a slat of LEDs. Due to the form of the recreated images, the tested
printer has four lines of LEDs. It is demonstrated by the fact that the obtained images contain four identical
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horizontal sets of graphic characters of the printed document, a fragment of which is shown in Figure 5. In
addition, the structure of the graphic characters contained in the recreated images is very different than in the
case of a typical laser printer.
Figure 8. Fragment of a recreated image for revealing emission source in the form of digital video standard DVI (image
inversion, original character size of 36 points); reception frequency fo= 365 MHz; BW = 50 MHz; sampling frequency
fs= 125 Ms/s.
The dissimilarity prevents data reading. This is caused by the LED slat operating method, which is
not controlled by a typical series signal. This detailed solution is not made available by the manufacturer.
One should suppose, however, that it is a parallel-serial signal, and the nature of the waveform is similar to
the analog video standard VGA. Consequently, a horizontal line of a predetermined length is exposed to a
continuous signal (stimulation of the corresponding number of LEDs in the slat) with constant voltage, unlike
a typical laser printer. The recreated image shows only the vertical and diagonal edges of the contained graphic
signs (Figure 9).
Figure 9. Fragment of a recreated image for revealing emission source in the form of a laser printer with an LED slat
(image inversion, original character size of 26 points, capital letters); reception frequency fo= 145 MHz, BW = 20 MHz;
sampling frequency fs= 250 Ms/s.
The appropriate recalibration of a fragment of an image (Figure 9) allows us to obtain more legible
characters (Figure 10). It should be emphasized, however, that the size of the printed characters was 26 points.
When printing the characters at a size of 14 points, similar to the commonly used 12 points, the obtained degree
of readability is not as high (Figure 11). The strings merge and form unreadable clusters of pixels of a different
color, which cannot be directly identified with the printed characters.
Many earlier publications discussed issues related to safe fonts as software support for the protection
of information against electromagnetic penetration. Their usefulness has been demonstrated in a wide range
of applications, ranging from sources in the form of graphic tracks of the analog standard VGA to the digital
DVI standard, and ending with sources in the form of conventional laser printers (one- and two-diode). Safe
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Figure 10. Calibrated fragment of the image presented in Figure 9.
Figure 11. Calibrated fragment of an image containing strings corresponding to the original size of 14 points (capital
letters, image inversion); reception frequency fo= 145 MHz, BW = 10 MHz; sampling frequency fs= 125 Ms/s
fonts have also become resistant to the operation of optical character recognition software, without taking the
learning stage into consideration.
When analyzing the commercial solution of photoconductor exposure technology, based on a slat of
LEDs, appropriate tests were also conducted for printing safe font characters. The strings were similar to
the characters presented in Figure 3. Due to the structure of safe font characters that have been designed
with the consideration of differential characteristics of the information penetration channel and drum exposure
technology, revealing emission signals are devoid of distinctive features that could help decide about correlating
them with the original information. Therefore, graphic elements that would indicate the occurrence of a given
character (Figures 12–14) cannot be identified in the obtained images.
It should be noticed that the property already occurs in the case when the signal–noise relation is
determined by the following relation:
SN R = 10 log10
Ps
Psz
,(1)
where PSis the signal power and Psz is the noise power, reaching values much higher than 0. This means
that the only information that is carried by the revealing emission is the fact that the printing device is in
operation. For printers based on a single laser diode and the use of safe fonts, the lack of effectiveness of the
electromagnetic infiltration process may be referred to when the SNR value is less than 0 [20,21].
4. Conclusion
As shown by many events everywhere in the world, the protection of information is currently a huge challenge.
This is especially true in cases where there is a possibility of noninvasive, i.e. unnoticed by the owner of the
protected data, acquisition of information. This is possible due to the occurrence of electromagnetic emissions
having characteristics of electronically processed information [22].
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Figure 12. Fragment of a recreated image and its enlarged part for revealing emission source in the form of a laser
printer with an LED slat for the printout of “symmetrical safe” font letters (image inversion, original character size of
26 points); reception frequency fo= 145 MHz, BW = 5 MHz; sampling frequency fs= 62.5 Ms/s: a) small letters, b)
big letters.
The costs of limiting the possibilities of the occurrence of revealing emission are extremely high. They
arise from the need to introduce structural changes to commercial devices, which are the basis for the special
solutions commonly known as TEMPEST solutions. Therefore, researchers are seeking new solutions that
will reduce the cost of information protection without reducing its capability level. Thus, all new technological
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Figure 13. Fragment of a recreated image and its enlarged part for revealing emission source in the form of a laser
printer with an LED slat for the printout of “asymmetrical safe” font letters (image inversion, original character size of
26 points); reception frequency fo= 145 MHz; BW = 5 MHz, sampling frequency fs= 62.5 Ms/s: a) small letters, b)
big letters.
devices appearing on the commercial market are now being analyzed. Some of them may prove useful throughout
the cycle of projects determining the level of protected information.
This article presents the results of research on revealing emission, analyses of images recreated based on
the emission, and the probability of electromagnetic eavesdropping of computer printers. The study involved two
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Figure 14. Fragment of a recreated image and its enlarged part for revealing emission source in the form of a laser
printer with an LED slat for the printout of “simple safe” font letters (image inversion, original character size of 26
points); reception frequency fo= 145 MHz; BW = 5 MHz, sampling frequency fs= 62.5 Ms/s.
types of printers: a printer using single diode laser technology for photoconductor exposing and a printer based
on LED slat technology. The text printed during the research and registration of unwanted signals contained
characters written in the commercial font Arial. Safe fonts (symmetrical safe, asymmetrical safe, and simple
safe) were also tested.
The research results show that photoconductor exposure technology, based on a slat of LEDs, is a solution
worthy of attention in terms of its capability to reduce the distinctive features of revealing emissions. Solutions
using slat LED technology and safe fonts provide a high level of protection against noninvasive data acquisition.
The use of only printers with LED slats makes the readability of data written with traditional fonts, as compared
to typical printers (single laser diode), much more difficult (Table).
Table. Comparison of susceptibility to infiltration of computer printers, depending on the font used in the processed
text document.
Font type Printer using a one diode Printer based on slat
laser system LED technology
Arial Susceptible Little resistance
Symmetrical safe Resistant Resistant
Asymmetrical safe Resistant Resistant
Simple safe Resistant Resistant
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The additional use of safe fonts prevents any perception (extracting elements from the background) of
text data. Printers with a single laser diode, for traditional fonts, are not a solution that can be considered
completely safe in terms of electromagnetism.
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... In this case, an increase of levels of measured emission by approximately several dB could enable the possibility of non-invasive acquisition of information (Figure 14) [20,21]. ...
... In this case, an increase of levels of measured emission by approximately several dB could enable the possibility of non-invasive acquisition of information ( Figure 14) [20,21]. ...
... In this case, an increase of levels of measured emission by approximately several dB could enable the possibility of non-invasive acquisition of information ( Figure 14) [20,21]. Examples of reconstructed images using recorded sensitive emissions on frequency 753 MHz for two different laptops of C type are shown in Figure 15. ...
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The protection of information against electromagnetic penetration is one of the most important aspects related to the protection of information against its non-invasive acquisition. Compared to the activities of cybercriminals, the use of electromagnetic emissions in the electromagnetic infiltration process does not leave any traces of activity, and the owner of the information is not aware of its loss. The most common activities of electromagnetic eavesdropping are related to the infiltration of emission sources, graphically revealing the processing of information using both an-alog and digital methods. This allows for the presentation of reconstructed data in the form of images. Correct display of the acquired information requires knowledge of raster parameters such as line length and the number of lines building the reconstructed image. Due to the lack of direct access to the intercepted device, knowledge in this field does not allow for the correct determination of the aforementioned parameters, and thus, for recreating an image that would contain legible and understandable data. Additionally, incorrect values of the parameters result in failure of further processing of the obtained image, e.g., by using a coherent summation of images. Therefore, it is necessary to propose a solution that will allow not so much to roughly define the raster parameters but to estimate them precisely. Moreover, it should enable the automation of the process after the implementation of an appropriate algorithm. The article proposes an algorithm for estimating the line length of the reconstructed image. The raster parameter estimated with the use of the algorithm allows for summarizing images several dozen times with a significant improvement in the image quality and readability of the data contained in it. The image summation algorithm is very often used as one of the main image processing methods in the electromagnetic infiltration process. Incorrect raster parameters often make coherent summation useless. The proposed algorithm for estimating the line length of the reconstructed image uses three methods of determining the line length of the image for a given accuracy. At the same time, criteria were indicated that must be met to determine the correct length of the image line for the assumed accuracy of estimation. Obtained results confirmed that the proposed methods and criteria are effective in the process of electromagnetic infiltration. These methods allow us to determine the line length of reconstructed images with accuracy up to 10 .
... In the same research centre, Ireneusz Kubiak investigated the influence of display brightness on the image recovery capabilities from CE electromagnetic radiation of Video Graphics Array (VGA) and DVI interfaces [15]. He also proposed an innovative method to counteract these security vulnerabilities by using security fonts [16] for personal computers (PCs) that can be successfully applied even to printing devices [17]. The effectiveness of the security fonts developed by Kubiak (symmetric and asymmetric fonts) and which have obtained Polish Office Pattern protection in the form of industrial design no. ...
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... It is known that the reception and analysis of the secondary electromagnetic emissions created during electronic data processing equipment operation allow the information processed by this equipment to be reconstructed [1][2][3][4]. In these cases, the aforementioned emissions directly constitute the information carrier [5][6][7]. ...
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... In this paper, analysis of useful signals [8] in the operation of LED arrays used inside printers ( Figure 2) shows that such a design was used by printer B in its photoconductor exposure system. Is this sufficient, however, to foil non-invasive information gathering? ...
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... The electromagnetic emissions with distinctive features can arise at any stage of processing of information (e.g. transmission, displaying on screen, printing) which occur in the electric form [1,[4][5][6][7][8]. Video signals are particularly dangerous (Figure 1) [9,10]. ...
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Nowadays the risk associated with the possibility of non-invasive obtaining information through the use of electromagnetic emissions is very real. In order to reduce the effect of the phenomenon are used the technical solutions which have however an effect on the appearance of modified devices and their purchase costs. All the time engineers look for new and cheaper solutions. Frequently, it is suggested that such solutions are ready and available on the market. An example would be a printer using a technology based on a slat of LEDs
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