LED Arrays of Laser Printers as Sources of Valuable Emissions for Electromagnetic Penetration Process
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Protection of information against electromagnetic eavesdropping is an important issue. Information may be derivable from the shape of an unintended electromagnetic signal. The resulting electromagnetic emanations can be correlated with processing of classified information. The problem extends to computer printers. This article presents a technical analysis of LED arrays used in monochrome computer printers and their contribution to unintentional electromagnetic emanations. We analysed two printers from different manufacturers, designated A and B. The forms of useful signals and their dependence on parameters of printing data are presented. Analyses were based on realistic type sizes and distribution of glyphs. Pictures were reconstructed from received radio frequency (RF) emanations. We observed differences in legibility of information receivable at a distance that we attribute to different ways used by printer designers to control the LED arrays, particularly the difference between relatively high voltage single-ended waveforms and lower-voltage differential signals. To decode the compromising emanations required knowledge of---or guessing---printer operating parameters including resolution, printing speed, and paper size. The optimal RF bandwidth for detecting individual pixels has been determined. Measurements were carried out across differences in construction and control of the LED arrays in tested printers, and the levels of RF emissions compared for selected operating modes (fast, high quality, or toner saving mode) of the printing device.
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