# Protecting Data at Risk of Unintentional Electromagnetic Emanation: TEMPEST Profiling
## 3 Sources of Electromagnetic Leakage
### Computer Monitors  
Computer monitors emit electromagnetic information due to three important factors used to display video images: (1) refresh rate, (2) horizontal frequency, and (3) pixel frequency [48]. Information from a computer screen can be reconstructed as it appears by monitoring and demodulating the radiation generated by the computer and its peripherals [1]. The video signals processed by the monitor, both video graphics array (VGA) and digital video interface/high-definition multimedia interface (DVI/HDMI), have strictly defined time frames and maintain the image framing principle [13; 49]. In [50], the authors conducted a study on EM interference unintentionally generated by electronic devices and showed that information leakage is a consequence of displayed video signals, from a TEMPEST perspective. The authors of [51] discussed the relation between the image recovery frequency and the compromising leakage degree. Furthermore, the authors analyzed the unwanted emissions from a display and introduced a secure font used against eavesdropping. The authors also emphasized the effectiveness of shielding against EM leakage, including display housings and Faraday cages. Advanced types of protection for display assemblies are listed in Table 1.  
A computer monitor can be equipped with a construction such as a metal mesh between two layers of glass or transparent foil. This construction directs most of the information towards the ground through the ground wires. Even if eavesdroppers manage to intercept the signals, they will not be able to reconstruct the information. Eavesdroppers often take advantage of the high-frequency spectrum to reproduce the image from the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Protection** & **Description** \\ \hline Transparent glass & Indium tin oxide (ITO) technology is applied to the glass surface. Disperses EMI while preserving display visibility. \\ Copper mesh & Cooper’s conductivity effects block EMI. Monitor and maintain its performance. \\ Silver coated frame & A frame surrounds the EMI glass, serves as a grounding mechanism, and creates a Faraday cage. Unwanted EMI energy is dissipated and interference neutralized. \\ Coatings & Shield coated through painting, spraying, dispensing, and electroplating. Typically used in lightweight applications (mobile devices). \\ Screen housing & The screen housing is based on the Faraday cage principle. Filter modules can be added to protect the cable connection to the screen. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Display shielding.  
monitor. The elimination of high-frequency components is therefore an advantage, as it makes the reproduction of EM signals more difficult [3].  
Several well-known manufacturers offer North Atlantic Treaty Organization (NATO) TEMPEST-protected monitors available in SDIP-27 Level A, B, or C versions. C2 company (Riga/Latvia) provides a wide range of equipment products and services, including computer monitors and large format displays offered in Level A, B, and C versions [52]. SIPAL (Turin/Italy) offers monitors available at TEMPEST Level B [53]. Eurotempset (Neuen/The Netherlands) specialized the production of TEMPEST Level A and B monitors [54]. Siltec (Pruszkov/Poland) is another reputable manufacturer offering Level A and B protected monitors [55]. Cissecure (Ashbrun, VA 20147/USA) is also widely known as a prominent manufacturer of TEMPEST monitors [56]. More detailed descriptions of the TEMPEST monitors and screens are given in Table 2.