Method for Evaluating Digital Video Electromagnetic Information Leakage from Video Cable

Chinese Journal of ElectronicsVolume 30, Issue 5 p. 978-985 Original Research PaperFree Access Method for Evaluating Digital Video Electromagnetic Information Leakage from Video Cable WANG Sen, Corresponding Author WANG Sen wangsen3356@126.com AECC Aero Engine Control System Institute, Wuxi, 214100 China Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this authorQIU Yang, QIU Yang Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this authorTIAN Jin, TIAN Jin Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this author WANG Sen, Corresponding Author WANG Sen wangsen3356@126.com AECC Aero Engine Control System Institute, Wuxi, 214100 China Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this authorQIU Yang, QIU Yang Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this authorTIAN Jin, TIAN Jin Electromagnetic Compatibility Laboratory, Xidian University, Xi'an, 710071 ChinaSearch for more papers by this author First published: 01 September 2021 https://doi.org/10.1049/cje.2021.07.009AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Electromagnetic emissions from electrical information equipment may contain useful information and lead to information leakage. In order to detect the electromagnetic information leakage of digital video cable, a rapid testing method for computer video information leakage detection without reconstructing the displayed image is proposed. In this method, the electromagnetic radiation spectrum of typical image transmitted by HDMI cable is simulated. Then the electromagnetic radiation spectrum of the computer when transmitting the same image as the typical image is obtained by conduction and radiation EMC test methods. Finally, the correlation between the spectrum of simulation and the test spectrum is analyzed, and the correlation coefficient is used to judge whether the computer has produced the electromagnetic leakage of video information. The analysis and test results show that when the correlation coefficient is greater than 0.5, the electromagnetic leakage of computer video information can be judged. I. Introduction With the development of technology and the popularization of electronic information devices, electric devices such as computers and various communication equipment have been widely used in both civil and military fields. In advanced information societies, information leakage has a strong impact on social and economic activities, especially military affairs. Information security has drawn much attention from the researchers and is becoming a hot topic in relevant field. One of the main reasons for information leakage is unintentional radiation of electromagnetic fields from computers[1,2]. Electric devices usually radiate electromagnetic wave when they are working, which may cause electromagnetic interference and leads to information security problems. This hidden information contained in radiation can often be reconstructed by intercepting such emissions, even if the emission levels are weak according to the regulated standards. This can be a potential information security threat, especially because information can then be stolen remotely from a distance with no trace[3,4]. In most cases, the net security and the information storage security are emphasized while the information security caused by the non-contacting electromagnetic radiation is often neglected[5,6]. Computer video information contains a large number of useful signals. The video data is transmitted serially in cable. It becomes the strongest emission part and easy to reconstruct video information after intercepting the leakage[7,8]. With the increase of computer screen resolution, radiation frequency range is becoming wider, and radiation characteristic is more obvious. The mainstream video display interface is Video graphic array (VGA). In recent years, the Digital video interface (DVI) and the High-definition multimedia interface (HDMI) video cable are increasingly popular. The research on electromagnetic information leakage is almost about VGA. DVI and HDMI digital video cable is researched rarely[9,10]. The transmission rate of digital video cable is faster than the analog video, and the frequency range of the electromagnetic radiation is higher. It's more suitable for long-distance reception[11,12]. In order to detect whether or not information is contained in electromagnetic disturbance radiated from computer, a direct method is reconstructing the display image by using a special receiver[13,14]. However, reconstruction of the image needs complex information processing technology. The special receivers are uncommon and expensive[15,16]. So the evaluation methods were proposed[17,18]. One evaluation method performs narrowband measurement by a receiving antenna and two spectrum analyzers that are commonly used for Electromagnetic interference (EMI) measurement, and classifies the disturbance on the basis of the frequency component of the horizontal synchronized signal when white and stripe pattern is displayed. Another method performs wideband measurement by using a current probe around the video signal lines, and measures the level of the frequency component of the display image[19,20]. For digital video information, 8 bit binary data is instead of the Red green blue (RGB) value ( 0 - 255 ) . The pixel data are transmitted after encoding. For DVI cables or HDMI cables, there is a complicated mapping between the bit pattern that after encoding with the gray value. This mapping varies with the center frequency of the receiver. The frequency range which contains the complete video information is related to the Fourier transform of the digital waveform. For the complex encodings are used in digital video signals for transmission, the relationship can be more complex[13]. To reconstruct the video information, not only the high sensitivity receiver and the high gain antenna, but also the complex signal processing technology is needed. It can lead to the detection is very complex. In this paper, a rapid testing method to evaluate the video information electromagnetic leakage from digital video cable is proposed. In next section, the source of video signals electromagnetic leakage is analyzed. A simulation method to calculate the spectrum of the digital video signal electromagnetic radiation is proposed. Firstly, the principle of electromagnetic information leakage from digital video cable is analyzed. The appropriate typical image is use to detect whether the video information is leakage. The video data can be obtained when the image displayed on screen. Then based on the transmitting principle of the digital video, the video data after encoding on video cable is presented in time domain. Through a Fast Fourier transform (FFT) algorithm, the radiation spectrum can be calculated. In Section III, the spectrum of radiated disturbance when the images are displayed is measured through near-field and far-field measurement. In Section IV, the correlation between the spectrum of simulation and the test spectrum is analyzed. Through the similarity, it can be determined whether or not the electromagnetic disturbance contains the video information. II. Analysis and Simulation For a desktop computer, there are many radiation sources. Electromagnetic disturbance from a computer can be categorized into two types: disturbance that contains the display image information and disturbance that does not. So the digital video system should be analyzed to find the radiation sources that contain information. 1. Analysis of digital video system Periodic averaging can lift a clearly readable image out of the background noise. Serial Gbit/s transmission formats used by modern digital video interface effectively modulate the signal, thereby making it even better suited for remote reception than emanations from analog system. The computer video displayed on the monitor is transmitted by video cables. The experiment reported that many papers demonstrate some types of flat-panel display do pose a realistic eavesdropping risk[18]. In particular, with some modern video interfaces, it is quite easy to configure the display of text in a way that maximizes the leaking signal strength. Anyway, the video cable used to connect the display turned to be the primary source of the leaking signal. Thus, signal leakage of HDMI digital video system focuses on the video cables. Fig. 1Open in figure viewerPowerPoint HDMI digital video transmission standard The HDMI digital video transmission standard[21] is shown in Fig.1. HDMI contains three Transition-minimized differential signaling (TMDS) transmission channels, as well as a separate TMDS clock channel. In one clock cycle, each TMDS channel transmit one pixel data. The TMDS channel transmits the RGB data of a pixel. For colorful images, the video data transmitted in the three channels on video cables are different. The received electromagnetic radiation signal is a composite signal of three channels. The signal each channel can not be distinguished clearly. It is because that video data are transmitted simultaneously and these parallel channels are in close proximity. So the regularity of superimposed field intensity and spectrum is not obvious on video cable. But for the black and white video images, the video data transmitted by the three channels on video cable is the same, which causes the characteristics of electromagnetic radiation more obvious. Then the video signal can be received easily, and the key part of the information leakage is usually a white background with black text image. Assumed the radiation field intensity of one channel is E → ( f ) £then, the total radiation field intensity of three-way RGB video signal on cable is as follow: E → t o t a l ( f ) = E → R ( f ) + E → G ( f ) + E → B ( f ) (1) For black and white image, the total radiation field intensity of the three channels is higher the one channel core cable. Therefore, the black and white image should be selected to detect the electromagnetic information leakage. The change speed of the image displayed is depended on the refresh rate of the monitor. If the transmission data of the image is periodic, then the electromagnetic signal from video cable is more obvious. The video data is transmitted from left top to right bottom. So the black and white stripe image is the best choice. 2. Simulation of information leakage on video cable Digital video is composed of a series of images by frames, which contain rich information. Assuming that the number of colors to be represented on the display panel is 16 million, the number of RGB pixel colors is 256. Image is represented by a two-dimensional matrix in the computer. f ( 0 , 0 ) f ( 0 , 1 ) ⋯ f ( 0 , M - 1 ) f ( 1 , 0 ) f ( 1 , 1 ) ⋯ f ( 1 , M - 1 ) ⋮ ⋮ ⋱ ⋮ f ( N - 1 , 0 ) f ( N - 1 , 1 ) ⋯ f ( n - 1 , M - 1 ) (2)where f ( x , y ) indicates a pixel value in image, M and N denote the pixel number in horizontal and vertical directions. Pixel data f ( x , y ) is from 0 to 255. Each pixel value of the image will be firstly converted to 8 bit binary data before transmission on HDMI cable. Then all these data will be serialized. After TMDS encoding and FFT transform, the final characteristics of the radiation spectrum of the cable is obtained. The video mode is followed Video electronic standard association (VESA) Display monitor timing (DMT) which is widely used. Then the electromagnetic radiation spectrum from video cable can be simulated and calculated. All these data can be utilized to match the results measured from the video cable when the image is on the screen. The process of the simulation are as following. Step 1: Given the image. The pixels number of the image is depends on the resolution of the display. Here taking 1920 × 1080 as an example, after the function of reading image, the pixel data of the total image can be obtained. For black and white image, the number of the RGB sub pixel is 0 or 255. The 0 represents the black pixel and 255 represents the white pixel. Step 2: The transmission mode of video data on the HDMI cable is serial, so the video data should be serialized firstly. Because it is a black and white picture, so RGB three-way data are consistent. The black pixel value 0 will be converted to 00000000 and the same to the white pixel, that is 255 → 11111111 . Step 3: All the video data will be encoded by the algorithm of TMDS. Each 8-bit character will be converted to 10-bit. The encoded characters provide an approximate DC balance as well as a reduction in the number of transitions in the data stream[11]. The encoding process for the active data period can be viewed in two stages. The first stage produces a transition-minimized 9-bit code word from the 8 bits input. The second stage produces a 10-bit code word. Step 4: Considering the horizontal and vertical blanking intervals, additional data (e.g. mode messages) is added at the end of every line of 1920 visible pixels and 1080 visible lines. The additional data is determined according to the HDMI standard. Step 5: Encoding the binary data using FFT. Here the method of Decimation-in-time Fast Fourier transform (DIT-FFT) is chosen. If the number of binary data to be transformed is N, then the discrete Fourier transform results of these data is given by D ( k ) = D F T [ d ( n ) ] = ∑ n = 0 N - 1 d ( n ) W N k n (3)where k = 0 , 1 , ⋯ , N - 1 , d ( n ) ∈ { 0 , 1 } , W N = e - j 2 π N . To make fast Fourier transform more efficiently, usually taking integral power of 2 points to calculate. When the FFT of a sequence with non-integral power of 2 is processed, sequence of integral power of 2 is obtained by the beginning data of the sequence. After butterfly computation and DIT-FFT, the transform results can be obtained. Then, the frequency-domain characters of the video cable electromagnetic radiation can be obtained when the image is displayed. 3. Simulation results Using MATLAB to simulate and calculate the video image. The resolution of the monitor is set as 1920 × 1080 and the refresh rate as 60Hz. According to Video Electronic Standard Association (VESA) standards[22] and High-Definition Multimedia Interface Specification[19]. Version 1.4, when the display resolution is 1920 × 1080 and frame rate is 60Hz, the pixel clock frequency is 148.5MHz. In order to reduce the amount of calculation and make the results clear, regulation white screen and black and white stripes images are chosen such as Fig.2. Fig. 2Open in figure viewerPowerPoint Black and white stripes. ( a) One black and one white; ( b) 480-black and 480-white According to the TMDS coding, the 10-bit encoded signal may be different because the previous bit balance. In order to match the transmission order, the binary words are shown in the least significant bit first. For one black and one white stripe picture, the encoded binary number is a combination of a, b, c and d, as shown in Fig.3. Fig. 3Open in figure viewerPowerPoint TMDS encoding schematic diagram of one black and one white As illustrated in Fig.3, the TMDS encoding result is a cycle consists of 40 binary numbers which will be transmitted on the HDMI video cable. The transmission speed of each channel on the video cable is 1.485Gbps. According to the knowledge of the discrete Fourier transform, some specific frequency points exist after DFT transform. These points are at the integral multiple of the repetition frequency. The repetition frequency is 1.485GHz/40=37.125MHz. Thus the specific frequency points are the integral multiple of 37.125MHz. These specific frequency points describe the radiation spectrum characters of the HDMI video cable when transmitting one black and one white pixels stripe image, as shown in Fig.4. For 480-black and 480-white stripe picture, the encoded binary number is a combination of a, b, c and d, as shown in Fig.5. As illustrated in Fig.5, the TMDS encoding result is also periodic. When 480 continuous black pixels are transmitted, the TMDS encoding results lead to a cycle of nine 10-bit words. The repetition frequency is 1.485GHz/90=16.5MHz, thus the specific frequency points are the integral multiple of 16.5MHz. When 480 continuous white pixels are transmitted, the TMDS coding consequences lead to a cycle of seven 10-bit words. The repetition frequency is 1.485GHz/70=21.2MHz, thus the specific frequency points are the integral multiple of 21.2MHz. These specific frequency points describe the radiation spectrum characters of the HDMI video cable when transmitting 480-black and 480-white pixels stripe image. The simulation result of the 480-black and 480-white is consistent with the above analysis, as shown in Fig.6. The large level was observed at 16.5MHz, 33MHz, 49.5MHz, 66MHz, 82.5MHz, 99MHz, 115.5MHz, 132MHz, which are the integral multiple of 16.5MHz repetition frequency, and 21.2 MHz, 42.4 MHz, 63.6 MHz, 84.8 MHz, 106 MHz, 127.2 MHz, which are the integral multiple of 21.2MHz repetition frequency. Fig. 4Open in figure viewerPowerPoint Simulation result of one black and one white Fig. 5Open in figure viewerPowerPoint TMDS encoding schematic diagram of 480-black and 480-white Fig. 6Open in figure viewerPowerPoint Simulation result of 480-black and 480-white Comparing simulation results of two images as shown in Fig.4 and Fig.6, it shows that electromagnetic radiation spectrums from video cable are different when transmits different images. However, if the images are regular, the electromagnetic spectrum characters are obvious which shows that one cycle is 148.5MHz. The spectrum distribution from 148.5MHz to 297MHz is the same as the spectrum distribution from 0 to 148.5MHz. The radiation intensity is gradually attenuate. III. Measurement For electromagnetic information leakage acquisition, the result measured by near-field reception is better than the far-field reception. The far-field reception always means greater noise and lower resolution. However, the practical information interception methods is based on far-field reception. The far-field reception continues to be one of the most challenging areas. So the measurement based on near-field is designed to verify the accuracy of the simulation, and the measurement based on far-field is designed to detect whether the electromagnetic disturbance contains the video information. 1. Near-field measurement Conduction measurements is designed to receive the frequency domain information of HDMI cable, which can verify the correctness of the simulation. In this method, the received electromagnetic signals contain less noise signal. Although other signals from other electronic components may couple to the HDMI cable, they are too small compared with the electromagnetic signals on the HDMI cable. The resolution of the display is 1920 × 1080 , and the refresh rate is 60Hz. The electromagnetic disturbance is measured in the anechoic chamber when the images shown in Fig.2 are displayed. The conduction measurement setup is illustrated in Fig.7. The video cable traverses the Radio frequency (RF) Current probe. The Resolution bandwidth (RBW) is 10 kHz. The sweep time of the spectrum analyzer t s w p is given as Ref.[14]: t s w p = ( N p t s - 1 ) · N p r d · t h (4) N ( p t s ) = ( f s p a n / R B W ) + 1 (5)where N p t s is the number of the measurement points, N p r d is the number of the horizontal scans of the display per measurement point, t h is the time of period of horizontal signals. The detected wave pattern is peak detector and max hold. The frequency spectrum measured when the one black and one white striped image are displayed is shown in Fig.8. For one black and one white striped image, by making a comparison of Fig.4 and Fig.8, the large level frequency points of simulation are all present in the test results. Results of conduction measurements clearly show the simulation results. The specific frequency points at 37.125MHz and the integral multiple are observed in the Fig.8. Fig.9 shows the frequency spectra measured when the 480-black and 480-white striped image are displayed. For 480-black and 480-white image, by making a comparison of Fig.6 and Fig.9, the large level frequency points of the simulation results also appear in measurement results. Simulation results are obvious especially in conduction measurement results from 148.5MHz to 445.5MHz. Fig. 7Open in figure viewerPowerPoint Measurement setup of the conduction Fig. 8Open in figure viewerPowerPoint Conduction test result of one black and one white Fig. 9Open in figure viewerPowerPoint Conduction test result of 480-black and 480-white 2. Far-field measurement Radiation measurement is designed to detect whether the electromagnetic information from the computer is leakage or not. As same with Near-field measurement, the resolution of the display is 1920 × 1080 , and refresh rate is 60Hz. The electromagnetic disturbance is measured in the anechoic chamber when the images which showed in Fig.2 are displayed. The measurement setup for evaluation of video information contained in radiated disturbance is illustrated in Fig.10. A log-periodic antenna was located in 1m in front of the HDMI cable, and its height was the same as the height of the center of the computer. The length of the HDMI cable is 3 meters. The antenna is connected to a spectrum analyzer through a preamp. The electromagnetic disturbance measured by vertical polarization in the anechoic chamber. The RBW is 30kHz, and the detected wave pattern is peak detector. Fig. 10Open in figure viewerPowerPoint Measurement setup of the radiation The radiation frequency spectrum measured when the one black and one white striped image are displayed is shown in Fig.11. By making a comparison of Fig.4 and Fig.11, the large level frequency points of simulation are all present in the measurement results. The results of radiation measurements clearly show the simulation results. The specific frequency points at 37.125MHz and the integral multiple are observed in the Fig.11. Fig. 11Open in figure viewerPowerPoint Radiation test result of one black and one white Fig.12 shows the frequency spectra measured when the 480-black and 480-white striped image is displayed. By making a comparison of Fig.6 and Fig.12, the large level frequency points of the simulation results also appear in measurement results. The simulation results are obvious especially in radiation measurement results from 297MHz to 594MHz. 3. Discussion In conduction measurement, some interference signals exist below 100MHz, which may be caused by other electronic components and circuits in the computer. The larger level electromagnetic signals measured are at the frequency points in simulation. Comparing the large level signals at the frequency points that in simulation, these interference signals are weak. The spectrum characters of the simulation are basically same as that obtained by conduction measurement. So this method can verify the correctness of simulation. Fig. 12Open in figure viewerPowerPoint Radiation test result of 480-black and 480-white. In radiation measurement, also some interference signals exist which may be caused by electromagnetic radiation interference of host and display monitor. But the peak points of simulation are all present in the test results. They are same as the distribution of frequency points, which proves the result that the electromagnetic disturbance contains the digital video information. It shows that computer video cable causes the electromagnetic information leakage. IV. Similarity Calculation In order to determine whether or not the video information is leakage from electromagnetic radiation, the spectrum similarity between the simulation and the measurement can evaluate it effectively. If the spectrum similarity between the simulation and the measurement is better, then the possibility of electromagnetic information leakage is higher. In mathematical statistics, cross-correlation coefficient is used to represent the correlation of two sequences. The electromagnetic radiation spectrum of simulation result is X, and the electromagnetic radiation spectrum of measurement result is Y. Cross-correlation coefficient of X Y can be expressed as follows: r X Y = C X Y C X C Y (6) C X Y = 1 N - 1 ∑ i = 1 N [ ( x i - x ¯ ) ( y i - y ¯ ) ] (7) C X = 1 N - 1 ∑ i = 1 N ( x i - x ¯ ) 2 (8) C Y = 1 N - 1 ∑ i = 1 N ( y i - y ¯ ) 2 (9)where x i is the sequence of the electromagnetic radiation spectrum of simulation X, y i is the sequence of the electromagnetic radiation spectrum of measurement Y, x ¯ is the average of sequence x i , y ¯ is the average of sequence y i , and N is the number of sequences. The value of cross-correlation coefficient r X Y is from –1 to 1. The positive and negative sign of the correlation coefficient only indicates the direction of correlation, and the absolute value indicates the degree of correlation. The degree of correlation is expressed as | r X Y | . The absolute value of the correlation coefficient is close to 1, the degree of correlation is high. The absolute value of the correlation coefficient is close to 0, then the two sequences are almost uncorrelated. The frequency points in simulation and measurement should be same, so interpolation method is used to ensure the number of sequences and the frequency points are same. After calculation, the cross-correlation coefficient of spectrum is 0.6922 between simulation and conduction measurement from 0 to 500MHz when one black and one white striped image is displayed, and the cross-correlation coefficient of spectrum is 0.5863 between simulation and radiation measurement from 200MHz to 600MHz. The cross-correlation coefficient of spectrum is 0.7358 between simulation and conduction measurement from 0 to 500MHz when 480 black and 480 white is displayed, and the cross-correlation coefficient of spectrum is 0.5384 between simulation and radiation measurement from 200MHz to 600MHz. The number of the sequence can affect the result of the calculation, but the trend of cross-correlation will not be changed. Here, the cross-correlation coefficient is chosen as 0.5. If the cross-correlation coefficient of spectrum between simulation and measurement is higher than 0.5, then the video information is leakage through electromagnetic radiation, otherwise not. The electromagnetic radiation measured from another computer with countermeasure when the one black and one white striped image displayed is shown in Fig.13. The cross-correlation coefficient between the simulation and measurement is 0.2462. Then it is determined that the video information is not leakage thorough electromagnetic radiation. In order to verify the feasibility of this method, a large number of tests have been carried out in specialized laboratories. In these tests, the probability of cross-correlation coefficient more than 0.5 exceeds 90% while the video image is reconstructed. The probability of cross-correlation coefficient less than 0.5 exceeds 95% while the video image can not be captured. This method is not only suitable for black and white stripes images, but also for other gray or black and white images. The more regular the image, the more accurate the calculation, and this method can be more effective. Fig. 13Open in figure viewerPowerPoint Wavelet denoising result of one black and one white radiation test V. Conclusions In this paper, a rapid method to evaluate the video information electromagnetic leakage without reconstructing image is proposed. Based on the digital video transmission principle, for the regular image, the radiation spectrum from the digital video cable is simulated. Then, the spectrum of radiated disturbance is measured by using only one spectrum analyzer that is commonly used for EMI measurement. Final, the spectrum between simulation and measurement is analyzed by calculating the cross-correlation coefficient. According to the cross-correlation coefficient, it can be easily determined whether the electromagnetic disturbance contains the video information and evaluate the electromagnetic information leakage. Biographies WANG Sen (corresponding author) was born in 1989. He received the B.E. degree in electronic science and technology from Yanshan University and the Ph.D. degree in Mechanical and Electronic Engineering from Xidian University. He is now working in AECC Aero Engine Control System Institute. His research interests include electromagnetic compatibility and information safety. QIU Yang was born in 1957. He received the B.S, M.S, and Ph.D. degrees in electromechanical engineering from Xidian University, Xi'an, China. In 1985, he joined the School of Electromechanical Engineering, Xidian University, as a Associate Professor. He is currently a Professor of the School of Electromechanical Engineering, Xidian University. His main research interests include electromagnetic compatibility, information security and high power electromagnetic pulse. TIAN Jin was born in 1972. She received the B.S and M.S. degrees in electromechanical engineering from Xidian University, Xi'an, China, in 1994 and 1997, respectively, and the Ph.D. degree in electromagnetic field and microwave from Xidian University in 2012. She is currently a Associate Professor of the School of Electromechanical Engineering, Xidian University. Her main research interests include electromagnetic compatibility, comprehensive evaluation for communication system. References 1Y-I. Hayashi, N. Homma, T. Watanabe, et al., “Introduction to the special section on electromagnetic information security”, IEEE Transactions on Electromagnetic Compatibility, Vol. 57, No. 3, pp. 539– 546, 2013. CrossrefGoogle Scholar 2D. G. Sun, J. Shi, D. 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