Study and validation of eavesdropping scenarios over a visible light communication channel

sensors Article Study and Validation of Eavesdropping Scenarios over a Visible Light Communication Channel Ignacio Marin-Garcia 1,2,*,† ID , Victor Guerra 2,† ID and Rafael Perez-Jimenez 2,† 1Facultad de Ingenieria en Electricidad y Computacion, Escuela Superior Politecnica del Litoral (ESPOL), P.O. Box 09-01-5863 Guayaquil, Ecuador 2 Instituto para el Desarrollo de las Telecomunicaciones (IDeTIC), Universidad de las Palmas de Gran Canaria, 35001 Las Palmas de Gran Canaria, Spain; [email protected] (V.G.); [email protected] (R.P.-J.) *Correspondence: [email protected]; Tel.: +34-685-616423 † These authors contributed equally to this work. Received: 3 October 2017; Accepted: 17 November 2017; Published: 21 November 2017 Abstract: The security and privacy provided by Visible Light Communication (VLC) technologies is an area that has been slightly addressed due to the misconception that, since light does not go through solid objects like walls, VLC-based communications cannot be eavesdropped on by outside observers. As an upcoming technology, VLC is expected to be used in multiple environments were, due to radio frequency RF overuse or limitations, RF solutions cannot or should not be employed. In this work, we study the eavesdropping characteristics of a VLC-based communication. To evaluate these concerns, a two-step process was followed. First, several simulations of a standardly used scenario were run. Later on, experimental tests were performed. Following those tests, the results of the simulations and the experimental tests were analyzed. The results of these simulations and tests seemed to indicate that VLC channels can be eavesdropped on without considerable difficulties. Furthermore, the results showed that sniffing attacks could be performed from areas outside the expected coverage of the VLC infrastructure. Finally, the use of the simulation such as the one implemented in this work to recognize places from which sniffing is possible helps determine the risk for eavesdropping that our VLC-based network has. Keywords: visible light communication; network security; data sniffing; eavesdropping; information assurance; communication 1. Introduction Visible light communication (VLC) is one of the newest technologies that has been developed for short and middle range data communication. The implementation of VLC is based on the use of the communication band between 380 nm and 780 nm, which is unlicensed. This technology potentially increases the available bandwidth for wireless communication systems, which currently is quite congested at Radio Frequency (RF) bands due to its massive use [1]. The use of VLC as a commercial solution is predicted to have a considerable market penetration in the near future due to the advantages the technology brings. VLC provides a flexible communication channel that, according to the standard [ 2 ], allows data transfer rates ranging from 11.67 kb/s to 96 Mb/s. This flexibility is based on the use of three operating modes (PHY I, II and III) of which two (PHY-I and PHY-II) are mandatory to implement. This standard also defines two types of modulation: On–Off Keying (OOK) and Variable Pulse-Position Modulation (VPPM) for the mandated operating modes. VLC uses a different kind of modulation, Color-Shift Keying (CSK), to achieve higher transfer rates for the third operating mode (PHY-III). Among the multiple benefits that VLC is supposed to bring into the market is the inherited security of the technology. This assumed security is based on the premise that (indoor) light data streams cannot Sensors 2017,17, 2687; doi:10.3390/s17112687 www.mdpi.com/journal/sensors Sensors 2017,17, 2687 2 of 18 be captured from outside users. Moreover, if a secure channel is required, the Advanced Encryption Standard (AES) with a modified Counter with cipher block chaining message authentication code (CCM*) optional Cryptosystem (AES-CCM*) is specified in the standard. However, no real proof of security has been provided, and no sniffing requirements have been determined. Reasonable concerns about security have been stated, and some general research into VLC security has been conducted. For example, Mostafa and Lutz in their paper [ 3 ] studied the use of null-steering and artificial noise strategies to achieve positive secrecy rates against eavesdropping attacks. In another paper [ 4 ], the same authors considered using friendly jamming to secure data transmissions through VLC. Additionally, in [ 5 ], Chow, et al. proposed using several LEDs, called intrusion-LEDs, to transmit an interference signal to create a secure area inside which VLC communication was eavesdropping-proof. In [ 6 ], Grzegorz Blinowski studied the risk of snooping, jamming and modifying VLC based communications. In the paper [ 7 ], Classem, et al. considered the theoretical eavesdropping possibility of VLC based communications through keyholes and door gaps. In [ 8 ], a lab test of VLC sniffing using readily available components was performed with positive results. Finally, in a different paper [ 9 ], Prasad, et al. compared Ultra-Wide-Band (UWB) and VLC for Data-Intensive and Security-Sensitive Applications. All the presented works begin to consider the feasibility of VLC for secure data communications and try to determine some security and secrecy boundaries for VLC transmissions. This concern is based on the expected large amounts of data that, in the near future, will be transmitted through VLC networks [ 10 , 11 ]. In addition to this, the information transmitted through VLC, like the one used in geolocation techniques [ 12 – 16 ], could be exploited for criminal activities, since the transmitted data could be of great interest for potential attackers. For the stated reasons, further understanding of the security limitations of VLC and its exploitation should be studied and understood for user protection. To be able to sniff two critical parameters must be accomplished. The power received by the eavesdropper must be enough to correctly “Read” the signal, and the bandwidth of the signal must be big enough so the signal can be discriminated. In this work, we proposed the study of eavesdropping on a VLC link. To accurately assess the possibilities of such an attack, simulations and practical experiments were performed. In the simulations, the evaluated channel model was used to determine the amount of received power as well as the signal quality. The test was run using an emitter and receiver over 25 m apart and a telescope to increase the gain of the receiver. This paper is organized as follows: Section 2describes the model used for the research. Section 3 presents the results from the simulation (Section 5.1) and from the experiments (Section 3.2). Section 4 puts into context the results of the previous section and examines the implication those results have for VLC-based systems security. Section 5describes the materials and methods used in both the simulation and experiments. Finally, some conclusions reached in this work are presented. 2. Working Model Indoor VLC links comprise Line-Of-Sight (LOS) and Non-Line-Of-Sight (NLOS) components. LOS components are modeled as the amount of energy radiated by the emitter that directly impacts the receiver. Mathematically, it is calculated as the solid-angle integral of the emitter’s radiation pattern. However, for small photoreceiver area-range relations, the solid angle differential can be approximated by dΩ≈Apd/d2, yielding as shown in Equation (1): Prx ≈Ptx m+1 2πcosm(θ)Glens Ae f f d2, (1) where Ptx is the overall optical emitted power, m is the directivity coefficient of the emitter (assumed a Lambertian one), θ is the elevation angle, G(ψ) is the lens gain which depends on the relative arrival angle ψ,Ae f f is the effective area of the receiver, and dis the link’s range. Sensors 2017,17, 2687 3 of 18 When considering indoor-to-outdoor (or vice versa) links, light travels through windows to reach the receiver. Depending on the type of window, the number of optical interfaces that are crossed vary. However, the simplest scenario implies a single-crystal window and, hence, two interfaces. When traversing an optical interface, light refracts following Snell’s law and an attenuation governed by Fresnel’s equation must be considered. This additional attenuation term can be included in Equation (1), yielding the following expression, Equation (2), in which Lw corresponds to the aforementioned two-interfaces Fresnel loss term and nwis the crystal’s refractive index: Prx ≈Ptx m+1 2πcosm(θ)Lw(θ,nw)Glens Ae f f d2. (2) If bandwidth estimation was needed, the impulse response could be approximated by the LOS component plus the first-bounce NLOS component. This final element depends on the receiver’s Field of View (FOV), the scenario’s geometry and the walls’ reflection patterns. For instance, in a scenario like the one shown in Figure 1, the eavesdropper’s bandwidth performance would be much higher than the indoor-located incumbents’ since the FOV limits the incoming contributions to a very narrow region centered on the emitters. Figure 1. Eavesdropping scenario. The emitter is located on a second floor room and the eavesdropper is located outside. The existence of an interference source, a street lamp, is shown in the figure. Generally, this situation will be kept in most scenarios, and it could be stated that an eavesdropper in an LOS case will be able to receive a signal with lesser inter-symbol interference (ISI). Nevertheless, there would be considerable received-power constraints and the necessity of high-gain optics, as well as the possibility of suffering sun-based interferences (Figure 2). In this type of situation, the Signal-to-Noise Ratio (SNR) could be approximated by using Equation (3): SNR ≈(PrxR(λ))2 2q[id+(Prx +Psunv)R(λ)]B+4kBTB Fn RL , (3) Sensors 2017,17, 2687 4 of 18 where R(λ) is the receiver’s responsivity, q is the electron’s charge, id is the photodiode’s darkness current, Psun is the average sun’s irradiance, v is the scenario’s albedo, kB is Boltzmann’s constant, T is the receiver’s temperature, B is the noise bandwidth, and Fn and RL are the amplifier’s noise figure and gain, respectively. Note that the sun generates both shot noise and offset level. This offset could be high enough to saturate the photoreceiver. Therefore, the receiver’s electrical topology must ensure a proper dynamic range. (a) Indoor scenario. (b) Outdoor scenario. Figure 2. Indoor and outdoor examples of noise sources. In ( a ), two interference sources are present: non Visible Light Communication (VLC) light fixtures and the light coming through a window; in ( b ), two interference sources are present: the non-VLC street light and the sun. 3. Results To validate the premise that VLC-based communication can be eavesdropped on from outside the premises as long as there is an “opening”, such as a window, two tests were conducted, a simulated one (Section 3.1) and an experimental one (Section 3.2). 3.1. Simulation Results For the environment simulation. a basic scenario proposed in the literature [ 17 – 19 ] was used. Distribution of luminaries and room configuration is shown in Figure 3. Further details can be found on Section 5.1. The definition of potential attack zones was based on pure geometrical considerations and assuming an LOS approach. A slice-based graph will be presented, showing how the number of available lamps varies depending on the position of the eavesdropper. The result of this first step, number of LEDs seen by the receiver using an LOS approach, is shown in Figure 4. It should be noted that, in the first 20 cm closest to the “window”, the number of LEDs viewed is zero. Sensors 2017,17, 2687 5 of 18 Figure 3. Room Configuration. In the scenario, the emitters are located in a room 6 m wide by 6 m deep by 3 m high. In the scenario’s room, there are nine luminaries with a power output of 12 W per luminary. Each luminary is composed of 100 individual LEDs and each luminary is a square one. Figure 4. Number of emitters (LEDs) viewed from the target area. The observation area is 50.00 by 26.00 m and is located at the height of 0.00 m. The window being observed is at 4.20 m of height and located between the 0.00 to the 6.00 m in the horizontal axis. Sensors 2017,17, 2687 6 of 18 The results of the simulation: number of LEDs seen with a 180 ◦ FOV and 20 ◦ FOV, power received ( Pi ), and bandwidth ( BW )), of the second step are listed on Table 1. The values corresponding to each of the columns are: •Position follows the hwidth,depth,heightiformat and is expressed in meters. •There are two “Viewed LEDs” columns: – The first column represents the number of LEDs that can be viewed from that position having an FOV of 180 ◦ . This variable represents the number of individual LEDs that a single attacker may observe from that position by eye. – The second column represents the number of LEDs that can be viewed from that position having an FOV of 20 ◦ . This variable represents the number of LEDs that an attacker observes using some help that restricts the FOV, down to 20 ◦ , but may increase the receiver gain such as the employment of the telescope used for the experiments (Section 3.2). • The power value, in the fourth column, is in mW and represents the receiver power in that position with an FOV of 20◦and no extra gain. • The bandwidth in MHz and, as in the previous value, it represents the bandwidth obtained with an FOV of 20◦. Additionally, to clarify which LEDs were being seen by the receiver at each of the points selected, and shown in Table 1, those LEDs were plotted as seen in Figure 5. The blue points in Figure 5represent the LEDs viewed by the receiver using an LOS approach and an FOV of 20 ◦ . In addition to those blue points, if the receiver had an FOV of 180 ◦ , the LEDs represented by the red points would also be seen by the receiver. Table 1. Simulation results. Position Coordinates (x,y,z)Viewed LEDs Power (mW/dBm) Bandwidth (MHz) FOV 180◦FOV 20◦ S1 h1.50, −9.00, 0.00i100 100 5.14/7.11 3760.00 S2 h1.50, −0.30, 0.00i254 54 30.44/14.83 24.66 S3 h1.50, 5.00, 0.00i300 100 42.97/16.33 8160.00 S4 h8.00, −9.00, 0.00i300 300 15.22/11.82 44.13 S5 h8.00, −4.00, 0.00i400 400 41.42/16.17 52.89 S6 h8.00, 5.00, 0.00i600 200 28.13/14.49 30.65 S7 h12.00, −9.00, 0.00i200 200 4.96/6.95 22.17 S8 h12.00, −0.30, 0.00i300 200 9.61/9.83 69.64 S9 h12.00, 5.00, 0.00i300 200 8.62/9.36 73.78 S10 h15.00, −9.00, 0.00i300 300 5.35/7.28 14.50 S11 h15.00, −4.00, 0.00i400 400 9.80/9.91 50.20 S12 h15.00, 5.00, 0.00i600 400 10.92/10.38 51.13 S13 h25.00, −9.00, 0.00i400 400 2.70/4.31 48.91 S14 h25.00, −0.30, 0.00i600 600 5.15/7.12 48.83 S15 h25.00, 5.00, 0.00i600 600 4.97/6.96 49.19 S16 h45.00, −9.00, 0.00i400 400 0.65/−1.87 6.42 S17 h45.00, −4.00, 0.00i400 400 0.69/−1.61 6.70 S18 h45.00, 5.00, 0.00i600 600 1.05/0.21 48.41 FOV: Field of View. Sensors 2017,17, 2687 7 of 18 (a)(b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) (m) (n) (o) (p) (q) (r) Figure 5. LEDs viewed from the receiver’s position: ( a ) S1; ( b ) S2; ( c ) S3; ( d ) S4; ( e ) S5; ( f ) S6; ( g ) S7; ( h ) S8; ( i ) S9; ( j ) S10; ( k ) S11; ( l ) S12; ( m ) S13; ( n ) S14; ( o ) S15; ( p ) S16; ( q ) S17; and ( r ) S18 as listed in Table 1. Blue points represent the LEDs viewed by the receiver with an FOV of 20 ◦ . Red points represent the LEDs viewed by the receiver, in addition to the blue ones, when the receiver has an FOV of 180◦. 3.2. Experimental Results For the experimental part of the work, a similar simulation was performed using an emitter located on the second floor of the IDeTIC building at the “Universidad de Las Palmas de Gran Canaria”. Of the possible points from which the eavesdropping was tried, four were chosen. The distances, direct and vertical of those positions, are listed in Table 2. Table 2. Sniffing positions. Position Coordinates Distance (m) ∆h(m)Azimuth T1 h13.90, −0.90, −10.65i17.53 10.65 37.50◦ T2 h13.90, 11.10, −10.65i20.73 10.65 35.50◦ T3 h19.60, −0.90, −8.25i21.28 8.25 28.00◦ T4 h20.40, 15.10, −7.85i26.57 7.85 20.00◦ The simulated power received in the positions shown on Table 2, the expected number of LEDs viewed from those positions as well as the tested power received in those areas are shown in Table 3. Power values are in mW, and viewed LEDs are a percentage of the total expected emitting LEDs on the lamp. In our case, a receiver as shown in Figure 6was used, and a ten by ten matrix of LEDs was assumed. Furthermore, in Figure 7, it can be seen which part of the lamp could be seen from the different locations specified in Table 2according to the computer simulation. The corresponding pictures are shown in Figure 8. Sensors 2017,17, 2687 8 of 18 Table 3. Sniffing results. Position Simulated Power (mW/dBm) Viewed LEDs Tested Power (mW/dBm) T1 0.9876/−0.0542 20% 0.0014/−28.5387 T2 0.8684/−0.6128 17% 0.0031/−25.0864 T3 1.1005/0.4159 100% 0.0087/−20.6048 T4 0.9251/−0.3381 64% 0.0044/−23.5655 Figure 6. Assembly used to eavesdrop on the VLC channel from position T1. (a) (b) (c) (d) Figure 7. Representations of the expected LEDs viewed from the different locations as simulated: (a) at T1; (b) at T2; (c) at T3; and (d) at T4 as defined in Table 2. During the test, the channel bandwidth could not be measured due to the hardware limitations. Nevertheless, in all cases, as it can be observed in Figure 9, the signal could be captured relatively clearly in all cases. Even when windows are between the emitter and the receiver, as in the cases shown in Figure 8b,d, the signal can be recognized, as is shown in Figure 10. Sensors 2017,17, 2687 9 of 18 (a) (b) (c) (d) Figure 8. Pictures of the target emitter from the diferent locations where the sniffer was positioned: ( a ) T1; ( b ) T2; ( c ) T3; and ( d ) T4 as defined in Table 2. In ( b , d ), it can be observed that the double panel window is between the emitter and the receiver. In ( a , c ), the receiver has a clear and direct view without obstacles of the emitter; and the window in which the emitter can be observed is highlighted in red. (a)(b) (c)(d) Figure 9. Captures of the signal from diferent locations: ( a ) at T1; ( b ) at T2; ( c ) at T3; and ( d ) at T4 as defined in Table 2. The emitted signal used an On-Off keying with non return to zero (OOK-NRZ) modulation scheme at a 512 bps data rate. Sensors 2017,17, 2687 16 of 18 Glens ≈ALens Aprojected ≈φlens φprojected 2 . (12) The total receiver’s gain included the gain from the telescope ( Glens ), the responsivity ( R ) of 0.3 A W , and an electronic gain ( Gelec ) of 70 dB. When Equation (13) was applied, we got an approximated total gain (G) of 13.34 V mW : G=Glens ·R·Gelec. (13) Four target positions at different heights and distances were selected for the test. The distances and height for each one of the locations are shown in Table 2. As can be observed, the positions were selected to emulate possible scenarios such as eavesdropping from a parking space or from outside the premises. The view of the emitter from the different locations as well as the general assembly used are shown in Figure 8. The distances were used to calculate the theoretical received power at each position and to compare those values with the ones obtained in the experiments as shown in Table 3. 6. Conclusions In this work, we proposed the study of eavesdropping on a VLC link. To assess such an attack, simulations were performed. To validate the results from the simulations, practical experiments were performed. The study proved the validity of using geometrical considerations to define potential attacks zones. This study served as the base to choose, from those potential attacks zones, the ones that seemed more adequate. Additionally, the simulations showed that even if the more significant amount of leakage was located directly in front of the window, there are significant leakages on the sides, and the higher power density happened to be not in the closest locations but in the middle range ones. The interference between sources decreased the bandwidth available for eavesdroppers. However, this issue affected both legit and non legit users so it can be minimized when a VLC system is implemented. An important aspect was the aperture of the receiver. This aperture will influence the outcome significantly, as well as the selection of the location from where the eavesdropping will be conducted. However attackers can modify their receiver’s aperture more freely than legit users. Although the simulations provide a good starting point, validating those results with experimental testing proved to be a necessity. Due to measurement and aiming errors, the resulting values vary significantly. However, the simulations and the experimental test proved that eavesdropping a VLC link is possible and that the attacker is less limited than previously expected. In turn, the simulations are useful for selecting areas, or positions, of interest from which the attack may be carried out. These positions increase the chance of capturing a clear signal and may be located outside the expected attacker’s area. The results seem to validate that it is possible to eavesdrop on a VLC-based channel from outside the premises, as it was previously accepted. Furthermore, this is exploited since the received power can be incremented while the noise can be decreased through optical means. Acknowledgments: This research was supported in part by the Escuela Superior Politecnica del Litoral (ESPOL), Ecuador. Author Contributions: Ignacio Marin-Garcia and Victor Guerra conceived and performed the experiments. Ignacio Marin-Garcia implemented the simulator, designed the experiments, analyzed the data and wrote the paper. Rafael Perez-Jimenez contributed with analysis tools and reviewed the work. Conflicts of Interest: The authors declare no conflict of interest. Sensors 2017,17, 2687 17 of 18 Abbreviations The following abbreviations are used in this manuscript: bps bits per second AES Advanced encryption Standard CBC-MAC Cipher Block Chaining Message Authentication Code CCM Counter with CBC-MAC CSK Color-Shift Keying FOV Field of View ISI Inter-Symbol Interference LED Light Emitting Diode LOS Line-of-Sight MIMO Multiple-Input Multiple-Output NLOS Non Line-of-Sight NRZ Non-Return-to-Zero RF Radio Frequency OOK On–Off Keying SNR Signal-to-Noise Ratio UWB Ultra-Wide Band VLC Visible Light Communication VPPM Variable Pulse-Position Modulation References 1. Jovicic, A.; Li, J.; Richardson, T. Visible light communication: Opportunities, challenges and the path to market. IEEE Commun. Mag. 2013,51, 26–32. 2. IEEE Standard for Local and Metropolitan Area Networks–Part 15.7: Short-Range Wireless Optical Communication Using Visible Light, 2011. Available online: http://ieeexplore.ieee.org/document/6016195/ (accessed on 1 October 2017). 3. 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