In this work, for the first time, we uncover that the level of security we have traditionally taken for granted on underwater wireless optical communication (UWOC) may not always be there. We first numerically investigate the security weaknesses of UWOC via Monte Carlo simulation. With the link distance increasing or the water becoming more turbid, the simulation results indicate that the possibility of information leakage increases, which may pose a great threat to the security of UWOC. By using a high-sensitivity multi-pixel photon counter (MPPC) placed aside the water tank, a 5-MHz square wave signal is successfully tapped at 1-m, 3-m, and 5-m underwater transmission distances, which preliminarily verifies the probability of information leakage. We further experimentally demonstrate an UWOC system with potential eavesdropping employing a 2.5-Gb/s orthogonal frequency division multiplexing (OFDM) signal. After transmitting through a 15-m underwater channel, the OFDM signal is eavesdropped by a mirror at 7.8 m. Both the normal receiver at 15 m and the eavesdropping receiver at 7.8 m can achieve a bit error rate (BER) below the forward error correction (FEC) limit of 3.8 × 10−3, which validates that UWOC indeed suffers potential safety hazard.
© 2017 Optical Society of America
The ocean, which contains huge untapped resources, is full of mysteries and temptations to human beings. Underwater wireless sensor networks (UWSNs), as platforms for marine environment monitoring, resource investigation, and offshore exploration, have gained great attention from both academic and industrial communities [1, 2]. In the past, underwater acoustic communication with low bandwidth and large propagation delay was generally adopted for data acquisition and exchange among spatially distributed sensor nodes as well as randomly deployed underwater vehicles . The communication quality is always significantly affected by noise, multipath effect, and Doppler spread. As a result, bit error rates (BERs) and packet loss probability are often high, which poses a serious threat to underwater acoustic communication [3, 4]. Nowadays underwater wireless optical communication (UWOC) with high bandwidth and low latency has motivated a worldwide interest and become a great complement to established acoustic communication in the new era of marine economy. In particular, UWOC is naturally regarded as a much securer solution compared with traditional underwater acoustic communication, because a very narrow light beam can be used as the information carrier. For this reason, while researchers engaged in studying the potential security issues of underwater acoustic communication and put forward a large number of effective solutions [5–11], the security issue of UWOC has been seldom investigated. Considerable work has been done in UWOC [12–24], but most of current studies mainly focused on how to improve the data rate-distance product of the UWOC system in tap water or simulated seawater. Laser diodes (LDs) and light emitting diodes (LEDs) used as light sources in UWOC systems are unconsciously considered to be secure, due to their directivity and impermeability. However, these features of LDs and LEDs should be considered with the combination of complex channel characteristics of UWOC links, which are usually studied by Monte Carlo simulation approach [25–27]. In real scenario, due to the inherently nonzero divergence angle of light beam and the scattering effect of water on light, the light spot diffuses as the transmission distance increases. The gradually diffused light beam may provide eavesdroppers with opportunities to wiretap or modify the transmitting signals, implying that the security of UWOC may become a serious and knotty problem in practical applications.
In this paper, for the first time, we uncover that UWOC exists security vulnerabilities. We first use Monte Carlo simulation to numerically study the security weaknesses of UWOC. Considering various link distance and water types, the simulation results prove that UWOC may suffer from serious security threat due to the scattering effect. To preliminarily investigate the probability of information leakage, we employ a high-sensitivity multi-pixel photon counter (MPPC) placed aside the light beam to successfully eavesdrop on a 5-MHz square wave signal at different transmission distances. We further experimentally demonstrate the UWOC system with potential eavesdropping based on a single-mode pigtailed green-light LD. In the experiment, 2.5-Gb/s orthogonal frequency division multiplexing (OFDM) signals transmitting through a 15-m underwater channel are eavesdropped by a mirror at 7.8 m. The BERs at 15 m and 7.8 m are 2.3173 × 10−3 and 1.9417 × 10−3, respectively, as the amplitude of the received signal at 15 m is 161.2 mV. The BERs at 15 m and 7.8 m are 1.6570 × 10−3 and 2.3301 × 10−3, respectively, as the amplitude of the received signal at 15 m is 129.5 mV. In these two cases, both the normal receiver at 15 m and the eavesdropping receiver at 7.8 m can simultaneously achieve BERs below the forward error correction (FEC) limit of 3.8 × 10−3. It is turned out that UWOC has the risk of information leakage.
2. Numerical study and preliminary validation on the security weaknesses of UWOC
Absorption and scattering are two dominant factors affecting light propagation in water. Absorption leads to the loss of light intensity and scattering causes the deflection of light from its original direction. We use the particle phase function (PPF) [28, 29] to model scattering. The absorption and the scattering coefficient are introduced, both of which are the function of wavelength λ and chlorophyll concentration . The attenuation coefficient is defined as the sum of and . The expressions are as follows [17, 28]:17, 28].
In this work, we numerically studied the potential influence of propagation distance and water type on the security of UWOC. Monte Carlo method was used to simulate the trajectories of emitted photons . In the simulation, the number of photons was 2 x 104. The divergence angle of the transmitter was 0.5 rad. The field of view of the detector was 180 degrees. Four typical water types were considered, including pure sea water, clear ocean water, coastal ocean water, and turbid harbor water. The corresponding coefficient values (,, and ) at 520 nm are detailed in Table 1 [26, 28].
Figure 1 illustrates the light intensity distribution at the receiver after transmitting through 20-m, 30-m, 40-m, and 50-m pure sea water. With the increase of the propagation distance, the light intensity became weaker and the beam spot got larger at the reception plane. It was attributed to the initial divergence angle and light scattering in water. There is no doubt that longer transmission distance will offer more opportunities for eavesdroppers to steal information.
Figure 2 presents the light intensity distribution at the reception plane after transmitting through 7.8-m pure sea water, clear ocean water, coastal ocean water, and turbid harbor water, respectively. For the four types of water, the corresponding attenuation coefficient increases successively, which results in the decrease of light intensity. In addition, it is obvious to see that the light spot becomes larger in more turbid water due to the scattering effect. It turns out that more turbid water will pose greater threat to UWOC security.
To preliminarily verify the probability of information leakage, we designed an experimental system employing a red-light LD and a high-sensitivity MPPC as shown in Fig. 3. A 5-MHz square wave signal was first output from a Tektronix AWG70002A arbitrary waveform generator (AWG). The sampling rate of the AWG was set at 20 MSamples/s and the amplitude of the output square wave signal was 0.5 V. To further adjust the voltage of the baseband square wave signal, a Mini-Circuits ZHL-6A-S + amplifier (AMP) and a key-press variable electrical attenuator (ATT) which was set at 8 dB were employed. Through a bias-tee (ZFBT-4R2GW + ), the baseband square wave signal was superimposed on a red-light LD (HL6501MG). The bias current of the LD was set at 78.0 mA. A water tank (length: 7 m, width: 0.4 m, height: 0.4 m) was filled with 542-L fresh tap water. 1-g Mg(OH)2 powder, acting as the scattering medium, was added to the water. The laser beam propagated from one end of the water tank to the other. The MPPC was located at the side of the water tank and its direction was adjusted to detect photons as many as possible. Note that the MPPC is very sensitivity to light, so the experiment was conducted in a dark room to reduce the influence of background light, as no narrow-band optical filter was used before the MPPC. The detected signals were captured by a Tektronix MSO 71254C mixed signal oscilloscope (MSO) and sent to a computer for demodulation. The sampling rate of the MSO was set at 125 MSamples/s. Figure 4 shows the waveforms of the captured 5-MHz square wave signal, when the MPPC was put at 1 m, 3 m, and 5 m away from the LD. All the waveforms are adequate for error free communication, indicating the potential risk of information leakage.
3. Experimental setup
Figure 5 illustrates the experimental setup to study the potential eavesdropping of UWOC. The transmitter module, the eavesdropping mirror at 7.8 m, the normal receiver module at 15 m, and the experiment site are shown in the insets of Fig. 5. A real-valued 32-QAM OFDM signal was generated offline. Table 2 lists the specific parameters of the OFDM signal. The generated OFDM signal was first loaded into the Tektronix AWG70002A AWG and then output from the AWG with the sampling rate of 5 GSamples/s. A Mini-Circuits ZHL-6A-S + AMP and an ATT which was set at 8 dB were employed to adjust the voltage of the baseband OFDM signal. Through an LD and a thermoelectric cooler (TEC) mount (Thorlabs LDM9LP), the baseband OFDM signal was superimposed on a single-mode pigtailed 520-nm LD (Thorlabs LP520-SF15). The bias current of the LD was set at 80.86 mA by using a LD controller (Thorlabs LDC205C). A temperature controller (Thorlabs TED 200 C) was used to stabilize the output dynamics of the LD. The laser beam was focused into the water by using an air-spaced doublet collimator (Thorlabs FB10FC-543), namely, Lens 1. Mirror 1 and Mirror 2 were separately placed at the transmitting and normal receiving ends in a water tank to help reflect the laser beam into and off the underwater channel. The water tank was filled with tap water and the underwater channel was 15 meters long. An eavesdropping mirror, Mirror 3, with the diameter of 50.8 mm, was located at 7.8 m away from the transmitter module. At the beginning, Mirror 3 was near to the laser beam. To ensure that the laser beams reflected by Mirror 3 were vertically focused into the receiving surface of the avalanche photo diode (APD), we kept the angle of Mirror 3 unchanged and made Mirror 3 move towards the laser beam step by step to intercept different amounts of light. The normal receiving end and the eavesdropping end shared the same receiver module, due to limited equipment availability. The laser beams reflected by Mirror 2 and Mirror 3 were first focused into a 1-GHz APD (Menlo Systems, APD210) by a plano-convex lens (Lens 2). Then the detected signals were captured by the Tektronix MSO 71254C MSO with the sampling rate of 100 GSamples/s and sent to a computer for demodulation. Note that the detector diameter of the APD we used is only 0.5 mm (smaller than that of the light spot), the received optical power by the active area of the APD cannot be measured accurately. Therefore, we record the corresponding amplitude of the OFDM signal captured by the MSO in the experiment, which is proportional to the optical power injected to the active area of the APD.
4. Experimental results
We first generated a 32-QAM OFDM signal with a gross data rate of 2.51 Gb/s. After removing the overheads of CP, training symbols, and FEC (7%), the net bit rate of the OFDM signal was 2.06 Gb/s. In the experiment, the OFDM signal transmitting through a 15-m underwater channel was intercepted by an eavesdropping mirror at 7.8 m (Mirror 3).
In Table 3, Position 1-6 denote different positions of Mirror 3 that can eavesdrop on different amounts of light. Without information leakage (Position 1), the amplitude and BER of the received signal at the normal receiving end were 184.8 mV and 9.9676 × 10−4, respectively. The corresponding constellation map which is well converged is shown in Fig. 6. When Mirror 3 gradually moves into the light path (Position 2-6), the amplitudes of the received signals at the normal receiving end gradually decrease and the corresponding BERs become degraded, as listed in Table 3. On the contrary, the BERs at the eavesdropping end get better and better. In particular, Position 3 and Position 4 are two positions where both the eavesdropper end and the normal receiving end could achieve a BER below the FEC limit of 3.8 × 10−3. Figure 7 presents the constellation maps of the received signals at the normal receiving end and the eavesdropping end, as Mirror 3 is at Position 2, Position 4, and Position 6. The three positions represented three different conditions: only the normal receiver side could communicate successfully (Position 2), both the normal receiver and eavesdropping sides could communicate successfully (Position 4) and only the eavesdropping side could communicate successfully (Position 6). Here, communicating successfully means the achieved BER is below the FEC limit.
When Mirror 3 is at Position 4, the BERs for different subcarriers at the normal receiving and eavesdropping sides are illustrated in Fig. 8. Note that most subcarriers have a BER of zero that is not suitable to be plotted in log scale. Higher BERs in the low-frequency region were attributed to larger beating noise among subcarriers. Figure 9 shows the captured waveform and spectrum of the received signal at the eavesdropping end, when Mirror 3 is at Position 4.
In this paper, for the first time, we numerically and experimentally investigate the security weaknesses of UWOC. However, in the past few years, most of the research effort has been devoted to improving the data rate-distance product of the UWOC system in tap water or simulated seawater. As the study of UWOC develops in depth, more and more researchers become aware of the considerable influence of complex underwater environment on UWOC [22, 30]. Nevertheless, security issues of UWOC, which are closely related to the scattering effect, have so far attracted very little attention. This work is the first step towards effective solutions to security issues of UWOC, which may rely on channel coding techniques.
In this paper, we reveal that UWOC has security vulnerabilities and potential risks. Monte Carlo simulation is first adopted to study the security weaknesses of UWOC. With the increased link distance or the deteriorated water quality, the effect of scattering turns more and more severe, which will offer attackers vast opportunities to eavesdrop on the information from the light path. To preliminarily investigate the probability of information leakage, a high-sensitivity MPPC placed aside the light beam is employed to successfully tap a 5-MHz square wave signal at 1-m, 3-m, and 5-m underwater transmission distances. We then further experimentally demonstrate an UWOC system with potential eavesdropping using a single-mode pigtailed green-emitting LD. 2.5-Gb/s OFDM signals transmitting through a 15-m underwater channel are intercepted by an eavesdropping mirror at 7.8 m. When the amplitude of the received signal at 15 m is 161.2 mV, the BERs at 15 m and 7.8 m are 2.3173 × 10−3 and 1.9417 × 10−3, respectively. When the amplitude of the received signal at 15 m is 129.5 mV, the BERs at 15 m and 7.8 m are 1.6570 × 10−3 and 2.3301 × 10−3, respectively. In the above two situations, both the eavesdropping receiver at 7.8 m and the normal receiver at 15 m can achieve BERs below the FEC limit of 3.8 × 10−3, which proves that UWOC suffers hidden dangers of information leakage.
National Natural Science Foundation of China (NSFC) (61671409, 61301141); The National Key Research and Development Program of China (2016YFC1401202, 2017YFC0306100, 2017YFC0306600).
The authors would like to thank Prof. Yong Liu from University of Electronic Science and Technology of China for valuable discussions on the security issues of UWOC at Photonics Asia 2016. The authors would like to thank Mr. Ming Liu for the assistance in the experiment.
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