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Numerical simulation analysis of information interception based on optical tunneling information signal from fiber optic communication systems in a channel leak shows a high level threats of information security of critical information infrastructure. Interception can be implemented covertly in the field with the probability of an error bit appearing, no more than in the communication line, even while preserving the structure of the optical fiber and with minimal damage to the cable’s protective shells.
Теги: informational and informative signals optical link and leakage channel optical network information security optical tunneling traffic intercept информационная безопасность оптических сетей информационный и информативный оптический сигнал оптические каналы связи и утечки оптическое туннелирование перехват трафика
Traffic Intercept in Optical Network: Method of Optical Tunneling
V. V. Grishachev
Institute for Information Sciences and Security Technologies Russian State University of the Humanities, Moscow, Russia
Numerical simulation analysis of information interception based on optical tunneling information signal from fiber optic communication systems in a channel leak shows a high level threats of information security of critical information infrastructure. Interception can be implemented covertly in the field with the probability of an error bit appearing, no more than in the communication line, even while preserving the structure of the optical fiber and with minimal damage to the cable’s protective shells.
Keywords: optical network information security, traffic intercept, optical link and leakage channel, informational and informative signals, optical tunneling
Received on: 24.10.2020
Accepted on: 24.11.2020
Problem of information security in optical communication networks
In the structure of modern communication systems, a decisive role is played by optical networks, which are based on fiber-optic information transmission systems. The transmission of information through an optical cable provides significant advantages over other communication channels, one of which is the high security of transmission from interception [1, 2]. It should be noted that the increased level of security is largely determined by the low level of knowledge of the methods of forming leakage channels and there are many technical solutions for gaining access to information transmitted via fiber-optic channels.
Interception of information traffic in communication networks is unauthorized access to information transmitted over communication networks using technical intelligence means, i. e. technical means not included in the standard infrastructure of the communication system [2–5]. The interception structure includes a regular channel and a communication line, in which an abnormal channel and a leak line are formed. Interception is based on the physical method of connecting to a standard communication channel. According to the connection method, the interception model includes two types (Fig. 1):
contact interception, formed by diverting a part of the information optical signal from the communication channel to the leakage channel (Fig. 1A, 1B);
remote interception, formed by registering optical and non-optical informative signals without or with impact on the communication channel (Fig. 1C, 1D).
The type of interception determines the technical reconnaissance means used and the effectiveness of their functioning, i. e. threat level of danger.
Remote interception. One of the main advantages of an optical channel in comparison with electrical communication channels is the absence of side electromagnetic radiation and pickup (SEMRP), but this statement is not absolute. As shown by numerical modeling [6, 7], the optical cable contains informative electromagnetic radiation at frequencies close to the modulation frequencies of the information signal of the optical carrier, parasitic electromagnetic radiation (PEMR), formed as a result of nonlinear optical transformations, the power assessment of which shows the possibility of forming a remote channel leaks. Also, the literature discusses remote interception based on nuclear magnetic resonance and other physical phenomena without conducting any analysis. All methods are based on the physical phenomena of the interaction of the information signal with the material of the communication channel, leading to informative modulations of the parameters of the environment.
Contact interception. Removing a part of the radiation from an optical fiber is a technical problem of fiber-optic technologies required for many purposes, such as combining and splitting an optical flow in a fiber and others. These technologies can also be successfully applied in interception techniques, especially since some devices with similar functions are used in the installation and operation of fiber-optic information transmission systems. Thus, in network monitoring services, network tap devices are used, which are presented in the form of fibers included in the break using standard fiber-optic coupler connectors and designed for regular control of information traffic [1, 2]. Another device, a commercial fiber-optic clothespin (e. g., FOD‑5503) is used in the installation and operation of an optical network for voice communication between installers at distances of more than 200 km, by outputting / inputting part of the radiation at the bend of the fiber [2].
The wide dissemination of these dual-purpose technical devices has led to the fact that only these two channels of leakage are discussed in the discussion of interception. Their danger is greatly exaggerated, since they are all easily detected by monitoring and security services either during installation or during operation. They are more or less effective for an internal intruder who, using his knowledge of the operation of a local network inside a protected perimeter, can connect either a traffic interceptor or a pin-coupler. Traffic interception in telecommunications becomes more complicated, since a channel break will be detected, the removal of a part of the information optical signal at a bend is too significant in magnitude, which will affect the network functionality.
Threat analysis shows that there are other more effective models of contact interception, which include traffic interception based on the diversion of a part of the optical radiation from the communication channel to the leakage channel by optical tunneling [2–3]. Although this contact interception has been qualitatively described for a long time, there is no physical description and assessments of its effectiveness. In this work, this gap is filled, which can make it possible to compile a more complete model of threats to information security to traffic in optical networks.
Interception methodology and efficiency assessment
Traffic interception in information cable networks has its own characteristics and in order to identify them, it is necessary to determine the technical characteristics of the formation and functioning of the leakage channel [2–5, 8–10]. The main components of the leakage channel are the method of physical access to the information signal and the formation of the informative signal, the implementation of the leakage channel and technical means of registration. Furthermore, it is necessary to determine the technical characteristics that determine the efficiency of the leakage channel.
Communication channel and leaks. A generalized block diagram of information traffic interception in optical networks is shown in Fig. 2. The standard communication channel consists of a transmitter (1), a communication channel (2) in the form of an optical cable and a receiver (3). The main parts of the leakage channel are the informative signal formation system (4), e. g., by diverting part of the radiation from the communication channel to the leakage channel, and the informative signal registration system (5). The operation of the leakage channel is determined by the method of generating an informative signal, which can be implemented in various ways.
The parameters of the communication channel are determined by the channel length (L), the power of the optical signal at the input (Pin), i. e. transmitter power, and at the output (Pout), i. e. at the entrance to the receiver, which determine the link budget (Pin – Pout in dB). The value of Pout is determined by the sensitivity of the receiver, which cannot be higher than this value. Also, the operation of the communication line is influenced by the noise characteristics of the line, specified by the signal-to-noise ratios at the SNRin input and SNRout output, which can be divided into electronic in active parts and optical in the communication channel.
The parameters of the leakage channel are determined by the distance (x) to the place of formation of the informative signal in the leakage channel, the choice of which has a significant effect on its efficiency. The closer the tap is to the transmitter, the higher the power of the information signal and the more power with less noise of the information signal (Pleak) can be diverted secretly from the monitoring systems. The efficiency of interception is largely due to the noise of the SNRleak leakage channel, which is mainly generated when connected to a standard communication channel and tapped into the leakage channel. In other technical devices of the channel leakage noise can be reduced by using low-noise receivers and amplifiers in comparison with standard devices of the communication channel, which will achieve better noise characteristics.
Leakage channel block diagram. Effective traffic registration can be implemented according to the scheme (Fig. 3), in which the diverted informative signal (1) is cut off from the communication channel by installing an optical isolator (2), subsequent amplification (3) and a converter (4). The optical isolator is made in the form of a Faraday element, circulator or other element and is designed to prevent backward optical radiation from the leakage channel to the communication channel, and, first of all, radiation from the optical amplifier. The removed optical signal should be small, in the limiting case, on the order of one or several photons, which makes it possible to increase the secrecy of radiation removal. Amplification allows you to increase the signal to the required power in the converter and subsequent decoding.
The task of the leakage channel is to intercept information traffic in the communication channel without losing information. In digital communication systems, this is due to the formation of a leakage channel, in which the probability of an erroneous bit (BERleak) will not be greater than the probability of an erroneous bit (BERlink) in the communication channel, i. e.
.
From these assumptions, if the main parameters of the communication channel are: (1) the optical budget of the Pin–Pout line, with which the receiver sensitivity is related; (2) signal-to-noise ratio at the SNRin input and SNRout output of the communication channel. Then, the main parameters of the leakage channel, based on the diversion of a part of the information optical signal to the leakage channel, are the Pleak power and the signal-to-noise ratio SNRleak of the informative optical signal.
Limitations on the power of the withdrawn informative signal are determined by: (1) the sensitivity of the optical receiver of the leakage channel, which must reliably record the signal with the probability of an erroneous bit appearing no greater than in the communication channel; (2) with a fraction of the power removed from the communication channel, which must be so small as not to be detected by the monitoring system. At a distance x from the transmitter, the power of the informative signal Pleak = Px depends on the power of the information signal P0 at the point of formation of the leakage channel, which allows us to introduce the concept of the power transfer coefficient from the communication channel to the leakage channel
.
Additional restrictions are associated with the signal-to-noise ratio of the informative signal: it should be no less than that of a standard receiver of the communication channel. To characterize the noise properties of channels, one can use the concept of the noise figure of channel elements [8–10], i. e. the ratio of the SNR at the input of the element to the SNR at the output, so that the integral noise factors of the communication channel and the leakage channel are determined, respectively, as
and .
Each of which is the product of the noise figures of the individual elements that make up the channel. For the leakage channel, it can be defined as the product
,
noise factors of the informative signal formation system (FT), transmission line (FL), amplifier (FA) and converter (receiver) (FR). The greatest contribution to the noise of the informative signal is made by the informative signal generation system, which is associated with the need to create it in the field conditions of the existing communication line, while other elements can be manufactured in advance by an industrial method with characteristics superior to standard elements.
A condition for a reliable assessment of the efficiency of the leakage channel is the requirement
,
then the probability of an erroneous bit appearing in the leakage channel will be no more than in the communication channel, i. e. .
The introduced parameters make it possible to analyze the efficiency of the leakage channels functioning based on the removal of a part of the optical radiation from the communication channel and to make assumptions on the effective protection of the communication channel from interception by individual types. In particular, a leakage channel based on diverting a part of optical radiation from a communication channel to a leakage channel based on optical tunneling is considered below.
Optical tunneling in traffic interception
The phenomenon of optical tunneling consists in the violation of total internal reflection at the core / cladding interface of an optical fiber, which is associated with the formation of a surface wave penetrating into the cladding with an exponentially decreasing intensity along the penetration depth, varying with the angle of incidence and wavelength [2, 3, 11–13]. The reflected wave penetrating into the shell leaves the boundary at a distance of several wavelengths and can be captured by another waveguide. Under the condition of phase matching in the main waveguide and the additional waveguide intercepting a part of the surface wave, the wave overflow occurs, which can be complete.
A model of optical power removal from one waveguide into an arc closely located waveguide is shown in Fig. 4. Two waveguide channels along the y axis with a width w with refractive indices n1 and n2 are separated by a distance s with a refractive index n3, which forms an optical contact along the fibers of length z along the z axis. Through the information communication channel, optical information radiation with power P0 propagates, in the area of the optical contact, part of the radiation with power Px passes into the informative leakage channel.
The transmission coefficient from the communication channel to the leakage channel for the case of optical tunneling can be determined by the coupled mode method [11–13] as
,
which depends on the coupling coefficient of optical modes (K) in the communication and leakage channels, the optical contact length z of the waveguides with optical losses in the communication channel α and the difference between the propagation constants Δβ between them.
In the approximation of small absorption (α << 1) and the same waveguides (Δβ ≈ 0), we obtain
.
The power drain from the communication channel to the leakage channel should not be large. It should not exceed the typical values of losses due to optical inhomogeneities of the welding type (less than 0.1 dB, about 0.01–0.02 dB), therefore, the power removed is always much less than 1 with , which makes it possible to determine the transmission coefficient in the form
.
As can be seen, the value of the transmission coefficient is determined by the coupling coefficient K between the waveguides of the communication and leakage channels, and the length of the optical contact z of the waveguides plays the role of a parameter with which one can influence the value of the transmission coefficient.
The integral transmission coefficient from the communication channel to the leakage channel can be assessed in the approximation of plane waveguides with a parabolic refractive index profile by the coupled mode method. For the assessment, we use the geometric and optical parameters of waveguides, which are close in magnitude to cylindrical fibers of optical communication systems [1, 2].
Geometric parameters: waveguide width of the order of the fiber core diameter – 8–9 µm (w / λ = 8), the optical contact width is less than or of the order of the fiber cladding thickness – ~60 µm with a 125 µm cladding diameter (s / λ ≤ 50).
The length of the optical contact of about 1 cm (z / λ = 104) is selected from the conditions of the technical possibility of fixing the extended contact with mechanical and adhesive devices, as well as from the condition of phase matching of coupled modes. The coherence length of the information optical signal is limited by the modulation frequency; therefore, for an information transmission rate of more than 10 GHz, the coherence length will be about 6 cm.
Optical parameters: the refractive indices of the waveguides are of the order of n1 = n2 = 1,45, the refractive index of the optical contact is n3 = 1,44 (i. e. n1 (n2) – n3 = 0,01) and the critical angle θc = 1,45328 rad (sin θc = n3 / n2 = 0,9931), i. e. limits of variation of the angle of incidence and sine of the angle of incidence .
The proposed approximations can be transmitted to cylindrical fibers with certain restrictions, but they allow one to determine the main parameters of optical tunneling in order of magnitude, which is quite sufficient for the task of assessing the efficiency of the leakage channel.
The coupling coefficient of optical modes K depends on the type of waveguides, the distance between the waveguides (s), the frequency or wavelength of the information signal (ω or λ), the waveguide propagation constant (), the refractive index of the waveguide medium (n), the wave number of the information signal in vacuum , the angle of incidence (θ, taking values from the critical angle of incidence to the direction of propagation along the axis of the waveguide π / 2). In the approximation of plane identical waveguides with n1 = n2, Δβ = 0 and a parabolic profile of the refractive index, the coupling coefficient [11] has the form
Then the transmission coefficient can be presented in a form convenient for numerical simulation
,
where dimensionless constants are introduced based on the adopted geometric and optical characteristics of waveguides:
; ; ; for и , λ = 1,6 µm, , , .
As a result of numerical simulation, the dependence of the transmission coefficient on the angle of incidence was obtained, which has a pronounced maximum (Fig. 5). The transmission coefficient is equal to 0 at critical angles of incidence; with an increase in the angle of incidence, it rapidly increases and reaches its maximum. The approach of the propagation direction to the paraxial rays leads to a rapid drop in the transmission coefficient to 0. Thus, the transmission coefficient has a maximum narrow in the angle of incidence, in which a small fraction of the information signal power is concentrated. The maximum value reaches 1 when the length of the optical contact is increased by more than 1 mm. This allows small portions of the information flow deviating from the axial direction of propagation to be completely diverted into the leakage channel, which is difficult to detect by network monitoring systems.
It can be seen from the plot of the dependence that the directions of propagation rays present in the structure of the flow close to the critical angle of incidence θc will effectively pass from the communication channel to the leakage channel. In the approximation of an optical contact with a length of about a hundred wavelengths (~ 160 μm) and a width of about 40 wavelengths (~ 65 μm), more than 1% of the power of all beams with an angle of incidence close to the critical angle will pass into the leakage channel. An increase in the length of the optical contact by a factor of 100 to 16 mm will lead to the involvement of beams more distant from the critical angle into the process of forming an informative signal with 100% transmission to the leakage channel. Beams closer to the critical angle will not have a significant effect on the transmission coefficient due to the violation of the wave synchronism between the channels. Thus, 100% transmission will occur in a narrow range of angles of incidence, limited by the width of the range of angles with maximum transmission, of the order of 0.003 rad from the total range of angles of the order of 0.1 rad. The integral power transfer ratio can be assessed at 1%. This type of dependence presupposes the possibility of forming a recorded informative signal in the leakage channel even for cylindrical fibers connected with optical glue without bleeding the fiber cladding to the core, which greatly simplifies the technique of removing the light flux and makes it hidden for monitoring.
Assessment of the distance from the transmitter of the communication line to the place of interception. The Pleak power of the informative signal at the point of interception is determined by the original power Pin of the transmitter, the loss α in the communication channel and the transmission coefficient κ, so that
.
The minimum power diverted into the leakage channel can be assessed at 10 photons, which is limited by the noise of the technical reconnaissance equipment of the leakage channel. The power limit for a data transfer rate of 100 Gb / s will be dBm (0.1 μW), hence the maximum interception range
.
For the integral transmission coefficient κ = 1%, transmitter power Pin = 10 dBm and loss in the communication channel α = 0.5 dB / km, the distance to which interception with the adopted reconnaissance technique will be possible will reach 60 km.
Noises of the informative signal in the leakage channel. When forming an informative signal by optical tunneling, the coupling coefficient of optical modes (K) experiences fluctuations due to a thermal change in the distance between channels (δs), the width of the spectrum of the information signal (δω), the direction of propagation (δθ), which causes fluctuations in the transmission coefficient and, consequently, distortion informative signal in the leakage channel, i. e. the appearance of multiplicative noise. Additive noise can be disregarded as the penetration from the external environment or internal light generation is negligible. At the point where the optical radiation is tapped off, the rms power of the information signal in the communication channel and the informative signal in the leakage channel
and ,
where the first terms and are the power of the useful part of the signal, and the second terms and are the power of the noise. RMS power of the useful part of the informative signal
and noise part
,
where are parasitic fluctuations of the transmission coefficient, which reduce the value of the useful part of the informative signal by and increase its noise part by the same amount, are additive noise, which we neglect. From here it is possible to obtain the relationship between the signal-to-noise ratio of the information and information signals in the form of leakage channel noise figure
.
Thus, it is assumed that the main contribution to the noise is provided by the method of forming the diversion of a part of the information signal from the communication channel to the leakage channel, the noise of other parts of the leakage channel is leveled by the choice of low-noise technical reconnaissance means.
The assessment of fluctuations in the transmission coefficient is determined by the relative fluctuations of the parameters of optical tunneling, such as the angle of incidence , width and length of the optical contact, the width of the waveguide , and others associated with thermal vibrations at the point of contact. In the approximation of smallness of the transfer coefficient, its value is associated with relative fluctuations in the accepted notation and approximations as
Taking into account the large length of the optical contact and the stability of the waveguide width, the transmission coefficient can be assessed by the approximation
.
Then the transmission noise figure is
для , i.e. .
This makes interception much more difficult, but the use of a less noisy receiver in comparison with a standard communication line receiver can achieve effective interception, i. e. with the probability of an erroneous bit appearing no larger than the standard communication channel.
Features of traffic interception in optical communication networks
Practical implementation of interception is possible when creating an effective and stable optical contact between the communication and leakage channels, which requires the fulfillment of certain conditions for access to the optical cable and technical solutions for forming a part of the optical radiation. Based on the qualitative analysis of the formation of an informative signal by the optical tunneling method, we can propose several structural schemes for the implementation of the leakage channel (Fig. 6). Despite the fact that the values of the transmission coefficient should be small, but even in this case, it is difficult to realize the tunneling of light through the protective shells. Therefore, first, it is necessary to free the fiber from all protective cladding to the optical cladding of the fiber, which is 125 µm in diameter; second, in the area of optical contact with the fiber of the communication channel, it is necessary to use a leakage channel in the form of a waveguide without a shell of a convenient shape, which turns into a conventional cylindrical fiber.
The formation of a stable optical contact can be done mechanically (Fig.6A) or adhesive (Fig.6B) without bending the fiber. The latter method is preferable, since it is more resistant to external influences – the boundaries of the optical glue perform focusing functions for the light tunneling into it and protective functions against external influences that can affect both the leakage channel and the communication channel. An additional small effect of external physical fields on the optical contact, including mechanical impact (i. e., bending), can increase the transmission coefficient. Another method (Fig. 6C) involves the use of optical glue as a second cladding around the first cladding of the optical fiber of the communication channel, into which the light tunnels and focuses on the input of the leakage channel fiber with a gradient lens at the end.
The proposed schemes make it possible to implement interception in the field with the simplest means of technical reconnaissance with minimal time investment. In particular, the formation of contact with the use of optical glue can be realized without completely destroying the protective sheaths of the cable, by introducing the glue and the fiber of the leakage channel through a small puncture in the cable with a hollow cylindrical tube similar to the needle of a medical syringe.
All this shows a high level of threat of this interception scenario, the counteraction of which requires the development of methods for protecting the cable system, the use of high-quality optical fiber, continuous monitoring of the state of the communication channel and other actions. In the current operating conditions of fiber-optic information transmission systems, the main ways to prevent interception is the use of high-quality optical cable with high-quality installation, which reduces the likelihood of covert connection.
AUTHOR
Grishachev Vladimir V., Cand of Science (Phys.-Math.), docent, associate professor Institute for Information Sciences and Security Technologies (IISST) Russian State University of the Humanities (RSUH), email: grishachev@mail.ru, Moscow, Russia.
ORCID: 0000-0002-7585-7282
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V. V. Grishachev
Institute for Information Sciences and Security Technologies Russian State University of the Humanities, Moscow, Russia
Numerical simulation analysis of information interception based on optical tunneling information signal from fiber optic communication systems in a channel leak shows a high level threats of information security of critical information infrastructure. Interception can be implemented covertly in the field with the probability of an error bit appearing, no more than in the communication line, even while preserving the structure of the optical fiber and with minimal damage to the cable’s protective shells.
Keywords: optical network information security, traffic intercept, optical link and leakage channel, informational and informative signals, optical tunneling
Received on: 24.10.2020
Accepted on: 24.11.2020
Problem of information security in optical communication networks
In the structure of modern communication systems, a decisive role is played by optical networks, which are based on fiber-optic information transmission systems. The transmission of information through an optical cable provides significant advantages over other communication channels, one of which is the high security of transmission from interception [1, 2]. It should be noted that the increased level of security is largely determined by the low level of knowledge of the methods of forming leakage channels and there are many technical solutions for gaining access to information transmitted via fiber-optic channels.
Interception of information traffic in communication networks is unauthorized access to information transmitted over communication networks using technical intelligence means, i. e. technical means not included in the standard infrastructure of the communication system [2–5]. The interception structure includes a regular channel and a communication line, in which an abnormal channel and a leak line are formed. Interception is based on the physical method of connecting to a standard communication channel. According to the connection method, the interception model includes two types (Fig. 1):
contact interception, formed by diverting a part of the information optical signal from the communication channel to the leakage channel (Fig. 1A, 1B);
remote interception, formed by registering optical and non-optical informative signals without or with impact on the communication channel (Fig. 1C, 1D).
The type of interception determines the technical reconnaissance means used and the effectiveness of their functioning, i. e. threat level of danger.
Remote interception. One of the main advantages of an optical channel in comparison with electrical communication channels is the absence of side electromagnetic radiation and pickup (SEMRP), but this statement is not absolute. As shown by numerical modeling [6, 7], the optical cable contains informative electromagnetic radiation at frequencies close to the modulation frequencies of the information signal of the optical carrier, parasitic electromagnetic radiation (PEMR), formed as a result of nonlinear optical transformations, the power assessment of which shows the possibility of forming a remote channel leaks. Also, the literature discusses remote interception based on nuclear magnetic resonance and other physical phenomena without conducting any analysis. All methods are based on the physical phenomena of the interaction of the information signal with the material of the communication channel, leading to informative modulations of the parameters of the environment.
Contact interception. Removing a part of the radiation from an optical fiber is a technical problem of fiber-optic technologies required for many purposes, such as combining and splitting an optical flow in a fiber and others. These technologies can also be successfully applied in interception techniques, especially since some devices with similar functions are used in the installation and operation of fiber-optic information transmission systems. Thus, in network monitoring services, network tap devices are used, which are presented in the form of fibers included in the break using standard fiber-optic coupler connectors and designed for regular control of information traffic [1, 2]. Another device, a commercial fiber-optic clothespin (e. g., FOD‑5503) is used in the installation and operation of an optical network for voice communication between installers at distances of more than 200 km, by outputting / inputting part of the radiation at the bend of the fiber [2].
The wide dissemination of these dual-purpose technical devices has led to the fact that only these two channels of leakage are discussed in the discussion of interception. Their danger is greatly exaggerated, since they are all easily detected by monitoring and security services either during installation or during operation. They are more or less effective for an internal intruder who, using his knowledge of the operation of a local network inside a protected perimeter, can connect either a traffic interceptor or a pin-coupler. Traffic interception in telecommunications becomes more complicated, since a channel break will be detected, the removal of a part of the information optical signal at a bend is too significant in magnitude, which will affect the network functionality.
Threat analysis shows that there are other more effective models of contact interception, which include traffic interception based on the diversion of a part of the optical radiation from the communication channel to the leakage channel by optical tunneling [2–3]. Although this contact interception has been qualitatively described for a long time, there is no physical description and assessments of its effectiveness. In this work, this gap is filled, which can make it possible to compile a more complete model of threats to information security to traffic in optical networks.
Interception methodology and efficiency assessment
Traffic interception in information cable networks has its own characteristics and in order to identify them, it is necessary to determine the technical characteristics of the formation and functioning of the leakage channel [2–5, 8–10]. The main components of the leakage channel are the method of physical access to the information signal and the formation of the informative signal, the implementation of the leakage channel and technical means of registration. Furthermore, it is necessary to determine the technical characteristics that determine the efficiency of the leakage channel.
Communication channel and leaks. A generalized block diagram of information traffic interception in optical networks is shown in Fig. 2. The standard communication channel consists of a transmitter (1), a communication channel (2) in the form of an optical cable and a receiver (3). The main parts of the leakage channel are the informative signal formation system (4), e. g., by diverting part of the radiation from the communication channel to the leakage channel, and the informative signal registration system (5). The operation of the leakage channel is determined by the method of generating an informative signal, which can be implemented in various ways.
The parameters of the communication channel are determined by the channel length (L), the power of the optical signal at the input (Pin), i. e. transmitter power, and at the output (Pout), i. e. at the entrance to the receiver, which determine the link budget (Pin – Pout in dB). The value of Pout is determined by the sensitivity of the receiver, which cannot be higher than this value. Also, the operation of the communication line is influenced by the noise characteristics of the line, specified by the signal-to-noise ratios at the SNRin input and SNRout output, which can be divided into electronic in active parts and optical in the communication channel.
The parameters of the leakage channel are determined by the distance (x) to the place of formation of the informative signal in the leakage channel, the choice of which has a significant effect on its efficiency. The closer the tap is to the transmitter, the higher the power of the information signal and the more power with less noise of the information signal (Pleak) can be diverted secretly from the monitoring systems. The efficiency of interception is largely due to the noise of the SNRleak leakage channel, which is mainly generated when connected to a standard communication channel and tapped into the leakage channel. In other technical devices of the channel leakage noise can be reduced by using low-noise receivers and amplifiers in comparison with standard devices of the communication channel, which will achieve better noise characteristics.
Leakage channel block diagram. Effective traffic registration can be implemented according to the scheme (Fig. 3), in which the diverted informative signal (1) is cut off from the communication channel by installing an optical isolator (2), subsequent amplification (3) and a converter (4). The optical isolator is made in the form of a Faraday element, circulator or other element and is designed to prevent backward optical radiation from the leakage channel to the communication channel, and, first of all, radiation from the optical amplifier. The removed optical signal should be small, in the limiting case, on the order of one or several photons, which makes it possible to increase the secrecy of radiation removal. Amplification allows you to increase the signal to the required power in the converter and subsequent decoding.
The task of the leakage channel is to intercept information traffic in the communication channel without losing information. In digital communication systems, this is due to the formation of a leakage channel, in which the probability of an erroneous bit (BERleak) will not be greater than the probability of an erroneous bit (BERlink) in the communication channel, i. e.
.
From these assumptions, if the main parameters of the communication channel are: (1) the optical budget of the Pin–Pout line, with which the receiver sensitivity is related; (2) signal-to-noise ratio at the SNRin input and SNRout output of the communication channel. Then, the main parameters of the leakage channel, based on the diversion of a part of the information optical signal to the leakage channel, are the Pleak power and the signal-to-noise ratio SNRleak of the informative optical signal.
Limitations on the power of the withdrawn informative signal are determined by: (1) the sensitivity of the optical receiver of the leakage channel, which must reliably record the signal with the probability of an erroneous bit appearing no greater than in the communication channel; (2) with a fraction of the power removed from the communication channel, which must be so small as not to be detected by the monitoring system. At a distance x from the transmitter, the power of the informative signal Pleak = Px depends on the power of the information signal P0 at the point of formation of the leakage channel, which allows us to introduce the concept of the power transfer coefficient from the communication channel to the leakage channel
.
Additional restrictions are associated with the signal-to-noise ratio of the informative signal: it should be no less than that of a standard receiver of the communication channel. To characterize the noise properties of channels, one can use the concept of the noise figure of channel elements [8–10], i. e. the ratio of the SNR at the input of the element to the SNR at the output, so that the integral noise factors of the communication channel and the leakage channel are determined, respectively, as
and .
Each of which is the product of the noise figures of the individual elements that make up the channel. For the leakage channel, it can be defined as the product
,
noise factors of the informative signal formation system (FT), transmission line (FL), amplifier (FA) and converter (receiver) (FR). The greatest contribution to the noise of the informative signal is made by the informative signal generation system, which is associated with the need to create it in the field conditions of the existing communication line, while other elements can be manufactured in advance by an industrial method with characteristics superior to standard elements.
A condition for a reliable assessment of the efficiency of the leakage channel is the requirement
,
then the probability of an erroneous bit appearing in the leakage channel will be no more than in the communication channel, i. e. .
The introduced parameters make it possible to analyze the efficiency of the leakage channels functioning based on the removal of a part of the optical radiation from the communication channel and to make assumptions on the effective protection of the communication channel from interception by individual types. In particular, a leakage channel based on diverting a part of optical radiation from a communication channel to a leakage channel based on optical tunneling is considered below.
Optical tunneling in traffic interception
The phenomenon of optical tunneling consists in the violation of total internal reflection at the core / cladding interface of an optical fiber, which is associated with the formation of a surface wave penetrating into the cladding with an exponentially decreasing intensity along the penetration depth, varying with the angle of incidence and wavelength [2, 3, 11–13]. The reflected wave penetrating into the shell leaves the boundary at a distance of several wavelengths and can be captured by another waveguide. Under the condition of phase matching in the main waveguide and the additional waveguide intercepting a part of the surface wave, the wave overflow occurs, which can be complete.
A model of optical power removal from one waveguide into an arc closely located waveguide is shown in Fig. 4. Two waveguide channels along the y axis with a width w with refractive indices n1 and n2 are separated by a distance s with a refractive index n3, which forms an optical contact along the fibers of length z along the z axis. Through the information communication channel, optical information radiation with power P0 propagates, in the area of the optical contact, part of the radiation with power Px passes into the informative leakage channel.
The transmission coefficient from the communication channel to the leakage channel for the case of optical tunneling can be determined by the coupled mode method [11–13] as
,
which depends on the coupling coefficient of optical modes (K) in the communication and leakage channels, the optical contact length z of the waveguides with optical losses in the communication channel α and the difference between the propagation constants Δβ between them.
In the approximation of small absorption (α << 1) and the same waveguides (Δβ ≈ 0), we obtain
.
The power drain from the communication channel to the leakage channel should not be large. It should not exceed the typical values of losses due to optical inhomogeneities of the welding type (less than 0.1 dB, about 0.01–0.02 dB), therefore, the power removed is always much less than 1 with , which makes it possible to determine the transmission coefficient in the form
.
As can be seen, the value of the transmission coefficient is determined by the coupling coefficient K between the waveguides of the communication and leakage channels, and the length of the optical contact z of the waveguides plays the role of a parameter with which one can influence the value of the transmission coefficient.
The integral transmission coefficient from the communication channel to the leakage channel can be assessed in the approximation of plane waveguides with a parabolic refractive index profile by the coupled mode method. For the assessment, we use the geometric and optical parameters of waveguides, which are close in magnitude to cylindrical fibers of optical communication systems [1, 2].
Geometric parameters: waveguide width of the order of the fiber core diameter – 8–9 µm (w / λ = 8), the optical contact width is less than or of the order of the fiber cladding thickness – ~60 µm with a 125 µm cladding diameter (s / λ ≤ 50).
The length of the optical contact of about 1 cm (z / λ = 104) is selected from the conditions of the technical possibility of fixing the extended contact with mechanical and adhesive devices, as well as from the condition of phase matching of coupled modes. The coherence length of the information optical signal is limited by the modulation frequency; therefore, for an information transmission rate of more than 10 GHz, the coherence length will be about 6 cm.
Optical parameters: the refractive indices of the waveguides are of the order of n1 = n2 = 1,45, the refractive index of the optical contact is n3 = 1,44 (i. e. n1 (n2) – n3 = 0,01) and the critical angle θc = 1,45328 rad (sin θc = n3 / n2 = 0,9931), i. e. limits of variation of the angle of incidence and sine of the angle of incidence .
The proposed approximations can be transmitted to cylindrical fibers with certain restrictions, but they allow one to determine the main parameters of optical tunneling in order of magnitude, which is quite sufficient for the task of assessing the efficiency of the leakage channel.
The coupling coefficient of optical modes K depends on the type of waveguides, the distance between the waveguides (s), the frequency or wavelength of the information signal (ω or λ), the waveguide propagation constant (), the refractive index of the waveguide medium (n), the wave number of the information signal in vacuum , the angle of incidence (θ, taking values from the critical angle of incidence to the direction of propagation along the axis of the waveguide π / 2). In the approximation of plane identical waveguides with n1 = n2, Δβ = 0 and a parabolic profile of the refractive index, the coupling coefficient [11] has the form
Then the transmission coefficient can be presented in a form convenient for numerical simulation
,
where dimensionless constants are introduced based on the adopted geometric and optical characteristics of waveguides:
; ; ; for и , λ = 1,6 µm, , , .
As a result of numerical simulation, the dependence of the transmission coefficient on the angle of incidence was obtained, which has a pronounced maximum (Fig. 5). The transmission coefficient is equal to 0 at critical angles of incidence; with an increase in the angle of incidence, it rapidly increases and reaches its maximum. The approach of the propagation direction to the paraxial rays leads to a rapid drop in the transmission coefficient to 0. Thus, the transmission coefficient has a maximum narrow in the angle of incidence, in which a small fraction of the information signal power is concentrated. The maximum value reaches 1 when the length of the optical contact is increased by more than 1 mm. This allows small portions of the information flow deviating from the axial direction of propagation to be completely diverted into the leakage channel, which is difficult to detect by network monitoring systems.
It can be seen from the plot of the dependence that the directions of propagation rays present in the structure of the flow close to the critical angle of incidence θc will effectively pass from the communication channel to the leakage channel. In the approximation of an optical contact with a length of about a hundred wavelengths (~ 160 μm) and a width of about 40 wavelengths (~ 65 μm), more than 1% of the power of all beams with an angle of incidence close to the critical angle will pass into the leakage channel. An increase in the length of the optical contact by a factor of 100 to 16 mm will lead to the involvement of beams more distant from the critical angle into the process of forming an informative signal with 100% transmission to the leakage channel. Beams closer to the critical angle will not have a significant effect on the transmission coefficient due to the violation of the wave synchronism between the channels. Thus, 100% transmission will occur in a narrow range of angles of incidence, limited by the width of the range of angles with maximum transmission, of the order of 0.003 rad from the total range of angles of the order of 0.1 rad. The integral power transfer ratio can be assessed at 1%. This type of dependence presupposes the possibility of forming a recorded informative signal in the leakage channel even for cylindrical fibers connected with optical glue without bleeding the fiber cladding to the core, which greatly simplifies the technique of removing the light flux and makes it hidden for monitoring.
Assessment of the distance from the transmitter of the communication line to the place of interception. The Pleak power of the informative signal at the point of interception is determined by the original power Pin of the transmitter, the loss α in the communication channel and the transmission coefficient κ, so that
.
The minimum power diverted into the leakage channel can be assessed at 10 photons, which is limited by the noise of the technical reconnaissance equipment of the leakage channel. The power limit for a data transfer rate of 100 Gb / s will be dBm (0.1 μW), hence the maximum interception range
.
For the integral transmission coefficient κ = 1%, transmitter power Pin = 10 dBm and loss in the communication channel α = 0.5 dB / km, the distance to which interception with the adopted reconnaissance technique will be possible will reach 60 km.
Noises of the informative signal in the leakage channel. When forming an informative signal by optical tunneling, the coupling coefficient of optical modes (K) experiences fluctuations due to a thermal change in the distance between channels (δs), the width of the spectrum of the information signal (δω), the direction of propagation (δθ), which causes fluctuations in the transmission coefficient and, consequently, distortion informative signal in the leakage channel, i. e. the appearance of multiplicative noise. Additive noise can be disregarded as the penetration from the external environment or internal light generation is negligible. At the point where the optical radiation is tapped off, the rms power of the information signal in the communication channel and the informative signal in the leakage channel
and ,
where the first terms and are the power of the useful part of the signal, and the second terms and are the power of the noise. RMS power of the useful part of the informative signal
and noise part
,
where are parasitic fluctuations of the transmission coefficient, which reduce the value of the useful part of the informative signal by and increase its noise part by the same amount, are additive noise, which we neglect. From here it is possible to obtain the relationship between the signal-to-noise ratio of the information and information signals in the form of leakage channel noise figure
.
Thus, it is assumed that the main contribution to the noise is provided by the method of forming the diversion of a part of the information signal from the communication channel to the leakage channel, the noise of other parts of the leakage channel is leveled by the choice of low-noise technical reconnaissance means.
The assessment of fluctuations in the transmission coefficient is determined by the relative fluctuations of the parameters of optical tunneling, such as the angle of incidence , width and length of the optical contact, the width of the waveguide , and others associated with thermal vibrations at the point of contact. In the approximation of smallness of the transfer coefficient, its value is associated with relative fluctuations in the accepted notation and approximations as
Taking into account the large length of the optical contact and the stability of the waveguide width, the transmission coefficient can be assessed by the approximation
.
Then the transmission noise figure is
для , i.e. .
This makes interception much more difficult, but the use of a less noisy receiver in comparison with a standard communication line receiver can achieve effective interception, i. e. with the probability of an erroneous bit appearing no larger than the standard communication channel.
Features of traffic interception in optical communication networks
Practical implementation of interception is possible when creating an effective and stable optical contact between the communication and leakage channels, which requires the fulfillment of certain conditions for access to the optical cable and technical solutions for forming a part of the optical radiation. Based on the qualitative analysis of the formation of an informative signal by the optical tunneling method, we can propose several structural schemes for the implementation of the leakage channel (Fig. 6). Despite the fact that the values of the transmission coefficient should be small, but even in this case, it is difficult to realize the tunneling of light through the protective shells. Therefore, first, it is necessary to free the fiber from all protective cladding to the optical cladding of the fiber, which is 125 µm in diameter; second, in the area of optical contact with the fiber of the communication channel, it is necessary to use a leakage channel in the form of a waveguide without a shell of a convenient shape, which turns into a conventional cylindrical fiber.
The formation of a stable optical contact can be done mechanically (Fig.6A) or adhesive (Fig.6B) without bending the fiber. The latter method is preferable, since it is more resistant to external influences – the boundaries of the optical glue perform focusing functions for the light tunneling into it and protective functions against external influences that can affect both the leakage channel and the communication channel. An additional small effect of external physical fields on the optical contact, including mechanical impact (i. e., bending), can increase the transmission coefficient. Another method (Fig. 6C) involves the use of optical glue as a second cladding around the first cladding of the optical fiber of the communication channel, into which the light tunnels and focuses on the input of the leakage channel fiber with a gradient lens at the end.
The proposed schemes make it possible to implement interception in the field with the simplest means of technical reconnaissance with minimal time investment. In particular, the formation of contact with the use of optical glue can be realized without completely destroying the protective sheaths of the cable, by introducing the glue and the fiber of the leakage channel through a small puncture in the cable with a hollow cylindrical tube similar to the needle of a medical syringe.
All this shows a high level of threat of this interception scenario, the counteraction of which requires the development of methods for protecting the cable system, the use of high-quality optical fiber, continuous monitoring of the state of the communication channel and other actions. In the current operating conditions of fiber-optic information transmission systems, the main ways to prevent interception is the use of high-quality optical cable with high-quality installation, which reduces the likelihood of covert connection.
AUTHOR
Grishachev Vladimir V., Cand of Science (Phys.-Math.), docent, associate professor Institute for Information Sciences and Security Technologies (IISST) Russian State University of the Humanities (RSUH), email: grishachev@mail.ru, Moscow, Russia.
ORCID: 0000-0002-7585-7282
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