External optical noise suppression of a fiber-optic communication channel to prevent eavesdropping on the acoustic-fiber-optic channel and leakage of speech information.

External optical noise suppression of a fiber-optic communication channel to prevent eavesdropping via an acoustic-fiber-optic channel of speech information leakage

Grishachev Vladimir Vasilievich,
Candidate of Physical and Mathematical Sciences, Associate Professor,
Khalyapin Dmitry Borisovich,
Candidate of Technical Sciences, Professor,
Shevchenko Natalia Andreevna
Institute of Information Sciences and Security Technologies,
Russian State Humanitarian University, Moscow

External optical noise suppression of a fiber-optic communication channel to prevent eavesdropping via an acoustic-fiber-optic channel and leakage of speech information

A method for preventing eavesdropping on an acoustic-fiber-optic channel of speech information leakage by adding optical radiation with a noise modulation spectrum to the information light flow is proposed. An experimental test of the efficiency of neutralizing the leakage channel based on the articulation method is carried out. Possible schemes for implementing the method, as well as its advantages and disadvantages, are discussed.

Protection of speech information is a pressing issue in modern society, which is related to the importance of the information that can be obtained from it when eavesdropping on confidential negotiations, conversations on special topics in commercial and government institutions [1]. In this regard, there is a significant need to analyze existing and implemented technologies for the operation of information systems for possible leakage of speech information.

The concept of an acoustic-fiber-optic leakage channel and methods for counteracting its use

Currently, one of the most promising areas of development of information technology is fiber-optic technologies, on the basis of which modern communication systems are built, such as trunk lines or local networks [2,3]. Fiber optics widely penetrate into institutions, citizens' apartments, therefore, if fiber-optic communications pass near or inside designated premises where confidential negotiations or conversations on special topics can be held, there is a real threat of leakage of acoustic (speech) confidential information.

The acoustic-fiber-optic channel is associated with unauthorized recording of speech information (eavesdropping) through regular fiber-optic channels for transmitting information of various purposes of a given institution [4,5]. In the leakage channel, the acoustic field from the information carrier affects the optical fiber of regular information systems built on fiber-optic technologies and causes modulation of the light flux in the optical fiber or network equipment at acoustic frequencies. Digital methods of information transmission, most often used in modern communication systems, make it possible to do this without disrupting the operation of the entire system, since the level of acoustic impact of the light flux slightly reduces the signal-to-noise ratio. The light flux can be formed both by regular equipment and specially created by the intruder. The speech-modulated light flux through regular fiber-optic communications can go far beyond the negotiation site, where it can be demodulated and recorded by the intruder. Signal propagation distances can reach hundreds of kilometers depending on the structure of the cable network, which is due to the low attenuation of the optical signal in the fiber. For example, modern fiber-optic telephones [6,7], used to set up communication systems, are used at distances over 200 km.

Currently, there are many methods and technical solutions for protecting speech information from leakage via side electromagnetic radiation and interference, vibroacoustic and acoustic channels [1]. The use by intruders of a new type of speech information leakage channel — acoustic-fiber-optic — can create serious problems for protection systems, which is associated with the widespread use of new information transmission technologies based on fiber-optic cable, as well as with non-standard physical principles of channel formation, technical counteraction to which does not currently exist in full.

All the main methods for counteracting speech information leakage through waveguide channels by influencing the channel environment can be conditionally divided into the following types:

soundproofing of the transmission channel environment — a passive method consisting of reducing the influence of acoustic impact on the transmission channel environment;
filtering of the information carrier in the transmission channel — a method consisting of not passing a signal with confidential speech information through the channel;
masking of the information carrier in the transmission channel — a method consisting of hiding it by adding a special masking signal;
noise pollution of the transmission channel environment — an active method consisting of creating artificial interference and noise at acoustic frequencies.

Each method has its own advantages and disadvantages. In particular, the last type of counteraction in relation to others is most effective when a quick, short-term solution to the problem of protecting confidential negotiations is required by simple, effective methods.

There are known methods for neutralizing the impact of acoustic fields on an optical cable by means of a special sound-insulating fiber and cable sheath, which reduces the impact of vibrations and sound on the parameters of light in fiber-optic communication lines. For example, cable manufacturers require optical cable resistance to vibration loads with acceleration up to 40 m/s2 in the frequency range from 10 to 200 Hz [8]. However, this does not provide complete protection against the acoustic-fiber-optic channel of speech information leakage, which is associated with the possibility of creating acoustic contact with the cable both unintentionally during installation and operation, and intentionally by an intruder.

There are known methods for neutralizing the local influence of acoustic fields on an optical cable by including special equipment in the fiber-optic communication line that restores the parameters of light pulses in it — repeaters, signal regenerators. For example, an optoelectronic decoupling device for 1 port SC ORU-1 or for 2 ports SC ORU-2 manufactured by FSUE KBPM (Moscow) [9]. The operating principle of the device is associated with the conversion of an optical signal into an electrical signal and subsequent reverse conversion into an optical signal cleared of noise, including that of an acoustic nature. In addition to special equipment, any active fiber-optic equipment in the network inevitably destroys the acousto-fiber-optic leakage channel, since the original digital modulation is restored in it, and noise effects disappear. But the use of special and/or active network equipment itself carries the risk of leakage formation, which requires special maintenance, placement near the protected room in a special cabinet, scheduled functionality checks, etc. In addition, such equipment requires special protection against leakage of side electromagnetic radiation and interference, which is associated with the electronic components of the equipment, with the electrical power supply circuits from the network.

There are known methods and devices for neutralizing acoustic channels of speech information leakage through various waveguide information transmission channels, for example, in telephone subscriber communication lines, electrical networks, other electrical cable systems, by acoustically polluting the environment near the line or the line itself. For example, the noise generator for the power supply network and grounding lines «Sonata-RS1» (ZAO «ANNA», Moscow) [10], designed for active protection against information leakage in the form of informative electrical signals arising in the power supply network, grounding system, utilities, etc.

However, such methods of counteracting eavesdropping for fiber-optic channels of information transmission are not known, which is due to the novelty of the problem. In this paper, one of the methods of protecting speech information is proposed based on adding an optical signal of the noise spectrum to the information signal.

Experiment on active protection of speech information from leakage through an acoustic-fiber-optic channel

The essence of the proposed technical solution for protection is that in order to neutralize the acoustic-fiber-optic leakage channel, optical radiation at acoustic frequencies with a noise or other special spectrum is introduced into the standard fiber-optic information transmission channel, which noises/masks the information light flow in the network. Thus, noise is noised/masked for any acoustic (speech) information that can be unauthorizedly transmitted over fiber-optic communications along with the information network traffic or instead of it using an external light source. The efficiency of the leakage channel can be controlled by the articulation method, which consists of determining the intelligibility of the transmitted speech. In the simplest case, intelligibility W is determined by the ratio of words correctly understood by the operator from the total number of transmitted test words, expressed as a percentage [1,11].

An experimental study of the influence of optical noise pollution of a fiber-optic communication channel on the efficiency of leakage channel formation was conducted on a created laboratory research stand, which includes the following elements (Fig. 1):

a communication line in the form of a dual multimode optical cable more than 30 m long together with an additional cable that closes the line in a ring;
a fiber-optic tester-phone «Rubin-021» with analog light modulation is placed at the end of the cable;
instead of a microphone, a laptop is connected to the tester via the linear output of the audio card, which plays recordings of test speech signals.

Fig. 1. Experimental setup for studying active protection of speech information from leakage via an acoustic-fiber-optic channel using the method of optical noise pollution of the communication channel.
1 — fiber-optic tester-phone «Rubin-021», 2 — dual fiber-optic cable,
3 — fiber-optic coupler,
4 — device for inputting an external noise optical signal from an MP3 player with audio files of various noise inputs via the tester-phone «Rubin-021»,
5 — headset (headphones) for speech intelligibility monitoring,
6 — computer for speech input.

Fig. 2. Results of an experimental study of the efficiency of an acoustic-fiber-optic
channel for speech information leakage based on the dependence of intelligibility W of test speech on the relative noise level SNR.

The light flux at the output of the tester-phone is modulated by the speech signal and, propagating along the fiber-optic cable ring, returns back to the tester-phone, where it is demodulated and listened to by the operator through headphones. The level of light modulation depth is adjusted programmatically on the laptop from 0 to 100% value. In order not to destroy the microphone amplifier of the tester-phone, the connection was made through dividers 1:10 and 1:100. The operator hears through the headphones a clear sound, which is not polluted by external noise or extraneous sounds and has an intelligibility of 100%. Although internal noises of the transmission line are recorded during listening, they do not affect the intelligibility of speech. The sound is almost clear when using a laser at a wavelength of 1310 nm, at a wavelength of 1550 nm minor noise appears.At the point of optical short-circuiting of the cable, a fiber-optic coupler is connected so that the external light introduced into the cable is distributed towards the photoreceiver detector of the tester-phone and is superimposed on the useful speech signal. To introduce the external light noise signal, a second tester-phone «Rubin-021» was used, to the microphone input of which a noise signal from a flash player was fed. The type of noise signal was selected from a set of noises previously recorded in it. The characteristics of this channel are similar to the main channel.

The experiment consisted of recording speech intelligibility W (in %) of a speech signal transmitted via an optical cable, depending on its level LV in dB and on the optical noise level LN in dB. The absolute values of the speech and noise signal levels are not important for the experiment, since only their difference in dB affects speech intelligibility. Thus, the signal-to-noise ratio SNR=(LV-LN). The values of the sound pressures of the signal and noise were recorded by a sound level meter by measuring the sound levels generated in the headphones from the speech and noise signals separately.

The objectivity of speech intelligibility measurement W is achieved by computer-generated speech generation based on a standard sound engine. In this case, the identity of speech in all main parameters can be considered very high. The use of various combinations of test words does not allow the operator to guess the spoken words, which increases the objectivity of calculating W. The accuracy is affected by the fact that word recognition by ear was performed by only one operator, whose objectivity was recorded by comparison with other independent operators. Our estimates of the accuracy of the measurement are about 5%.

The results of the experimental studies are presented in Fig. 2. The dependence W(SNR) has a standard form, observed in acoustically noisy communication channels. At the value of the speech signal-to-noise ratio SNR<0, speech intelligibility is less than 50% and quickly decreases as it decreases to the point of incomprehensibility, and at SNR>0, we have well-understood speech with intelligibility above 50%. The value of the noise level SNR=0 is a boundary value, allowing us to estimate the necessary depth of noise modulation to neutralize the leakage channel, which is consistent with the estimates for other types of leakage channels.

The obtained experimental results can be used in the development of devices for protecting speech information from leakage via an acoustic-fiber-optic channel.

Analysis of experimental studies and recommendations for their use

Prevention of eavesdropping via fiber-optic communications by introducing additional radiation into the fiber-optic channel is implemented using special devices. A possible design of such a device is presented in the form of a generalized block diagram in Fig. 3. The protection device is a broadband light generator with a noise modulation spectrum at the fiber-optic output, which is implemented on the basis of standard or specially created elements. As a light source, it is necessary to use a generator of a broadband radiation spectrum modulated by a special noise signal at acoustic frequencies. The use of a broadband light source allows creating noise/masking of the fiber-optic channel over the entire spectral range and excludes the use of special narrowband light sources to form a leakage channel. In fact, the use of external narrow-band light sources in the leak channel is a dangerous way to eavesdrop, since it allows you to get a high signal-to-noise ratio by installing selective optical filters that cut off all other emissions in the communication channel.

Fig. 3. Generalized block diagram of the method and device for optical noise pollution of a fiber-optic information transmission channel.
1 — a broadband light generator with a noise modulation spectrum at the fiber-optic output,
2 — broadband light source, 3 — generator of electric signal at audio frequencies (player),
4 — fiber-optic coupler.

The device is connected to the fiber-optic channel of the standard system at the detachable connection points and can operate on a permanent basis or only during confidential meetings and negotiations. When switched off, the device should not affect the operation of the communication system. When switched on, the protection device, depending on the operating modes, can either completely neutralize the operation of the communication system when the intensity of the external optical noise signal exceeds the intensity of other signals (information) in the communication channel, or have no effect on the operation when the intensity of the noise signal is insignificant in comparison with the same signals.

Radiation in an optical cable can be introduced into the optical fiber of standard systems at the places of detachable connections using standard connecting adapters or without breaking the fiber using a special device for input/output of an optical signal on a microbend. The simplest input is using a standard optical splitter of the 2->1 format. The generator is connected to one of the 2 inputs. The direction in which it is necessary to produce interference is selected by connecting the remaining connectors of the splitter that close the communication line. For example, turning on the splitter (Fig. 3) allows protection against eavesdropping in the direction to the right of the device, by which the light from the generator is distributed. Radiation can also be introduced into an optical fiber and output back using radiation input/output devices on a bend, such as RNOTOM 550 (Hakuto Co., Ltd., Japan) [12], FOD 5503 (KB Volokonno-Opticheskikh Instrumenty, Russia) [13] or an input/output device for optical power without fiber breakage FCD-10B from EXFO (Canada) [14]. However, input/output devices have limited efficiency in terms of wavelengths and fiber coating color, which limits their practical use.

Conclusion

The paper discusses a method for optical jamming of a communication channel and the practical operation of a device to prevent eavesdropping through an acoustic-optical fiber channel for leakage of speech information. High efficiency of the method is shown when implemented correctly, and disadvantages associated with the need for jamming over a wide optical spectrum are also noted. The work is a development of the Application for Invention [15].

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