Information protection devices for VHF radio communications. Scramblers.

#Scramblers, #scrambler, #ultrasound

Information security devices for VHF radio communications.

Ovchinnikov A.M., Lazin A.S.

Source: Special Equipment» magazine

The article discusses information security devices — scramblers, which can be used in VHF radio communications.

Possible principles of their implementation and main parameters are described.

Specific models of scramblers are considered and their technical characteristics are given.

Many security services and various security structures use VHF radio communications in their work to organize dispatch communications between employees when guarding facilities, escorting vehicles, holding any events, etc.

In this case, there is often a need to ensure the protection of information transmitted in local radio networks.

There are two main methods of closing speech signals: analog scrambling and speech sampling with subsequent encryption.

Scramblers

Analog scramblers convert the original speech signal (without converting it into digital form) in such a way that when listening to the channel using a radio station not equipped with such a device, significant difficulties are created for the intelligibility of the transmitted information.

In this case, when scrambling, the converted speech signal, possessing the properties of unintelligibility and unrecognizability, occupies the same frequency band of the spectrum as the original signal.

In digital information security devices (ultrasound), speech components are converted into a digital data stream and mixed with some pseudo-random sequence generated by a key generator.

The closed voice message obtained in this way is transmitted to the communication channel, at the receiving end of which the reverse transformations are performed in order to obtain an open speech signal.

The main technical characteristics of scramblers that must be assessed when choosing a specific type of information protection device are the level of information closure, residual intelligibility, quality of signal recovery, impact on radio station parameters, level of technical performance (dimensions, consumption, possibility of installation in various types of stations, etc.).

The most important characteristic of a scrambler for a user who wants to ensure the protection of information in their communication channels is the level of information closure.

It should be noted that this concept is conditional, since to date no clear standards or rules have been developed on this matter.

At the same time, all ultrasound can be conditionally divided into means of protecting information from unintentional interception (i.e. from wiretapping by persons who do not use special means of intercepting other people's conversations) and means of protecting information from unauthorized access (i.e. from wiretapping by potential competitors whose goal is precisely to intercept other people's conversations, and who are equipped with special technical means for this purpose).

Naturally, the means of protecting information from unauthorized access, in turn, can be divided into several levels, providing different levels of closing conversations depending on the degree of equipment of the competitor.

In the literature, there is often a classification of ultrasound by the time it takes to decrypt information after it is recorded.

However, such a classification is more applicable to complex cryptographic systems for military purposes, rather than to VHF radio networks, where the value of the information being transmitted is not so great, and the danger is usually only the interception of conversations in real time.

Therefore, it is more appropriate to use the cost of technical means that ensure the interception of messages in real time as a criterion for the level of information closure for VHF radio communications.

Residual intelligibility is understood as the degree of possibility of restoring a speech message when listening to it using technical means that are not equipped with this type of ultrasound.

To some extent, residual intelligibility is an integral part of the information closure level characteristic, however, residual intelligibility characterizes the degree of unintelligibility of messages when listening to them without the use of special means, and the information closure level shows the security of negotiations and during deliberate interception using special equipment.

Residual intelligibility can be quantitatively assessed by the percentage of restored fragments of the message when listening to negotiations using radio equipment that is not equipped with this type of ultrasound.

(Often, the requirements for residual intelligibility are specified as the maximum permissible speech intelligibility class according to GOST 16600-72.)

Naturally, the highest level of information protection is provided by scramblers with minimal residual intelligibility.

It should be noted, however, that almost all known analog speech scramblers retain residual intelligibility to some extent.

The speech signal being listened to, protected by a scrambler, retains information about the speech rate, and pauses are captured. With simple protection methods, an experienced operator can make out (depending on the availability of information about the topic of the ongoing negotiations) from 10 to 50% of the transmitted information.

The quality of signal restoration is the most important operational characteristic of scramblers.

It is determined by signal distortions during its frequency or time transformations.

It can be expressed by intelligibility (not to be confused with residual intelligibility!) and recognizability of the restored speech.

Acceptable or commercial quality of speech reconstructed at the receiving end is considered to be such that the listener can easily identify the speaker's voice and the meaning of the spoken message.

An accurate quantitative assessment of speech intelligibility can be obtained by measuring the percentage of correctly transmitted test messages during lengthy and labor-intensive articulation tests.

The influence of scramblers on the parameters of radio stations is manifested, first of all, in the deterioration of their sensitivity due to a decrease in the signal-to-noise ratio at the receiver input.

In addition, during signal conversions, associated with changing any of the conversion parameters over time, a certain time interval is required for synchronization of the timers of the transmitting and receiving sides. This forces the operator to pause between pressing the “transmit” push-to-talk button on the radio station and the start of speech.

As a rule, a decrease in residual intelligibility is accompanied by a deterioration in both the quality of signal recovery and the parameters of radio stations. This is explained by the fact that, wishing to increase the degree of information protection, developers resort to various tricks that lead to additional distortions of either the spectrum or the time parameters of the signal.

For users of VHF radio communications, the level of technical performance of scramblers is extremely important.

Since scramblers are structurally small microelectronic units that are installed inside radio station housing, it is preferable to choose analog ultrasound with minimal dimensions and power consumption.

Naturally, minimizing dimensions allows to expand the applicability of scramblers, since it is possible to install them in a larger number of radio equipment.

Most security structures are equipped with professional VHF radio communication equipment from foreign companies such as Motorola, Kenwood, Yaesu, Standard, Alinco, Icom, etc., using analog types of signal modulation (frequency or phase).

For this type of radio equipment, in the overwhelming majority of cases, analog speech scramblers are used as information security devices.

Signal conversion in analog scramblers is carried out in the spectral or time domain.

For small-sized scramblers used in VHF radio stations, the following types of signal conversion are most often used:

— frequency inversion of the signal;

— splitting the speech signal frequency band into several sub-ranges (usually two) and frequency inversion of the spectrum in each relative to the average frequency of the sub-range;

— splitting the speech signal frequency band into several sub-ranges (usually four) and their frequency permutations;

— splitting the signal into speech segments and their permutations in time.

In terms of their operating mode, analog ultrasound systems can be either static systems, in which the signal conversion parameters remain unchanged throughout the entire transmission of a speech message, or dynamic systems, in which the conversion parameters change over time (for example, changing the frequency of the speech signal band splitting with a certain periodicity).

Frequency inversion is the simplest type of analog scrambling.

In such a scrambler (frequency inverter), the speech spectrum is transformed, equivalent to rotating the frequency band of the speech signal around a certain midpoint, which acts as a certain key of the system.

This achieves the effect of transforming low frequencies into high ones, and vice versa. This method ensures a low level of information closure, since when intercepting, the frequency value corresponding to the midpoint of the inversion in the spectrum band of the speech signal is easily established.

In practice, to intercept conversations conducted using radio equipment equipped with a frequency inverter, it is enough to have a similar radio station with the ability to select the inversion frequency ($500-1000).

In addition, it should be noted that the residual intelligibility of scramblers with frequency inversion is quite high. At the same time, frequency inverters have the maximum quality of signal restoration and practically do not worsen the intelligibility and recognizability of speech.

A scrambler with dividing the speech signal band into sub-ranges with frequency inversion of the signal provides some increase in the level of information closure.in each sub-range (band-shift inverter).

In this case, the split point acts as the system key. However, this method of analog scrambling does not provide reliable closure and has a fairly high residual intelligibility.

Just as in the case of frequency inversion, to intercept conversations, it is enough to have a radio station equipped with a similar scrambler, with the ability to select the signal split frequency.

Band scramblers that use the method of dividing the speech signal band into several sub-bands with frequency permutations of these sub-bands have approximately the same characteristics in terms of the level of information closure and residual intelligibility as band-shift inverters.

In this case, the system key is the code combination of the frequency band permutation.

An additional increase in the level of information closure is provided by changing the parameters of signal transformation in time (dynamic scramblers).

For methods of frequency signal conversion, such key parameters can be the inversion frequency (for a frequency inverter), the frequency of signal band splitting (for a band-shift inverter), a combination of frequency permutation of signal sub-ranges (for a band-scrambler).

In this case, the level of protection is determined by the number of gradations of the signal parameter, the length of the key, i.e. the number of possible combinations of the parameter, the rate of change of the parameter.

Theoretically, to intercept messages in real time in communication channels protected by scramblers with time-varying conversion parameters, it is necessary to use special technical means that allow one to first determine the key sequence (i.e., the rules for changing the signal conversion parameters), and then adapt to the found key sequence.

At the same time, for many specific types of scramblers with dynamically changing parameters, interception of conversations can be carried out using simpler methods.

Dynamic scramblers are usually significantly more expensive than scramblers with fixed signal conversion parameters, have a greater impact on the characteristics of radio equipment, and require initial synchronization, which limits their use for some applications.

Along with frequency, analog scramblers can use time signal conversions.

The simplest type of time transformation is time inversion, in which the original signal is divided into a sequence of time segments and each of them is transmitted inversely in time — from the end to the beginning.

It has been shown that in order to achieve unintelligibility of slow speech, the segment length must be about 250 ms. This means that the delay in the communication channel will be 500 ms, which may be unacceptable in some cases.

To increase the level of closure, a method of permutation of time segments of the speech signal within a fixed frame is used.

The permutation rule is the key to the system, by changing which the degree of speech closure can be significantly increased.

Residual intelligibility depends on the duration of the signal segments and the frame and decreases with an increase in the latter.

A decrease in the delay time can be obtained by using a scrambler with permutation of time segments with a sliding window.

In it, the number of combinations of possible permutations is limited in such a way that the delay of any segment does not exceed the established maximum value.

Each segment of the original speech signal has a time window, within which it can occupy an arbitrary place during scrambling.

This window slides in time as each new segment of the signal enters it. The delay is reduced to the duration of the window.

It should be noted that there are other more complex types of analog scrambling, such as fast Fourier transform (FFT), various combinations of time and frequency transformation, etc., however they have not yet found wide application in VHF radio communications, due to the complexity of their hardware implementation in acceptable dimensions and with low power consumption.

Ultrasound devices are produced both by manufacturers of VHF radio communication equipment for radio stations of their own manufacture (Kenwood, Standard, etc.), and by companies specializing in the production of scramblers for short-range VHF radio communication equipment.

On the domestic market of ultrasound devices, the latter are represented by such companies as Transcrypt, Selectone, Midian, etc.

In addition, domestic information protection tools designed for joint operation with imported radio stations are also known.

Let's present the main, most well-known models.

The largest number of known models of scramblers implement frequency signal inversion. They all have similar parameters.

Among the first models of scramblers to appear on the domestic market were those by Selectone, such as the ST-20 and the later model ST-022.

ST-20 operates in the frequency range of 300-2400 Hz and provides signal inversion relative to 8 possible frequency values ​​in the range from 2.6 to 3.7 kHz (the inversion frequency is set by software).

The supply voltage range is from 5.2 to 18 V, the consumption current is less than 4 mA.

The operating temperature range is from minus 30 to +70 C.

Overall dimensions – 20.96×38.20×3.81 mm.

The ST-022 model differs from the ST-20 in its extended frequency range (up to 3000 Hz) and supply voltage (up to 24 V), as well as slightly smaller dimensions (20x25x4 mm).

The cost of Selectone scramblers is within $40-60 (2010).

The simplest models of Transcrypt scramblers SC20-400 and SC20-401 have characteristics similar to the ST-20 and ST-022;

Speech frequency range, 4 inversion frequency options, supply voltage — from 5 to 12 V, current consumption — 3 mA, operating temperature range — from minus 20 to +60 C, overall dimensions — 39x21x4 mm.

The cost of scramblers is about $50 (2010).

The family of frequency inverters from Midian provides the user with a choice of a specific model for use in their radio equipment.

VPU-1 and VPU-8 are scramblers of different dimensions for use in duplex radio stations (depending on the modification, they have different inversion frequencies);

VPU-2 (15 programmable inversion frequencies) and VPU-7 (one fixed inversion frequency) are miniature scramblers that provide only simplex operation.

The cost of scramblers is from $ 35 to 70 (2010), their parameters are presented in Table 1

Kenwood produces the KVS-1 scrambler for use with its radio stations. Its parameters are also presented in Table 1.

The cost of the scrambler is $ 80 (2010).

A similar scrambler of the RSC-32 brand is produced by the Russian Scientific and Technical Center “INTER-VOK”.

Table 1.

More complex signal conversion is offered by band-shift inverters developed by the Scientific and Technical Center “INTER-VOK”.

The operating principle of the microassemblies 04ХК011 (“Sonet”), 04ХК012, 04ХК014А, 04ХК015А, 04ХК017А consists in dividing the speech spectrum into two parts, low-frequency and high-frequency, each of which unfolds around its own mid-frequencies.

All of them operate in the speech frequency range of 300-3400 Hz, have small dimensions and low current consumption (within a few mA).

It is possible to change the frequency of the speech signal band division 04ХК014 — 1 gradation, 04ХК015 — 2, 04ХК016А — 3, 04ХК017 and 04ХК018А — 4, 04ХК011 and 04ХК012 — 32 gradations.

All microassemblies operate from a supply voltage of +5 V, except for 04ХК011, which ensures operability in the range from 6 to 13 V

The specified scramblers have a higher degree of information closure compared to frequency inverters.

The technical data indicate that the scramblers provide residual speech intelligibility (i.e. intelligibility when listening to third parties) of no more than 10%.

At the same time, high speech quality is guaranteed when listening to a radio station equipped with a similar scrambler (maintaining class 1 intelligibility when measured using the GOST 16600-72 method).

Scramblers have very small dimensions (minimum overall dimensions — 15x15x6.5 mm — micro assembly 04ХК018А) and are installed inside the case of almost any radio station.

The cost of scramblers, including the cost of installing them in a radio station, ranges from $50 to $80 (2010).

The most famous representative of the class of bandpass scramblers in Russia is the CVS-240 scrambler of the Standard brand (trade mark of the Marantz concern), which is installed in radio equipment produced by this manufacturer.

Its operating principle is to divide the speech signal band into 4 sub-bands and to rearrange these sub-bands. The rearrangement code is set using jumpers.

According to subjective assessments (no precise tests were conducted), this scrambler provides a degree of information closure comparable to band-shift inverters.

At the same time, when installing the scrambler in a radio station of the HX240 type of the Standard brand, a sharp decrease in speech intelligibility and recognition was obtained.

The cost of the scrambler is $ 170 (price in 2010).

A wide range of dynamic scramblers is produced by Tramscrypt.

Their operating principle is similar to frequency inverters with a change in the signal inversion frequency over time. The number of possible inversion frequencies is 16.

Different models of scramblers from this company differ in the rate of change of the variable parameter: from 1 time per second for SC20-406J and SC20-410 to 1000 times per second for SC20-460 and SC20-500.

The scrambler parameters are presented in Table 2.

In addition to the actual signal conversion function, Transcrypt modules provide additional functions such as selective and emergency calling, selective access, and the ability to remotely control the radio station via a radio channel.

These modules are installed in radio stations of various companies, in particular Motorola Standard Johnson.

The scramblers are guaranteed to operate in the operating temperature range of -30…+60 C.

Table 2

There is an opinion about the high level of information secrecy provided by these scramblers, which makes radio users buy modules from Transcrypt, despite their high cost.

However, the validity of this opinion seems, to put it mildly, exaggerated.

Despite the large length of the key sequence (pseudo-random sequence of changing the frequency of signal band splitting), the actual number of values ​​of the variable parameter is small (16) and does not provide reliable closure.

As studies of radio equipment equipped with scramblers have shown, the SC20-410 model can make out from 50 to 80% of the transmitted information when listening to conversations using a radio station with an installed frequency inverter with a fixed signal inversion frequency.

The operating principle of scramblers requires synchronization of transmitting and receiving radio equipment, which requires the operator to pause after pressing the “transmit” push-to-talk button on the radio station.

For modules from Transcrypt, the synchronization time is 300-500 ms

A similar family of dynamic scramblers MOT-TVS/VPU-10 is produced by Midian.

The parameters of the VPU-10 scrambler are similar to the parameters of the SC20-406J and SC20-410, i.e. the inversion frequency is switched once per second.

TVS-1 and TVS-2 scramblers have high speed (exact data on the frequency of switching the inversion frequency is not available), and also provide additional service functions similar to the scramblers of the Transcrypt company.

More detailed information on the technical parameters of the dynamic scramblers of the Midian company is not available

Time conversions are carried out by a scrambler developed by the Research Institute of Microelectronic Equipment (NIIMA) «Progress» (symbol Ml).

It is made in the form of an unpackaged micromodule on a ceramic substrate using specialized LSI delta encoder and delta decoder, encryption device and memory.

The speech signal arriving at the input is converted by the adaptive delta codec encoder into digital form and recorded in memory as a sequence of speech segments.

A frame is formed from 32 segments.

In accordance with the recorded code, the encryption device creates a pseudo-random sequence of permutations of these segments in the frame.

After time transformations, the adaptive delta codec decoder restores the original speech signal within each segment at the scrambler output.

In the reception mode, frame synchronization and inverse transformations are performed to restore the original signal.

The use of specialized integrated circuits allows for satisfactory quality of speech signal restoration and low (3-5%) residual intelligibility with a power consumption of 75 mW. The overall dimensions of the scrambler are 42x15x3 mm.

As can be seen from the review, users of VHF radio communications have a wide choice of information protection tools for their radio stations and local radio networks.

You can purchase analog scramblers from companies that supply mobile radio communications.

Specialists from these companies will suggest a scrambler model and install it in your radio stations.

At the same time, we would still like to recommend that those wishing to purchase VHF radio communications equipment that provides information security approach the problem in a comprehensive manner when purchasing radio equipment, that is, at the initial stage, determine the requirements for the level of information security so as not to revise their approach to this issue during the operation of radio equipment, which is usually more difficult and expensive.

When purchasing a large batch of radio communications equipment, it is advisable to consult with independent specialists (and not with a company interested in selling, first of all, its own products).

In addition, as can be seen from the review of the presented models, it should be taken into account that analog scramblers will never provide protection against intentional eavesdropping on your conversations.

At the same time, the financial costs of those planning to intercept your conversations will not be an obstacle for them. For reliable protection of the transmitted information, it is necessary to use radio stations with digital signal transmission with a specific masking algorithm.

Such protection can be ensured by using significantly more expensive digital radio stations or by using digital ultrasound for analog radio equipment, which is being developed by a number of organizations.

Literature

1. S.V. Dvoryankin, D.V. Devochkin «Methods of Closing Speech Signals in Telephone Channels» «Confident», No. 5 July-September 1995

2. V. Mochkin «Microelectronic Devices for Protecting Speech Information for Radio Stations and Telephones», CHIP NEWS, No. 5 b 1997

3. V.V. Fomin, V.N. Dudnik, V.E. Lepin, T.V. Bateneva, M.S. Podlubny «Method of encoding speech signals for radio and telephone communication devices» — Sat. «Radio Communication Engineering», issue 3 1997r