Means of protection of low-current lines.

Means of protection of low-current lines.

Eliseev Andrey Andreevich
MEANS OF PROTECTION OF LOW-CURRENT LINES

Source: magazine «Special Equipment»

When discussing confidential information in any premises, there is always a possibility of its leakage due to the conversion of sound pressure into electrical signals by various elements of technical means located in this room. These are, as a rule, analog and digital telephone sets of administrative and city PBXs, loudspeakers of the warning system, three-program receivers, secondary electric clocks, etc.

Electrical signals of speech information through the circuits of the specified technical means extend beyond the premises and can be received at significant distances.

Reception can be carried out either by directly listening to them at the output of a low-frequency amplifier, or by high-frequency probing through the specified circuits with subsequent detection.

The article examines the existing level of information protection against leakage through the above-mentioned channels and formulates recommendations for its improvement.

Note that for analog and digital telephones, the protection means (PM) that perform their functions in call waiting mode are considered.

Passive and active means of protection are used to block this leakage channel. The first samples used either passive or active means of protection. For example, purely passive means of protection (PPP) are the products «Granit-8M», «Korund», and active means of protection (APP) are «Granit-12». Both PSZ and ASZ have their own characteristic advantages and disadvantages.

The advantages of PSZ are:

1. Relative simplicity of the electrical circuit and small dimensions.
2. PSZ does not require power sources.
3. PSZ are included in the break of the circuits of technical means of information transmission (TMI) and, therefore, the failure of some elements of the electrical circuit is detected during operation.
4. Relatively low cost.

The disadvantages of PSZ are:

1. The lack of operational control of proper operation leads to the need to carry out routine maintenance, which requires disconnecting the PSS and using complex and expensive measuring equipment in the measurement process. As a result, the costs of these works, taking into account the payment of technical personnel, are many times higher than the cost of the PSS itself, and, consequently, the cost of protecting information from leakage increases sharply.
2. In the interval between scheduled maintenance, there is a risk of deliberately bypassing the PSZ (short-circuiting) or replacing it with an externally indistinguishable dummy.
3. The possibility of a sharp decrease in its special properties with active methods of influence, for example, when a high-frequency signal is applied to it.

The advantages of the ASZ are:

1. The possibility of constructing an ASZ with a circuit for built-in continuous monitoring of its operability, which sharply reduces or completely eliminates operating costs.
2. Possibility of constructing an ASZ that is resistant to active impacts, in which its protective properties are fully preserved.
3. Impossibility of implementing an undetectable bypass, blocking or reduction of the protective properties of the ASZ.

The disadvantages of the ASZ are:

1. Relative complexity of the electrical circuit.
2. To prevent the operation of the APS from being blocked by breaking the power supply circuit, it is necessary to have a guaranteed power source or a backup battery in the APS, the capacity of which should be sufficient for several days of continuous operation.
3. Higher cost compared to the PSZ.

An analysis of the advantages and disadvantages of the PSZ and APS shows that the optimal means of protection can be built on the basis of their combination. In addition, to guarantee the exclusion of information leakage, it is very useful to have two lines of defense.

The following requirements should be imposed on combined SZ:

1. As part of a combined SIS, it is advisable to use a PSZ with detectable failures and an ASZ with a circuit for signaling correct operation and a guaranteed power source.
2. PSZ should be included in the break of the TSPI circuits, and the ASZ — in parallel to them.
3. Prospective SIS should have, as a rule, two lines of information protection against leakage, representing an optimal combination of ASZ and PSZ in order to completely eliminate or maximally complicate its interception. The costs of intercepting information should be economically unprofitable in comparison with the costs of its protection.
4. The cost of information protection should be determined primarily by the cost of the information protection system, and not by the cost of its operation.

The optimal solution is to have no need for any preventive maintenance or checks during the entire service life of the information protection system.

The table shows some parameters of existing and prospective information protection systems for analog and digital telephones, single- and three-program receivers, public address system loudspeakers, and secondary clocks.

The first three products have been used to protect information for a long time, and the products “MP-1A”, “MP-1C”, “MP-2”, “MP-3”, “MP-5”, “Gran-300” — since 1997, 1998.

Comparison table.

The table shows that the products “Korund”, “Granit-8M”, “Buket”, “Gran-300”, “MP-3” and “MP-5” are PSZ. The product “Korund” is the simplest PSZ, the only function of which is to protect information from leakage in the presence of the acoustoelectric effect in analog two-wire TA. The operating principle is to suppress the information signal by > 80 dB using a diode-capacitive divider.

The products “Granit-8M”, “Buket” and “Gran-300” are analogs. They simultaneously perform the functions of the product “Korund” and introduce attenuation for high-frequency signals in the band of 30 kHz — 30 MHz > 70 dB.

The “Gran-300” product in call waiting mode at frequencies of 300 Hz – 32 MHz introduces attenuation into the acoustoelectric signal of at least 60 dB and additionally performs blocking of the telephone set if the handset of any parallel telephone set was previously picked up.

The MP-3 product eliminates information leakage along the TSPI power supply circuit when exposed to acoustic impact and the supply voltage is disconnected. Note that, unlike the products discussed above, which provide the necessary attenuation for the information signal using only one circuit, the MP-3 simultaneously implements both a power supply circuit break using relay contacts and attenuation using a diode-capacitive circuit, which together provides attenuation at a frequency of f =1 kHz more than 90 dB. The interval of scheduled maintenance is determined only by the probability of abnormal closure of the relay contacts. All other faults are detectable.

Photo 1. Power supply network protection device “MP-3”

The MP-5 product is designed to protect public address system loudspeakers or single-program receivers from leakage of acoustic signals from the room. In the absence of warning signals (or broadcast signals), the loudspeaker is switched off using relay contacts. When a standard signal appears after t <5 ms, the loudspeaker switches on and this state is maintained if t pause <10 s. With these parameters, the MP-5 product does not affect the quality of the message. With the loudspeaker switched off, the acoustoelectric signal measured at a frequency of f =1 kHz, before entering the broadcast line, it undergoes attenuation of < 90 dB, which ensures the exclusion of information leakage from the room along the broadcast chain.

Photo 2. Device for protecting the loudspeaker of the warning system “MP-5”

Product “MP-4” is an ASZ, and products “MP-1A”, “MP-1C”, “MP-2”, “MP-6” and “MP-7” include both ASZ and PSZ.

The MP-4 product is a masking interference generator with an optical operation control circuit that operates every 2 minutes and indicates proper operation by briefly lighting the LED. The circuit is very economical. The continuous operation time from a Krona battery is > 10,000 h. Operating costs — battery replacement once a year.

The MP-1A and MP-1C products protect information from leakage from analog and digital telephones in call waiting mode, respectively. They simultaneously use both PSZ and ASZ. The PSZ are built on the principle of the Granit-8 product, and the ASZ — on the principle of the Granit-11 and Granit-12 products. The essential thing about the MP-1A and MP1-1C products is that, while surpassing the specified Granit-type products in all special parameters, they are an order of magnitude or more better in terms of weight and size characteristics and power consumption, which allows them to be placed inside telephone sockets of various types. Operating costs for the MP-1A product are not required, and for the MP-1C product they are reduced to periodic audio monitoring of the presence of MP in the subscriber line.

Photo 3. Product “MP-1A” in a Russian case

Photo 4. Product “MP-1C” in a European socket without a cover

The “MP-2” product includes a PSZ (disconnection of a three-program receiver from a broadcast line using relay contacts) and an ASZ (MP generator) to create a second line of defense. Operating costs are periodic monitoring of the MP level and shape in the control socket.

Photo 5. The “MP-2” product board

The MP-6 product is a development of the SZ based on the MP-1A and MP-1C. It is universal and can be used to protect both analog and digital telephones. During the normal operation of the MP-6 product from the subscriber line, the backup power source (Krona battery) is not used. If the subscriber line is broken, the product automatically switches to power from the Krona battery. The continuous operation time from the Krona battery is > 1500 h. The presence of an optical control circuit for the MP and the state of the battery discharge reduces operating costs to almost zero, since the battery life reaches 5 years.

The MP-7 product is in the final stage of development. It is based on the MP-6 product with the addition of a function for protecting loudspeaker circuits of the TA.

The recommendations presented are of a general nature and can be useful in the development of PSZ and ASZ and for other types of TSPI.