Confidentiality of wireless calls

Since wireless intercoms transmit voice information in a wide space, anyone can listen to the conversation between intercom users without permission with the help of appropriate technical means. Whether intentional or unintentional, the content of the call may be eavesdropped and leaked. The consequences depend on the identity of the caller and the content of the call. For the vast majority of intercom users, the content of the call does not care about being eavesdropped, and there will be no adverse consequences even if the content of the call is leaked. Therefore, most intercom users do not require the intercom to have a confidentiality function. Generally, it is just to prevent interference and set up sub-audio and digital sub-audio. However, for some users who do not want others to know what they are talking about when talking about work content and personal privacy on the intercom, they hope that the intercom should have a confidentiality function. These users, such as government, public security, and commercial intercoms, have certain requirements for communication confidentiality. There are also some special users such as security departments and the military who have higher requirements for communication confidentiality. Different confidentiality requirements for call confidentiality vary according to the nature of the work, and the confidentiality settings and performance are different for different requirements. Different confidentiality equipment is used, and the cost is also different. What level of confidentiality depends on the needs of the user, the content of the call user, and the ability of the eavesdropper.
Most walkie-talkies now have privacy functions such as CTCSS and DCS. Generally speaking, when these privacy functions are set on the walkie-talkie, the content of the conversation cannot be heard without knowing the private line code used. However, this is not a true confidentiality function. Because its confidentiality function is very limited, it is only useful for unintentional eavesdroppers, but useless for intentional eavesdroppers. Users who need confidentiality requirements cannot rely on "private" lines to keep confidentiality.

The voice confidentiality we are talking about here is to disguise the call information so that unauthorized people cannot understand the content of the call. The disguised voice is called plain language, the process of disguising the voice information is called voice encryption, and the encrypted voice information is called secret language. The set of rules and procedures used by the encryptor is called encryption algorithm, and the algorithm is usually controlled by a set of keys. The receiver is called the recipient, who uses the key to use the opposite algorithm to encrypt and restore the secret language to understand the plain language. This process is called decryption or decryption. The process of eavesdroppers deducing the plain language from the secret language is called decryption. The eavesdropper must decrypt the secret language to understand the plain language. In the intercom voice encryption, the voice information of the call is processed to destroy its characteristics and reduce its intelligibility. After the intercom adopts the confidentiality function, almost all confidentiality settings will affect the voice quality to varying degrees. Therefore, when choosing the confidentiality level, pay attention to this issue and find the best balance between confidentiality and voice quality. Most confidentiality devices encrypt and then decrypt the voice signal.

It is very sensitive to channel distortion, interference and background noise. When selecting confidentiality equipment, cost factors should also be considered. The higher the confidentiality function, the higher the cost of the confidentiality equipment. Therefore, when selecting confidentiality equipment, multiple factors such as voice quality, confidentiality level and confidentiality equipment cost should be considered comprehensively. The voice confidentiality technology of walkie-talkies is mainly divided into analog voice confidentiality and digital voice confidentiality technology. Frequency domain scrambling is the earliest analog voice confidentiality technology used in voice, and it is still widely used. There are three common technologies for frequency domain scrambling. Frequency inversion is the voice confidentiality technology currently used by most imported walkie-talkies. As the name suggests, frequency inversion is to exchange the high frequency and low frequency of the signal, that is, to move the high frequency part of the signal to the low frequency band and the low frequency part to the high frequency band. The signal after frequency inversion has the same frequency band range as the original signal. Since the frequency components of the original voice signal are scrambled, the intelligibility is reduced, which plays a role in voice confidentiality. The same frequency inversion is used at the receiving end to restore the signal. This frequency inversion is also the simplest voice scrambler, which is essentially an encoder. Common frequency inversions do not have a key, so its confidentiality is limited. In addition, there is band-shifted frequency scrambling, which introduces a key based on a frequency scrambler. Both parties can use the agreed key for confidential communication. In this way, the band-shifted frequency scrambler becomes a true voice security machine. Another is frequency band division, also called frequency band scrambling, which is also the most commonly used technology in analog voice security machines. The method is to divide the original signal spectrum into several equal sub-bands, and then rearrange the order of these sub-bands to achieve scrambling, and some sub-bands can also be scrambled. The three technologies of frequency domain scrambling, frequency scrambler, band-shifted frequency scrambler and band splitter mainly use modulators and filters to achieve frequency shifting, frequency measurement and division. Time domain scrambling technology is mainly implemented by traditional frequency modulation and filters. It can be implemented by analog technology such as delay line, or by digital technology, so it is widely used in current voice security equipment. This technology is divided into time period inversion (dividing the voice signal into time periods of a certain length, and inverting the signal of each time period in time). Time unit scrambling (divide the voice signal into equal time segments, called frames, and then divide each frame into smaller time segments, called segments. Generally, each frame contains 8-16 segments, and then scramble the time order of each segment in a frame, which is similar to time division switching, so it is also called time division scrambling) and time sampling scrambling (sample the voice signal and group it according to a certain length, then scramble the order of each group of sampled signals, and then restore it to its original state at the receiving end). Two-dimensional scrambling is to combine the two most commonly used technologies in voice analog encryption, frequency domain and time domain scrambling, to form a more effective confidentiality system. The confidentiality of this two-dimensional scrambling is quite good, and the residual intelligibility of the encrypted voice is almost zero. The disadvantages are high cost, poor voice quality, and sensitivity to the channel. The analog-to-digital-to-analog scrambling technology is to digitize the analog voice signal first, and then scramble it when using digital signal processing technology. Finally, it is converted into an analog encrypted signal with basically unchanged bandwidth through a digital-to-analog converter and sent out, and the receiving end restores it to a voice signal in the opposite process.

Digital voice security technology can achieve a high degree of confidentiality and overcome the difficulty of achieving both confidentiality and voice quality in analog voice security technology. Therefore, digital voice security technology is mostly used in important occasions. The digital security system includes voice encoders, decoders, digital modulators, demodulators, and channel encoders and decoders. The working principle is that the analog voice signal is digitally encoded into a digital signal through a voice encoder (analog-to-digital encoder), and then converted into a digital encrypted voice signal through an encryptor. The channel encoder performs error correction, and then the digital modulator converts the digital signal into an analog signal suitable for transmission through the transmitting channel. At the receiving end, the demodulator restores it to a digital signal, and then the channel decoder, decryptor, and digital-to-analog converter restore the voice signal. The encryptor uses sequence cipher technology and block encryption technology. Since the output encrypted signal of digital voice security technology has pseudo-random characteristics, it is just a piece of noise for eavesdroppers, and the intelligibility is zero.

In summary, the type of voice security technology to be used depends on the needs of the user. For general security requirements, simple frequency domain scrambling technology is used, for higher security requirements, two-dimensional encryption technology is used, and for higher security requirements, digital voice security technology is preferred.