Abstract
One of the most important requirements for speech channel is to provide information security. Speech scrambling is one of the preferred methods for secure speech communications over the narrowband channels. That confirms the relevance of works focused on improvement and investigation of operation features of speech scrambling devices. The article shows the advantages of using scramblers in narrowband voice communication systems and presents a detailed analysis of strength to hacking of communication channels secured by frequency-domain scrambling technique. The investigation of strength to hacking is based on the developed digital signal processing algorithm which realizes the band scrambler. The developed algorithm does not require synchronization between the scrambler and the descrambler which is achieved by using of sliding window FFT technique. Presented experimental results show that the lowest level of the scrambled signal residual intelligibility can be achieved when low frequency bands of speech spectrum are permuted with high frequency ones. Also the article shows that if amount of bands is less than 32 the scrambled signal can be hacked by a simple frequency spectrum inversion in the entire frequency band of the signal. During scrambling with amount of bands greater than 32, there are variants of band permutation that ensure the level of residual intelligibility near 10‑20 % and that is secured against hacking by the spectrum inversion. For a real-time speech communication system, the maximum number of scrambler bands is limited by two factors: the delay time for the scrambling-descrambling process and inserted distortions. It was shown that the maximum amount of bands should be less or equal to 256 for scrambling time delay of 128 ms, communication channel bandwidth of 4 kHz and the level of distortions that causes reduction of intelligibility not lower than 90 %. The presented information can be used in development of frequency-domain scramblers and for selection scrambler keys, which are optimal by the criteria of strength to hacking, minimum of residual intelligibility and level of inserted distortions.
Highlights
Along with the development of computer technologies, methods and means for transmission of information, approaches to ensure information security are being developed
It should be noted that the following aspects of implementation and application of band scramblers are not fully represented in the available literature sources and reviews of technical solutions: 1) relevance of declared advantages of the scrambling methods over the encryption ones taking into account the modern techniques of compression and transmission of a stream of speech samples through various communication channels in particular reasonableness of using scramblers in narrowband communication systems; 2) optimal splitting of the speech signal spectrum for frequency domain scrambling and recommendations for choosing of a band permutation order for providing minimal level of speech residual intelligibility in a secured communication channel; 3) estimation of the scrambled signal security level, especially, its strength for a various hacking attempts
Comparative analysis of scrambling and encryption methods has shown that scrambling is the only method that provides secure transmission of voice information in real-time over narrowband communication channels with a signal-to-noise ratio of 0 dB or less
Summary
Along with the development of computer technologies, methods and means for transmission of information, approaches to ensure information security are being developed. Examples of narrowband communication systems are general- and special-purpose wired telephone lines as well as special, operational-technological and amateur radio systems up to the VHF band. In such systems, along with the requirements of reliability and efficiency, issues of the information transmission security are of importance. Due to its versatility, scrambling devices are being actively produced by such manufacturers as Transcrypt International (USA), Communico (USA), MX COM (USA), MIDIAN Electronics (USA), Selectone (USA) and used in commercially available special-purposes radio stations One of such radio stations are the ORIONRS-1S and the ORION-RV-1S transceivers manufactured by ORION Radio Plant PJSC (Ternopil, Ukraine). It confirms the relevance of the works connected with improvement of the speech scrambling devices and investigation of its operation features
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