# A Class of Perfectly Secret Autonomous Low-Bit-Rate Voice Communication Systems
## 8 Conclusions  
The paper presents a perfectly secret voice communication system based on a MELPe vocoder with a speed of 1.2 kb/s and Vernam's cipher. The generation and distribution of secret keys rely on two sources of common randomness and the SKD protocol, which requires the use of an additional authenticated channel. The primary source of CR is a specially designed LP synthesizer, which ensures the required amount of innovative entropy based on a local source of randomness. By selecting appropriate security margins and synthesizer parameters, such as sample rate and SNR, the system designer can achieve the desired secret key rate of 1.2 kb/s, thereby maintaining the perfect secrecy of the Vernam cipher. The maximum security margin reaches approximately 460 bits, ensuring negligible leakage of the generated secret keys.  
The requirement for an additional reliable authenticated public channel can be seen as the trade-off for achieving perfect secrecy. However, the asynchronous operation of the voice coder and cipher system in the main channel relative to the system components utilizing the public channel significantly mitigates implementation challenges, enhancing practical usability. An experimental evaluation of the vPCP-V system demonstrates the robustness of this approach across various communication channels, including the hardest one: GSM 3G and VoLTE, achieving an acceptable BER on the order of \(10^{-3}\).  
While the proposed APS-VCS system meets the stringent requirements of perfect secrecy and autonomy, several directions for enhancement and integration with emerging technologies warrant further exploration. Let us mention just a few of the most interesting, such as the optimization of the SKD protocol, multiplexing of the main and public channels, and refining synthesizer models by exploring alternative excitation methods (such as neural vocoder-based synthesis, for example). A particularly promising direction for future research is hybrid architectures that combine SKD with QKD. While QKD provides provable security guarantees, its current deployment limitations (e.g., infrastructure requirements) make direct substitution challenging. However, incorporating QKD-based key exchange as an additional security layer for key refreshing could significantly enhance long-term robustness.  
By addressing these future directions, the APS-VCS system can continue to evolve as a promising solution in the domain of perfectly secret voice communication. The findings presented in this paper contribute to the broader domain of secure speech transmission and highlight the potential for further innovation in autonomous cryptographic communication systems.  
Conceptualization, M.M. and J.R.; methodology, M.M., J.R. and Z.B.; software, J.R.; validation, M.M., J.R., S.C. and Z.K.; formal analysis, M.M., Z.B., M.P. and D.P.; investigation, M.M. and J.R.; resources, Z.B.; data curation, S.C. and Z.K.; writing--original draft preparation, M.M. and J.R.; writing--review and editing, M.M., J.R., S.C., Z.K. and Z.B.; visualization, M.M. and J.R.; supervision, M.M. and Z.B.; project administration, M.M., Z.B., M.P. and D.P.; funding acquisition, Z.B. All authors have read and agreed to the published version of the manuscript.  
The research is funded by the Vlatacom Institute of High Technologies under Project #164 EEC_Keys.  
The raw data supporting the conclusions of this article will be made available by the authors upon request.  
[PERSON] would like to thank his daughter [PERSON], who participated in the creation and implementation of the founding of the so-called Cyprus experiment [43], in which it was shown for the first time that it is practically possible to implement the SKD protocol over EEG signals, which opened up a whole series of practical implementations, including the implementation shown in this paper.