The core objective is to securely transmit digital data over widely accessible voice channels like VoIP or GSM networks. Encryption at the highest security level is applied digitally, but a challenge lies in selecting a modulation technique that effectively converts encrypted digital data into analogue signals capable of navigating the obstacles inherent in voice channels. These channels are lossy and optimized for speech transmission, employing sophisticated signal processing techniques such as Voice Activity Detection (VAD) to maintain speech quality and suppress noise. This paper introduces a novel system design and approach for transmitting Data over Voice (DoV) channels using Offset Quadrature Amplitude Modulation (OQAM) combined with Filter Bank Multicarrier (FBMC) modulation. FBMC/OQAM generates speech-like waveforms that can evade detection by non-speech signal detection mechanisms inherent in voice channels, marking its debut in this domain. In order to demonstrate the feasibility of the proposed system, we have simulated the distortion of FBMC/OQAM signals when processed by an AMR-NB codec using MATLAB®. Subsequently, the performance of the proposed modulation technique is evaluated across various real voice channels, considering actual time and phase deformations, real-time voice transmission bit-rate constraints and possible bit errors, which all demand specific waveform structures for synchronization and algorithms for correct demodulation and decoding at the receiver. Evaluation is done for waveforms that were sent through GSM and VoIP channels. The obtained results demonstrate a favorable bit error rate (BER) on the 3G channel compared to existing methods, such as conventional Orthogonal Frequency Division Modulation (OFDM), thereby proving that FBMC/OQAM is suitable for secure DoV transmission. Finally, the decoded speech from all channels is assessed using Perceptual Evaluation of Speech Quality (PESQ) metric.

This paper proposes a novel approach to speech-like signal design for reliable real-time transmission of encrypted data over voice channels. The structure of the digitally encrypted information-carrying segment relies on the known concept of the OFDM/QPSK modulation. The segment is extended with time-synchronization sequences, amplitude variations and silence intervals, all necessary for successful passing through realistic channels. Phase distortion has been identified as a significant source of error and inspired the proposal of a phase-shift compensation and a time-and-phase fine-tuning algorithms. Finally, this paper outlines a complete secure real-time communication system. Transmission quality of the resulting speech-like scheme is first evaluated on the AMR codec at various compression rates, followed by the analysis over realistic voice channels. The proposed secure DoV system achieves a mean BER of $2.45\cdot 10^{-2}$, $1.11\cdot 10^{-2}$ and $4.83\cdot 10^{-3}$ over Signal, Telegram and 3G cellular network voice channels, respectively.

This paper presents an autonomous perfectly secure low-bit-rate voice communication system (APS-VCS) based on the mixed-excitation linear prediction voice coder (MELPe), Vernam cipher, and sequential key distillation (SKD) protocol by public discussion. An authenticated public channel can be selected in a wide range, from internet connections to specially leased radio channels. We found the source of common randomness between the locally synthesized speech signal at the transmitter and the reconstructed speech signal at the receiver side. To avoid information leakage about open input speech, the SKD protocol is not executed on the actual transmitted speech signal but on artificially synthesized speech obtained by random selection of the linear spectral pairs (LSP) parameters of the speech production model. Experimental verification of the proposed system was performed on the Vlatacom Personal Crypto Platform for Voice encryption (vPCP-V). Empirical measurements show that with an adequate selection of system parameters for voice transmission of 1.2 kb/s, a secret key rate (KR) of up to 8.8 kb/s can be achieved, with a negligible leakage rate (LR) and bit error rate (BER) of order ${10}^{-3}$ for various communications channels, including GSM 3G and GSM VoLTE networks. At the same time, by ensuring perfect secrecy within symmetric encryption systems, it further highlights the importance of the symmetry principle in the field of information-theoretic security. To our knowledge, this is the first autonomous, perfectly secret system for low-bit-rate voice communication that does not require explicit prior generation and distribution of secret keys.