Perfectly Secret System for Voice Transmission Based on Common Randomness

This paper presents a system for secure digital voice signal transmission over the GSM mobile network (Global System for Mobile Communication). The cryptographic part of the system is based on Vernam cipher, which requires the same speed of generating secret symmetric keys as the basic information flow in the main channel. The system for generating and distributing secret keys for Vernam cipher is realized based on the correlation properties of the locally synthesized speech signal of the transmitter and the corresponding receiving speech signal. By applying the method of sequential distillation of secret keys (SKD) on the mentioned source of common randomness, the generation speed of up to 34 kb/s is ensured, which is more than enough to maintain the absolute secrecy of Vernam cipher. The price that needs to be paid is the necessity of having an additional authenticated public channel through which the SKD protocol is performed. The privacy amplification block, as a necessary part of the SKD protocol, was implemented using machine learning methods, in order to reliably estimate the conditional Renyi entropy of the second order. According to our knowledge, this is the first autonomous, perfectly secret system for voice transmission over the GSM network without explicit prior generation and distribution of secret keys.