The current increasing need for privacy-preserving voice communications is
leading to new ideas for securing voice transmission. This paper refers to a
relatively new concept of sending encrypted speech as pseudo-speech in the
audio domain over digital voice communication infrastructures, like 3G cellular
network and VoIP.

This work presents a novel distortion-tolerant speech encryption scheme for
secure voice communications over voice channels that combines the robustness of
analog speech scrambling and elevated security offered by digital ciphers like
AES-CTR. The system scrambles vocal parameters of a speech signal (loudness,
pitch, timbre) using distance-preserving pseudo-random translations and
rotations on a hypersphere of parameters. Next, scrambled parameters are
encoded to a pseudo-speech signal adapted to transmission over digital voice
channels equipped with voice activity detection. Upon reception of this
pseudo-speech signal, the legitimate receiver restores distorted copies of the
initial vocal parameters. Despite some deciphering errors, an integrated
neural-based vocoder based on the LPCNet architecture reconstructs an
intelligible speech.

The experimental implementation of this speech encryption scheme has been
tested by simulations and sending an encrypted signal over FaceTime between two
iPhones 6 connected to the same WiFi network. Moreover, speech excerpts
restored from encrypted signals were evaluated by a speech quality assessment
on a group of about 40 participants. The experiments demonstrated that the
proposed scheme produces intelligible speech with a gracefully progressive
quality degradation depending on the channel noise. Finally, the preliminary
computational analysis suggested that the presented setting may operate on
high-end portable devices in nearly real-time.

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