CallShield: Secure Caller Authentication over Real-Time Audio Channels

TL;DR

CallShield introduces a novel real-time audio-based caller authentication system using neural watermarking and a secure protocol, achieving over 99% success rate with minimal delay and high audio quality in telephony environments.

Contribution

It is the first system to authenticate callers entirely over the audio channel without relying on speech transcription or internet connectivity, using neural watermarking and a lightweight security protocol.

Findings

Achieves over 99.2% success rate on clean audio

Maintains high audio quality with PESQ > 4.2 and STOI > 0.94

Operates reliably under various channel distortions

Abstract

We present CallShield, the first caller identity authentication system that operates entirely at the audio layer, without relying on speech transcription, internet connectivity, or trusted infrastructure. CallShield introduces a real-time neural watermarking technique that enables per-bit embedding and recovery within 40-millisecond frames of live 8 kHz speech. This capability allows CallShield to transform the real-time audio channel into a noisy serial communication medium. To ensure reliable data transmission, CallShield implements a low-bitrate data link protocol that provides basic frame synchronization along with error detection, correction, and recovery. For caller authentication, CallShield adopts a secure and lightweight symmetric-key protocol that relies on pairwise shared secrets among trusted contacts. The system completes the full authentication process in an average of 63 seconds, including up to three retransmission attempts, making it suitable for real-time deployment. Extensive experiments under realistic telephony conditions demonstrate that CallShield achieves an overall authentication success rates exceeding 99.2% on clean audio and over 95% under common distortions, aided by selective retransmission of failed messages. Additionally, CallShield maintains high audio quality, achieving PESQ scores above 4.2 and STOI scores above 0.94 on clean speech, and exhibits robustness across a wide range of channel distortions, validating its practical viability for secure, real-time caller authentication.