Robust covert communication over a public audio channel using spread spectrum

Abstract

We present a set of watermarking techniques for effective covert communication over an audio signal carrier. Watermark robustness is enabled using: (i) redundant spread-spectrum for prevention against desynchronization attacks and (ii) psycho-acoustic frequency masking (PAFM). We show that PAFM impacts the balance of "ones" and "zeros" in the part of the spread-spectrum sequence used for watermark detection and propose a modified covariance test to compensate for that anomaly. The covert message is combined with the spreadspectrum sequence and additionally permuted in time to improve correlation convergence as well as the security of individual message bits. We have incorporated these techniques into a system capable of reliably detecting watermarks in audio that has been modified using a composition of attacks that degrade the original audio characteristics well beyond the limit of acceptable quality. Such attacks include: fluctuating scaling in time and frequency, editing, recompression, noise addition, resampling, normalization, and filtering.