Secrets Leaking Through Quicksand: Covert Channels in Approximate Computing.

Approximate computing (AxC) has emerged as an attractive architectural paradigm especially for artificial-intelligence applications, yet its security implications are being neglected. We demonstrate a novel covert channel where the malicious sender modulates transmission by switching between regular and AxC realizations of the same computational task. The malicious receiver identifies the transmitted information by either reading out the workload statistics or by creating controlled congestion. We demonstrate the channel on both an Android simulator and an actual smartphone and systematically study measures to increase its robustness. The achievable transmission rates are comparable with earlier covert channels based on power consumption, but the malicious behavior of our channel is more stealthy and less detectable.