Establishing and maintaining secure communications in the Internet of Things (IoT) is vital to protect smart devices. Zero-interaction pairing (ZIP) and zero-interaction authentication (ZIA) enable IoT devices to establish and maintain secure communications without user interaction by utilizing devices' ambient context, e.g., audio. For autonomous operation, ZIP and ZIA require the context to have enough entropy to resist attacks and complete in a timely manner. Despite the low-entropy context being the norm, like inside an unoccupied room, the research community has yet to come up with ZIP and ZIA schemes operating under such conditions. We propose HARDZIPA, a novel approach that turns commodity IoT actuators into injecting devices, generating high-entropy context. Here, we combine the capability of IoT actuators to impact the environment, e.g., emitting a sound, with a pseudorandom number generator (PRNG) featured by many actuators to craft hard-to-predict context stimuli. To demonstrate the feasibility of HARDZIPA, we implement it on off-the-shelf IoT actuators, i.e., smart speakers, lights, and humidifiers. We comprehensively evaluate HARDZIPA, collecting over 80 hours of various context data in real-world scenarios. Our results show that HARDZIPA is able to thwart advanced active attacks on ZIP and ZIA schemes, while doubling the amount of context entropy in many cases, which allows two times faster pairing and authentication.

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