  The low-power radio technologies open up many opportunities to facilitate
Internet-of-Things (IoT) into our daily life, while their minimalist design
also makes IoT devices vulnerable to many active attacks. Recent advances use
an antenna array to extract fine-grained physical-layer signatures to identify
the attackers, which adds burdens in terms of energy and hardware cost to IoT
devices. In this paper, we present ShieldScatter, a lightweight system that
attaches low-cost tags to single-antenna devices to shield the system from
active attacks. The key insight of ShieldScatter is to intentionally create
multi-path propagation signatures with the careful deployment of tags. These
signatures can be used to construct a sensitive profile to identify the
location of the signals' arrival, and thus detect the threat. In addition, we
also design a tag-random scheme and a multiple receivers combination approach
to detect a powerful attacker who has the strong priori knowledge of the
legitimate user. We prototype ShieldScatter with USRPs and tags to evaluate our
system in various environments. The results show that even when the powerful
attacker is close to the legitimate device, ShieldScatter can mitigate 95% of
attack attempts while triggering false alarms on just 7% of legitimate traffic.
