  The lightweight protocols and 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 due to the lack of sophisticated security protocols. Recent advances
advocate the use of an antenna array to extract fine-grained physical-layer
signatures to mitigate these active attacks. However, it adds burdens in terms
of energy consumption and hardware cost that IoT devices cannot afford. To
overcome this predicament, we present ShieldScatter, a lightweight system that
attaches battery-free backscatter 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 backscatter tags. These signatures can be used to construct a
sensitive profile to identify the location of the signals' arrival, and thus
detect the threat. We prototype ShieldScatter with USRPs and ambient
backscatter tags to evaluate our system in various environments. The
experimental results show that even when the attacker is located only 15 cm
away from the legitimate device, ShieldScatter with merely three backscatter
tags can mitigate 97% of spoofing attack attempts while at the same time
trigger false alarms on just 7% of legitimate traffic.
