  Secure Device-to-Device (D2D) communication is becoming increasingly
important with the ever-growing number of Internet-of-Things (IoT) devices in
our daily life. To achieve secure D2D communication, the key agreement between
different IoT devices without any prior knowledge is becoming desirable.
Although various approaches have been proposed in the literature, they suffer
from a number of limitations, such as low key generation rate and short pairing
distance. In this paper, we present InaudibleKey, an inaudible acoustic
signal-based key generation protocol for mobile devices. Based on acoustic
channel reciprocity, InaudibleKey exploits the acoustic channel frequency
response of two legitimate devices as a common secret to generating keys.
InaudibleKey employs several novel technologies to significantly improve its
performance. We conduct extensive experiments to evaluate the proposed system
in different real environments. Compared to state-of-the-art works,
InaudibleKey improves key generation rate by 3-145 times, extends pairing
distance by 3.2-44 times, and reduces information reconciliation counts by
2.5-16 times. Security analysis demonstrates that InaudibleKey is resilient to
a number of malicious attacks. We also implement InaudibleKey on modern
smartphones and resource-limited IoT devices. Results show that it is
energy-efficient and can run on both powerful and resource-limited IoT devices
without incurring excessive resource consumption.
