  The physical-layer key generation is a lightweight technique to generate
secret keys from wireless channels for resource-constrained Internet of things
(IoT) applications. The security of key generation relies on spatial
decorrelation, which assumes that eavesdroppers observe uncorrelated channel
measurements when they are located over a half-wavelength away from legitimate
users. Unfortunately, there is no experimental validation for communications
environments when there are large-scale and small-scale fading effects.
Furthermore, while the current key generation work mainly focuses on
short-range communications techniques such as WiFi and ZigBee, the exploration
with long-range communications, e.g., LoRa, is rather limited. This paper
presents a LoRa-based key generation testbed and reveals a new
colluding-eavesdropping attack that perceives and utilizes large-scale fading
effects in key generation channels, by using multiple eavesdroppers circularly
around a legitimate user. We formalized the attack and validated it through
extensive experiments conducted under both indoor and outdoor environments. It
is corroborated that the attack reduces secret key capacity when large-scale
fading is predominant. We further investigated potential defenses by proposing
a conditional entropy and high-pass filter-based countermeasure to estimate and
eliminate large-scale fading associated components. The experimental results
demonstrated that the countermeasure can significantly improve the key
generation's security when there are both varying large-scale and small-scale
fading effects. The key bits generated by legitimate users have a low key
disagreement rate (KDR) and validated by the NIST randomness tests. On the
other hand, eavesdroppers' average KDR is increased to 0.49, which is no better
than a random guess.
