Secure Data Timestamping in Synchronization-Free LoRaWAN

Low-power wide-area network technologies such as LoRaWAN are important for achieving ubiquitous connectivity required by the Internet of Things. Due to limited bandwidth, LoRaWAN is primarily for applications of collecting low-rate monitoring data from geographically distributed sensors. In these applications, sensor data timestamping is often a critical system function. This paper considers a synchronization-free approach of timestamping the uplink data at the LoRaWAN gateway, which can give milliseconds accuracy. Its key advantages are simplicity and no extra overhead, commensurate with the scarce communication resources of LoRaWAN. However, we show that this low-overhead approach is susceptible to a frame delay attack that can be implemented by a combination of stealthy jamming and delayed replay. To address this threat, we propose a SoftLoRa gateway design that integrates a commodity LoRaWAN gateway with a low-power software-defined radio receiver to track the inherent frequency biases of LoRaWAN end devices. With a set of efficient signal processing algorithms that are designed based on LoRaWAN's modulation method, our frequency bias estimation achieves a resolution of 0.14 parts-per-million (ppm) of the channel's central frequency. This resolution is sufficient to detect the attack that introduces an additional frequency bias of one or more ppm. We evaluate our approach in various indoor and outdoor environments. In summary, this paper presents an attack-aware and low-overhead approach to timestamping the data generated by LoRaWAN end devices.