On the Energy Cost of Channel Based Key Agreement

Besides security, energy consumption is a major concern for devices in the Internet of Things (IoT). We compare the energy consumption of two key agreement schemes -- Channel-Based Key Agreement (CBKA) and Elliptic Curve Diffie-Hellman (ECDH) -- in the IoT setting, using Wi-Fi as wireless communication interface. While ECDH is a well-studied protocol, CBKA has received attention only in recent years. Several publications proposed CBKA as a low-energy alternative to ECDH, but they did not address the energy cost of communication. For a fair comparison, we implemented the schemes on a 32-bit ARM Cortex M3-based IoT platform and measured the respective energy consumption for computation and communication. Our results show that the limiting factor for CBKA over Wi-Fi is the energy cost of communication, in particular the cost of acquiring the Received Signal Strength Indicator (RSSI) values. Even in an optimal scenario, CBKA must not measure more than ca. 300 RSSI values to be more energy efficient than ECDH. This is at most 1/5 of RSSI values required by CBKA implementations reported in the literature. As an optimization, we present a refined CBKA protocol which can save up to 25% of the energy compared to existing protocols by exploiting inherent data exchanges for entropy extraction.