Optimizing Cryptography in Energy Harvesting Applications
Charles Suslowicz, Archanaa S. Krishnan, Patrick R. Schaumont · 2017
The Internet of Things will need to support ubiquitous and continuous connectivity to resource constrained and energy constrained devices. To this end, we consider the optimization of cryptographic protocols under energy harvesting conditions. Traditionally, computing using energy harvesting power sources is handled as a case of intermittent-computing: working towards the completion of a goal under uncertain energy supply. In our work we consider the often ignored case when there is harvested energy available but there are no useful operations to complete. In cryptographic protocols, this can occur while the protocol waits for the next message. To avoid waste, we partition cryptographic algorithms into an offline portion and an online portion, where only the online portion has a real-time dependency to the availability of data. The offline portion is precomputed with the result stored as a coupon for the remaining online operation. We show that this structure brings multiple benefits including decreased response latency, a smaller energy store requirement, and reduced energy waste in a harvester supported system. We present a case study of two canonical cryptographic applications: true random number generation and bulk-encryption. We analyze the precomputed implementations on an MSP430 with ferroelectric RAM and an ARM Cortex M4 with nonvolatile flash memory. Our solutions avoid energy waste during the offline phase, and they offer gains in energy efficiency during the online phase of up to 57 times for bulk-encryption and over 100 times for random number generation.