Powering Privacy: On the Energy Demand and Feasibility of Anonymity Networks on Smartphones

Many different anonymity networks have been designed and implemented over the last 20 years. These networks protect communication and metadata through multi-layered encryption and cover traffic. However, there is little research on whether such networks are actually practical on smartphones with limited battery power. This is important as these are the dominant computing devices today. Previous research suggests that cryptographic operations and background radio transmissions are the two main contributors to energy consumption when running such software on mobile devices. We develop and open-source a test setup that measures actual energy consumption, including side-effects that evade simple models. With this setup we explore the costs of cryptography, radio communication, and background scheduling. We find that radio communication dominates overall power consumption, while cryptographic operations (asymmetric and symmetric) are negligible for typical anonymity network workloads. We also investigate the feasibility of running anonymity networks continuously to protect the metadata of all communication. For a 12-hour usage period, a mobile Tor client on a 4G network requires an additional 4 percentage points of battery charge which appears practical and is at least as efficient as the commercial VPN clients that we tested. However, a mix network client that continuously sends cover traffic requires up to 30 percentage points for the same period. Such costs are unlikely to be acceptable to many users.