  Edge devices are increasingly in charge of storing privacy-sensitive data, in
particular implantables, wearables, and nearables can potentially collect and
process high-resolution vital signs 24/7. Storing and performing computations
over such data in a privacy-preserving fashion is of paramount importance. We
present KEVLAR-TZ, an application-level trusted cache designed to leverage ARM
TrustZone, a popular trusted execution environment available in consumer-grade
devices. To facilitate the integration with existing systems and IoT devices
and protocols, KEVLAR-TZ exposes a REST-based interface with connection
endpoints inside the TrustZone enclave. Furthermore, it exploits the on-device
secure persistent storage to guarantee durability of data across reboots. We
fully implemented KEVLAR-TZ on top of the OP-TEE framework, and experimentally
evaluated its performance. Our results showcase performance trade-offs, for
instance in terms of throughput and latency, for various workloads, and we
believe our results can be useful for practitioners and in general developers
of systems for TrustZone. KEVLAR-TZ is available as open-source at
https://github.com/mqttz/kevlar-tz/.
