  The implementation, deployment and testing of secure services for Internet of
Things devices is nowadays still at an early stage. Several frameworks have
recently emerged to help developers realize such services, abstracting the
complexity of the many types of underlying hardware platforms and software
libraries. Assessing the performance and usability of a given framework remains
challenging, as they are largely influenced by the application and workload
considered, as well as the target hardware. Since 15 years, ARM processors are
providing support for TrustZone, a set of security instructions that realize a
trusted execution environment inside the processor. OP-TEE is a free-software
framework to implement trusted applications and services for TrustZone. In this
short paper we show how one can leverage OP-TEE for implementing a secure
service (i.e., a key-value store). We deploy and evaluate the performance of
this trusted service on common Raspberry Pi hardware platforms. We report our
experimental results with the data store and also compare it against OP-TEE's
built-in secure storage.
