TrustZoneM

TrustZone-M is ARM’s hardware-assisted security extension for ARMv8-M microcontrollers that partitions a device’s software and resources into two distinct execution environments: a secure world and a non-secure world. This separation enables security-critical code and data to run isolated from ordinary application software, while still sharing the same physical processor and peripherals. The goal is to provide a trusted execution environment on low-power microcontrollers suitable for embedded and Internet of Things devices.

The architectural basis of TrustZone-M relies on memory-attribute and security-state controls built into ARMv8-M cores. A Security Attribution Unit (SAU) marks memory regions as secure or non-secure, occasionally complemented by an Implementation Defined Attribution Unit (IDAU) for additional configuration. A non-secure callable mechanism allows controlled entry points from the non-secure world to secure services. Transitions between worlds are governed by defined rules so that secure operations, cryptographic keys, and security-sensitive data remain protected from non-secure software.

Software support centers around enabling secure boot, trusted firmware, and secure services within the secure world, while the non-secure world runs general application code. ARM provides a reference implementation called Trusted Firmware-M (TF-M) to manage secure boot, secure storage, cryptographic services, and access to secure peripherals. This framework helps developers implement end-to-end security with a relatively small performance and footprint impact suitable for resource-constrained devices.

Applications of TrustZone-M span IoT devices, consumer electronics, and automotive components, where hardware-enforced isolation helps reduce attack surfaces and protect sensitive assets. While it does not equate to a full-fledged multi-core security enclave, TrustZone-M offers a practical security partitioning approach for Cortex-M based systems.