TrustZone is a hardware-based security feature developed by ARM to support secure and non-secure execution environments on the same processor. It's a key part of ARMv8-M architecture, used to build secure embedded applications.

🔐 What is ARM TrustZone?

TrustZone creates two separate security domains:

Secure World – runs sensitive code (e.g., cryptography, secure boot, key management)
Non-Secure World – runs regular application code (e.g., UI, communication)

Each world:

Has its own memory, peripherals, and interrupts
Can’t access the Secure World unless explicitly allowed

This separation is enforced by the processor, memory protection units (MPUs), and bus-level access rules.

🧠 Key Concepts of TrustZone

Concept	Description
Secure Attribution	Determines whether a memory region or peripheral is secure
NSC (Non-Secure Callable)	Gateway functions callable from the Non-Secure side
TZ-M	TrustZone for ARM Cortex-M (used in STM32)
SAU/IDAU	Secure Attribution Unit (SAU) defines secure/non-secure regions
Secure Boot	Ensures device starts in a known secure state

🧩 TrustZone in STM32 Microcontrollers

Only STM32 microcontrollers based on ARM Cortex-M33, like the STM32L5, STM32U5, or STM32H5 series, support TrustZone.

✅ Supported STM32 Families:

🧱 STM32 Implementation Highlights:

Feature	Description
Dual Image Firmware	Application split into Secure and Non-Secure binaries
Secure Boot (SBSFU)	ST’s Secure Boot and Secure Firmware Update (part of X-CUBE-SBSFU)
TrustZone-Aware IDE	STM32CubeIDE allows configuration of Secure/Non-Secure regions
Memory Isolation	Flash and RAM regions configured via SAU/IDAU
Peripheral Isolation	Only secure code can access secure peripherals unless explicitly exposed

🛠️ How to Implement TrustZone on STM32

1. Enable TrustZone

Select a TrustZone-capable MCU (e.g., STM32L552)
In STM32CubeMX, enable TrustZone support

2. Configure Secure and Non-Secure Worlds

Define memory regions as Secure or Non-Secure
Setup peripherals similarly
Configure NSC region for callable secure APIs

3. Develop Secure/Non-Secure Firmware

In STM32CubeIDE:
- Create two separate projects: Secure and Non-Secure
- Secure firmware can expose APIs using __attribute__((cmse_nonsecure_entry))

4. Use Secure Boot (Optional but recommended)

ST provides SBSFU (Secure Boot and Secure Firmware Update) as middleware

5. Debug and Test

STM32CubeIDE supports debugging of both Secure and Non-Secure contexts

📦 ST Resources

X-CUBE-TZ: TrustZone demo projects
X-CUBE-SBSFU: Secure Bootloader package
STM32L5/STM32U5 Reference Manuals
AN5394: Guidelines for TrustZone on STM32

🛡️ Advantages of TrustZone

Benefit	Why it matters
Hardware-level security	Not just software isolation
Protect sensitive code	Keep keys, crypto functions in secure world
Mitigate attacks	Stops buffer overflow or firmware injection from reaching critical functions
No need for dual-core	Achieves isolation on a single core

What is TrustZone, and how is it implemented on STM32 microcontrollers?