Security features on STM32U0 MCUs

This article contains an overview of the security features[1] available on STM32U0 MCUs. The table below contains detailed information based on the different product lines.

Security features embedded on: STM32U031 picto.png STM32U073 picto.png STM32U083 picto.png
Secure Boot and Firmware Update
OEMiSB YES YES YES
OEMiRoT NO NO YES
Isolation
HDP YES YES YES
Cryptography
ST crypto lib YES YES YES
Crypto libraries YES YES YES
Silicon device life cycle
RDP with regression YES YES YES
Product state NO NO NO
Debug authentication NO NO NO

1. Secure Boot and Firmware Update

When a product's code needs securing, it becomes necessary to embed a Root of Trust (RoT). This RoT ensures the next stage of security ensuring the code integrity and its authenticity. By definition, a RoT must be based on an immutable code handling the reset of the platform. The RoT is the foundation to build a secure product.

STM32U0 boot path

On STM32U0, the implementation of the root of trust is based on boot lock and HDP features allowing to design robust secure boot solutions.
Two solutions are proposed:

  1. OEMiSB
  2. OEMiRoT

1.1. OEMiSB

OEMiSB stands for OEM immutable (unchangeable) Secure Boot. OEMiSB offers only one service:

  • The Secure Boot (root of trust service) is an immutable code, which is always executed after a system reset. It checks static protections (options bytes), activates runtime protections, and then verifies the integrity of the user application code before every execution.

OEMiSB for STM32U0 detailed

1.2. OEMiRoT

OEMiRoT stands for OEM immutable (unchangeable) Root of Trust and acts as a first boot stage. It is based on the MCUboot open-source software[2] provided with STM32CubeU0.

OEMiRoT offers two services:

  • The Secure Boot (root of trust service) is an immutable code, which is always executed after a system reset. It checks static protections (option bytes), activates runtime protections, and verifies the authenticity and integrity of the user application code before every execution.
  • The Secure Firmware Update application is an immutable code that detects new firmware image candidates. It checks the version (version downgrade prevention), authenticity, and integrity before installing it after decryption.

OEMiRoT for STM32U0 detailed

2. Isolation

2.1. Securable memory area by Hide Data Protect (HDP)

The main purpose of the securable memory area is to protect a specific part of the flash memory against undesired access. Following a system reset, the code in the securable memory area is executable only until this area is hidden. It remains inaccessible until a system reset occurs. This allows implementing software security services with root parameters including secret parameters owned only by the secure boot.

2.2. Securable memory area extension

The HDP area can be extended though a dedicated volatile register indicating the number of supplementary pages added to the securable memory area.

3. Cryptography

Means of performing cryptography depends on the STM32U0 product line.

Cryptography features embedded on: STM32U031 picto.png STM32U073 picto.png STM32U083 picto.png
Hardware Tiny AES NO NO YES
Hardware HASH NO NO NO
Hardware SAES NO NO NO
Hardware PKA NO NO NO
ST crypto lib YES YES YES
Crypto libraries YES YES YES

STM32Cube HAL can be used to work with the crypto IP.

3.1. ST crypto lib

ST crypto library v4[3] is available for the STM32U0 MCUs.
ST crypto library is delivered through X-CUBE-CRYPTOLIB with many examples.

3.2. Crypto libraries

Third-party libraries such as Mbed™ are available for STM32U0 MCUs.
Mbed™ library implements PSA crypto API. Mbed™ is distributed as source code.

4. Silicon device life cycle

The flexible device life cycle scheme is based on the readout protection (RDP) mechanism, including support for product decommissioning (auto-erase):


5. References