Introduction
This application note describes how to get a Secure Boot and Secure Firmware Update solution on Arm
®
TrustZone
®
STM32
microcontrollers based on the Arm
®
Cortex
®
‑M33 processor. It also provides a top-level comparison of this solution versus
the X-CUBE-SBSFU solution, which applies to non-TrustZone
®
STM32 microcontrollers based on the Arm
®
Cortex
®
‑M0,
Cortex
®
‑M3, Cortex
®
‑M4, or Cortex
®
‑M7 processors. It provides as well top-level integration guidelines for the Secure Boot and
Secure Firmware Update solution.
For Arm
®
TrustZone
®
STM32 microcontrollers, a Secure Boot and Secure Firmware Update solution is provided in the
corresponding STM32Cube MCU Package. Contrary to the solution proposed in the X-CUBE-SBSFU STM32Cube Expansion
Package, it is based on the open-source TF‑M (Trusted Firmware for Arm
®
Cortex
®
‑M) reference implementation.
This application note applies to all TrustZone
®
STM32 microcontrollers (refer to Table 1). However, in this document, the
STM32L5 Series is used as an example.
Depending on the TrustZone
®
STM32 microcontroller, TF‑M-based application available in the STM32Cube MCU Package may
differ. Refer to the user manual of the TFM application (complete implementation of TF‑M) of the considered Arm
®
TrustZone
®
STM32 microcontroller (see Section 2 References) to get a precise description of the solution.
To get more information about the open-source TF‑M reference implementation, refer to [TF‑M].
Table 1. Applicable products
Type Product series
Microcontrollers STM32L5 Series, STM32U5 Series
Overview of Secure Boot and Secure Firmware Update solution
on Arm
®
TrustZone
®
STM32 microcontrollers
AN5447
Application note
AN5447 - Rev 3 - August 2021
For further information contact your local STMicroelectronics sales office.
www.st.com
1 General information
Throughout this application note, the terminology X-CUBE-SBSFU refers to the Secure Boot and Secure
Firmware Update solution available in the X-CUBE-SBSFU STM32Cube Expansion Package, whereas the
terminology SBSFU refers to the Secure Boot and Secure Firmware Update solution available in the STM32Cube
MCU Packages of Arm
®
TrustZone
®
STM32 microcontrollers (STM32CubeL5 is used as an example).
Table 2 presents the definition of acronyms that are relevant for a better understanding of this document.
Table 2. List of acronyms
Acronym Definition
AEAD Authenticated encryption with associated data
AES Advanced encryption standard
CBC AES cipher block chaining
CTR AES counter mode
EAT Entity attestation token
ECDSA Elliptic curve digital signature algorithm
GCM AES Galois/counter mode
HDP Hide protection
HUK Hardware unique key
ITS Internal trusted storage
KMS Key management services
MAC Message authentication code
MPU Memory protection unit
OEM Original equipment manufacturer
OTFDEC On-the-fly decryption
PKCS Public-key cryptography standard
PSA Platform security architecture. Framework for securing devices
RDP Read protection
RoT Root of Trust
RSA Rivest–Shamir–Adleman algorithm
SBSFU Secure Boot and Secure Firmware Update
SST Secure storage service. Secure storage service provided by TF‑M
TBSA-M
Trusted base system architecture for Arm
®
Cortex
®
-M
TF‑M
Trusted Firmware for M-class Arm
®
processors. TF‑M provides a reference implementation of secure
world software for Armv8-M
TFM Name of the TF‑M-based application with complete functionalities in the STM32Cube MCU Package
TZ
TrustZone
®
WRP Write protection
Note: Arm and TrustZone are registered trademarks of Arm Limited (or its subsidiaries) in the US and or elsewhere.
AN5447
General information
AN5447 - Rev 3
page 2/22
