ST AN4023 APPLICATION NOTE

AN4023

Application note

STM32 secure firmware upgrade (SFU) overview

Introduction

One of the advantages of using a microcontroller is its flexibility and its ability to reprogram
the built-in Flash memory, which provides the ability to upgrade the microcontroller remotely
with new firmware versions, adding new features and correcting potential issues.

This upgrade process must be performed in a secure way in order to prevent malicious
users from copying the firmware for other manufacturers. Secure download and upgrade
solutions assume that the firmware binary is sent to a device in an encrypted form, and that
the device can receive the encrypted binary, decrypt it, and check the version number and
code authenticity/integrity before saving it in the Flash memory.

Three main actors are involved in the secure firmware upgrade solution:

1. STMicroelectronics: the STM32 device manufacturer, responsible for programming the
secure firmware upgrade solution provided as binary by the OEM.

2. OEM: Original equipment manufacturer: STMicroelectronics direct customer and the
final product (based on STM32 devices) owner.

3. OEM-CM: OEM subcontractor: responsible for device personalization and firmware
download. Its environment is considered as non-secure.

This solution is based on two main phases:

1. Device personalization: The STM32 device (where the firmware will be loaded)
receives the necessary data from a hardware security module (HSM) and generates an
encryption key which it saves in its internal Flash memory.

This phase can be bypassed, removing the need for investment in an HSM. In this
case, the chip must be initially personalized with an OEM key instead of an OEM
master key to make it ready for secure firmware download/upgrade.

2. Device firmware secure download and upgrade: The STM32 device receives the
encrypted firmware from a host station (personal computer or similar). It decrypts the
received firmware, checks the version and firmware authenticity and then performs a
firmware upgrade.

For more details about the complete solution, please contact your local ST sales
representative.

March 2012 Doc ID 022593 Rev 1 1/3

www.st.com

Revision history AN4023

Revision history

Table 1. Document revision history

Date Revision Changes

06-Mar-2012 1 Initial release.

2/3 Doc ID 022593 Rev 1