How to enable secure boot on STM32 MPU

Applicable for

1. Article purpose[edit source]

The main purpose of this article is to give main steps on how to enable security on MPU platforms.

2. Overview[edit source]

Warning

Make sure that a device with Secure boot enabled is used: this is mentioned in the chip part number, for STM32MP13 and STM32MP15, otherwise the device will become permanently unusable.

A STM32 MPU offers multiple ways to control and enforce security on the device. It exists some hardware and software mechanisms that can be adapted by the users to set the MPU to the expected security level. These security protection mechanisms are offered to users and fully customizable.

3. Device life cycle[edit source]

The STM32MP1 proposes different hardware state that manage different protection levels when changing state.

There are 3 modes defined at hardware level:

Device states

OPEN state: By default the device is in open state. Authentication is not mandatory. It can be used to test authentication as error does not prevent FSBL from being started. Debugger is enabled with a limited access.

CLOSED state: Once product secrets keys have been provisioned into the device, it can be closed. On closed devices, authentication is mandatory. An authentication error prevents FSBL from being started. All debug access are default turn off.

RMA state: A closed device can be put once in RMA state and back again in CLOSED state. When the device is in RMA state, all ROM code features are disabled.

On STM32MP15x lines , it is possible to go back to close state.

4. Secure Boot[edit source]

The secure boot is essential to ensure the integrity and security of the platform at runtime.

The STM32 MPU trusted boot chain is designed to guarantee such a secure boot sequence.

STM32 MPU rely on ROM code and TF-A security features to guarantee a complete

It performs the following tasks:

Configuration of the platform firewall, which is the foundation for a safe execution of the platform
Configuration of the platform debug capabilities
Verification of the integrity (thanks to a hash algorithm) and authentication (using asymmetric cryptography algorithms) of the started software components, including the Secure and non-secure worlds.
Decryption of the loaded binary (Optional)

TF-A is the recommended open source bootloader. Its implementation supports the trusted boot and peripheral access control with firewall.

4.1. Enable secure boot[edit source]

The authentication is the minimal setup required when enabling the secure boot.

Encryption is an extended security feature (only available on STM32MP13x lines ) that can be enabled at each firmware level at the beginning or in a second step.

To enable the secure boot, the following steps must be done:

Generate authentication keys (and encryption if needed)
Provision keys in the chip
Prepare software to support trusted boot features by signing the loaded firmware
Validate the secure boot feature for each software
Close the device

4.2. STM32MPU keys[edit source]

The secure boot relies on different keys fit the secure boot implementation.

STM32MP15x lines :

PKH (Mandatory): Hash (SHA256) of the ECC Public Key (ECDSA is the authentication algorithm STM32MP15_OTP_mapping) used to compare the public root key. This file is automatically generated by the KeyGen_tool.

- ROM code used it to compare against the public key included in the STM32 header.
- TF-A BL2 used it to compare against the public root key from the trusted certificate.

STM32MP13x lines :

PKHTH (Mandatory): Hash (SHA256) of the 8 ECC Public Keys Hashes Table (ECDSA is the authentication algorithm STM32MP13_OTP_mapping) used to compare the public root key. This file is automatically generated by the KeyGen_tool.

- ROM code used it to compare against the public key table hash included in the STM32 header.
- TF-A BL2 used it to compare against the public key table hash included in the STM32 header.
EDMK (Optional): Encryption Decryption Master key used to decrypt the BL2 by the ROM and by the TF-A BL2 to decrypt FIP binaries. The firmware encryption is an optional feature and could be later enabled. Key is a 128 bit random value.

It can be generated using following commands:

- Linux:
```
  dd if=/dev/random of=edmk.bin bs=1 count=16 
```
- Windows: Use openssl<refs>https://www.openssl.org/</refs> rand command
BHK (Optional): The Boot Hardware key is located in the backup registers and can be used to decrypt secrets as directly connected to the SAES for firmware decryption. It is not used by the ROM code.

Warning

The defined keys are used by ROM code and could not be changed. By default, OpenSTLinux uses the same key for the next stage boot loader TF-A BL2 but it can be updated to define specific keys.

4.3. Provisioning[edit source]

The key provisioning or others secrets provisioning is an important step to deploy security management in STM32MPU.

There are different ways to provision the OTP:

U-Boot stm32key command to write authentication and encryption keys.
STM32CubeProgrammer to write all OTP.
U-Boot fuse command to directly write OTP. By default only non secure OTP are accessible for read/write access to the OTP.
Secure Secret Provisioning for an overall secure provisioning process.

4.4. Firmware secure boot[edit source]

The secure boot implementation is specific to each software described:

the STM32 MPU ROM code secure boot article for the ROM code authentication process based on STM32 header.
the TF-A BL2 trusted board boot using FIP.
the coprocessor authentication.

4.4.1. ROM code secure boot validation[edit source]

When the chip is in secure open state, it is possible to check if the key provisioning is OK or not.

Once the FSBL image is signed using Signing_tool, it is possible to test the secure boot.

The authentication result is transmitted to TF-A and, at boot time on board, you must see the following line in TF-A trace:
NOTICE: Bootrom authentication succeeded or NOTICE: Bootrom authentication failed.

Example:

NOTICE:  CPU: STM32MP157CAA Rev.?
NOTICE:  Model: STMicroelectronics STM32MP157C eval daughter on eval mother
NOTICE:  Board: MB1263 Var1.0 Rev.C-01
NOTICE:  Bootrom authentication succeeded
INFO:    Reset reason (0x15):
INFO:      Power-on Reset (rst_por)
INFO:    PMIC version = 0x10

4.4.2. TF-A BL2 trusted board boot validation[edit source]

Once the key provisioning is done, the TF-A BL2 will check the Root Key Hash.

Boot process will go to the next boot loader.
Block the boot process with error authentication error -80 in case of incorrect provisioning.

5. Close the device[edit source]

Closing the device is the most important security feature as:

it ensures that only signed binary can be used
it closes the debug access

Closing the device can be done using:

U-Boot stm32key command.
STM32CubeProgrammer to write OTP.
U-Boot fuse command to directly write OTP.
Secure Secret Provisioning that close the device at the end of the process.