- Last edited 2 months ago ago
How to secure STM32 MPU
1 Article purpose
The main purpose of this article is to give main steps on how to enable security on MPU platforms.
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 Secure Boot
The secure boot is a mandatory feature to ensure the integrity and security of the platform at runtime.
The STM32 MPU trusted boot chain is designed to answer such requirements, relying on ROM code and TF-A security features to guarantee a complete chain of trust.
It performs the following tasks:
- 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)
- Configuration of the platform, which is the foundation for a safe execution of the platform
- Configuration of the platform debug capabilities
TF-A is the recommended open source bootloader. Its implementation supports the trusted boot and peripheral access control with.
3.1 Enable secure boot
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
- Close the device
- Prepare software to support trusted boot features by signing the loaded firmware
- Flash the binaries on the boot device
3.2 STM32MPU keys
The secure boot relies on different keys to manage the secure boot implementation.
- PKH (Mandatory): Hash (SHA256) of the ECC Public Key (ECDSA is the authentication algorithm STM32MP15_OTP_mapping). This hash is automatically generated by the KeyGen_tool in a dedicated file.
- 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.
- PKHTH (Mandatory): Hash (SHA256) of the 8 ECC Public Keys Hashes Table (ECDSA is the authentication algorithm STM32MP13_OTP_mapping). This hash is automatically generated by the KeyGen_tool in a dedicated file.
- 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:
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.
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.
3.4 Firmware secure boot
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.
3.4.1 ROM code secure boot validation
When the chip is in secure opened state, it is possible to check if the key provisioning procedure is OK or not.
Once the FSBL image is signed using Signing_tool, it is possible to test the secure boot.
The ROM code authentication result is transmitted to TF-A BL2 during boot sequence. The following trace should appear in the TF-A console:
NOTICE: Bootrom authentication succeeded or NOTICE: Bootrom authentication failed.
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
3.4.2 TF-A BL2 trusted board boot validation
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.
4 Close the device
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.