How to deploy SSP: a step-by-step approach

Applicable for

STM32MP13x lines, STM32MP15x lines

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1 Article purpose

This article explains, step by step, how to run the SSP process from the OEM secret generation till the chip provisioning. The SSP overview is explained here.

This example uses the STM32CubeProgrammer (STM32CubeProg) and its associated tools to manage the complete SSP.

2 OEM secret management

The aim of SSP is to protect OEM secrets. It includes multiple secrets:

• Secrets to be provisioned in OTP
• OEM authentication keys for Secure Boot
• OEM secrets encryption key

2.1 Key generation

2.1.1 Authentication keys

The initial keys to generate for STM32 MPUs are the authentication keys. This is the root of trust of the STM32 MPU secure boot. The PKHTH (on STM32MP13xC/F lines ) or PKH (on STM32MP15xC/F lines ) is automatically installed during the SSP process in the dedicated OTP.

To generate the keys, you can use the KeyGen_tool:

• On STM32MP13xC/F lines :

Example to generate eight ECC key pairs (Prime256v1) using AES_256_cbc encryption:

 STM32MP_KeyGen_CLI -abs /home/user/KeyFolder/ -pwd testkey1 testkey2 testkey3 testkey4 testkey5 testkey6 testkey7 testkey8 -n 8

• On STM32MP15xC/F lines :

Example to generate one ECC key pair (Prime256v1) using AES_256_cbc encryption:

 STM32MP_KeyGen_CLI -abs /home/user/KeyFolder/ -pwd testkey

2.1.2 Encryption key

The SSP process uses a symmetric encryption to protect the OEM secrets exchanged with the chip. This encryption/decryption uses an AES encryption scheme using:

• a 128-bit symmetric key
• a 128-bit nonce value

Both values are provisioned in a HSM, and then securely sent to a ST verified chip using a secured transfer flow.
The two values can be directly generated with the STM32 Trusted Package Creator available in STM32CubeProgrammer , which also further manages the secret file encryption with the given key:

Another solution is to prepare both keys using the KeyGen_tool (till version 2.12.0) with the command:

 STM32MP_KeyGen_CLI -rand 16 enc_key.bin
 STM32MP_KeyGen_CLI -rand 16 nonce.bin

2.2 Secret file

2.2.1 Secret file creation

Warning

There is actually no tool to generate the file. Therefore, the secret file must be generated manually. A hexadecimal tool must be used to generate the file. ssp-secrets/ssp_secret_template.bin is an empty file with the correct size for STM32MP1 series.

The secret file, which must be generated by the OEM, must represent the OTP 59 to 95 on STM32MP1 series, named Free for user.

On STM32MP1 series:

• the words 57 and 58 are reserved for mac_address.
• the final file size must be (96 - 59) * 4 bytes = 148 bytes.

Information

On the STM32 boards, the word 59 is already fused to store the ST board ID. For SSP testing purpose on an STM32 MPU board, the content of the word 59 must be skipped by using the 0x00000000 value in the secret file.

2.2.2 Secret file encryption

The STM32 Trusted Package Creator is used to encrypt and prepare the final secret file used for provisioning.
This encrypted final file contains:

• The Root of trust:
  • On STM32MP15xC/F lines , the PKH which is the public_key.pem file (178 bytes).
  • On STM32MP13xC/F lines , the PKHTH which is the public key hash table hash file (32 bytes) generated by KeyGen_tool.
• the RMA password
• the OEM secrets

All the previous generated keys must be properly selected in the STM32 Trusted Package Creator interface to generate the final encrypted file (.ssp). The chip and RMA password(s) must be registered in the interface.

2.3 HSM provisioning

The HSMv2 must be provisioned with the OEM associated SSP file.

The STM32 Trusted Package Creator has a dedicated SSP HSM provisioning interface to select:

• the encryption key used to encrypt the SSP secret file
• the nonce used to encrypt the SSP secret file
• the personalization data file (used to identify the chip):
  • STM32MP1 for STM32MP1 series
    • 5010100D file for STM32MP13xC/F lines  rev.Y (1.2)
    • 5000200A file for STM32MP15xC/F lines  rev.Z (2.0) & rev.Y (2.1)

• the maximum number of images to program, which depends also on the HSMv2 used.

3 SSP Firmware management

On STM32MP1 series, a specific firmware is used to:

• exchange SSP communication with the host,
• burn the OTP in fuses.

This part is managed using a specific TF-A BL2 that is delivered with each ecosystem version and must be updated regarding the user's board.
It only includes a limited part of the TF-A BL2 scope:

• only serial boot support
• dedicated SSP feature set

Warning

An important requirement is the VDDCORE regulator. The interaction between the ROM code and TF-A BL2 requires to maintain the VDDCORE during reset transition. This is done by the TF-A BL2 SSP default firmware when using a PMIC. It must be taken in account for a power discret board not to stop the VDDCORE during a software system reset.

3.1 TF-A customization

The main TF-A firmware is ready for all targets except the board device tree. It must be customized in the same way as for TF-A BL2.

3.2 Build TF-A BL2 SSP

Because it uses the same firmware, the way of building and configuring the firmware is described in the How_to_configure_TF-A_BL2 page.

The specific part is the following one:

• dedicated flag to enable the SSP part: STM32MP_SSP
• no TRUSTED_BOARD_BOOT enabled to limit the BL2 binary size

Example:

• Command line for the UART mode on STM32MP13xC/F lines  using STM32MP135x-DK Discovery kit  :

 make ARM_ARCH_MAJOR=7 ARCH=aarch32 PLAT=stm32mp1 STM32MP_UART_PROGRAMMER=1 \
      STM32MP13=1 STM32MP_SSP=1 DTB_FILE_NAME=stm32mp135f-dk.dtb

• Command line for the USB mode on STM32MP15xC/F lines  using STM32MP157x-EV1 Evaluation board :

 make ARM_ARCH_MAJOR=7 ARCH=aarch32 PLAT=stm32mp1 STM32MP_USB_PROGRAMMER=1 \
      STM32MP15=1 STM32MP_SSP=1 DTB_FILE_NAME=stm32mp157c-ev1.dtb

3.3 Signing TF-A BL2 SSP

This TF-A BL2 SSP firmware MUST be signed with the OEM authentication key generated at step 1. The authentication is made by the ROM code using the OEM public key given by the HSM during the process.

This is made in the same way as TF-A BL2 using the Signing_tool:
Example :

  STM32MP_SigningTool_CLI -bin tfa-ssp.stm32 -pubk OEMpublicKey.pem –prvk OEMprivateKey.pem –pwd testkey

4 Production processing

At that stage, the following parts are available:

• SSP encrypted secret file
• provisioned HSM
• TF-A BL2 SSP signed firmware

All that material must be given to the untrusted production to start the SSP process.

4.1 Test case using STM32CubeProgrammer_Cli

For an evaluation purpose, STM32CubeProgrammer can be used to test the SSP processing chain.

On the host, the STM32CubeProgrammer must be installed. HSM is plugged into the host smartcard reader. The board is connected, its chip is virgin. The serial boot mode is selected and the power is ON.

The following command can be used for a USB SSP provisioning:

   STM32_Programmer_CLI -c port=usb1 -ssp <ssp_file_path> <tf-a-bl2-ssp-path> hsm=1 slot=1

5 SSP : Final state

At that stage, the device is provisioned with all the OEM secrets:

• OEM PKH or PKHTH
• OTP secrets

The chip is now in the secure close state; The secure boot is mandatory.
Enabling secure boot is the final step to use the chip.

The SSP process cannot be restarted anymore.