How to start with OEMuRoT on STM32N6 MCUs

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Literature

Prerequisites

  • Hardware
    • STM32HN6 discovery board
    • Discovery MB1860- STM32N6 (need USBC cable)

Required tools

  • IAR: v9.40.1 + IAR patch to support STM32N6 (delivered with V0.5.0) + IAR Patch EWARMv9_STM32N6xx_V0.6.2
    • IAR patch is available in the STM32CubeFW: STM32Cube_FW_N6_Vx.x.x\Utilities\PC_Software
  • STM32CubeN6 FW version 1.0.0 or later
  • STM32CubeProgrammer version 2.18.0
  • Tera Term / Putty or equivalent terminal emulator


  • STM32Cube Firmware
    • Download the STM32Cube_FW_N6 Cube firmware (it is recommended to place it close to form the C: to avoid long windows paths).
    • A directory STM32N6-DK is included in "STM32Cube_FW_N6\Projects" .
  • Open the env.bat file If the STM32CubeProgrammer has not been installed in the default folder:C:\Program Files\STMicroelectronics\STM32Cube\STM32CubeProgrammer, the customized installation path needs to be updated:

1. Introduction

OEMuRoT properties

  • It is a secure boot with secure update capabilities
  • stored in an external flash memory
  • Code provided in STM32CubeN6 firmware
  • MCU Boot based
  • OEMuRoT crypto keys stored in OTP area

2. Step 1 Configuration management - secrets provisioning

The secrets are provisioned in OTP fuses. Once fused, the secrets can not be modified anymore.

To provision secrets, the device must be in Secured_unlocked state and in development mode. While in development mode, debug is open which allows STM32CubeProgrammer to program OTP fuses.

2.1. BootRom secrets provisioning

BootRom secrets are:

  • Keys used to sign the OEMuRoT_Boot firmware
  • Keys used to encrypt/decrypt the OEMuRoT_Boot firmware
  • RMA password

To generate and provision the secrets, if not already done, refer to the following chapters:

Security:How to install a trusted application on STM32N6 MCUs#Keys Generation

Security:How to install a trusted application on STM32N6 MCUs#Keys provisioning

2.2. OEMuRoT secrets provisioning

Warning
The keys_generation.bat script will overwrite all keys in the OEMuRoT/Keys folder. Make sure to back up any existing keys before running this script.

Launch the script keys_generation.bat  to generate the OEMuRoT secrets in Keys folder

Folder path: Projects\STM32N6570-DK\ROT_Provisioning\OEMuROT

  • Configure your board in Dev Boot mode:

  • If you are using STM32CubeFW N6 v1.0.0,

Open keys-programming.bat script with an editor and replace line 15:

set connect_no_reset=-c port=SWD ap=1 mode=Hotplug

by

set connect_no_reset=-c port=SWD ap=1

Save the modifications.

  • Launch the script keys_programming.bat

Folder path: Projects\STM32N6570-DK\ROT_Provisioning\OEMuROT\Keys

Warning
Once programmed, OTP fuses are locked and can't be reprogrammed.

2.3. Debug Authentication password provisioning

When using STM32CubeFW N6 v1.0.0, you must create your password via STM32CubeProgrammer:

  • Open the STM32CubeProgrammer and connect your board in Dev Boot mode:

  • Go to the shield and select PROV tab.
  • Choose a password (size between 4 and 16 characters)
  • Select the path where you want to provision the password
  • Click "Start Provisioning"

Once the success messages received, the password is provisioned in OTP and can't be changed anymore.

The generated file "password.bin" will be used to do Debug Authentication

Warning
Once programmed, OTP fuses are locked and can't be reprogrammed.

2.4. OEMuRoT provisioning script

The script drives OEM to

  • Provision secrets
  • Compile OEMuRoT_Boot firmware
  • Compile User application Secure and Nonsecure
  • Generate trusted images

Then, script automatically flashes

  • OEMuRoT_Boot image
  • user application image(s)
  • User data

Launch provisioning.bat script from folder: Projects\STM32N6570-DK\ROT_Provisioning\OEMuROT.

The step 1 of the script is done. Press a key and start step 2.

3. Step 2: Images generation

3.1. OEMuROT_Boot image generation

OEMuROT_Boot firmware secure boot properties:

  • Executed at HDP level 1
  • Launches a user application in HDP Level 2
  • Secure firmware update

Many Secure boot capabilities' parameters can be updated like:

  • Number of user application images and data images
  • Load and Run in external or internal RAM / Execute in place option
  • SWAP or overwrite mode

OEMuRoT default parameters in STM32CubeFW:

  • Load and run Internal RAM
  • Overwrite mode
  • 1 Secure application image
  • 1 Nonsecure application image
  • 1 secure data image
  • 1 nonsecure data image

To generated application images:

  1. Open the postbuild.bat file located in Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Boot\EWARM The postbuild The script is automatically launched after the IAR project compilation. By default, this script signs the OEMuRoT_Boot firmware, but does not encrypt it. In this step by step we are showing how to encrypt and sign the OEMuRoT_Boot firmware so the script must be edited. In order to encrypt the OEMuRoT_Boot firmware, the crypted variable must be set to 1:
Warning
If using STM32Cubeprogrammer v2.21 version or more recent, add -align option in the command line in postbuild.bat file:

"%stm32signingtoolcli%" -bin "%binary_file%" %scmd% %enccmd% %optionflag% -o "%trusted_binary_file%" -hv 2.3 -align-s

  1. Save the postbuild.bat script and close it.
  2. Launch the Project.eww file in the Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Boot\EWARM folder
  3. Compile the project


After build success, the trusted image is created during the postbuild script.

3.2. Application image generation

OEMuRoT_Appli is an example of secure and non secure application managed by OEMuRoT

To generate application images:

  1. Launch the Project.eww file in Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Appli\EWARM folder
  2. Select the project secure
  3. Compile the project
  4. Select the project Nonsecure
  5. Compile the project


User application must be flashed encrypted and signed

The trusted images have been signed and encrypted during the postbuild script but

this could be done thanks to the STM32TrustePackageCreator tool

3.3. Data images generation

Data secure and nonsecure provided in STM32CubeFW

To update the data examples provided (optional)

  • Launch STM32TrustedPackageCreator

  • Enter the xml file corresponding to data image secure or nonsecure in Projects\STM32N6570-DK\ROT_Provisioning\OEMuROT\Images
  • Update the parameters (input/ output..)
  • Click on

⇒ an encrypted and signed file is generated: s_data_enc_sign.bin


4. OEMuRoT - Execute application

  • Set the device in flash boot mode and open Tera Term:

5. Load a new user application

In this example, we update the non secure application, but the same process can be applied for the secure application, the secure Datas, or/and the non secure Datas.

First, the non secure application display can be modified before updating a new application.

  1. Open the project non secure in IAR
  2. Open main.c file nonsecure
  3. Modify line 47
  4. Rebuild the code → a new trusted image nonsecure is generated

Go back to the Tera Term window, with the device connected in Flash boot mode and press reset to display the application menu. To load a new nonsecure application:

  1. Enter “2” to select “New Fw Image”
  2. Select “3” “Download Nonsecure App Image”


Follow the instructions:

  1. In Tera Term panel, go to File → Transfer → YMODEM → Send
  2. Select the new secure application to download: rot_tz_ns_app_enc_sign.bin

rot_tz_ns_app_init_enc_sign.bin contains magic at the end of the binary file. It is used by the provisioning script.

rot_tz_ns_app_enc_sign.bin does not contain the magic and it is used to download a new application. The size of this binary is smaller, the download time is thus reduced.


Result:

  1. You get the Success message
  2. Press the reset button and see the update displayed

6. Lock the device

To lock the device, refer to the wiki article:Security:How to install a trusted application on STM32N6 MCUs#Lock the device and execute trusted application


7. Load new OEMuRoT (FSBL) in Secured-locked state

OEMuRoT stands for OEM Updatable root of trust. It means that the OEMuROT_Boot firmware, the first stage bootloader that is called by the BootRom, can be updated. Even if the device is in secured-locked state this firmware can be updated.

First we will generate a new OEMuROT_Boot firmware. This firmware will have the same functionalities than the OEMuROT_Boot firmware already installed but the display to Teraterm will be different in order to visualize the modifications easily.

  • To load this new OEMuRoT_Boot firmware we need specified files that you can load from Github.

Please load the application from Github:BootRom_traces

Download the code:

click to "Code", then "Download ZIP" :


Copy/paste the folderLoad_new_FSBL_locked_state to Projects\STM32N6570-DK\ROT_Provisioning folder:

  • First modify the OEMuROT_Boot firmware code.

When a new OEMuRoT_Boot version is uploaded, its version must be upper than the previous one. By default the version installed with STM32CubeFW is 0. In this example we will install version 1.

Open the OEMuROT_Boot firmware IAR project : double click on Project.eww located in Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Boot\EWARM folder.

Add a line of code before the firmware jumps to the user application : at line 153 of the file bl2_main.c located in Middlewares\Third_Party\mcuboot\bl2\ext\mcuboot folder add, for example indicate which version number you are uploading :

    BOOT_LOG_INF("OEMuRoT_Boot firmware_Test version 1");

  • Rebuild the project: a new binary file OEMuROT_Boot.bin is generated. This binary file is automatically generated to Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Boot\Binary folder.
Information
Don't use the file OEMuROT_Boot_Trusted.bin as it is automatically signed by the postbuild script with a version number =0. The version number must be incremented so we will sign the OEMuRoT_boot.bin file with an upper version number.
  • Rename OEMuROT_Boot.bin to binary.bin and copy/ paste it to Projects\STM32N6570-DK\ROT_Provisioning/Load_new_FSBL_locked_state\NOR_Binary folder :

  • Go to Projects\STM32N6570-DK\ROT_Provisioning/Load_new_FSBL_locked_state folder
  • The script Sign_newFSBL.bat will sign the new binary generated with the keys programmed in OTP.

Launch the script Sign_newFSBL.bat

  • Two new signed binaries are generated in NOR_Binary folder:
    • binary_trusted.bin : the new FSBL to be loaded
    • OpenBootloader_STM32N6570-DK-trusted.stm32 : the signed OpenBootloader

  • Connect your board in Serial mode:

  • Open STM32CubeProgrammerand connect the board via USB1:

  • Go to "Memory & File editing" tab and select "Open file" :

  • Open the FlashLayout.tsv file located in Projects\STM32N6570-DK\ROT_Provisioning\Load_new_FSBL_locked_state\NOR_Binary folder
  • Browse the Binaries path and download the file → this will load the new FSBL

Success message:

  • Set the device now in Flash boot mode to execute the new FSBL

8. Debug Authentication

  • Open the STM32CubeProgrammer
  • The N6 Discovery board must be configured in Flash boot mode.
  • Select the shield , then go to DA tab.
  • Click to Discover and the discover window is displayed:


After this discover command, you have the possibility to open the debug.

Select the password file password.bin generated in "OEMuRoT secrets provisioning" paragraph, and click to "Open Debug" :

You get this success message:


Debug nonsecure is now opened. The board is automatically connected to the STM32CubeProgrammer.

In the "Memory & File editing" tab, you can read the nonsecure Data @ 0X2404000:


Disconnect STM32CubeProgrammer and you can debug the non secure application in IAR.

Don't disconnect your board (don't power ON/OFF the board), to let the debug opened.

Launch the Project.eww file in Projects\STM32N6570-DK\Applications\ROT\OEMuROT_Appli\EWARM folder

Modify the Project nonsecure properties:

  1. Right click on Project-ns project and select "Options..."
  2. In "Debugger" select "ST-LINK" driver
  3. In "ST-LINK" select "ST-LINK/V3" emulator. Then choose access port = 1 and set Interface speed to 2MHZ.

Click OK

  • Add a breakpoint in main.c file nonsecure, line 195
  • Select "Project -> Attach to Running Target"

You can now debug the nonsecure code in IAR.


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