Set up secure boot | ConnectCore MP25

1. Configure secure boot

To build signed artifacts, modify your conf/local.conf file to include the following:

conf/local.conf

# Required to include trustfence support.
INHERIT += "trustfence"

Digi Embedded Yocto generates a PKI tree by default under a new folder called trustfence inside your project. To specify a custom path for the PKI tree, see Advanced options.

1.1. Hardware unique key (HUK)

The hardware unique key (HUK) is a symmetric encryption key stored in the OTP bits of the platform. The HUK is not used directly to encrypt data. Instead, it is used as a derivative for other keys. This means you can use different keys for different encryption purposes and know that the keys cannot be used to access data on a different device, even if they are leaked.

Each chip has a unique HUK pre-provisioned by STMicroelectronics.

1.2. U-Boot environment encryption

When TrustFence is enabled, the U-Boot environment is encrypted by default using a key derived from the chip hardware unique key (HUK).

To explicitly disable U-Boot environment encryption, see Advanced options.

2. Build your target images

Once TrustFence is enabled and configured in your Digi Embedded Yocto project, you can build your target image. For example:

$ bitbake dey-image-webkit

When the build process finishes, several secure artifacts appear in the deployment directory:

tf-a-ccmp25-dvk-optee-emmc_Signed.stm32: This is the signed ARM Trusted Firmware for booting from the eMMC.
fip-ccmp25-dvk-optee-emmc_Signed.bin: This is the signed FIP image with the U-Boot bootloader and Trusted Execution Environment (OP-TEE).
fitImage-ccmp25-dvk.bin: This is a signed FIT image that contains the following artifacts with signed hashes:
- The Linux kernel
- The device tree
- Device tree overlays
- Boot script

The fitImage-ccmp25-dvk.bin is contained inside the <image-name>-boot.vfat image file.

Additionally, when not provided, TrustFence keys are generated at the specified TRUSTFENCE_SIGN_KEYS_PATH location (by default, a subfolder in the project called trustfence/).

For additional details on TrustFence keys, see Summary of TrustFence keys.

These files need to be securely stored in order to be used in the manufacturing of secure devices.

3. Program the signed artifacts

Deploy the built artifacts. See Update firmware using system images for reference.

The default U-Boot is prepared to boot a regular Linux kernel with Image.gz format. This is determined by variable dboot_kernel_var. When using the install_linux_fw_uuu.sh script to deploy TrustFence images, this variable is automatically set to boot a FIT image.

If you use a different method to deploy the images, you must set this variable manually in U-Boot as follows:

=> setenv dboot_kernel_var fitimage
=> saveenv

4. Boot the signed artifacts

Reset the device. Check the new messages that appear:

NOTICE:  CPU: STM32MP255CAL Rev.Y
NOTICE:  Model: Digi International ConnectCore MP25 Development Kit
NOTICE:  Reset reason: Power-on reset (por_rstn) (0x2035)
NOTICE:  Bootrom authentication failed
NOTICE:  BL2: v2.10-stm32mp2-r1.0(release):dey-5.0-r1.3-v2.10-21-g8a2c4b322-dirty(8a2c4b32)
NOTICE:  BL2: Built : 11:53:45, Apr  9 2025
NOTICE:  TRUSTED_BOARD_BOOT support enabled
NOTICE:  ROTPK is not deployed on platform. Skipping ROTPK verification.
NOTICE:  ROTPK is not deployed on platform. Skipping ROTPK verification.
NOTICE:  BL2: Booting BL31
NOTICE:  BL31: v2.10-stm32mp2-r1.0(release):dey-5.0-r1.3-v2.10-21-g8a2c4b322-dirty(8a2c4b32)
NOTICE:  BL31: Built : 11:53:45, Apr  9 2025
[    0.000000] SCP-firmware 2.13.0-intree-optee-os-dey-5.0-r1.3-v4.0-14-gb56
[    0.000000]
[    0.000000] [FWK] Module initialization complete!


U-Boot dub-2023.10-r2.1-git (Mar 28 2025 - 11:12:09 +0000)

[...]

The Bootrom authentication failed message appears. This means the ROM loader detected that it is booting a signed image, but it could not authenticate it. This is expected because you have not yet programmed the hash of the eight ECC public keys hashes table in the OTP bits.

The next message is TRUSTED_BOARD_BOOT support enabled. This means the TF-A image (or first-stage bootloader) has been built with authentication support enabled; that is, FSBL will either authenticate the second-stage boot loader or refuse to boot.

The next message is ROTPK is not deployed on platform. Skipping ROTPK verification. This displays because the Root Of Trust Public Key (ROTPK) has not yet been programmed in the OTP bits, so the FSBL skips the authentication of the second-stage bootloader and just boots it.

Following that, OP-TEE runs and then U-Boot starts.

The U-Boot environment encryption feature is enabled by default, so U-Boot will import the MAC addresses (and any other write-once environment variables) from the previous (not encrypted) environment, and will reset any other variable to the default values. After that, a save operation will overwrite the environment with encrypted data.

To ensure that both copies of the environment get encrypted, save the environment twice right after booting a U-Boot with environment encryption support, as follows:

=> saveenv
Saving Environment to MMC...
Writing to MMC(0)... done
=> saveenv
Saving Environment to MMC...
Writing to redundant MMC(0)... done

4.1. Boot the FIT image

As part of the Digi Embedded Yocto project, the U-Boot recipe includes a U-Boot boot script to boot the ConnectCore MP25 Development Kit. This script determines the variant of the ConnectCore MP25 SoM and the version of the carrier board and selects the device tree overlays to apply on top of the base device tree (wireless, Bluetooth, etc.) before booting the kernel. This boot script is embedded and signed in the FIT image and is run by default when you start the device.

The default U-Boot (without TrustFence) is prepared to boot a regular Linux kernel with zImage format. This is determined by variable dboot_kernel_var. When using the install_linux_fw_uuu.sh script to deploy TrustFence images, this variable is automatically set to boot a FIT image.

If you used a different method to deploy the images, you must set this variable manually in U-Boot as follows:

=> setenv dboot_kernel_var fitimage
=> saveenv

Let U-Boot run, or run the boot command if you stopped at the U-Boot prompt. Check the new messages that appear:

## Executing script at 94000000
sha256+ sha256,rsa2048:fitimg+ Booting from system A

## Loading fitImage: fitImage-ccmp25-dvk.bin
11710394 bytes read in 301 ms (37.1 MiB/s)
## Loading kernel from FIT Image at 84000000 ...

Explanation of output:

The FIT image is loaded into RAM memory and the boot script is authenticated against the public key.
Authentication succeeds and the boot script prints the message: "sha256,rsa2048:fitimg+ sha256+ Booting from system A".
The boot script calls the dboot command to boot the system. While this command typically loads the image into RAM, in this case the FIT image is not reloaded because it was already loaded to run the boot script.

Next, U-Boot successfully authenticates the default configuration:

## Loading kernel from FIT Image at 84000000 ...
   Using 'conf-ccmp25-dvk.dtb' configuration
   Verifying Hash Integrity ... OK
   Trying 'kernel-1' kernel subimage
     Description:  Linux kernel
     Created:      2025-04-08  14:32:11 UTC
     Type:         Kernel Image
     Compression:  gzip compressed
     Data Start:   0x840000e8
     Data Size:    11545299 Bytes = 11 MiB
     Architecture: AArch64
     OS:           Linux
     Load Address: 0x88000000
     Entry Point:  0x88000000
     Hash algo:    sha256
     Hash value:   a1fac39ca6bf3a858b8f82992abc02de85904994dc12b98c44a005a10a2ea198
     Sign algo:    sha256,rsa2048:fitimg
     Sign value:   a16a06512b2230b2884560455cbd6c072c321923b424f50bd72e26da2a27337ce6a8ebd2efcac87c9172c16264c4b8e48c35eca705a0d76b490d05ccc6f8ce2b42d2725bbe008433cf51f264616911aba07e
     Timestamp:    2025-04-08  14:32:11 UTC
   Verifying Hash Integrity ... sha256+ sha256,rsa2048:fitimg- OK

Explanation of output:

The first item in the default configuration is the Linux kernel.
U-Boot also authenticates the kernel image successfully.
Similar messages appear for the base device tree and the selected overlays that the boot script populated as a list in U-Boot environment variable overlays.
After all the artifacts have been authenticated, the Linux system boots.

The FIT image is the only file that’s stored in the linux partition, as it includes all the required files to boot the system.

5. Secure the device

The final step in configuring secure boot for a device is burning the secure OTP configuration.

The secure OTP configuration can only be written once and is irreversible.

To secure the device:

5.1. Program the Public Key Hash Table Hash for FSBL-A or FSBL-M (OEM-KEY1)

The OEM-KEY1 OTP bits (words 144..151) hold the hash of the eight ECC public keys hashes table. For additional details on TrustFence keys, see Summary of TrustFence keys.

The ConnectCore MP25 has two boot stages:

Initially, the ROM loader uses the OEM-KEY1 OTP bits to authenticate the FSBL (TF-A). In open devices, the FSBL will boot in any case and the log reports whether the authentication was successful or failed.
Next, the FSBL (built with authentication support when TrustFence is enabled) authenticates the second stage bootloader (SSBL), the FIP image. If the OEM-KEY1 is not programmed, it skips the authentication and boots it; if the OEM-KEY1 is programmed, it validates the signature and refuses to boot if the authentication fails.

To program the OEM-KEY1:

Check the current status of the device:

{pu}trustfence status
* OEM-KEY1 fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* OEM-KEY2 fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* FIP-EDMK fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* EDMK1 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* EDMK2 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* Secure boot:  [OPEN]

The command reports the OEM-KEY1 is not programmed and the device is open.

Transfer the publicKeysHashHashes.bin (by default inside the trustfence/ folder in your project) into an address in RAM, for example via TFTP:
```
=> tftp ${loadaddr} publicKeysHashHashes.bin
```

Read the contents of that RAM address interpreted as OEM-KEY1. This command prints the hexadecimal bytes that form the OEM-KEY1:

=> trustfence read_pkhth ${loadaddr}
Read OEM-KEY1 at 0x84000000
OEM-KEY1 OTP 144: [84000000] b53a0bb4
OEM-KEY1 OTP 145: [84000004] 6a809bfa
OEM-KEY1 OTP 146: [84000008] 5c461350
OEM-KEY1 OTP 147: [8400000c] f8979d6f
OEM-KEY1 OTP 148: [84000010] ad1e36dd
OEM-KEY1 OTP 149: [84000014] e573053f
OEM-KEY1 OTP 150: [84000018] db27c6da
OEM-KEY1 OTP 151: [8400001c] 5d297f18

Verify the hexadecimal values match the contents of the publicKeysHashHashes.bin that corresponds to the PKI tree you are using to sign the files. Remember to back up this PKI tree to be able to sign future images.

Program the OEM-KEY1 in the OTP bits (irreversible):

=> trustfence prog_pkhth ${loadaddr}
OEM-KEY1 OTP 144: 00000000 lock : 00000000
OEM-KEY1 OTP 145: 00000000 lock : 00000000
OEM-KEY1 OTP 146: 00000000 lock : 00000000
OEM-KEY1 OTP 147: 00000000 lock : 00000000
OEM-KEY1 OTP 148: 00000000 lock : 00000000
OEM-KEY1 OTP 149: 00000000 lock : 00000000
OEM-KEY1 OTP 150: 00000000 lock : 00000000
OEM-KEY1 OTP 151: 00000000 lock : 00000000
OEM-KEY1 is not locked!
OEM-KEY1 is free!
Writing OEM-KEY1 with
OEM-KEY1 OTP 144: [84000000] b53a0bb4
OEM-KEY1 OTP 145: [84000004] 6a809bfa
OEM-KEY1 OTP 146: [84000008] 5c461350
OEM-KEY1 OTP 147: [8400000c] f8979d6f
OEM-KEY1 OTP 148: [84000010] ad1e36dd
OEM-KEY1 OTP 149: [84000014] e573053f
OEM-KEY1 OTP 150: [84000018] db27c6da
OEM-KEY1 OTP 151: [8400001c] 5d297f18
Warning: Programming fuses is an irreversible operation!
         This may brick your system.
         Use this command only if you are sure of what you are doing!

Really perform this fuse programming? <y/N> y
Fuse OEM-KEY1 OTP 144 : b53a0bb4
Fuse OEM-KEY1 OTP 145 : 6a809bfa
Fuse OEM-KEY1 OTP 146 : 5c461350
Fuse OEM-KEY1 OTP 147 : f8979d6f
Fuse OEM-KEY1 OTP 148 : ad1e36dd
Fuse OEM-KEY1 OTP 149 : e573053f
Fuse OEM-KEY1 OTP 150 : db27c6da
Fuse OEM-KEY1 OTP 151 : 5d297f18
OEM-KEY1 updated !
=>

At this point, the OEM-KEY1 has been programmed but the device is still open and can still boot any unsigned images.

Reset the device, and check the new messages that appear:

NOTICE:  CPU: STM32MP255CAL Rev.Y
NOTICE:  Model: Digi International ConnectCore MP25 Development Kit
NOTICE:  Bootrom authentication succeeded
NOTICE:  BL2: v2.10-stm32mp2-r1.0(release):dey-5.0-r1.3-v2.10-19-g38a33604d-dirty(38a33604)
NOTICE:  BL2: Built : 14:55:23, Mar 17 2025
NOTICE:  TRUSTED_BOARD_BOOT support enabled
NOTICE:  BL2: Booting BL31
NOTICE:  BL31: v2.10-stm32mp2-r1.0(release):dey-5.0-r1.3-v2.10-19-g38a33604d-dirty(38a33604)
NOTICE:  BL31: Built : 14:55:23, Mar 17 2025
[    0.000000] SCP-firmware 2.13.0-intree-optee-os-dey-5.0-r1.3-v4.0-14-gb56
[    0.000000]
[    0.000000] [FWK] Module initialization complete!


U-Boot dub-2023.10-r2.1-git (Mar 28 2025 - 11:12:09 +0000)

[...]

=>

The Bootrom authentication succeeded message appears. This means the ROM loader detected that it is booting a signed image and it could successfully authenticate it against the programmed OEM-KEY1.

If the boot continues at this point, the FIP image was authenticated successfully. If authentication fails, the FIP image won’t boot at all, even if the device is open.

Check the TrustFence status:

=> trustfence status
* OEM-KEY1 fuses:  [PROGRAMMED] [LOCKED]
* OEM-KEY2 fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* FIP-EDMK fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* EDMK1 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* EDMK2 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* Secure boot:  [OPEN]

On open devices you can still boot unsigned images without authentication support (without TrustFence).

5.2. Close the device

This step is irreversible and could brick your device.

Before closing the device:

Verify you have programmed signed, not encrypted, TF-A and FIP images in the eMMC.
Verify you have programmed the OEM-KEY1 in the OTP bits.
Verify the FSBL shows Bootrom authentication succeeded.
Verify the FSBL shows TRUSTED_BOARD_BOOT support enabled.
Verify the images boot up to Linux.
Make sure you have a backup of the generated PKI tree (by default in the trustfence/ folder in your project).

To close the device:

=> trustfence close
OTP 18: closed status: 0 lock : 00000000
OEM-KEY1 OTP 144: b53a0bb4 lock : 40000000
OEM-KEY1 OTP 145: 6a809bfa lock : 40000000
OEM-KEY1 OTP 146: 5c461350 lock : 40000000
OEM-KEY1 OTP 147: f8979d6f lock : 40000000
OEM-KEY1 OTP 148: ad1e36dd lock : 40000000
OEM-KEY1 OTP 149: e573053f lock : 40000000
OEM-KEY1 OTP 150: db27c6da lock : 40000000
OEM-KEY1 OTP 151: 5d297f18 lock : 40000000
Warning: Programming fuses is an irreversible operation!
         This may brick your system.
         Use this command only if you are sure of what you are doing!

Really perform this fuse programming? <y/N> y
Device is closed !

After that, the device will only boot properly signed images.

Reboot the device

Check the TrustFence status:

=> trustfence status
* OEM-KEY1 fuses:  [PROGRAMMED] [LOCKED]
* OEM-KEY2 fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* FIP-EDMK fuses:       [NOT PROGRAMMED] [NOT LOCKED]
* EDMK1 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* EDMK2 fuses:  [NOT PROGRAMMED] [NOT LOCKED]
* Secure boot:  [CLOSED]

Advanced options

Disable signing of images

Signing of images is automatically enabled as part of TrustFence. To explicitly disable the generation of signed images, define TRUSTFENCE_SIGN to 0:

conf/local.conf

# Disable signed images
TRUSTFENCE_SIGN = "0"

The use case for disabling image signing is to separate the generation of the image from the signing process. For instance, you may want to avoid managing security keys on development machines and instead only sign images on a separate, more secure computer.

Use a custom PKI tree path

Digi Embedded Yocto creates by default a folder called trustfence/ inside your project to hold all keys.

To customize the location of the TrustFence keys set variable TRUSTFENCE_SIGN_KEY_PATH in your project’s conf/local.conf:

conf/local.conf

# Use a custom path to the signature keys and certificates.
TRUSTFENCE_SIGN_KEYS_PATH = "/mnt/secure/PKI_tree"

Use custom FIT sign keys

Digi Embedded Yocto generates random keys to sign the artifacts in the FIT image, but you can optionally provide custom ones:

If it doesn’t already exist, create a subdirectory fit inside your PKI tree path to hold the key pairs.
```
$ mkdir <your-trustfence-sign-keys-path>/fit
```
Optionally, edit your project config file conf/local.conf and set the following variables to give your keys custom names. (If not provided, the default filenames are fitcfg and fitimg.)
conf/local.conf
```
TRUSTFENCE_FIT_CFG_SIGN_KEYNAME = "my-fitcfg"
TRUSTFENCE_FIT_IMG_SIGN_KEYNAME = "my-fitimg"
```

Generate an RSA private key to sign the FIT image nodes and another one to sign the configuration nodes:

$ openssl genrsa -F4 -out <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.key 2048
$ openssl genrsa -F4 -out <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.key 2048

Generate public certificates out of the private keys:

$ openssl req -batch -new -x509 -key <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.key -out <your-trustfence-sign-keys-path>/fit/<your-fitcfg-name>.crt
$ openssl req -batch -new -x509 -key <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.key -out <your-trustfence-sign-keys-path>/fit/<your-fitimg-name>.crt

Rebuild your project.

Disable U-Boot environment encryption

To explicitly disable U-Boot environment encryption, set TRUSTFENCE_ENCRYPT_ENVIRONMENT to 0 as follows:

conf/local.conf

# Disable U-Boot environment encryption
TRUSTFENCE_ENCRYPT_ENVIRONMENT = "0"

When flashing U-Boot without the environment encryption feature in a device with an encrypted environment, all the values will be lost. Be sure to save any important data such as MAC addresses before you execute this procedure.