This topic contains detailed instructions to set up secure boot in your device:

1. Configure secure boot

To build signed artifacts, modify your conf/local.conf file to enable TrustFence:

conf/local.conf
# Required to include TrustFence support.
INHERIT += "trustfence"

1.1. Encryption of images

Image encryption is disabled by default on the ConnectCore 91 due to the poor decryption performance of the i.MX91.

To enable image encryption, modify your conf/local.conf file to include the following:

conf/local.conf
TRUSTFENCE_ENCRYPT = "1"
Enabling image encryption increases the boot time by over six seconds.

1.2. PKI tree

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

1.3. U-Boot environment encryption

When TrustFence is enabled, the U-Boot environment is encrypted using a random key derived from the chip’s hardware unique key (HUK), only when the device is closed.

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

2. Build your image

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

$ bitbake core-image-base

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

  • imx-boot-ccimx91-dvk.bin: These are the default U-Boot images. They are not signed.

  • imx-boot-signed-ccimx91-dvk.bin: These are the signed U-Boot images. Like default U-Boot images, they are specific for each variant.

  • imx-boot-encrypted-ccimx91-dvk.bin: These are signed and encrypted U-Boot images specific for each variant.

  • SRK_efuses.bin: This is a file containing the hash of the SRK public keys. It will be required when setting up the device for secure boot.

  • A core-image-base-ccimx91-dvk.boot.vfat image containing the following:

    • fitImage-ccimx91-dvk.bin: Signed FIT image that contains the following artifacts: the linux kernel, the device tree, device tree overlays and the boot script.

  • A core-image-base-ccimx91-dvk.recovery.vfat image containing the following:

    • fitImage-ccimx91-dvk.bin: Signed FIT image that contains the following artifacts: the linux kernel, the device tree, device tree overlays and the boot script.

  • A core-image-base-ccimx91-dvk.wic.gz compressed microSD card image able to boot the closed device once decompressed.

    If secure boot encryption is enabled, the microSD card image will be able to boot the closed device into U-Boot, but will not boot the OS.

    The core-image-base-ccimx91-dvk.recovery.vfat image is only used in systems based on single boot and encrypted rootfs.

TrustFence generates keys at the path specified by variable TRUSTFENCE_KEYS_PATH (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. Provision the keys on the OTP bits

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

3.1. Program the SRK eFuse

The SRK fuses hold the hash of the SRK public keys. In open devices, they are never used. In closed devices, they are used to validate the public key contained in signed firmware images.

Before closing the device, you must store the hash of the public keys in the SRK OTP bits on the device. This allows the ROM loader to validate the public key included in signed firmware images. When building images with TrustFence, Digi Embedded Yocto generates a file named SRK_efuses.bin, which you can use to program the SRK efuses.

  1. Check the TrustFence status:

    => trustfence status
* SRK fuses:            [NOT PROGRAMMED]
* Secure boot:          [OPEN]
* Encrypted U-Boot:     [NO]
* AHAB events:           [ERRORS PRESENT!]

    The output shows:

    • The SRK e-fuses are not programmed

    • The device is still in open configuration

    • U-Boot is not encrypted

    • There are AHAB errors (expected because, without SRK, no authentication can succeed)

  2. From the U-Boot prompt, load the SRK_efuses.bin file to memory, for example using TFTP:

    => tftp ${loadaddr} SRK_efuses.bin
Using FEC device
TFTP from server 192.168.129.10; our IP address is 192.168.42.30
Filename 'deploy/SRK_efuses.bin'.
Load address: 0x12000000
Loading: #
         15.6 KiB/s
done
Bytes transferred = 32 (20 hex)

  3. Dump the contents in memory

    => md.b ${loadaddr} 20
40480000: ae 82 d3 c8 be 53 de c5 27 42 3c 05 44 b2 1f 2b  .....S..'B<.D..+
40480010: 46 26 20 9c 9d 81 64 d6 4a b7 17 ff a9 f6 38 bd  F& ...d.J.....8.
    Information in the console log may vary.

  4. Verify the hexadecimal values match the contents of the SRK_efuses.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.

  5. Program the device using the trustfence prog_srk command:

    => trustfence prog_srk ${loadaddr} ${filesize}
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>
    The filesize environment variable is automatically calculated from the previous tftp command to be equal to the size (in bytes) of the SRK_efuses.bin file.

4. Program the signed artifacts

4.1. Program the signed bootloader

The signed bootloader can be flashed like any other bootloader image:

=> update uboot tftp imx-boot-signed-ccimx91-dvk.bin

4.2. Reboot and check the TrustFence status

Reset the device, and check the result of command trustfence status:

=> trustfence status
* SRK fuses:            [PROGRAMMED]
   Key 0:               [OK]
   Key 1:               [OK]
   Key 2:               [OK]
   Key 3:               [OK]
* Secure boot:          [OPEN]
* Encrypted U-Boot:     [NO]
* AHAB events:           [NO ERRORS]

The output shows:

  • The SRK e-fuses are programmed

  • All keys are ok (none revoked)

  • The device is still in open configuration

  • U-Boot is not encrypted

  • There are no AHAB errors

If secure boot events were present, you can get additional information with the ahab_status command to understand why the signature verification failed. This U-Boot command dumps extra debug information from the High Assurance Boot ROM. See the NXP secure boot application notes for more information on event decoding.

4.3. Program the signed system images

By default, Digi Embedded Yocto produces signed, non-encrypted artifacts for the ConnectCore 91.

Once you have verified the bootloader authenticates and boots correctly, you can proceed to deploy the rest of artifacts. See Update firmware using system images for reference.

5. 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 93 Development Kit. This script determines the variant of the ConnectCore 91 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 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 84002000
sha256+ Booting from system A
Skip re-loading of FIT image
Moving Image from 0x84000000 to 0x98000000, end=98c38000

Explanation of output:

  • The FIT image authenticated and loaded into RAM memory; the boot script is executed

  • Image integrity is verified and the boot script prints the message: "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-ccimx91-dvk.dtb' configuration
   Trying 'kernel-1' kernel subimage
     Description: Linux kernel
     Created: 2025-02-04 17:13:02 UTC
     Type: Kernel Image
     Compression: gzip compressed
     Data Start: 0x840000e8
     Data Size: 12715417 Bytes = 12.1 MiB
     Architecture: AArch64
     OS: Linux
     Load Address: 0x80400000
     Entry Point: 0x80400000
     Hash algo: sha256
     Hash value: 8f37609439ceac1a731e78681a12481ff7015a0388564b61e853c925cb1d0cef
   Verifying Hash Integrity ... sha256+ OK

Explanation of output:

  • The first item in the default configuration is the Linux kernel.

  • U-Boot authenticates and verifies the integrity of 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.

6. Close the device

This step is irreversible and could brick your device.

Before closing the device:

  • Verify the SRK e-fuses are programmed.

  • Verify the device boots the signed U-Boot.

  • Verify there are no AHAB events.

  • Make sure you can boot the device via Recover your device#,USB recovery.

  • Make sure you have a backup of the generated PKI tree (by default in the trustfence/ folder in your project).

To close a device:

=> trustfence close
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>

Reboot the device. You’ll see an error message when U-Boot tries to read the environment:

Loading Environment from MMC... Decrypting... Error in blob decapsulation: 536873242
Environment is unencrypted!
Resetting to defaults (read-only variables like MAC addresses will be kept).
## Resetting to default environment

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 closed device that has 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

Check the TrustFence status:

=> trustfence status
* SRK fuses:            [PROGRAMMED]
   Key 0:               [OK]
   Key 1:               [OK]
   Key 2:               [OK]
   Key 3:               [OK]
* Secure boot:          [CLOSED]
* Encrypted U-Boot:     [NO]
* AHAB events:           [NO ERRORS]

Now your device is closed and it will only boot properly signed images.

7. Program the signed and encrypted system artifacts on a closed device

By default, system artifacts encryption is disabled on the ConnectCore 91, so you can skip this section.

Refer to Encryption of images for information about enabling encryption before proceeding with the instructions in this section.

Once the device has been closed, the encryption uses the unique and secure OTPMK (One Time Programmable Master Key). The update process encrypts the DEK (Data Encryption Key) into an encrypted blob and stores it in the U-Boot partition. This must happen with the unique secure OTPMK, so be sure final programming happens after the device has been closed and restarted.

7.1. Program the encrypted bootloader

There are two ways to flash an encrypted U-Boot image:

Secure a new DEK

Use this method if you are using encrypted images for the first time, or if you want to change the DEK. In this case, you need two input artifacts:

  • The signed and encrypted U-Boot image

  • The data encryption key (DEK) in plain text

To program the signed and encrypted U-Boot image with a new DEK:

=> trustfence update tftp imx-boot-encrypted-ccimx91-dvk.bin dek.bin

This programs the U-Boot image and encrypts and stores the DEK in the uboot partition.

On an open device, the encryption uses the test master key, while on a closed device the encryption uses the unique and secure OTPMK. For that reason, the final programming of the encrypted U-Boot image must be done after closing the device and resetting.

The DEK blob is secured only if a closed device was used. When using an open device, the DEK blob is not secured and both the DEK in plain text and the decrypted U-Boot could be recovered from the media.

Reuse an existing DEK

Use this method if you are updating a bootloader that’s already encrypted with the same DEK. In this case you don’t need to send the plain text DEK anymore.

To program the signed and encrypted U-Boot image reusing an existing DEK:

=> trustfence update tftp imx-boot-encrypted-ccimx91-dvk.bin

The U-Boot image being flashed must have been encrypted with the existing DEK secured in the device. If the device does not have a DEK secured, the command will fail.

If a different DEK is used, the device will stop booting.
When updating to a new U-Boot major version, Digi recommends installing first the signed-only version, rebooting and then installing the encrypted one.

7.2. Reboot and check the TrustFence status

Reset the device, and check the result of command trustfence status:

=> trustfence status
* SRK fuses:            [PROGRAMMED]
   Key 0:               [OK]
   Key 1:               [OK]
   Key 2:               [OK]
   Key 3:               [OK]
* Secure boot:          [CLOSED]
* Encrypted U-Boot:     [YES]
* AHAB events:           [NO ERRORS]

The output shows:

  • The SRK e-fuses are programmed

  • All keys are ok (none revoked)

  • The device is closed

  • U-Boot is encrypted

  • There are no AHAB errors

7.3. Program the encrypted system artifacts

The rest of the encrypted system artifacts (Linux kernel, device tree files, boot scripts) can be flashed using standard procedures as long as:

  • You use a closed device.

  • U-Boot has been encrypted with the same key.

To program the linux partition:

=> update linux tftp core-image-base-ccimx91-dvk.boot.vfat

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"

This also disables the encryption of images (bootloader, system artifacts).

Disable encryption

Digi Embedded Yocto provides encryption support for the following elements:

  • U-Boot environment

  • Bootloader

  • System artifacts (Linux kernel, device trees, boot scripts).

When you enable TrustFence, the following encryption is automatically enabled:

  • U-Boot environment

The following sections describe how to explicitly disable the encryption of these elements:

Disable U-Boot environment encryption

To explicitly disable U-Boot environment encryption, set:

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.

Disable encryption of images

To disable the encryption of images (bootloader, system artifacts), set:

conf/local.conf
# Disable encryption of images
TRUSTFENCE_ENCRYPT = "0"

Use a custom PKI tree path

You can use the following parameters to customize the location of the sensitive keys:

  • TRUSTFENCE_KEYS_PATH: Path to a folder containing the PKI tree. If the folder does not exist or does not contain a PKI tree, a new PKI tree will be generated automatically. The default value is a new folder trustfence in the Digi Embedded Yocto project home location.

conf/local.conf
# Use a custom path to the signature keys and certificates.
TRUSTFENCE_KEYS_PATH = "/home/username/trustfence/mydevice"

  • TRUSTFENCE_DEK_ENCRYPT_KEYNAME: Filename of the Data Encryption Key (DEK). When provided, it must be a 128-, 192- or 256-bit binary file. Otherwise, a random 256-bit key will be generated automatically. The default value is dek.bin.

conf/local.conf
# Use a custom filename for the Data Encryption Key
TRUSTFENCE_DEK_ENCRYPT_KEYNAME = "my-custom-dek.bin"

Read-only rootfs authentication

You can choose to build a read-only rootfs (see Read-only root file system). When TrustFence is enabled, the resulting rootfs image is signed and the bootloader authenticates it before booting. This guarantees the rootfs comes from a trusted source.