Set up secure boot

When enabling TrustFence, the U-Boot environment will be encrypted by default using the CAAM OTPMK and the secure internal unique key.

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

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 will allow the ROM loader to validate the public key included in signed firmware images. When building signed U-Boot images, Yocto generates a file named SRK_efuses.bin, which can be used to program the SRK efuses from the U-Boot shell in a safe way following this procedure:

To simplify key management, Digi recommends you program all devices with the same set of four keys.

See Revoke a key for steps to take if any of these keys are compromised.

To close a device, issue Digi’s U-Boot trustfence close command as follows and then reset the target:

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

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:

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:

Use the following command to flash the encrypted U-Boot image:

With the above command, the U-Boot image is flashed and the DEK is secured and stored 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.

The device now boots into a signed and encrypted U-Boot.

You can always recover a closed device by booting a properly signed U-Boot from recovery media. See Recover your device.

When a new U-Boot image is required (for example, for new features) but the same DEK is used, you don’t need to send the plain text DEK again. This is useful for updates performed in the field.

In this case, use the same command as when securing a new DEK but omit the DEK argument:

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

The rest of the encrypted artifacts (other than U-Boot) can be flashed using standard procedures for programming a boot partition. However, to use any encrypted artifact, the following additional requirements must be met:

To program the linux partition containing all the encrypted artifacts:

To program the recovery partition containing all the encrypted artifacts:

The device now boots into a trusted Linux kernel.

To check the CST versions supported by Digi Embedded Yocto, run the following command in your Digi Embedded Yocto project:

If more than one version appears and you want to use a different CST version than the latest one supported, set the variable PREFERRED_VERSION_trustfence-cst-native in your conf/local.conf:

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

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

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

To explicitly disable root file system encryption, set TRUSTFENCE_ENCRYPT_ROOTFS to 0 as follows:

Then, generate an SWU package and install it on the target to disable the rootfs encryption.

You can choose to build a read-only rootfs (see [{XREF_yocto-read-only-rootfs_t}]). 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.