Security

With increasing digitalization and networking, the protection of embedded systems against unauthorized access and targeted attacks is more important than ever. Guaranteeing this type of security, along with functional security, is a major challenge in electronics design. PHYTEC supports you in minimizing risks by considering security requirements during the development of our hardware and board support packages. On top of these deployment ready solutions, we support you with individual project consulting on complex security principles.

Security is a process which regards all parts of a device and all development phases of its lifetime.

Introduction to Yocto

This document assumes that you are familiar with our Yocto BSPs. The basics of our BSPs are explained in the document Yocto Reference Manual (L-813e_7).

Introduction to the BSP with Security Support

Short Crypto Refresher

Recommended Security Requirements

As of the writing of this manual, recommendations apply for key lengths, certificates, and hash-values. These recommendations come from BSI (Bundesamt für Sicherheit in der Informationstechnik) and NIST (National Institute of Standards and Technology).

Distro yogurt-secure

What is Distro yogurt-secure?

Distro yogurt-secure is a sample yocto distro like yogurt with additional security configurations:

• Activated DISTRO_FEATURE secureboot for building signed bootloader and FIT-Image

• Image phytec-headless-image is configured for filesystem encryption with FIT-Image, which includes a ramdisk and a boot init script.

  Note

  This example for filesystem encryption works only with SD card / eMMC, not with Nand / NOR Flash.

• Activated DISTRO_FEATURE protectionshield for password protection in bootloader and kernel

Enable yogurt-secure

Change the distro in conf/local.conf from yogurt to yogurt-secure.

 MACHINE ?= "phyboard-mira-imx6-5"
 DISTRO ?= "yogurt-secure"

Secure Boot

Why Secure Boot?

Chain of Trust

Secure boot is used to ensure that only trustworthy, signed software can be executed on the controller. This is the first stage of the Chain-of-Trust. With the Chain-of-Trust, signed programs are always started by other previously verified programs. This ensures that even the end application is at the highest layer of trustworthiness

Process flow:

• Trusted ROM-bootloader verifies software image before they are executed
• bootloader verifies linux-kernel image and device tree (fit-image)
• linux-kernel verifies file-system and customer application data

Getting Started

NXP Controller

Information about Secure Boot on NXP Controller:  https://www.nxp.com/docs/en/application-note/AN4581.pdf
HABv4 RVT guidelines and recommendations:  https://www.nxp.com/docs/en/application-note/AN12263.pdf

• Code Signing Tools (CST) in version 3.1.0 or 3.2.0 (install recipes are available in the BSP)
  
  • Account required on NXP website:  https://www.nxp.com/webapp/sps/download/license.jsp?colCode=IMX_CST_TOOL
  • Download the cst-3.2.0.tar.gz file
  • Unpack the cst-3.2.0.tar.gz file to cst-3.2.0.tar file.
  • Place the cst-3.2.0.tar file in the yocto download folder

Enable Secure Boot Support in Yocto

Enable Secure Boot in your own distro

• To enable secure boot, add the DISTRO_FEATURE "secureboot" to your distro configuration file conf/distro/xyz.conf:

  DISTRO_FEATURES += "secureboot"                                                  
  DISTRO_FEATURES_NATIVE += "secureboot"                                           
  DISTRO_FEATURES_NATIVESDK += "secureboot"

• Active image signing during the build process. The class "secureboot" needs to be inherited in your distro configuration:

  INHERIT += "secureboot"

Configuration Class - Path to the certificates

The path to the keys and certificates used for code-signing during the build process can be changed in the secureboot.bbclass file:

CERT_PATH ??= "${OEROOT}/../../phytec-dev-ca"

For more information about keys and certificates, refer to Change PKI-Tree from phytec-dev-ca to the own PKI

Bootloader

i.MX6 with Barebox

Configuration Class - Signing Bootloader

All variables to adjust the bootloader signing process can be found in secureboot.bbclass:

BOOTLOADER_SIGN ??= "true"                                                       
BOOTLOADER_SIGN[type] = "boolean"

BOOTLOADER_SIGN_IMG_PATH_mx6 ??= "${CERT_PATH}/nxp_habv4_pki/crts/IMG1_1_sha256_4096_65537_v3_usr_crt.pem"
BOOTLOADER_SIGN_CSF_PATH_mx6 ??= "${CERT_PATH}/nxp_habv4_pki/crts/CSF1_1_sha256_4096_65537_v3_usr_crt.pem"
BOOTLOADER_SIGN_SRKFUSE_PATH_mx6 ??= "${CERT_PATH}/nxp_habv4_pki/crts/SRK_1_2_3_4_table.bin"

BOOTLOADER_SIGN:  enable the bootloader signing
BOOTLOADER_SIGN_IMG_PATH, BOOTLOADER_SIGN_CSF_PATH, BOOTLOADER_SIGN_SRKFUSE_PATH:  paths to the certificates and SRKFUSE-table required to sign the bootloader

Linux Kernel in the FIT-Image

FIT-Image Generation 

First, add a FIT-Image recipe in your BSP layer. A possible location for this recipe could be recipes-images/fitimage/

You can find two example recipes for Fit-Image generation in the meta-yorgurt layer at yocto/source/meta-yogurt/recipes-images/fitimage/.

• phytec-simple-fitimage.bb:  example for a fit-image with kernel an device tree of selected machine
• phytec-secureboot-ramdisk-fitimage.bb:  example for a fit-image with kernel, device tree of selected machine, and a ramdisk with file encryption support enable

To create a fit-image during the build process you need to inherit the Fit-Image class in your build configuration:

INHERIT += "fitimage"

• To create the manifest file, you may either use the built-in class mechanism or provide a custom manifest.
• For using the built-in bundle generation, you need to specify some variables:
  • FITIMAGE_SLOTS:  Use this to list all slot classes for which the FIT-Image should contain images. A value of "kernel fdt", for example, will create a manifest with images for two slot classes - kernel and devicetree.
  • FITIMAGE_SLOT_<slotclass>:  For each slot class, set this to the image (recipe) name which builds the artifact you intend to place in the slot class.
  • FITIMAGE_SLOT_<slotclass>[type]:  For each slot class, set this to the type of image you intend to place in this slot. Possible types are:  kernel, fdt, or ramdisk.
  • FITIMAGE_SLOT_<slotclass>[file]:  For slot type kernel and fdt, set this to the file of image you intend to place in this slot.
  • FITIMAGE_SLOT_<slotclass>[fstype]:  For slot type ramdisk, set this to the filesystem type of image you intend to place in this slot.

Configuration Class - Signing FIT-Image

In the configuration class secureboot.bbclass, you can set all relevant variables for signing the FIT-Image.

All variables to adjust the fit-image signing process can be found in secureboot.bbclass:

FITIMAGE_SIGN ??= "true"                                                         
FITIMAGE_SIGN[type] = "boolean"

FITIMAGE_SIGNER ??= "customer"                                                   
FITIMAGE_PUBKEY_SIGNATURE_PATH ??= "${WORKDIR}/signature_node.dtsi"

FITIMAGE_SIGN_KEY_PATH_mx6 ??= "${CERT_PATH}/fit/FIT-4096.key"                   
FITIMAGE_HASH_mx6 ??= "sha256"                                                   
FITIMAGE_SIGNATURE_ENCRYPTION_mx6 ??= "rsa4096"                                  
FITIMAGE_SIGNER_VERSION_mx6 ??= "vPD20.0.0"

FITIMAGE_SIGN:  Activation the FIT-Image signing
FITIMAGE_SIGNER:  Name of the FIT-Image signer, which is part of the FIT-Image certificate
FITIMAGE_PUBKEY_SIGNATURE_PATH:  Automatically created manifest with public key of the FITIMAGE_SIGN_KEY_PATH for storing in the bootloader device tree
FITIMAGE_SIGN_KEY_PATH:  Path to the signing key
FITIMAGE_HASH:  Hash function for the FIT-Image artifacts. sha1 and sha256 are supported
FITIMAGE_SIGNATURE_ENCRYPTION:  Key type for FIT-Image certificate signing. rsa2048 and rsa4096 are supported
FITIMAGE_SIGNER_VERSION:  Version of the signer, which is part of the FIT-Image certificate

Start the Build Process

Refer to Start the Build.

The phytec-headless-image has support for FIT-Image creation with an example for file encryption with SD-Card/eMMC only, not for NAND/NOR Flash. The phytec-headless-image include a FIT-Image with Kernel, devicetree, and ramdisk.

BSP Images

All images generated by Bitbake are deployed to yocto/build/deploy/images/<machine>. The following list shows all additionally generated files for the i.MX 6 SoC, phyboard-mira-imx6-5machine with activated Secure Boot:

• Barebox:barebox-s.bin, barebox-us.bin
• FIT-Image:fitImage.fitimg with kernel, device tree, and ramdisk
• FIT-Image manifest: phytec-secureboot-ramdisk-fitimage-phyboard-mira-imx6-5.its
• RAM-disk: phytec-secureboot-ramdisk-image-phyboard-mira-imx6-5.cpio.gz
• Root filesystem: phytec-headless-image-phyboard-mira-imx6-5.tar.gz
• SD card image: phytec-headless-image-phyboard-mira-imx6-5.sdcard with barebox-s.bin

Booting and Updating the System

Refer to Introduction to the BSP with Security Support for booting from different flashs and updating the software. Please use the signed bootloader images and Kernel or FIT-Images to program to the flash. The SD card image is ready to use with signed bootloader.

Activate Secure Boot on the Device

The final step to activate secure boot on your device is burning the secure eFuse configuration.

Program the SRK eFuse with Barebox

The SRK fuses contain the hash value 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 the yocto-secure distro for the first time, the bootloader image is signed with PHYTEC's development keys. Yocto stores these development keys to yocto/phytec-dev-ca. Please refer to Change PKI-Tree from phytec-dev-ca to the own PKI.

For NXP i.MX Controllers with HABv4 (i.MX6, i.MX7, i.MX8M, i.MX8M mini), you will need the SRK_1_2_3_4_fuse.bin file. An example file is located in yocto/phytec-dev-ca/nxp_habv4_pki/crts/SRK_1_2_3_4_fuse.bin.

Check the Current state of your device

bootloader$ hab -i

You will receive:

Current SRK hash: 0000000000000000000000000000000000000000000000000000000000000000       
devel mode

Burn the eFuse with the right SRK_1_2_3_4_fuse.bin!

bootloader$ hab -p -s SRK_1_2_3_4_fuse.bin

You will receive:

ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
barebox@PHYTEC phyCORE-i.MX6 Quad with eMMC:/

Verify new SRK eFuse

bootloader$ hab -i

You will receive:

Current SRK hash: 3425849ab41a49b07ba0b6d5e7dc92fd7cc80dc1a904bdd8e49f4e705953029b      
devel mode

Lock the device with Barebox

Lock down your device

bootloader$ hab -p -l

You will receive:

ocotp0: reloading shadow registers...
ocotp0: reloading shadow registers...
Device successfully locked down

Filesystem Encryption

Root filesystem encryption adds another layer of security to your product. It uses the kernel's cryptographic support to encrypt all the data you store in the root filesystem. Attempting to access this data without the correct encryption key returns random, meaningless bytes.

Boot Process flow:

• bootloader verifies FIT-Image with linux-kernel image, device tree, and ramdisk before they are executed
• Linux kernel executes the ramdisk (read only filesystem)
• The bootscript loads the encrypted filesystem encryption key with the CAAM unit in the RAM and encrypts the filesystem. After the encryption, the root filesystem will be switched and the boot process continues.

Requirements for Filesystem Encryption

• File encryption support for block device (SD card, eMMC) or MTD device (NAND, NOR)
• Master Key Storage to securely store the encryption key

Master Key Storage

NXP i.MX CAAM

The i.MX6 processors include hardware encryption through NXP's Cryptographic Accelerator and Assurance Module (CAAM, also known as SEC4). The CAAM combines functions to create a modular and scalable acceleration and assurance engine.

The kernel 4.19.100 of the BSP includes patches picked from Mailinglist to introduce a new key type, "secure key", into the kernels keyring. this key type is required to use NXPs CAAM module for sorting the filesystem encryption key as blob.

More information about the patches can be found here: https://patchwork.kernel.org/project/linux-security-module/list/?submitter=181647)
More information about the CAAM module can be found in the corresponding NXP reference Manual: https://www.nxp.com/docs/en/reference-manual/i.MX_Reference_Manual_Linux.pdf

Additionally, yogurt-secure includes patches to use this new key type directly with dm-crypt.

Refer to Key Generation for Filesystem Encryption for key generation and key storage.

Load an encrypted key

kernel$ keyctl add secure rootfs "load `cat /mnt_secrets/secrets/secure_key.blob`" @u

Starting the Build Process

Filesystem encryption is not included in the DISTRO_FEATURE secureboot. You need to enable it manually.

You will find an example on how to enable filesystem encryption in your distro in meta-yogurt:

• phytec-secureboot-ramdisk-fitimage
• phytec-secureboot-ramdisk-image

Filesystem encryption has several dependencies:

Build All Necessary Images and SD card / eMMC Image

host$ bitbake phytec-headless-image

When you change something on the boot init script, then you should clean at least the recipes for ramdisk and FIT-Image, too. If you need a new SD card / eMMC image, then clean the image for encryption partition, too

Setup Root Filesystem Encryption on your Device

The filesystem encryption ensures the target has a unique key or a equal key per device.

The filesystem encryption process flow:

• The filesystem encryption key is generated and stored encrypted with CAAM.
• The partition is formatted.
• Encryption is initialized.
  
• Data is copied to the encrypted partition.

1. First Boot of the Device

1. From a high-level point of view, an eMMC device is like an SD card. Therefore, it is possible to flash the image <name>.sdcard from the Yocto build system directly to the SD card or eMMC. The image contains the signed bootloader, signed FIT-Image, and root filesystem.
2. Boot the system. At the first boot, the device stops with following message because there is no encrypted key stored in the folder / secrets:

[ERROR]: No secure_key.blob for Fileencryption in folder /mnt_secrets/secrets


(none) login:

The Protection Shield Level low is in default with user: root and password: root

2. Key Generation for Filesystem Encryption

You can choose between:

• One individual filesystem encryption key for every device:

kernel$ keyctl add secure rootfs "new 64" @u

• The same filesystem encryption key for many devices

Create the filesystem encryption key one-time and copy to an external device:

kernel$ cat /dev/hwrng | tr -dc a-f0-9 | head -c${1:-128} > keyfile128

Import the key to the kernel keyring:

kernel$ keyctl add secure rootfs "load_plain `cat keyfile128`" @u

Both variants can used with our implementation. The symmetric filesystem encryption key is stored, encrypted with an individual device key in the processor (NXP CAAM unit). The result is named key blob.

The type of the key blob created is red. Red key blobs store the encrypted filesystem encryption key for usage at runtime in the RAM. The alternative is black key blob, which store the encrypted filesystem encryption key for usage in a secure memory. Black key blobs are not supported in the driver at the moment.

• Show the created or imported filesystem encryption key:

kernel$ keyctl show
Session Keyring
1064738065 --alswrv      0 65534  keyring: _uid_ses.0
 635505777 --alswrv      0 65534   \_ keyring: _uid.0
 347540047 --als-rv      0     0       \_ secure: rootfs

The key type of the filesystem encryption key in the kernel keyring is secure. This is a new key type, which is not part in the mainline kernel at the moment.

• Key Blob export to the filesystem /mnt_secrets/secrets:

kernel$ mkdir /mnt_secrets/secrets
kernel$ keyctl pipe 347540047 > /mnt_secrets/secrets/secure_key.blob

3. Install Encrypted Filesystem

The file encryption example uses the Linux kernel device-mapper crypto target (dm-crypt).

Please copy the filesystem image, <IMAGENAME>-<MACHINE>.tar.gz, to a USB drive fso that it can be installed on the target.

• Create a mapper /dev/dm-0 for
  • SD card Partition: /dev/mmcblk0p2
  • eMMC Partition: /dev/mmcblk3p2

kernel$ dmsetup create encrootfs --table "0 $(blockdev --getsz  /dev/mmcblk0p2) crypt aes-xts-plain64 :64:secure:rootfs 0 /dev/mmcblk0p2 0"

• Format the new mapper with ext4 filesystem:

kernel$ mkfs.ext4 /dev/dm-0

You will receive:

mke2fs 1.44.5 (15-Dec-2018)
ext2fs_check_if_mount: Can't check if filesystem is mounted due to missing mtab file while determining whether /dev/dm-0 is mounted.
64-bit filesystem support is not enabled.  The larger fields afforded by this feature enable full-strength checksumming.  Pass -O 64bit to rectify.
Creating filesystem with 230352 1k blocks and 57768 inodes
Filesystem UUID: 90ee890d-b4b4-431c-87de-2adb6bb006ef
Superblock backups stored on blocks:
        8193, 24577, 40961, 57345, 73729, 204801, 221185

Allocating group tables: done
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

• Mount the formatted mapper:

kernel$ mount /dev/dm-0 /newroot

• Mount the USB drive with filesystem image <IMAGENAME>-<MACHINE>.tar.gz:

kernel$ mkdir mnt
kernel$ mount /dev/sda1 /mnt

• Untar <IMAGENAME>-<MACHINE>.tar.gz rootfs image to it:

kernel$ tar xvfz /mnt/phytec-headless-image-phyboard-mira-imx6-5.tar.gz -C /newroot/

• Sync the filesystem:

kernel$ sync

• Unmount the partition:

kernel$ umount /newroot

• Remove mapper:

kernel$ cryptsetup remove encrootfs

• If you have a RAUC A/B system, then repeat these instructions for:
  • SD card Partition: /dev/mmcblk0p4
  • eMMC Partition: /dev/mmcblk3p4
• After the installation, power off the system:

kernel$ poweroff -f

• Restart the system. After a sucessfull installation, the system will boot to the kernel login console.

Recover a System

If your filesystem is damaged or the secure_key.blob is deleted, then you can reinstall the encrypted filesystem with the following commands.

• Stop booting in the bootloader.
• Add linux bootargs:

bootloader$ global linux.bootargs.rescue="rescue=1"

• Boot the FIT-Image:

bootloader$ boot

• The boot process can be stopped with a login console in the ramdisk like First Boot of the Device.

Hint for RAUC Updates

The RAUC handler on the target only backs up and restores the secrets for boot0 partition. 

The RAUC handler must be reworked to handle the updates for an encrypted filesystem. Additionally, the data in the RAUC bundles should be encrypted during the transition to the target with a different key.

Protection Shield Level

The DISTRO_FEATURE protection shield offers an easy way to switch between different protection levels. You can use the low protection level during your development and easily switch to a higher level for your final build when entering series production.

Password Protection

The following table shows the different protection shield levels for password protection.

The password protection depends on the quality of the password. The default password for user root is root, which is a very low quality password.

Enable Protection Shield Support in Yocto

Enable Protection Shield in Your Own Distro

To enable the protection shield add the DISTRO_FEATURE protection shield to your distro configuration file conf/distro/xyz.conf. The variables PROTECTION_SHIELD_LEVEL and PROTECTION_SHIELD_ROOT_PASSWORD must be set:

 DISTRO_FEATURES += "protectionshield"                                            
 PROTECTION_SHIELD_LEVEL = "shieldlow"                                            
 #user root password for shield low and midle                                     
 #password quality decide about the protection level                              
 PROTECTION_SHIELD_ROOT_PASSWORD ?= 'root'

Enable Protection Shield Level in Your Kernel Image

To enable protection shield in your linux kernel image, you need to include security/userauthentication.inc into your image recipe.  For an example, please take a look at recipes-images/images/phytec-headless-image.bb included in yogurt-secure.

If you use a ramdisk for your filesystem, you will need to include security/userauthentication.inc into your ramdisk recipe, too. For an example, please take a look at recipes-images/images/phytec-secureboot-ramdisk-image.bb included in yogurt-secure.

Additional Device Security Measures

To further protect your device, it is important to reduce attack vectors. Start by securing development features like JTAG and serial downloader.

Burn eFuses from the Kernel

For activation or deactivation of controller features, is necessary to write and read eFuses. In the kernel of the BSP is a driver to access the OTP (One Time Programmable) eFuses, which is deactivated by default.

Activate the Freescale On-Chip OTP driver in your kernel config:

CONFIG_FSL_OTP=y

Access to the eFuses on the target with cat and echo:

kernel$ cat /sys/fsl_otp/HW_OCOTP_MAC1

You will receive the first part of the programmed MAC address:

0x502d

Secure JTAG

Most embedded devices provide a JTAG interface for debugging purposes. However, if left unprotected, this interface can become an important attack vector on the systems in series production. The i.MX6 series System JTAG Controller (SJC) allows you to regulate JTAG access with three security modes using OTP (One Time Programmable) eFuses:

Additional information about JTAG Security can be found here: https://www.nxp.com/docs/en/application-note/AN4686.pdf

Disabled Debugging Mode

Set JTAG to "Disabled debugging" mode:

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<23 | 1<<22))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Secure JTAG Mode

Set JTAG to "Secure" mode:

1. Generate a secret response key

   You can choose between Identical Response Keys on each device or unique response keys for every device

   kernel$ cat /dev/hwrng | tr -dc a-f0-9 | head -c${1:-14} | sed 's,^\([[:xdigit:]]\{6\}\)\([[:xdigit:]]\{8\}\),0x00\1 0x\2,g'

   You will receive, for example:

   0x00edcba9 0x87654321

2. Write secret response key:

   kernel$ echo 0x87654321 > /sys/fsl_otp/HW_OCOTP_RESP0
   kernel$ echo 0x00edcba9 > /sys/fsl_otp/HW_OCOTP_HSJC_RESP1

3. Lock access to secret response key:

   kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_LOCK) | 1<<6 ))")
   kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_LOCK

4. Set Secure JTAG mode:

   kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 0<<23 | 1<<22))")
   kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Disabled JTAG Mode

Set JTAG to "Disabled":

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<20 ))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Disable HAB JTAG Enabling

When JTAG is set in "Secure", an option to enable HAB JTAG DEBUG becomes available. By writing '1' to HAB_JDE (HAB JTAG DEBUG ENABLE) bit in the e-fuse controller module, the JTAG is opened, regardless of its security mode. If this feature is not required, it is highly recommended this be disabled. 

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<27 ))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Disable Serial Downloader

The i.MX6 Controller supports the burning of different eFuse to disable the USB Serial Download Protocol (SDP) and Force Internal Boot Only to enhance device security.

For more information on i.MX6 controllers, please visit https://community.nxp.com/docs/DOC-334996

Below is a list of PHYTEC Products and varients that include the controller version with the feature for Disable serial downloader and Force Internal Boot:

For phyFLEX-i.MX6 and phyCARD-i.MX6 support, ask your sales contact.

Disable SDP Mode Completely

Disabling the serial download support is recommended for security enabled configurations:

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<0 ))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Disable Read Access in SDP

Disabling only read access via SDP. Writing will still be possible.

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<2 ))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Force Internal Boot

Ensure the device always boots in INTERNAL BOOT mode, ignoring BOOT_MODE pins. This setting is recommended for security enabled configurations.

This feature is supported in the listed PHYTEC product versions Disable Serial Downloader.

kernel$ value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_CFG5) | 1<<31 ))")
kernel$ echo $value > /sys/fsl_otp/HW_OCOTP_CFG5

Keys and Certificates

Public Key Infrastructure Tree (PKI tree)

To use secure boot with barebox and kernel image, several keys and certificates are required to sign the images. The key and certificate creation is a manual process and the public key infrastructure (PKI) tree must be in place before you start your build. This BSP includes the phytec development pki-tree as an example. You are obligated to create your own pki-tree with your own keys and certificates.

PHYTEC Development Keys (phytec-dev-ca)

The included phytec-dev-ca example consists of a self-signed main-ca and three derived sub-ca's for bootloader, Fit-Image, and RAUC updates.

The recipes for Bootloader, FIT-Image, and RAUC depend on the recipe phytec-dev-ca. If you build the BSP for the first time, the PHYTEC development keys are downloaded in yocto/phytec-dev-ca und used to sign the Bootloader, FIT-Image, and the RAUC bundles.

NXP HABv4 Key Structure

The NXP HABv4 Key structure is for signing the bootloader, which is verified from the ROM bootloader of the NXP controller with the HABv4 module. The i.MX6, i.MX6UL, i.MX7, i.MX8M, and i.MX8M Mini include a HABv4 module for secure boot.

• CA (Certification Authority): This certificate is used to sign the SRK keys and establish the authority of other keys. There is only one CA certificate per PKI tree. This certificate is never used on the target and has no requirements. An existing certificate can be used as CA during the generation of all these keys. The rest of the keys and certificates are always generated and have special requirements, as they are directly used on the target.
• SRK (Super Root Keys): This certificate is used to sign the CSF and IMG certificates. There are up to four SRK certificates per PKI tree (each one is used to sign one CSF and one IMG certificates). See revoke a key for more information on having multiple SRK certificates.
• CSF (Command Sequence File): This certificate is used to validate the CSF region.
• IMG: This certificate is used to validate the bootloader.

Create Your Own PKI Tree

We included a script in meta-yogurt/scripts/ to build your own PKI tree. The script helps you generate a main-ca, bootloader keys, Fit-Image keys, and RAUC keys.

Requirements

To generate keys and certficates for the bootloader, you well need two scripts from the NXP CST-Tool.

• Copy st-3.2.0/release/keys/hab4_pki_tree.sh and cst-3.2.0/release/linux64/bin/srktool into the yocto/source/meta-yogurt folder.
• Check the configuration files for main-ca, Fit-Image, and RAUC in meta-yogurt/scripts/, adjust to your requirements.

1. Create the main-ca

host$ ./scripts/openssl-ca.sh -m

You will receive:

Creation of the Main CA for Bootloader, FIT-Image and
rauc bundle Signning
The Main CA based on RSA4096
No pass phrase for protecting the Main CA private keys
Please put a pass phrase (store it in the key_pass.txt):

2. Create bootloader signing and verification keys and certificate

host$ ./scripts/openssl-ca.sh -b -s

You will receive:

serial: a positive 32-bit number that uniquely identifies
the certificate per certification authority e.g. 12345678 ?08154711

Main ca exist, please use existing CA in SRK creation Tool with
CA key name: /home/user/yocto/sources/meta-yogurt/openssl-ca/main-ca/private/mainca-rsa.key
CA certificate name:  /home/user/yocto/sources/meta-yogurt/openssl-ca/main-ca/mainca-rsa.crt
Do you want start the SRK creation with hab4_pki_tree.sh (y/n)y
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
the script  hab4_pki_tree.sh is started
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Do you want to use an existing CA key (y/n)?:y
Enter CA key name:/home/user/yocto/sources/meta-yogurt/openssl-ca/main-ca/private/mainca-rsa.key
Enter CA certificate name:/home/user/yocto/sources/meta-yogurt/openssl-ca/main-ca/mainca-rsa.crt
Do you want to use Elliptic Curve Cryptography (y/n)?:n
Enter key length in bits for PKI tree: 4096
Enter PKI tree duration (years): 10
How many Super Root Keys should be generated? 4
Do you want the SRK certificates to have the CA flag set? (y/n)?: y

• The serial content must be a positive 32-bit number that uniquely identifies the certificate per certification authority.
• You can use an existing key as CA key by answering the first question with 'y' and then providing the path of the given created main-ca certificate and the key for the certificate to be used as CA.
• If asked about using ECC cryptography, answer 'n'. The RSA keys are used for the signature.
• The following key sizes are supported: 1024, 2048, and 4096.
• The PKI duration is used to compute the expiration date for the certificates.

• You must generate four keys (for key revocation purposes).
• The last question regarding the “CA flag” in the SRK must be answered as 'y'

The generated Keys, Certificates (IMG1_1_sha256_4096_65537_v3_usr_crt.pem, CSF1_1_sha256_4096_65537_v3_usr_crt.pem), and SRK_1_2_3_5_fuse.bin are in the folder /home/user/yocto/sources/meta-yogurt/openssl-ca/nxp_habv4_pki/crts/.

3. Create FIT-Image key

host$ ./scripts/openssl-ca.sh -f

You will receive:

Creation of Signing Keys for FIT-Image
Enter key length in bits for PKI tree:4096

• The following key sizes are supported: 2048 and 4096.

4. Create RAUC keys

host$ ./scripts/openssl-ca.sh -r

You will receive:

Creation of Signing Keys for rauc bundles
Enter key length in bits for rauc PKI tree: 2048

• The following key sizes are supported: 2048.

Change PKI-Tree from phytec-dev-ca to Your Own PKI

In the configuration class yocto/source/meta-yogurt/secureboot.bbclass is the path to youe PKI tree initial defined.

CERT_PATH ??= "${OEROOT}/../../phytec-dev-ca"

If you want change the path, then reinit the CERT_PATH ?= in your layer or overwrite the CERT_PATH in the secureboot.class.

The name of your PKI tree must have a name other then phytec-dev-ca. The recipe for phytec-dev-ca use the name phytec-dev-ca as parameter for the clean command.

After the CERT_PATH has been changed, you must clean and rebuild the bootloader, FIT-Image, RAUC bundles, and the rootfs!

Encryption Keys

Revoke a key

Revoke NXP SRK Key

Although securing the device involves programming the hash of four public keys into the eFuses, only one key (number 1 by default) is used in the secure boot process. If the key gets compromised, it can be revoked and a different key used.

To use a different key for the signature of bootloader images, change the following variables in yocto/sources/meta-yogurt/classes/secureboot.bbclass:

BOOTLOADER_SIGN_IMG_PATH_mx6 ??= "${CERT_PATH}/nxp_habv4_pki/crts/IMG1_1_sha256_4096_65537_v3_usr_crt.pem"
BOOTLOADER_SIGN_CSF_PATH_mx6 ??= "${CERT_PATH}/nxp_habv4_pki/crts/CSF1_1_sha256_4096_65537_v3_usr_crt.pem"

The following keys are available:

The kernel driver support to burn eFuses must be activated: Burn eFuses from the Kernel.

Revoke Key Slot 0

value=$(printf '0x%04x' "$(( $(cat /sys/fsl_otp/HW_OCOTP_SRK_REVOKE) | 1<<0 | 0 << 1 | 0<< 2 ))")
echo $value > /sys/fsl_otp/HW_OCOTP_SRK_REVOKE

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