As discussed in the earlier blog, it is becoming very important, in an embedded system, to ensure authenticity of the firmware before running it. Also, the system has to be made tamper proof against further hacking, especially for remotely managed internet connected IoT applications.

To prevent breach of security, the software can be strengthened with various techniques based on the underlying MCU and peripheral set. This blog discusses in particular how it can be done for iMx RT1020 based devices using the High Assurance Boot (HAB) mechanism as recommended by NXP.

Secure Boot Concepts

NXP’s HAB uses the mechanism of asymmetric encryption to protect its firmware. To give a quick introduction to asymmetric encryption, it is essentially creating a pair of keys in a way that one of the keys can encrypt the message and other can decrypt the message (and vice versa). It is mathematically impossible to use the same key used for encryption to decrypt the message. Also, with increased key sizes, it will be highly resource consuming to decrypt a message without the other pair.

Thus, with asymmetric encryption, it is enough to protect one of the keys (private key) and other can be shared (public key). The message encrypted by the private key can only be decrypted by the public key. Further if the public key (or at least its hash) is stored in a location that can not be modified such as On Time Programmable Flash, it will be impossible for any one to compromise the system. An attempt to modify the public key will be nullified because of the check with the OTP memory.

The high level of the above sequence can be capture in the below sequence diagram.

Secure Boot iMX RT 1020 HAB process

During the device provisioning process, the public and private key pairs are generated and private key is secured in the provisioning system. Hash for the public key is generated and stored in the device OTP area, which prevents further modification.

In the code signing sequence, the firmware image is hashed and encrypted using the private key. The final image generated is comprised of the firmware image, its encrypted image along with the public key and the same is programmed on to the boot memory.

During the bootup sequence, the HAB logic extracts the individual components of the signed image and validates to authenticity of the public key by comparing the computed hash and that stored in the OTP fuses. It is impossible to create public key such that the hash is same there by preventing any attempt of overriding the public key by external parties. Then it proceeds to calculate the hash of the firmware. It is compared with a hash generated by decrypting the encrypted hash using the public key. If it is a match, it proceeds to boot. If it fails in any of the place, the boot is aborted.

Code Signing for i.Mx RT1020

NXP provides all the tools necessary for generating public-private key pairs, code signing and blowing boot flashes such as MfgTool, elftosb, cst etc.

The device can be programmed using two methods: Device Boot and Secure Boot. The Device boot mode can be used during development purposes and secure boot for final programming. If the device is once programmed in Secure boot mode, it is not possible to revert back to Dev Boot mode and all further firmware has to be signed properly. The programming process is carried out by Flashloader tools such as- elftosb tool for boot image creation, Mfg tool for boot image programming.

Dev Boot Mode

To program the device, use the Mfgtool.

  • Create unsigned boot_image.sb using elftosb tool from SREC format of the application image (app.s19 file).
  • Make sure the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-DevBoot
  • Import the boot_image.sb file to …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from …/Tools/elftosb/linux/amd64/
  • After generating the boot_image.sb and placing it in the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware
  • Change the device’s boot mode into serial downloader mode and connect it to the host PC
  • Run the MfgTool and press the start button to program the target.
  • To exit MfgTool, click “Stop” and “Exit” in turn

Secure Boot Mode

To program the OTP flash once with hash of the public key, use the Mfgtool as follows

  • Check whether the device is in serial downloader mode
  • Generate the private/public keys using CST tool and create fuse.bin and fuse.table files.
  • Make sure the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-Burnfuse
  • Create and import the enable_hab.sb file to the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from the directory …/Flashloader_RT1020_1.0_GA/Tools/elftosb/linux/amd64/
  • After programming the above mentioned enable_hab.sb file successfully, the device will be ready for secure boot.

The above process of programming the fuse has to be executed only once. Further mode to program the device with signed image, use the Mfgtool as follows

  • Create signed boot_image.sb using elftosb tool from SREC format of the application image (app.s19 file).
  • Check whether the file inside the Mfg tool is available in the name – cfg.ini
  • The content inside the file should be in the following format : chip → MXRT102X, name → MXRT102X-SecureBoot
  • Import the signed boot_image.sb file to the following directory …/Tools/mfgtools-rel/Profiles/MXRT102X/OS Firmware from the directory …/Flashloader_RT1020_1.0_GA/Tools/elftosb/linux/amd64/

The details of the process can be obtained from NXP i.Mx1020 product page. Once secured, it will be impossible to run unauthorized software.

Same concepts can be extended to OTA updates so that the new firmware can be authenticated even before programming.

About Embien :

Embien has been actively developing IoT devices that form important part of a larger network with huge ramifications on security. We have been using advanced tools and techniques to prevent unauthorized access and tampering of the device. Get in touch with us to get your design unprecedented security.

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