SecureBoot and Encryption

Last updated 2 months ago

Summary

In order to encrypt your firmware, you will need to build it from source. Our firmware source code can be found here, along with instructions on how to build it. Below you will find specific instructions on how generate keys, build and flash encrypted firmware.

  1. Obtain keys (for Secure Boot and Flash Encryption)

  2. Flash keys and parameters in efuses

  3. Compile bootloader and application with make SECURE=on

  4. Flash: bootloader-digest at address 0x0 and encrypted; all the others (partitions and application) encrypted, too.

Prerequisites

Firstly you will need to setup the tool chain and download the source code. detailed instructions on how to achieve this can be found here. Once you have complete this, you will need to open a terminal in the esp32 folder of the firmware source code repo.

Next you will need keys for Flash Encryption and Secure Boot; they can be generated randomly with the following commands:

python $IDF_PATH/components/esptool_py/esptool/espsecure.py generate_flash_encryption_key flash_encryption_key.bin
python $IDF_PATH/components/esptool_py/esptool/espsecure.py generate_signing_key secure_boot_signing_key.pem

The Secure Boot key secure_boot_signing_key.pem has to be transformed into secure-bootloader-key.bin, to be burnt into efuses. This can be done in 2 ways:

python $IDF_PATH/components/esptool_py/esptool/espsecure.py extract_public_key --keyfile secure_boot_signing_key.pem signature_verification_key.bin

or, as an artifact of the make build process, on the same directory level as Makefile

make BOARD=GPY SECURE=on TARGET=boot

To flash the keys (flash_encryption_key.bin and secure-bootloader-key.bin) into the efuses (write and read protected) run the following commands (ignoring the lines that start with #):

Note: Irreversible operations

# Burning Encryption Key
python $IDF_PATH/components/esptool_py/esptool/espefuse.py --port /dev/ttyUSB0 burn_key flash_encryption flash_encryption_key.bin
# Burning Secure Boot Key
python $IDF_PATH/components/esptool_py/esptool/espefuse.py --port /dev/ttyUSB0 burn_key secure_boot secure-bootloader-key.bin
# Enabling Flash Encryption mechanism
python $IDF_PATH/components/esptool_py/esptool/espefuse.py --port /dev/ttyUSB0 burn_efuse FLASH_CRYPT_CNT
# Configuring Flash Encryption to use all address bits together with Encryption key (max value 0x0F)
python $IDF_PATH/components/esptool_py/esptool/espefuse.py --port /dev/ttyUSB0 burn_efuse FLASH_CRYPT_CONFIG 0x0F
# Enabling Secure Boot mechanism
python $IDF_PATH/components/esptool_py/esptool/espefuse.py --port /dev/ttyUSB0 burn_efuse ABS_DONE_0

If the keys are not written in efuse, before flashing the bootloader, then random keys will be generated by the ESP32, they can never be read nor re-written, so bootloader can never be updated. Even more, the application can be re-flashed (by USB) just 3 more times.

Makefile options

make BOARD=GPY SECURE=on SECURE_KEY=secure_boot_signing_key.pem ENCRYPT_KEY=flash_encryption_key.bin TARGET=[boot|app]
  • SECURE=on is the main flag; it's not optional

  • if SECURE=on the following defaults are set:

    • encryption is enable

    • secure_boot_signing_key.pem is the secure boot key, located relatively to Makefile

    • flash_encryption_key.bin is the flash encryption key, located relatively to Makefile

For flashing the bootloader digest and the encrypted versions of all binaries:

make BOARD=GPY SECURE=on flash

Flashing

For flashing the bootloader-reflash-digest.bin has to be written at address 0x0, instead of the bootloader.bin (at address 0x1000).

Build is done using SECURE=on option; additionally, all the binaries are pre-encrypted.

make BOARD=GPY clean
make BOARD=GPY SECURE=on TARGET=boot
make BOARD=GPY SECURE=on TARGET=app
make BOARD=GPY SECURE=on flash

Manual flash command:

python $IDF_PATH/components/esptool_py/esptool/esptool.py --chip esp32 --port /dev/ttyUSB0 --baud 921600 --before no_reset --after no_reset write_flash -z --flash_mode dio --flash_freq 80m --flash_size detect 0x0 build/GPY/release/bootloader/bootloader-reflash-digest.bin_enc 0x8000 build/GPY/release/lib/partitions.bin_enc 0x10000 build/GPY/release/gpy.bin_enc_0x10000

OTA update

The OTA should be done using the pre-encrypted application image.

Because the encryption is done based on the physical flash address, there are 2 application binaries generated:

  • gpy.bin_enc_0x10000 which has to be written at default factory address: 0x10000

  • gpy.bin_enc_0x1A0000 which has to be written at the ota_0 partition address (0x1A0000)

Hint: on MicroPython interface, the method pycom.ota_slot() responds with the address of the next OTA partition available (either 0x10000 or 0x1A0000).