Signing Binaries

Binaries can be optionally signed as part of a build automatically using CMake code, there is also the ability to use west sign to sign binaries too, this page describes the former, the latter is documented on Signing Binaries.

MCUboot / imgtool

The Zephyr build system has special support for signing binaries for use with the MCUboot bootloader using the imgtool program provided by its developers. You can both build and sign this type of application binary in one step by setting some Kconfig options. If you do, west flash will use the signed binaries.

Here is an example workflow, which builds and flashes MCUboot, as well as the Hello World application for chain-loading by MCUboot. Run these commands from the zephyrproject workspace you created in the Getting Started Guide.

west build -b YOUR_BOARD zephyr/samples/hello_world --sysbuild -d build-hello-signed -- \
     -DSB_CONFIG_BOOTLOADER_MCUBOOT=y

west flash -d build-hello-signed

Notes on the above commands:

  • YOUR_BOARD should be changed to match your board

  • The singing key value is the insecure default provided and used by MCUboot for development and testing

  • You can change the hello_world application directory to any other application that can be loaded by MCUboot, such as the SMP server sample.

For more information on these and other related configuration options, see:

  • SB_CONFIG_BOOTLOADER_MCUBOOT: build the application for loading by MCUboot

  • SB_CONFIG_BOOT_SIGNATURE_KEY_FILE: the key file to use when singing images. If you have your own key, change this appropriately

  • CONFIG_MCUBOOT_EXTRA_IMGTOOL_ARGS: optional additional command line arguments for imgtool

  • CONFIG_MCUBOOT_GENERATE_CONFIRMED_IMAGE: also generate a confirmed image, which may be more useful for flashing in production environments than the OTA-able default image

  • On Windows, if you get “Access denied” issues, the recommended fix is to run pip3 install imgtool, then retry with a pristine build directory.

If your west flash runner uses an image format supported by imgtool, you should see something like this on your device’s serial console when you run west flash -d build-hello-signed:

*** Booting Zephyr OS build zephyr-v2.3.0-2310-gcebac69c8ae1  ***
[00:00:00.004,669] <inf> mcuboot: Starting bootloader
[00:00:00.011,169] <inf> mcuboot: Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
[00:00:00.021,636] <inf> mcuboot: Boot source: none
[00:00:00.027,374] <inf> mcuboot: Swap type: none
[00:00:00.115,142] <inf> mcuboot: Bootloader chainload address offset: 0xc000
[00:00:00.123,168] <inf> mcuboot: Jumping to the first image slot
*** Booting Zephyr OS build zephyr-v2.3.0-2310-gcebac69c8ae1  ***
Hello World! nrf52840dk_nrf52840

Whether west flash supports this feature depends on your runner. The nrfjprog and pyocd runners work with the above flow. If your runner does not support this flow and you would like it to, please send a patch or file an issue for adding support.

Extending signing externally

The signing script used when running west flash can be extended or replaced to change features or introduce different signing mechanisms. By default with MCUboot enabled, signing is setup by the cmake/mcuboot.cmake file in Zephyr which adds extra post build commands for generating the signed images. The file used for signing can be replaced from a sysbuild scope (if being used) or from a zephyr/zephyr module scope, the priority of which is:

  • Sysbuild

  • Zephyr property

  • Default MCUboot script (if enabled)

From sysbuild, -D<target>_SIGNING_SCRIPT can be used to set a signing script for a specific image or -DSIGNING_SCRIPT can be used to set a signing script for all images, for example:

west build -b <board> <application> -DSIGNING_SCRIPT=<file>

The zephyr property method is achieved by adjusting the SIGNING_SCRIPT property on the zephyr_property_target, ideally from by a module by using:

if(CONFIG_BOOTLOADER_MCUBOOT)
  set_target_properties(zephyr_property_target PROPERTIES SIGNING_SCRIPT ${CMAKE_CURRENT_LIST_DIR}/custom_signing.cmake)
endif()

This will include the custom signing CMake file instead of the default Zephyr one when projects are built with MCUboot signing support enabled. The base Zephyr MCUboot signing file can be used as a reference for creating a new signing system or extending the default behaviour.