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Signing Binaries

The west sign extension command can be used to sign a Zephyr application binary for consumption by a bootloader using an external tool. In some configurations, west sign is also used to invoke an external, post-processing tool that “stitches” the final components of the image together. Run west sign -h for command line help.

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.

If you use this feature, you don’t need to run west sign yourself; the build system will do it for you.

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 bootloader/mcuboot/boot/zephyr -d build-mcuboot
west build -b YOUR_BOARD zephyr/samples/hello_world -d build-hello-signed -- \
     -DCONFIG_BOOTLOADER_MCUBOOT=y \
     -DCONFIG_MCUBOOT_SIGNATURE_KEY_FILE=\"bootloader/mcuboot/root-rsa-2048.pem\"

west flash -d build-mcuboot
west flash -d build-hello-signed

Notes on the above commands:

YOUR_BOARD should be changed to match your board
The CONFIG_MCUBOOT_SIGNATURE_KEY_FILE 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:

CONFIG_BOOTLOADER_MCUBOOT: build the application for loading by MCUboot
CONFIG_MCUBOOT_SIGNATURE_KEY_FILE: the key file to use with west sign. 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-mcuboot:

*** 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
[00:00:00.027,313] <wrn> mcuboot: Failed reading image headers; Image=0
[00:00:00.035,064] <err> mcuboot: Unable to find bootable image

Then, you should see something like this 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.

rimage

rimage configuration uses a different approach that does not rely on Kconfig or CMake but on west config instead, similar to Permanent CMake Arguments.

Signing involves a number of “wrapper” scripts stacked on top of each other: west flash invokes west build which invokes cmake and ninja which invokes west sign which invokes imgtool or rimage. As long as the signing parameters desired are the default ones and fairly static, these indirections are not a problem. On the other hand, passing imgtool or rimage options through all these layers can causes issues typical when the layers don’t abstract anything. First, this usually requires boilerplate code in each layer. Quoting whitespace or other special characters through all the wrappers can be difficult. Reproducing a lower west sign command to debug some build-time issue can be very time-consuming: it requires at least enabling and searching verbose build logs to find which exact options were used. Copying these options from the build logs can be unreliable: it may produce different results because of subtle environment differences. Last and worst: new signing feature and options are impossible to use until more boilerplate code has been added in each layer.

To avoid these issues, rimage parameters can bet set in west config instead. Here’s a workspace/.west/config example:

[sign]
# Not needed when invoked from CMake
tool = rimage

[rimage]
# Quoting is optional and works like in Unix shells
# Not needed when rimage can be found in the default PATH
path = "/home/me/zworkspace/build-rimage/rimage"

# Not needed when using the default development key
extra-args = -i 4 -k 'keys/key argument with space.pem'

In order to support quoting, values are parsed by Python’s shlex.split() like in One-Time CMake Arguments.

The extra-args are passed directly to the rimage command. The example above has the same effect as appending them on command line after -- like this: west sign --tool rimage -- -i 4 -k 'keys/key argument with space.pem'. In case both are used, the command-line arguments go last.