The Python program
scripts/imgtool.py can be used to perform the
operations that are necessary to manage keys and sign images. Using
this script should be preferred to the manual steps described in
This program is written for Python3, and has several dependencies on Python libraries. These can be installed using ‘pip3’:
pip3 install --user -r scripts/requirements.txt
This tool currently supports rsa-2048, rsa-3072, ecdsa-p256 and ed25519 keys. You can generate a keypair for one of these types using the ‘keygen’ command:
./scripts/imgtool.py keygen -k filename.pem -t rsa-2048
or use rsa-3072, ecdsa-p256, or ed25519 for the type. The key type used should match what mcuboot is configured to verify.
This key file is what is used to sign images, this file should be protected, and not widely distributed.
You can add the
-p argument to
keygen, which will cause it to
prompt for a password. You will need to enter this password in every
time you use the private key.
Incorporating the public key into the code
There is a development key distributed with mcuboot that can be used for testing. Since this private key is widely distributed, it should never be used for production. Once you have generated a production key, as described above, you should replace the public key in the bootloader with the generated one.
For Zephyr, the keys live in the file
mynewt, follow the instructions in
doc/signed_images.md to generate
the key file.
./scripts/imgtool.py getpub -k filename.pem
will extract the public key from the given private key file, and output it as a C data structure. You can replace or insert this code into the key file.
Image signing takes an image in binary or Intel Hex format intended for the primary slot and adds a header and trailer that the bootloader is expecting:
Usage: imgtool.py sign [OPTIONS] INFILE OUTFILE Create a signed or unsigned image Options: -k, --key filename --align [1|2|4|8] [required] -v, --version TEXT [required] -H, --header-size INTEGER [required] --pad-header Add --header-size zeroed bytes at the beginning of the image -S, --slot-size INTEGER Size of the slot where the image will be written [required] --pad Pad image to --slot-size bytes, adding trailer magic -M, --max-sectors INTEGER When padding allow for this amount of sectors (defaults to 128) --overwrite-only Use overwrite-only instead of swap upgrades -e, --endian [little|big] Select little or big endian -E, --encrypt filename Encrypt image using the provided public key -h, --help Show this message and exit.
The main arguments given are the key file generated above, a version field to place in the header (1.2.3 for example), the alignment of the flash device in question, and the header size.
The header size depends on the operating system and the particular
flash device. For Zephyr, it will be configured as part of the build,
and will be a small power of two. By default, the Zephyr build system will
already prepended a zeroed header to the image. If another build system is
in use that does not automatically add this zeroed header,
be passed and the
--header-size will be added by imgtool. If
is used with an Intel Hex file,
--header-size bytes will be subtracted from
the load address (in Intel Hex terms, the Extended Linear Address record) to
adjust for the new bytes prepended to the file. The load address of all data
existing in the file should not change.
--slot-size argument is required and used to check that the firmware
does not overflow into the swap status area (metadata). If swap upgrades are
not being used,
--overwrite-only can be passed to avoid adding the swap
status area size when calculating overflow.
--pad argument will place a trailer on the image that
indicates that the image should be considered an upgrade. Writing this image
in the secondary slot will then cause the bootloader to upgrade to it.