Image tool

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 doc//documentation/signed-images/.

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

Managing keys

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 boot/zephyr/keys.c. For mynewt, follow the instructions in doc//documentation/signed-images/ 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. However, when the MCUBOOT_HW_KEY config option is enabled, this last step is unnecessary and can be skipped.

Signing images

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

  INFILE and OUTFILE are parsed as Intel HEX if the params have .hex
  extension, otherwise binary format is used

Options:
  -k, --key filename
  --public-key-format [hash|full]
  --align [1|2|4|8]             [required]
  -v, --version TEXT            [required]
  -s, --security-counter TEXT   Specify the value of security counter. Use
                                the `auto` keyword to automatically generate
                                it from the image version.
  -d, --dependencies TEXT
  --pad-sig                     Add 0-2 bytes of padding to ECDSA signature
                                (for mcuboot <1.5)
  -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
  --confirm                     When padding the image, mark it as confirmed
  -M, --max-sectors INTEGER     When padding allow for this amount of
                                sectors (defaults to 128)
  --boot-record sw_type         Create CBOR encoded boot record TLV. The
                                sw_type represents the role of the software
                                component (e.g. CoFM for coprocessor
                                firmware). [max. 12 characters]
  --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
  --save-enctlv                 When upgrading, save encrypted key TLVs
                                instead of plain keys. Enable when
                                BOOT_SWAP_SAVE_ENCTLV config option was set.
  -L, --load-addr INTEGER       Load address for image when it should run
                                from RAM.
  -x, --hex-addr INTEGER        Adjust address in hex output file.
  -R, --erased-val [0|0xff]     The value that is read back from erased
                                flash.
  -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, --pad-header can be passed and the --header-size will be added by imgtool. If --pad-header 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.

The --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.

The optional --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.

A dependency can be specified in the following way: -d "(image_id, image_version)". The image_id is the number of the image which the current image depends on. The image_version is the minimum version of that image to satisfy compliance. For example -d "(1, 1.2.3+0)" means this image depends on Image 1 which version has to be at least 1.2.3+0.

The --public-key-format argument can be used to distinguish where the public key is stored for image authentication. The hash option is used by default, in which case only the hash of the public key is added to the TLV area (the full public key is incorporated into the bootloader). When the full option is used instead, the TLV area will contain the whole public key and thus the bootloader can be independent from the key(s). For more information on the additional requirements of this option, see the design document.