# 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/signed_images.md`.

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](#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](#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/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.

## [Signing images](#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

    Options:
      -k, --key filename
      --align [1|2|4|8]          [required]
      -v, --version TEXT         [required]
      -d, --dependencies TEXT
      -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, `--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.