98 lines
4.2 KiB
Markdown
98 lines
4.2 KiB
Markdown
# Image tool
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The Python program `scripts/imgtool.py` can be used to perform the
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operations that are necessary to manage keys and sign images. Using
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this script should be preferred to the manual steps described in
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`doc/signed_images.md`.
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This program is written for Python3, and has several dependencies on
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Python libraries. These can be installed using 'pip3':
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pip3 install --user -r scripts/requirements.txt
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## Managing keys
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This tool currently supports rsa-2048 and ecdsa-p256 keys. You can
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generate a keypair for one of these types using the 'keygen' command:
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./scripts/imgtool.py keygen -k filename.pem -t rsa-2048
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or use ecdsa-p256 for the type. The key type used should match what
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mcuboot is configured to verify.
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This key file is what is used to sign images, this file should be
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protected, and not widely distributed.
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You can add the `-p` argument to `keygen`, which will cause it to
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prompt for a password. You will need to enter this password in every
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time you use the private key.
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## Incorporating the public key into the code
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There is a development key distributed with mcuboot that can be used
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for testing. Since this private key is widely distributed, it should
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never be used for production. Once you have generated a production
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key, as described above, you should replace the public key in the
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bootloader with the generated one.
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For Zephyr, the keys live in the file `boot/zephyr/keys.c`. For
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mynewt, follow the instructions in `doc/signed_images.md` to generate
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the key file.
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./scripts/imgtool.py getpub -k filename.pem
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will extract the public key from the given private key file, and
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output it as a C data structure. You can replace or insert this code
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into the key file.
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## Signing images
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Image signing takes an image in binary or Intel Hex format intended for Slot 0
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and adds a header and trailer that the bootloader is expecting:
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Usage: imgtool.py sign [OPTIONS] INFILE OUTFILE
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Create a signed or unsigned image
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Options:
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-k, --key filename
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--align [1|2|4|8] [required]
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-v, --version TEXT [required]
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-H, --header-size INTEGER [required]
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--pad-header Add --header-size zeroed bytes at the beginning
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of the image
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-S, --slot-size INTEGER Size of the slot where the image will be written
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[required]
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--pad Pad image to --slot-size bytes, adding trailer
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magic
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-M, --max-sectors INTEGER When padding allow for this amount of sectors
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(defaults to 128)
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--overwrite-only Use overwrite-only instead of swap upgrades
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-e, --endian [little|big] Select little or big endian
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-E, --encrypt filename Encrypt image using the provided public key
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-h, --help Show this message and exit.
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The main arguments given are the key file generated above, a version
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field to place in the header (1.2.3 for example), the alignment of the
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flash device in question, and the header size.
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The header size depends on the operating system and the particular
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flash device. For Zephyr, it will be configured as part of the build,
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and will be a small power of two. By default, the Zephyr build system will
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already prepended a zeroed header to the image. If another build system is
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in use that does not automatically add this zeroed header, `--pad-header` can
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be passed and the `--header-size` will be added by imgtool. If `--pad-header`
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is used with an Intel Hex file, `--header-size` bytes will be subtracted from
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the load address (in Intel Hex terms, the Extended Linear Address record) to
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adjust for the new bytes prepended to the file. The load address of all data
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existing in the file should not change.
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The `--slot-size` argument is required and used to check that the firmware
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does not overflow into the swap status area (metadata). If swap upgrades are
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not being used, `--overwrite-only` can be passed to avoid adding the swap
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status area size when calculating overflow.
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The optional `--pad` argument will place a trailer on the image that
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indicates that the image should be considered an upgrade. Writing
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this image in slot 1 will then cause the bootloader to upgrade to it.
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