Image tool

The Python program imgtool.py can be used to perform the operations that are necessary to manage keys and sign images.

This program is written for Python3, and has several dependencies on Python libraries. These can be installed using 'pip3' manually:

pip3 install --user pycrypto
pip3 install --user pyasn1
pip3 install --user ecdsa

or, on Ubuntu, using the package manager:

sudo apt-get install python3-crypto python3-pyasn1 python3-ecdsa

Managing keys

This tool currently supports rsa-2048 and ecdsa-p256 keys. You can generate a keypair for one of these types using the 'keygen' command:

./imgtool.py keygen -k filename.pem -t rsa-2048

or use ecdsa-p256 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.

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.

./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

Image signing takes a binary image intended for Slot 0 and adds a header and trailer that the bootloader is expecting:

usage: imgtool.py sign [-h] -k filename --align ALIGN -v VERSION -H
                       HEADER_SIZE [--pad PAD] [--rsa-pkcs1-15]
                       infile outfile

positional arguments:
  infile
  outfile

optional arguments:
  -h, --help            show this help message and exit
  -k filename, --key filename
  --align ALIGN
  -v VERSION, --version VERSION
  -H HEADER_SIZE, --header-size HEADER_SIZE
  --included-header     Image has gap for header
  --pad PAD             Pad image to this many bytes, adding trailer magic
  --rsa-pkcs1-15        Use old PKCS#1 v1.5 signature algorithm

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 header will be prepended to the image. If --included-header is given, the image must start with header-size bytes of zeros, and the header will be overwritten over these bytes.

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 slot 1 will then cause the bootloader to upgrade to it.

Lastly, the --rsa-pkcs1-15 will cause the tool to use the older, deprecated pkcs#1 v1.5 signing algorithm when using RSA. This can be enabled in the bootloader as well, and may be needed if you are using an older version of the bootloader.