MCUboot's Image Signing and Key Management Tool

2.4.0 · active · verified Sun Apr 12

imgtool is a Python library and command-line tool primarily designed for image signing and key management within the MCUboot ecosystem. It facilitates secure firmware updates for embedded systems by providing functionalities to generate cryptographic key pairs, extract public keys, and sign firmware images with necessary headers and trailers for bootloader verification. The library is actively maintained as a core component of the open-source MCUboot project, with releases generally aligning with MCUboot's development cycle.

Warnings

gotcha When installing from the MCUboot source repository, ensure all Python dependencies are manually installed using `pip install -r scripts/requirements.txt`. Installing the `imgtool` PyPI package generally handles these dependencies automatically.
Fix: Use `pip install imgtool` for simpler dependency management, or manually install dependencies from `scripts/requirements.txt` if using the script from the MCUboot source tree.
breaking The `imgtool sign` command requires several critical arguments (`--key`, `--version`, `--header-size`, `--align`, `--slot-size`) that are specific to your MCUboot and target hardware configuration. Incorrect values for these arguments can lead to unsigned images or images that are not accepted by the bootloader.
Fix: Always consult your specific MCUboot build configuration (e.g., Kconfig options in Zephyr) to determine the correct values for `--header-size`, `--align`, and `--slot-size`.
breaking Never use the development key pair provided with the MCUboot repository for production firmware signing. These keys are publicly known and compromise the security of your device.
Fix: Always generate your own unique, strong cryptographic key pairs for production use, and protect your private keys diligently.
deprecated The `--rsa-pkcs1-15` option for RSA signing uses an older, deprecated PKCS#1 v1.5 signing algorithm. While it might be supported by older bootloader versions, it's recommended to use more modern and secure signing schemes.
Fix: Avoid using `--rsa-pkcs1-15` unless absolutely necessary for backward compatibility with older MCUboot versions. Default RSA signing without this flag uses a more robust algorithm.

Install

pip install imgtool Install from PyPI

Quickstart

This quickstart demonstrates how to generate a cryptographic key pair and then use it to sign a dummy firmware image. The `sign` command requires specific parameters (`--header-size`, `--align`, `--slot-size`) that must match your target MCUboot configuration.

# 1. Create a dummy binary image file
!echo "Hello, MCUboot! This is a test image content." > my_firmware.bin

# 2. Generate an ECDSA P256 key pair
!imgtool keygen -k my_key.pem -t ecdsa-p256

# 3. Sign the firmware image
# Note: --header-size, --align, and --slot-size are critical and depend on your MCUboot configuration
# These values are examples; use values appropriate for your target system.
!imgtool sign \
    --key my_key.pem \
    --version 1.0.0 \
    --header-size 0x200 \
    --align 8 \
    --slot-size 0x20000 \
    my_firmware.bin signed_firmware.bin

print("Signed firmware created as signed_firmware.bin")
print("You can inspect the key and signed image structure using 'imgtool getpub' or 'imgtool dump'")

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