Essential MCUboot configuration items

The following page provides an overview of the key configuration items necessary for understanding and configuring MCUboot’s behavior. The Sysbuild (System build) Kconfig options generally override the MCUboot’s Kconfig option. Sysbuild options aim to establish a coherent environment for the entire system, while MCUboot options focus specifically on the bootloader. This means that many sysbuild options configure the MCUboot, the application, and the build system settings to ensure compatibility.

Supported image configurations

MCUboot primarily functions as a dual-bank bootloader, managing one bank as the executable and the other as an upgrade candidate or an alternative executable. In MCUboot terminology, image refers to a specific type of executable code, typically the application image. In the dual-bank scheme, two banks (or slots) are associated with each image.

MCUboot can be configured to support multiple images, enabling it to operate as an enhanced dual-bank bootloader. This configuration allows for more complex update scenarios and greater flexibility in managing different sets of firmware:

• CONFIG_UPDATEABLE_IMAGE_NUMBER - Specifies the number of images that MCUboot can handle.

• SB_CONFIG_MCUBOOT_UPDATEABLE_IMAGES - Configures the number of images supported by MCUboot at the sysbuild level.

Operational modes of MCUboot

MCUboot supports various operational modes that dictate its behavior towards the application images it manages. The choice of mode determines the specific scheme of the bootloader’s operations for the images supported by a given MCUboot instance:

MCUboot modes

Mode name	Description	MCUboot Kconfig option	Sysbuild Kconfig option	Multiple images
Swap using scratch	Implements a dual-bank image swapping algorithm utilizing a scratch area, accommodating memories with varying erase block sizes.	CONFIG_BOOT_SWAP_USING_OFFSET	SB_CONFIG_MCUBOOT_MODE_SWAP_SCRATCH	Yes
Swap using move	Executes a dual-bank image swapping algorithm by moving sectors, offering greater efficiency than scratch-based swaps, suitable only for memories with consistent erase block sizes.	CONFIG_BOOT_SWAP_USING_MOVE	SB_CONFIG_MCUBOOT_MODE_SWAP_USING_MOVE	Yes
Swap using offset	Introduces a new dual-bank image swapping algorithm that moves sectors with optimizations for enhanced speed, applicable to memories with uniform erase block sizes.	CONFIG_BOOT_SWAP_USING_OFFSET	–	Yes
Overwrite	Employs a straightforward dual-bank image overwrite algorithm that directly replaces the image.	CONFIG_BOOT_UPGRADE_ONLY	SB_CONFIG_MCUBOOT_MODE_OVERWRITE_ONLY	Yes
Direct-XIP	Facilitates dual-bank image execution directly from storage, updating by uploading a new image to an alternate bank, eliminating the need for swapping or overwriting NVM.	CONFIG_BOOT_DIRECT_XIP	SB_CONFIG_MCUBOOT_MODE_DIRECT_XIP	Yes
Direct-XIP with revert	Enables dual-bank image execution directly from storage with additional support for reverting to a previous image if necessary, enhancing system reliability.	CONFIG_BOOT_DIRECT_XIP and CONFIG_BOOT_DIRECT_XIP_REVERT	SB_CONFIG_MCUBOOT_MODE_DIRECT_XIP_WITH_REVERT	Yes
Firmware loader	Provides a dual-bank image firmware loading mode that allows dynamic selection of the image bank for booting the application, accommodating banks of different sizes.	CONFIG_BOOT_FIRMWARE_LOADER	SB_CONFIG_MCUBOOT_MODE_FIRMWARE_UPDATER	No
Single application	Supports a single application image mode, utilized when only one application image is necessary and dual-bank operations are not required.	CONFIG_SINGLE_APPLICATION_SLOT	SB_CONFIG_MCUBOOT_MODE_SINGLE_APP	No

Signature types

MCUboot supports various signature types. The signature type specifies the algorithm used to sign the image. You can calculate each signature on a hash of the image, prepared by MCUboot (referred to as pre-hash signatures). Notably, the Ed25519 signature can also be directly calculated on the image itself.

MCUboot signature types :header-rows: 1 :widths: auto

Signature	Description	MCUboot Kconfig option	Sysbuild Kconfig option	Signed material
RSA	Utilizes RSA for digital signatures, supporting key sizes of 2048 and 3072 bits.	CONFIG_BOOT_SIGNATURE_TYPE_RSA, CONFIG_BOOT_SIGNATURE_TYPE_RSA_LEN	SB_CONFIG_BOOT_SIGNATURE_TYPE_RSA	Image hash
ECDSA P-256	Employs the elliptic curve digital signature algorithm using the P-256 curve for enhanced security.	CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256	SB_CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256	Image hash
Ed25519	Uses the Edwards curve digital signature algorithm with Ed25519.	CONFIG_BOOT_SIGNATURE_TYPE_ED25519	SB_CONFIG_BOOT_SIGNATURE_TYPE_ED25519	Image hash, image itself (pure Ed25519)
None	Indicates the absence of a signature; the image is unchecked but its hash is verified for integrity.	CONFIG_BOOT_SIGNATURE_TYPE_NONE	SB_CONFIG_BOOT_SIGNATURE_TYPE_NONE	Not applicable

Public key storage

MCUboot supports two methods for storing the public key used for image verification:

• Embedded in the image - The public key is compiled in the MCUboot instance. For this method, no additional configuration is required.

• Stored in the KMU - The public key is stored in the Key Management Unit (KMU) of the nRF54L devices that support this hardware peripheral. You can enable it using the SB_CONFIG_MCUBOOT_SIGNATURE_USING_KMU sysbuild Kconfig option. This implementation supports up to three keys and includes a key revocation scheme. You can manage these features through the CONFIG_BOOT_SIGNATURE_KMU_SLOTS and CONFIG_BOOT_SIGNATURE_KMU_SLOTS MCUboot Kconfig options.

MCUboot image hash algorithms

MCUboot supports several hash algorithms to calculate the integrity of the image hash. Typically, the default hash algorithm is sufficient for most applications, as MCUboot selects the most relevant one based on the system configuration. However, you can customize it to meet specific requirements.

MCUboot image hash algorithms :header-rows: 1 :widths: auto

Hash	MCUboot Kconfig option	Comments
SHA-256	CONFIG_BOOT_IMG_HASH_ALG_SHA256	The default hash. Compatible with all pre-hash signature types.
SHA-384	CONFIG_BOOT_IMG_HASH_ALG_SHA384	Currently not utilized by nRF Connect SDK.
SHA-512	CONFIG_BOOT_IMG_HASH_ALG_SHA512	Restricted to use with Ed25519 signatures.

MCUboot recovery protocol

Mcuboot supports serial recovery protocols compatible with MCUmgr, enabling device programming via serial connection instead of J-Link. This feature allows direct upload of applications to the executable image bank and, depending on configuration, to other banks as well. To use this feature, enable the CONFIG_MCUBOOT_SERIAL Kconfig option.