Full modem firmware update from flash device

The fmfu_fdev library provides an API for applying a full modem firmware update from a flash device.

Functionality 

The modem firmware and metadata stored in the flash device must conform to a specific serialization format. For more information, see Full modem upgrades.

The serialized modem firmware contains the hash of the firmware and a signature. These fields are used to pre-validate the modem firmware before it is programmed to the modem, ensuring that the data about to be written corresponds to the data that have been signed. Once the modem firmware is pre-validated, it is written to the modem using the nrf_modem_bootloader.h API.

Serialization 

The modem firmware is serialized using the following CDDL scheme.

;
; Copyright (c) 2021 Nordic Semiconductor ASA
;
; SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
;


Segment = (
    target_addr: uint,
    len: uint
)

Segments = [ + Segment ]

Manifest = [
    version: uint,
    compat: uint,
    blob_hash: bstr .size 32,
    segments: bstr .cbor Segments
]

Wrapper = #6.18(COSE_Sign1_Manifest)

COSE_Sign1_Manifest = [
    protected: bstr .cbor header_map,
    unprotected: {},
    payload : bstr .cbor Manifest,
    signature : bstr .size 256
]

header_map = {
    1 => sign_RSA256  ; algorithm identifier
}

sign_RSA256 = -37 ; RSASSA-PSS, SHA256

Sig_structure1 = [
    context : "Signature1",
    body_protected : bstr .cbor header_map,
    external_aad : '',   ; 0 length bstr
    payload : bstr
]

The resulting serialized firmware file uses the .cbor extension. A generated decoder, modem_update_decode.c, is used for parsing the data serialized with this format.