A Firmware Update Architecture for Internet of Things
draft-ietf-suit-architecture-16

Document Type Active Internet-Draft (suit WG)
Authors Brendan Moran  , Hannes Tschofenig  , David Brown  , Milosch Meriac 
Last updated 2021-03-24 (latest revision 2021-01-27)
Replaces draft-moran-suit-architecture
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SUIT                                                            B. Moran
Internet-Draft                                             H. Tschofenig
Intended status: Informational                               Arm Limited
Expires: July 31, 2021                                          D. Brown
                                                                  Linaro
                                                               M. Meriac
                                                              Consultant
                                                        January 27, 2021

         A Firmware Update Architecture for Internet of Things
                    draft-ietf-suit-architecture-16

Abstract

   Vulnerabilities in Internet of Things (IoT) devices have raised the
   need for a reliable and secure firmware update mechanism suitable for
   devices with resource constraints.  Incorporating such an update
   mechanism is a fundamental requirement for fixing vulnerabilities but
   it also enables other important capabilities such as updating
   configuration settings as well as adding new functionality.

   In addition to the definition of terminology and an architecture this
   document motivates the standardization of a manifest format as a
   transport-agnostic means for describing and protecting firmware
   updates.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 31, 2021.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   5
     2.1.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Stakeholders  . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Functions . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Invoking the Firmware . . . . . . . . . . . . . . . . . . . .  13
     4.1.  The Bootloader  . . . . . . . . . . . . . . . . . . . . .  14
   5.  Types of IoT Devices  . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Single MCU  . . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Single CPU with Secure - Normal Mode Partitioning . . . .  16
     5.3.  Symmetric Multiple CPUs . . . . . . . . . . . . . . . . .  16
     5.4.  Dual CPU, shared memory . . . . . . . . . . . . . . . . .  16
     5.5.  Dual CPU, other bus . . . . . . . . . . . . . . . . . . .  17
   6.  Manifests . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Securing Firmware Updates . . . . . . . . . . . . . . . . . .  19
   8.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  25
   12. Informative References  . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Firmware updates can help to fix security vulnerabilities, and
   performing updates is an important building block in securing IoT
   devices.  Due to rising concerns about insecure IoT devices the
   Internet Architecture Board (IAB) organized a 'Workshop on Internet
   of Things (IoT) Software Update (IOTSU)' [RFC8240] to take a look at
   the bigger picture.  The workshop revealed a number of challenges for

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   developers and led to the formation of the IETF Software Updates for
   Internet of Things (SUIT) working group.

   Developing secure Internet of Things (IoT) devices is not an easy
   task and supporting a firmware update solution requires skillful
   engineers.  Once devices are deployed, firmware updates play a
   critical part in their lifecycle management, particularly when
   devices have a long lifetime, or are deployed in remote or
   inaccessible areas where manual intervention is cost prohibitive or
   otherwise difficult.  Firmware updates
   for IoT devices are expected to work automatically, i.e. without user
   involvement.  Conversely, non-IoT devices are expected to account for
   user preferences and consent when scheduling updates.  Automatic
   updates that do not require human intervention are key to a scalable
   solution for fixing software vulnerabilities.

   Firmware updates are done not only to fix bugs, but also to add new
   functionality and to reconfigure the device to work in new
   environments or to behave differently in an already deployed context.

   The manifest specification has to allow that

   -  The firmware image is authenticated and integrity protected.
      Attempts to flash a maliciously modified firmware image or an
      image from an unknown, untrusted source must be prevented.  In
      examples this document uses asymmetric cryptography because it is
      the preferred approach by many IoT deployments.  The use of
      symmetric credentials is also supported and can be used by very
      constrained IoT devices.

   -  The firmware image can be confidentiality protected so that
      attempts by an adversary to recover the plaintext binary can be
      mitigated or at least made more difficult.  Obtaining the firmware
      is often one of the first steps to mount an attack since it gives
      the adversary valuable insights into the software libraries used,
      configuration settings and generic functionality.  Even though
      reverse engineering the binary can be a tedious process modern
      reverse engineering frameworks have made this task a lot easier.

   Authentication and integrity protection of firmware images must be
   used in a deployment but the confidential protection of firmware is
   optional.

   While the standardization work has been informed by and optimized for
   firmware update use cases of Class 1 devices (according to the device
   class definitions in RFC 7228 [RFC7228]), there is nothing in the
   architecture that restricts its use to only these constrained IoT
   devices.  Moreover, this architecture is not limited to managing

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   firmware and software updates, but can also be applied to managing
   the delivery of arbitrary data, such as configuration information and
   keys.  Unlike higher end devices, like laptops and desktop PCs, many
   IoT devices do not have user interfaces; and support for unattended
   updates is, therefore, essential for the design of a practical
   solution.  Constrained IoT devices often use a software engineering
   model where a developer is responsible for creating and compiling all
   software running on the device into a single, monolithic firmware
   image.  On higher end devices application software is, on the other
   hand, often downloaded separately and even obtained from developers
   different to the developers of the lower level software.  The details
   for how to obtain those application layer software binaries then
   depends heavily on the platform, programming language used and the
   sandbox in which the software is executed.

   While the IETF standardization work has been focused on the manifest
   format, a fully interoperable solution needs more than a standardized
   manifest.  For example, protocols for transferring firmware images
   and manifests to the device need to be available as well as the
   status tracker functionality.  Devices also require a mechanism to
   discover the status tracker(s) and/or firmware servers, for example
   using pre-configured hostnames or DNS-SD [RFC6763].  These building
   blocks have been developed by various organizations under the
   umbrella of an IoT device management solution.  The LwM2M protocol
   [LwM2M] is one IoT device management protocol.

   There are, however, several areas that (partially) fall outside the
   scope of the IETF and other standards organizations but need to be
   considered by firmware authors, as well as device and network
   operators.  Here are some of them, as highlighted during the IOTSU
   workshop:

   -  Installing firmware updates in a robust fashion so that the update
      does not break the device functionality of the environment this
      device operates in.  This requires proper testing and offering
      recovery strategies when a firmware update is unsuccessful.

   -  Making firmware updates available in a timely fashion considering
      the complexity of the decision making process for updating
      devices, potential re-certification requirements, the length of a
      supply chain an update needs to go through before it reaches the
      end customer, and the need for user consent to install updates.

   -  Ensuring an energy efficient design of a battery-powered IoT
      device because a firmware update, particularly radio communication
      and writing the firmware image to flash, is an energy-intensive
      task for a device.

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   -  Creating incentives for device operators to use a firmware update
      mechanism and to demand the integration of it from IoT device
      vendors.

   -  Ensuring that firmware updates addressing critical flaws can be
      obtained even after a product is discontinued or a vendor goes out
      of business.

   This document starts with a terminology followed by the description
   of the architecture.  We then explain the bootloader and how it
   integrates with the firmware update mechanism.  Subsequently, we
   offer a categorization of IoT devices in terms of their hardware
   capabilities relevant for firmware updates.  Next, we talk about the
   manifest structure and how to use it to secure firmware updates.  We
   conclude with a more detailed example.

2.  Conventions and Terminology

2.1.  Terms

   This document uses the following terms:

   -  Firmware Image: The firmware image, or simply the "image", is a
      binary that may contain the complete software of a device or a
      subset of it.  The firmware image may consist of multiple images,
      if the device contains more than one microcontroller.  Often it is
      also a compressed archive that contains code, configuration data,
      and even the entire file system.  The image may consist of a
      differential update for performance reasons.

      The terms, firmware image, firmware, and image, are used in this
      document and are interchangeable.  We use the term application
      firmware image to differentiate it from a firmware image that
      contains the bootloader.  An application firmware image, as the
      name indicates, contains the application program often including
      all the necessary code to run it (such as protocol stacks, and
      embedded operating system).

   -  Manifest: The manifest contains meta-data about the firmware
      image.  The manifest is protected against modification and
      provides information about the author.

   -  Microcontroller (MCU for microcontroller unit): An MCU is a
      compact integrated circuit designed for use in embedded systems.
      A typical microcontroller includes a processor, memory (RAM and
      flash), input/output (I/O) ports and other features connected via
      some bus on a single chip.  The term 'system on chip (SoC)' is

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      often used interchangeably with MCU, but MCU tends to imply more
      limited peripheral functions.

   -  Rich Execution Environment (REE): An environment that is provided
      and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
      potentially in conjunction with other supporting operating systems
      and hypervisors; it is outside of the TEE.  This environment and
      applications running on it are considered un-trusted.

   -  Software: Similar to firmware, but typically dynamically loaded by
      an Operating System.  Used interchangeably with firmware in this
      document.

   -  System on Chip (SoC): An SoC is an integrated circuit that
      contains all components of a computer, such as CPU, memory, input/
      output ports, secondary storage, a bus to connect the components,
      and other hardware blocks of logic.

   -  Trust Anchor: A trust anchor, as defined in [RFC6024], represents
      an authoritative entity via a public key and associated data.  The
      public key is used to verify digital signatures, and the
      associated data is used to constrain the types of information for
      which the trust anchor is authoritative.

   -  Trust Anchor Store: A trust anchor store, as defined in [RFC6024],
      is a set of one or more trust anchors stored in a device.  A
      device may have more than one trust anchor store, each of which
      may be used by one or more applications.  A trust anchor store
      must resist modification against unauthorized insertion, deletion,
      and modification.

   -  Trusted Applications (TAs): An application component that runs in
      a TEE.

   -  Trusted Execution Environments (TEEs): An execution environment
      that runs alongside of, but is isolated from, an REE.  For more
      information about TEEs see [I-D.ietf-teep-architecture].

2.2.  Stakeholders

   The following stakeholders are used in this document:

   -  Author: The author is the entity that creates the firmware image.
      There may be multiple authors involved in producing firmware
      running on an IoT device.  Section 5 talks about those IoT device
      deployment cases.

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   -  Device Operator: The device operator is responsible for the day-
      to-day operation of a fleet of IoT devices.  Customers of IoT
      devices, as the owners of IoT devices - such as enterprise
      customers or end users - interact with their IoT devices
      indirectly through the device operator via web or smart phone
      apps.

   -  Network Operator: The network operator is responsible for the
      operation of a network to which IoT devices connect.

   -  Trust Provisioning Authority (TPA): The TPA distributes trust
      anchors and authorization policies to devices and various
      stakeholders.  The TPA may also delegate rights to stakeholders.
      Typically, the Original Equipment Manufacturer (OEM) or Original
      Design Manufacturer (ODM) will act as a TPA, however complex
      supply chains may require a different design.  In some cases, the
      TPA may decide to remain in full control over the firmware update
      process of their products.

   -  User: The end-user of a device.  The user may interact with
      devices via web or smart phone apps, as well as through direct
      user interfaces.

2.3.  Functions

   -  (IoT) Device: A device refers to the entire IoT product, which
      consists of one or many MCUs, sensors and/or actuators.  Many IoT
      devices sold today contain multiple MCUs and therefore a single
      device may need to obtain more than one firmware image and
      manifest to successfully perform an update.

   -  Status Tracker: The status tracker has a client and a server
      component and performs three tasks: 1) It communicates the
      availability of a new firmware version.  This information will
      flow from the server to the client.
      2) It conveys information about software and hardware
      characteristics of the device.  The information flow is from the
      client to the server.
      3) It can remotely trigger the firmware update process.  The
      information flow is from the server to the client.

      For example, a device operator may want to read the installed
      firmware version number running on the device and information
      about available flash memory.  Once an update has been triggered,
      the device operator may want to obtain information about the state
      of the firmware update.  If errors occurred, the device operator
      may want to troubleshoot problems by first obtaining diagnostic
      information (typically using a device management protocol).

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      We make no assumptions about where the server-side component is
      deployed.  The deployment of status trackers is flexible: they may
      be found at cloud-based servers or on-premise servers, or they may
      be embedded in edge computing devices.  A status tracker server
      component may even be deployed on an IoT device.  For example, if
      the IoT device contains multiple MCUs, then the main MCU may act
      as a status tracker towards the other MCUs.  Such deployment is
      useful when updates have to be synchronized across MCUs.

      The status tracker may be operated by any suitable stakeholder;
      typically the Author, Device Operator, or Network Operator.

   -  Firmware Consumer: The firmware consumer is the recipient of the
      firmware image and the manifest.  It is responsible for parsing
      and verifying the received manifest and for storing the obtained
      firmware image.  The firmware consumer plays the role of the
      update component on the IoT device, typically running in the
      application firmware.  It interacts with the firmware server and
      with the status tracker client (locally).

   -  Firmware Server: The firmware server stores firmware images and
      manifests and distributes them to IoT devices.  Some deployments
      may require a store-and-forward concept, which requires storing
      the firmware images/manifests on more than one entity before
      they reach the device.  There is typically some interaction
      between the firmware server and the status tracker and these two
      entities are often physically separated on different devices for
      scalability reasons.

   -  Bootloader: A bootloader is a piece of software that is executed
      once a microcontroller has been reset.  It is responsible for
      deciding what code to execute.

3.  Architecture

   More devices today than ever before are connected to the Internet,
   which drives the need for firmware updates to be provided over the
   Internet rather than through traditional interfaces, such as USB or
   RS-232.  Sending updates over the Internet requires the device to
   fetch the new firmware image as well as the manifest.

   Hence, the following components are necessary on a device for a
   firmware update solution:

   -  the Internet protocol stack for firmware downloads.  Because
      firmware images are often multiple kilobytes, sometimes exceeding
      one hundred kilobytes, for low-end IoT devices and even several
      megabytes for IoT devices running full-fledged operating systems

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      like Linux, the protocol mechanism for retrieving these images
      needs to offer features like congestion control, flow control,
      fragmentation and reassembly, and mechanisms to resume interrupted
      or corrupted transfers.

   -  the capability to write the received firmware image to persistent
      storage (most likely flash memory).

   -  a manifest parser with code to verify a digital signature or a
      message authentication code.

   -  the ability to unpack, to decompress and/or to decrypt the
      received firmware image.

   -  a status tracker.

   The features listed above are most likely offered by code in the
   application firmware image running on the device rather than by the
   bootloader itself.  Note that cryptographic algorithms will likely
   run in a trusted execution environment, on a separate MCU, in a
   hardware security module, or in a secure element rather than in the
   same context with the application code.

   Figure 1 shows the architecture where a firmware image is created by
   an author, and made available to a firmware server.  For security
   reasons, the author will not have the permissions to upload firmware
   images to the firmware server and to initiate an update directly.
   Instead, authors will make firmware images available to the device
   operators.  Note that there may be a longer supply chain involved to
   pass software updates from the author all the way to the party that
   can then finally make a decision to deploy it with IoT devices.

   As a first step in the firmware update process, the status tracker
   server needs to inform the status tracker client that a new firmware
   update is available.  This can be accomplished via polling (client-
   initiated), push notifications (server-initiated), or more complex
   mechanisms (such as a hybrid approach):

   -  Client-initiated updates take the form of a status tracker client
      proactively checking (polling) for updates.

   -  With Server-initiated updates the server-side component of the
      status tracker learns about a new firmware version and determines
      which devices qualify for a firmware update.  Once the relevant
      devices have been selected, the status tracker informs these
      devices and the firmware consumers obtain those images and
      manifests.  Server-initiated updates are important because they
      allow a quick response time.  Note that in this mode the client-

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      side status tracker needs to be reachable by the server-side
      component.  This may require devices to keep reachability
      information on the server-side up-to-date and state at NATs and
      stateful packet filtering firewalls alive.

   -  Using a hybrid approach the server-side of the status tracker
      pushes notifications of availability of an update to the client
      side and requests the firmware consumer to pull the manifest and
      the firmware image from the firmware server.

   Once the device operator triggers an update via the status tracker,
   it will keep track of the update process on the device.  This allows
   the device operator to know what devices have received an update and
   which of them are still pending an update.

   Firmware images can be conveyed to devices in a variety of ways,
   including USB, UART, WiFi, BLE, low-power WAN technologies, mesh
   networks and many more.  At the application layer a variety of
   protocols are also available: MQTT, CoAP, and HTTP are the most
   popular application layer protocols used by IoT devices.  This
   architecture does not make assumptions about how the firmware images
   are distributed to the devices and therefore aims to support all
   these technologies.

   In some cases it may be desirable to distribute firmware images using
   a multicast or broadcast protocol.  This architecture does not make
   recommendations for any such protocol.  However, given that broadcast
   may be desirable for some networks, updates must cause the least
   disruption possible both in metadata and firmware transmission.  For
   an update to be broadcast friendly, it cannot rely on link layer,
   network layer, or transport layer security.  A solution has to rely
   on security protection applied to the manifest and firmware image
   instead.  In addition, the same manifest must be deliverable to many
   devices, both those to which it applies and those to which it does
   not, without a chance that the wrong device will accept the update.
   Considerations that apply to network broadcasts apply equally to the
   use of third-party content distribution networks for payload
   distribution.

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                                                       +----------+
                                                       |          |
                                                       |  Author  |
                                                       |          |
                                                       +----------+
                        Firmware + Manifest                 |
               +----------------------------------+         | Firmware +
               |                                  |         | Manifest
               |                               ---+-------  |
               |                           ----   |       --|-
               |                         //+----------+     | \\
              -+--                      // |          |     |   \
         ----/ |  ----                |/   | Firmware |<-+  |    \
       //      |      \\              |    | Server   |  |  |     \
      /        |        \             /    |          |  +  +      \
     /         |         \           /     +----------+   \ /       |
    / +--------+--------+ \         /                      |        |
   /  |        v        |  \       /                       v        |
  |   | +------------+  |   |     |          +----------------+      |
  |   | |  Firmware  |  |                    |     Device     |      |
  |   | |  Consumer  |  |   |     |          |     Management |      |
 |    | +------------+  |    |    |          |                |      |
 |    | +------------+  |    |    |          |    +--------+  |      |
 |    | |  Status    |<-+--------------------+->  |        |  |      |
 |    | |  Tracker   |  |    |    |          |    | Status |  |      |
 |    | |  Client    |  |    |    |          |    | Tracker|  |     |
  |   | +------------+  |   |     |          |    | Server |  |     |
  |   |    Device       |   |      |         |    +--------+  |     |
  |   +-----------------+   |       \        |                |    /
   \                       /         \       +----------------+   /
    \       Network       /           \                          /
     \     Operator      /             \     Device Operator    /
       \\             //                \ \                   //
         ----     ----                     ----           ----
             -----                             -----------

                          Figure 1: Architecture.

   Firmware images and manifests may be conveyed as a bundle or
   detached.  The manifest format must support both approaches.

   For distribution as a bundle, the firmware image is embedded into the
   manifest.  This is a useful approach for deployments where devices
   are not connected to the Internet and cannot contact a dedicated
   firmware server for the firmware download.  It is also applicable
   when the firmware update happens via a USB sticks or short range
   radio technologies (such as Bluetooth Smart).

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   Alternatively, the manifest is distributed detached from the firmware
   image.  Using this approach, the firmware consumer is presented with
   the manifest first and then needs to obtain one or more firmware
   images as dictated in the manifest.

   The pre-authorisation step involves verifying whether the entity
   signing the manifest is indeed authorized to perform an update.  The
   firmware consumer must also determine whether it should fetch and
   process a firmware image, which is referenced in a manifest.

   A dependency resolution phase is needed when more than one component
   can be updated or when a differential update is used.  The necessary
   dependencies must be available prior to installation.

   The download step is the process of acquiring a local copy of the
   firmware image.  When the download is client-initiated, this means
   that the firmware consumer chooses when a download occurs and
   initiates the download process.  When a download is server-initiated,
   this means that the status tracker tells the device when to download
   or that it initiates the transfer directly to the firmware consumer.
   For example, a download from an HTTP/1.1-based firmware server is
   client-initiated.  Pushing a manifest and firmware image to the
   Package resource of the LwM2M Firmware Update object [LwM2M] is
   server-initiated update.

   If the firmware consumer has downloaded a new firmware image and is
   ready to install it, to initiate the installation, it may

   -  either need to wait for a trigger from the status tracker,

   -  or trigger the update automatically,

   -  or go through a more complex decision making process to determine

   the appropriate timing for an update.  Sometimes the final decision
   may require confirmation of the user of the device for safety
   reasons.

   Installation is the act of processing the payload into a format that
   the IoT device can recognize and the bootloader is responsible for
   then booting from the newly installed firmware image.  This process
   is different when a bootloader is not involved.  For example, when an
   application is updated in a full-featured operating system, the
   updater may halt and restart the application in isolation.  Devices
   must not fail when a disruption, such as a power failure or network
   interruption, occurs during the update process.

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4.  Invoking the Firmware

   Section 3 describes the steps for getting the firmware image and the
   manifest from the author to the firmware consumer on the IoT device.
   Once the firmware consumer has retrieved and successfully processed
   the manifest and the firmware image it needs to invoke the new
   firmware image.  This is managed in many different ways, depending on
   the type of device, but it typically involves halting the current
   version of the firmware, handing control over to a firmware with a
   higher privilege/trust level (the firmware verifier), verifying the
   new firmware's authenticity & integrity, and then invoking it.

   In an execute-in-place microcontroller, this is often done by
   rebooting into a bootloader (simultaneously halting the application &
   handing over to the higher privilege level) then executing a secure
   boot process (verifying and invoking the new image).

   In a rich OS, this may be done by halting one or more processes, then
   invoking new applications.  In some OSs, this implicitly involves the
   kernel verifying the code signatures on the new applications.

   The invocation process is security sensitive.  An attacker will
   typically try to retrieve a firmware image from the device for
   reverse engineering or will try to get the firmware verifier to
   execute an attacker-modified firmware image.  The firmware verifier
   will therefore have to perform security checks on the firmware image
   before it can be invoked.  These security checks by the firmware
   verifier happen in addition to the security checks that took place
   when the firmware image and the manifest were downloaded by the
   firmware consumer.

   The overlap between the firmware consumer and the firmware verifier
   functionality comes in two forms, namely

   -  A firmware verifier must verify the firmware image it boots as
      part of the secure boot process.  Doing so requires meta-data to
      be stored alongside the firmware image so that the firmware
      verifier can cryptographically verify the firmware image before
      booting it to ensure it has not been tampered with or replaced.
      This meta-data used by the firmware verifier may well be the same
      manifest obtained with the firmware image during the update
      process.

   -  An IoT device needs a recovery strategy in case the firmware
      update / invocation process fails.  The recovery strategy may
      include storing two or more application firmware images on the
      device or offering the ability to invoke a recovery image to
      perform the firmware update process again using firmware updates

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      over serial, USB or even wireless connectivity like Bluetooth
      Smart.  In the latter case the firmware consumer functionality is
      contained in the recovery image and requires the necessary
      functionality for executing the firmware update process, including
      manifest parsing.

   While this document assumes that the firmware verifier itself is
   distinct from the role of the firmware consumer and therefore does
   not manage the firmware update process, this is not a requirement and
   these roles may be combined in practice.

   Using a bootloader as the firmware verifier requires some special
   considerations, particularly when the bootloader implements the
   robustness requirements identified by the IOTSU workshop [RFC8240].

4.1.  The Bootloader

   In most cases the MCU must restart in order to hand over control to
   the bootloader.  Once the MCU has initiated a restart, the bootloader
   determines whether a newly available firmware image should be
   executed.  If the bootloader concludes that the newly available
   firmware image is invalid, a recovery strategy is necessary.  There
   are only two approaches for recovering from an invalid firmware:
   either the bootloader must be able to select a different, valid
   firmware, or it must be able to obtain a new, valid firmware.  Both
   of these approaches have implications for the architecture of the
   update system.

   Assuming the first approach, there are (at least) three firmware
   images available on the device:

   -  First, the bootloader is also firmware.  If a bootloader is
      updatable then its firmware image is treated like any other
      application firmware image.

   -  Second, the firmware image that has to be replaced is still
      available on the device as a backup in case the freshly downloaded
      firmware image does not boot or operate correctly.

   -  Third, there is the newly downloaded firmware image.

   Therefore, the firmware consumer must know where to store the new
   firmware.  In some cases, this may be implicit, for example replacing
   the least-recently-used firmware image.  In other cases, the storage
   location of the new firmware must be explicit, for example when a
   device has one or more application firmware images and a recovery
   image with limited functionality, sufficient only to perform an
   update.

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   Since many low end IoT devices do not use position-independent code,
   either the bootloader needs to copy the newly downloaded application
   firmware image into the location of the old application firmware
   image and vice versa or multiple versions of the firmware need to be
   prepared for different locations.

   In general, it is assumed that the bootloader itself, or a minimal
   part of it, will not be updated since a failed update of the
   bootloader poses a reliability risk.

   For a bootloader to offer a secure boot functionality it needs to
   implement the following functionality:

   -  The bootloader needs to fetch the manifest from nonvolatile
      storage and parse its contents for subsequent cryptographic
      verification.

   -  Cryptographic libraries with hash functions, digital signatures
      (for asymmetric crypto), message authentication codes (for
      symmetric crypto) need to be accessible.

   -  The device needs to have a trust anchor store to verify the
      digital signature.  (Alternatively, access to a key store for use
      with the message authentication code.)

   -  There must be an ability to expose boot process-related data to
      the application firmware (such as to the status tracker).  This
      allows sharing information about the current firmware version, and
      the status of the firmware update process and whether errors have
      occurred.

   -  Produce boot measurements as part of an attestation solution.  See
      [I-D.ietf-rats-architecture] for more information. (optional)

   -  The bootloader must be able to decrypt firmware images, in case
      confidentiality protection was applied.  This requires a solution
      for key management. (optional)

5.  Types of IoT Devices

   There are billions of MCUs used in devices today produced by a large
   number of silicon manufacturers.  While MCUs can vary significantly
   in their characteristics, there are a number of similiaries allowing
   us to categorize in groups.

   The firmware update architecture, and the manifest format in
   particular, needs to offer enough flexibility to cover these common
   deployment cases.

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5.1.  Single MCU

   The simplest, and currently most common, architecture consists of a
   single MCU along with its own peripherals.  These SoCs generally
   contain some amount of flash memory for code and fixed data, as well
   as RAM for working storage.  A notable characteristic of these SoCs
   is that the primary code is generally execute in place (XIP).  Due to
   the non-relocatable nature of the code, the firmware image needs to
   be placed in a specific location in flash since the code cannot be
   executed from an arbitrary location in flash.  Hence, when the
   firmware image is updated it is necessary to swap the old and the new
   image.

5.2.  Single CPU with Secure - Normal Mode Partitioning

   Another configuration consists of a similar architecture to the
   previous, with a single CPU.  However, this CPU supports a security
   partitioning scheme that allows memory (in addition to other things)
   to be divided into secure and normal mode.  There will generally be
   two images, one for secure mode, and one for normal mode.  In this
   configuration, firmware upgrades will generally be done by the CPU in
   secure mode, which is able to write to both areas of the flash
   device.  In addition, there are requirements to be able to update
   either image independently, as well as to update them together
   atomically, as specified in the associated manifests.

5.3.  Symmetric Multiple CPUs

   In more complex SoCs with symmetric multi-processing support,
   advanced operating systems, such as Linux, are often used.  These
   SoCs frequently use an external storage medium, such as raw NAND
   flash or eMMC.  Due to the higher quantity of resources, these
   devices are often capable of storing multiple copies of their
   firmware images and selecting the most appropriate one to boot.  Many
   SoCs also support bootloaders that are capable of updating the
   firmware image, however this is typically a last resort because it
   requires the device to be held in the bootloader while the new
   firmware is downloaded and installed, which results in down-time for
   the device.  Firmware updates in this class of device are typically
   not done in-place.

5.4.  Dual CPU, shared memory

   This configuration has two or more heterogeneous CPUs in a single SoC
   that share memory (flash and RAM).  Generally, there will be a
   mechanism to prevent one CPU from unintentionally accessing memory
   currently allocated to the other.  Upgrades in this case will

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   typically be done by one of the CPUs, and is similar to the single
   CPU with secure mode.

5.5.  Dual CPU, other bus

   This configuration has two or more heterogeneous CPUs, each having
   their own memory.  There will be a communication channel between
   them, but it will be used as a peripheral, not via shared memory.  In
   this case, each CPU will have to be responsible for its own firmware
   upgrade.  It is likely that one of the CPUs will be considered the
   primary CPU, and will direct the other CPU to do the upgrade.  This
   configuration is commonly used to offload specific work to other
   CPUs.  Firmware dependencies are similar to the other solutions
   above, sometimes allowing only one image to be upgraded, other times
   requiring several to be upgraded atomically.  Because the updates are
   happening on multiple CPUs, upgrading the two images atomically is
   challenging.

6.  Manifests

   In order for a firmware consumer to apply an update, it has to make
   several decisions using manifest-provided information and data
   available on the device itself.  For more detailed information and a
   longer list of information elements in the manifest consult the
   information model specification [I-D.ietf-suit-information-model],
   which offers justifications for each element, and the manifest
   specification [I-D.ietf-suit-manifest] for details about how this
   information is included in the manifest.

   Table 1 provides examples of decisions to be made.

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   +----------------------------+--------------------------------------+
   |                   Decision | Information Elements                 |
   +----------------------------+--------------------------------------+
   |  Should I trust the author | Trust anchors and authorization      |
   |           of the firmware? | policies on the device               |
   |                            |                                      |
   |      Has the firmware been | Digital signature and MAC covering   |
   |                 corrupted? | the firmware image                   |
   |                            |                                      |
   |   Does the firmware update | Conditions with Vendor ID, Class ID  |
   |      apply to this device? | and Device ID                        |
   |                            |                                      |
   |   Is the update older than | Sequence number in the manifest (1)  |
   |       the active firmware? |                                      |
   |                            |                                      |
   |     When should the device | Wait directive                       |
   |          apply the update? |                                      |
   |                            |                                      |
   |      How should the device | Manifest commands                    |
   |          apply the update? |                                      |
   |                            |                                      |
   |      What kind of firmware | Unpack algorithms to interpret a     |
   |              binary is it? | format.                              |
   |                            |                                      |
   | Where should the update be | Dependencies on other manifests and  |
   |                  obtained? | firmware image URI in Manifest       |
   |                            |                                      |
   |  Where should the firmware | Storage Location and Component       |
   |                 be stored? | Identifier                           |
   +----------------------------+--------------------------------------+

                    Table 1: Firmware Update Decisions.

   (1): A device presented with an old, but valid manifest and firmware
   must not be tricked into installing such firmware since a
   vulnerability in the old firmware image may allow an attacker to gain
   control of the device.

   Keeping the code size and complexity of a manifest parsers small is
   important for constrained IoT devices.  Since the manifest parsing
   code may also be used by the bootloader it can be part of the trusted
   computing base.

   A manifest may be used to protect not only firmware images but also
   configuration data such as network credentials or personalization
   data related to firmware or software.  Personalization data
   demonstrates the need for confidentiality to be maintained between
   two or more stakeholders that both deliver images to the same device.

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   Personalization data is used with Trusted Execution Environments
   (TEEs), which benefit from a protocol for managing the lifecycle of
   trusted applications (TAs) running inside a TEE.  TEEs may obtain TAs
   from different authors and those TAs may require personalization
   data, such as payment information, to be securely conveyed to the
   TEE.  The TA's author does not want to expose the TA's code to any
   other stakeholder or third party.  The user does not want to expose
   the payment information to any other stakeholder or third party.

7.  Securing Firmware Updates

   Using firmware updates to fix vulnerabilities in devices is important
   but securing this update mechanism is equally important since
   security problems are exacerbated by the update mechanism: update is
   essentially authorized remote code execution, so any security
   problems in the update process expose that remote code execution
   system.  Failure to secure the firmware update process will help
   attackers to take control over devices.

   End-to-end security mechanisms are used to protect the firmware image
   and the manifest.  The following assumptions are made to allow the
   firmware consumer to verify the received firmware image and manifest
   before updating software:

   -  Authentication ensures that the device can cryptographically
      identify the author(s) creating firmware images and manifests.
      Authenticated identities may be used as input to the authorization
      process.  Not all entities creating and signing manifests have the
      same permissions.  A device needs to determine whether the
      requested action is indeed covered by the permission of the party
      that signed the manifest.  Informing the device about the
      permissions of the different parties also happens in an out-of-
      band fashion and is a duty of the Trust Provisioning Authority.

   -  Integrity protection ensures that no third party can modify the
      manifest or the firmware image.  To accept an update, a device
      needs to verify the signature covering the manifest.  There may be
      one or multiple manifests that need to be validated, potentially
      signed by different parties.  The device needs to be in possession
      of the trust anchors to verify those signatures.  Installing trust
      anchors to devices via the Trust Provisioning Authority happens in
      an out-of-band fashion prior to the firmware update process.

   -  For confidentiality protection of the firmware image, it must be
      done in such a way that the intended firmware consumer(s), other
      authorized parties, and no one else can decrypt it.  The
      information that is encrypted individually for each device/
      recipient must be done in a way that is usable with Content

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      Distribution Networks, bulk storage, and broadcast protocols.  For
      confidentiality protection of firmware images the author needs to
      be in possession of the certificate/public key or a pre-shared key
      of a device.  The use of confidentiality protection of firmware
      images is optional.

   A manifest specification must support different cryptographic
   algorithms and algorithm extensibility.  Moreover, since RSA- and
   ECC-based signature schemes may become vulnerable to quantum-
   accelerated key extraction in the future, unchangeable bootloader
   code in ROM is recommended to use post-quantum secure signature
   schemes such as hash-based signatures [RFC8778].  A bootloader author
   must carefully consider the service lifetime of their product and the
   time horizon for quantum-accelerated key extraction.  The worst-case
   estimate, at time of writing, for the time horizon to key extraction
   with quantum acceleration is approximately 2030, based on current
   research [quantum-factorization].

   When a device obtains a monolithic firmware image from a single
   author without any additional approval steps, the authorization flow
   is relatively simple.  There are, however, other cases where more
   complex policy decisions need to be made before updating a device.

   In this architecture the authorization policy is separated from the
   underlying communication architecture.  This is accomplished by
   separating the entities from their permissions.  For example, an
   author may not have the authority to install a firmware image on a
   device in critical infrastructure without the authorization of a
   device operator.  In this case, the device may be programmed to
   reject firmware updates unless they are signed both by the firmware
   author and by the device operator.

   Alternatively, a device may trust precisely one entity, which does
   all permission management and coordination.  This entity allows the
   device to offload complex permissions calculations for the device.

8.  Example

   Figure 2 illustrates an example message flow for distributing a
   firmware image to a device.  The firmware and manifest are stored on
   the same firmware server and distributed in a detached manner.

   +--------+    +-----------------+    +-----------------------------+
   |        |    | Firmware Server |    |         IoT Device          |
   | Author |    | Status Tracker  |    | +------------+ +----------+ |
   +--------+    | Server          |    | |  Firmware  | |Bootloader| |
     |           +-----------------+    | |  Consumer  | |          | |
     |                   |              | +------------+ +----------+ |

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     |                   |              |      |                |     |
     |                   |              |  +-----------------------+  |
     | Create Firmware   |              |  | Status Tracker Client |  |
     |--------------+    |              |  +-----------------------+  |
     |              |    |               `''''''''''''''''''''''''''''
     |<-------------+    |                     |        |       |
     |                   |                     |        |       |
     | Upload Firmware   |                     |        |       |
     |------------------>|                     |        |       |
     |                   |                     |        |       |
     | Create Manifest   |                     |        |       |
     |---------------+   |                     |        |       |
     |               |   |                     |        |       |
     |<--------------+   |                     |        |       |
     |                   |                     |        |       |
     | Sign Manifest     |                     |        |       |
     |-------------+     |                     |        |       |
     |             |     |                     |        |       |
     |<------------+     |                     |        |       |
     |                   |                     |        |       |
     | Upload Manifest   |                     |        |       |
     |------------------>|  Notification of    |        |       |
     |                   |  new firmware image |        |       |
     |                   |----------------------------->|       |
     |                   |                     |        |       |
     |                   |                     |Initiate|       |
     |                   |                     | Update |       |
     |                   |                     |<-------|       |
     |                   |                     |        |       |
     |                   |   Query Manifest    |        |       |
     |                   |<--------------------|        .       |
     |                   |                     |        .       |
     |                   |   Send Manifest     |        .       |
     |                   |-------------------->|        .       |
     |                   |                     | Validate       |
     |                   |                     | Manifest       |
     |                   |                     |--------+       |
     |                   |                     |        |       |
     |                   |                     |<-------+       |
     |                   |                     |        .       |
     |                   |  Request Firmware   |        .       |
     |                   |<--------------------|        .       |
     |                   |                     |        .       |
     |                   | Send Firmware       |        .       |
     |                   |-------------------->|        .       |
     |                   |                     | Verify .       |
     |                   |                     | Firmware       |
     |                   |                     |--------+       |

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     |                   |                     |        |       |
     |                   |                     |<-------+       |
     |                   |                     |        .       |
     |                   |                     | Store  .       |
     |                   |                     | Firmware       |
     |                   |                     |--------+       |
     |                   |                     |        |       |
     |                   |                     |<-------+       |
     |                   |                     |        .       |
     |                   |                     |        .       |
     |                   |                     |        .       |
     |                   |                     |        |       |
     |                   |                     | Update |       |
     |                   |                     |Complete|       |
     |                   |                     |------->|       |
     |                   |                              |       |
     |                   |  Firmware Update Completed   |       |
     |                   |<-----------------------------|       |
     |                   |                              |       |
     |                   |  Reboot                      |       |
     |                   |----------------------------->|       |
     |                   |                     |        |       |
     |                   |                     |        |       |
                         |                     |        |Reboot |
     |                   |                     |        |------>|
     |                   |                     |        |       |
     |                   |                     |        .       |
     |                   |                 +---+----------------+--+
     |                   |                S|   |                |  |
     |                   |                E|   | Verify         |  |
     |                   |                C|   | Firmware       |  |
     |                   |                U|   | +--------------|  |
     |                   |                R|   | |              |  |
     |                   |                E|   | +------------->|  |
     |                   |                 |   |                |  |
     |                   |                B|   | Activate new   |  |
     |                   |                O|   | Firmware       |  |
     |                   |                O|   | +--------------|  |
     |                   |                T|   | |              |  |
     |                   |                 |   | +------------->|  |
     |                   |                P|   |                |  |
     |                   |                R|   | Boot new       |  |
     |                   |                O|   | Firmware       |  |
     |                   |                C|   | +--------------|  |
     |                   |                E|   | |              |  |
     |                   |                S|   | +------------->|  |
     |                   |                S|   |                |  |
     |                   |                 +---+----------------+--+

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     |                   |                     |        .       |
     |                   |                     |        |       |
     |                   |                     .        |       |
     |                   |  Device running new firmware |       |
     |                   |<-----------------------------|       |
     |                   |                     .        |       |
     |                   |                              |       |

            Figure 2: First Example Flow for a Firmware Update.

   Figure 3 shows an exchange that starts with the status tracker
   querying the device for its current firmware version.  Later, a new
   firmware version becomes available and since this device is running
   an older version the status tracker server interacts with the device
   to initiate an update.

   The manifest and the firmware are stored on different servers in this
   example.  When the device processes the manifest it learns where to
   download the new firmware version.  The firmware consumer downloads
   the firmware image with the newer version X.Y.Z after successful
   validation of the manifest.  Subsequently, a reboot is initiated and
   the secure boot process starts.  Finally, the device reports the
   successful boot of the new firmware version.

    +---------+   +-----------------+    +-----------------------------+
    | Status  |   | Firmware Server |    | +------------+ +----------+ |
    | Tracker |   | Status Tracker  |    | |  Firmware  | |Bootloader| |
    | Server  |   | Server          |    | |  Consumer  | |          | |
    +---------+   +-----------------+    | |  +Status   | +----------+ |
         |                |              | |  Tracker   |        |     |
         |                |              | |  Client    |        |     |
         |                |              | +------------+        |     |
         |                |              |      |  IoT Device    |     |
         |                |               `''''''''''''''''''''''''''''
         |                |                     |                |
         |        Query Firmware Version        |                |
         |------------------------------------->|                |
         |        Firmware Version A.B.C        |                |
         |<-------------------------------------|                |
         |                |                     |                |
         |         <<some time later>>          |                |
         |                |                     |                |
       _,...._         _,...._                  |                |
     ,'       `.     ,'       `.                |                |
    |   New     |   |   New     |               |                |
    \ Manifest  /   \ Firmware  /               |                |
     `.._   _,,'     `.._   _,,'                |                |
         `''             `''                    |                |

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         |            Push manifest             |                |
         |----------------+-------------------->|                |
         |                |                     |                |
         |                '                     |                '
         |                |                     | Validate       |
         |                |                     | Manifest       |
         |                |                     |---------+      |
         |                |                     |         |      |
         |                |                     |<--------+      |
         |                | Request firmware    |                |
         |                | X.Y.Z               |                |
         |                |<--------------------|                |
         |                |                     |                |
         |                | Firmware X.Y.Z      |                |
         |                |-------------------->|                |
         |                |                     |                |
         |                |                     | Verify         |
         |                |                     | Firmware       |
         |                |                     |--------------+ |
         |                |                     |              | |
         |                |                     |<-------------+ |
         |                |                     |                |
         |                |                     | Store          |
         |                |                     | Firmware       |
         |                |                     |-------------+  |
         |                |                     |             |  |
         |                |                     |<------------+  |
         |                |                     |                |
         |                |                     |                |
         |                |                     | Trigger Reboot |
         |                |                     |--------------->|
         |                |                     |                |
         |                |                     |                |
         |                |                     | __..-------..._'
         |                |                    ,-'               `-.
         |                |                   |      Secure Boot    |
         |                |                   `-.                 _/
         |                |                     |`--..._____,,.,-'
         |                |                     |                |
         | Device running firmware X.Y.Z        |                |
         |<-------------------------------------|                |
         |                |                     |                |
         |                |                     |                |

           Figure 3: Second Example Flow for a Firmware Update.

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9.  IANA Considerations

   This document does not require any actions by IANA.

10.  Security Considerations

   This document describes terminology, requirements and an architecture
   for firmware updates of IoT devices.  The content of the document is
   thereby focused on improving security of IoT devices via firmware
   update mechanisms and informs the standardization of a manifest
   format.

   An in-depth examination of the security considerations of the
   architecture is presented in [I-D.ietf-suit-information-model].

11.  Acknowledgements

   We would like to thank the following persons for their feedback:

   -  Geraint Luff

   -  Amyas Phillips

   -  Dan Ros

   -  Thomas Eichinger

   -  Michael Richardson

   -  Emmanuel Baccelli

   -  Ned Smith

   -  Jim Schaad

   -  Carsten Bormann

   -  Cullen Jennings

   -  Olaf Bergmann

   -  Suhas Nandakumar

   -  Phillip Hallam-Baker

   -  Marti Bolivar

   -  Andrzej Puzdrowski

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   -  Markus Gueller

   -  Henk Birkholz

   -  Jintao Zhu

   -  Takeshi Takahashi

   -  Jacob Beningo

   -  Kathleen Moriarty

   -  Bob Briscoe

   -  Roman Danyliw

   -  Brian Carpenter

   -  Theresa Enghardt

   -  Rich Salz

   -  Mohit Sethi

   -  Eric Vyncke

   -  Alvaro Retana

   -  Barry Leiba

   -  Benjamin Kaduk

   -  Martin Duke

   -  Robert Wilton

   We would also like to thank the WG chairs, Russ Housley, David
   Waltermire, and Dave Thaler, for their support and their reviews.

12.  Informative References

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture",
              draft-ietf-rats-architecture-08 (work in progress),
              December 2020.

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   [I-D.ietf-suit-information-model]
              Moran, B., Tschofenig, H., and H. Birkholz, "An
              Information Model for Firmware Updates in IoT Devices",
              draft-ietf-suit-information-model-08 (work in progress),
              October 2020.

   [I-D.ietf-suit-manifest]
              Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
              "A Concise Binary Object Representation (CBOR)-based
              Serialization Format for the Software Updates for Internet
              of Things (SUIT) Manifest", draft-ietf-suit-manifest-11
              (work in progress), December 2020.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-13 (work in
              progress), November 2020.

   [LwM2M]    OMA, ., "Lightweight Machine to Machine Technical
              Specification, Version 1.0.2", February 2018,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_0_2-20180209-A/
              OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.

   [quantum-factorization]
              Jiang, S., Britt, K., McCaskey, A., Humble, T., and S.
              Kais, "Quantum Annealing for Prime Factorization",
              December 2018,
              <https://www.nature.com/articles/s41598-018-36058-z>.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <https://www.rfc-editor.org/info/rfc6024>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC8240]  Tschofenig, H. and S. Farrell, "Report from the Internet
              of Things Software Update (IoTSU) Workshop 2016",
              RFC 8240, DOI 10.17487/RFC8240, September 2017,
              <https://www.rfc-editor.org/info/rfc8240>.

Moran, et al.             Expires July 31, 2021                [Page 27]
Internet-Draft   A Firmware Update Architecture for IoT     January 2021

   [RFC8778]  Housley, R., "Use of the HSS/LMS Hash-Based Signature
              Algorithm with CBOR Object Signing and Encryption (COSE)",
              RFC 8778, DOI 10.17487/RFC8778, April 2020,
              <https://www.rfc-editor.org/info/rfc8778>.

Authors' Addresses

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

   David Brown
   Linaro

   EMail: david.brown@linaro.org

   Milosch Meriac
   Consultant

   EMail: milosch@meriac.com

Moran, et al.             Expires July 31, 2021                [Page 28]