SUIT                                                            B. Moran
Internet-Draft                                               Arm Limited
Intended status: Informational                                 M. Meriac
Expires: October 11, 2019 March 16, 2020                                       Consultant
                                                           H. Tschofenig
                                                             Arm Limited
                                                                D. Brown
                                                                  Linaro
                                                          April 09,
                                                      September 13, 2019

     A Firmware Update Architecture for Internet of Things Devices
                    draft-ietf-suit-architecture-05
                    draft-ietf-suit-architecture-06

Abstract

   Vulnerabilities with Internet of Things (IoT) devices have raised the
   need for a solid and secure firmware update mechanism that is also
   suitable for constrained devices.  Incorporating such update
   mechanism to fix vulnerabilities, to update configuration settings as
   well as adding new functionality is recommended by security experts.

   This document lists requirements and describes an architecture for a
   firmware update mechanism suitable for IoT devices.  The architecture
   is agnostic to the transport of the firmware images and associated
   meta-data.

   This version of the document assumes asymmetric cryptography and a
   public key infrastructure.  Future versions may also describe a
   symmetric key approach for very constrained devices.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on October 11, 2019. March 16, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6   7
     3.1.  Agnostic to how firmware images are distributed . . . . .   7
     3.2.  Friendly to broadcast delivery  . . . . . . . . . . . . .   7   8
     3.3.  Use state-of-the-art security mechanisms  . . . . . . . .   7   8
     3.4.  Rollback attacks must be prevented  . . . . . . . . . . .   8
     3.5.  High reliability  . . . . . . . . . . . . . . . . . . . .   8   9
     3.6.  Operate with a small bootloader . . . . . . . . . . . . .   8   9
     3.7.  Small Parsers . . . . . . . . . . . . . . . . . . . . . .   9  10
     3.8.  Minimal impact on existing firmware formats . . . . . . .   9  10
     3.9.  Robust permissions  . . . . . . . . . . . . . . . . . . .   9  10
     3.10. Operating modes . . . . . . . . . . . . . . . . . . . . .   9  10
     3.11. Suitability to software and personalization data  . . . .  12
   4.  Claims  . . . . . . . . . . . . . . . . . . . . . . . . . . .  11  13
   5.  Communication Architecture  . . . . . . . . . . . . . . . . .  12  13
   6.  Manifest  . . . . . . . . . . . . . . . . . . . . . . . . . .  16  17
   7.  Device Firmware Update Examples . . . . . . . . . . . . . . .  17  18
     7.1.  Single CPU SoC  . . . . . . . . . . . . . . . . . . . . .  17  18
     7.2.  Single CPU with Secure - Normal Mode Partitioning . . . .  17  18
     7.3.  Dual CPU, shared memory . . . . . . . . . . . . . . . . .  17  18
     7.4.  Dual CPU, other bus . . . . . . . . . . . . . . . . . . .  17  18
   8.  Bootloader  . . . . . . . . . . . . . . . . . . . . . . . . .  18  19
   9.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  20  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21  24
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  22  24
   12. Mailing List Information  . . . . . . . . . . . . . . . . . .  22  25
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23  26
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24  27
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  24  27
     14.2.  Informative References . . . . . . . . . . . . . . . . .  24  27
     14.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25  28

1.  Introduction

   When developing IoT devices, one of the most difficult problems to
   solve is how to update the firmware on the device.  Once the device
   is deployed, firmware updates play a critical part in its lifetime,
   particularly when devices have a long lifetime, are deployed in
   remote or inaccessible areas where manual intervention is cost
   prohibitive or otherwise difficult.  Updates to the firmware of an
   IoT device are done to fix bugs in software, to add new
   functionality, and to re-configure the device to work in new
   environments or to behave differently in an already deployed context.

   The firmware update process, among other goals, has to ensure that

   -  The firmware image is authenticated and integrity protected.
      Attempts to flash a modified firmware image or an image from an
      unknown source are prevented.

   -  The firmware image can be confidentiality protected so that
      attempts by an adversary to recover the plaintext binary can be
      prevented.  Obtaining the firmware is often one of the first steps
      to mount an attack since it gives the adversary valuable insights
      into used software libraries, configuration settings and generic
      functionality (even though reverse engineering the binary can be a
      tedious process).

   More details about the security goals are discussed in Section 5 and
   requirements are described in Section 3.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

   This document uses the following terms:

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

   -  Firmware Image: The firmware 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.  The image may consist of a differential
      update for performance reasons.  Firmware is the more universal
      term.  Both terms are used in this document and are
      interchangeable.

   -  Bootloader: A bootloader is a piece of software that is executed
      once a microcontroller has been reset.  It is responsible for
      deciding whether to boot a firmware image that is present or
      whether to obtain and verify a new firmware image.  Since the
      bootloader is a security critical component its functionality may
      be split into separate stages.  Such a multi-stage bootloader may
      offer very basic functionality in the first stage and resides in
      ROM whereas the second stage may implement more complex
      functionality and resides in flash memory so that it can be
      updated in the future (in case bugs have been found).  The exact
      split of components into the different stages, the number of
      firmware images stored by an IoT device, and the detailed
      functionality varies throughout different implementations.  A more
      detailed discussion is provided in Section 8.

   -  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
      often used for these types of devices.

   -  System on Chip (SoC): An SoC is an integrated circuit that
      integrates all components of a computer, such as CPU, memory,
      input/output ports, secondary storage, etc.

   -  Homogeneous Storage Architecture (HoSA): A device that stores all
      firmware components in the same way, for example in a file system
      or in flash memory.

   -  Heterogeneous Storage Architecture (HeSA): A device that stores at
      least one firmware component differently from the rest, for
      example a device with an external, updatable radio, or a device
      with internal and external flash memory.

   -  Trusted Execution Environments (TEEs): An execution environment
      that runs alongside of, but is isolated from, an REE.

   -  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.

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

   For more information about TEEs see [I-D.ietf-teep-architecture].

   The following entities are used:

   -  Author: The author is the entity that creates the firmware image.
      There may be multiple authors in a system either when a device
      consists of multiple micro-controllers or when the the final
      firmware image consists of software components from multiple
      companies.

   -  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, if present.

   -  (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 succesfully perform an update.  The terms device and
      firmware consumer are used interchangably since the firmware
      consumer is one software component running on an MCU on the
      device.

   -  Status Tracker: The status tracker offers device management
      functionality to monitor retrieve information about the installed firmware update process.  A status
      tracker may, for example, want
      on a device and other device characteristics (including free
      memory and hardware components), to know what obtain the state of the
      firmware update cycle the device is currently in.

   -  Firmware Server: The firmware server stores firmware images and
      manifests in, 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 trigger
      the device.

   -  Device Operator: update process.  The actor responsible for the day-to-day
      operation of a fleet deployment of IoT devices.

   - status trackers is flexible
      and they may be used as cloud-based servers, on-premise servers,
      embedded in edge computing device (such as Internet access
      gateways or protocol translation gateways), or even in smart
      phones and tablets.  While the IoT device itself runs the client-
      side of the status tracker it will most likely not run a status
      tracker itself unless it acts as a proxy for other IoT devices in
      a protocol translation or edge computing device node.  How much
      functionality a status tracker includes depends on the selected
      configuration of the device management functionality and the
      communication environment it is used in.  In a generic networking
      environment the protocol used between the client and the server-
      side of the status tracker need to deal with Internet
      communication challenges involving firewall and NAT traversal.  In
      other cases, the communication interaction may be rather simple.
      This architecture document does not impose requirements on the
      status tracker.

   -  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 but those
      entities are often physically separated on different devices for
      scalability reasons.

   -  Device Operator: The actor responsible for the day-to-day
      operation of a fleet of IoT devices.

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

   In addition to the entities in the list above there is an orthogonal
   infrastructure with a Trust Provisioning Authority (TPA) distributing
   trust anchors and authorization permissions to various entities in
   the system.  The TPA may also delegate rights to install, update,
   enhance, or delete trust anchors and authorization permissions to
   other parties in the system.  This infrastructure overlaps the
   communication architecture and different deployments may empower
   certain entities while other deployments may not.  For example, in
   some cases, the Original Design Manufacturer (ODM), which is a
   company that designs and manufactures a product, may act as a TPA and
   may decide to remain in full control over the firmware update process
   of their products.

   The terms 'trust anchor' and 'trust anchor store' are defined in
   [RFC6024]:

   -  "A trust anchor 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."

   -  "A trust anchor store 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.

3.  Requirements

   The firmware update mechanism described in this specification was
   designed with the following requirements in mind:

   -  Agnostic to how firmware images are distributed

   -  Friendly to broadcast delivery

   -  Use state-of-the-art security mechanisms

   -  Rollback attacks must be prevented

   -  High reliability

   -  Operate with a small bootloader

   -  Small Parsers

   -  Minimal impact on existing firmware formats

   -  Robust permissions

   -  Diverse modes of operation

   -  Suitability to software and personalization data

3.1.  Agnostic to how firmware images are distributed

   Firmware images can be conveyed to devices in a variety of ways,
   including USB, UART, WiFi, BLE, low-power WAN technologies, etc.  and
   use different protocols (e.g., CoAP, HTTP).  The specified mechanism
   needs to be agnostic to the distribution of the firmware images and
   manifests.

3.2.  Friendly to broadcast delivery

   This architecture does not specify any specific broadcast protocol.
   However, given that broadcast may be desirable for some networks,
   updates must cause the least disruption possible both in metadata and
   payload transmission.

   For an update to be broadcast friendly, it cannot rely on link layer,
   network layer, or transport layer security.  In addition, the same
   message 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.

3.3.  Use state-of-the-art security mechanisms

   End-to-end security between the author and the device, as shown in
   Section 5, is used to ensure that the device can verify firmware
   images and manifests produced by authorized authors.

   The use of post-quantum secure signature mechanisms, such as hash-
   based signatures, should be explored.  A migration to post-quantum
   secure signatures would require significant effort, therefore,
   mandatory-to-implement support for post-quantum secure signatures is
   a goal.

   A mandatory-to-implement set of algorithms has to be defined offering
   a key length of 112-bit symmetric key or security or more, as
   outlined in Section 20 of RFC 7925 [RFC7925].  This corresponds to a
   233 bit ECC key or a 2048 bit RSA key.

   If the firmware image is to be encrypted, it must be done in such a
   way that every intended recipient can decrypt it.  The information
   that is encrypted individually for each device must be an absolute
   minimum, for example AES Key Wrap [RFC5649], in order to maintain
   friendliness to Content Distribution Networks, bulk storage, and
   broadcast protocols.

3.4.  Rollback attacks must be prevented

   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.

3.5.  High reliability

   A power failure at any time must not cause a failure of the device.
   A failure to validate any part of an update must not cause a failure
   of the device.  One way to achieve this functionality is to provide a
   minimum of two storage locations for firmware and one bootable
   location for firmware.  An alternative approach is to use a 2nd stage
   bootloader with build-in full featured firmware update functionality
   such that it is possible to return to the update process after power
   down.

   Note: This is an implementation requirement rather than a requirement
   on the manifest format.

3.6.  Operate with a small bootloader

   The

   Throughout this document we assume that the bootloader must be minimal, containing only flash support,
   cryptographic primitives and optionally a recovery mechanism.  The
   recovery mechanism itself is used in case
   distinct from the update process failed role of the fw consumer and therefore does not
   manage the firmware update process.  This may
   include support give the impression
   that the bootloader itself is a completely separate component, which
   is mainly responsible for selecting a firmware image to boot.

   The overlap between the firmware update process and the bootloader
   functionality comes in two forms, namely

   -  First, a bootloader 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 bootloader 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 bootloader may well be the same manifest obtained with
      the firmware image during the update process (with the severable
      fields stripped off).

   -  Second, an IoT device needs a recovery strategy in case the
      firmware update / boot process fails.  The recovery strategy may
      include storing two or more firmware images on the device or
      offering the ability to have a second stage bootloader perform the
      firmware update process again using firmware updates over serial,
      USB or even a
   limited version of wireless connectivity standard like a limited version of
      Bluetooth Smart.  Such a recovery mechanism must provide security at
   least at  In the same level as latter case the full featured firmware update
   functionalities.

   The bootloader needs to verify fw consumer functionality
      is contained in the received manifest second stage bootloader and to install requires the
      necessary functionality for executing the bootable firmware image.  The update process,
      including manifest parsing.

   In general, it is assumed that the bootloader should itself, or a minimal
   part of it, will not require
   updating be updated since a failed update of the
   bootloader poses a risk in reliability.  If more
   functionality is required in the bootloader, it must use a two-stage
   bootloader, with the first stage comprising the functionality defined
   above.

   All information necessary for a device to make a decision about the
   installation of a firmware update must fit into the available RAM of
   a constrained IoT device.  This prevents flash write exhaustion.
   This is typically not a difficult requirement to accomplish because
   there are not other task/processing running while the bootloader is
   active (unlike it may be the case when running the application
   firmware).

   Note: This is an implementation requirement.

3.7.  Small Parsers

   Since parsers are known sources of bugs they must be minimal.
   Additionally, it must be easy to parse only those fields that are
   required to validate at least one signature or MAC with minimal
   exposure.

3.8.  Minimal impact on existing firmware formats

   The design of the firmware update mechanism must not require changes
   to existing firmware formats.

3.9.  Robust permissions

   When a device obtains a monolithic firmware image from a single
   author without any additional approval steps then 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.

3.10.  Operating modes

   There are three broad classifications of update operating modes.

   -  Client-initiated Update
   -  Server-initiated Update

   -  Hybrid Update

   Client-initiated updates take the form of a firmware consumer on a
   device proactively checking (polling) for new firmware images.

   Server-initiated updates are important to consider because timing of
   updates may need to be tightly controlled in some high- reliability
   environments.  In this case the status tracker determines what
   devices qualify for a firmware update.  Once those devices have been
   selected the firmware server distributes updates to the firmware
   consumers.

   Note: This assumes that the status tracker is able to reach the
   device, which may require devices to keep reachability information at
   the status tracker up-to-date.  This may also require keeping state
   at NATs and stateful packet filtering firewalls alive.

   Hybrid updates are those that require an interaction between the
   firmware consumer and the status tracker.  The status tracker pushes
   notifications of availability of an update to the firmware consumer,
   and it then downloads the image from a firmware server as soon as
   possible.

   An alternative view to the operating modes is to consider the steps a
   device has to go through in the course of an update:

   -  Notification

   -  Pre-authorisation

   -  Dependency resolution

   -  Download

   -  Installation

   The notification step consists of the status tracker informing the
   firmware consumer that an update is available.  This can be
   accomplished via polling (client-initiated), push notifications
   (server-initiated), or more complex mechanisms.

   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-based firmware server is client-
   initiated.  Pushing a manifest and firmware image to the transfer to
   the Package resource of the LwM2M Firmware Update object [LwM2M] is
   server-initiated.

   If the firmware consumer has downloaded a new firmware image and is
   ready to install it, it may need to wait for a trigger from the
   status tracker to initiate the installation, may trigger the update
   automatically, or may go through a more complex decision making
   process to determine the appropriate timing for an update (such as
   delaying the update process to a later time when end users are less
   impacted by the update process).

   Installation is the act of processing the payload into a format that
   the IoT device can recognise and the bootloader is responsible for
   then booting from the newly installed firmware image.

   Each of these steps may require different permissions.

4.  Claims

   Claims in the manifest offer a

3.11.  Suitability to software and personalization data

   The work on a standardized manifest format initially focused on the
   most constrained IoT devices and those devices contain code put
   together by a single author (although that author may obtain code
   from other developers, some of it only in binary form).

   Later it turns out that other use cases may benefit from a
   standardized manifest format also for conveying software and even
   personalization data alongside software.  Trusted Execution
   Environments (TEEs), for example, greatly 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 be conveyed to the TEE.

   To support this wider range of use cases the manifest format should
   therefore be extensible to convey other forms of payloads as well.

4.  Claims

   Claims in the manifest offer a way to convey instructions to a device
   that impact the firmware update process.  To have any value the
   manifest containing those claims must be authenticated and integrity
   protected.  The credential used to must be directly or indirectly
   related to the trust anchor installed at the device by the Trust
   Provisioning Authority.

   The baseline claims for all manifests are described in
   [I-D.ietf-suit-information-model].  For example, there are:

   -  Do not install firmware with earlier metadata than the current
      metadata.

   -  Only install firmware with a matching vendor, model, hardware
      revision, software version, etc.

   -  Only install firmware that is before its best-before timestamp.

   -  Only allow a firmware installation if dependencies have been met.

   -  Choose the mechanism to install the firmware, based on the type of
      firmware it is.

5.  Communication Architecture

   Figure 1 shows the communication architecture where a firmware image
   is created by an author, and uploaded to a firmware server.  The
   firmware image/manifest is distributed to the device either in a push
   or pull manner using the firmware consumer residing on the device.
   The device operator keeps track of the process using the status
   tracker.  This allows the device operator to know and control what
   devices have received an update and which of them are still pending
   an update.

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

                          Figure 1: Architecture.

   End-to-end security mechanisms are used to protect the firmware image
   and the manifest although Figure 2 does not show the manifest itself
   since it may be distributed independently.

                              +-----------+
  +--------+                  |           |                   +--------+
  |        |  Firmware Image  | Firmware  |   Firmware Image  |        |
  | Device |<-----------------| Server    |<------------------| Author |
  |        |                  |           |                   |        |
  +--------+                  +-----------+                   +--------+
       ^                                                          *
       *                                                          *
       ************************************************************
                          End-to-End Security

                      Figure 2: End-to-End Security.

   Whether the firmware image and the manifest is pushed to the device
   or fetched by the device is a deployment specific decision.

   The following assumptions are made to allow the firmware consumer to
   verify the received firmware image and manifest before updating
   software:

   -  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.

   -  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 also a duty of the Trust Provisioning Authority.

   -  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 deployment specific.

   There are different types of delivery modes, which are illustrated
   based on examples below.

   There is an option for embedding a firmware image into a 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 stick or via Bluetooth Smart.
   Figure 3 shows this delivery mode graphically.

                /------------\                 /------------\
               /Manifest with \               /Manifest with \
               |attached      |               |attached      |
               \firmware image/               \firmware image/
                \------------/  +-----------+  \------------/
    +--------+                  |           |                 +--------+
    |        |<.................| Firmware  |<................|        |
    | Device |                  | Server    |                 | Author |
    |        |                  |           |                 |        |
    +--------+                  +-----------+                 +--------+

                Figure 3: Manifest with attached firmware.

   Figure 4 shows an option for remotely updating a device where the
   device fetches the firmware image from some file server.  The
   manifest itself is delivered independently and provides information
   about the firmware image(s) to download.

                                /------------\

                /--------\                     /--------\
               /          \                   /          \
               | Manifest |                   | Manifest |
               \          /                   \          /
                \--------/                     \--------/
                               +-----------+
   +--------+                  \------------/                  |           |                 +--------+
   |        |<..............................................>|        |<.................| Status    |................>|        |
   | Device |                  | Tracker   |              -- | Author |
   |        |<-                |           |            ---  |        |
   +--------+  --              +-----------+          ---    +--------+
                 --                                 ---
                   ---                            ---
                      --       +-----------+    --
                        --     |           |  --
         /------------\   --   | Firmware  |<-    /------------\
        /              \    -- | Server    |     /              \
        |   Firmware   |       |           |     |   Firmware   |
        \              /       +-----------+     \              /
         \------------/                           \------------/

          Figure 4: Independent retrieval of the firmware image.

   This architecture does not mandate a specific delivery mode but a
   solution must support both types.

6.  Manifest

   In order for a device to apply an update, it has to make several
   decisions about the update:

   -  Does it trust the author of the update?

   -  Has the firmware been corrupted?

   -  Does the firmware update apply to this device?

   -  Is the update older than the active firmware?

   -  When should the device apply the update?

   -  How should the device apply the update?

   -  What kind of firmware binary is it?

   -  Where should the update be obtained?

   -  Where should the firmware be stored?

   The manifest encodes the information that devices need in order to
   make these decisions.  It is a data structure that contains the
   following information:

   -  information about the device(s) the firmware image is intended to
      be applied to,

   -  information about when the firmware update has to be applied,

   -  information about when the manifest was created,

   -  dependencies on other manifests,

   -  pointers to the firmware image and information about the format,

   -  information about where to store the firmware image,

   -  cryptographic information, such as digital signatures or message
      authentication codes (MACs).

   The manifest information model is described in
   [I-D.ietf-suit-information-model].

7.  Device Firmware Update Examples

   Although these documents attempt to define a firmware update
   architecture that is applicable to both existing systems, as well as
   yet-to-be-conceived systems; it is still helpful to consider existing
   architectures.

7.1.  Single CPU SoC

   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.  These systems either have a single
   firmware image, or an immutable bootloader that runs a single image.
   A notable characteristic of these SoCs is that the primary code is
   generally execute in place (XIP).  Combined with the non-relocatable
   nature of the code, firmware updates need to be done in place.

7.2.  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.

7.3.  Dual CPU, shared memory

   This configuration has two or more CPUs in a single SoC that share
   memory (flash and RAM).  Generally, they will be a protection
   mechanism to prevent one CPU from accessing the other's memory.
   Upgrades in this case will typically be done by one of the CPUs, and
   is similar to the single CPU with secure mode.

7.4.  Dual CPU, other bus

   This configuration has two or more 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 a master, 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.

8.  Bootloader

   More devices today than ever before are being 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 RS232.  Updating a device over the Internet requires the
   device to fetch not only the firmware image but also the manifest.
   Hence, the following building blocks are necessary for a firmware
   update solution:

   -  the Internet protocol stack for (possibly large) firmware
      downloads,

   -  the capability to write the received firmware image to persistent
      storage (most likely flash memory) prior to performing the update,

   -  the ability to unpack, decompress or otherwise process the
      received firmware image,

   -  the features to verify an image and a manifest, including digital
      signature verification or checking a message authentication code,

   -  a manifest parsing library, and

   -  integration of the device into a device management server to
      perform automatic firmware updates and to track their progress.

   All these features are most likely offered by the application, i.e.
   firmware consumer, running on the device (except for basic security
   algorithms that may run either on a trusted execution environment or
   on a separate hardware security MCU/module) rather than by the
   bootloader itself.

   Once manifests have been processed and firmware images successfully
   downloaded and verified the device needs to hand control over to the
   bootloader.  In most cases this requires the MCU to restart.  Once
   the MCU has initiated a restart, the bootloader takes over control
   and determines whether the newly downloaded firmware image should be
   executed.

   The boot process is security sensitive because the firmware images
   may, for example, be stored in off-chip flash memory giving attackers
   easy access to the image for reverse engineering and potentially also
   for modifying the binary.  The bootloader will therefore have to
   perform security checks on the firmware image before it can be
   booted.  These security checks by the bootloader happen in addition
   to the security checks that happened when the firmware image and the
   manifest were downloaded.

   The manifest may have been stored alongside the firmware image to
   allow re-verification of the firmware image during every boot
   attempt.  Alternatively, secure boot-specific meta-data may have been
   created by the application after a successful firmware download and
   verification process.  Whether to re-use the standardized manifest
   format that was used during the initial firmware retrieval process or
   whether it is better to use a different format for the secure boot-
   specific meta-data depends on the system design.  The manifest format
   does, however, have the capability to serve also as a building block
   for secure boot with its severable elements that allow shrinking the
   size of the manifest by stripping elements that are no longer needed.

   If the application image contains the firmware consumer
   functionality, as described above, then it is necessary that a
   working image is left on the device to ensure that the bootloader can
   roll back to a working firmware image to re-do the firmware download
   since the bootloader itself does not have enough functionality to
   fetch a firmware image plus manifest from a firmware server over the
   Internet.  A multi-stage bootloader may soften this requirement at
   the expense of a more sophisticated boot process.

   For a bootloader to offer a secure boot mechanism it needs to provide
   the following features:

   -  ability to access security algorithms, such as SHA-256 to compute
      a fingerprint over the firmware image and a digital signature
      algorithm.

   -  access keying material directly or indirectly to utilize the
      digital signature.  The device needs to have a trust anchor store.

   -  ability to expose boot process-related data to the application
      firmware (such as to the device management software).  This allows
      a device management server to determine whether the firmware
      update has been successful and, if not, what errors occurred.

   -  to (optionally) offer attestation information (such as
      measurements).

   While the software architecture of software architecture of the bootloader and its security
   mechanisms are implementation-specific, the manifest can be used to
   control the firmware download from the Internet in addition to
   augmenting secure boot process.  These building blocks are highly
   relevant for the design of the manifest.

9.  Example

   Figure 5 illustrates an example message flow for distributing a
   firmware image to a device starting with an author uploading the new
   firmware to firmware server and creating a manifest.  The firmware
   and manifest are stored on the same firmware server.  This setup does
   not use a status tracker and the firmware consumer component is
   therefore responsible for periodically checking whether a new
   firmware image is available for download.

   +--------+    +-----------------+      +------------+ +----------+
   | Author |    | Firmware Server |      |FW Consumer | |Bootloader|
   +--------+    +-----------------+      +------------+ +----------+
     |                   |                     |                +
     | Create Firmware   |                     |                |
     |--------------+    |                     |                |
     |              |    |                     |                |
     |<-------------+    |                     |                |
     |                   |                     |                |
     | Upload Firmware   |                     |                |
     |------------------>|                     |                |
     |                   |                     |                |
     | Create Manifest   |                     |                |
     |---------------+   |                     |                |
     |               |   |                     |                |
     |<--------------+   |                     |                |
     |                   |                     |                |
     | Sign Manifest     |                     |                |
     |-------------+     |                     |                |
     |             |     |                     |                |
     |<------------+     |                     |                |
     |                   |                     |                |
     | Upload Manifest   |                     |                |
     |------------------>|                     |                |
     |                   |                     |                |
     |                   |   Query Manifest    |                |
     |                   |<--------------------|                |
     |                   |                     |                |
     |                   |   Send Manifest     |                |
     |                   |-------------------->|                |
     |                   |                     | Validate       |
     |                   |                     | Manifest       |
     |                   |                     |---------+      |
     |                   |                     |         |      |
     |                   |                     |<--------+      |
     |                   |                     |                |
     |                   |  Request Firmware   |                |
     |                   |<--------------------|                |
     |                   |                     |                |
     |                   | Send Firmware       |                |
     |                   |-------------------->|                |
     |                   |                     | Verify         |
     |                   |                     | Firmware       |
     |                   |                     |--------------+ |
     |                   |                     |              | |
     |                   |                     |<-------------+ |
     |                   |                     |                |
     |                   |                     | Store          |
     |                   |                     | Firmware       |
     |                   |                     |-------------+  |
     |                   |                     |             |  |
     |                   |                     |<------------+  |
     |                   |                     |                |
     |                   |                     |                |
     |                   |                     | Trigger 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|   |                |  |
     |                   |                 +---+----------------+--+
     |                   |                     |                |

            Figure 5: First Example Flow for a Firmware Upate.

   Figure 6 shows an example follow with the device using a status
   tracker.  For editorial reasons the author publishing the manifest at
   the status tracker and the firmware image at the firmware server is
   not shown.  Also omitted is the secure boot process following the
   successful firmware update process.

   The exchange starts with the device interacting with the status
   tracker; the details of such exchange will vary with the different
   device management systems being used.  In any case, the status
   tracker learns about the firmware version of the devices it manages.
   In our example, the device under management is using firmware version
   A.B.C.  At a later point in time the author uploads a new firmware
   along with the manifest to the firmware server and the status
   tracker, respectively.  While there is no need to store the manifest
   and the firmware on different servers this example shows a common
   pattern used in the industry.  The status tracker may then
   automatically, based on human intervention or based on a more complex
   policy decide to inform the device about the bootloader and its security
   mechanisms are implementation-specific, newly available firmware
   image.  In our example, it does so by pushing the manifest can be used to
   control the FW
   consumer.  The firmware download from the Internet in addition to
   augmenting secure boot process.  These building blocks are highly
   relevant for the design of consumer downloads the manifest.

9.  Example

   The following example message flow illustrates a possible interaction
   for distributing a firmware image to a device starting with an author
   uploading
   the new firmware to firmware server and creating a newer version X.Y.Z after successful validation of the manifest.  The firmware
   Subsequently, a reboot is initiated and manifest are stored on the same firmware
   server.

   +--------+ secure boot process
   starts.

    +---------+   +-----------------+      +------------+ +----------+    |-----------------------------.
    | Author Status  |   | Firmware Server |      |FW Consumer    | |Bootloader|
   +--------+    +-----------------+ +------------+ +----------+ |
    | Tracker |                +   | Create Firmware                 |    | |FW Consumer |
     |--------------- |Bootloader| |
    +---------+   +-----------------+    | +------------+ +----------+ |
         |                |              |      |  IoT Device    |
     |<--------------     |
         |                |               `''''''''''''''''''''''''''''
         |                |                     |                |
         | Upload        Query Firmware Version        |                |                |
     |------------------>|                     |                |
     |                   |                     |                |
     | Create Manifest   |                     |                |
     |----------------   |                     |                |
     |               |   |                     |                |
     |<---------------   |
         |------------------------------------->|                |
         |        Firmware Version A.B.C        |                |
         |<-------------------------------------|                |
         |                | Sign Manifest                     |                |
         |
     |--------------         <<some time later>>          |                |
         |                |                     |                |
       _,...._         _,...._                  |                |
     |<-------------
     ,'       `.     ,'       `.                |                |
    |   New     |   |   New     |               |                | Upload
    \ Manifest  /   \ Firmware  /               |                |                |
     |------------------>|                     |                |
     |                   |                     |                |
     |                   |   Query Manifest
     `.._   _,,'     `.._   _,,'                |                |
         `''             `''                    |                   |<--------------------|                |
         |            Push manifest             |                |
         |----------------+-------------------->|                |
         |                |   Send Manifest                     |                |
         |                   |-------------------->|                '                     |                '
         |                |                     | Validate       |
         |                |                     | Manifest       |
         |                |                     |---------+      |
         |                |                     |         |      |
         |                |                     |<--------+      |
         |                | Request firmware    |                |
         |                |  Request Firmware X.Y.Z               |                |
         |                |<--------------------|                |
         |                |                     |                |
         |                | Send Firmware X.Y.Z      |                |
         |                |-------------------->|                |
         |                |                     | Verify         |
     |                   |                     | Firmware       |
     |                   |                     |--------------- |                |
         |                |                     | |
     |                   |                     |<-------------- |
     |                   |                     |                |
     |                   |                     | Store Verify         |
         |                |                     | Firmware       |
         |                |                     |--------------  |
     |                   |                     |--------------+ |
         |                |                     |              |                     |<------------- |
         |                |                     |<-------------+ |
         |                |                     |                |
         |                |                     |                     | Reboot         |
     |                   |                     |--------------->|
     |                   |                     |                |
     |                   |                     | Verify Store          |
         |                |                     | Firmware       |
         |                |                     |-------------+  | ---------------|
         |                |                     |             |  |
         |                |                     |<------------+  | -------------->|
         |                |                     |                |
         |                |                     | Activate new                |
         |                |                     | Firmware Trigger Reboot |
         |                |                     |--------------->|
         | ---------------|                |                     |                |
         |                |                     |                |
         | -------------->|                |                     | __..-------..._'
         |                |                    ,-'               `-.
         |                |                   |      Secure Boot new       |
     |                   |                     | Firmware       |
     |                   |                     | ---------------|
     |    |
         |                |                   `-.                 _/
         |                |                   |                     | -------------->|                     |`--..._____,,.,-'
         |                |                     |                |

            Figure 5: 6: Second Example Flow for a Firmware Upate.

10.  IANA Considerations

   This document does not require any actions by IANA.

11.  Security Considerations

   Firmware updates fix security vulnerabilities and are considered to
   be an important building block in securing IoT devices.  Due to the
   importance of firmware updates for IoT devices the Internet
   Architecture Board (IAB) organized a 'Workshop on Internet of Things
   (IoT) Software Update (IOTSU)', which took place at Trinity College
   Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
   the big picture.  A report about this workshop can be found at
   [RFC8240].  A standardized firmware manifest format providing end-to-
   end security from the author to the device will be specified in a
   separate document.

   There are, however, many other considerations raised during the
   workshop.  Many of them are outside the scope of standardization
   organizations since they fall into the realm of product engineering,
   regulatory frameworks, and business models.  The following
   considerations are outside the scope of this document, namely

   -  installing firmware updates in a robust fashion so that the update
      does not break the device functionality of the environment this
      device operates in.

   -  installing firmware updates in a timely fashion considering the
      complexity of the decision making process of updating devices,
      potential re-certification requirements, and the need for user
      consent to install updates.

   -  the distribution of the actual firmware update, potentially in an
      efficient manner to a large number of devices without human
      involvement.

   -  energy efficiency and battery lifetime considerations.

   -  key management required for verifying the digital signature
      protecting the manifest.

   -  incentives for manufacturers to offer a firmware update mechanism
      as part of their IoT products.

12.  Mailing List Information

   The discussion list for this document is located at the e-mail
   address suit@ietf.org [1].  Information on the group and information
   on how to subscribe to the list is at
   https://www1.ietf.org/mailman/listinfo/suit [2]

   Archives of the list can be found at: https://www.ietf.org/mail-
   archive/web/suit/current/index.html [3]

13.  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

   -  Markus Gueller

   -  Henk Birkholz

   -  Jintao Zhu

   -  Takeshi Takahashi

   -  Jacob Beningo

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

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>. 1997.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925, DOI
              10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>. <https://www.rfc-
              editor.org/info/rfc7925>.

14.2.  Informative References

   [I-D.ietf-suit-information-model]
              Moran, B., Tschofenig, H., and H. Birkholz, "Firmware
              Updates for Internet of Things Devices - An Information
              Model for Manifests", draft-ietf-suit-information-model-02 draft-ietf-suit-information-model-03
              (work in progress), July 2019.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Wheeler, D., Atyeo, A., and D.
              Liu, "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-03 (work in
              progress), January July 2019.

   [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>.

   [RFC5649]  Housley, R. and M. Dworkin, "Advanced Encryption Standard
              (AES) Key Wrap with Padding Algorithm", RFC 5649, DOI
              10.17487/RFC5649, September 2009,
              <https://www.rfc-editor.org/info/rfc5649>. <https://www.rfc-
              editor.org/info/rfc5649>.

   [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>.

   [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>.

14.3.  URIs

   [1] mailto:suit@ietf.org

   [2] https://www1.ietf.org/mailman/listinfo/suit

   [3] https://www.ietf.org/mail-archive/web/suit/current/index.html

Authors' Addresses

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com

   Milosch Meriac
   Consultant

   EMail: milosch@meriac.com

   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@arm.com

   David Brown
   Linaro

   EMail: david.brown@linaro.org