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SUIT                                                            B. Moran
Internet-Draft                                                 M. Meriac
Intended status: Informational                             H. Tschofenig
Expires: September 3, 2018                                   Arm Limited
                                                          March 02, 2018


     A Firmware Update Architecture for Internet of Things Devices
                    draft-moran-suit-architecture-02

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

   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 http://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 September 3, 2018.








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Copyright Notice

   Copyright (c) 2018 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
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Agnostic to how firmware images are distributed . . . . .   6
     3.2.  Friendly to broadcast delivery  . . . . . . . . . . . . .   6
     3.3.  Uses state-of-the-art security mechanisms . . . . . . . .   6
     3.4.  Rollback attacks must be prevented  . . . . . . . . . . .   6
     3.5.  High reliability  . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Operates with a small bootloader  . . . . . . . . . . . .   7
     3.7.  Small Parsers . . . . . . . . . . . . . . . . . . . . . .   7
     3.8.  Minimal impact on existing firmware formats . . . . . . .   7
     3.9.  Robust permissions  . . . . . . . . . . . . . . . . . . .   7
   4.  Claims  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Manifest  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. Mailing List Information  . . . . . . . . . . . . . . . . . .  15



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   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  Threat Model, User Stories, Security Requirements,
                and Usability Requirements . . . . . . . . . . . . .  17
     A.1.  Threat Model  . . . . . . . . . . . . . . . . . . . . . .  17
     A.2.  Threat Descriptions . . . . . . . . . . . . . . . . . . .  17
       A.2.1.  Threat MFT1: Old Firmware . . . . . . . . . . . . . .  17
       A.2.2.  Threat MFT2: Mismatched Firmware  . . . . . . . . . .  17
       A.2.3.  Threat MFT3: Offline device + Old Firmware  . . . . .  18
       A.2.4.  Threat MFT4: The target device misinterprets the type
               of payload  . . . . . . . . . . . . . . . . . . . . .  18
       A.2.5.  Threat MFT5: The target device installs the payload
               to the wrong location . . . . . . . . . . . . . . . .  18
       A.2.6.  Threat MFT6: Redirection  . . . . . . . . . . . . . .  19
       A.2.7.  Threat MFT7: Payload Verification on Boot . . . . . .  19
       A.2.8.  Threat MFT8: Unauthenticated Updates  . . . . . . . .  19
       A.2.9.  Threat MFT9: Unexpected Precursor images  . . . . . .  19
       A.2.10. Threat MFT10: Unqualified Firmware  . . . . . . . . .  19
       A.2.11. Threat MFT11: Reverse Engineering Of Firmware Image
               for Vulnerability Analysis  . . . . . . . . . . . . .  20
     A.3.  Security Requirements . . . . . . . . . . . . . . . . . .  21
       A.3.1.  Security Requirement MFSR1: Monotonic Sequence
               Numbers . . . . . . . . . . . . . . . . . . . . . . .  21
       A.3.2.  Security Requirement MFSR2: Vendor, Device-type
               Identifiers . . . . . . . . . . . . . . . . . . . . .  21
       A.3.3.  Security Requirement MFSR3: Best-Before Timestamps  .  21
       A.3.4.  Security Requirement MFSR4: Signed Payload Descriptor  21
       A.3.5.  Security Requirement MFSR5: Cryptographic
               Authenticity  . . . . . . . . . . . . . . . . . . . .  22
       A.3.6.  Security Requirement MFSR6: Rights Require
               Authenticity  . . . . . . . . . . . . . . . . . . . .  22
       A.3.7.  Security Requirement MFSR7: Firmware encryption . . .  23
     A.4.  User Stories  . . . . . . . . . . . . . . . . . . . . . .  23
       A.4.1.  Use Case MFUC1: Installation Instructions . . . . . .  23
       A.4.2.  Use Case MFUC2: Reuse Local Infrastructure  . . . . .  23
       A.4.3.  Use Case MFUC3: Modular Update  . . . . . . . . . . .  24
       A.4.4.  Use Case MFUC4: Multiple Authorisations . . . . . . .  24
       A.4.5.  Use Case MFUC5: Multiple Payload Formats  . . . . . .  24
       A.4.6.  Use Case MFUC6: IP Protection . . . . . . . . . . . .  24
     A.5.  Usability Requirements  . . . . . . . . . . . . . . . . .  24
       A.5.1.  Usability Requirement MFUR1 . . . . . . . . . . . . .  24
       A.5.2.  Usability Requirement MFUR2 . . . . . . . . . . . . .  24
       A.5.3.  Usability Requirement MFUR3 . . . . . . . . . . . . .  25
       A.5.4.  Usability Requirement MFUR4 . . . . . . . . . . . . .  25
       A.5.5.  Usability Requirement MFUR5 . . . . . . . . . . . . .  25



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     A.6.  Manifest Fields . . . . . . . . . . . . . . . . . . . . .  25
       A.6.1.  Manifest Field: Timestamp . . . . . . . . . . . . . .  25
       A.6.2.  Manifest Field: Vendor ID Condition . . . . . . . . .  26
       A.6.3.  Manifest Field: Class ID Condition  . . . . . . . . .  26
       A.6.4.  Manifest Field: Precursor Image Digest Condition  . .  26
       A.6.5.  Manifest Field: Best-Before timestamp . . . . . . . .  26
       A.6.6.  Manifest Field: Payload Format  . . . . . . . . . . .  26
       A.6.7.  Manifest Field: Storage Location  . . . . . . . . . .  27
       A.6.8.  Manifest Field: URIs  . . . . . . . . . . . . . . . .  27
       A.6.9.  Manifest Field: Digests . . . . . . . . . . . . . . .  27
       A.6.10. Manifest Field: Size  . . . . . . . . . . . . . . . .  27
       A.6.11. Manifest Field: Signature . . . . . . . . . . . . . .  27
       A.6.12. Manifest Field: Directives  . . . . . . . . . . . . .  27
       A.6.13. Manifest Field: Aliases . . . . . . . . . . . . . . .  28
       A.6.14. Manifest Field: Dependencies  . . . . . . . . . . . .  28
       A.6.15. Manifest Field: Content Key Distribution Method . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  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 or where manual intervention is cost
   prohibitive or otherwise difficult.  The need for a firmware update
   may be to fix bugs in software, to add new functionality, or to re-
   configure the device.

   The firmware update process has to ensure that

   -  The firmware image is authenticated and attempts to flash a
      malicious firmware image 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 plaintext binary is often one of the
      first steps for an attack to mount an attack.

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:




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   -  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.

   The following entities are used:

   -  Author: The author is the entity that creates the firmware image,
      signs and/or encrypts it and attaches a manifest to it.  The
      author is most likely a developer using a set of tools.

   -  Device: The device is the recipient of the firmware image and the
      manifest.  The goal is to update the firmware of the device.

   -  Untrusted Storage: Firmware images and manifests are stored on
      untrusted fileservers or cloud storage infrastructure.  Some
      deployments may require storage of the firmware images/manifests
      to be stored on various entities before they reach the device.

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

   -  Uses state-of-the-art security mechanisms

   -  Rollback attacks must be prevented.

   -  High reliability

   -  Operates with a small bootloader

   -  Small Parsers

   -  Minimal impact on existing firmware formats

   -  Robust permissions



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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

   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.  Uses 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 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.  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.

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 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



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   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.  Operates with a small bootloader

   The bootloader must be minimal, containing only flash support,
   cryptographic primitives and optionally a recovery mechanism.  The
   recovery mechanism is used in case the update process failed and may
   include support for firmware updates over serial, USB or even a
   limited version of Bluetooth Smart.  Such a recovery mechanism must
   provide security at least at the same level as the full featured
   firmware update functionalities.

   The bootloader needs to verify the received manifest and to install
   the bootable firmware image.  The bootloader should not require
   updating since a failed update 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.

   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 which are
   required to validate at least one signature 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

   A device may have many modules that require updating individually.
   It may also need to trust several actors in order to authorize an
   update.  For example, a firmware author may not have the authority to
   install firmware on a device in critical infrastructure without the



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   authorization of a device operator.  In this case, the device should
   reject firmware updates unless they are signed both by the firmware
   author and by the device operator.  To facilitate complex use-cases
   such as this, updates require several permissions.

4.  Claims

   When a simple set of permissions fails to encapsulate the rules
   required for a device make decisions about firmware, claims can be
   used instead.  Claims represent a form of policy.  Several claims can
   be used together, when multiple actors should have the rights to set
   policies.

   Some example claims are:

   -  Trust the actor identified by the referenced public key.

   -  Three actors are trusted identified by their public keys.
      Signatures from at least two of these actors are required to trust
      a manifest.

   -  The actor identified by the referenced public key is authorized to
      create secondary policies

   The baseline claims for all manifests are described in Appendix A.
   In summary, they 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 install firmware with metadata signed by a trusted actor.

   -  Only allow an actor to exercise rights on the device via a
      manifest if that actor has signed the manifest.

   -  Only allow a firmware installation if all required rights have
      been met through signatures (one or more) or manifest dependencies
      (one or more).

   -  Use the instructions provided by the manifest to install the
      firmware.

   -  Any authorized actor may redirect any URI.



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   -  Install any and all firmware images that are linked together with
      manifest dependencies.

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

5.  Architecture

   We start the architectural description with the security model.  It
   is based on end-to-end security.  Figure 1 illustrates the security
   model where a firmware image and the corresponding manifest are
   created by an author and verified by the device.  The firmware image
   is integrity protected and may be encrypted.  The manifest is
   integrity protected and authenticated.  When the author is ready to
   distribute the firmware image it is conveyed using some communication
   channel to the device, which will typically involve the use of
   untrusted storage.  Examples of untrusted storage are FTP servers,
   Web servers or USB sticks.

                              +-----------+
  +--------+  Firmware Image  |           |   Firmware Image  +--------+
  |        |  + Manifest      | Untrusted |   + Manifest      |        |
  | Device |<-----------------| Storage   |<------------------| Author |
  |        |                  |           |                   |        |
  +--------+                  +-----------+                   +--------+
       ^                                                          *
       *                                                          *
       ************************************************************
                          End-to-End Security

                      Figure 1: End-to-End Security.

   Whether the firmware image and the manifest is pushed to the device
   or fetched by the device is outside the scope of this work and
   existing device management protocols can be used for efficiently
   distributing this information.

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

   -  To accept an update, a device needs to decide whether the author
      signing the firmware image and the manifest is authorized to make
      the updates.  We use public key cryptography to accomplish this.
      The device verifies the signature covering the manifest using a
      digital signature algorithm.  The device is provisioned with a
      trust anchor that is used to validate the digital signature
      produced by the author.  This trust anchor is potentially
      different from the trust anchor used to validate the digital



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      signature produced for other protocols (such as device management
      protocols).  This trust anchor may be provisioned to the device
      during manufacturing or during commissioning.

   -  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.

   There are different types of delivery modes, which are illustrates
   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 server for
   download of the firmware.  It is also applicable when the firmware
   update happens via a USB stick or via Bluetooth Smart.  Figure 2
   shows this delivery mode graphically.

                /------------\                 /------------\
               /Manifest with \               /Manifest with \
               |attached      |               |attached      |
               \firmware image/               \firmware image/
                \------------/  +-----------+  \------------/
    +--------+                  |           |                 +--------+
    |        |<.................| Untrusted |<................|        |
    | Device |                  | Storage   |                 | Author |
    |        |                  |           |                 |        |
    +--------+                  +-----------+                 +--------+

                Figure 2: Manifest with attached firmware.

   Figure 3 shows an option for remotely updating a device where the
   device fetches the firmware image from some file server.  The
   manifest itself is delivery independently and provides information
   about the firmware image(s) to download.
















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                                /------------\
                               /              \
                               |   Manifest   |
                               \              /
    +--------+                  \------------/                +--------+
    |        |<..............................................>|        |
    | Device |                                             -- | Author |
    |        |<-                                         ---  |        |
    +--------+  --                                     ---    +--------+
                  --                                 ---
                    ---                            ---
                       --       +-----------+    --
                         --     |           |  --
          /------------\   --   | Untrusted |<-    /------------\
         /              \    -- | Storage   |     /              \
         |   Firmware   |       |           |     |   Firmware   |
         \              /       +-----------+     \              /
          \------------/                           \------------/

          Figure 3: 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?





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   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 intented to
      be applied to,

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

   -  information about when the manifest was created,

   -  dependencies to 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.

   The manifest format is described in a companion document.

7.  Example Flow

   The following example message flow illustrates the interaction for
   distributing a firmware image to a device starting with an author
   uploading the new firmware to untrusted storage and creating a
   manifest.

   +--------+    +-----------------+      +------+
   | Author |    |Untrusted Storage|      |Device|
   +--------+    +-----------------+      +------+
     |                   |                     |
     | Create Firmware   |                     |
     |---------------    |                     |
     |              |    |                     |
     |<--------------    |                     |
     |                   |                     |
     | Upload Firmware   |                     |
     |------------------>|                     |
     |                   |                     |
     | Create Manifest   |                     |
     |----------------   |                     |
     |               |   |                     |
     |<---------------   |                     |
     |                   |                     |
     | Sign Manifest     |                     |
     |--------------     |                     |
     |             |     |                     |



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     |<-------------     |                     |
     |                   |                     |
     | Upload Manifest   |                     |
     |------------------>|                     |
     |                   |                     |
     |                   |   Query Manifest    |
     |                   |<--------------------|
     |                   |                     |
     |                   |   Send Manifest     |
     |                   |-------------------->|
     |                   |                     |
     |                   |                     | Validate Manifest
     |                   |                     |------------------
     |                   |                     |                 |
     |                   |                     |<-----------------
     |                   |                     |
     |                   |  Request Firmware   |
     |                   |<--------------------|
     |                   |                     |
     |                   | Send Firmware       |
     |                   |-------------------->|
     |                   |                     |
     |                   |                     | Verify Firmware
     |                   |                     |---------------
     |                   |                     |              |
     |                   |                     |<--------------
     |                   |                     |
     |                   |                     | Store Firmware
     |                   |                     |--------------
     |                   |                     |             |
     |                   |                     |<-------------
     |                   |                     |
     |                   |                     | Reboot
     |                   |                     |-------
     |                   |                     |      |
     |                   |                     |<------
     |                   |                     |
     |                   |                     | Bootloader validates
     |                   |                     | Firmware
     |                   |                     |----------------------
     |                   |                     |                     |
     |                   |                     |<---------------------
     |                   |                     |
     |                   |                     | Bootloader activates
     |                   |                     | Firmware
     |                   |                     |----------------------
     |                   |                     |                     |
     |                   |                     |<---------------------



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     |                   |                     |
     |                   |                     | Bootloader transfers
     |                   |                     | control to new Firmware
     |                   |                     |----------------------
     |                   |                     |                     |
     |                   |                     |<---------------------
     |                   |                     |

               Figure 4: Example Flow for a Firmware Upate.

8.  IANA Considerations

   This document does not require any actions by IANA.

9.  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].  This document (and associated specifications) offer a
   standardized firmware manifest format providing end-to-end security
   from the author to the device.

   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's
      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.




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

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

10.  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

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

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

   -  David Brown

   -  Jim Schaad

   -  Carsten Bormann

   -  Cullen Jennings

   -  Olaf Bergmann

   -  Suhas Nandakumar

   -  Phillip Hallam-Baker




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   -  Marti Bolivar

   -  Andrzej Puzdrowski

   -  Markus Gueller

   We would also like to thank the WG chairs, Russ Housley, David
   Waltermire, Dave Thaler and the responsible security area director,
   Kathleen Moriarty, for their support and their reviews.

12.  References

12.1.  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>.

12.2.  Informative References

   [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>.

   [STRIDE]   Microsoft, "The STRIDE Threat Model", January 2018.

12.3.  URIs

   [1] mailto:suit@ietf.org




















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Appendix A.  Threat Model, User Stories, Security Requirements, and
             Usability Requirements

A.1.  Threat Model

   This appendix aims to provide information about the threats that were
   considered, the security requirements that are derived from those
   threats and the fields that permit implementation of the security
   requirements.  This model uses the S.T.R.I.D.E.  [STRIDE] approach.
   Each threat is classified according to:

   -  Spoofing Identity

   -  Tampering with data

   -  Repudiation

   -  Information disclosure

   -  Denial of service

   -  Elevation of privilege

A.2.  Threat Descriptions

A.2.1.  Threat MFT1: Old Firmware

   Classification: Escalation of Privilege

   An attacker sends an old, but valid manifest with an old, but valid
   firmware image to a device.  If there is a known vulnerability in the
   provided firmware image, this may allow an attacker to exploit the
   vulnerability and gain control of the device.

   Threat Escalation: If the attacker is able to exploit the known
   vulnerability, then this threat can be escalated to ALL TYPES.

A.2.2.  Threat MFT2: Mismatched Firmware

   Classification: Denial of Service

   An attacker sends a valid firmware image, for the wrong type of
   device, signed by an actor with firmware installation permission on
   both types of device.  The firmware is verified by the device
   positively because it is signed by an actor with the appropriate
   permission.  This could have wide-ranging consequences.  For devices
   that are similar, it could cause minor breakage, or expose security




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   vulnerabilities.  For devices that are very different, it is likely
   to render devices inoperable.

A.2.3.  Threat MFT3: Offline device + Old Firmware

   Classification: Escalation of Privilege

   An attacker targets a device that has been offline for a long time
   and runs an old firmware version.  The attacker sends an old, but
   valid manifest to a device with an old, but valid firmware image.
   The attacker-provided firmware is newer than the installed one but
   older than the most recently available firmware.  If there is a known
   vulnerability in the provided firmware image then this may allow an
   attacker to gain control of a device.  Because the device has been
   offline for a long time, it is unaware of any new updates.  As such
   it will treat the old manifest as the most current.

   Threat Escalation: If the attacker is able to exploit the known
   vulnerability, then this threat can be escalated to ALL TYPES.

A.2.4.  Threat MFT4: The target device misinterprets the type of payload

   Classification: Denial of Service

   If a device misinterprets the type of the firmware image, it may
   cause a device to install a firmware image incorrectly.  An
   incorrectly installed firmware image would likely cause the device to
   stop functioning.

   Threat Escalation: An attacker that can cause a device to
   misinterpret the received firmware image may gain escalation of
   privilege and potentially expand this to all types of threat.

A.2.5.  Threat MFT5: The target device installs the payload to the wrong
        location

   Classification: Denial of Service

   If a device installs a firmware image to the wrong location on the
   device, then it is likely to break.  For example, a firmware image
   installed as an application could cause a device and/or an
   application to stop functioning.

   Threat Escalation: An attacker that can cause a device to
   misinterpret the received code may gain escalation of privilege and
   potentially expand this to all types of threat.





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A.2.6.  Threat MFT6: Redirection

   Classification: Denial of Service

   If a device does not know where to obtain the payload for an update,
   it may be redirected to an attacker's server.  This would allow an
   attacker to provide broken payloads to devices.

A.2.7.  Threat MFT7: Payload Verification on Boot

   Classification: All Types

   An attacker replaces a newly downloaded firmware after a device
   finishes verifying a manifest.  This could cause the device to
   execute the attacker's code.  This attack likely requires physical
   access to the device.  However, it is possible that this attack is
   carried out in combination with another threat that allows remote
   execution.

A.2.8.  Threat MFT8: Unauthenticated Updates

   Classification: All Types

   If an attacker can install their firmware on a device, by
   manipulating either payload or metadata, then they have complete
   control of the device.

A.2.9.  Threat MFT9: Unexpected Precursor images

   Classification: Denial of Service

   An attacker sends a valid, current manifest to a device that has an
   unexpected precursor image.  If a payload format requires a precursor
   image (for example, delta updates) and that precursor image is not
   available on the target device, it could cause the update to break.

   Threat Escalation: An attacker that can cause a device to install a
   payload against the wrong precursor image could gain escalation of
   privilege and potentially expand this to all types of threat.

A.2.10.  Threat MFT10: Unqualified Firmware

   Classification: Denial of Service, Escalation of Privilege

   This threat can appear in several ways, however it is ultimately
   about interoperability of devices with other systems.  The owner or
   operator of a network needs to approve firmware for their network in
   order to ensure interoperability with other devices on the network,



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   or the network itself.  If the firmware is not qualified, it may not
   work.  Therefore, if a device installs firmware without the approval
   of the network owner or operator, this is a threat to devices and the
   network.

   Example 1: We assume that OEMs expect the rights to create firmware,
   but that Operators expect the rights to qualify firmware as fit-for-
   purpose on their networks.

   An attacker obtains a manifest for a device on Network A.  They send
   that manifest to a device on Network B.  Because Network A and
   Network B are different, and the firmware has not been qualified for
   Network B, the target device is disabled by this unqualified, but
   signed firmware.

   This is a denial of service because it can render devices inoperable.
   This is an escalation of privilege because it allows the attacker to
   make installation decisions that should be made by the Operator.

   Example 2: Multiple devices that interoperate are used on the same
   network.  Some devices are manufactured by OEM A and other devices by
   OEM B.  These devices communicate with each other.  A new firmware is
   released by OEM A that breaks compatibility with OEM B devices.  An
   attacker sends the new firmware to the OEM A devices without approval
   of the network operator.  This breaks the behaviour of the larger
   system causing denial of service and possibly other threats.  Where
   the network is a distributed SCADA system, this could cause
   misbehaviour of the process that is under control.

   Threat Escalation: If the firmware expects configuration that is
   present in Network A devices, but not Network B devices, then the
   device may experience degraded security, leading to threats of All
   Types.

A.2.11.  Threat MFT11: Reverse Engineering Of Firmware Image for
         Vulnerability Analysis

   Classification: All Types

   An attacker wants to mount an attack on an IoT device.  To prepare
   the attack he or she retrieves the provided firmware image and
   performs reverse engineering of the firmware image to analyze it for
   specific vulnerabilities.








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A.3.  Security Requirements

   The security requirements here are a set of policies that mitigate
   the threats described in the previous section.

A.3.1.  Security Requirement MFSR1: Monotonic Sequence Numbers

   Only an actor with firmware installation authority is permitted to
   decide when device firmware can be installed.  To enforce this rule,
   Manifests MUST contain monotonically increasting sequence numbers.
   Manifests MAY use UTC epoch timestamps to coordinate monotonically
   increasting sequence numbers across many actors in many locations.
   Devices MUST reject manifests with sequence numbers smaller than any
   onboard sequence number.

   N.B.  This is not a firmware version.  It is a manifest sequence
   number.  A firmware version may be rolled back by creating a new
   manifest for the old firmware version with a later sequence number.

   Mitigates: Threat MFT1

A.3.2.  Security Requirement MFSR2: Vendor, Device-type Identifiers

   Devices MUST only apply firmware that is intended for them.  Devices
   MUST know with fine granularity that a given update applies to their
   vendor, model, hardware revision, software revision.  Human-readable
   identifiers are often error-prone in this regard, so unique
   identifiers SHOULD be used.

   Mitigates: Threat MFT2

A.3.3.  Security Requirement MFSR3: Best-Before Timestamps

   Firmware MAY expire after a given time.  Devices MAY provide a secure
   clock (local or remote).  If a secure clock is provided and the
   Firmware manifest has a best-before timestamp, the device MUST reject
   the manifest if current time is larger than the best-before time.

   Mitigates: Threat MFT3

A.3.4.  Security Requirement MFSR4: Signed Payload Descriptor

   All descriptive information about the payload MUST be signed.  This
   MUST include:

   -  The location to store the payload





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   -  The payload digest, in each state of installation (encrypted,
      plaintext, installed, etc.)

   -  The payload size

   -  The payload format

   -  Where to obtain the payload

   -  All instructions or parameters for applying the payload

   -  Any rules that identify whether or not the payload can be used on
      this device

   Mitigates: Threats MFT5, MFT6, MFT7, MFT9

A.3.5.  Security Requirement MFSR5: Cryptographic Authenticity

   The authenticity of an update must be demonstrable.  Typically, this
   means that updates must be digitally signed.  Because the manifest
   contains information about how to install the update, the manifest's
   authenticity must also be demonstrable.  To reduce the overhead
   required for validation, the manifest contains the digest of the
   firmware image, rather than a second digitial signature.  The
   authenticity of the manifest can be verified with a digital
   signature, the authenticity of the firmware image is tied to the
   manifest by the use of a fingerprint of the firmware image.

   Mitigates: Threat MFT8

A.3.6.  Security Requirement MFSR6: Rights Require Authenticity

   If a device grants different rights to different actors, exercising
   those rights MUST be accompanied by proof of those rights, in the
   form of proof of authenticity.  Authenticity mechanisms such as those
   required in MFSR5 are acceptable but need to follow the end-to-end
   security model.

   For example, if a device has a policy that requires that firmware
   have both an Authorship right and a Qualification right and if that
   device grants Authorship and Qualification rights to different
   parties, such as an OEM and an Operator, respectively, then the
   firmware cannot be installed without proof of rights from both the
   OEM and the Operator.

   Mitigates: MFT10





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A.3.7.  Security Requirement MFSR7: Firmware encryption

   Firmware images must be encrypted to prevent third parties, including
   attackers, from reading the content of the firmware image and to
   reverse engineer the code.

   Mitigates: MFT11

A.4.  User Stories

   User stories provide expected use cases.  These are used to feed into
   usability requirements.

A.4.1.  Use Case MFUC1: Installation Instructions

   As an OEM for IoT devices, I want to provide my devices with
   additional installation instructions so that I can keep process
   details out of my payload data.

   Some installation instructions might be:

   -  Specify a package handler

   -  Use a table of hashes to ensure that each block of the payload is
      validate before writing.

   -  Run post-processing script after the update is installed

   -  Do not report progress

   -  Pre-cache the update, but do not install

   -  Install the pre-cached update matching this manifest

   -  Install this update immediately, overriding any long-running
      tasks.

A.4.2.  Use Case MFUC2: Reuse Local Infrastructure

   As an Operator of IoT devices, I would like to tell my devices to
   look at my own infrastructure for payloads so that I can manage the
   traffic generated by firmware updates on my network and my peers'
   networks.








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A.4.3.  Use Case MFUC3: Modular Update

   As an OEM of IoT devices, I want to divide my firmware into
   frequently updated and infrequently updated components, so that I can
   reduce the size of updates and make different parties responsible for
   different components.

A.4.4.  Use Case MFUC4: Multiple Authorisations

   As an Operator, I want to ensure the quality of a firmware update
   before installing it, so that I can ensure a high standard of
   reliability on my network.  The OEM may restrict my ability to create
   firmware, so I cannot be the only authority on the device.

A.4.5.  Use Case MFUC5: Multiple Payload Formats

   As a OEM or Operator of devices, I want to be able to send multiple
   payload formats to suit the needs of my update, so that I can
   optimise the bandwidth used by my devices.

A.4.6.  Use Case MFUC6: IP Protection

   As an OEM or developer for IoT devices, I want to protect the IP
   contained in the firmware image, such as the utilized algorithms.
   The need for protecting IP may have also been imposed on my due to
   the use of some third party code libraries.

A.5.  Usability Requirements

   The following usability requirements satisfy the user stories listed
   above.

A.5.1.  Usability Requirement MFUR1

   It must be possible to write additional installation instructions
   into the manifest.

   Satisfies Use-Case MFUC1

A.5.2.  Usability Requirement MFUR2

   It must be possible to redirect payload fetches.  This applies where
   two manifests are used in conjunction.  For example, an OEM manifest
   specifies a payload and signs it, and provides a URI for that
   payload.  An Operator creates a second manifest, with a dependency on
   the first.  They use this second manifest to override the URIs
   provided by the OEM, directing them into their own infrastructure
   instead.



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   Satisfies Use-Case MFUC2

A.5.3.  Usability Requirement MFUR3

   It MUST be possible to link multiple manifests together so that a
   multi-component update can be described.  This allows multiple
   parties with different permissions to collaborate in creating a
   single update for the IoT device, across multiple components.

   Satisfies Use-Case MFUC2, MFUC3

A.5.4.  Usability Requirement MFUR4

   It MUST be possible to sign a manifest multiple times so that
   signatures from multiple parties with different permissions can be
   required in order to authorise installation of a manifest.

   Satisfies Use-Case MFUC4

A.5.5.  Usability Requirement MFUR5

   The manifest format MUST accommodate any payload format that an
   operator or OEM wishes to use.  Some examples of payload format would
   be:

   -  Binary

   -  Elf

   -  Differential

   -  Compressed

   -  Packed configuration

   Satisfies Use-Case MFUC5

A.6.  Manifest Fields

   Each manifest field is anchored in a security requirement or a
   usability requirement.  The manifest fields are described below and
   justified by their requirements.

A.6.1.  Manifest Field: Timestamp

   A monotonically increasing sequence number

   Implements: Security Requirement MFSR1.



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A.6.2.  Manifest Field: Vendor ID Condition

   Vendor IDs MUST be unique.  This is to prevent similarly, or
   identically named entities from different geographic regions from
   colliding in their customer's infrastructure.  Recommended practice
   is to use type 5 UUIDs with the vendor's domain name and the UUID DNS
   prefix.  Other options include type 1 and type 4 UUIDs.

   Implements: Security Requirement MFSR2, MFSR4.

A.6.3.  Manifest Field: Class ID Condition

   Class Identifiers MUST be unique within a Vendor ID.  This is to
   prevent similarly, or identically named devices colliding in their
   customer's infrastructure.  Recommended practice is to use type 5
   UUIDs with the model, hardware revision, etc. and use the Vendor ID
   as the UUID prefix.  Other options include type 1 and type 4 UUIDs.
   A device "Class" is defined as any device that can run the same
   firmware without modification.  Classes MAY be implemented in a more
   granular way.  Classes MUST NOT be implemented in a less granular
   way.  Class ID can encompass model name, hardware revision, software
   revision.  Devices MAY have multiple Class IDs.

   Implements: Security Requirement MFSR2, MFSR4.

A.6.4.  Manifest Field: Precursor Image Digest Condition

   When a precursor image is required by the payload format, a precursor
   image digest condition MUST be present in the conditions list.

   Implements: Security Requirement MFSR4

A.6.5.  Manifest Field: Best-Before timestamp

   This field tells a device the last application time.  This is only
   usable in conjunction with a secure clock.

   Implements Security Requirement MFSR3

A.6.6.  Manifest Field: Payload Format

   The format of the payload must be indicated to devices is in an
   unambiguous way.  This field provides a mechanism to describe the
   payload format, within the signed metadata.

   Implements Security Requirement MFSR4, Usability Requirement MFUR5





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A.6.7.  Manifest Field: Storage Location

   This field tells the device which component is being updated.  The
   device can use this to establish which permissions are necessary and
   the physical location to use.

   Implements Security Requirement MFSR4

A.6.8.  Manifest Field: URIs

   This field is a list of weighted URIs that the device uses to select
   where to obtain a payload.

   Implements Security Requirement MFSR4

A.6.9.  Manifest Field: Digests

   This field is a map of digests, each for a separate stage of
   installation.  This allows the target device to ensure authenticity
   of the payload at every step of installation.

   Implements Security Requirement MFSR4

A.6.10.  Manifest Field: Size

   The size of the payload in bytes.

   Implements Security Requirement MFSR4

A.6.11.  Manifest Field: Signature

   This is not strictly a manifest field.  Instead, the manifest is
   wrapped by a standardised authentication container, such as a COSE or
   CMS signature object.  The authentication container MUST support
   multiple actors and multiple authentications.

   Implements Security Requirement MFSR5, MFSR6, MFUR4

A.6.12.  Manifest Field: Directives

   A list of instructions that the device should execute, in order, when
   installing the payload.

   Implements Usability Requirement MFUR1







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A.6.13.  Manifest Field: Aliases

   A list of URI/Digest pairs.  A device should build an alias table
   while paring a manifest tree and treat any aliases as top-ranked URIs
   for the corresponding digest.

   Implements Usability Requirement MFUR2

A.6.14.  Manifest Field: Dependencies

   A list of URI/Digest pairs that refer to other manifests by digest.
   The manifests that are linked in this way must be acquired and
   installed simultaneously in order to form a complete update.

   Implements Usability Requirement MFUR3

A.6.15.  Manifest Field: Content Key Distribution Method

   Encrypting firmware images requires symmetric content encryption
   keys.  Since there are several methods to protect or distribute the
   symmetric content encryption keys, the manifest contains a field for
   the Content Key Distribution Method.  One examples for such a Content
   Key Distribution Method is the usage of Key Tables, pointing to
   content encryption keys, which themselves are encrypted using the
   public keys of devices.

   Implements: Security Requirement MFSR7.

Authors' Addresses

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com


   Milosch Meriac
   Arm Limited

   EMail: Milosch.Meriac@arm.com


   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@gmx.net





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