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Versions: (draft-moran-suit-architecture) 00 01

SUIT                                                            B. Moran
Internet-Draft                                             H. Tschofenig
Intended status: Standards Track                             Arm Limited
Expires: January 3, 2019                                     H. Birkholz
                                                          Fraunhofer SIT
                                                           July 02, 2018


 Firmware Updates for Internet of Things Devices - An Information Model
                             for Manifests
                  draft-ietf-suit-information-model-01

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.

   One component of such a firmware update is the meta-data, or
   manifest, that describes the firmware image(s) and offers appropriate
   protection.  This document describes all the information that must be
   present in the manifest.

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 January 3, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

   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
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   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 . . . . . . . . . . . . . . . . .   5
   3.  Motivation for Manifest Fields  . . . . . . . . . . . . . . .   5
     3.1.  Threat Model  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Threat Descriptions . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Threat MFT1: Old Firmware . . . . . . . . . . . . . .   6
       3.2.2.  Threat MFT2: Mismatched Firmware  . . . . . . . . . .   6
       3.2.3.  Threat MFT3: Offline device + Old Firmware  . . . . .   7
       3.2.4.  Threat MFT4: The target device misinterprets the type
               of payload  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.5.  Threat MFT5: The target device installs the payload
               to the wrong location . . . . . . . . . . . . . . . .   7
       3.2.6.  Threat MFT6: Redirection  . . . . . . . . . . . . . .   8
       3.2.7.  Threat MFT7: Payload Verification on Boot . . . . . .   8
       3.2.8.  Threat MFT8: Unauthenticated Updates  . . . . . . . .   8
       3.2.9.  Threat MFT9: Unexpected Precursor images  . . . . . .   8
       3.2.10. Threat MFT10: Unqualified Firmware  . . . . . . . . .   9
       3.2.11. Threat MFT11: Reverse Engineering Of Firmware Image
               for Vulnerability Analysis  . . . . . . . . . . . . .  10
       3.2.12. Threat MFT12: Overriding Critical Manifest Elements .  10
     3.3.  Security Requirements . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Security Requirement MFSR1: Monotonic Sequence
               Numbers . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.2.  Security Requirement MFSR2: Vendor, Device-type



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               Identifiers . . . . . . . . . . . . . . . . . . . . .  11
       3.3.3.  Security Requirement MFSR3: Best-Before Timestamps  .  11
       3.3.4.  Security Requirement MFSR5: Cryptographic
               Authenticity  . . . . . . . . . . . . . . . . . . . .  12
       3.3.5.  Security Requirement MFSR4a: Authenticated Payload
               Type  . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.6.  Security Requirement MFSR4b: Authenticated Storage
               Location  . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.7.  Security Requirement MFSR4c: Authenticated Remote
               Resource Location . . . . . . . . . . . . . . . . . .  12
       3.3.8.  Security Requirement MFSR4d: Secure Boot  . . . . . .  13
       3.3.9.  Security Requirement MFSR4e: Authenticated precursor
               images  . . . . . . . . . . . . . . . . . . . . . . .  13
       3.3.10. Security Requirement MFSR4f: Authenticated Vendor and
               Class IDs . . . . . . . . . . . . . . . . . . . . . .  13
       3.3.11. Security Requirement MFSR4f: Authenticated Vendor and
               Class IDs . . . . . . . . . . . . . . . . . . . . . .  13
       3.3.12. Security Requirement MFSR6: Rights Require
               Authenticity  . . . . . . . . . . . . . . . . . . . .  13
       3.3.13. Security Requirement MFSR7: Firmware encryption . . .  14
       3.3.14. Security Requirement MFSR8: Access Control Lists  . .  14
     3.4.  User Stories  . . . . . . . . . . . . . . . . . . . . . .  14
       3.4.1.  Use Case MFUS1: Installation Instructions . . . . . .  15
       3.4.2.  Use Case MFUS2: Override Non-Critical Manifest
               Elements  . . . . . . . . . . . . . . . . . . . . . .  15
       3.4.3.  Use Case MFUS3: Modular Update  . . . . . . . . . . .  16
       3.4.4.  Use Case MFUS4: Multiple Authorisations . . . . . . .  16
       3.4.5.  Use Case MFUS5: Multiple Payload Formats  . . . . . .  16
       3.4.6.  Use Case MFUS6: Prevent Confidential Information
               Disclosures . . . . . . . . . . . . . . . . . . . . .  16
       3.4.7.  Use Case MFUS7: Prevent Devices from Unpacking
               Unknown Formats . . . . . . . . . . . . . . . . . . .  16
       3.4.8.  Use Case MFUS8: Specify Version Numbers of Target
               Firmware  . . . . . . . . . . . . . . . . . . . . . .  17
       3.4.9.  Use Case MFUS9: Enable devices to choose between
               images  . . . . . . . . . . . . . . . . . . . . . . .  17
     3.5.  Usability Requirements  . . . . . . . . . . . . . . . . .  17
       3.5.1.  Usability Requirement MFUR1 . . . . . . . . . . . . .  17
       3.5.2.  Usability Requirement MFUR2 . . . . . . . . . . . . .  17
       3.5.3.  Usability Requirement MFUR3 . . . . . . . . . . . . .  18
       3.5.4.  Usability Requirement MFUR4 . . . . . . . . . . . . .  19
       3.5.5.  Usability Requirement MFUR5 . . . . . . . . . . . . .  19
       3.5.6.  Usability Requirement MFUR6 . . . . . . . . . . . . .  19
       3.5.7.  Usability Requirement MFUR7 . . . . . . . . . . . . .  19
       3.5.8.  Usability Requirement MFUR8 . . . . . . . . . . . . .  20
   4.  Manifest Information Elements . . . . . . . . . . . . . . . .  20
     4.1.  Manifest Element: version identifier of the manifest
           structure . . . . . . . . . . . . . . . . . . . . . . . .  20



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     4.2.  Manifest Element: Monotonic Sequence Number . . . . . . .  20
     4.3.  Manifest Element: Vendor ID Condition . . . . . . . . . .  20
       4.3.1.  Example: Domain Name-based UUIDs  . . . . . . . . . .  21
     4.4.  Manifest Element: Class ID Condition  . . . . . . . . . .  21
       4.4.1.  Example 1: Different Classes  . . . . . . . . . . . .  21
       4.4.2.  Example 2: Upgrading Class ID . . . . . . . . . . . .  22
       4.4.3.  Example 3: Shared Functionality . . . . . . . . . . .  22
     4.5.  Manifest Element: Precursor Image Digest Condition  . . .  23
     4.6.  Manifest Element: Required Image Version List . . . . . .  23
     4.7.  Manifest Element: Best-Before timestamp condition . . . .  23
     4.8.  Manifest Element: Payload Format  . . . . . . . . . . . .  23
     4.9.  Manifest Element: Processing Steps  . . . . . . . . . . .  24
     4.10. Manifest Element: Storage Location  . . . . . . . . . . .  24
       4.10.1.  Example 1: Two Storage Locations . . . . . . . . . .  24
       4.10.2.  Example 2: File System . . . . . . . . . . . . . . .  24
       4.10.3.  Example 3: Flash Memory  . . . . . . . . . . . . . .  24
     4.11. Manifest Element: Component Identifier  . . . . . . . . .  25
     4.12. Manifest Element: URIs  . . . . . . . . . . . . . . . . .  25
     4.13. Manifest Element: Payload Digest  . . . . . . . . . . . .  25
     4.14. Manifest Element: Size  . . . . . . . . . . . . . . . . .  25
     4.15. Manifest Element: Signature . . . . . . . . . . . . . . .  26
     4.16. Manifest Element: Directives  . . . . . . . . . . . . . .  26
     4.17. Manifest Element: Aliases . . . . . . . . . . . . . . . .  26
     4.18. Manifest Element: Dependencies  . . . . . . . . . . . . .  26
     4.19. Manifest Element: Content Key Distribution Method . . . .  27
     4.20. Manifest Element: XIP Address . . . . . . . . . . . . . .  27
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  28
   Appendix A.  Mailing List Information . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The information model describes all the information elements required
   to secure firmware updates of IoT devices from the threats described
   in Section 3.1 and enable the user stories captured in Section 3.4.
   These threats and user stories are not intended to be an exhaustive
   list of the threats against IoT devices, nor of the possible use
   cases of firmware update; instead they are intended to describe the
   threats against firmware update in isolation and provide sufficient
   motivation to provide information elements that cover a wide range of
   use cases.  The information model does not define the encoding,
   ordering, or structure of information elements, only their semantics.




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   Because the information model covers a wide range of user stories and
   a wide range of threats, not all information elements apply to all
   scenarios.  As a result, many information elements could be
   considered optional to implement and optional to use, depending on
   which threats exist in a particular system and which use cases are
   required.  Elements marked as mandatory provide baseline security and
   usability properties that are expected to be required for most
   applications.  Those elements are mandatory to implement and
   mandatory to use.  Elements marked as recommended provide important
   security or usability properties that are needed on most devices.
   Elements marked as optional enable security or usability properties
   that are useful in some applications.

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 terms defined in [I-D.ietf-suit-architecture].
   The term 'Operator' refers to both, Device and Network Operator.

3.  Motivation for Manifest Fields

   The following sub-sections describe the threat model, user stories,
   security requirements, and usability requirements.

3.1.  Threat Model

   The following sub-sections aim 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




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   This threat model only covers elements related to the transport of
   firmware updates.  It explicitly does not cover threats outside of
   the transport of firmware updates.  For example, threats to an IoT
   device due to physical access are out of scope.

3.2.  Threat Descriptions

3.2.1.  Threat MFT1: Old Firmware

   Classification: Elevation 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.

   Mitigated by: MFSR1

3.2.2.  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
   vulnerabilities.  For devices that are very different, it is likely
   to render devices inoperable.

   Mitigated by: MFSR2

   Example:

   Suppose that two vendors, Vendor A and Vendor B, adopt the same trade
   name in different geographic regions, and they both make products
   with the same names, or product name matching is not used.  This
   causes firmware from Vendor A to match devices from Vendor B.

   If the vendors are the firmware authorities, then devices from Vendor
   A will reject images signed by Vendor B since they use different
   credentials.  However, if both devices trust the same firmware
   authority, then, devices from Vendor A could install firmware
   intended for devices from Vendor B.



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3.2.3.  Threat MFT3: Offline device + Old Firmware

   Classification: Elevation 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.

   Mitigated by: MFSR3

3.2.4.  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 elevation of
   privilege and potentially expand this to all types of threat.

   Mitigated by: MFSR4a

3.2.5.  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 elevation of privilege and
   potentially expand this to all types of threat.




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   Mitigated by: MFSR4b

3.2.6.  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.

   Mitigated by: MFSR4c

3.2.7.  Threat MFT7: Payload Verification on Boot

   Classification: Elevation of Privilege

   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.

   Threat Escalation: If the attacker is able to exploit a known
   vulnerability, or if the attacker can supply their own firmware, then
   this threat can be escalated to ALL TYPES.

   Mitigated by: MFSR4d

3.2.8.  Threat MFT8: Unauthenticated Updates

   Classification: Elevation of Privilege

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

   Threat Escalation: If the attacker is able to exploit a known
   vulnerability, or if the attacker can supply their own firmware, then
   this threat can be escalated to ALL TYPES.

   Mitigated by: MFSR5

3.2.9.  Threat MFT9: Unexpected Precursor images

   Classification: Denial of Service





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   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 elevation of
   privilege and potentially expand this to all types of threat.

   Mitigated by: MFSR4e

3.2.10.  Threat MFT10: Unqualified Firmware

   Classification: Denial of Service, Elevation 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,
   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.

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

   Mitigated by: MFSR6, MFSR8

3.2.10.1.  Example 1: Multiple Network Operators with a Single Device
           Operator

   In this example let us assume that Device Operators expect the rights
   to create firmware but that Network Operators expect the rights to
   qualify firmware as fit-for-purpose on their networks.  Additionally
   assume that an Device Operators manage devices that can be deployed
   on any network, including Network A and B in our example.

   An attacker may obtain a manifest for a device on Network A.  Then,
   this attacker sends that manifest to a device on Network B.  Because
   Network A and Network B are under control of different Operators, and
   the firmware for a device on Network A has not been qualified to be
   deployed on Network B, the target device on Network B is now in
   violation of the Operator B's policy and may get disabled by this
   unqualified, but signed firmware.




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

3.2.10.2.  Example 2: Single Network Operator with Multiple Device
           Operators

   Multiple devices that interoperate are used on the same network and
   communicate with each other.  Some devices are manufactured and
   managed by Device Operator A and other devices by Device Operator B.
   A new firmware is released by Device Operator A that breaks
   compatibility with devices from Device Operator B.  An attacker sends
   the new firmware to the devices managed by Device Operator A 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.

3.2.11.  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.

   Mitigated by: MFSR7

3.2.12.  Threat MFT12: Overriding Critical Manifest Elements

   Classification: Elevation of Privilege

   An authorised actor, but not the firmware authority, uses an override
   mechanism (MFUS2) to change an information element in a manifest
   signed by the firmware authority.  For example, if the authorised
   actor overrides the digest and URI of the payload, the actor can
   replace the entire payload with a payload of their choice.

   Threat Escalation: By overriding elements such as payload
   installation instructions or firmware digest, this threat can be
   escalated to all types.

   Mitigated by: MFSR8






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3.3.  Security Requirements

   The security requirements here are a set of policies that mitigate
   the threats described in Section 3.1.

3.3.1.  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 increasing sequence numbers.
   Manifests MAY use UTC epoch timestamps to coordinate monotonically
   increasing sequence numbers across many actors in many locations.  If
   UTC epoch timestamps are used, they MUST NOT be treated as times,
   they MUST be treated only as sequence numbers.  Devices MUST reject
   manifests with sequence numbers smaller than any onboard sequence
   number.

   Note: 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

   Implemented by: Manifest Element: Monotonic Sequence Number

3.3.2.  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

   Implemented by: Manifest Elements: Vendor ID Condition, Class ID
   Condition

3.3.3.  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

   Implemented by: Manifest Element: Best-Before timestamp condition



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3.3.4.  Security Requirement MFSR5: Cryptographic Authenticity

   The authenticity of an update must be demonstrable.  Typically, this
   means that updates must be digitally authenticated.  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 digital signature.  The
   authenticity of the manifest can be verified with a digital signature
   or Message Authentication Code, the authenticity of the firmware
   image is tied to the manifest by the use of a digest of the firmware
   image.

   Mitigates: Threat MFT8

   Implemented by: Signature, Payload Digest

3.3.5.  Security Requirement MFSR4a: Authenticated Payload Type

   The type of payload (which may be independent of format) MUST be
   authenticated.  For example, the target must know whether the payload
   is XIP firmware, a loadable module, or serialized configuration data.

   Mitigates: MFT4

   Implemented by: Manifest Elements: Payload Format, Storage Location

3.3.6.  Security Requirement MFSR4b: Authenticated Storage Location

   The location on the target where the payload is to be stored MUST be
   authenticated.

   Mitigates: MFT5

   Implemented by: Manifest Elements: Storage Location

3.3.7.  Security Requirement MFSR4c: Authenticated Remote Resource
        Location

   The location where a target should find a payload MUST be
   authenticated.

   Mitigates: MFT6

   Implemented by: Manifest Elements: URIs






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3.3.8.  Security Requirement MFSR4d: Secure Boot

   The target SHOULD verify firmware at time of boot.  This requires
   authenticated payload size, and digest.

   Mitigates: MFT7

   Implemented by: Manifest Elements: Payload Digest, Size

3.3.9.  Security Requirement MFSR4e: Authenticated precursor images

   If an update uses a differential compression method, it MUST specify
   the digest of the precursor image and that digest MUST be
   authenticated.

   Mitigates: MFT9

   Implemented by: Manifest Elements: Precursor Image Digest Condition

3.3.10.  Security Requirement MFSR4f: Authenticated Vendor and Class IDs

   The identifiers that specify firmware compatibility MUST be
   authenticated to ensure that only compatible firmware is installed on
   a target device.

   Mitigates: MFT2

   Implemented By: Manifest Elements: Vendor ID Condition, Class ID
   Condition

3.3.11.  Security Requirement MFSR4f: Authenticated Vendor and Class IDs

   The identifiers that specify firmware compatibility MUST be
   authenticated to ensure that only compatible firmware is installed on
   a target device.

   Mitigates: MFT2

   Implemented By: Manifest Elements: Vendor ID Condition, Class ID
   Condition

3.3.12.  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.



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   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 a Device Operator and a Network Operator,
   respectively, then the firmware cannot be installed without proof of
   rights from both the Device and the Network Operator.

   Mitigates: MFT10

   Implemented by: Signature

3.3.13.  Security Requirement MFSR7: Firmware encryption

   The manifest information model must enable encrypted payloads.
   Encryption helps to prevent third parties, including attackers, from
   reading the content of the firmware image.  This can protect against
   confidential information disclosures and discovery of vulnerabilities
   through reverse engineering.  Therefore the manifest must convey the
   information required to allow an intended recipient to decrypt an
   encrypted payload.

   Mitigates: MFT11

   Implemented by: Manifest Element: Content Key Distribution Method

3.3.14.  Security Requirement MFSR8: Access Control Lists

   If a device grants different rights to different actors, then an
   exercise of those rights must be validated against a list of rights
   for the actor.  This typically takes the form of an Access Control
   List (ACL).  ACLs are applied to two scenarios:

   1.  An ACL decides which elements of the manifest may be overridden
       and by which actors.

   2.  An ACL decides which component identifier/storage identifier
       pairs can be written by which actors.

   Mitigates: MFT12, MFT10

   Implemented by: Client-side code, not specified in manifest.

3.4.  User Stories

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





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3.4.1.  Use Case MFUS1: Installation Instructions

   As an Device Operator, 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:

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

   -  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.

   Satisfied by: MFUR1

3.4.2.  Use Case MFUS2: Override Non-Critical Manifest Elements

   As a Network Operator, I would like to be able to override the non-
   critical information in the manifest so that I can control my devices
   more precisely.  This assumes that the Device Operator delegated
   rights about the device to the Network Operator.

   Some examples of potentially overridable information:

   -  URIs: this allows the Network Operator to direct devices to their
      own infrastructure in order to reduce network load.

   -  Conditions: this allows the Network Operator to pose additional
      constraints on the installation of the manifest.

   -  Directives: this allows the Network Operator to add more
      instructions such as time of installation.

   -  Processing Steps: If an intermediary performs an action on behalf
      of a device, it may need to override the processing steps.  It is
      still possible for a device to verify the final content and the
      result of any processing step that specifies a digest.  Some
      processing steps should be non-overridable.

   Satisfied by: MFUR2, MFUR3




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3.4.3.  Use Case MFUS3: Modular Update

   As an Operator, 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.

   Satisfied by: MFUR3

3.4.4.  Use Case MFUS4: Multiple Authorisations

   As a Device Operator, I want to ensure the quality of a firmware
   update before installing it, so that I can ensure interoperability of
   all devices in my product family.  I want to restrict the ability to
   make changes to my devices to require my express approval.

   Satisfied by: MFUR4, MFSR8

3.4.5.  Use Case MFUS5: Multiple Payload Formats

   As an Operator, 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.

   Satisfied by: MFUR5

3.4.6.  Use Case MFUS6: Prevent Confidential Information Disclosures

   As an firmware author, I want to prevent confidential information
   from being disclosed during firmware updates.  It is assumed that
   channel security is adequate to protect the manifest itself against
   information disclosure.

   Satisfied by: MFSR7

3.4.7.  Use Case MFUS7: Prevent Devices from Unpacking Unknown Formats

   As a Device Operator, I want devices to determine whether they can
   process a payload prior to downloading it.

   In some cases, it may be desirable for a third party to perform some
   processing on behalf of a target.  For this to occur, the third party
   MUST indicate what processing occurred and how to verify it against
   the Trust Provisioning Authority's intent.

   This amounts to overriding Processing Steps and URIs.

   Satisfied by: MFUR6, MFUR2



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3.4.8.  Use Case MFUS8: Specify Version Numbers of Target Firmware

   As a Device Operator, I want to be able to target devices for updates
   based on their current firmware version, so that I can control which
   versions are replaced with a single manifest.

   Satisfied by: MFUR7

3.4.9.  Use Case MFUS9: Enable devices to choose between images

   As a developer, I want to be able to sign two or more versions of my
   firmware in a single manifest so that I can use a very simple
   bootloader that chooses between two or more images that are executed
   in-place.

   Satisfied by: MFUR8

3.5.  Usability Requirements

   The following usability requirements satisfy the user stories listed
   above.

3.5.1.  Usability Requirement MFUR1

   It must be possible to provide all information necessary for the
   processing of a manifest into the manifest.

   Satisfies: User story MFUS1

   Implemented by: Manifest Element: Directives

3.5.2.  Usability Requirement MFUR2

   It must be possible to redirect payload fetches.  This applies where
   two manifests are used in conjunction.  For example, a Device
   Operator creates a manifest specifying a payload and signs it, and
   provides a URI for that payload.  A Network Operator creates a second
   manifest, with a dependency on the first.  They use this second
   manifest to override the URIs provided by the Device Operator,
   directing them into their own infrastructure instead.  Some devices
   may provide this capability, while others may only look at canonical
   sources of firmware.  For this to be possible, the device must fetch
   the payload, whereas a device that accpets payload pushes will ignore
   this feature.

   Satisfies: User story MFUS2

   Implemented by: Manifest Element: Aliases



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3.5.3.  Usability Requirement MFUR3

   It must be possible express the requirement to install one or more
   payloads from one or more authorities so that a multi-payload 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.

   This requirement effectively means that it must be possible to
   construct a tree of manifests on a multi-image target.

   Because devices can be either HeSA or HoSA both the storage system
   and the storage location within that storage system must be possible
   to specify.  In a HoSA device, the payload location may be as simple
   as an address, or a file path.  In a HeSA device, the payload
   location may be scoped by a component identifier.  It is expedient to
   consider that all HoSA devices are HeSA devices with a single
   component.

3.5.3.1.  Example 1: Multiple Microcontrollers

   An IoT device with multiple microcontrollers in the same physical
   device (HeSA) will likely require multiple payloads with different
   component identifiers.

3.5.3.2.  Example 2: Code and Configuration

   A firmware image can be divided into two payloads: code and
   configuration.  These payloads may require authorizations from
   different actors in order to install (see MFSR6 and MFSR8).  This
   structure means that multiple manifests may be required, with a
   dependency structure between them.

3.5.3.3.  Example 3: Multiple Chunks

   A firmware image can be divided into multiple functional blocks for
   separate testing and distribution.  This means that code would need
   to be distributed in multiple payloads.  For example, this might be
   desirable in order to ensure that common code between devices is
   identical in order to reduce distribution bandwidth.

   Satisfies: User story MFUS2, MFUS3

   Implemented by Manifest Element: Dependencies, StorageIdentifier,
   ComponentIdentifier






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3.5.4.  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: User story MFUS4

   Implemented by: COSE Signature (or similar)

3.5.5.  Usability Requirement MFUR5

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

   -  Binary

   -  Elf

   -  Differential

   -  Compressed

   -  Packed configuration

   -  Intel HEX

   -  S-Record

   Satisfies: User story MFUS5

   Implemented by: Manifest Element: Payload Format

3.5.6.  Usability Requirement MFUR6

   The manifest format must accommodate nested formats, announcing to
   the target device all the nesting steps and any parameters used by
   those steps.

   Satisfies: User story MFUS6

   Implemented by: Manifest Element: Processing Steps

3.5.7.  Usability Requirement MFUR7

   The manifest format must provide a method to specify multiple version
   numbers of firmware to which the manifest applies, either with a list
   or with range matching.



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   Satisfies: User story MFUS8

   Implemented by: Manifest Element: Required Image Version List

3.5.8.  Usability Requirement MFUR8

   The manifest format must provide a mechanism to list multiple
   equivalent payloads by Execute-In-Place Installation Address,
   including the payload digest and, optionally, payload URIs.

   Satisfies: User story MFUS9

   Implemented by: Manifest Element: XIP Address

4.  Manifest Information Elements

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

4.1.  Manifest Element: version identifier of the manifest structure

   An identifier that describes which iteration of the manifest format
   is contained in the structure.

   This element is MANDATORY and must be present in order to allow
   devices to identify the version of the manifest data model that is in
   use.

4.2.  Manifest Element: Monotonic Sequence Number

   A monotonically increasing sequence number.  For convenience, the
   monotonic sequence number MAY be a UTC timestamp.  This allows global
   synchronisation of sequence numbers without any additional
   management.

   This element is MANDATORY and is necessary to prevent malicious
   actors from reverting a firmware update against the wishes of the
   relevant authority.

   Implements: Security Requirement MFSR1.

4.3.  Manifest Element: 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




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   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.

   This ID is OPTIONAL but RECOMMENDED and helps to distinguish between
   identically named products from different vendors.

   Implements: Security Requirement MFSR2, MFSR4f.

4.3.1.  Example: Domain Name-based UUIDs

   Vendor A creates a UUID based on their domain name:

   vendorId = UUID5(DNS, "vendor-a.com")

   Because the DNS infrastructure prevents multiple registrations of the
   same domain name, this UUID is guaranteed to be unique.  Because the
   domain name is known, this UUID is reproducible.  Type 1 and type 4
   UUIDs produce similar guarantees of uniqueness, but not
   reproducibility.

4.4.  Manifest Element: Class ID Condition

   A device "Class" is defined as any device that can accept the same
   firmware update without modification.  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.  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.

   Note Well: Class ID is not a human-readable element.  It is intended
   for match/mismatch use only.

   This ID is OPTIONAL but RECOMMENDED and allows devices to determine
   applicability of a firmware in an unambiguous way.

   Implements: Security Requirement MFSR2, MFSR4f.

4.4.1.  Example 1: Different Classes

   Vendor A creates product Z and product Y.  The firmware images of
   products Z and Y are not interchangeable.  Vendor A creates UUIDs as
   follows:




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   -  vendorId = UUID5(DNS, "vendor-a.com")

   -  ZclassId = UUID5(vendorId, "Product Z")

   -  YclassId = UUID5(vendorId, "Product Y")

   This ensures that Vendor A's Product Z cannot install firmware for
   Product Y and Product Y cannot install firmware for Product Z.

4.4.2.  Example 2: Upgrading Class ID

   Vendor A creates product X.  Later, Vendor A adds a new feature to
   product X, creating product X v2.  Product X requires a firmware
   update to work with firmware intended for product X v2.

   Vendor A creates UUIDs as follows:

   -  vendorId = UUID5(DNS, "vendor-a.com")

   -  XclassId = UUID5(vendorId, "Product X")

   -  Xv2classId = UUID5(vendorId, "Product X v2")

   When product X receives the firmware update necessary to be
   compatible with product X v2, part of the firmware update changes the
   class ID to Xv2classId.

4.4.3.  Example 3: Shared Functionality

   Vendor A produces two products, product X and product Y.  These
   components share a common core (such as an operating system), but
   have different applications.  The common core and the applications
   can be updated independently.  To enable X and Y to receive the same
   common core update, they require the same class ID.  To ensure that
   only product X receives application X and only product Y receives
   application Y, product X and product Y must have different class IDs.
   The vendor creates Class IDs as follows:

   -  vendorId = UUID5(DNS, "vendor-a.com")

   -  XclassId = UUID5(vendorId, "Product X")

   -  YclassId = UUID5(vendorId, "Product Y")

   -  CommonClassId = UUID5(vendorId, "common core")

   Product X matches against both XclassId and CommonClassId.  Product Y
   matches against both YclassId and CommonClassId.



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4.5.  Manifest Element: 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.  The
   precursor image may be installed or stored as a candidate.

   This element is MANDATORY for differential updates.  Otherwise, it is
   not needed.

   Implements: Security Requirement MFSR4e

4.6.  Manifest Element: Required Image Version List

   When a payload applies to multiple versions of a firmware, the
   required image version list specifies which versions must be present
   for the update to be applied.  This allows the update author to
   target specific versions of firmware for an update, while excluding
   those to which it should not be applied.

   Where an update can only be applied over specific predecessor
   versions, that version MUST be specified by the Required Image
   Version List.

   This element is OPTIONAL.

   Implements: MFUR7

4.7.  Manifest Element: Best-Before timestamp condition

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

   This element is OPTIONAL and MAY enable use cases where a secure
   clock is provided and firmware is intended to expire regularly.

   Implements: Security Requirement MFSR3

4.8.  Manifest Element: Payload Format

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

   This element is MANDATORY and MUST be present to enable devices to
   decode payloads correctly.

   Implements: Security Requirement MFSR4a, Usability Requirement MFUR5




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4.9.  Manifest Element: Processing Steps

   A list of all payload processors necessary to process a nested format
   and any parameters needed by those payload processors.  Each
   Processing Step SHOULD indicate the expected digest of the payload
   after the processing is complete.  Processing steps are distinct from
   Directives in that Directives apply to the manifest as a whole,
   whereas Processing Steps apply to an individual payload and provide
   instructions on how to unpack it.

   Implements: Usability Requirement MFUR6

4.10.  Manifest Element: Storage Location

   This element 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.

   This element is MANDATORY and MUST be present to enable devices to
   store payloads to the correct location.

   Implements: Security Requirement MFSR4b

4.10.1.  Example 1: Two Storage Locations

   A device supports two components: an OS and an application.  These
   components can be updated independently, expressing dependencies to
   ensure compatibility between the components.  The firmware authority
   chooses two storage identifiers:

   -  OS

   -  APP

4.10.2.  Example 2: File System

   A device supports a full filesystem.  The firmware authority chooses
   to make the storage identifier the path at which to install the
   payload.  The payload may be a tarball, in which case, it unpacks the
   tarball into the specified path.

4.10.3.  Example 3: Flash Memory

   A device supports flash memory.  The firmware authority chooses to
   make the storage identifier the offset where the image should be
   written.





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4.11.  Manifest Element: Component Identifier

   In a heterogeneous storage architecture, a storage identifier is
   insufficient to identify where and how to store a payload.  To
   resolve this, a component identifier indicates which part of the
   storage architecture is targeted by the payload.  In a homogeneous
   storage architecture, this element is unnecessary.

   This element is OPTIONAL and only necessary in heterogeneous storage
   architecture devices.

   Implements: MFUR3

4.12.  Manifest Element: URIs

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

   This element is OPTIONAL and only needed when the target device does
   not intrinsically know where to find the payload.

   Note: Devices will typically require URIs.

   Implements: Security Requirement MFSR4c

4.13.  Manifest Element: Payload Digest

   This element contains the digest of the payload.  This allows the
   target device to ensure authenticity of the payload.  It MUST be
   possible to specify more than one payload digest, indexed by Manifest
   Element: XIP Address.

   This element is MANDATORY and fundamentally necessary to ensure the
   authenticity and integrity of the payload.

   Implements: Security Requirement MFSR4d, Usability Requirement MFUR8

4.14.  Manifest Element: Size

   The size of the payload in bytes.

   This element is MANDATORY and informs the target device how big of a
   payload to expect.  Without it, devices are exposed to some classes
   of denial of service attack.

   Implements: Security Requirement MFSR4d





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4.15.  Manifest Element: Signature

   This is not strictly a manifest element.  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.

   This element is MANDATORY and represents the foundation of all
   security properties of the manifest.

   Implements: Security Requirement MFSR5, MFSR6, MFUR4

4.16.  Manifest Element: Directives

   A list of instructions that the device should execute, in order, when
   processing the manifest.  This information is distinct from the
   information necessary to process a payload (Processing Steps) and
   applies to the whole manifest including all payloads that it
   references.  Directives include information such as update timing
   (For example, install only on Sunday, at 0200), procedural
   considerations (for example, shut down the equipment under control
   before executing the update), pre and post-installation steps (for
   example, run a script).

   This element is OPTIONAL and enables some use cases.

   Implements: Usability Requirement MFUR1

4.17.  Manifest Element: Aliases

   A list of Digest/URI 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.

   This element is OPTIONAL and enables some use cases.

   Implements: Usability Requirement MFUR2

4.18.  Manifest Element: Dependencies

   A list of Digest/URI 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.

   This element is MANDATORY to use in deployments that include both
   multiple authorities and multiple payloads.

   Implements: Usability Requirement MFUR3



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4.19.  Manifest Element: 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 element
   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.  This MAY be included in a decryption step
   contained in Processing Steps.

   This element is MANDATORY to use for encrypted payloads,

   Implements: Security Requirement MFSR7.

4.20.  Manifest Element: XIP Address

   In order to support XIP systems with multiple possible base
   addresses, it is necessary to specify which address the payload is
   linked for.

   For example a microcontroller may have a simple bootloader that
   chooses one of two images to boot.  That microcontroller then needs
   to choose one of two firmware images to install, based on which of
   its two images is older.

   Implements: MFUR8

5.  Security Considerations

   Security considerations for this document are covered in Section 3.

6.  IANA Considerations

   This document does not require any actions by IANA.

7.  Acknowledgements

   We would like to thank our working group chairs, Dave Thaler, Russ
   Housley and David Waltermire, for their review comments and their
   support.

   We would like to thank the participants of the 2018 Berlin SUIT
   Hackathon and the June 2018 virtual design team meetings for their
   discussion input.  In particular, we would like to thank Koen
   Zandberg, Emmanuel Baccelli, Carsten Bormann, David Brown, Markus
   Gueller, Frank Audun Kvamtro, Oyvind Ronningstad, Michael Richardson,
   Jan-Frederik Rieckers Francisco Acosta, Anton Gerasimov, Matthias



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   Waehlisch, Max Groening, Daniel Petry, Gaetan Harter, Ralph Hamm,
   Steve Patrick, Fabio Utzig, Paul Lambert, Benjamin Kaduk, Said
   Gharout, and Milen Stoychev.

8.  References

8.1.  Normative References

   [I-D.ietf-suit-architecture]
              Moran, B., Meriac, M., Tschofenig, H., and D. Brown, "A
              Firmware Update Architecture for Internet of Things
              Devices", draft-ietf-suit-architecture-01 (work in
              progress), July 2018.

   [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>.

8.2.  Informative References

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005, <https://www.rfc-
              editor.org/info/rfc4122>.

   [STRIDE]   Microsoft, "The STRIDE Threat Model", May 2018,
              <https://msdn.microsoft.com/en-us/library/
              ee823878(v=cs.20).aspx>.

8.3.  URIs

   [1] mailto:suit@ietf.org


















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Appendix A.  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

Authors' Addresses

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com


   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@gmx.net


   Henk Birkholz
   Fraunhofer SIT

   EMail: henk.birkholz@sit.fraunhofer.de























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