Strong Assertions of IoT Network Access Requirements
draft-ietf-suit-mud-03

SUIT                                                            B. Moran
Internet-Draft                                             H. Tschofenig
Intended status: Standards Track                             Arm Limited
Expires: 14 September 2023                                 13 March 2023

          Strong Assertions of IoT Network Access Requirements
                         draft-ietf-suit-mud-03

Abstract

   The Manufacturer Usage Description (MUD) specification describes the
   access and network functionality required a device to properly
   function.  The MUD description has to reflect the software running on
   the device and its configuration.  Because of this, the most
   appropriate entity for describing device network access requirements
   is the same as the entity developing the software and its
   configuration.

   A network presented with a MUD file by a device allows detection of
   misbehavior by the device software and configuration of access
   control.

   This document defines a way to link a SUIT manifest to a MUD file
   offering a stronger binding between the two.

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
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   This Internet-Draft will expire on 14 September 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Advantages over previous MUD url reporting mechanisms . . . .   4
   5.  Extensions to SUIT  . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Under [RFC8520], devices report a URL to a MUD manager in the
   network.  RFC 8520 envisions different approaches for conveying the
   information from the device to the network such as:

   *  DHCP,

   *  IEEE802.1AB Link Layer Discovery Protocol (LLDP), and

   *  IEEE 802.1X whereby the URL to the MUD file would be contained in
      the certificate used in an EAP method.

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   The MUD manager then uses the the URL to fetch the MUD file, which
   contains access and network functionality required a device to
   properly function.

   The MUD manager must trust the service from which the URL is fetched
   and to return an authentic copy of the MUD file.  This concern may be
   mitigated using the optional signature reference in the MUD file.
   The MUD manager must also trust the device to report a correct URL.
   In case of DHCP and LLDP the URL is unprotected.  When the URL to the
   MUD file is included in a certificate then it is authenticated and
   integrity protected.  A certificate created for use with network
   access authentication is typically not signed by the entity that
   wrote the software and configured the device, which leads to
   conflation of local network access rights with rights to assert all
   network access requirements.

   There is a need to bind the entity that creates the software and
   configuration to the MUD file because only that entity can attest the
   network access requirements of the device.  This specification
   defines an extension to the SUIT manifest to include a MUD file (per
   reference or by value).  When combining a manufacturer usage
   description with a manifest used for software/firmware updates
   (potentially augmented with attestation) then a network operator can
   get more confidence in the description of the access and network
   functionality required a device to properly function.

2.  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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Workflow

   The intended workflow is as follows:

   *  At the time of onboarding, devices report their manifest in use to
      the MUD manager.

   *  If the SUIT_MUD_container has been severed, the suit-reference-uri
      can be used to retrieve the complete manifest.

   *  The manifest authenticity is verified by the MUD manager, which
      enforces that the MUD file presented is also authentic and as
      intended by the device software vendor.

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   *  Each time a device is updated, rebooted, or otherwise
      substantially changed, it will execute an attestation.

      -  Among other claims in the Entity Attestation Token (EAT)
         [I-D.ietf-rats-eat], the device will report its software
         digest(s), configuration digest(s), primary manifest URI, and
         primary manifest digest to the MUD manager.

      -  The MUD manager can then validate these attestation reports in
         order to check that the device is operating with the expected
         version of software and configuration.

      -  Since the manifest digest is reported, the MUD manager can look
         up the corresponding manifest.

   *  If the MUD manager does not already have a full copy of the
      manifest, it can be acquired using the reference URI.

   *  Once a full copy of the manifest is provided, the MUD manager can
      verify the device attestation report

   *  The MUD manager acquires the MUD file from the MUD url.

   *  The MUD manager verifies the MUD file signature using the provided
      Subject Key Identifier.

   *  Then, the MUD manager can apply any appropriate policy as
      described by the MUD file.

4.  Advantages over previous MUD url reporting mechanisms

   Binding within the firmware manifest has several advantages over
   other MUD url reporting mechanisms:

   *  The MUD url is tightly coupled to device firmware version.

   *  The device does not report the url, so the device cannot tamper
      with the url.

   *  The onus is placed on the software author to provide a MUD file
      that describes their device.

   *  The Manifest Author (software signer) explicitly authorizes a key
      to sign MUD files, providing a tight coupling between the party
      that knows device behavior best (the manifest author) and the
      party that declares device behavior (MUD file signer).

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   *  Network operators do not need to know, a priori, which MUD url to
      use for each device; this can be harvested from the device's
      manifest and only replaced if necessary.

   *  A network operator can still replace a MUD url:

      -  By providing a manifest that overrides the MUD url.

      -  By replacing the MUD url in network infrastructure.

   *  Devices can be quarantined if they do not attest a known software
      version.

   *  Devices cannot lie about which MUD url to use.

5.  Extensions to SUIT

   To enable strong assertions about the network access requirements
   that a device should have for a particular software/configuration
   pair a MUD url is added to SUIT along with a subject-key identifier,
   according to [RFC7093], mechanism 4 (the keyIdentifier is composed of
   the hash of the DER encoding of the SubjectPublicKeyInfo value).  The
   Subject Key Identifier MUST be constructed with mechanism 4.

   The following CDDL describes the extension to the SUIT_Manifest
   structure:

   $$severable-manifest-members-choice-extensions //= (
     suit-manifest-mud => SUIT_Digest / SUIT_MUD_container
   )

   The SUIT_Envelope is also amended:

   $$SUIT_severable-members-extensions //= (
     suit-manifest-mud => bstr .cbor SUIT_MUD_container
   )

   SUIT_MUD_container = {
       suit-mud-url => #6.32(tstr),
       suit-mud-ski => SUIT_Digest,
   }

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6.  Security Considerations

   This specification links MUD files to other IETF technologies,
   particularly to SUIT manifests, for improving security protection and
   ease of use.  By including MUD files (per reference or by value) in
   SUIT manifests an extra layer of protection has been created and
   synchronization risks can be minimized.  If the MUD file and the
   software/firmware loaded onto the device gets out-of-sync a device
   may be firewalled and, with firewalling by networks in place, the
   device may stop functioning.

7.  IANA Considerations

   suit-manifest-mud must be added as an extension point to the SUIT
   manifest registry.

8.  Normative References

   [I-D.ietf-rats-eat]
              Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-19, 19
              December 2022, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-19>.

   [I-D.ietf-suit-manifest]
              Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
              O. Rønningstad, "A Concise Binary Object Representation
              (CBOR)-based Serialization Format for the Software Updates
              for Internet of Things (SUIT) Manifest", Work in Progress,
              Internet-Draft, draft-ietf-suit-manifest-22, 27 February
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              suit-manifest-22>.

   [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>.

   [RFC7093]  Turner, S., Kent, S., and J. Manger, "Additional Methods
              for Generating Key Identifiers Values", RFC 7093,
              DOI 10.17487/RFC7093, December 2013,
              <https://www.rfc-editor.org/info/rfc7093>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

Authors' Addresses

   Brendan Moran
   Arm Limited
   Email: brendan.moran.ietf@gmail.com

   Hannes Tschofenig
   Arm Limited
   Email: hannes.tschofenig@gmx.net

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