Strong Assertions of IoT Network Access Requirements
draft-ietf-suit-mud-06

SUIT                                                            B. Moran
Internet-Draft                                               Arm Limited
Intended status: Standards Track                           H. Tschofenig
Expires: 25 April 2024                                   23 October 2023

          Strong Assertions of IoT Network Access Requirements
                         draft-ietf-suit-mud-06

Abstract

   The Manufacturer Usage Description (MUD) specification describes the
   access and network functionality required for 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.

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   This Internet-Draft will expire on 25 April 2024.

Copyright Notice

   Copyright (c) 2023 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
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   4
     4.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Extensions to SUIT  . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Under [RFC8520], devices report a MUD URL to a MUD Manager in the
   network, which then interacts with a MUD File Server to ultimately
   obtain the MUD file.  The following figure shows the MUD
   architecture.

       .......................................
       .                      ____________   .           _____________
       .                     |            |  .          |             |
       .                     |    MUD     |-->get URL-->|    MUD      |
       .                     |  Manager   |  .(https)   | File Server |
       .  End system network |____________|<-MUD file<-<|_____________|
       .                             .       .
       .                             .       .
       . ________                _________   .
       .|        |              | router  |  .
       .| Device |--->MUD URL-->|   or    |  .
       .|________|              | switch  |  .
       .                        |_________|  .
       .......................................

   RFC 8520 envisions different approaches for conveying the MUD URL
   from the device to the network such as:

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   *  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.

   The MUD Manager uses the MUD URL to fetch the MUD file, which
   contains connectivity-related functionality required for a device to
   properly function.

   The MUD Manager must trust the MUD File Server from which the MUD
   file is fetched 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 MUD URL.  In case of DHCP and LLDP the URL is likely
   unprotected.

   When the MUD URL 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 a conflation of rights.

   There is a need to bind the entity that creates the software/
   configuration to the MUD file because only that entity can attest the
   connectivity requirements of the device.

   This specification defines an extension to the Software Updates for
   Internet of Things (SUIT) manifest format [I-D.ietf-suit-manifest] to
   include a MUD URL.  When combining a MUD URL with a manifest used for
   software/firmware updates then a network operator can get more
   confidence in the description of the connectivity requirements for 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, and assumes an attestation
   mechanism between the device and the MUD Manager:

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   *  At the time of onboarding, devices report their manifest in use to
      the MUD Manager via some form of attestation evidence and a
      conveyance protocol.  The normative specification of these
      mechanisms is out of scope for this document.  .

      -  An example of an attestation evidence format is the Entity
         Attestation Token (EAT) [I-D.ietf-rats-eat].  Among other
         claims, the device could report its software digest(s), and the
         manifest URI in the EAT "manifests" claim to the MUD Manager.
         This approach assumes that attestation evidence includes a link
         to the SUIT manifest via the "manifests" claim (see
         Section 4.2.15 of [I-D.ietf-rats-eat]) and that this evidence
         can be carried in either a network access authentication
         protocol (for eample an EAP method) or some onboarding protocol
         like FIDO Device Onboard (FDO).

      -  The MUD Manager can then (with the help of the Verifier)
         validate the evidence in order to check that the device is
         operating with the expected version of software and
         configuration.

      -  Since a URL to the manifest is contained in the Evidence, the
         MUD Manager can look up the corresponding manifest.

   *  If the SUIT_MUD_container, see Section 5, has been severed, the
      MUD Manager can use the suit-reference-uri to retrieve the
      complete SUIT 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.

   *  The MUD Manager acquires the MUD file from the MUD URL found in
      the SUIT manifest.

   *  The MUD Manager verifies the MUD file signature using the Subject
      Key Identifier (SKI) provided in the SUIT manifest.

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

   Each time a device is updated, rebooted, or otherwise substantially
   changed, it will execute the remote attestation procedures again.

4.  Operational Considerations

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4.1.  Pros

   The approach described in this document has several advantages over
   other MUD URL reporting mechanisms:

   *  The MUD URL is tightly coupled to device software/firmware
      version.

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

   *  The onus is placed on the software/firmware author to provide a
      MUD file that describes the behavior of the software running on a
      device.

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

   *  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 in a SUIT manifest:

      -  By providing a SUIT manifest that overrides the MUD URL.

      -  By replacing the MUD URL in their network infrastructure.

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

   *  Devices cannot lie about which MUD URL to use.

4.2.  Cons

   This mechanism relies on the use of SUIT manifests to encode the MUD
   URL.  Conceptually, the MUD file is similar to a Software Bill of
   Material (SBOM) but focuses on the external visible communication
   behavior, which is essential for network operators, rather than
   describing the software libraries contained within the device itself.
   The SUIT manifest must then be conveyed to the network during
   onboarding or during the network access authentication step.  To
   accomplish the transport of the manifest attestation evidence is
   used, which needs to be available at the protocol of choice.

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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 the SUIT manifest along with a subject key
   identifier (ski).  The subject key identifier MUST be generated
   according to the process defined in [I-D.ietf-cose-key-thumbprint]
   and the SUIT_Digest structure MUST be populated with the selected
   hash algorithm and obtained fingerprint.  The subject key identifier
   corresponds to the key used in the MUD signature file described in
   Section 13.2 of [RFC8520].

   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
   )

   The SUIT_MUD_container structure is defined as follows:

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

6.  Security Considerations

   This specification links MUD files to SUIT manifests for improving
   security protection and ease of use.  By including MUD URLs 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

   IANA is requested to add a new value to the SUIT manifest elements
   registry created with [I-D.ietf-suit-manifest]:

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   *  Label: TBD1 [[Value allocated from the standards action address
      range]]

   *  Name: Manufacturer Usage Description (MUD)

   *  Reference: [[TBD: This document]]

   IANA is requested to add a new value to the SUIT envelope elements
   registry created with [I-D.ietf-suit-manifest]:

   *  Label: TBD2 [[Value allocated from the standards action address
      range]]

   *  Name: Manufacturer Usage Description (MUD)

   *  Reference: [[TBD: This document]]

8.  Normative References

   [I-D.ietf-cose-key-thumbprint]
              Isobe, K., Tschofenig, H., and O. Steele, "CBOR Object
              Signing and Encryption (COSE) Key Thumbprint", Work in
              Progress, Internet-Draft, draft-ietf-cose-key-thumbprint-
              04, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              key-thumbprint-04>.

   [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-22, 14
              October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-22>.

   [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-24, 23 October
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              suit-manifest-24>.

   [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/rfc/rfc2119>.

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   [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/rfc/rfc8174>.

   [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/rfc/rfc8520>.

Authors' Addresses

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

   Hannes Tschofenig
   Email: hannes.tschofenig@gmx.net

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