MUD-Based RATS Resources Discovery

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Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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Table of Contents

1. Introduction

Verifiers, Endorsers, and Attesters are roles defined in the RATS Architecture [RFC9334]. In the RATS architecture, the Relying Party roles depend on the Verifier to bear the burden of Evidence appraisal and to generate corresponding Attestation Results for them. Attestation Results compose a believable chunk of information that can be digested by Relying Parities in order to assess an Attester's trustworthiness. The assessment of a remote peer's trustworthiness is vital to determine whether any future protocol interaction between a Relying Party and a remote Attester can be considered secure. To create these Attestation Results to be consumed by Relying Parties, the Attestation Evidence an Attester generates has to be appraised by one or more appropriate Verifiers.¶

This document defines a procedure that enables the discovery of resources or services in support of RATS, including:¶

1. Reference Values,¶
2. Trust Anchors,¶
3. Endorsements and Endorsement Distribution APIs,¶
4. (remote) Verifier APIs,¶
5. Transparency Logs, or¶
6. Appraisal Policies.¶

MUD URIs can be embedded in any data item that was signed with trusted key material. One common way to establish trust in a signed data item is to associate the signing key material with a trust anchor via a certification path (see [RFC4949] for trust anchor and certification path). This document defines the use of MUD URIs embedded in two types of signed data items that typically are trusted via certification paths:¶

1. Secure Device Identifiers (IEEE 802.1AR DevIDs) as defined by [RFC8520] and¶
2. Entity Attestation Tokens (EAT) as defined by [I-D.ietf-rats-eat].¶

DevIDs and EATs (essentially CWTs ) are two very prominent examples of "trustworthy documents" (TDs) with a binary format and the embedding of MUD URIs in theses TDs can be applied to other TD types, for example, Selective Disclosure CWTs [I-D.ietf-spice-sd-cwt]. Other TDs are out-of-scope of this specification, though. The TDs are typically enrolled on Attesters by manufacturers or provisioned by supply chain entities with appropriate authority. The TDs can be presented to local Network Management Systems, AAA-services (e.g., via IEEE 802.1X), or other points of first contact (POFC), for example, [RFC8071]. These POFC are typically trusted third parties (TTP) that can digest the TDs and then base trust decisions on the associated certification paths and trust anchors. If a TD presented by the Attester is deemed to be trusted by a local trust authority, the MUD URI embedded is considered to be a trusted source for viable resources and services in support of remote attestation of the Attester.¶

This specification does not define the shape or format of any resource or service that is referenced by the MUD file. In support of a unified mechanism to categorize the formats of referenced resources, a conceptual message wrapper (CMW, [I-D.ietf-rats-msg-wrap] is used for each type of resource. An example of a referenced resource is a CoRIM tag [I-D.ietf-rats-corim].¶

2. MUD URIs in Trusted Documents (TDs)

This document does neither modify nor augment the definition about how to compose a MUD URI. The two types of trusted documents (TDs) covered by this specification are Secure Device Identifiers and Entity Attestation Tokens.¶

3. MUD File Signatures

As the resources required by a Verifier's appraisal procedures have to be trustworthy, a MUD signature file for a corresponding MUD File MUST be available. The MUD File MUST include a reference to its MUD signature file via the 'mud-signature' statement. The MUD File Signature generation as specified in Section 13.1 of [RFC8520] applies. If a MUD file changed (i.e., the checking of the MUD File Signature fails) or the corresponding MUD File Signer certificate is expired (see Section 13.2 of [RFC8520], the reference in the changed MUD File MUST point to a new valid MUD Signature File and that new MUD File Signature MUST be available. If a corresponding MUD File Signer certificate is expired (see Section 13.2 of [RFC8520]) or a MUD File Signature referenced by a MUD File cannot be checked successfully, the MUD File MUST NOT be trusted.¶

4. Trusting MUD URIs and MUD Files

The level of assurance about the authenticity of a MUD URI embedded in a TD is based on the level of trust put into the corresponding trust anchor associated with the key material that signed the TD. If it is not possible to establish a level of trust towards the entity that signed a TD, the embedded MUD URI SHOULD NOT be trusted. In some usage scenarios it might suffice to trust a MUD File, if the referenced MUD File Signer's certificate is not expired, but that behavior is NOT RECOMMENDED.¶

The level of assurance about the authenticity of a MUD file is based on the level of trust put into the entity that created the corresponding MUD File Signer's certificate. If it is not possible to establish a level of trust into the corresponding trust anchor associated with the MUD Signer's certificate, the MUD File that references that MUD Signer MUST NOT be trusted.¶

5. Specification of RATS MUD Files Referenced by MUD URIs

The MUD URI embedded in a TD presented by an Attester points to a MUD File. MUD URIs typically point to a piece of data that is a YANG-modeled XML file with a structure specified in the style of a YANG module definition ([RFC7950] and corresponding updates: [RFC8342], [RFC8526]). This document specifies a YANG module augment definition for generic MUD files to create RATS MUD files. The following definition MUST be used, if a MUD URI points to a RATS MUD file.¶

<CODE BEGINS>

module: ietf-mud-rats
  augment /mud:mud:
    +--rw ras
    |  +--rw ras-uris*   inet:uri
    +--rw rim
    |  +--rw rim-uris*   inet:uri
    +--rw edt
       +--rw edt-uris*   inet:uri

<CODE ENDS>

<CODE BEGINS> file [email protected]
module ietf-mud-rats {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-mud-rats";
  prefix "mud-rats";

  import ietf-mud {
    prefix "mud";
  }

  import ietf-inet-types {
    prefix "inet";
  }

  organization
    "IETF RATS (Remote ATtestation procedureS) Working Group";

  contact
    "WG Web: http://tools.ietf.org/wg/rats/
     WG List: [email protected]
     Author: Eliot Lear <[email protected]>
     Author: Henk Birkholz <[email protected]>";

  description
    "This YANG module augments the ietf-mud model to provide for three
     optional lists to enable Remote Attestation Procedures so that
     this device type may be used as a controller for other
     MUD-enabled devices.

     Copyright (c) 2020 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC XXXX
     (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
      for full legal notices.

     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 (RFC 2119) (RFC 8174) when, and only when,
     they appear in all capitals, as shown here.";

  revision 2020-03-09 {
    description
      "Initial proposed standard.";
       reference "RFC XXXX: MUD Extension to find RATS supply chain
       entity resources: remote attestation services, endorsement
       documents, and reference integrity measurement";
  }

  grouping mud-rats-grouping {
    description
      "Grouping to locate RATS services";
    container ras {
      description
        "Lists of Remote Attestation Service
         (RAS/Verifiers) candidates.";
      leaf-list ras-uris {
        type inet:uri;
        description
          "A list of Verifiers that can appraise evidence produced by
           the entity that presents a DevID including this MUD URI.";
      }
    }
    container rim {
      description
        "Lists of Reference Integrity Measurement (RIM) candidates.";
      leaf-list rim-uris {
        type inet:uri;
        description
          "A list of RIM CoSWID that provide reference integrity
           measurements represented as signed CoSWID using
           the CoSWID RIM extension.";
      }
    }
    container edt {
      description
        "List of Endorsements for Roots of Trusts (e.g. Endorsement
         Key Certificates).";
      leaf-list edt-uris {
        type inet:uri;
        description
          "A list of Endorsements that vouch for the characteristics
           of Roots of Trusts the entity possesses.";
      }
    }
  }
  augment "/mud:mud" {
    uses mud-rats-grouping;
    description
      "add mud-rats URI resources";
  }
}
<CODE ENDS>

6. Privacy Considerations

The privacy considerations of RFC 9334 apply.¶

7. Security Considerations

The trust model and Security Considerations of RFC 8520 and RFC 9334 apply.¶

8. IANA Considerations

RFC Editor: Please replace "RFCthis" with the RFC number assigned to this document.¶

RFC Editor: This document uses the CPA (code point allocation) convention described in [I-D.bormann-cbor-draft-numbers]. For each usage of the term "CPA", please remove the prefix "CPA" from the indicated value and replace the residue with the value assigned by IANA; perform an analogous substitution for all other occurrences of the prefix "CPA" in the document. Finally, please remove this note.¶

9. References

Authors' Addresses