EAT Measured Component

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Table of Contents

1. Introduction

Section 4.2.16 of [I-D.ietf-rats-eat] defines a Measurements claim that:¶

• "[c]ontains descriptions, lists, evidence or measurements of the software that exists on the entity or any other measurable subsystem of the entity."¶

This claim allows for different measurement formats, each identified by a different CoAP Content-Format (Section 12.3 of [RFC7252]). Currently, the only specified format is CoSWID of type "evidence", as per Section 2.9.4 of [RFC9393].¶

This document introduces a "measured component" format that can be used with the EAT Measurements claim in addition to or as an alternative to CoSWID.¶

The term "measured component" refers to any measurable object on a target environment, that is, an object whose state can be sampled and digested. This includes, for example: the invariant part of a firmware component that is loaded in memory at startup time, a run-time integrity check (RTIC), a file system object, or a CPU register.¶

2. Conventions and Definitions

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.¶

In this document, CDDL [RFC8610] [RFC9165] [I-D.ietf-cbor-cddl-modules] [I-D.ietf-cbor-cddl-more-control] is used to describe the data formats.¶

3. Information Model

A "measured component" information element includes the digest of the component's sampled state along with metadata that helps in identifying the component. Optionally, any entities responsible for signing the installed component can also be specified.¶

The information model of a "measured component" is described in Table 1.¶

4. Data Model

The data model is inspired by the "PSA software component" claim (Section 4.4.1 of [I-D.tschofenig-rats-psa-token]), which has been refactored to take into account the recommendations about new EAT claims design in Appendix E of [I-D.ietf-rats-eat].¶

;# import digest from RFCYYYY as corim

measured-component = [
  id:           component-id
  measurement:  corim.digest
  ? signers:    [ + signer-type ]
]

;# import sw-version-type from RFCXXXX as eat

component-id = [
  name:      text
  ? version: eat.sw-version-type
]

signer-type = bytes

mc-cbor = bstr .cbor measured-component
mc-json = tstr .json measured-component

; EAT CBOR (`.feature "cbor"`)
$measurements-body-cbor /= mc-cbor            ; native
$measurements-body-cbor /= tstr .b64u mc-json ; tunnel

; EAT JSON (`.feature "json"`)
$measurements-body-json /= mc-json            ; native
$measurements-body-json /= tstr .b64u mc-cbor ; tunnel

5. Examples

(The examples are CBOR only. JSON examples will be added in a future version of this document.)¶

The example in Figure 2 is a measured component with all the fields populated.¶

[
  / id / [
    / name / "boot loader X",
    / version / [
      "1.2.3rc2",
      16384 / semver /
    ]
  ],
  / measurement / [
    / alg / "sha-256",
    / val / h'3996003d486fb91ffb056f7d03f2b2992b215b31dbe7af4b37
              3431fc7d319da3'
  ],
  / signers / [
    h'492e9b676c21f6012b1ceeb9032feb4141a880797355f6675015ec59c5
      1ca1ec',
    h'4277bb97ba7b51577a0d38151d3e08b40bdf946753f5b5bdeb814d6ff5
      7a8a5e'
  ]
]

The example in Figure 3 is the same measured component as above but used as the format of a measurements claim in a EAT claims-set.¶

Note that the example uses a CoAP Content-Format value from the experimental range (65000), which will change to the value assigned by IANA for the application/measured-component+cbor Content-Format.¶

Note also that the array contains only one measured component, but additional entries could be added if the measured TCB is made of multiple, individually measured components.¶

{
  273: [
    [
      65000, / using a CoAP C-F from the experimental range /
      <<
        [
          / id / [
            / name / "boot loader X",
            / version / [
              "1.2.3rc2",
              16384 / semver /
            ]
          ],
          / measurement / [
            / alg / "sha-256",
            / val / h'3996003d486fb91ffb056f7d03f2b2992b215b31db
                      e7af4b373431fc7d319da3'
          ],
          / signers / [
            h'492e9b676c21f6012b1ceeb9032feb4141a880797355f66750
              15ec59c51ca1ec',
            h'4277bb97ba7b51577a0d38151d3e08b40bdf946753f5b5bdeb
              814d6ff57a8a5e'
          ]
        ]
      >>
    ]
  ]
}

6. Security and Privacy Considerations

The Name and Version of a component could provide an attacker with detailed information about the running software and configuration settings of the device. This information could also expose private details regarding the device. The stability requirement for the component's Name could potentially enable tracking.¶

7. IANA Considerations

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

Appendix A. Open Issues

The list of currently open issues for this documents can be found at https://github.com/thomas-fossati/draft-fft-rats-eat-measured-component/issues.¶

Note to RFC Editor: please remove before publication.¶

Acknowledgments

The authors would like to thank Carl Wallace, Carsten Bormann, Giridhar Mandyam and Laurence Lundblade for providing comments, reviews and suggestions that greatly improved this document.¶

Authors' Addresses