Internet-Draft Evidence Transformations February 2025
Draper & Smith Expires 15 August 2025 [Page]
Workgroup:
Remote ATtestation ProcedureS
Internet-Draft:
draft-smith-rats-evidence-trans-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Draper
Altera
N. Smith
Intel

Evidence Transformations

Abstract

Remote Attestation Procedures (RATS) enable Relying Parties to assess the trustworthiness of a remote Attester and therefore to decide whether to engage in secure interactions with it - or not. Evidence about trustworthiness can be rather complex and it is deemed unrealistic that every Relying Party is capable of the appraisal of Evidence. Therefore that burden is typically offloaded to a Verifier. In order to conduct Evidence appraisal, a Verifier requires fresh Evidence from an Attester. Before a Verifier can appraise Evidence it may require transformation to an internal representation. This document specifies Evidence transformation methods for DICE and SPDM formats to the CoRIM internal representation.

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 https://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 15 August 2025.

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Table of Contents

1. Introduction

Remote Attestation Procedures (RATS) enable Relying Parties to assess the trustworthiness of a remote Attester and therefore to decide whether to engage in secure interactions with it - or not. Evidence about trustworthiness can be rather complex and it is deemed unrealistic that every Relying Party is capable of the appraisal of Evidence. Therefore that burden is typically offloaded to a Verifier. In order to conduct Evidence appraisal, a Verifier requires fresh Evidence from an Attester. Before a Verifier can appraise Evidence it may require transformation to an internal representation. This document specifies Evidence transformation methods for DICE and SPDM formats to the CoRIM internal representation.

1.1. Terminology

This document uses terms and concepts defined by the RATS architecture. For a complete glossary see Section 4 of [RFC9334]. Addintional RATS architecture is found in [I-D.ietf-rats-endorsements]. RATS architecture terms and concepts are always referenced as proper nouns, i.e., with Capital Letters.

In this document, an Evidence structure describes an external representation. There are many possible Evidence structures including [I-D.ietf-rats-eat] and [RFC5280]. The bytes composing the CoRIM data structure are the same either way.

The terminology from CoRIM [I-D.ietf-rats-corim], CBOR [STD94], CDDL [RFC8610] and COSE [STD96] applies.

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.

2. Verifier Reconciliation

This specification assumes the reader is familiar with Verifier Reconsiliation as described in Section 2 of [I-D.ietf-rats-corim]. It describes how a Verifier should process the CoRIM to enable CoRIM authors to convey their intended meaning and how a Verifier reconciles its various inputs. Evidence is one of its inputs. The Verifier is expected to create an internal representation from an external representation. By using an internal representation, the Verifier processes Evidence inputs such that they can be appraised consistently.

This specification defines format transformations for Evidence in DICE [DICE.Attest], SPDM [SPDM], and concise evidence [TCG.CE] formats that are transformed into a Verifier's internal representation. This specification uses the CoMID internal representation (Section 8.2.1 of [I-D.ietf-rats-corim]) as the transformation target. Other internal representations are possible but out of scope for this specification.

3. Transforming DICE Certificate Extension Evidence

This section defines how Evidence from an X.509 certificate containing a DICE certificate extension [DICE.Attest] is transformed into an internal representation that can be processed by Verifiers.

Verifiers supporting the DICE certificate extension Evidence SHOULD implement this transformation.

This specification defines transformation methods for two DICE certificate extensions DiceTcbInfo and DiceMultiTcbInfo. These extensions are identified by the following object identifiers:

Each DiceTcbInfo entry in a MultiTcbInfo is converted to a CoRIM ECT (see Section 8.2.1 of [I-D.ietf-rats-corim]) using the transformation steps in this section. Each DiceMultiTcbInfo entry is independent of the others such that each is transformed to a separate ECT entry. A list of Evidence ECTs (i.e., ae = [ + ECT]) is constructed using CoRIM attestation evidence internal representation (see Section 8.2.1.1 of [I-D.ietf-rats-corim]).

For each DiceTcbInfo (DTI) entry in a DiceMultiTcbInfo allocate an ECT structure.

Step 1.

An ae entry is allocated.

Step 2.

The cmtype of the ECT is set to evidence.

Step 3.

The DiceTcbInfo (DTI) entry populates the ae ECT.

i

The DTI entry populates the ae ECT environment-map

ii

The DTI entry populates the ae ECT elemenet-list.

iii

The DTI entry populates the ae ECT elemenet-list.flags. Foreach f in DTI.OperationalFlags and each m in DTI.OperationalFlagsMask:

Step 4.

The signer of the certificate containing DTI is copied to the ECT.authority field. The signer identity MUST be expressed using $crypto-key-type-choice. A profile or other arrangement is used to coordinate which $crypto-key-type-choice is used for both Evidence and Reference Values.

The completed ECT is added to the ae list.

4. Transforming TCG Concise Evidence

This section defines how Evidence from TCG [TCG.CE] is transformed into an internal representation that can be processed by Verifiers.

Verifiers supporting the TCG Concise Evidence format SHOULD implement this transformation.

Concise evidence may be recognized by any of the following registered types:

Table 1
CBOR tag C-F ID TN Tag Media Type
#6.571 10571 #6.1668557429 "application/ce+cbor"

A Concise Evidence entry is converted to a CoRIM ECT (see Section 8.2.1 of [I-D.ietf-rats-corim]) using the transformation steps in this section. A list of Evidence ECTs (i.e., ae = [ + ECT]) is constructed using CoRIM attestation evidence internal representation (see Section 8.2.1.1 of [I-D.ietf-rats-corim]). The Concise Evidence scheme uses CoRIM CDDL definitions to define several Evidence representations called triples. Cases where Concise Evidence CDDL is identical to CoRIM CDDL the transformation logic uses the structure names in common.

4.1. Transforming the ce.evidence-triples

The ce.evidence-triples structure is a list of evidence-triple-record. An evidence-triple-record consists of an environment-map and a list of measurement-map. For each evidence-triple-record an ae ECT is constructed.

Step 1.

An ae ECT entry is allocated.

Step 2.

The cmtype of the ECT is set to evidence.

Step 3.

The Concise Evidence (CE) entry populates the ae ECT environment fields.

    • copy(CE.evidence-triple-record.environment-map, ECT.environment.environment-map).

i

For each ce in CE.[ + measurement-map]; and each ect in ECT.[ + element-list]:

    • copy(ce.mkey, ect.element-map.element-id)

    • copy(ce.mval, ect.element-map.element-claims`)

Step 4.

The signer of the envelope containing CE is copied to the ECT.authority field. For example, a CE may be wrapped by an EAT token [I-D.ietf-rats-eat] or DICE certificate [DICE.Attest]. The signer identity MUST be expressed using $crypto-key-type-choice. A profile or other arrangement is used to coordinate which $crypto-key-type-choice is used for both Evidence and Reference Values.

Step 5.

If CE has a profile, the profile is converted to a $profile-type-choice then copied to the ECT.profile` field.

The completed ECT is added to the ae list.

4.2. Transforming the ce.identity-triples

The ce.identity-triples structure is a list of ev-identity-triple-record. An ev-identity-triple-record consists of an environment-map and a list of $crypto-key-type-choice. For each ev-identity-triple-record an ae ECT is constructed where the $crypto-key-type-choice values are copied as ECT Evidence measurement values. The ECT internal representation accommodates keys as a type of measurement. In order for the $crypto-key-type-choice keys to be verified a CoRIM identity-triples claim MUST be asserted.

Step 1.

An ae ECT entry is allocated.

Step 2.

The cmtype of the ECT is set to evidence.

Step 3.

The Concise Evidence (CE) entry populates the ae ECT environment fields.

    • copy(CE.ce-identity-triple-record.environment-map, ECT.environment.environment-map).

    • copy(null, ECT.element-list.element-map.element-id).

i

For each cek in CE.[ + $crypto-key-type-choice ]; and each ect in ECT.element-list.element-map.element-claims.intrep-keys.[ + typed-crypto-key ]:

    • copy(cek, ect.key)

    • set( &(identity-key: 1), ect.key-type)

Step 4.

The signer of the envelope containing CE is copied to the ECT.authority field. For example, a CE may be wrapped by an EAT token [I-D.ietf-rats-eat] or DICE certificate [DICE.Attest]. The signer identity MUST be expressed using $crypto-key-type-choice. A profile or other arrangement is used to coordinate which $crypto-key-type-choice is used for both Evidence and Reference Values.

Step 5.

If CE has a profile, the profile is converted to a $profile-type-choice then copied to the ECT.profile` field.

The completed ECT is added to the ae list.

4.3. Transforming the ce.attest-key-triples

The ce.attest-key-triples structure is a list of ev-attest-key-triple-record. An ev-attest-key-triple-record consists of an environment-map and a list of $crypto-key-type-choice. For each ev-attest-key-triple-record an ae ECT is constructed where the $crypto-key-type-choice values are copied as ECT Evidence measurement values. The ECT internal representation accommodates keys as a type of measurement. In order for the $crypto-key-type-choice keys to be verified a CoRIM attest-key-triples claim MUST be asserted.

Step 1.

An ae ECT entry is allocated.

Step 2.

The cmtype of the ECT is set to evidence.

Step 3.

The Concise Evidence (CE) entry populates the ae ECT environment fields.

    • copy(CE.ce-attest-key-triple-record.environment-map, ECT.environment.environment-map).

    • copy(null, ECT.element-list.element-map.element-id).

i

For each cek in CE.[ + $crypto-key-type-choice ]; and each ect in ECT.element-list.element-map.element-claims.intrep-keys.[ + typed-crypto-key ]:

    • copy(cek, ect.key)

    • set( &(attest-key: 0), ect.key-type)

Step 4.

The signer of the envelope containing CE is copied to the ECT.authority field. For example, a CE may be wrapped by an EAT token [I-D.ietf-rats-eat] or DICE certificate [DICE.Attest]. The signer identity MUST be expressed using $crypto-key-type-choice. A profile or other arrangement is used to coordinate which $crypto-key-type-choice is used for both Evidence and Reference Values.

Step 5.

If CE has a profile, the profile is converted to a $profile-type-choice then copied to the ECT.profile` field.

The completed ECT is added to the ae list.

5. Transforming SPDM Evidence

This section defines how Evidence from SPDM [SPDM] is transformed into an internal representation that can be processed by Verifiers.

Verifiers supporting the SPDM Evidence format SHOULD implement this transformation.

The SPDM measurements are converted to concise-evidence which has a format that is similar to CoRIM triples-map (their semantics follows the matching rules described above). The TCG DICE Concise Evidence Binding for SPDM specification [TCG.CE] describes a process for converting the SPDM Measurement Block to Concise Evidence. Subsequently the transformation steps defined in Section 4.

6. Security and Privacy Considerations

There are no security and privacy considerations.

7. IANA Considerations

There are no IANA considerations.

8. References

8.1. Normative References

[DICE.Attest]
Trusted Computing Group (TCG), "DICE Attestation Architecture", Version 1.2, Revision 1 , , <https://trustedcomputinggroup.org/wp-content/uploads/DICE-Attestation-Architecture-Version-1.2-rc-1_9January25.pdf>.
[DICE.CoRIM]
Trusted Computing Group (TCG), "DICE Endorsement Architecture for Devices", Version 1.0, Revision 0.38 , , <https://trustedcomputinggroup.org/wp-content/uploads/TCG-Endorsement-Architecture-for-Devices-V1-R38_pub.pdf>.
[I-D.ietf-rats-corim]
Birkholz, H., Fossati, T., Deshpande, Y., Smith, N., and W. Pan, "Concise Reference Integrity Manifest", Work in Progress, Internet-Draft, draft-ietf-rats-corim-06, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-corim-06>.
[I-D.ietf-rats-endorsements]
Thaler, D., Birkholz, H., and T. Fossati, "RATS Endorsements", Work in Progress, Internet-Draft, draft-ietf-rats-endorsements-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-endorsements-05>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9334]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, , <https://www.rfc-editor.org/rfc/rfc9334>.
[SPDM]
Distributed Management Task Force, "Security Protocol and Data Model (SPDM)", Version 1.3.0 , , <https://www.dmtf.org/sites/default/files/standards/documents/DSP0274_1.3.0.pdf>.
[TCG.CE]
Trusted Computing Group, "TCG DICE Concise Evidence Binding for SPDM", Version 1.00, Revision 0.54 , , <https://trustedcomputinggroup.org/wp-content/uploads/TCG-DICE-Concise-Evidence-Binding-for-SPDM-Version-1.0-Revision-54_pub.pdf>.
[X.690]
International Telecommunications Union, "Information technology — ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, , <https://www.itu.int/rec/T-REC-X.690>.

8.2. Informative References

[DICE.Layer]
Trusted Computing Group, "DICE Layering Architecture", Version 1.0, Revision 0.19 , , <https://trustedcomputinggroup.org/wp-content/uploads/DICE-Layering-Architecture-r19_pub.pdf>.
[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-31, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-eat-31>.
[RFC5280]
Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, , <https://www.rfc-editor.org/rfc/rfc5280>.
[RFC7942]
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <https://www.rfc-editor.org/rfc/rfc7942>.
[RFC8610]
Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC9090]
Bormann, C., "Concise Binary Object Representation (CBOR) Tags for Object Identifiers", RFC 9090, DOI 10.17487/RFC9090, , <https://www.rfc-editor.org/rfc/rfc9090>.
[STD66]
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <https://www.rfc-editor.org/rfc/rfc3986>.
[STD94]
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
[STD96]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.

Contributors

The authors would like to thank the following people for their valuable contributions to the specification.

Henk Birkholz

Email: henk.birkholz@ietf.contact

Yogesh Deshpande

Email: yogesh.deshpande@arm.com

Thomas Fossati

Email: Thomas.Fossati@linaro.org

Dionna Glaze

Email: dionnaglaze@google.com

Acknowledgments

The authors would like to thank James D. Beaney, Francisco J. Chinchilla, Vincent R. Scarlata, and Piotr Zmijewski for review feedback.

Authors' Addresses

Andrew Draper
Altera
Ned Smith
Intel