Use of Remote Attestation with Certificate Signing Requests

Internet-Draft	Remote Attestation with CSRs	March 2024
Ounsworth, et al.	Expires 2 September 2024	[Page]

Abstract

A PKI end entity requesting a certificate from a Certification Authority (CA) may wish to offer believable claims about the protections afforded to the corresponding private key, such as whether the private key resides on a hardware security module or the protection capabilities provided by the hardware.¶

This document defines a new PKCS#10 attribute attr-evidence and CRMF extension ext-evidence that allows placing any Evidence data, in any pre-existing format, along with any certificates needed to validate it, into a PKCS#10 or CRMF CSR.¶

Including Evidence along with a CSR can help to improve the assessment of the security posture for the private key, and the trustworthiness properties of the submitted key to the requested certificate profile. These Evidence Claims can include information about the hardware component's manufacturer, the version of installed or running firmware, the version of software installed or running in layers above the firmware, or the presence of hardware components providing specific protection capabilities or shielded locations (e.g., to protect keys).¶

1. Introduction

When requesting a certificate from a Certification Authority (CA), a PKI end entity may wish to include Evidence of the security properties of its environments in which the private keys are stored in that request. This Evidence can be appraised by authoritative entities, such as a Registration Authority (RA) or a CA, or associated trusted Verifiers as part of validating an incoming certificate request against given certificate policies. Regulatory bodies are beginning to require proof-of-hardware residency for certain classifications of cryptographic keys. At the time of writing, the most notable example is the Code-Signing Baseline Requirements [CSBR] document maintained by the CA/Browser Forum, which requires compliant CAs to "ensure that a Subscriber’s Private Key is generated, stored, and used in a secure environment that has controls to prevent theft or misuse".¶

This specification defines an attribute and an extension that allow for conveyance of Evidence in Certificate Signing Requests (CSRs) in either PKCS#10 [RFC2986] or Certificate Request Message Format (CRMF) [RFC4211] payloads which provides an elegant and automatable mechanism for meeting requirements such as those in the CA/B Forum's [CSBR].¶

As outlined in the RATS Architecture [RFC9334], an Attester (typically a device) produces a signed collection of Claims that constitutes Evidence about its running environment. While the term "attestation" is not defined in RFC 9334, it was later defined in [I-D.ietf-rats-tpm-based-network-device-attest], it refers to the activity of producing and appraising remote attestation Evidence. A Relying Party may consult an Attestation Result produced by a Verifier that has appraised the Evidence in making policy decisions about the trustworthiness of the Target Environment being assessed via appraisal of Evidence. Section 3 provides the basis to illustrate in this document how the various roles in the RATS architecture map to a certificate requester and a CA/RA.¶

At the time of writing, several standard and several proprietary attestation technologies are in use. This specification thereby is intended to be as technology-agnostic as it is feasible with respect to implemented remote attestation technologies. Instead, it focuses on (1) the conveyance of Evidence via CSRs while making minimal assumptions about content or format of the transported Evidence and (2) the conveyance of sets of certificates used for validation of Evidence. The certificates typically contain one or more certification paths rooted in a device manufacture trust anchor and the leaf certificate being on the device in question; the latter is the Attestation Key that signs the Evidence statement.¶

This document specifies a CSR Attribute (or Extension for Certificate Request Message Format (CRMF) CSRs) for carrying Evidence. Evidence can be placed into an EvidenceStatement along with an OID to identify its type and optionally a hint to the Relying Party about how to verify it. A set of EvidenceStatements may be grouped together along with the set of CertificateAlternatives needed to validate them to form a EvidenceBundle. One or more EvidenceBundles may be placed into the id-aa-evidence CSR Attribute (or CRFM Extension).¶

A CSR may contain one or more Evidence payloads, for example Evidence asserting the storage properties of a private key as well Evidence asserting firmware version and other general properties of the device, or Evidence signed using different cryptographic algorithms.¶

With these attributes, additional information information is available to an RA or CA which may be used to decide whether to issue a certificate and what certificate profile to apply. The scope of this document is, however, limited to the conveyance of Evidence within CSR. The exact format of the Evidence being conveyed is defined in various standard and proprietary specifications.¶

4. Information Model

4.1. Interaction with an HSM

This specification is intended to be applicable both in cases where the CSR is constructed internally or externally to the attesting environment, from the point of view of the calling application.¶

Cases where the CSR is generated internally to the attesting environment are straightforward: the HSM generates and embeds the Evidence and the corresponding certification paths when constructing the CSR.¶

Cases where the CSR is generated externally may require extra round-trips of communication between the CSR generator and the attesting environment, first to obtain the necessary Evidence about the subject key, and then to use the subject key to sign the CSR. For example, consider a CSR generated by a popular crypto library about a subject key stored on a PKCS#11 [PKCS11] device.¶

As an example, assuming that the HSM is, or contains, the attesting environment, and some cryptographic library is assembling a CSR by interacting with the HSM over some network protocol, then the interaction would conceptually be:¶

Figure 2: Example interaction between CSR generator and HSM.

4.2. Implementation Strategies

To support a number of different use cases for the transmission of Evidence in a CSR (together with certification paths) the structure shown in Figure 3 is used.¶

On a high-level, the structure can be explained as follows: A PKCS#10 attribute or a CRMF extension contains one or more EvidenceBundle structures. Each EvidenceBundle contains one or more EvidenceStatement structures as well as one or more CertificateAlternatives which enable to carry various format of certificates.¶

Figure 3: Information Model for CSR Evidence Conveyance.

The following use cases are supported:¶

Single Attester, which only distributes Evidence without any certification paths, i.e. the Verifier is assumed to be in possession of the certification paths already or there is no certification paths because the Verifier directly trusts the Attester key. As a result a single EvidenceBundle is included in a CSR that contains a single EvidenceStatement without the CertificateAlternatives structure. Figure 4 shows this use case.¶

Figure 4: Use Case 1: Single Attester without Certification Path.

A single Attester, which shares Evidence together with a certification path. The CSR conveys a single EvidenceBundle with a single EvidenceStatement and a single CertificateAlternatives structure. Figure 5 shows this use case.¶

Figure 5: Use Case 2: Single Attester with Certification Path.

In a Composite Device, which contains multiple Attesters, a collection of Evidence statements is obtained. Imagine that each Attester returns its Evidence together with a certification path. As a result, multiple EvidenceBundle structures, each carrying an EvidenceStatement and the corresponding CertificateAlternative structure with the certification path as provided by each Attester, are included in the CSR. This may result in certificates being duplicated across multiple EvidenceBundles. This approach does not require any processing capabilities by a lead Attester since the information is merely forwarded. Figure 6 shows this use case.¶

Figure 6: Use Case 3: Multiple Attesters in Composite Device.

In the last scenario, we also assume a Composite Device with additional processing capabilities of the Leader Attester, which parses the certification path provided by all Attesters in the device and removes redundant certificate information. The benefit of this approach is the reduced transmission overhead. There are several implementation strategies and we show two in Figure 7.¶

Figure 7: Use Case 4: Multiple Attesters in Composite Device (with Optimization).

In implementation strategy (4a) we assume that each Attester is provisioned with a unique end-entity certificate. Hence, the certification path will at least differ with respect to the end-entity certificates. The Lead Attester will therefore create multiple EvidenceBundle structures, each will carry an EvidenceStatement followed by a certification path in the CertificateAlternative structures containing most likely only the end-entity certificate. The shared certification path is carried in the first entry of the EvidenceBundle sequence to allow path validation to take place immediately after processing the first structure. This implementation strategy may place extra burden on the Relying Party to parse EvidenceBundles and reconstruct certification path if the Verifier requires each EvidenceStatement to be accompanied with a complete certification path.¶

Implementation strategy (4b), as an alternative, requires the Lead Attester to merge all certification paths into a single EvidenceBundle containing a single de-duplicated sequence of CertificateAlternatives structures. This means that each EvidenceBundle is self-contained and any EvidenceStatement can be verified using only the sequence of CertificateAlternatives in its bundle, but Verifiers will have to do proper certification path building since the sequence of CertificateAlternatives is now a cert bag and not a certification path. This implementation strategy may place extra burden on the Attester in order to allow the Relying Party to treat the Evidence and Certificates as opaque content. It also may place extra burden on the Verifier since this implementation strategy requires it to be able to perform X.509 path building over a bag of certificates that may be out of order or contain extraneous certificates.¶

Note: This specification does not mandate optimizing certification path since there is a trade-off between the Attester implementation complexity and the transmission overhead.¶

5. ASN.1 Elements

5.1. Object Identifiers

We reference id-pkix and id-aa, both defined in [RFC5912].¶

We define:¶

-- Arc for Evidence types
id-ata OBJECT IDENTIFIER ::= { id-pkix (TBD1) }

5.2. Evidence Attribute and Extension

By definition, Attributes within a PKCS#10 CSR are typed as ATTRIBUTE and within a CRMF CSR are typed as EXTENSION. This attribute definition contains one or more Evidence bundles of type EvidenceBundle which each contain one or more Evidence statements of a type EvidenceStatement along with an optional certification path. This structure allows for grouping Evidence statements that share a certification path.¶

EVIDENCE-STATEMENT ::= TYPE-IDENTIFIER

EvidenceStatementSet EVIDENCE-STATEMENT ::= {
   ... -- None defined in this document --
}

This is a mapping and ASN.1 Types for Evidence Statements to the OIDs that identify them. These mappings are are used to construct or parse EvidenceStatements. These would typically be Evidence Statement formats defined in other IETF standards, defined by other standards bodies, or vendor proprietary formats along with the OIDs that identify them.¶

This list is left empty in this document. However, implementers should populate it with the formats that they wish to support.¶

EvidenceHint ::= CHOICE {
     rfc822Name [0] IA5String,
     dNSName    [1] IA5String,
     uri        [2] IA5String,
     text       [3] UTF8String
}

EvidenceStatements ::= SEQUENCE SIZE (1..MAX) OF EvidenceStatement

EvidenceStatement ::= SEQUENCE {
   type   EVIDENCE-STATEMENT.&id({EvidenceStatementSet}),
   stmt   EVIDENCE-STATEMENT.&Type({EvidenceStatementSet}{@type}),
   hint   EvidenceHint OPTIONAL
}

The type is on OID indicating the format of the data contained in stmt.¶

The hint is intended for an Attester to indicate to the Relying Party (RP) which Verifier should be invoked to parse this statement. In many cases, the type OID will already uniquely indicate which Verifier to invoke; for example because the OID indicates a proprietary Evidence format for which the RP has corresponding proprietary Verifier. However, in some cases it may still be ambiguous, or the type may indicate another layer of conceptual message wrapping in which case it is helpful to the RP to bring this hint outside of the statement. It is assumed that the RP must be pre-configured with a list of trusted Verifiers and that the contents of this hint can be used to look up the correct Verifier. Under no circumstances must the RP be tricked into contacting an unknown and untrusted Verifier since the returned Attestation Result must not be relied on. The format and contents of the hint are out of scope of this document.¶

EvidenceBundles ::= SEQUENCE SIZE (1..MAX) OF EvidenceBundle

EvidenceBundle ::= SEQUENCE
{
  evidence EvidenceStatements,
  certs SEQUENCE SIZE (1..MAX) OF CertificateAlternatives OPTIONAL
}

id-aa-evidence OBJECT IDENTIFIER ::= { id-aa TBDAA }

-- For PKCS#10
attr-evidence ATTRIBUTE ::= {
  TYPE EvidenceBundles
  IDENTIFIED BY id-aa-evidence
}

-- For CRMF
ext-evidence EXTENSION ::= {
  SYNTAX EvidenceBundles
  IDENTIFIED BY id-aa-evidence
}

The Extension variant is intended only for use within CRMF CSRs and MUST NOT be used within X.509 certificates due to the privacy implications of publishing Evidence about the end entity's hardware environment. See Section 7 for more discussion.¶

The certs contains a set of certificates that may be needed to validate the contents of an Evidence statement contained in evidence. The set of certificates should contain the object that contains the public key needed to directly validate the evidence. The remaining elements should chain that data back to an agreed upon trust anchor used for attestation. No order is implied, it is up to the Attester and its Verifier to agree on both the order and format of certificates contained in certs.¶

A CSR MAY contain one or more instances of attr-evidence or ext-evidence. This means that the SEQUENCE OF EvidenceBundle is redundant with the ability to carry multiple attr-evidence or ext-evidence at the CSR level; either mechanism MAY be used for carrying multiple Evidence bundles.¶

5.3. CertificateAlternatives

This is an ASN.1 CHOICE construct used to represent an encoding of a broad variety of certificate types.¶

CertificateAlternatives ::=
   CHOICE {
      cert          [0] Certificate,
      typedCert     [1] TypedCert,
      typedFlatCert [2] TypedFlatCert,
      ...
   }

"Certificate" is a standard X.509 certificate that MUST be compliant with RFC 5280. Enforcement of this constraint is left to the relying parties.¶

"TypedCert" is an ASN.1 construct that has the characteristics of a certificate, but is not encoded as an X.509 certificate. The certType Field (below) indicates how to interpret the certBody field. While it is possible to carry any type of data in this structure, it's intended the content field include data for at least one public key formatted as a SubjectPublicKeyInfo (see [RFC5912]).¶

TYPED-CERT ::= TYPE-IDENTIFIER

CertType ::= TYPED-CERT.&id

TypedCert ::= SEQUENCE {
              certType     TYPED-CERT.&id({TypedCertSet}),
              content     TYPED-CERT.&Type ({TypedCertSet}{@certType})
          }

TypedCertSet TYPED-CERT ::= {
   ... -- None defined in this document --
      }

"TypedFlatCert" is a certificate that does not have a valid ASN.1 encoding. These are often compact or implicit certificates used by smart cards. certType indicates the format of the data in the certBody field, and ideally refers to a published specification.¶

TypedFlatCert ::= SEQUENCE {
    certType OBJECT IDENTIFIER,
    certBody OCTET STRING
}

6. IANA Considerations

IANA is requested to open two new registries, allocate a value from the "SMI Security for PKIX Module Identifier" registry for the included ASN.1 module, and allocate values from "SMI Security for S/MIME Attributes" to identify two Attributes defined within.¶

6.1. Module Registration - SMI Security for PKIX Module Identifier

Decimal: IANA Assigned - Replace TBDMOD¶
Description: CSR-ATTESTATION-2023 - id-mod-pkix-attest-01¶
References: This Document¶

6.2. Object Identifier Registrations - SMI Security for S/MIME Attributes

Evidence Statement¶
- Decimal: IANA Assigned - Replace TBDAA¶
- Description: id-aa-evidence¶
- References: This Document¶

6.3. "SMI Security for PKIX Evidence Statement Formats" Registry

IANA is asked to create a registry for Evidence Statement Formats within the SMI-numbers registry, allocating an assignment from id-pkix ("SMI Security for PKIX" Registry) for the purpose.¶

Decimal: IANA Assigned - replace TBD1¶
Description: id-ata¶
References: This document¶
Initial contents: None¶
Registration Regime: Specification Required. Document must specify an EVIDENCE-STATEMENT definition to which this Object Identifier shall be bound.¶

Columns:¶

Decimal: The subcomponent under id-ata¶
Description: Begins with id-ata¶
References: RFC or other document¶

6.4. Attestation Evidence OID Registry

IANA is asked to create a registry that helps developers to find OID/Evidence mappings.¶

Registration requests are evaluated using the criteria described in the registration template below after a three-week review period on the [[TBD]] mailing list, with the advice of one or more Designated Experts [RFC8126]. However, to allow for the allocation of values prior to publication, the Designated Experts may approve registration once they are satisfied that such a specification will be published.¶

Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register attestation evidence: example").¶

IANA must only accept registry updates from the Designated Experts and should direct all requests for registration to the review mailing list.¶

6.4.1. Registration Template

The registry has the following columns:¶

OID: The OID number, which has already been allocated. IANA does not allocate OID numbers for use with this registry.¶
Description: Brief description of the use of the Evidence and the registration of the OID.¶
Reference(s): Reference to the document or documents that register the OID for use with a specific attestation technology, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.¶
Change Controller: For Standards Track RFCs, list the "IESG". For others, give the name of the responsible party. In most cases the third party requesting registration in this registry will also be the party that registered the OID.¶

6.4.2. Initial Registry Contents

The initial registry contents is shown in the table below. It lists one entry for the Conceptual Message Wrapper (CMW) [I-D.ietf-rats-msg-wrap].¶

Table 1: Initial Contents of the Attestation Evidence OID Registry
OID	Description	Reference(s)	Change Controller
2 23 133 5 4 9	Conceptual Message Wrapper	[TCGDICE1.1]	TCG

EDNOTE: This is currently under debate with our contacts at TCG about which OID they want used for the initial registry.¶

The current registry values can be retrieved from the IANA online website.¶

7. Security Considerations

A PKCS#10 or CRMF Certification Request message typically consists of a distinguished name, a public key, and optionally a set of attributes, collectively signed by the entity requesting certification. In general usage, the private key used to sign the CSR MUST be different from the Attesting Key utilized to sign Evidence about the Target Environment, though exceptions MAY be made where CSRs and Evidence are involved in bootstrapping the Attesting Key. To demonstrate that the private key applied to sign the CSR is generated, and stored in a secure environment that has controls to prevent theft or misuse (including being non-exportable / non-recoverable), the Attesting Environment has to collect claims about this secure environment (or Target Environment, as shown in Figure 8).¶

Figure 8 shows the interaction inside an Attester. The Attesting Environment, which is provisioned with an Attestation Key, retrieves claims about the Target Environment. The Target Environment offers key generation, storage and usage, which it makes available to services. The Attesting Environment collects these claims about the Target Environment and signs them and exports Evidence for use in remote attestation via a CSR.¶

Figure 8: Interaction between Attesting and Target Environment

Figure 8 places the CSR library outside the Attester, which is a valid architecture for certificate enrollment. The CSR library may also be located inside the trusted computing base. Regardless of the placement of the CSR library, an Attesting Environment MUST be able to collect claims about the Target Environment such that statements about the storage of the keying material can be made. For the Verifier, the provided Evidence must allow an assessment to be made whether the key used to sign the CSR is stored in a secure location and cannot be exported.¶

Evidence communicated in the attributes and structures defined in this document are meant to be used in a CSR. It is up to the Verifier and to the Relying Party (RA/CA) to place as much or as little trust in this information as dictated by policies.¶

This document defines the transport of Evidence of different formats in a CSR. Some of these encoding formats are based on standards while others are proprietary formats. A Verifier will need to understand these formats for matching the received claim values against policies.¶

Policies drive the processing of Evidence at the Verifier: the Verifier's Appraisal Policy for Evidence will often be based on specifications by the manufacturer of a hardware security module, a regulatory agency, or specified by an oversight body, such as the CA Browser Forum. The Code-Signing Baseline Requirements [CSBR] document is an example of such a policy that has been published by the CA Browser Forum and specifies certain properties, such as non-exportability, which must be enabled for storing publicly-trusted code-signing keys. Other policies influence the decision making at the Relying Party when evaluating the Attestation Result. The Relying Party is ultimately responsible for making a decision of what information in the Attestation Result it will accept. The presence of the attributes defined in this specification provide the Relying Party with additional assurance about an Attester. Policies used at the Verifier and the Relying Party are implementation dependent and out of scope for this document. Whether to require the use of Evidence in a CSR is out-of-scope for this document.¶

7.1. Freshness

Evidence generated by an Attester generally needs to be fresh to provide value to the Verifier since the configuration on the device may change over time. Section 10 of [RFC9334] discusses different approaches for providing freshness, including a nonce-based approach, the use of timestamps and an epoch-based technique. The use of nonces requires that nonce to be provided by the Relying Party in some protocol step prior to Evidence and CSR generation, and the use of timestamps requires synchronized clocks which cannot be guaranteed in all operating environments. Epochs also require (unidirectional) communication prior to Evidence and CSR generation. This document only specifies how to carry existing Evidence formats inside a CSR, and so issues of synchronizing freshness data is left to be handled, for example, via certificate management protocols. Developers, operators, and designers of protocols, which embed Evidence-carrying-CSRs, MUST consider what notion of freshness is appropriate and available in-context; thus the issue of freshness is left up to the discretion of protocol designers and implementers.¶

In the case of Hardware Security Modules (HSM), the definition of "fresh" is somewhat ambiguous in the context of CSRs, especially considering that non-automated certificate enrollments are often asynchronous, and considering the common practice of re-using the same CSR for multiple certificate renewals across the lifetime of a key. "Freshness" typically implies both asserting that the data was generated at a certain point-in-time, as well as providing non-replayability. Certain use cases may have special properties impacting the freshness requirements. For example, HSMs are typically designed to not allow downgrade of private key storage properties; for example if a given key was asserted at time T to have been generated inside the hardware boundary and to be non-exportable, then it can be assumed that those properties of that key will continue to hold into the future.¶

7.2. Publishing evidence in an X.509 extension

This document specifies and Extension for carrying Evidence in a CRMF Certificate Signing Request (CSR), but it is intentionally NOT RECOMMENDED for a CA to copy the ext-evidence or ext-evidenceCerts extensions into the published certificate. The reason for this is that certificates are considered public information and the Evidence might contain detailed information about hardware and patch levels of the device on which the private key resides. The certificate requester has consented to sharing this detailed device information with the CA but might not consent to having these details published. These privacy considerations are beyond the scope of this document and may require additional signaling mechanisms in the CSR to prevent unintended publication of sensitive information, so we leave it as "NOT RECOMMENDED".¶

7.3. Type OID and verifier hint

The EvidenceStatement includes both a type OID and a free form hint field with which the Attester can provide information to the Relying Party about which Verifier to invoke to parse a given piece of Evidence. Care should be taken when processing these data since at the time they are used, they are not yet verified. In fact, they are protected by the CSR signature but not by the signature from the Attester and so could be maliciously replaced in some cases. The authors' intent is that the type OID and hint will allow an RP to select between Verifier with which it has pre-established trust relationships, such as Verifier libraries that have been compiled in to the RP application. As an example, the hint may take the form of an FQDN to uniquely identify a Verifier implementation, but the RP MUST NOT blindly make network calls to unknown domain names and trust the results. Implementers should also be cautious around type OID or hint values that cause a short-circuit in the verification logic, such as None, Null, Debug, empty CMW contents, or similar values that could cause the Evidence to appear to be valid when in fact it was not properly checked.¶

8. References

8.1. Normative References

[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>.
[RFC2986]: Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, November 2000, <https://www.rfc-editor.org/rfc/rfc2986>.
[RFC4211]: Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, DOI 10.17487/RFC4211, September 2005, <https://www.rfc-editor.org/rfc/rfc4211>.
[RFC5912]: Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, June 2010, <https://www.rfc-editor.org/rfc/rfc5912>.
[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>.
[RFC9334]: Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, January 2023, <https://www.rfc-editor.org/rfc/rfc9334>.

8.2. Informative References

[CSBR]: CA/Browser Forum, "Baseline Requirements for Code-Signing Certificates, v.3.3", June 2023, <https://cabforum.org/wp-content/uploads/Baseline-Requirements-for-the-Issuance-and-Management-of-Code-Signing.v3.3.pdf>.
[I-D.bft-rats-kat]: Brossard, M., Fossati, T., and H. Tschofenig, "An EAT-based Key Attestation Token", Work in Progress, Internet-Draft, draft-bft-rats-kat-02, 10 February 2024, <https://datatracker.ietf.org/doc/html/draft-bft-rats-kat-02>.
[I-D.ietf-rats-msg-wrap]: Birkholz, H., Smith, N., Fossati, T., and H. Tschofenig, "RATS Conceptual Messages Wrapper (CMW)", Work in Progress, Internet-Draft, draft-ietf-rats-msg-wrap-04, 27 February 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-rats-msg-wrap-04>.
[I-D.ietf-rats-tpm-based-network-device-attest]: Fedorkow, G., Voit, E., and J. Fitzgerald-McKay, "TPM-based Network Device Remote Integrity Verification", Work in Progress, Internet-Draft, draft-ietf-rats-tpm-based-network-device-attest-14, 22 March 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-rats-tpm-based-network-device-attest-14>.
[I-D.tschofenig-rats-psa-token]: Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and T. Fossati, "Arm's Platform Security Architecture (PSA) Attestation Token", Work in Progress, Internet-Draft, draft-tschofenig-rats-psa-token-22, 21 February 2024, <https://datatracker.ietf.org/doc/html/draft-tschofenig-rats-psa-token-22>.
[PKCS11]: OASIS, "PKCS #11 Cryptographic Token Interface Base Specification Version 2.40", April 2015, <http://docs.oasis-open.org/pkcs11/pkcs11-base/v2.40/os/pkcs11-base-v2.40-os.html>.
[RFC7030]: Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013, <https://www.rfc-editor.org/rfc/rfc7030>.
[RFC8126]: Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/rfc/rfc8126>.
[TCGDICE1.1]: Trusted Computing Group, "DICE Attestation Architecture", January 2024, <https://trustedcomputinggroup.org/wp-content/uploads/DICE-Attestation-Architecture-Version-1.1-Revision-18_pub.pdf>.
[TPM20]: Trusted Computing Group, "Trusted Platform Module Library Specification, Family 2.0, Level 00, Revision 01.59", November 2019, <https://trustedcomputinggroup.org/resource/tpm-library-specification/>.

Appendix A. Examples

This section provides two non-normative examples for embedding Evidence in CSRs. The first example embeds Evidence produced by a TPM in the CSR. The second example conveys an Arm Platform Security Architecture token, which provides claims about the used hardware and software platform, into the CSR.¶

At the time of writing, the authors are not aware of registered OIDs for these evidence formats, and so we leave the OIDs as TBD1 / TBD2.¶

A.1. Extending EvidenceStatementSet

As defined in Section 5.2, EvidenceStatementSet acts as a way to provide an ASN.1 compiler or runtime parser with a list of OBJECT IDENTIFIERs that are known to represent EvidenceStatements -- and are expected to appear in an EvidenceStatement.type field, along with the ASN.1 type that should be used to parse the data in the associated EvidenceStatement.stmt field. Essentially this is a mapping of OIDs to data structures. Implementers are expected to populate it with mappings for the Evidence types that their application will be handling.¶

This specification aims to be agnostic about the type of data being carried, and therefore does not specify any mandatory-to-implement Evidence types.¶

As an example of how to populate EvidenceStatementSet, implementing the TPM 2.0 and PSA Evidence types given below would result in the following EvidenceStatementSet definition:¶

EvidenceStatementSet EVIDENCE-STATEMENT ::= {
  --- TPM 2.0
  { Tcg-attest-certify IDENTIFIED BY tcg-attest-certify },
  ...,

  --- PSA
  { OCTET STRING IDENTIFIED BY { 1 3 6 1 5 5 7 1 99 } }
}

A.2. TPM V2.0 Evidence in CSR

This section describes TPM2 key attestation for use in a CSR.¶

A.2.1. TCG Key Attestation Certify

There are several ways in TPM2 to provide proof of a key's properties. (i.e., key attestation). This description uses the simplest and most generally expected to used which is the TPM2_Certify and the TPM2_ReadPublic commands.¶

A.2.2. TCG OIDs

The OIDs in this section are defined by TCG TCG has a registered arc of 2.23.133¶

id-tcg OBJECT IDENTIFIER ::= { 2 23 133 }¶

id-tcg-kp-AIKCertificate OBJECT IDENTIFIER ::= { id-tcg 8 3 }¶

id-tcg-attest OBJECT IDENTIFIER ::= { id-tcg TBD }¶

id-tcg-attest-certify OBJECT IDENTIFIER ::= { id-tcg-attest 1 }¶

A.2.3. TPM2 AttestationStatement

The EvidenceStatement structure contains a sequence of two fields: a type and a stmt. The 'type' field contains the OID of the Evidence format and it is set to tcg-attest-certify. The content of the structure shown below is placed into the stmt, which is a concatenation of existing TPM2 structures. These structures will be explained in the rest of this section.¶

Tcg-attest-certify ::= SEQUENCE {
  tcg-attest-certify-tpm2b_attest       TPM2B_ATTEST,
  tcg-attest-certify-tpmt_signature     TPMT_SIGNATURE,
  tcg-attest-certify-tpm2b_public   [0] TPM2B_PUBLIC OPTIONAL,
  tcg-kp-AIKCertificate             [1] OCTET STRING OPTIONAL
}

The tcg-kp-AIKCertificate field contains the AIK Certificate in RFC 5280 format.¶

A.3. Introduction to TPM2 concepts

The definitions in the following sections are defined by the TPM2 and various TCG defined specification including the TPM2 set of specifications. Those familiar with TPM2 concepts may skip to Appendix A.2.3 which defines an ASN.1 structure specific for bundling a TPM attestation into an EvidenceStatement, and Appendix A.3.2 which provides the example. For those unfamiliar with TPM2 concepts this section provides only the minimum information to understand TPM2 Attestation in CSR and is not a complete description of the technology in general.¶

A.3.1. TCG Objects and Key Attestation

This provides a brief explanation of the relevant TPM2 commands and data structures needed to understand TPM2 Attestation used in this RFC. NOTE: The TPM2 specification used in this explanation is version 1.59, section number cited are based on that version. Note also that the TPM2 specification comprises four documents: Part 1: Architecture; Part 2: Structures; Part 3: Commands; Part 4: Supporting Routines.¶

Note about convention: All structures starting with TPM2B_ are:¶

a structure that is a sized buffer where the size of the buffer is contained in a 16-bit, unsigned value.¶
The first parameter is the size in octets of the second parameter. The second parameter may be any type.¶

A full explanation of the TPM structures is outside the scope of this document. As a simplification references to TPM2B_ structures will simply use the enclosed TPMT_ structure by the same name following the '_'.¶

A.3.1.1. TPM2 Object Names

All TPM2 Objects (e.g., keys are key objects which is the focus of this specification). A TPM2 object name is persistent across the object's life cycle whether the TPM2 object is transient or persistent.¶

A TPM2 Object name is a concatenation of a hash algorithm identifier and a hash of the TPM2 Object's TPMT_PUBLIC.¶

     Name ≔ nameAlg || HnameAlg (handle→publicArea)
     nameAlg is a TCG defined 16 bit algorithm identifier

publicArea is the TPMT_PUBLIC structure for that TPM2 Object.¶

The size of the Name field can be derived by examining the nameAlg value, which defines the hashing algorithm and the resulting size.¶

The Name field is returned in the TPM2B_ATTEST data field.¶

     typedef struct {
          TPM_GENERATED magic;
          TPMI_ST_ATTEST type;
          TPM2B_NAME qualifiedSigner;
          TPM2B_DATA extraData;
          TPMS_CLOCK_INFO clockInfo;
          UINT64 firmwareVersion;
          TPMU_ATTEST attested;
     } TPMS_ATTEST;

where for a key object the attested field is¶

     typedef struct {
          TPM2B_NAME name;
          TPM2B_NAME qualifiedName;
     } TPMS_CERTIFY_INFO;

A.3.1.2. TPM2 Public Structure

Any TPM2 Object has an associated TPM2 Public structure defined as TPMT_PUBLIC. This is defined below as a 'C' structure. While there are many types of TPM2 Objects each with its own specific TPMT_PUBLIC structure (handled by the use of 'unions') this document will specifically define TPMT_PUBLIC for a TPM2 key object.¶

     typedef struct {
          TPMI_ALG_PUBLIC type;
          TPMI_ALG_HASH nameAlg;
          TPMA_OBJECT objectAttributes;
          TPM2B_DIGEST authPolicy;
          TPMU_PUBLIC_PARMS parameters;
          TPMU_PUBLIC_ID unique;
     } TPMT_PUBLIC;

Where: * type and nameAlg are 16 bit TCG defined algorithms. * objectAttributes is a 32 bit field defining properties of the object, as shown below¶

     typedef struct TPMA_OBJECT {
          unsigned Reserved_bit_at_0 : 1;
          unsigned fixedTPM : 1;
          unsigned stClear : 1;
          unsigned Reserved_bit_at_3 : 1;
          unsigned fixedParent : 1;
          unsigned sensitiveDataOrigin : 1;
          unsigned userWithAuth : 1;
          unsigned adminWithPolicy : 1;
          unsigned Reserved_bits_at_8 : 2;
          unsigned noDA : 1;
          unsigned encryptedDuplication : 1;
          unsigned Reserved_bits_at_12 : 4;
          unsigned restricted : 1;
          unsigned decrypt : 1;
          unsigned sign : 1;
          unsigned x509sign : 1;
          unsigned Reserved_bits_at_20 : 12;
     } TPMA_OBJECT;

authPolicy is the Policy Digest needed to authorize use of the object.¶
Parameters are the object type specific public information about the key.¶
- For key objects, this would be the key's public parameters.¶
unique is the identifier for parameters¶

The size of the TPMT_PUBLIC is provided by the following structure:¶

     typedef struct {
          UINT16     size;
          TPMT_PUBLIC publicArea;
     } TPM2B_PUBLIC;

A.3.1.3. TPM2 Signatures

TPM2 signatures use a union where the first field (16 bits) identifies the signature scheme. The example below shows an RSA signature where TPMT_SIGNATURE->sigAlg will indicate to use TPMS_SIGNATURE_RSA as the signature.¶

     typedef struct {
          TPMI_ALG_SIG_SCHEME sigAlg;
          TPMU_SIGNATURE signature;
     } TPMT_SIGNATURE;

     typedef struct {
          TPMI_ALG_HASH hash;
          TPM2B_PUBLIC_KEY_RSA sig;
     } TPMS_SIGNATURE_RSA;

A.3.1.4. Attestation Key

The uniquely identifying TPM2 key is the Endorsement Key (the EK). As this is a privacy sensitive key, the EK is not directly used to attest to any TPM2 asset. Instead, the EK is used by an Attestation CA to create an Attestation Key (the AK). The AK is assumed trusted by the Verifier and is assume to be loaded in the TPM during the execution of the process described in the subsequent sections. The description of how to create the AK is outside the scope of this document.¶

A.3.1.5. Attester Processing

The only signed component is the TPM2B_ATTEST structure, which returns only the (key's) Name and the signature computed over the Name but no detailed information about the key. As the Name is comprised of public information, the Name can be calculated by the Verifier but only if the Verify knows all the public information about the Key.¶

The Attester's processing steps are as follows:¶

Using the TPM2 command TPM2_Certify obtain the TPM2B_ATTEST and TPMT_SIGNATURE structures from the TPM2. The signing key for TPMT_SIGNATURE is an Attention Key (or AK), which is assumed to be available to the TPM2 upfront. More details are provided in Appendix A.3.1.4 ¶

The TPM2 command TPM2_Certify takes the following input:¶

TPM2 handle for Key (the key to be attested to)¶
TPM2 handle for the AK (see Appendix A.3.1.4)¶

It produces the following output:¶

TPM2B_ATTEST in binary format¶
TPMT_SIGNATURE in binary format¶

Then, using the TPM2 command TPM2_ReadPublic obtain the Keys TPM2B_PUBLIC structure. While the Key's public information can be obtained by the Verifier in a number ways, such as storing it from when the Key was created, this may be impractical in many situations. As TPM2 provided a command to obtain this information, this specification will include it in the TPM2 Attestation CSR extension.¶

The TPM2 command TPM2_ReadPublic takes the following input:¶

TPM2 handle for Key (the key to be attested to)¶

It produces the following output:¶

TPM2B_PUBLIC in binary format¶

A.3.1.6. Verifier Processing

The Verifier has to perform the following steps once it receives the Evidence:¶

Verify the TPM2B_ATTEST using the TPMT_SIGNATURE.¶
Use the Key's "expected" Name from the provided TPM2B_PUBLIC structure. If Key's "expected" Name equals TPM2B_ATTEST->attestationData then returned TPM2B_PUBLIC is the verified.¶

A.3.2. Example Structures

TODO -- a full CSR would be great.¶

A.4. PSA Attestation Token in CSR

The Platform Security Architecture (PSA) Attestation Token is defined in [I-D.tschofenig-rats-psa-token] and specifies claims to be included in an Entity Attestation Token (EAT). [I-D.bft-rats-kat] defines key attestation based on the EAT format. In this section the platform attestation offered by [I-D.tschofenig-rats-psa-token] is combined with key attestation by binding the key attestation token (KAT) to the platform attestation token (PAT) with the help of the nonce. For details see [I-D.bft-rats-kat]. The resulting KAT-PAT bundle is, according to Section 5.1 of [I-D.bft-rats-kat], combined in a CMW collection [I-D.ietf-rats-msg-wrap].¶

The encoding of this KAT-PAT bundle is shown in the example below.¶

EvidenceBundles
 +
 |
 +-> EvidenceBundle
      +
      |
      +->  EvidenceStatement
            +
            |
            +-> type: OID for CMW Collection
            |         1 3 6 1 5 5 7 1 TBD
            |
            +-> stmt: KAT/PAT CMW Collection

The value in EvidenceStatement->stmt is based on the KAT/PAT example from Section 6 of [I-D.bft-rats-kat] and the result of CBOR encoding the CMW collection shown below (with line-breaks added for readability purposes):¶

{
  "kat":
    h'd28443A10126A058C0A30A5820B91B03129222973C214E42BF31D68
      72A3EF2DBDDA401FBD1F725D48D6BF9C8171909C4A40102200121
      5820F0FFFA7BA35E76E44CA1F5446D327C8382A5A40E5F29745DF
      948346C7C88A5D32258207CB4C4873CBB6F097562F61D5280768C
      D2CFE35FBA97E997280DBAAAE3AF92FE08A101A40102200121582
      0D7CC072DE2205BDC1537A543D53C60A6ACB62ECCD890C7FA27C9
      E354089BBE13225820F95E1D4B851A2CC80FFF87D8E23F22AFB72
      5D535E515D020731E79A3B4E47120584056F50D131FA83979AE06
      4E76E70DC75C070B6D991AEC08ADF9F41CAB7F1B7E2C47F67DACA
      8BB49E3119B7BAE77AEC6C89162713E0CC6D0E7327831E67F3284
      1A',
  "pat":
    h'd28443A10126A05824A10A58205CA3750DAF829C30C20797EDDB794
      9B1FD028C5408F2DD8650AD732327E3FB645840F9F41CAB7F1B7E
      2C47F67DACA8BB49E3119B7BAE77AEC6C89162713E0CC6D0E7327
      831E67F32841A56F50D131FA83979AE064E76E70DC75C070B6D99
      1AEC08AD'
}