Internet-Draft PKI-based Attestation Evidence July 2024
Ounsworth, et al. Expires 9 January 2025 [Page]
Workgroup:
RATS
Internet-Draft:
draft-ounsworth-rats-pkix-evidence-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
M. Ounsworth
Entrust
R. Kettlewell
Entrust
JPF
Crypto4A
H. Tschofenig
Siemens
T. Reddy
Nokia
M. Wiseman
Beyond Identity

PKI-based Attestation Evidence

Abstract

This document specifies ASN.1 structures produced by an Attester as part of the remote attestation procedures and constitute Evidence.

This document follows the Remote ATtestation procedureS (RATS) architecture where Evidence is sent by an Attester and processed by a Verifier.

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 9 January 2025.

Table of Contents

1. Introduction

Trusted execution environments, like secure elements and hardware security modules (HSMs), are now widely used, which provide a safe environment to place cryptographic key material and security sensitive code which uses it, such as signing and decryption services, secure boot, secure storage, and other essential security functions. These security functions are typically exposed through a narrow and well-defined interface, and can be used by operating system libraries and applications.

Increasingly, parties that rely on these secure elements want evidence that the security sensitive operations are in fact being performed within a secure element. This evidence can pertain to the secure element platform itself, or to the storage and protection properties of the cryptographic keys, or both. This is generally referred to as remote attestation, and is covered by the Remote ATtestation procedureS (RATS) architecture [RFC9344]. This document species an evidence data format specified in ASN.1 and re-using many data structures from the PKIX ASN.1 modules [RFC5912] so to be a convenient format for secure elements and verifiers that are designed primarily for use within X.509 Public Key Infrastructures.

When a Certificate Signing Request (CSR) library is requesting a certificate from a Certification Authority (CA), a PKI end entity may wish to provide Evidence of the security properties of the trusted execution environment in which the private key is stored. This Evidence is to be verified by a Relying Party, such as the Registration Authority or the Certification Authority as part of validating an incoming CSR against a given certificate policy. [I-D.ietf-lamps-csr-attestation] defines how to carry Evidence in either PKCS#10 [RFC2986] or Certificate Request Message Format (CRMF) [RFC4211].

[I-D.ietf-lamps-csr-attestation] is agnostic to the content and the encoding of Evidence. To offer interoperability it is necessary to define a format that is utilized in a specific deployment environment. Hardware security modules and other trusted execution environments, which have been using ASN.1-based encodings for a long time prefer the use of the same format throughout their software ecosystem. For those use cases this specification has been developed.

This specification re-uses the claims defined in [I-D.ietf-rats-eat]. While the encoding of the claims is different to what is defined in [I-D.ietf-rats-eat], the semantics of the claims is retained. This specification is not an EAP profile, as defined in Section 6 of [I-D.ietf-rats-eat]

This specification was designed to meet the requirements published by the CA Browser Forum to convey properties about hardware security models, such as non-exportability, which must be enabled for storing publicly-trusted code-signing keys. Hence, this specification is supposed to be used with the attestation extension for Certificate Signing Requests (CSRs), see [I-D.ietf-lamps-csr-attestation].

There are, however, other use cases where remote attestation may also be used, such as

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.

This document re-uses the terms defined in [RFC9334] related to remote attestation. Readers of this document are assumed to be familiar with the following terms: Evidence, Claim, Attestation Result, Attester, Verifier, and Relying Party.

3. Attestation Evidence

This specification defines the following Evidence format, which contains a set of claims. To protect Evidence against modification, it is protected with a digital signature.

PkixEvidenceStatement ::= SEQUENCE {
  tbsEvidence TBSEvidenceStatement
  signatureValues SEQUENCE SIZE (1..MAX) OF BIT STRING,
  relatedCertificates [0] IMPLICIT SEQUENCE of Certificate OPTIONAL
  -- As defined in RFC 5280
}

TBSEvidenceStatement ::= SEQUENCE {
  version INTEGER,
  claims SEQUENCE SIZE (1..MAX) OF EVIDENCE-CLAIM,
  signatureInfos SEQUENCE SIZE (1..MAX) OF SignatureInfo
}

EVIDENCE-CLAIM ::= TYPE-IDENTIFIER

-- TYPE-IDENTIFIER definition from X.681
TYPE-IDENTIFIER ::= CLASS
{
    &id OBJECT IDENTIFIER UNIQUE,
    &Type
}
WITH SYNTAX {&Type IDENTIFIED BY &id}

SignatureInfo ::= SEQUENCE {
   signatureAlgorithm AlgorithmIdentifier,
   sid [0] SignerIdentifier OPTIONAL
}

SignerIdentifier ::= SEQUENCE {
   keyId [0] EXPLICIT OCTET STRING OPTIONAL,
   subjectKeyIdentifier [1] EXPLICIT SubjectPublicKeyInfo OPTIONAL,
     -- As defined in RFC 5280
   certificate [2] EXPLICIT Certificate OPTIONAL,
     -- As defined in RFC 5280
   certHash [3] EXPLICIT CertHash OPTIONAL
}

CertHash ::= SEQUENCE {
    hash AlgorithmIdentifier,
    value OCTET STRING
}
-- There is bound to already exist an ASN.1 structure for this somewhere.

AlgorithmIdentifier ::= SEQUENCE {
   algorithm OBJECT IDENTIFIER,
   parameters ANY DEFINED BY algorithm OPTIONAL
}

version MUST be set to 1.

4. Signing and Verification Procedure

EDNOTE: Can we start our versions at some number to avoid versions that Crypto4A has already used?

4.1. Signing Procedure

  1. The message to be signed is the TBSEvidenceStatement, including the SignatureInfo for each of the signatures to be performed.

  2. Each signature is computed in parallel and placed into index of the signatureValues SEQUENCE that matches the position of the corresponding SignatureInfo in the signatureInfos sequence.

The signer MUST produce one signature per signatureInfo, it MUST NOT omit signatures and MUST NOT produce a subset of the signatures listed in signatureInfos.

4.2. Verification Procedure

  1. The message to be verified is the TBSEvidenceStatement.

  2. For each signatureInfo, the corresponding verification public key and signature algorithm is found according to the information contained in the SignatureInfo for that signature and any accompanying certificates or key material.

  3. For each signature, the message is verified using the value from the corresponding element of the signatureValue sequence.

  4. The PkixEvidenceStatement SHOULD be considered valid if and only if all signatures are valid; i.e. multiple signatures are to be treated as an AND mode. This item is a recommendation and not a hard requirement since verification policy is of course at the discretion of the Verifier.

EDNOTE: the major change here from the original Crypto4A QASM Attestation is that the original only includes the claims in the signature, whereas this includes everything, including the version, list of signature algorithms. This prevents possible attacks where those values are manipulated by attackers. We should debate whether the certificates should be protected by the signature. Pro: generally better for security to sign everything. Con: in some contexts, it may be difficult to have the certificates prior to signing, but that's ok because most evidence carrier formats also allow you to attach the signatures externally.

5. Claims

Since no claims are marked as MANDATORY, the sequence 'claims' may be constituted of differing claims from one instance to the next. This is expected as each evidence statement may be providing information to support different use cases.

Once an evidence statement is signed, the Attester is guaranteeing that all of the claims carried by the evidence statement are true.

It is important to note that multiple claims in the sequence may have the same 'id'. Implementers should ensure that this case is handled by verifying logic.

For ease of reading, claims have been separated into two lists: "platform claims" and "key claims".

5.1. Platform Claims

| Claim          | OID      | Value        | Section           | Status       |
| --------       | -------- | ------------ | ----------------- | ------------ |
| Oemid          | TBD      | UTF8String   | {{sect-deviceID}} | RECOMMENDED  |
| Hwmodel        | TBD      | UTF8String   | {{sect-deviceID}} | RECOMMENDED  |
| Hwversion      | TBD      | UTF8String   | {{sect-deviceID}} | RECOMMENDED  |
| Hwserial       | TBD      | UTF8String   | {{sect-deviceID}} | RECOMMENDED  |
| Ueid           | TBD      | UTF8String   | {{sect-ueid}    } | OPTIONAL     |
| Sueid          | TBD      | UTF8String   | {{sect-sueid}}    | OPTIONAL     |
| EnvID          | TBD      | UTF8String   | {{sect-envID}}    | OPTIONAL     |
| Swname         | TBD      | UTF8String   | {{sect-swID}}     | RECOMMENDED  |
| Swversion      | TBD      | UTF8String   | {{sect-swID}}     | RECOMMENDED  |
| Oemboot        | TBD      | BOOLEAN      | {{sect-oemboot}}  | RECOMMENDED  |
| Location       | TBD      | ???          | {{sect-location}} | OPTIONAL     |
| Dbgstat        | TBD      | CHOICE       | {{sect-dbgstat}}  | RECOMMENDED  |
| Uptime         | TBD      | INTEGER      | {{sect-uptime}}   | OPTIONAL     |
| Bootcount      | TBD      | INTEGER      | {{sect-bootcount}}| OPTIONAL     |
| Bootseed       | TBD      | BIT STRING   | {{sect-bootseed}} | OPTIONAL     |
| Dloas          | TBD      | SEQUENCE OF Dloa | {{sect-dloas}}    | OPTIONAL     |
| Endorsements   | TBD      | SEQUENCE of Endorsement | {{sect-endorsements}} | OPTIONAL |
| Manifests      | TBD      | ??           | {{sect-manifests}} | OPTIONAL    |
| Measurements   | TBD      | ??           | {{sect-measurements}} | OPTIONAL    |
| Measres        | TBD      | ??           | {{sect-measres}}   | OPTIONAL    |
| Submods        | TBD      | ??           | {{sect-submods}}   | OPTIONAL    |
| Iat            | TBD      | Time         | {{sect-iat}}       | RECOMMENDED |
| FipsMode       | TBD      | Boolean      | {{sect-fipsmode}}  | RECOMMENDED |
| VendorInfo     | TBD      | TYPE-IDENTIFIER | {{sect-vendorinfo}}| OPTIONAL    |
| NestedEvidences| TBD      | SEQUENCE OF PkixEvidenceStatement | {{sect-nestedevidences}} | OPTIONAL |
| Nonce          | TBD      | OCTET STRING | {{sect-nonce}}     | OPTIONAL    |

5.2. Key Claims

| Claim          | OID      | Value        | Section           | Status       |
| --------       | -------- | ------------ | ----------------- | ------------ |
| KeyId          | TBD      | IA5String    | {{sect-keyid}}    | OPTIONAL     |
| PubKey         | TBD      | OCTET STRING | {{sect-pubkey}}   | RECOMMENDED  |
| Purpose        | TBD      | CHOICE       | {{sect-purpose}}  | RECOMMENDED  |
| NonExportable  | TBD      | BOOLEAN      | {{sect-nonexportable}} | RECOMMENDED |
| Imported       | TBD      | BOOLEAN      | {{sect-imported}} | RECOMMENDED  |
| KeyExpiry      | TBD      | Time         | {{sect-keyexpiry}}| OPTIONAL     |

Even though no specific claims are required, a Verifier or Relying Party MAY reject an Evidence claim if it is missing information required by the appraisal policy. For example, a Relying Party which requires a FIPS-certified device SHOULD reject Evidence if it does not contain sufficient information to determine the FIPS certification status of the device.

5.3. Device Identifier

Devices assigned a Universal Entity ID compliant with RATS EAT SHOULD provide this in the Ueid or Sueid claim. Devices with a traditional human-readable serial number SHOULD provide this in the Hwserial claim. Both MAY be provided.

The set {OemID, Hwmodel, Hwversion, Hwserial}, when provided, SHOULD represent a universally unique identification of the device. Where applicable, {OemID, Hwmodel, Hwversion} SHOULD match the way the device is identified in relevant endorsements, such as published FIPS or Common Criteria certificates.

5.3.1. ueid (Universal Entity ID) Claim

The "ueid" claim conveys a UEID, which identifies an individual manufactured entity. This identifier is a globally unique and permanent identifier. See Section 4.2.1 of [I-D.ietf-rats-eat] for a description of this claim. Three types of UEIDs are defined, which are distinguished via a type field.

The ueid claim is defined as follows:

   id-ce-evidence-ueid OBJECT IDENTIFIER ::=
         { id-ce TBD_evidence TBD_ueid }

   claim_ueid ::= SEQUENCE {
       type    INTEGER ( RAND(1), EUI(2), IMEI(3),...),
       value   OCTET STRING
   }

5.3.2. sueids (Semi-permanent UEIDs) Claim (SUEIDs)

The "sueids" claim conveys one or more semi-permanent UEIDs (SUEIDs). An SUEID has the same format, characteristics and requirements as a UEID, but MAY change to a different value on entity life-cycle events while the ueid claim is permanent. An entity MAY have both a UEID and SUEIDs, neither, one or the other.

There MAY be multiple SUEIDs and each has a text string label the purpose of which is to distinguish it from others.

See Section 4.2.2 of [I-D.ietf-rats-eat] for a description of this claim.

The sueids claim is defined as follows:

   id-ce-evidence-sueids OBJECT IDENTIFIER ::=
         { id-ce TBD_evidence TBD_sueids }

   claim_sueids ::= SEQUENCE {
       label   OCTET STRING,
       type    INTEGER ( RAND(1), EUI(2), IMEI(3),...),
       value   OCTET STRING
   }

5.3.3. oemid (Hardware OEM Identification) Claim

The "oemid" claim identifies the Original Equipment Manufacturer (OEM) of the hardware.

See Section 4.2.3 of [I-D.ietf-rats-eat] for a description of this claim.

The value of this claim depends on the type of OEMID and three types of IDs are defined:

  • OEMIDs using a 128-bit random number. Section 4.2.3.1 of [I-D.ietf-rats-eat] defines this type.

  • an IEEE based OEMID using a global registry for MAC addresses and company IDs. Section 4.2.3.1 of [I-D.ietf-rats-eat] defines this type.

  • OEMIDs using Private Enterprise Numbers maintained by IANA. Section 4.2.3.3 of [I-D.ietf-rats-eat] defines this type.

The oemid claim is defined as follows:

   id-ce-evidence-oemid OBJECT IDENTIFIER ::=
         { id-ce TBD_evidence TBD_oemid }

   claim_oemid ::= SEQUENCE {
       type    INTEGER ( PEN(1), IEEE(2), RANDOM(3),...),
       value   OCTET STRING
   }

Editor's Note: The value for the PEN is numeric. For the other two types it is a binary string.

5.3.4. hwmodel (Hardware Model) Claim

The "hwmodel" claim differentiates hardware models, products and variants manufactured by a particular OEM, the one identified by OEM ID. It MUST be unique within a given OEM ID. The concatenation of the OEM ID and "hwmodel" give a global identifier of a particular product. The "hwmodel" claim MUST only be present if an "oemid" claim is present.

See Section 4.2.4 of [I-D.ietf-rats-eat] for a description of this claim.

The hwmodel claim is defined as follows:

   id-ce-evidence-hwmodel OBJECT IDENTIFIER ::=
         { id-ce TBD_evidence TBD_hwmodel }

   claim_hwmodel ::= OCTET STRING

5.3.5. hwversion (Hardware Version) Claim

The "hwversion" claim is a text string the format of which is set by each manufacturer. A "hwversion" claim MUST only be present if a "hwmodel" claim is present.

See Section 4.2.5 of [I-D.ietf-rats-eat] for a description of this claim.

The hwversion claim is defined as follows:

   id-ce-evidence-hwversion OBJECT IDENTIFIER ::=
         { id-ce TBD_evidence TBD_hwwversion }

   hwversion ::= OCTET STRING

5.4. Environment Identifier

EnvID EVIDENCE-CLAIM ::= UTF8String IDENTIFIED BY TBD

This claim MAY be used to identify a partition within a cryptographic device, or a logical environment that spans multiple cryptographic devices such as a Security World or a cloud tenant. The format of these identifiers will be vendor or environment specific.

5.5. Software Identifier

Swname EVIDENCE-CLAIM ::= UTF8String IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.6
Swversion EVIDENCE-CLAIM ::= UTF8String IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.7

SwName and Swversion together identify the device firmware and SHOULD match the way the firmware is identified in relevant endorsements, such as published FIPS or Common Criteria certificates.

5.6. OEM Boot

Oemboot EVIDENCE-CLAIM ::= BOOLEAN IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.8

5.7. Dbgstat (Debug Status)

The "dbgstat" claim applies to entity-wide or submodule-wide debug facilities and diagnostic hardware built into chips. It applies to any software debug facilities related to privileged software that allows system-wide memory inspection, tracing or modification of non-system software like user mode applications.

See Section 4.2.9 of [I-D.ietf-rats-eat] for a description of this claim and the semantic of the values in the enumerated list.

The dbgstat claim is defined as follows:

Dbgstat EVIDENCE-CLAIM ::= CHOICE {
    enabled                         [0] IMPLICIT NULL,
    disabled                        [1] IMPLICIT NULL,
    disabled-Since-Boot             [2] IMPLICIT NULL,
    disabled-Permanently            [3] IMPLICIT NULL,
    disabled-Fully-and-Permanently  [4] IMPLICIT NULL
}
  -- semantics defined in rats-eat-4.2.9

5.8. Location

Location EVIDENCE-CLAIM ::= ???? IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.10

Most HSMs will likely not know their own physical location, but cryptographic modules on mobile devices may.

5.9. Uptime

The "uptime" claim contains the number of seconds that have elapsed since the entity or submodule was last booted.

Uptime EVIDENCE-CLAIM ::= INTEGER IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.11

5.10. Bootcount {sect-bootcount}

The "bootcount" claim contains a count of the number times the entity or submodule has been booted. Support for this claim requires a persistent storage on the device.

Bootcount EVIDENCE-CLAIM ::= INTEGER IDENTIFIER BY TBD
  -- semantics defined in rats-eat-4.2.12

5.11. Bootseed

The "bootseed" claim contains a value created at system boot time that allows differentiation of attestation reports from different boot sessions of a particular entity (e.g., a certain UEID).

This value is usually public. It is not a secret and MUST NOT be used for any purpose that a secret seed is needed, such as seeding a random number generator.

Bootseed EVIDENCE-CLAIM ::= BIT STRING IDENTIFIED BY TBD
  -- semantics defined in rats-eat-4.2.13

5.12. dloas (Digital Letters of Approval)

The "dloas" claim conveys one or more Digital Letters of Approval (DLOAs). A DLOA is a document that describes a certification that an entity has received. Examples of certifications represented by a DLOA include those issued by Global Platform and those based on Common Criteria. The DLOA is unspecific to any particular certification type or those issued by any particular organization.

Dloas EVIDENCE-CLAIM ::= SEQUENCE SIZE (1..MAX) OF Dloa

Dloa ::= SEQUENCE IDENTIFIED BY TBD {
    dloaRegistrar IA5STRING,
    dloaPlatformLabel UTF8STRING,
    dloaApplicationLabal [0] IMPLICIT UTF8String OPTIONAL
}
  -- semantics defined in rats-eat-4.2.14

5.13. Endorsements

This claim allows referencing third party endorsements; for example from the device vendor or a certification such as FIPS or Common Criteria. The content MAY be referenced by URI, or placed directly inline, but either way, the endorsement content or its URI MUST be known by the attester at the time that the evidence is generated.

Endorsements EVIDENCE-CLAIM ::= SEQUENCE SIZE (1..MAX) OF Endorsement

Endorsement ::= CHOICE IDENTIFIED BY TBD {
    uri     [0] IMPLICIT IA5String,
    content [1] IMPLICIT OCTET STRING
}

EDNOTE: this needs a bit of thought about what types of endorsements we will likely see, and whether OCTET STRING is the right format.

5.14. Manifests

TODO -- rats-eat-4.2.15

5.15. Measurements

TODO -- rats-eat-4.2.16

5.17. Submods (Submodules)

TODO -- rats-eat-4.2.18

5.18. iat (Issuance Time)

The time at which the evidence was created. Here we differ from the iat claim in rats-eat-4.3.1 in that we use the PKIX time format Time instead of the 64-bit CBOR time structure.

Iat EVIDENCE-CLAIM ::= Time

It is recognized that many HSMs, especially if air-gapped, will not have an accurate system clock. If the system is not anticipated to have a reliable clock, then this claim SHOULD be omitted and the Nonce claim used instead.

5.19. intuse (Intended Use)

Intuse EVIDENCE-CLAIM ::= CHOICE IDENTIFIED BY TBD {
    generic              [1] IMPLICIT NULL,
    registration         [2] IMPLICIT NULL,
    provisioning         [3] IMPLICIT NULL,
    certificateIssuance  [4] IMPLICIT NULL,
    proofOfPossession    [5] IMPLICIT NULL
}
  -- semantics defined in rats-eat-4.3.3

Note: tags intentionally started at 1 to align with EAT. If the IANA registry of intended use claims is extended, then the this CHOICE MAY be extended using the same tag values as indicated in the EAT registry.

5.20. FipsMode

The cryptographic module was booted in FIPS mode, including the required self-tests and any other requiremnts of its FIPS certificate.

Note to verifiers and relying parties: "FIPS Mode" does not imply "FIPS Certified". For example, a device may have a FIPS Mode even if the device was never submitted for FIPS certification. This claim SHOULD only be taken in conjunction with a valid FIPS certification for this hardware and software version, and appraising any other claims as required by the FIPS certification.

FipsMode EVIDENCE-CLAIM ::= BOOLEAN IDENTIFIED BY TBD

5.21. VendorInfo

This claim provides a place for vendor to place propriatary data; i.e. any proprietary data that does not fit in any other claim.

VendorInfo ::= TYPE-IDENTIFIER IDENTIFIED BY TBD

Vendors must specify an OID and data type for their VendorInfo, and communicate this to verifiers who wish to parse this data.

5.22. NestedEvidences

NestedEvidences EVIDENCE-CLAIM ::= SEQUENCE OF PkixEvidenceStatement IDENTIFIED BY TBD

Composite devices may produce multiple signed evidence statements that need to be signed in a hiearchical manner. PkixEvidenceStatements MAY be nested.

5.23. Nonce

The "nonce" claim is used to provide freshness.

The Nonce claim is used to carry the challenge provided by the caller to demonstrate freshness of the generated token. The following constraints apply to the nonce-type:

  • The length must be reasonable as it may be processed by end entities with limited resources. Therefore, it is RECOMMENDED that the length does not exceed 64 bytes.

  • Only a single nonce value is conveyed.

The nonce claim is defined as follows:

Nonce EVIDENCE-CLAIM ::= OCTET STRING IDENTIFIED BY TBD

See Section 4.1 of [I-D.ietf-rats-eat] for a description of this claim.

5.24. KeyId

An identifier for the subject key. The format MAY be vendor-specific, but MUST be an ASCII value (IA5String).

KeyId EVIDENCE-CLAIM ::= IA5String IDENTIFIED BY TBD

5.25. PubKey

The subject public key being attested by this evidence.

PubKey EVIDENCE-CLAIM ::= OCTET STRING IDENTIFIED BY TBD

5.26. Purpose

TODO: align with PKCS#11 Purposes

Purpose EVIDENCE-CLAIM ::= CHOICE IDENTIFIED BY TBD {
   ... Sign, Decrypt, Unwrap, etc..

}

5.27. NonExportable

TODO align with PKCS#11

NonExportable EVIDENCE-CLAIM ::= BOOLEAN IDENTIFIED BY TBD

5.28. Imported

TODO align with PKCS#11

Imported EVIDENCE-CLAIM ::= BOOLIAN IDENTIFIED BY TBD

5.29. KeyExpiry

If the key has a known expiry time or "not after" date.

KeyExpiry EVIDENCE-CLAIM ::= Time

5.30. Unrecognized claims

This document does not define an exhaustive list of claims. New claims may be added in the future, including proprietary ones. As such, parsers SHOULD expect to encounter unrecognized claims, and to handle them gracefully.

In general, the correct behaviour for a verifier will be to start with an appraisal policy of claims to look for, and where appropriate the expected values (for example, FipsMode: true), and any additional claims that may be in the evidence SHOULD be ignored.

6. Evidence Claims Certificate Extension

This section specifies the syntax and semantics of the Evidence Claims certificate extension which provides a list of claims associated with the certificate subject appraised by the CA.

The Evidence Claims certificate extension MAY be included in public key certificates [RFC5280]. The Evidence Claims certificate extension MUST be identified by the following object identifier:

     id-pe-evidenceclaims OBJECT IDENTIFIER  ::=
        { iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanisms(5) pkix(7) id-pe(1) 34 }

This extension MUST NOT be marked critical.

The Evidence Claims extension MUST have the following syntax:

EvidenceClaims ::= SET SIZE (1..MAX) OF EVIDENCE-CLAIM

The EvidenceClaims represents an unsigned version of the evidence claims appraised by the CA. It MUST contain at least one claim. For privacy reasons, the CA MAY include only a subset of the EvidenceClaims that were presented to it, for example in an EvidenceBundle in a CSR. The CA may include in their certificate profile a list of verified evidence claims (identified by OID) that MAY be copied from the CSR to the certificate, while any other claims MUST NOT be copied. By removing the signature from the evidence, the CA is asserting that it has has verified the Evidence to chain to a root that the CA trusts, but it is not required to disclose in the final certificate what that root is.

See Section 8 for a discussion of privacy concerns related to re-publishing Evidence into a certificate.

6.1. ASN.1 Module

This section provides an ASN.1 Module [X.680] for the Evidence Claims certificate extension, and it follows the conventions established in [RFC5912] and [RFC6268].

<CODE BEGINS>
     EvidenceClaimsCertExtn
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-evidenceclaims(TBD) }

     DEFINITIONS IMPLICIT TAGS ::=
     BEGIN

     IMPORTS
       EXTENSION
       FROM PKIX-CommonTypes-2009  -- RFC 5912
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkixCommon-02(57) } ;

     -- Evidence Claims Certificate Extension

     ext-EvidenceClaims EXTENSION ::= {
       SYNTAX EvidenceClaims
       IDENTIFIED BY id-pe-evidenceclaims }

     -- EvidenceClaims Certificate Extension OID

     id-pe-evidenceclaims OBJECT IDENTIFIER ::=
        { iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanisms(5) pkix(7) id-pe(1) 34 }

     -- Evidence Claims Certificate Extension Syntax

     EvidenceClaims ::= SET SIZE (1..MAX) OF EVIDENCE-CLAIM

     END

<CODE ENDS>

7. Implementation Considerations

7.1. API for requesting evidence from an attesting device

While it is strictly outside the scope of this document to specify how a calling application can request evidence from a cryptographic device, two modes are suggested.

7.1.1. Request by ID and claim profile

In this mode, the calling application request evidence about a given entity -- for example, a given EnvID or a given KeyID -- and the cryptographic device assembles a PkixEvidenceStatement containing as many claims as it is able to populate. Implementers may have named evidence profiles if it is desirable for the cryptographic device to respond with multiple different sets of claims.

7.1.2. Request by claim set

In this mode, the calling application pre-constructs a sequence of EVIDENCE-CLAIM which is passed in to the attesting device. As a response, the attesting device returns a structure of type PkixEvidenceStatement which includes all the expected signatures.

This mode is useful for attesting devices with more resources and used in situations where the supported evidence profiles may not be known during implementation.

It is left to the implementer to choose the way that the desired claims are submitted to the attesting device, including which types of claims are recognized and how much information is provided by the caller.

However, when using this mode: - an attesting device MUST reject the production of a PkixEvidenceStatement if any requested claim is not recognized; and, - an attesting device MUST reject the production of a PkixEvidenceStatement if any requested claim is not supported by the observed state (claim is deemed false).

The use of this mode implies that the attesting device contains the logic necessary to interpret and verify the submitted claims.

8. Privacy Considerations

8.1. Publishing Evidence in a certificate

The extension MUST NOT publish in the certificate any privacy-sensitive information that could compromise the end device. What counts as privacy-sensitive will vary by use case. For example, consider a few scenarios:

First, consider a Hardware Security Module (HSM) backing a public code-signing service. The model and firmware patch level could be considered sensitive as it could give an attacker an advantage in exploiting known vulnerabilities against un-patched systems.

Second, consider a certificate issued to a end-user mobile computing device, any sort of unique identifier could be used as a super-cookie for tracking purposes.

Third, consider small IoT devices such as un-patchable wireless sensors. Here there may be no privacy concerns and in fact knowing exact hardware and firmware version information could help edge gateways to deny network access to devices with known vulnerabilities.

Beyond that, a CA MUST have a configurable mechanism to control which information is to be copied from the provided Evidence into the certificate, for example this could be configured within a certificate profile or Certificate Practice Statement (CPS) and this must be considered on a case-by-base basis. To protect end-user privacy, CA operators should err on the side of caution and exclude information that is not clearly essential for security verification by relying parties. Avoiding unnecessary claims also mitigates the risk of targeted attacks, where an attacker could exploit knowledge of hardware versions, models, etc.

9. Security Considerations

This specification re-uses the claims from the EAT specification and relies on the security protection offered by digital signatures. This digital signature is computed with the Attestation Key available on the device, see Section 12.1 of [RFC9334] for considerations regarding the generation, the use and the protection of these Attestation Keys. Since the Attester located at the end entity creates the Evidence with claims defined in this document. This document inherits the remote attestation architecture described in [RFC9334]. With the re-use of the claims from [I-D.ietf-rats-eat] the security and privacy considerations apply also to this document even though the encoding in this specification is different from the encoding of claims discussed by [I-D.ietf-rats-eat].

Evidence contains information that may be unique to a device and may therefore allow to single out an individual device for tracking purposes. Deployments that have privacy requirements must take appropriate measures to ensure that claim values can only identify a group of devices and that the Attestation Keys are used across a number of devices as well.

To verify the Evidence, the primary need is to check the signature and the correct encoding of the claims. To produce the Attestation Result, the Verifier will use Endorsements, Reference Values, and Appraisal Policies. The policies may require that certain claims must be present and that their values match registered reference values. All claims may be worthy of additional appraisal.

10. IANA Considerations

TBD: OIDs for all the claims listed in this document.

10.1. OIDs for Evidence Claims Certificate Extension

For the EvidenceClaims certificate extension in Section 6, IANA is requested to assign an object identifier (OID) for the certificate extension. The OID for the certificate extension should be allocated in the "SMI Security for PKIX Certificate Extension" registry (1.3.6.1.5.5.7.1).

For the ASN.1 Module in Section 6.1, IANA is requested to assign an object identifier (OID) for the module identifier. The OID for the module should be allocated in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

11. References

11.1. Normative References

[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-28, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-eat-28>.
[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>.
[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>.
[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>.

11.2. Informative References

[I-D.ietf-lamps-csr-attestation]
Ounsworth, M., Tschofenig, H., Birkholz, H., Wiseman, M., and N. Smith, "Use of Remote Attestation with Certification Signing Requests", Work in Progress, Internet-Draft, draft-ietf-lamps-csr-attestation-10, , <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-csr-attestation-10>.
[RFC2986]
Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc5912>.
[RFC6268]
Schaad, J. and S. Turner, "Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI 10.17487/RFC6268, , <https://www.rfc-editor.org/rfc/rfc6268>.
[RFC9344]
Asaeda, H., Ooka, A., and X. Shao, "CCNinfo: Discovering Content and Network Information in Content-Centric Networks", RFC 9344, DOI 10.17487/RFC9344, , <https://www.rfc-editor.org/rfc/rfc9344>.
[X.680]
ITU-T, "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", n.d., <https://www.itu.int/rec/T-REC-X.680>.

Appendix A. Acknowledgements

This specification is the work of a design team created by the chairs of the LAMPS working group. This specification has been developed based on discussions in that design team.

The following persons, in no specific order, contributed to the work: Richard Kettlewell, Chris Trufan, Bruno Couillard, Jean-Pierre Fiset, Sander Temme, Jethro Beekman, Zsolt Rózsahegyi, Ferenc Pető, Mike Agrenius Kushner, Tomas Gustavsson, Dieter Bong, Christopher Meyer, Michael StJohns, Carl Wallace, Michael Ricardson, Tomofumi Okubo, Olivier Couillard, John Gray, Eric Amador, Johnson Darren, Herman Slatman, Tiru Reddy, Thomas Fossati, Corey Bonnell, Argenius Kushner, James Hagborg.

Appendix B. ASN.1 Module

TBD: Full ASN.1 goes in here.

Authors' Addresses

Mike Ounsworth
Entrust Limited
2500 Solandt Road – Suite 100
Ottawa, Ontario K2K 3G5
Canada
Richard Kettlewell
Entrust Limited
United Kingdom
Jean-Pierre Fiset
Crypto4A Technologies Inc.
1550A Laperriere Ave
Ottawa, Ontario K1Z 7T2
Canada
Hannes Tschofenig
Siemens
Tirumaleswar Reddy
Nokia
Bangalore
Karnataka
India
Monty Wiseman
Beyond Identity
United States of America