Internet-Draft CoRIM PSA Profile March 2023
Fossati, et al. Expires 11 September 2023 [Page]
Workgroup:
Remote ATtestation ProcedureS
Internet-Draft:
draft-fdb-rats-psa-endorsements-02
Published:
Intended Status:
Informational
Expires:
Authors:
T. Fossati
Arm Ltd
Y. Deshpande
Arm Ltd
H. Birkholz
Fraunhofer SIT

A CoRIM Profile for Arm's Platform Security Architecture (PSA)

Abstract

PSA Endorsements include reference values, endorsed values, cryptographic key material and certification status information that a Verifier may need in order to appraise attestation Evidence produced by a PSA device. This memo defines PSA Endorsements as a profile of the CoRIM data model.

Discussion Venues

This note is to be removed before publishing as an RFC.

Discussion of this document takes place on the Remote ATtestation ProcedureS Working Group mailing list (rats@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/rats/.

Source for this draft and an issue tracker can be found at https://github.com/thomas-fossati/corim-psa.

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 11 September 2023.

1. Introduction

PSA Endorsements include reference values, endorsed values, cryptographic key material and certification status information that a Verifier needs in order to appraise attestation Evidence produced by a PSA device [PSA-TOKEN]. This memo defines PSA Endorsements as a profile of the CoRIM data model [CoRIM].

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.

An understanding of the [CoRIM] data model is a prerequisite.

The reader is also assumed to be familiar with the terms defined in Section 2.1 of [PSA-TOKEN] and in Section 4 of [RATS-ARCH].

3. PSA Endorsements

PSA Endorsements describe an attesting device in terms of the hardware and firmware components that make up its PSA Root of Trust (RoT). This includes the identification and expected state of the device as well as the cryptographic key material needed to verify Evidence signed by the device's PSA RoT. Additionally, PSA Endorsements can include information related to the certification status of the attesting device.

There are three basic types of PSA Endorsements:

  • Reference Values (Section 3.3), i.e., measurements of the PSA RoT firmware;
  • Attestation Verification Claims (Section 3.4), i.e., cryptographic keys that are used to verify signed Evidence produced by the PSA RoT, along with the identifiers that bind the keys to their device instances;
  • Certification Claims (Section 3.5), i.e., metadata that describe the certification status associated with a PSA device;

There is a fourth PSA Endorsement type that aims at covering more advanced Verifier use cases (e.g., the one described in Section 7 of [TEEP]):

  • Software Relations (Section 3.6), used to model upgrade and patch relationships between software components.

3.1. PSA Endorsement Profile

PSA Endorsements are carried in one or more CoMIDs inside a CoRIM.

The profile attribute in the CoRIM MUST be present and MUST be set to the URI http://arm.com/psa/iot/1 as shown in Figure 1.

/ corim-map / {
  / corim.profile / 3: 32("http://arm.com/psa/iot/1")
  / ... /
}
Figure 1: PSA IoT version 1, CoRIM profile

The list of all, and only, the CoMIDs that are currently "active" (i.e., CoMIDs that contain triples that can be used for appraisal) is provided in a CoBOM tag.

TODO CoBOM example

3.2. PSA Endorsements to PSA RoT Linkage

Each PSA Endorsement - be it a Reference Value, Attestation Verification Claim or Certification Claim - is associated with an immutable PSA RoT. The linkage between a PSA Endorsement and its PSA RoT is made by means of the unique PSA RoT identifier known as Implementation ID (see Section 3.2.2 of [PSA-TOKEN]).

In order to support PSA Implementation IDs, the CoMID type $class-id-type-choice is extended as follows:

; from draft-tschofenig-rats-psa-token
psa-implementation-id-type = bytes .size 32

tagged-implementation-id-type = #6.600(implementation-id-type)

$class-id-type-choice /= tagged-implementation-id-type

Besides, a PSA Endorsement can be associated with a specific instance of a certain PSA RoT - as is the case for Attestation Verification Claims. A PSA Endorsement is associated with a PSA RoT instance by means of the Instance ID (see Section 3.2.1 of [PSA-TOKEN]) and its "parent" Implementation ID.

These identifiers are typically found in the subject of a CoMID triple, encoded in an environment-map as shown in Figure 2.

/ environment-map / {
  / comid.class / 0 : {
    / comid.class-id / 0 :
      / tagged-impl-id-type / 600(
        h'61636d652d696d706c656d656e746174
          696f6e2d69642d303030303030303031'
      ),
      / comid.vendor / 1 : "ACME Ltd.",
      / comid.model /  2 : "Roadrunner 1.0"
  },
  / comid.instance / 1 :
    / tagged-ueid-type / 550(
      h'01
        4ca3e4f50bf248c39787020d68ffd05c
        88767751bf2645ca923f57a98becd296'
    )
}
Figure 2: Example PSA RoT Identification

Optional vendor and model can be specified as well. Together, they are interpreted as a unique identifier of the product that embeds the PSA RoT. It is RECOMMENDED to consistently provide a product identifier.

3.3. Reference Values

Reference Values carry measurements and other metadata associated with the updatable firmware in a PSA RoT. When appraising Evidence, the Verifier compares Reference Values against the values found in the Software Components of the PSA token (see Section 3.4.1 of [PSA-TOKEN]).

When there is more than one measurement associated to a certain PSA RoT, the measurements are spread across multiple reference-triple-records and, in certain cases, across multiple CoMIDs. A single CoBOM MUST completely describe the updatable PSA RoT.

The elements of the psa-software-component map defined in Section 4.4.1 of [PSA-TOKEN] are matched against CoMID measurement-map entries as follows:

Table 1: PSA Software Component Mappings
PSA Evidence PSA Endorsement Description
measurement-type measurement-values-map.name Section 4.4.1.1 of [PSA-TOKEN]
measurement-value measurement-values-map.digests[*][1] Section 4.4.1.2 of [PSA-TOKEN]
version measurement-values-map.version.version Section 4.4.1.3 of [PSA-TOKEN]
measurement-desc measurement-values-map.digests[*][0]  
signer-id authorized-by[0] Section 4.4.1.4 of [PSA-TOKEN]

The digests array MUST contain at least one entry and MAY contain more than one entry if multiple digests (obtained with different hash algorithms) of the same measured component exist.

The authorized-by in the measurement-map MUST have exactly one entry of type tagged-thumbprint-type (CBOR tag 557) containing the signer-id.

The example in Figure 3 shows a CoMID encoding a PSA Endorsement of type Reference Value for a firmware measurement associated with Implementation ID acme-implementation-id-000000001.

/ concise-mid-tag / {
  / comid.tag-identity / 1 : {
    / comid.tag-id / 0 : h'3f06af63a93c11e4979700505690773f'
  },
  / comid.triples / 4 : {
    / comid.reference-triples / 0 : [
      [
        / environment-map / {
          / comid.class / 0 : {
            / comid.class-id / 0 :
              / tagged-impl-id-type / 600(
                h'61636d652d696d706c656d656e746174
                  696f6e2d69642d303030303030303031'
              ),
            / comid.vendor / 1 : "ACME Ltd.",
            / comid.model /  2 : "Roadrunner 1.0"
          }
        },
        [
          / measurement-map / {
            / comid.mval / 1 : {
              / comid.version / 0 : {
                / version / 0: "1.3.5"
              },
              / comid.digests / 2 : [
                [
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'44aa336af4cb14a8
                                    79432e53dd6571c7
                                    fa9bccafb75f4882
                                    59262d6ea3a4d91b'
                ]
              ],
              / comid.name / 11 : "PRoT"
            },
            / authorized-by / 2 : [
              557([
                / hash-alg-id / "sha-256",
                / hash-value /  h'acbb11c7e4da2172
                                  05523ce4ce1a245a
                                  e1a239ae3c6bfd9e
                                  7871f7e5d8bae86b'
              ])
            ]
          }
        ]
      ]
    ]
  }
}
Figure 3: Example Reference Value

3.4. Attestation Verification Claims

An Attestation Verification Claim carries the verification key associated with the Initial Attestation Key (IAK) of a PSA device. When appraising Evidence, the Verifier can use the Implementation ID and Instance ID claims (see Section 3.2) to look up the verification key that it SHALL use to check the signature on the Evidence. This allows the Verifier to prove (or disprove) the Attester's claimed identity.

Each verification key is provided alongside the corresponding device Instance and Implementation IDs (and, possibly, a product identifier) in an attest-key-triple-record. Specifically:

  • The Instance and Implementation IDs are encoded in the environment-map as shown in Figure 2;
  • The IAK public key is carried in the comid.key entry in the verification-key-map. The IAK public key is a PEM-encoded SubjectPublicKeyInfo [RFC5280]. There MUST be only one verification-key-map in an attest-key-triple-record;
  • The optional comid.keychain entry MUST NOT be set by a CoMID producer that uses the profile described in this document, and MUST be ignored by a CoMID consumer that is parsing according to this profile.

The example in Figure 4 shows the PSA Endorsement of type Attestation Verification Claim carrying a secp256r1 EC public IAK associated with Instance ID 4ca3...d296.

/ concise-mid-tag / {
  / comid.tag-identity / 1 : {
    / comid.tag-id / 0 : h'3f06af63a93c11e4979700505690773f'
  },
  / comid.triples / 4 : {
    / comid.attest-key-triples / 3 : [
      [
        / environment-map / {
          / comid.class / 0 : {
            / comid.class-id / 0 :
              / tagged-impl-id-type / 600(
                h'61636d652d696d706c656d656e746174
                  696f6e2d69642d303030303030303031'
              ),
              / comid.vendor / 1 : "ACME Ltd.",
              / comid.model /  2 : "Roadrunner 1.0"
          },
          / comid.instance / 1 :
            / tagged-ueid-type / 550(
              h'01
                4ca3e4f50bf248c39787020d68ffd05c
                88767751bf2645ca923f57a98becd296'
            )
        },
        [
          / verification-key-map / {
            / comid.key / 0 :
              "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgA
ETl4iCZ47zrRbRG0TVf0dw7VFlHtv18HInY
hnmMNybo+A1wuECyVqrDSmLt4QQzZPBECV8
ANHS5HgGCCSr7E/Lg=="
          }
        ]
      ]
    ]
  }
}
Figure 4: Example Attestation Verification Claim

3.5. Certification Claims

PSA Certified [PSA-CERTIFIED] defines a certification scheme for the PSA ecosystem. A product - either a hardware component, a software component, or an entire device - that is verified to meet the security criteria established by the PSA Certified scheme is warranted a PSA Certified Security Assurance Certificate (SAC). A SAC contains information about the certification of a certain product (e.g., the target system, the attained certification level, the test lab that conducted the evaluation, etc.), and has a unique Certificate Number.

The linkage between a PSA RoT -- comprising the immutable part as well as zero or more of the mutable components -- and the associated SAC is provided by a Certification Claim, which binds the PSA RoT Implementation ID and the software component identifiers with the SAC unique Certificate Number. When appraising Evidence, the Verifier can use the Certification Claims associated with the identified Attester as ancillary input to the Appraisal Policy, or to enrich the produced Attestation Result.

A Certification Claim is encoded as a conditional-endorsement-triple-record.

The SAC is encoded in a psa-cert-num that extends the measurement-values-map:

$$measurement-values-map-extension //= (
  &(psa-cert-num: 100) => psa-cert-num-type
)

psa-cert-num-type = text .regexp "[0-9]{13} - [0-9]{5}"

The conditional-endorsement-triple-record is constructed as follows:

  • The Implementation ID of the immutable PSA RoT to which the SAC applies is encoded as a tagged-impl-id-type in the environment-map of the stateful-environment-record;
  • Any software component that is part of the certified PSA RoT is encoded as a reference value (see Section 3.3) in the measurement-map of the stateful-environment-record;
  • The unique SAC Certificate Number is encoded as psa-cert-num in the measurement-values-map.

The example in Figure 5 shows a Certification Claim that associates Certificate Number 1234567890123 - 12345 to Implementation ID acme-implementation-id-000000001 and a single "PRoT" software component with version "1.3.5".

/ concise-mid-tag / {
  / comid.tag-identity / 1 : {
    / comid.tag-id / 0 : h'dbb0508ac658421c99c904124bab59ca'
  },
  / comid.triples / 4 : {
    / comid.conditional-endorsement-triple / 9 : [
      [
        / stateful-environment-record / [
          / environment-map / {
            / comid.class / 0 : {
              / comid.class-id / 0 :
                / tagged-impl-id-type / 600(
                  h'61636d652d696d706c656d656e746174
                    696f6e2d69642d303030303030303031'
                ),
              / comid.vendor / 1 : "ACME Ltd.",
              / comid.model /  2 : "Roadrunner 1.0"
            }
          },
          / measurement-map / {
            / comid.mval / 1 : {
              / comid.version / 0 : {
                / version / 0: "1.3.0"
              },
              / comid.digests / 2 : [
                [
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'44aa336af4cb14a8
                                    79432e53dd6571c7
                                    fa9bccafb75f4882
                                    59262d6ea3a4d91b'
                ]
              ],
              / comid.name / 11 : "PRoT"
            },
            / authorized-by / 2 : [
              557([
                / hash-alg-id / "sha-256",
                / hash-value /  h'acbb11c7e4da2172
                                  05523ce4ce1a245a
                                  e1a239ae3c6bfd9e
                                  7871f7e5d8bae86b'
              ])
            ]
          }
        ],
        / measurement-values-map / {
           / psa.cert-num / 100 : "1234567890123 - 12345"
        }
      ]
    ]
  }
}
Figure 5: Example Certification Claim

3.6. Software Upgrades and Patches

In order to model software lifecycle events such as updates and patches, this profile defines a new triple that conveys the following semantics:

  • SUBJECT: a software component
  • PREDICATE: (non-critically / critically) (updates / patches)
  • OBJECT: another software component

The triple is reified and used as the object of another triple, psa-swrel-triple-record, whose subject is the embedding environment.

comid.psa-swrel-triples = TBD2

$$triples-map-extension //= (
  comid.psa-swrel-triples => [ + psa-swrel-triple-record ]
)

psa.updates = 1
psa.patches = 2

psa-swrel-rel = [
  type: psa.updates / psa.patches
  security-critical: bool ; true means it's a fix for a security bug
]

sw-rel = [
  new: comid.measurement-map ; the "new" firmware
  rel: psa-swrel-rel         ; patches/updates and the security flag
  old: comid.measurement-map ; the "old" firmware
]

psa-swrel-triple-record = [
  environment-map
  sw-rel
]

An example of a security critical update involving versions "1.2.5" and "1.3.0" of software component "PRoT" within the target environment associated with Implementation ID acme-implementation-id-000000001 is shown in Figure 6.

/ concise-mid-tag / {
  / comid.tag-identity / 1 : {
    / comid.tag-id / 0 : h'3f06af63a93c11e4979700505690773f'
  },
  / comid.triples / 4 : {
    / comid.psa-swrel-triples / 5 : [
      [
        / environment-map / {
          / comid.class-id / 0 :
          / tagged-impl-id-type / 600(
            h'61636d652d696d706c656d656e746174
              696f6e2d69642d303030303030303031'
          ),
          / comid.vendor / 1 : "ACME Ltd.",
          / comid.model /  2 : "Roadrunner 1.0"
        },

        / sw-rel / [
          / new / {
            / comid.mval / 1 : {
              / comid.version / 0 : {
                / version / 0: "1.3.0"
              },
              / comid.digests / 2 : [
                [
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'44aa336af4cb14a8
                                    79432e53dd6571c7
                                    fa9bccafb75f4882
                                    59262d6ea3a4d91b'
                ]
              ],
              / comid.name / 11 : "PRoT"
            },
            / authorized-by / 2 : [
              557([
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'acbb11c7e4da2172
                                    05523ce4ce1a245a
                                    e1a239ae3c6bfd9e
                                    7871f7e5d8bae86b'
              ])
            ]
          },

          / rel / [
            / type / 1, / psa.updates /
            / security-critical / true
          ],

          / old / {
            / comid.mval / 1 : {
              / comid.version / 0 : {
                / version / 0: "1.2.5"
              },
              / comid.digests / 2 : [
                [
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'98b06c3f4bfeb294
                                    f69dae2bbe7d4be0
                                    750e258a86414d90
                                    a17cda9e2e775337'
                ]
              ],
              / comid.name / 11 : "PRoT"
            },
            / authorized-by / 2 : [
              557([
                  / hash-alg-id / "sha-256",
                  / hash-value /  h'acbb11c7e4da2172
                                    05523ce4ce1a245a
                                    e1a239ae3c6bfd9e
                                    7871f7e5d8bae86b'
              ])
            ]
          }
        ]
      ]
    ]
  }
}
Figure 6: Example Critical Software Upgrade

5. IANA Considerations

5.1. CBOR Tag Registrations

IANA is requested to allocate the following tag in the "CBOR Tags" registry [IANA.cbor-tags], preferably with the specified value:

Table 2: CoRIM CBOR Tags
Tag Data Item Semantics
600 tagged bytes PSA Implementation ID (Section 3.2 of RFCthis)

5.2. CoRIM Profile Registration

IANA is requested to register the following profile value in the TODO CoRIM registry.

Table 3: PSA profile for CoRIM
Profile Value Type Semantics
http://arm.com/psa/iot/1 uri The CoRIM profile specified by this document

5.3. CoMID Codepoints

5.3.1. CoMID Triples Map Extension

IANA is requested to register the following codepoints to the "CoMID Triples Map" registry.

Table 4: PSA CoMID Triples
Index Item Name Specification
50 comid.psa-swrel-triples RFCthis

5.3.2. CoMID Measurement Values Map Extension

Table 5: Measurement Values Map Extensions
Key Item Name Item Type Specification
100 comid.psa-cert-num psa-cert-num Section 3.5 of RFCthis

References

Normative References

[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-01, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-corim-01>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags", <https://www.iana.org/assignments/cbor-tags>.
[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-11, , <https://datatracker.ietf.org/doc/html/draft-tschofenig-rats-psa-token-11>.
[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>.

Informative References

[PSA-CERTIFIED]
"PSA Certified", , <https://www.psacertified.org>.
[RATS-ARCH]
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>.
[TEEP]
Pei, M., Tschofenig, H., Thaler, D., and D. M. Wheeler, "Trusted Execution Environment Provisioning (TEEP) Architecture", Work in Progress, Internet-Draft, draft-ietf-teep-architecture-19, , <https://datatracker.ietf.org/doc/html/draft-ietf-teep-architecture-19>.

Authors' Addresses

Thomas Fossati
Arm Ltd
Yogesh Deshpande
Arm Ltd
Henk Birkholz
Fraunhofer SIT