Internet DRAFT - draft-xyz-rats-psa-endorsements
draft-xyz-rats-psa-endorsements
RATS T. Fossati
Internet-Draft Y. Deshpande
Intended status: Informational Arm Ltd
Expires: 13 January 2022 H. Birkholz
Fraunhofer SIT
12 July 2021
Arm's Platform Security Architecture (PSA) Attestation Verifier
Endorsements
draft-xyz-rats-psa-endorsements-00
Abstract
PSA Endorsements include reference values, cryptographic key material
and certification status information that a Verifier needs in order
to appraise attestation Evidence produced by a PSA device. This memo
defines such PSA Endorsements as a profile of the CoRIM data model.
Status of This Memo
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
3. PSA Endorsements . . . . . . . . . . . . . . . . . . . . . . 3
3.1. PSA Endorsements to PSA RoT Linkage . . . . . . . . . . . 3
3.2. Reference Values . . . . . . . . . . . . . . . . . . . . 4
3.3. Attestation Verification Claims . . . . . . . . . . . . . 6
3.4. Certification Claims . . . . . . . . . . . . . . . . . . 8
3.5. Endorsements Block List . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . . 9
Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
PSA Endorsements include reference 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 such 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.
The reader is assumed to be familiar with the terms defined in
Section 2.1 of [PSA-TOKEN] and in Section 4 of [RATS-ARCH].
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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 3.2), i.e., measurements of the PSA RoT
firmware;
* Attestation Verification Claims (Section 3.3), i.e., cryptographic
keys that can be 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 3.4), i.e., metadata that describe
the certification status associated with a PSA device.
There is also a fourth category of PSA Endorsements:
* Endorsements Block List (Section 3.5),
that is used to invalidate previously provisioned Endorsements.
3.1. 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. A PSA Endorsement is associated to its PSA RoT by
means of the unique PSA RoT identifier known as Implementation ID
(see Section 3.2.2 of [PSA-TOKEN]). Besides, a PSA Endorsement can
be associated with a specific instance of a certain PSA RoT - as in
the case of 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 1.
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/ environment-map / {
/ comid.class / 0 : {
/ comid.class-id / 0 :
/ tagged-impl-id-type / 47115(
h'61636d652d696d706c656d656e746174696f6e2d69642d303030303030
303031'
)
},
/ comid.instance / 1 :
/ tagged-ueid-type / 48000(
h'014ca3e4f50bf248c39787020d68ffd05c88767751bf2645ca923f57a98b
ecd296'
)
}
Figure 1: Example PSA RoT Identification
3.2. 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]).
Each measurement is encoded in a "measurement-map" of a CoMID
"reference-triple-record". Since a "measurement-map" can encode one
or more measurements, a single "reference-triple-record" can carry as
many measurements as needed, provided they belong to the same PSA RoT
carried in the subject of the "reference value" triple.
The identifier of a measurement is encoded in a "psa-refval-id"
object as follows:
psa-refval-id = {
? psa.measurement-type => text
? psa.version => text
psa.signer-id => psa.hash-type
? psa.measurement-desc => text
}
psa.hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
psa.measurement-type = 1
psa.version = 4
psa.signer-id = 5
psa.measurement-desc = 6
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The semantics of the codepoints in the "psa-refval-id" map are
equivalent to those in the "psa-software-component" map defined in
Section 3.4.1 of [PSA-TOKEN].
In order to support PSA Reference Value identifiers, the "$measured-
element-type-choice" CoMID type is extended as follows:
tagged-psa-refval-id = #6.TBD(psa-refval-id)
$measured-element-type-choice /= tagged-psa-refval-id
and automatically bound to the "comid.mkey" in the "measurement-map".
The raw measurement is encoded in a "digests-type" object in the
"measurements-value-map". The "digests-type" array MUST contain only
one entry. If multiple digests of the same measured component exist
(obtained with different hash algorithms), a different
"psa.measurement-desc" MUST be used in the identifier.
The example in Figure 2 shows the PSA Endorsement of type Reference
Value for a firmware measurement associated with Implementation ID
"acme-implementation-id-000000001".
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/ 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 / 47115(
h'61636d652d696d706c656d656e746174696f6e2d69642d303030
303030303031'
)
}
},
/ measurement-map / {
/ comid.mkey / 0 : TBD({
/ psa.measurement-type / 1 : "PRoT",
/ psa.version / 4 : "1.3.5",
/ psa.signer-id / 5 : h'acbb11c7e4da217205523ce4ce1
a245ae1a239ae3c6bfd9e7871f7e5d8bae86b'
}),
/ comid.mval / 1 : {
/ comid.digests / 2 : [
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'44aa336af4cb14a879432e53dd6571c7fa9bcc
afb75f488259262d6ea3a4d91b'
]
}
}
]
}
}
Figure 2: Example Reference Value
3.3. 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 uses the Implementation ID and
Instance ID claims (see Section 3.1) to retrieve the verification key
that it must 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 in an "attest-key-triple-record".
Specifically:
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* The Instance and Implementation IDs are encoded in the
environment-map as shown in Figure 1;
* The IAK public key is carried in the "comid.key" entry in the
"verification-key-map". The IAK public key is encoded as a
COSE_Key according to Section 7 of [RFC8152]. There MUST be only
one "verification-key-map" in an "identity-triple-record";
* The optional "comid.keychain" entry MUST NOT be set by a producer
and MUST be ignored by a consumer.
The example in Figure 3 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 / 47115(
h'61636d652d696d706c656d656e746174696f6e2d69642d303030
303030303031'
)
},
/ comid.instance / 1 :
/ tagged-ueid-type / 48000(
h'014ca3e4f50bf248c39787020d68ffd05c88767751bf2645ca923f
57a98becd296'
)
},
/ verification-key-map / {
/ comid.key / 0 : {
/ kty / 1 : 2, / EC2 /
/ crv / -1 : 1, / P-256 /
/ x / -2 : h'30a0424cd21c2944838a2d75c92b37e76ea20d9f008
93a3b4eee8a3c0aaf',
/ y / -3 : h'e04b65e92456d9888b52b379bdfbd51ee869ef1f0fc
65b6659695b6cce08'
}
}
]
}
}
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Figure 3: Example Attestation Verification Claim
3.4. 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 and a related SAC is provided by a
Certification Claim, which binds the PSA RoT Implementation ID 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 in an "psa-cert-triple-record",
which extends the "$$triples-map-extension" socket, as follows:
comid.psa-cert-triples = 4
$$triples-map-extension //= (
comid.psa-cert-triples => one-or-more<psa-cert-triple-record>
)
psa-cert-triple-record = [
tagged-impl-id-type,
psa-cert-num-type
]
psa-cert-num-type = text .regexp "[0-9]{13} - [0-9]{5}"
* The Implementation ID to which the SAC applies is encoded in the
"tagged-impl-id-type";
* The unique SAC Certificate Number is encoded in the "psa-cert-num-
type".
A single CoMID can carry one or more Certification Claims.
The example in Figure 4 shows a Certification Claim for Certificate
Number "1234567890123 - 12345" and Implementation ID "acme-
implementation-id-000000001".
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/ concise-mid-tag / {
/ comid.tag-identity / 1 : {
/ comid.tag-id / 0 : h'3f06af63a93c11e4979700505690773f'
},
/ comid.triples / 4 : {
/ comid.psa-cert-triples / 4 : [
/ tagged-impl-id-type / 47115(
h'61636d652d696d706c656d656e746174696f6e2d69642d303030303030
303031'
),
/ psa-cert-num-type / "1234567890123 - 12345"
]
}
}
Figure 4: Example Certification Claim with `supplement` Link-Relation
3.5. Endorsements Block List
The following three "blocklist" claims:
* "reference-blocklist-triple"
* "attest-key-blocklist-triple"
* "cert-blocklist-triple"
are defined with the same syntax but opposite semantics with regards
to their "positive" counterparts to allow invalidating previously
provisioned endorsements from the acceptable set.
4. Security Considerations
TODO
5. IANA Considerations
TODO
Acknowledgements
TODO
References
Normative References
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[CoRIM] Birkholz, H., Fossati, T., Deshpande, Y., Smith, N., and
W. Pan, "Concise Reference Integrity Manifest", Work in
Progress, Internet-Draft, draft-birkholz-rats-corim-00, 12
July 2021, <https://www.ietf.org/archive/id/draft-
birkholz-rats-corim-00.txt>.
[PSA-TOKEN]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T.
Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-08, 24 March 2021,
<https://www.ietf.org/archive/id/draft-tschofenig-rats-
psa-token-08.txt>.
[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/info/rfc2119>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[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/info/rfc8174>.
Informative References
[PSA-CERTIFIED]
"PSA Certified", 2021, <https://www.psacertified.org>.
[RATS-ARCH]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
12, 23 April 2021, <https://www.ietf.org/archive/id/draft-
ietf-rats-architecture-12.txt>.
Authors' Addresses
Thomas Fossati
Arm Ltd
Email: thomas.fossati@arm.com
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Yogesh Deshpande
Arm Ltd
Email: yogesh.deshpande@arm.com
Henk Birkholz
Fraunhofer SIT
Email: henk.birkholz@sit.fraunhofer.de
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