Internet DRAFT - draft-fdb-rats-psa-endorsements
draft-fdb-rats-psa-endorsements
RATS T. Fossati
Internet-Draft Linaro
Intended status: Informational Y. Deshpande
Expires: 5 September 2024 Arm Ltd
H. Birkholz
Fraunhofer SIT
4 March 2024
Arm's Platform Security Architecture (PSA) Attestation Verifier
Endorsements
draft-fdb-rats-psa-endorsements-04
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 2
3. PSA Endorsements . . . . . . . . . . . . . . . . . . . . . . 3
3.1. PSA Endorsement Profile . . . . . . . . . . . . . . . . . 3
3.2. PSA Endorsements to PSA RoT Linkage . . . . . . . . . . . 4
3.3. Reference Values . . . . . . . . . . . . . . . . . . . . 5
3.3.1. Software Upgrades and Patches . . . . . . . . . . . . 8
3.4. Attestation Verification Claims . . . . . . . . . . . . . 10
3.5. Certification Claims . . . . . . . . . . . . . . . . . . 11
3.6. Endorsements Block List . . . . . . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
5.1. CBOR Tag Registrations . . . . . . . . . . . . . . . . . 14
5.2. CoRIM Profile Registration . . . . . . . . . . . . . . . 14
5.3. CoMID Codepoints . . . . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . . 15
Informative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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.
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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].
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 five 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 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 3.5), i.e., metadata that describe
the certification status associated with a PSA device.
* Software Relations (Section 3.3.1), used to model upgrade and
patch relationships between software components;
* Endorsements Block List (Section 3.6), used to invalidate
previously provisioned Endorsements.
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 have a
single entry 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
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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. 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]).
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 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 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
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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. Consistently providing a product identifier is RECOMMENDED.
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]).
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
identified in the subject of the "reference value" triple. A single
reference-triple-record SHALL completely describe the updatable PSA
RoT.
The identifier of a measured software component is encoded in a psa-
swcomp-id object as follows:
psa-swcomp-id = {
psa.measurement-type => text
psa.version => text
psa.signer-id => psa.hash-type
}
psa.hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
psa.measurement-type = 1
psa.version = 4
psa.signer-id = 5
The semantics of the codepoints in the psa-swcomp-id map are
equivalent to those in the psa-software-component map defined in
Section 3.4.1 of [PSA-TOKEN]. The psa-swcomp-id MUST uniquely
identify a given software component within the PSA RoT / product.
In order to support PSA Reference Value identifiers, the CoMID type
$measured-element-type-choice is extended as follows:
tagged-psa-swcomp-id = #6.601(psa-swcomp-id)
$measured-element-type-choice /= tagged-psa-swcomp-id
and automatically bound to the comid.mkey in the measurement-map.
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The raw measurement is encoded in a digests-type object in the
measurement-values-map. The digests-type array MUST contain at least
one entry. The digests-type array MAY contain more than one entry if
multiple digests (obtained with different hash algorithms) of the
same measured component exist.
The example in Figure 3 shows a CoMID a 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 / 600(
h'61636d652d696d706c656d656e746174
696f6e2d69642d303030303030303031'
),
/ comid.vendor / 1 : "ACME Ltd.",
/ comid.model / 2 : "Roadrunner 1.0"
}
},
[
/ measurement-map / {
/ comid.mkey / 0 : 601({
/ psa.measurement-type / 1 : "PRoT",
/ psa.version / 4 : "1.3.5",
/ psa.signer-id / 5 : h'acbb11c7e4da2172
05523ce4ce1a245a
e1a239ae3c6bfd9e
7871f7e5d8bae86b'
}),
/ comid.mval / 1 : {
/ comid.digests / 2 : [
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'44aa336af4cb14a8
79432e53dd6571c7
fa9bccafb75f4882
59262d6ea3a4d91b'
]
}
}
]
]
]
}
}
Figure 3: Example Reference Value
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3.3.1. 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 = 5
$$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: psa-swcomp-id ; identifier of the "new" firmware
rel: psa-swrel-rel ; patches, updates and the security flag
old: psa-swcomp-id ; identifier of the "old" firmware
]
psa-swrel-triple-record = [
environment-map
sw-rel
]
An example of a security critical update involving versions "1.3.5"
and "1.4.0" of software component "PRoT" within the target
environment associated with Implementation ID acme-implementation-
id-000000001 is shown in Figure 4.
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/ 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 / {
/ psa.measurement-type / 1 : "PRoT",
/ psa.version / 4 : "1.4.0",
/ psa.signer-id / 5 : h'acbb11c7e4da2172
05523ce4ce1a245a
e1a239ae3c6bfd9e
7871f7e5d8bae86b'
},
/ rel / [
/ type / 1, / psa.updates /
/ security-critical / true
],
/ old / {
/ psa.measurement-type / 1 : "PRoT",
/ psa.version / 4 : "1.3.5",
/ psa.signer-id / 5 : h'acbb11c7e4da2172
05523ce4ce1a245a
e1a239ae3c6bfd9e
7871f7e5d8bae86b'
}
]
]
]
}
}
Figure 4: Example Critical Software Upgrade
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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 uses the Implementation ID and
Instance ID claims (see Section 3.2) to retrieve 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 5 shows the PSA Endorsement of type Attestation
Verification Claim carrying a secp256r1 EC public IAK associated with
Instance ID 4ca3...d296.
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/ 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 5: 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
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(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 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 => [ + psa-cert-triple-record ]
)
psa.immutable-rot = 1
psa.mutable-rot = 2
psa-rot-descriptor = {
psa.immutable-rot => psa-implementation-id-type
psa.mutable-rot => [ * psa-swcomp-id ]
}
psa-cert-triple-record = [
psa-rot-descriptor
psa-cert-num-type
]
psa-cert-num-type = text .regexp "[0-9]{13} - [0-9]{5}"
* The Implementation ID of the immutable PSA RoT to which the SAC
applies is encoded as a tagged-impl-id-type in the psa.immutable-
rot of the psa-rot-descriptor;
* Any software component that is part of the certified PSA RoT is
encoded as a psa-swcomp-id (see Section 3.3) in the psa.mutable-
rot of the psa-rot-descriptor;
* The unique SAC Certificate Number is encoded in the psa-cert-num-
type.
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A single CoMID can carry one or more Certification Claims.
The example in Figure 6 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'3f06af63a93c11e4979700505690773f'
},
/ comid.triples / 4 : {
/ comid.psa-cert-triples / 4 : [
[
/ psa-rot-descriptor / {
/ psa.immutable-rot / 1 :
h'61636d652d696d706c656d656e746174
696f6e2d69642d303030303030303031',
/ psa.mutable-rot / 2 : [
/ psa-swcomp-id / {
/ psa.measurement-type / 1 : "PRoT",
/ psa.version / 4 : "1.3.5",
/ psa.signer-id / 5 : h'acbb11c7e4da2172
05523ce4ce1a245a
e1a239ae3c6bfd9e
7871f7e5d8bae86b'
}
]
},
/ psa-cert-num-type / "1234567890123 - 12345"
]
]
}
}
Figure 6: Example Certification Claim with `supplement` Link-Relation
3.6. Endorsements Block List
// This is work in progress. It may change or be removed in the
// future.
The following three "blocklist" claims:
* reference-blocklist-triple
* attest-key-blocklist-triple
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* 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
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:
+=====+==============+===================================+
| Tag | Data Item | Semantics |
+=====+==============+===================================+
| 600 | tagged bytes | PSA Implementation ID |
| | | (Section 3.2 of RFCTHIS) |
+-----+--------------+-----------------------------------+
| 601 | tagged map | PSA Software Component Identifier |
| | | (Section 3.3 of RFCTHIS) |
+-----+--------------+-----------------------------------+
Table 1: CoRIM CBOR Tags
5.2. CoRIM Profile Registration
IANA is requested to register the following profile value in the
// TODO CoRIM registry.
+==========================+======+============================+
| Profile Value | Type | Semantics |
+==========================+======+============================+
| http://arm.com/psa/iot/1 | uri | The CoRIM profile |
| | | specified by this document |
+--------------------------+------+----------------------------+
Table 2: PSA profile for CoRIM
5.3. CoMID Codepoints
IANA is requested to register the following codepoints to the "CoMID
Triples Map" registry.
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+=======+=========================+===============+
| Index | Item Name | Specification |
+=======+=========================+===============+
| 4 | comid.psa-cert-triples | RFCTHIS |
+-------+-------------------------+---------------+
| 5 | comid.psa-swrel-triples | RFCTHIS |
+-------+-------------------------+---------------+
Table 3: PSA CoMID Triples
Acknowledgements
// TODO
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-03, 23
October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-corim-03>.
[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-22, 21 February 2024,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-22>.
[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>.
[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, May 2008,
<https://www.rfc-editor.org/rfc/rfc5280>.
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[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>.
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 (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
Authors' Addresses
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org
Yogesh Deshpande
Arm Ltd
Email: yogesh.deshpande@arm.com
Henk Birkholz
Fraunhofer SIT
Email: henk.birkholz@sit.fraunhofer.de
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