RATS Y. Deshpande
Internet-Draft Arm Ltd
Intended status: Informational T. Fossati
Expires: 4 September 2025 Linaro
3 March 2025
Arm's Confidential Computing Architecture (Arm CCA) Attestation Verifier
Endorsements
draft-ydb-rats-cca-endorsements-01
Abstract
Arm Confidential Computing Architecture (CCA) Endorsements comprise
of reference values and cryptographic key material that a Verifier
needs in order to appraise Attestation Evidence produced by an Arm
CCA system.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/yogeshbdeshpande/draft-cca-rats-endorsements.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 4 September 2025.
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Table of Contents
1. Introduction
2. Conventions and Definitions
3. Arm CCA Endorsements
3.1. Arm CCA Platform Endorsements
3.1.1. Arm CCA Platform Endorsement Profile
3.1.2. Arm CCA Platform Endorsements linkage to CCA Platform
3.1.3. Reference Values
3.1.4. Attestation Verification Claims
3.2. Arm CCA Realm Endorsements
3.2.1. Realm Endorsements linkage to Realm
3.2.2. Arm CCA Realm Endorsement Profile
3.2.3. Reference Values
4. Security Considerations
5. IANA Considerations
5.1. CBOR Tag Registrations
6. Acknowledgements
7. References
7.1. Normative References
7.2. Informative References
Contributors
Authors' Addresses
1. Introduction
Arm CCA Endorsements include reference values and cryptographic key
material information that a Verifier needs in order to appraise
attestation Evidence produced by a CCA System [CCA-TOKEN]. This memo
defines such CCA 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 A7.2.1 of [CCA-TOKEN] and in Section 4 of [RATS-ARCH].
3. Arm CCA Endorsements
Arm CCA attestation scheme is a composite attestation scheme which
comprises a CCA Platform Attestation & a Realm Attestation[CCA-ARCH].
Hence appraisal of Arm CCA attestation needs endorsements for both
CCA Platform and CCA Realm. This draft documents both the CCA
platform and realm endorsements.
3.1. Arm CCA Platform Endorsements
There are two types of CCA Platform Endorsements:
* Reference Values (Section 3.1.3), i.e., measurements of the CCA
Platform firmware.
* Attestation Verification Claims (Section 3.1.4), i.e.,
cryptographic keys that can be used to verify signed attestation
token produced by the CCA platform, along with the identifiers
that bind the keys to their platform instances.
3.1.1. Arm CCA Platform Endorsement Profile
Arm CCA platform 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/cca/ssd/1 as shown in
Figure 1.
/ corim-map / {
/ corim.profile / 3: [
32("http://arm.com/cca/ssd/1")
]
/ ... /
}
Figure 1: CCA platform profile version 1, CoRIM profile
3.1.2. Arm CCA Platform Endorsements linkage to CCA Platform
Each CCA Platform Endorsement - be it a Reference Value or
Attestation Verification Claim is associated with a unique CCA
platform identifier. A CCA platform identifier known as CCA Platform
Implementation ID (see Section A7.2.3.2.3 of [CCA-TOKEN]) uniquely
identifies a class of CCA platform to which the manufacturer/endorser
links the supplied Endorsements (Reference Values & Attestation
Verification Keys) for a CCA platform.
In order to support CCA Implementation IDs, the CoMID type $class-id-
type-choice is extended as follows Figure 2:
cca-implementation-id-type = bytes .size 32
tagged-implementation-id-type = #6.600(implementation-id-type)
$class-id-type-choice /= tagged-implementation-id-type
Figure 2: Example CCA Platform Implementation ID
Besides, a CCA Endorsement can be associated with a specific instance
of a certain CCA Platform implementation - as is the case of
Attestation Verification Claims. A CCA Attestation Verification
Claims are associated with a CCA platform instance by means of the
Instance ID (see Section A7.2.3.2.4 of [CCA-TOKEN]) and its platform
Implementation ID.
These identifiers are typically found in the subject of a CoMID
triple, encoded in an environment-map as shown in Figure 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'
)
}
Figure 3: Example CCA Platform Identification
Optional vendor and model can be specified as well. Together, they
are interpreted as a unique identifier of the CCA platform.
Consistently providing a product identifier is RECOMMENDED.
3.1.3. Reference Values
Reference Values carry measurements and other metadata associated
with the updatable firmware of CCA platform. The CCA platform is a
collective term used to identify all the hardware and firmware
components that comprise a CCA system. Specifically these include
the following:
* CCA system security domain
* Monitor security domain
* Realm Management Security domain
When appraising Evidence, the Verifier compares Reference Values
against:
* The values found in the Software Components of the CCA platform
token (see Section A7.2.3.2.7 of [CCA-TOKEN]).
* The value set in the platform configuration of the CCA platform
token (see Section A7.2.3.2.5 of [CCA-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 CCA platform
identified in the subject of the "reference value" triple. A single
reference-triple-record SHALL completely describe the CCA platform
measurements.
3.1.3.1. CCA Platform Software Components
For the Reference Values of CCA platform software components the
identifier of a measured software component is encoded in a arm-
swcomp-id object as follows Figure 4:
arm-swcomp-id = {
arm.measurement-type => text
arm.version => text
arm.signer-id => arm.hash-type
}
arm.hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
arm.measurement-type = 1
arm.version = 4
arm.signer-id = 5
Figure 4: Example SW Component ID
The semantics of the codepoints in the arm-swcomp-id map are
equivalent to those in the cca-platform-sw-component map defined in
Section A7.2.3.2.7 of [CCA-TOKEN]. The arm-swcomp-id MUST uniquely
identify a given software component within the CCA platform /
product.
In order to support CCA Reference Value identifiers, the CoMID type
$measured-element-type-choice is extended as followsFigure 5:
tagged-arm-swcomp-id = #6.601(arm-swcomp-id)
$measured-element-type-choice /= tagged-arm-swcomp-id
Figure 5: Example SW Component ID Extension
and automatically bound to the comid.mkey in the measurement-map.
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. Refer below Figure 6.
3.1.3.2. CCA Platform Configuration
A Reference value for CCA platform configuration describes the set of
chosen implementation options of the CCA platform. As an example,
these may include a description of the level of physical memory
protection which is provided.
CCA platform configuration reference value represent vendor specific
variable length data. As a result, in the CCA platform CoRIM
profile, it is represented in a measurement-values-map using raw-
values set to tagged-bytes to express a variable length byte string,
representing platform configuration data.
$raw-value-type-choice /= tagged-bytes
3.1.3.3. Complete Representation
The complete representation of CCA Platform Reference Values is given
in Figure 6 & Figure 7.
/ 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({
/ arm.measurement-type / 1 : "PRoT",
/ arm.version / 4 : "1.3.5",
/ arm.signer-id / 5 : h'acbb11c7e4da217205523ce4ce
1a245ae1a239ae3c6bfd9e7871
f7e5d8bae86b'
}),
/ comid.mval / 1 : {
/ comid.digests / 2 : [
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'44aa336af4cb14a879432e53dd6571c7
fa9bccafb75f488259262d6ea3a4d91b'
]
}
}
]
]
]
}
}
Figure 6: Example CCA SW Component Reference Value
/ 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 : 602({
/ "cca.platform-config-id" / 1 : "any-value"
}),
/ comid.mval / 1 : {
/ comid.raw-value / 4 : {
/ tagged-bytes / 560(
h'67b28b6c39cc40a19117ab5b05911e37'
)
}
}
}
]
]
]
}
}
Figure 7: Example CCA Platform Configuration Reference Values
3.1.4. Attestation Verification Claims
Attestation Verification Claim carries the verification key
associated with the Initial Attestation Key (IAK) of a CCA platform.
When appraising Evidence, the Verifier uses the Implementation ID and
Instance ID claims (see Section 3.1.2) to retrieve the verification
key that it SHALL use to check the signature on the CCA platform
token. This allows the Verifier to prove (or disprove) the
Attester's claimed identity.
Each verification key is provided alongside the corresponding CCA
platform Instance and Implementation IDs (and, possibly, a CCA
platform product identifier) in an attest-key-triple-record.
Specifically:
* The Instance and Implementation IDs are encoded in the
environment-map as shown in Figure 3
* The IAK public key is set using $crypto-key-type-choice set to
tagged-pkix-base64-key-type. The IAK public key is a PEM-encoded
SubjectPublicKeyInfo [RFC5280]. There MUST be only one key in an
attest-key-triple-record;
The example in Figure 8 shows the CCA Endorsement of type Attestation
Verification Key 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'
)
},
[
/ tagged-pkix-base64-key-type / 554(
"MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgA
ETl4iCZ47zrRbRG0TVf0dw7VFlHtv18HInY
hnmMNybo+A1wuECyVqrDSmLt4QQzZPBECV8
ANHS5HgGCCSr7E/Lg=="
)
]
]
]
}
}
Figure 8: Example CCA Platform Attestation Verification Key
3.2. Arm CCA Realm Endorsements
Arm CCA Realm provides a protected execution environment for
applications executing within a Realm. A Realm Endorsements comprise
of:
* Reference Values (Section 3.2.3), i.e., measurements of the
configuration and contents of a Realm at the time of its
activation along with measurements of software running inside
Realm, which can be extended during the lifetime of a Realm.
Please note that there are no Realm Trust Anchor Endorsements needed
from supply chain as they are present inline in the Attestation
Evidence.
3.2.1. Realm Endorsements linkage to Realm
Each Realm has a unique execution context and hence a unique Realm
instance. Each Realm is uniquely identified in the Arm CCA system.
For a Realm its Endorsements are associated to this unique instance.
The Realm instance is a vendor defined variable length identifier.
Hence in this profile, it is represented in a CoMID inside an
environment-map with $instance-id-type-choice set to tagged-bytes,
i.e. an opaque, variable-length byte string. In this profile of CCA
Endorsements, the Realm Initial Measurements are set in tagged-bytes
to represent Realm instance.
When supplying Realm Endorsements, a supplier of one or more Realms
may wish to identify itself. Hence the following class related
elements in the environment-map of a comid can be used. See Figure 9
In the class-map select vendor name and/or class-id set as UUID
representing unique identity of the Realm owner.
$class-id-type-choice /= tagged-uuid-type
vendor => tstr to represent vendor name
/ environment-map / {
/ comid.class / 0 : {
/ comid.class-id / 0 :
/ tagged-uuid-type / 37(
h'67b28b6c34cc40a19117ab5b05911e37'
),
/ comid.vendor / 1 : "Workload Client Ltd",
},
/ comid.instance / 1 :
/ tagged-bytes / 560(
h'67b28b6c39cc40a19117ab5b05911e37'
)
}
Figure 9: CCA realm identifiers
3.2.2. Arm CCA Realm Endorsement Profile
Arm CCA Realm Endorsements are carried in a CoMID 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/cca/realm/1 as shown in
Figure 10.
/ corim-map / {
/ corim.profile / 3: [
32("http://arm.com/cca/realm/1")
]
/ ... /
}
Figure 10: CCA realm profile version 1, CoRIM profile
3.2.3. Reference Values
Reference Values carry measurements and other metadata associated
with the CCA Realm.
Realm reference values comprise of:
1. Realm Initial Measurements (RIM)
2. Realm Extended Measurements (REMs)
3. Realm Personalization Value (RPV)
RIM and REMs are encoded in a measurement-values-map (in a
measurement-map) of a CoMID reference-triple-record. Inside
measurement-values-map these measurements are carried as integrity-
registers map. Integrity Registers map is used to group together one
or more measured objects pertaining to an environment. Please refer
[CoRIM] for details about Integrity Register map.
All the measured objects in an Integrity Registers map are explicitly
named. In the context of Realms, the measured objects are RIM and
REMs. Inside Integrity Register map, RIM is uniquely identified by
the name "rim", while REMs which is an array of measurements from
1..4 are uniquely identified by the coresponding name "rem0".."rem3".
Realm Personalization Value, (RPV) is an optional identity used by a
Realm endorser to uniquely identify multiple Realms which all have
the same RIM. RPV if provided is a fixed length 64 bytes identifier.
In this profile, RPV is represented using Raw Value Measurements in a
measurement-values-map, with raw value type choice set to tagged-
bytes. See Figure 11
$raw-value-type-choice /= tagged-bytes
Given below is the complete example of a Realm Endorsements.
/ 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-uuid-type / 37(
h'67b28b6c34cc40a19117ab5b05911e37'
),
/ comid.vendor / 1 : "Workload Client Ltd"
},
/ comid.instance / 1 :
/ tagged-bytes / 560(
h'67b28b6c39cc40a19117ab5b05911e37'
)
},
[
/ measurement-map / {
/ comid.mval / 1 : {
/ comid.raw-value / 4 : {
/ tagged-bytes / 560(
h'ABABABABABABABABABABABABABABABABABABABABABABABABA
BABABABABABABABABABABABABABABABABABABABABABABABAB
ABABABABABABABABABABABABABABAB',
)
},
/ comid.integrity-registers / 14 : {
"rim": [
[
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'44aa336af4cb14a879432e53dd6571c7
fa9bccafb75f488259262d6ea3a4d91b'
]
],
"rem0": [
[
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'50aa341af9cb20a879440e58dd6581c1
4fa14bccafb75f488259262d6ea3a4d9'
]
],
"rem1": [
[
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'50aa341af9cb20a879440e59dd6581c1
4fa14bccafb75f488259262d6ea3a4d9'
]
],
"rem2": [
[
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'50aa341af9cb20a879440e5add6581c1
4fa14bccafb75f488259262d6ea3a4d9'
]
],
"rem3": [
[
/ hash-alg-id / 1, / sha256 /
/ hash-value / h'50aa341af9cb20a879440e5bdd6581c1
4fa14bccafb75f488259262d6ea3a4d9'
]
]
}
}
}
]
]
]
}
}
Figure 11: CCA realm identifiers
4. Security Considerations
// TODO
5. IANA Considerations
5.1. CBOR Tag Registrations
IANA is requested to allocate the following tags in the "CBOR Tags"
registry [IANA.cbor-tags], preferably with the specified value:
+=====+==============+===================================+
| Tag | Data Item | Semantics |
+=====+==============+===================================+
| 600 | tagged bytes | CCA Implementation ID |
| | | (Section 3.1.2 of RFCTHIS) |
+-----+--------------+-----------------------------------+
| 601 | tagged map | CCA Software Component Identifier |
| | | (Section 3.1.3 of RFCTHIS) |
+-----+--------------+-----------------------------------+
Table 1: CoRIM CBOR Tags
6. Acknowledgements
// TODO
7. References
7.1. Normative References
[CCA-ARCH] Arm, "Learn the architecture - Introducing Arm
Confidential Compute Architecture", May 2023,
<https://developer.arm.com/documentation/den0125/0300>.
[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-06, 18
October 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-corim-06>.
[IANA.cbor-tags]
IANA, "Concise Binary Object Representation (CBOR) Tags",
<https://www.iana.org/assignments/cbor-tags>.
[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>.
[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>.
7.2. Informative References
[CCA-TOKEN]
"Arm's Confidential Compute Architecture Reference
Attestation Token", 2022,
<https://datatracker.ietf.org/doc/draft-ffm-rats-cca-
token/>.
[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>.
Contributors
Simon Frost
Arm Limited
Email: Simon.Frost@arm.com
Sergei Trofimov
Arm Limited
Email: Sergei.Trofimov@arm.com
Authors' Addresses
Yogesh Deshpande
Arm Ltd
Email: yogesh.deshpande@arm.com
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org