Internet-Draft | CoRIM PSA Profile | March 2023 |
Fossati, et al. | Expires 11 September 2023 | [Page] |
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.¶
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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].¶
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].¶
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:¶
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]):¶
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.¶
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¶
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.¶
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.¶
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-record
s 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:¶
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
.¶
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:¶
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
;¶
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
.¶
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
:¶
$$measurement-values-map-extension //= ( &(psa-cert-num: 100) => psa-cert-num-type ) psa-cert-num-type = text .regexp "[0-9]{13} - [0-9]{5}"¶
The conditional-endorsement-triple-record
is constructed as follows:¶
tagged-impl-id-type
in the environment-map
of the
stateful-environment-record
;¶
measurement-map
of the
stateful-environment-record
;¶
psa-cert-num
in the
measurement-values-map
.¶
The example in Figure 5 shows a Certification Claim that
associates Certificate Number 1234567890123 - 12345
to Implementation ID
acme-implementation-id-000000001
and a single "PRoT" software component with
version "1.3.5".¶
In order to model software lifecycle events such as updates and patches, this profile defines a new triple that conveys the following semantics:¶
The triple is reified and used as the object of another triple,
psa-swrel-triple-record
, whose subject is the embedding environment.¶
comid.psa-swrel-triples = TBD2 $$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.¶
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) |
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 |
IANA is requested to register the following codepoints to the "CoMID Triples Map" registry.¶
Index | Item Name | Specification |
---|---|---|
50 | comid.psa-swrel-triples | RFCthis |
Key | Item Name | Item Type | Specification |
---|---|---|---|
100 | comid.psa-cert-num |
psa-cert-num
|
Section 3.5 of RFCthis |
TODO¶