Arm's Platform Security Architecture (PSA) Attestation Verifier Endorsements

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 (), i.e., measurements of the PSA RoT firmware; Attestation Verification Claims (), 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 (), i.e., metadata that describe the certification status associated with a PSA device. Software Relations (), used to model upgrade and patch relationships between software components; Endorsements Block List (), used to invalidate previously provisioned Endorsements.

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 .

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

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 ) 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 .

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.

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 ). 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:

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 . 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:

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

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.

[ + 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 .

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 ) 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 ; 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 . 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 shows the PSA Endorsement of type Attestation Verification Claim carrying a secp256r1 EC public IAK associated with Instance ID 4ca3...d296.

Certification Claims 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 in an psa-cert-triple-record, which extends the $$triples-map-extension socket, as follows:

[ + 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 ) in the psa.mutable-rot of the psa-rot-descriptor; 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 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".

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 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.