RFC 9783: Arm's Platform Security Architecture (PSA) Attestation Token

4.1.1. Nonce

The EAT [EAT] "nonce" (claim key 10) is used to carry the challenge provided by the caller to demonstrate freshness of the generated token.¶

Since the EAT nonce claim offers flexibility for different attestation technologies, this specification applies the following constraints to the nonce-type:¶

This claim MUST be present in a PSA attestation token.¶

psa-nonce = (
    nonce-label => psa-hash-type
)

4.1.2. Client ID

The Client ID claim represents the security domain of the caller.¶

In PSA, a security domain is represented by a signed integer whereby negative values represent callers from the NSPE and where positive IDs represent callers from the SPE. The value 0 is not permitted.¶

For an example definition of client IDs, see the PSA Firmware Framework [PSA-FF].¶

It is essential that this claim is checked in the verification process to ensure that a security domain, i.e., an attestation endpoint, cannot spoof a report from another security domain.¶

This claim MUST be present in a PSA attestation token.¶

psa-client-id-nspe-type = -2147483648...0
psa-client-id-spe-type = 1..2147483647

psa-client-id-type = psa-client-id-nspe-type / psa-client-id-spe-type

psa-client-id = (
    psa-client-id-key => psa-client-id-type
)

4.2.1. Instance ID

The Instance ID claim represents the unique identifier of the IAK. The full definition is in [PSA-SM].¶

The EAT ueid (claim key 256) of type RAND is used. The following constraints apply to the ueid-type:¶

This claim MUST be present in a PSA attestation token.¶

psa-instance-id-type = bytes .size 33

psa-instance-id = (
    ueid-label => psa-instance-id-type
)

4.2.2. Implementation ID

The Implementation ID claim uniquely identifies the hardware assembly of the immutable PSA RoT. A verification service uses this claim to locate the details of the PSA RoT implementation from an Endorser or manufacturer. Such details are used by a verification service to determine the security properties or certification status of the PSA RoT implementation.¶

The value and format of the ID is decided by the manufacturer or a particular certification scheme. For example, the ID could take the form of a product serial number, database ID, or other appropriate identifier.¶

This claim MUST be present in a PSA attestation token.¶

Note that this identifies the PSA RoT implementation, not a particular instance. To uniquely identify an instance, see the Instance ID claim Section 4.2.1.¶

psa-implementation-id-type = bytes .size 32

psa-implementation-id = (
    psa-implementation-id-key => psa-implementation-id-type
)

4.2.3. Certification Reference

The Certification Reference claim is used to link the class of chip and PSA RoT of the attesting device to an associated entry in the PSA Certification database. The Certification Reference claim MUST be represented as a string made of nineteen numeric characters: a thirteen-digit EAN-13 [EAN-13] followed by a dash "-" and the five-digit versioning information described in [PSA-Cert-Guide].¶

Linking to the PSA Certification entry can still be achieved if this claim is not present in the token by making an association at a Verifier between the reference value and other token claim values, for example, the Implementation ID.¶

This claim MAY be present in a PSA attestation token.¶

psa-certification-reference-type = text .regexp "[0-9]{13}-[0-9]{5}"

psa-certification-reference = (
    ? psa-certification-reference-key =>
        psa-certification-reference-type
)

4.3.1. Security Lifecycle

The Security Lifecycle claim represents the current lifecycle state of the PSA RoT. The state is represented by an integer that is divided to convey a major state and a minor state. A major state is mandatory and defined by [PSA-SM]. A minor state is optional and 'IMPLEMENTATION DEFINED'. The PSA security lifecycle state and implementation state are encoded as follows:¶

The PSA lifecycle states are illustrated in Figure 4. For PSA, a Verifier can only trust reports from the PSA RoT when it is in SECURED or NON_PSA_ROT_DEBUG major states.¶

This claim MUST be present in a PSA attestation token.¶

The CDDL representation is shown below. Table 1 provides the mappings between Figure 4 and the data model.¶

psa-lifecycle-unknown-type = 0x0000..0x00ff
psa-lifecycle-assembly-and-test-type = 0x1000..0x10ff
psa-lifecycle-psa-rot-provisioning-type = 0x2000..0x20ff
psa-lifecycle-secured-type = 0x3000..0x30ff
psa-lifecycle-non-psa-rot-debug-type = 0x4000..0x40ff
psa-lifecycle-recoverable-psa-rot-debug-type = 0x5000..0x50ff
psa-lifecycle-decommissioned-type = 0x6000..0x60ff

psa-lifecycle-type =
    psa-lifecycle-unknown-type /
    psa-lifecycle-assembly-and-test-type /
    psa-lifecycle-psa-rot-provisioning-type /
    psa-lifecycle-secured-type /
    psa-lifecycle-non-psa-rot-debug-type /
    psa-lifecycle-recoverable-psa-rot-debug-type /
    psa-lifecycle-decommissioned-type

psa-lifecycle = (
    psa-lifecycle-key => psa-lifecycle-type
)

psa-lifecycle-unknown-type is not shown in Figure 4; it represents an invalid state that must not occur in a system.¶

4.3.2. Boot Seed

The "bootseed" claim contains a value created at system boot time that allows differentiation of attestation reports from different boot sessions of a particular entity (i.e., a certain Instance ID).¶

The EAT "bootseed" (claim key 268) is used. The following constraints apply to the binary-data type:¶

This claim MAY be present in a PSA attestation token.¶

psa-boot-seed-type = bytes .size (8..32)

psa-boot-seed = (
    boot-seed-label => psa-boot-seed-type
)

4.4.1. Software Components

The Software Components claim is a list of software components that includes all the software (both code and configuration) loaded by the PSA RoT. This claim MUST be included in attestation tokens produced by an implementation conformant with [PSA-SM].¶

Each entry in the Software Components list describes one software component using the attributes described in the following subsections. Unless explicitly stated, the presence of an attribute is OPTIONAL.¶

Note that a Relying Party will typically see the result of the appraisal process from the Verifier in form of an Attestation Result rather than the PSA token from the attesting endpoint as described in [RFC9334]. Therefore, a Relying Party is not expected to understand the Software Components claim. Instead, it is for the Verifier to check this claim against the available Reference Values and provide an answer in form of a "high-level" Attestation Result, which may or may not include the original Software Components claim.¶

psa-software-component = {
  ? &(measurement-type: 1) => text
    &(measurement-value: 2) => psa-hash-type
  ? &(version: 4) => text
    &(signer-id: 5) => psa-hash-type
  ? &(measurement-desc: 6) => text
}

psa-software-components = (
    psa-software-components-key => [ + psa-software-component ]
)

4.5.1. Verification Service Indicator

The Verification Service Indicator claim is a hint used by a Relying Party to locate a verification service for the token. The value is a text string that can be used to locate the service (typically, a URL specifying the address of the verification service API). A Relying Party may choose to ignore this claim in favor of other information.¶

psa-verification-service-indicator-type = text

psa-verification-service-indicator = (
    ? psa-verification-service-indicator-key =>
        psa-verification-service-indicator-type
)

It is assumed that the Relying Party is pre-configured with a list of trusted verification services and that the contents of this hint can be used to look up the correct one. Under no circumstances must the Relying Party be tricked into contacting an unknown and untrusted verification service since the returned Attestation Result cannot be relied on.¶

Note: This hint requires the Relying Party to parse the content of the PSA token. Since the Relying Party may not be in possession of a trust anchor to verify the digital signature, it uses the hint in the same way as it would treat any other information provided by an external party, which includes attacker-provided data.¶

4.5.2. Profile Definition

The Profile Definition claim encodes the unique identifier that corresponds to the EAT profile described by this document. This allows a receiver to assign the intended semantics to the rest of the claims found in the token.¶

The EAT eat_profile (claim key 265) is used.¶

The URI encoding MUST be used.¶

The value MUST be tag:psacertified.org,2023:psa#tfm for the profile defined in Section 5.2.¶

Future profiles derived from the baseline PSA profile SHALL create their unique value as described in Section 4.5.2.1.¶

This claim MUST be present in a PSA attestation token.¶

See Section 4.6 for considerations about backwards compatibility with previous versions of the PSA attestation token format.¶

psa-profile-type = "tag:psacertified.org,2023:psa#tfm"

psa-profile = (
    profile-label => psa-profile-type
)

5.1.1. Token Encoding and Signing

The PSA attestation token is encoded in CBOR [STD94] format. The CBOR representation of a PSA token MUST be "valid" according to the definition in Section 1.2 of RFC 8949 [STD94]. Besides, only definite-length string, arrays, and maps are allowed. Given that a PSA Attester is typically found in a constrained device, it MAY NOT emit CBOR preferred serializations (Section 4.1 of RFC 8949 [STD94]). Therefore, the Verifier MUST be a variation-tolerant CBOR decoder.¶

Cryptographic protection is obtained by wrapping the psa-token claims set in a COSE Web Token (CWT) [RFC8392]. For asymmetric key algorithms, the signature structure MUST be a tagged (18) COSE_Sign1. For symmetric key algorithms, the signature structure MUST be a tagged (17) COSE_Mac0.¶

Acknowledging the variety of markets, regulations, and use cases in which the PSA attestation token can be used, the baseline profile does not impose any strong requirement on the cryptographic algorithms that need to be supported by Attesters and Verifiers. The flexibility provided by the COSE format should be sufficient to deal with the level of cryptographic agility needed to adapt to specific use cases. It is RECOMMENDED that commonly adopted algorithms are used, such as those discussed in [COSE-ALGS]. It is expected that receivers will accept a wider range of algorithms while Attesters would produce PSA tokens using only one such algorithm.¶

The CWT CBOR tag (61) is not used. An application that needs to exchange PSA attestation tokens can wrap the serialized COSE_Sign1 or COSE_Mac0 in the media type defined in Section 10.2 or the CoAP Content-Format defined in Section 10.3.¶

A PSA token is always directly signed by the PSA RoT. Therefore, a PSA-token claims set (Section 4) is never carried in a Detached EAT bundle (Section 5 of [EAT]).¶

5.1.2. Freshness Model

The PSA token supports the freshness models for attestation Evidence based on nonces and epoch handles (Section 10.2 and Section 10.3 of [RFC9334]) using the "nonce" claim to convey the nonce or epoch handle supplied by the Verifier. No further assumption on the specific remote attestation protocol is made.¶

Note that use of epoch handles is constrained by the type restrictions imposed by the eat_nonce syntax. For use in PSA tokens, it must be possible to encode the epoch handle as an opaque binary string between 8 and 64 octets.¶

5.1.3. Synopsis

Table 3 presents a concise view of the requirements described in the preceding sections.¶

10.1.1. Client ID Claim

Claim Name:

psa-client-id¶

Claim Description:

PSA Client ID¶

JWT Claim Name:

N/A¶

Claim Key:

2394¶

Claim Value Type(s):

signed integer¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.1.2 of RFC 9783¶

10.1.2. Security Lifecycle Claim

Claim Name:

psa-security-lifecycle¶

Claim Description:

PSA Security Lifecycle¶

JWT Claim Name:

N/A¶

Claim Key:

2395¶

Claim Value Type(s):

unsigned integer¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.3.1 of RFC 9783¶

10.1.3. Implementation ID Claim

Claim Name:

psa-implementation-id¶

Claim Description:

PSA Implementation ID¶

JWT Claim Name:

N/A¶

Claim Key:

2396¶

Claim Value Type(s):

byte string¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.2.2 of RFC 9783¶

10.1.4. Certification Reference Claim

Claim Name:

psa-certification-reference¶

Claim Description:

PSA Certification Reference¶

JWT Claim Name:

N/A¶

Claim Key:

2398¶

Claim Value Type(s):

text string¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.2.3 of RFC 9783¶

10.1.5. Software Components Claim

Claim Name:

psa-software-components¶

Claim Description:

PSA Software Components¶

JWT Claim Name:

N/A¶

Claim Key:

2399¶

Claim Value Type(s):

array¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.4.1 of RFC 9783¶

10.1.6. Verification Service Indicator Claim

Claim Name:

psa-verification-service-indicator¶

Claim Description:

PSA Verification Service Indicator¶

JWT Claim Name:

N/A¶

Claim Key:

2400¶

Claim Value Type(s):

text string¶

Change Controller:

Hannes Tschofenig¶

Specification Document(s):

Section 4.5.1 of RFC 9783¶

10.3.1. Registry Contents

Media Type:

application/eat+cwt; eat_profile="tag:psacertified.org,2023:psa#tfm"¶

Encoding:

-¶

ID:

10003¶

Reference:

RFC 9783¶

Media Type:

application/eat+cwt; eat_profile="tag:psacertified.org,2019:psa#legacy"¶

Encoding:

-¶

ID:

10004¶

Reference:

RFC 9783¶