]> University of Applied Sciences Bonn-Rhein-Sieg

Germany Hannes.Tschofenig@gmx.net

Arm Limited

Simon.Frost@arm.com

Arm Limited

Mathias.Brossard@arm.com

HP Labs

adrianlshaw@acm.org

Linaro

thomas.fossati@linaro.org

Remote Attestation EAT profile Evidence RATS IoT Arm's Platform Security Architecture (PSA) is a family of hardware and firmware security specifications, along with open-source reference implementations, aimed at helping device makers and chip manufacturers integrate best-practice security into their products. Devices that comply with PSA can generate attestation tokens as described in this document, which serve as the foundation for various protocols, including secure provisioning and network access control. This document specifies the structure and semantics of the PSA attestation token. The PSA attestation token is a profile of the Entity Attestation Token (EAT). This specification describes the claims used in an attestation token generated by PSA-compliant systems, how these claims are serialized for transmission, and how they are cryptographically protected. This Informational document is published as an Independent Submission to improve interoperability with Arm's architecture. It is not a standard nor a product of the IETF.

Introduction The Platform Security Architecture (PSA) is a set of hardware and firmware specifications backed by reference implementations and a security certification program . The security specifications have been published by Arm, while the certification program and reference implementations are the result of a collaborative effort by companies from multiple sectors, including evaluation laboratories, IP semiconductor vendors, and security consultancies. The main objective of the PSA initiative is to assist device manufacturers and chip makers in incorporating best-practice security measures into their products. Many devices now have Trusted Execution Environments (TEEs) that provide a safe space for security-sensitive code, such as cryptography, secure boot, secure storage, and other essential security functions. These security functions are typically exposed through a narrow and well-defined interface, and can be used by operating system libraries and applications. As outlined in the Remote ATtestation procedureS (RATS) Architecture , an Attester produces a signed collection of Claims that constitutes Evidence about its Target Environment. This document focuses on the output provided by PSA's Initial Attestation API . This output corresponds to Evidence in and, as a design decision, the PSA attestation token is a profile of the Entity Attestation Token (EAT) . Note that there are other profiles of EAT available for use with different use cases and by different attestation technologies, such as and . Since the PSA tokens are also consumed by services outside the device, there is an actual need to ensure interoperability. Interoperability needs are addressed here by describing the exact syntax and semantics of the attestation claims, and defining the way these claims are encoded and cryptographically protected. Further details on concepts expressed below can be found in the PSA Security Model documentation . As mentioned in the abstract, this memo documents a vendor extension to the RATS architecture and is not a standard.

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 when, and only when, they appear in all capitals, as shown here. The terms Attester, Relying Party, Verifier, Attestation Result, Target Environment, Attesting Environment, and Evidence are defined in . We use the term "receiver" to refer to Relying Parties and Verifiers. We use the terms Evidence, "PSA attestation token", and "PSA token" interchangeably. The terms "sender" and Attester are used interchangeably. Likewise, we use the terms Verifier and "verification service" interchangeably.

Root of Trust (RoT):

The minimal set of software, hardware, and data that has to be implicitly trusted in the platform; there is no software or hardware at a deeper level that can verify that the RoT is authentic and unmodified. An example of RoT is an initial bootloader in ROM, which contains cryptographic functions and credentials, running on a specific hardware platform.

Secure Processing Environment (SPE):

A platform's processing environment for software that provides confidentiality and integrity for its runtime state, from software and hardware, outside of the SPE. Contains trusted code and trusted hardware. (Equivalent to a TEE, "secure world", or "secure enclave".)

Non-Secure Processing Environment (NSPE):

The security domain (Application domain) outside of the SPE that typically contains the application firmware, real-time operating systems, applications, and general hardware. (Equivalent to Rich Execution Environment (REE), or "normal world".)

In this document, the structure of data is specified in Concise Data Definition Language (CDDL) .

PSA Attester Model outlines the structure of the PSA Attester according to the conceptual model described in .

PSA Attester | Boot |<---+ Attestation | | | | | | | W | State | R | Service | | | | | '-----+------' '-----' '-------------' | | | '----------------------|----------------------------------------' | | .------------+--------------+---------------. | | .--------|-------------|--------------|----------------|--------. | | | | | | | | | | | .------o-----. .-----o-------. .----o--------. .-----o------. | | | | | Updateable | | Application | | Application | | PSA RoT | | | | | | PSA RoT | | RoT | | Loader | | Parameters | | | | | '------------' '-------------' '-------------' '------------' | | | | Target Environment | | | '---------------------------------------------------------------' | '-------------------------------------------------------------------' Legend: ---> read ---> write ---o measure R W ]]>

The PSA Attester is a relatively straightforward embodiment of the RATS Attester with exactly one Attesting Environment and one or more Target Environments. The Attesting Environment is responsible for collecting the information to be represented in PSA claims and to assemble them into Evidence. The Attesting Environment is made of two cooperating components:

• Executing at boot-time, the Main Bootloader measures the Target Environments (i.e., loaded software components and all the relevant PSA RoT parameters) and stores the recorded information in secure memory (Main Boot State). See .

  PSA Attester Boot Phase | | '--------|-------------|-------------|----' | | | ]]>

• The Initial Attestation Service (executing at run-time in SPE) answers requests coming from NSPE via the PSA attestation API , collects and formats the claims from Main Boot State, and uses the Initial Attestation Key (IAK) to sign them and produce Evidence. See .

The word "Initial" in "Initial Attestation Service" refers to a limited set of Target Environments, namely those representing the first foundational stages establishing the chain of trust of a PSA device. Collecting measurements from Target Environments after this initial phase is outside the scope of this specification. Extensions of this specification could collect up-to-date measurements from additional Target Environments and define additional claims for use within those environments, but these are, by definition, custom.

PSA Attester Run-Time Phase | | | | PSA Token | | +---------->| | | | ]]>

The Target Environments can be of four types, some of which may or may not be present depending on the device architecture:

• (A subset of) the PSA RoT parameters, including Instance and Implementation IDs.
• The updateable PSA RoT, including the Secure Partition Manager and all PSA RoT services.
• The (optional) Application RoT, that is any application-defined security service possibly making use of the PSA RoT services.
• The loader of the application software running in NSPE.

A reference implementation of the PSA Attester is provided by .

PSA Claims This section describes the claims to be used in a PSA attestation token. A more comprehensive treatment of the EAT profiles defined by PSA is found in . CDDL along with text descriptions is used to define each claim independent of encoding. The following CDDL types are reused by different claims: Two conventions are used to encode the Right-Hand-Side (RHS) of a claim. The postfix -label is used for EAT-defined claims and the postfix -key is used for PSA-originated claims.

Caller Claims

Nonce The 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:

• The length MUST be either 32, 48, or 64 bytes.
• Only a single nonce value is conveyed. The array notation MUST NOT be used for encoding the nonce value.

This claim MUST be present in a PSA attestation token. psa-nonce = ( nonce-label => psa-hash-type )

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 . 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 )

Target Identification Claims

Instance ID The Instance ID claim represents the unique identifier of the IAK. The full definition is in . The EAT ueid (claim key 256) of type RAND is used. The following constraints apply to the ueid-type:

• The length MUST be 33 bytes.
• The first byte MUST be 0x01 (RAND) followed by the 32-byte unique identifier of the IAK. provides implementation options for deriving the IAK unique identifier from the IAK itself.

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 )

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 . psa-implementation-id-type = bytes .size 32 psa-implementation-id = ( psa-implementation-id-key => psa-implementation-id-type )

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 followed by a dash "-" and the five-digit versioning information described in . 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 )

Target State Claims

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 . A minor state is optional and 'IMPLEMENTATION DEFINED'. The PSA security lifecycle state and implementation state are encoded as follows:

• major[15:8] - PSA security lifecycle state, and
• minor[7:0] - IMPLEMENTATION DEFINED state.

The PSA lifecycle states are illustrated in . 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.

PSA Lifecycle States | Decommissioned |<--------' '----------------' ]]>

The CDDL representation is shown below. provides the mappings between and the data model. psa-lifecycle-type )]]> psa-lifecycle-unknown-type is not shown in ; it represents an invalid state that must not occur in a system. Lifecycle States Mappings

CDDL	Lifecycle States
psa-lifecycle-unknown-type
psa-lifecycle-assembly-and-test-type	Assembly and Test
psa-lifecycle-psa-rot-provisioning-type	PSA RoT Provisioning
psa-lifecycle-secured-type	Secured
psa-lifecycle-non-psa-rot-debug-type	Non-Recoverable PSA RoT Debug
psa-lifecycle-recoverable-psa-rot-debug-type	Recoverable PSA RoT Debug
psa-lifecycle-decommissioned-type	Decommissioned

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:

• The length MUST be between 8 and 32 bytes.

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 )

Software Inventory Claims

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 . 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 . 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. 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 ] )]]>

Measurement Type The Measurement Type attribute (key=1) is a short string representing the role of this software component. The following measurement types MAY be used for code measurements:

"BL":

a Boot Loader

"PRoT":

a component of the PSA Root of Trust

"ARoT":

a component of the Application Root of Trust

"App":

a component of the NSPE application

"TS":

a component of a Trusted Subsystem

The same labels with a "_CONFIG" postfix (e.g., "PRoT_CONFIG") MAY be used for configuration measurements. This attribute SHOULD be present in a PSA software component unless there is a very good reason to leave it out, for example, in networks with severely constrained bandwidth where sparing a few bytes really makes a difference.

Measurement Value The Measurement Value attribute (key=2) represents a hash of the invariant software component in memory at startup time. The value MUST be a cryptographic hash of 256 bits or stronger. This attribute MUST be present in a PSA software component.

Version The Version attribute (key=4) is the issued software version in the form of a text string. The value of this attribute will correspond to the entry in the original signed manifest of the component.

Signer ID The Signer ID attribute (key=5) uniquely identifies the signer of the software component. The identification is typically accomplished by hashing the signer's public key. The value of this attribute will correspond to the entry in the original manifest for the component. This can be used by a Verifier to ensure the components were signed by an expected trusted source. This attribute MUST be present in a PSA software component to be compliant with .

Measurement Description The Measurement Description attribute (key=6) contains a string identifying the hash algorithm used to compute the corresponding Measurement Value. The string SHOULD be encoded according to "Hash Name String" in the "Named Information Hash Algorithm Registry" .

Verification Claims The following claims, although part of Evidence, do not reflect the internal state of the Attester. Instead, they aim to help receivers, including Relying Parties, in processing the received attestation Evidence.

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.

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 . Future profiles derived from the baseline PSA profile SHALL create their unique value as described in . This claim MUST be present in a PSA attestation token. See 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 )

URI Structure for the Derived Profile Identifiers A new profile is associated with a unique string. The string MUST use the URI fragment syntax defined in . The string SHOULD be short to avoid unnecessary overhead. To avoid collisions, profile authors SHOULD communicate their intent upfront to use a certain string that uses the inquiry form on the website . To derive the value to be used for the eat_profile claim, the string is added as a fragment to the tag:psacertified.org,2023:psa tag URI . For example, a hypothetical profile using only COSE_Mac0 with the AES Message Authentication Code (AES-MAC) may decide to use the string "aes-mac". The eat_profile value would then be tag:psacertified.org,2023:psa#aes-mac.

Backwards Compatibility Considerations An earlier draft of this document identified by the PSA_IOT_PROFILE_1 profile, used claim key values from the "private use range" of the CWT Claims registry. These claim keys have now been deprecated. provides the mappings between the deprecated and new claim keys. Claim Key Mappings

	PSA_IOT_PROFILE_1	tag:psacertified.org,2023:psa#tfm
Nonce	-75008	10 (EAT nonce)
Instance ID	-75009	256 (EAT euid)
Profile Definition	-75000	265 (EAT eat_profile)
Client ID	-75001	2394
Security Lifecycle	-75002	2395
Implementation ID	-75003	2396
Boot Seed	-75004	268 (EAT bootseed)
Certification Reference	-75005	2398
Software Components	-75006	2399
Verification Service Indicator	-75010	2400

The new profile introduces three further changes:

• The "bootseed" claim is now optional and of variable length (see ).
• The "No Software Measurements" claim has been retired.
• The "Certification Reference" claim syntax changed from EAN-13 to EAN-13+5 (see ).

To simplify the transition to the token format described in this document, it is RECOMMENDED that Verifiers accept tokens encoded according to the old profile (PSA_IOT_PROFILE_1) as well as to the profile defined in this document (tag:psacertified.org,2023:psa#tfm), at least for the time needed to their devices to upgrade.

Profiles This document defines a baseline with common requirements that all PSA profiles must satisfy. (Note that this does not apply to .) This document also defines a "TFM" profile () that builds on the baseline while constraining the use of COSE algorithms to improve interoperability between Attesters and Verifiers. Baseline and TFM are what the EAT calls a "partial" and "full" profile, respectively. See for further details regarding profiles.

Baseline Profile

Token Encoding and Signing The PSA attestation token is encoded in CBOR format. The CBOR representation of a PSA token MUST be "valid" according to the definition in Section of RFC 8949 . 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 of RFC 8949 ). 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) . 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 . 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 or the CoAP Content-Format defined in . A PSA token is always directly signed by the PSA RoT. Therefore, a PSA-token claims set () is never carried in a Detached EAT bundle ().

Freshness Model The PSA token supports the freshness models for attestation Evidence based on nonces and epoch handles (Section and Section of ) 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.

Synopsis presents a concise view of the requirements described in the preceding sections. Baseline Profile

Issue	Profile Definition
CBOR/JSON	CBOR MUST be used.
CBOR Encoding	Definite length maps and arrays MUST be used.
CBOR Encoding	Definite length strings MUST be used.
CBOR Serialization	Variant serialization MAY be used.
COSE Protection	COSE_Sign1 and/or COSE_Mac0 MUST be used.
Algorithms	SHOULD be used.
Detached EAT Bundle Usage	Detached EAT bundles MUST NOT be sent.
Verification Key Identification	Any identification method listed in .
Endorsements	See .
Freshness	Nonce or epoch ID-based.
Claims	Those defined in . As per general EAT rules, the receiver MUST NOT error out on claims it does not understand.

Profile TFM The TFM profile is appropriate for the code base implemented in and should apply for most derivative implementations. If an implementation changes the requirements described below, then a different profile value should be used () to ensure interoperability. This includes a restriction of the profile to a subset of the COSE Protection scheme requirements. presents a concise view of the requirements. The value of the eat_profile MUST be tag:psacertified.org,2023:psa#tfm. TF-M Profile

Issue	Profile Definition
CBOR/JSON	See .
CBOR Encoding	See .
CBOR Encoding	See .
CBOR Serialization	See .
COSE Protection	COSE_Sign1 or COSE_Mac0 MUST be used.
Algorithms	The receiver MUST accept ES256, ES384, and ES512 with COSE_Sign1 and HMAC256/256, HMAC384/384, and HMAC512/512 with COSE_Mac0; the sender MUST send one of these.
Detached EAT Bundle Usage	See .
Verification Key Identification	Claim-Based Key Identification () using Instance ID.
Endorsements	See .
Freshness	See .
Claims	See .

Collated CDDL psa-boot-seed-type ) 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 ) psa-certification-reference-type = text .regexp "[0-9]{13}-[0-9]{5}" psa-certification-reference = ( ? psa-certification-reference-key => psa-certification-reference-type ) psa-implementation-id-type = bytes .size 32 psa-implementation-id = ( psa-implementation-id-key => psa-implementation-id-type ) psa-instance-id-type = bytes .size 33 psa-instance-id = ( ueid-label => psa-instance-id-type ) psa-nonce = ( nonce-label => psa-hash-type ) psa-profile-type = "tag:psacertified.org,2023:psa#tfm" psa-profile = ( profile-label => psa-profile-type ) 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-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 ] ) psa-verification-service-indicator-type = text psa-verification-service-indicator = ( ? psa-verification-service-indicator-key => psa-verification-service-indicator-type )]]>

Scalability Considerations IAKs (see ) can be either raw public keys or certified public keys. Certified public keys require the manufacturer to run the certification authority (CA) that issues X.509 certificates for the IAKs. (Note that operating a CA is a complex and expensive task that may be unaffordable to certain manufacturers.) Using certified public keys offers better scalability properties when compared to using raw public keys, namely:

• Storage requirements for the Verifier are minimized; the same manufacturer's trust anchor is used for any number of devices.
• The provisioning model is simpler and more robust since there is no need to notify the Verifier about each newly manufactured device.

Furthermore, existing and well-understood revocation mechanisms can be readily used. The IAK's X.509 certificates can be inlined in the PSA token using the x5chain COSE header parameter at the cost of an increase in the PSA token size. and provide guidance for profiling X.509 certificates used in IoT deployments. Note that the exact split between pre-provisioned and inlined certificates may vary depending on the specific deployment. In that respect, x5chain is quite flexible. It can contain the end entity (EE) certification only, the EE and a partial chain, or the EE and the full chain up to the trust anchor (see for the details). Constraints around network bandwidth and computing resources available to endpoints, such as network buffers, may dictate a reasonable split point.

PSA Token Verification To verify the token, the primary need is to check correct encoding and signing as detailed in . The key used for verification is either supplied to the Verifier by an authorized Endorser along with the corresponding Attester's Instance ID or inlined in the token using the x5chain header parameter as described in . If the IAK is a raw public key and the Instance ID claim is used to assist in locating the key used to verify the signature covering the CWT token. If the IAK is a certified public key, X.509 path construction and validation () up to a trusted CA MUST be successful before the key is used to verify the token signature. This also includes revocation checking. The Verifier typically has a policy where it compares some claims in this profile to reference values registered with it for a given deployment. This verification process serves to confirm that the device is endorsed by the manufacturer supply chain. The policy may require that the relevant claims must have a match to a registered reference value. All claims may be worthy of additional appraisal. It is likely that most deployments would include a policy with appraisal for the following claims:

• Implementation ID: The value of the Implementation ID can be used to identify the verification requirements of the deployment.
• Software Component, Measurement Value: This value can uniquely identify a firmware release from the supply chain. In some cases, a Verifier may maintain a record for a series of firmware releases being patches to an original baseline release. A verification policy may then allow this value to match any point on that release sequence or expect some minimum level of maturity related to the sequence.
• Software Component, Signer ID: Where present in a deployment, this could allow a Verifier to operate a more general policy than that for Measurement Value as above by allowing a token to contain any firmware entries signed by a known Signer ID without checking for a uniquely registered version.
• Certification Reference: If present, this value could be used as a hint to locate security certification information associated with the attesting device. An example could be a reference to a certificate.

AR4SI Trustworthiness Claims Mappings defines an information model that Verifiers can employ to produce Attestation Results. AR4SI provides a set of standardized appraisal categories and tiers that greatly simplifies the task of writing Relying Party policies in Multi-Attester environments. The contents of are intended as guidance for implementing a PSA Verifier that computes its results using AR4SI. The table describes which PSA Evidence claims (if any) are related to which AR4SI trustworthiness claim, and therefore what the Verifier must consider when deciding if and how to appraise a certain feature associated with the PSA Attester. AR4SI Claims mappings

Trustworthiness Vector claims	Related PSA claims
"configuration"	Software Components ()
"executables"	ditto
"file-system"	N/A
"hardware"	Implementation ID ()
"instance-identity"	Instance ID (). The Security Lifecycle () can also impact the derived identity.
"runtime-opaque"	Indirectly derived from "executables", "hardware", and "instance-identity". The Security Lifecycle () can also be relevant, e.g., any debug state will expose otherwise protected memory.
"sourced-data"	N/A
"storage-opaque"	Indirectly derived from "executables", "hardware", and "instance-identity".

This document does not prescribe what value must be chosen based on each possible situation. When assigning specific Trustworthiness Claim values, an implementation is expected to follow the algorithm described in .

Endorsements, Reference Values, and Verification Key Material defines a protocol based on the data model that can be used to convey PSA Endorsements, Reference Values, and verification key material to the Verifier.

Security and Privacy Considerations This specification reuses the EAT specification and therefore the CWT specification. Hence, the security and privacy considerations of those specifications apply here as well. Since CWTs offer different ways to protect the token, this specification profiles those options and allows signatures using public key cryptography as well as message authentication codes (MACs). COSE_Sign1 is used for digital signatures and COSE_Mac0 for MACs as defined in the COSE specification . Note, however, that the use of MAC authentication is NOT RECOMMENDED due to the associated infrastructure costs for key management and protocol complexities. A PSA Attester MUST NOT provide Evidence to an untrusted challenger, as it may allow attackers to interpose and trick the Verifier into believing the attacker is a legitimate Attester. This is especially relevant to protocols that use PSA attestation tokens to authenticate the attester to a Relying Party. Attestation tokens contain information that may be unique to a device. Therefore, they may allow to single out an individual device for tracking purposes. Deployments that have privacy requirements must take appropriate measures to ensure that the token is only used to provision anonymous/pseudonym keys.

IANA Considerations

CBOR Web Token Claims Registration IANA has registered the following claims in the "CBOR Web Token (CWT) Claims" registry .

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):

of RFC 9783

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):

of RFC 9783

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):

of RFC 9783

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):

of RFC 9783

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):

of RFC 9783

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):

of RFC 9783

Media Types This document does not register any new media types. To indicate that the transmitted content is a PSA attestation token, applications can use the application/eat+cwt media type defined in with the eat_profile parameter set to tag:psacertified.org,2023:psa#tfm (or tag:psacertified.org,2019:psa#legacy if the token is encoded according to the old profile; see ).

CoAP Content-Formats Registration IANA has registered two CoAP Content-Format IDs in the First Come First Served range of the "CoAP Content-Formats" registry :

• One for the application/eat+cwt media type with the eat_profile parameter equal to tag:psacertified.org,2023:psa#tfm.
• Another for the application/eat+cwt media type with the eat_profile parameter equal to tag:psacertified.org,2019:psa#legacy.

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

References Normative References Arm GS1 Arm PSA Certified IANA IANA Informative References IANA Trusted Firmware Trusted Firmware commit: 0b49b00195b7733d6fe74e8f42ed4d7b81242801 Arm PSA Certified Arm

Examples The following examples show PSA attestation tokens for a hypothetical system comprising a single measured software component. The attesting device is in a lifecycle state () of SECURED. The attestation has been requested from a client residing in the SPE. The example in illustrates the case where the IAK is an asymmetric key. A COSE Sign1 envelope is used to wrap the PSA-token claims set. illustrates the case where the IAK is a symmetric key and a COSE Mac0 envelope is used instead. The claims sets are identical, except for the Instance ID which is synthesized from the key material. The examples have been created using the iat-verifier tool .

COSE Sign1 Token { / ueid / 256: h'01020202020202020202020202 0202020202020202020202020202020202020202', / psa-implementation-id / 2396: h'00000000000000000000000000 00000000000000000000000000000000000000', / eat_nonce / 10: h'01010101010101010101010101 01010101010101010101010101010101010101', / psa-client-id / 2394: 2147483647, / psa-security-lifecycle / 2395: 12288, / eat_profile / 265: "tag:psacertified.org,2023:psa#tfm", / bootseed / 268: h'0000000000000000', / psa-software-components / 2399: [ { / signer ID / 5: h'0404040404040404040404040404040 404040404040404040404040404040404', / measurement value / 2: h'0303030303030303030303030303030 303030303030303030303030303030303', / measurement type / 1: "PRoT" } ] } The JWK representation of the IAK used for creating the COSE Sign1 signature over the PSA token is: { "kty": "EC", "crv": "P-256", "alg": "ES256", "x": "Tl4iCZ47zrRbRG0TVf0dw7VFlHtv18HInYhnmMNybo8", "y": "gNcLhAslaqw0pi7eEEM2TwRAlfADR0uR4Bggkq-xPy4", "d": "Q__-y5X4CFp8QOHT6nkL7063jN131YUDpkwWAPkbM-c" } The resulting COSE object is: 18([ h'A10126', {}, h'A81901005821010202020202020202020202020202020202020202020202 02020202020202020219095C5820000000000000000000000000000000000000 00000000000000000000000000000A5820010101010101010101010101010101 010101010101010101010101010101010119095A1A7FFFFFFF19095B19300019 010978217461673A7073616365727469666965642E6F72672C323032333A7073 612374666D19010C48000000000000000019095F81A305582004040404040404 0404040404040404040404040404040404040404040404040402582003030303 0303030303030303030303030303030303030303030303030303030301645052 6F54', h'786E937A4C42667AF3847399319CA95C7E7DBABDC9B50FDB8DE3F6BFF4AB 82FF80C42140E2A488000219E3E10663193DA69C75F52B798EA10B2F7041A90E 8E5A' ]) which has the following base16 encoding: d28443a10126a0590100a819010058210102020202020202020202020202 0202020202020202020202020202020202020219095c5820000000000000 00000000000000000000000000000000000000000000000000000a582001 010101010101010101010101010101010101010101010101010101010101 0119095a1a7fffffff19095b19300019010978217461673a707361636572 7469666965642e6f72672c323032333a7073612374666d19010c48000000 000000000019095f81a30558200404040404040404040404040404040404 040404040404040404040404040404025820030303030303030303030303 0303030303030303030303030303030303030303016450526f545840786e 937a4c42667af3847399319ca95c7e7dbabdc9b50fdb8de3f6bff4ab82ff 80c42140e2a488000219e3e10663193da69c75f52b798ea10b2f7041a90e 8e5a

COSE Mac0 Token { / ueid / 256: h'01C557BD4FADC83F756FCA2CD5 EA2DCC8B82159BB4E7453D6A744D4EECD6D0AC60', / psa-implementation-id / 2396: h'00000000000000000000000000 00000000000000000000000000000000000000', / eat_nonce / 10: h'01010101010101010101010101 01010101010101010101010101010101010101', / psa-client-id / 2394: 2147483647, / psa-security-lifecycle / 2395: 12288, / eat_profile / 265: "tag:psacertified.org,2023:psa#tfm", / psa-boot-seed / 268: h'0000000000000000', / psa-software-components / 2399: [ { / signer ID / 5: h'0404040404040404040404040404040 404040404040404040404040404040404', / measurement value / 2: h'0303030303030303030303030303030 303030303030303030303030303030303', / measurement type / 1: "PRoT" } ] } The JWK representation of the IAK used for creating the COSE Mac0 signature over the PSA token is: ========== NOTE: '\\' line wrapping per RFC 8792 ========== { "kty": "oct", "alg": "HS256", "k": "3gOLNKyhJXaMXjNXq40Gs2e5qw1-i-Ek7cpH_gM6W7epPTB_8imqNv8k\ \bBKVlk-s9xq3qm7E_WECt7OYMlWtkg" } The resulting COSE object is: 17([ h'A10105', {}, h'A8190100582101C557BD4FADC83F756FCA2CD5EA2DCC8B82159BB4E7453D 6A744D4EECD6D0AC6019095C5820000000000000000000000000000000000000 00000000000000000000000000000A5820010101010101010101010101010101 010101010101010101010101010101010119095A1A7FFFFFFF19095B19300019 010978217461673A7073616365727469666965642E6F72672C323032333A7073 612374666D19010C48000000000000000019095F81A305582004040404040404 0404040404040404040404040404040404040404040404040402582003030303 0303030303030303030303030303030303030303030303030303030301645052 6F54', h'CF88D330E7A5366A95CF744A4DBF0D50304D405EDD8B2530E243EDDBD317 7820' ]) which has the following base16 encoding: d18443a10105a0590100a8190100582101c557bd4fadc83f756fca2cd5ea 2dcc8b82159bb4e7453d6a744d4eecd6d0ac6019095c5820000000000000 00000000000000000000000000000000000000000000000000000a582001 010101010101010101010101010101010101010101010101010101010101 0119095a1a7fffffff19095b19300019010978217461673a707361636572 7469666965642e6f72672c323032333a7073612374666d19010c48000000 000000000019095f81a30558200404040404040404040404040404040404 040404040404040404040404040404025820030303030303030303030303 0303030303030303030303030303030303030303016450526f545820cf88 d330e7a5366a95cf744a4dbf0d50304d405edd8b2530e243eddbd3177820

Acknowledgments Thank you for help with the CDDL. Thanks to , , , , and for ideas, comments, and suggestions.

Contributors Security Theory LLC

lgl@securitytheory.com

Arm Limited

Tamas.Ban@arm.com

Arm Limited

Sergei.Trofimov@arm.com