Internet-Draft PSA Attestation Token December 2023
Tschofenig, et al. Expires 3 June 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-tschofenig-rats-psa-token-17
Published:
Intended Status:
Informational
Expires:
Authors:
H. Tschofenig
S. Frost
Arm Limited
M. Brossard
Arm Limited
A. Shaw
HP Labs
T. Fossati
Linaro

Arm's Platform Security Architecture (PSA) Attestation Token

Abstract

The Platform Security Architecture (PSA) is a family of hardware and firmware security specifications, as well as open-source reference implementations, to help device makers and chip manufacturers build best-practice security into products. Devices that are PSA compliant can produce attestation tokens as described in this memo, which are the basis for many different protocols, including secure provisioning and network access control. This document specifies the PSA attestation token structure and semantics.

The PSA attestation token is a profiled Entity Attestation Token (EAT).

This specification describes what claims are used in an attestation token generated by PSA compliant systems, how these claims get serialized to the wire, and how they are cryptographically protected.

This document is produced through the Independent RFC Stream and was not subject to the IETF's approval process.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 3 June 2024.

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Table of Contents

1. Introduction

Trusted execution environments are now present in many devices, which provide a safe environment to place 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. Various APIs have been developed by Arm as part of the Platform Security Architecture [PSA] framework. This document focuses on the output provided by PSA's Initial Attestation API [PSA-API]. Since the 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 [PSA-SM].

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 terms Attester, Relying Party, Verifier, Attestation Result, and Evidence are defined in [RFC9334]. 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.

RoT:

Root of Trust, 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 Root of Trust 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.

SPE:

Secure Processing Environment, 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 Trusted Execution Environment (TEE), or "secure world".)

NSPE:

Non Secure Processing Environment, the security domain outside of the SPE, the Application domain, typically containing the application firmware, operating systems, and general hardware. (Equivalent to Rich Execution Environment (REE), or "normal world".)

3. PSA Attester Model

Figure 1 outlines the structure of the PSA Attester according to the conceptual model described in Section 3.1 of [RFC9334].

Verifier Evidence Attesting Environment Main Main Initial Bootloader Boot Attestation State Service Updateable Application Application PSA RoT PSA RoT RoT Loader Parameters Target Environment
Figure 1: PSA Attester

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. It is made of two cooperating components:

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

A reference implementation of the PSA Attester is provided by [TF-M].

4. PSA Claims

This section describes the claims to be used in a PSA attestation token.

CDDL [RFC8610] along with text descriptions is used to define each claim independent of encoding. The following CDDL type(s) are reused by different claims:

psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64

4.1. Caller Claims

4.1.1. Nonce

The Nonce claim is used to carry the challenge provided by the caller to demonstrate freshness of the generated token.

The EAT [EAT] nonce (claim key 10) is used. The following constraints apply 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
)

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. Target Identification Claims

4.2.1. Instance ID

The Instance ID claim represents the unique identifier of the Initial Attestation Key (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:

  • The length MUST be 33 bytes.

  • The first byte MUST be 0x01 (RAND) followed by the 32-bytes key hash.

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 implementation 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. It MUST be represented as a string made of nineteen numeric characters: a thirteen-digit [EAN-13], followed by a dash "-", followed by 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.

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. Target State Claims

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:

  • version[15:8] - PSA security lifecycle state, and

  • version[7:0] - IMPLEMENTATION DEFINED state.

The PSA lifecycle states are illustrated in Figure 2. 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.

Enrol Provisioning Lockdown Verifier Secured Blocklist Non-PSA RoT Debug Recoverable PSA RoT Debug Terminate Decommissioned
Figure 2: PSA Lifecycle States 
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
)

4.3.2. Boot Seed

The Boot Seed claim represents a value created at system boot time that will allow differentiation of reports from different boot sessions.

This claim MAY be present in a PSA attestation token.

If present, it MUST be between 8 and 32 bytes.

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

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

4.4. Software Inventory Claims

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, as described in [RFC9334], a relying party will typically see the result of the verification process from the Verifier in form of an attestation result, rather than the PSA token from the attesting endpoint. 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 endorsements and provide an answer in form of an "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 ]
)
4.4.1.1. Measurement Type

The Measurement Type attribute (key=1) is 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 "-cfg" postfix (e.g., "PRoT-cfg") 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.

4.4.1.2. 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.

4.4.1.3. 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.

4.4.1.4. Signer ID

The Signer ID attribute (key=5) is the hash of a signing authority public key for the software component. 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 [PSA-SM].

4.4.1.5. 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 [IANA-HashFunctionTextualNames].

4.5. Verification Claims

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
)

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
)
4.5.2.1. 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 Section 3.5 of [RFC3986].

The string SHOULD be short to avoid unnecessary overhead.

To avoid collisions, profile authors SHOULD communicate upfront their intent to use a certain string using the enquiry form on the [PSACertified] 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 [RFC4151].

For example, an 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.

4.6. Backwards Compatibility Considerations

A previous version of this specification (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 retired and their use is deprecated.

Table 1 provides the mappings between the deprecated and new claim keys.

Table 1: 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 2397
Certification Reference -75005 2398
Software Components -75006 2399
Verification Service Indicator -75010 2400

The new profile introduces three further changes:

  • the "Boot Seed" claim is now optional and of variable length (see Section 4.3.2),

  • the "No Software Measurements" claim has been retired,

  • the "Certification Reference" claim syntax changed from EAN-13 to EAN-13+5 (see Section 4.2.3).

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 new profile (tag:psacertified.org,2023:psa#tfm), at least for the time needed to their devices to upgrade.

5. Profiles

This document defines a baseline with common requirements that all PSA profiles must satisfy.

This document also defines a profile (Section 5.2) that builds on the baseline while constraining the use of COSE algorithms to improve interoperability between PSA Attesters and Verifiers.

5.1. Baseline Profile

5.1.1. Token Encoding and Signing

The PSA attestation token is encoded in CBOR [RFC8949] format. 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 [RFC8949]). 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 serialised COSE_Sign1 or COSE_Mac0 in the media type defined in Section 11.2 or the CoAP Content-Format defined in Section 11.3.

A PSA token is always directly signed by the PSA RoT. Therefore, a PSA 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 2 presents a concise view of the requirements described in the preceding sections.

Table 2: 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 [COSE-ALGS] SHOULD be used
Detached EAT Bundle Usage Detached EAT bundles MUST NOT be sent
Verification Key Identification Any identification method listed in Appendix F.1 of [EAT]
Endorsements See Section 10.2
Freshness nonce or epoch ID based
Claims Those defined in Section 4. As per general EAT rules, the receiver MUST NOT error out on claims it does not understand.

5.2. Profile TFM

This profile is appropriate for the code base implemented in [TF-M] and should apply for most derivative implementations. If an implementation changes the requirements described below then, to ensure interoperability, a new profile value should be used (Section 4.5.2.1). This includes a restriction of the profile to a subset of the COSE Protection scheme requirements.

Table 3 presents a concise view of the requirements.

The value of the eat_profile MUST be tag:psacertified.org,2023:psa#tfm.

Table 3: TF-M Profile
Issue Profile Definition
CBOR/JSON See Section 5.1
CBOR Encoding See Section 5.1
CBOR Encoding See Section 5.1
CBOR Serialization See Section 5.1
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 Section 5.1
Verification Key Identification Claim-Based Key Identification (Appendix F.1.4 of [EAT]) using Implementation ID and Instance ID
Endorsements See Section 10.2
Freshness See Section 5.1
Claims See Section 5.1

6. Collated CDDL 

psa-token = {
    psa-nonce
    psa-instance-id
    psa-verification-service-indicator
    psa-profile
    psa-implementation-id
    psa-client-id
    psa-lifecycle
    psa-certification-reference
    ? psa-boot-seed
    psa-software-components
}

psa-client-id-key = 2394
psa-lifecycle-key = 2395
psa-implementation-id-key = 2396
psa-boot-seed-key = 2397
psa-certification-reference-key = 2398
psa-software-components-key = 2399
psa-verification-service-indicator-key = 2400

nonce-label = 10
ueid-label = 256
profile-label = 265

psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64

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

psa-boot-seed = (
    psa-boot-seed-key => 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
)

7. Scalability Considerations

IAKs 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 certs for the IAKs. (Note that operating a CA is a complex and expensive task that may be unaffordable to certain manufacturers.)

If applicable, such approach provides sensibly better scalability properties compared to using raw public keys, namely:

The IAK's X.509 cert can be inlined in the PSA token using the x5chain COSE header parameter [COSE-X509] at the cost of an increase in the PSA token size. Section 4.4 of [TLS12-IoT] and Section 15 of [TLS13-IoT] provide guidance for profiling X.509 certs used in IoT deployments. Note that the exact split between pre-provisioned and inlined certs may vary depending on the specific deployment. In that respect, x5chain is quite flexible: it can contain the end-entity (EE) cert only, the EE and a partial chain, or the EE and the full chain up to the trust anchor (see Section 2 of [COSE-X509] for the details). Deciding on a sensible split point may depend on constraints around network bandwidth and computing resources available to the endpoints (especially network buffers).

8. Implementation Status

Implementations of this specification are provided by the Trusted Firmware-M project [TF-M], [IAT-VERIFIER], the Veraison project [Veraison], and the Xclaim [Xclaim] library. All four implementations are released as open-source software.

9. Security and Privacy Considerations

This specification re-uses 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 [STD96]. 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 attestation evidence to an untrusted challenger, as it may allow attackers to interpose and trick the verifier into believing the attacker is a legitimate attester.

Attestation tokens contain information that may be unique to a device and 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.

10. Verification

To verify the token, the primary need is to check correct encoding and signing as detailed in Section 5.1.1. 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 Section 7. If the IAK is a raw public key, the Instance and Implementation ID claims are used (together with the kid in the COSE header, if present) 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 (Section 6 of [X509]) up to a trusted CA MUST be successful before the key is used to verify the token signature.

In addition, the Verifier will typically operate a policy where values of some of the claims in this profile can be compared to reference values, registered with the Verifier for a given deployment, in order 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:

10.1. AR4SI Trustworthiness Claims Mappings

[RATS-AR4SI] 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 Table 4 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.

Table 4: AR4SI Claims mappings
Trustworthiness Vector claims Related PSA claims
configuration Software Components (Section 4.4.1)
executables ditto
file-system N/A
hardware Implementation ID (Section 4.2.2)
instance-identity Instance ID (Section 4.2.1). The Security Lifecycle (Section 4.3.1) can also impact the derived identity.
runtime-opaque Indirectly derived from executables, hardware, and instance-identity. The Security Lifecycle (Section 4.3.1) can also be relevant: for example, 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 Section 2.3.3 of [RATS-AR4SI].

10.2. Endorsements, Reference Values and Verification Key Material

[PSA-Endorsements] defines a protocol based on the [RATS-CoRIM] data model that can be used to convey PSA Endorsements, Reference Values and verification key material to the Verifier.

11. IANA Considerations

11.1. CBOR Web Token Claims Registration

IANA is requested to make permanent the following claims that have been assigned via early allocation in the "CBOR Web Token (CWT) Claims" registry [IANA-CWT].

11.1.1. 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 RFCthis

11.1.2. 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 RFCthis

11.1.3. 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 RFCthis

11.1.4. Boot Seed Claim

  • Claim Name: psa-boot-seed

  • Claim Description: PSA Boot Seed

  • JWT Claim Name: N/A

  • Claim Key: 2397

  • Claim Value Type(s): byte string

  • Change Controller: Hannes Tschofenig

  • Specification Document(s): Section 4.3.2 of RFCthis

11.1.5. 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 RFCthis

11.1.6. 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 RFCthis

11.1.7. 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 RFCthis

11.2. Media Types

No new media type registration is requested. To indicate that the transmitted content is a PSA Attestation Token, applications can use the application/eat+cwt media type defined in [EAT-MEDIATYPES] with the eat_profile parameter set to tag:psacertified.org,2023:psa#tfm (or PSA_IOT_PROFILE_1 if the token is encoded according to the old profile, see Section 4.6).

11.3. CoAP Content-Formats Registration

IANA is requested to register two CoAP Content-Format IDs in the "CoAP Content-Formats" registry [IANA-CoAP-Content-Formats]:

  • 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 PSA_IOT_PROFILE_1

The Content-Formats should be allocated from the Expert review range (0-255).

11.3.1. Registry Contents

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

  • Encoding: -

  • Id: [[To-be-assigned by IANA]]

  • Reference: RFCthis

  • Media Type: application/eat+cwt; eat_profile="PSA_IOT_PROFILE_1"

  • Encoding: -

  • Id: [[To-be-assigned by IANA]]

  • Reference: RFCthis

12. References

12.1. Normative References

[COSE-ALGS]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, , <https://www.rfc-editor.org/rfc/rfc9053>.
[EAN-13]
GS1, "International Article Number - EAN/UPC barcodes", , <https://www.gs1.org/standards/barcodes/ean-upc>.
[EAT]
Lundblade, L., Mandyam, G., O'Donoghue, J., and C. Wallace, "The Entity Attestation Token (EAT)", Work in Progress, Internet-Draft, draft-ietf-rats-eat-22, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-eat-22>.
[EAT-MEDIATYPES]
Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media Types", Work in Progress, Internet-Draft, draft-ietf-rats-eat-media-type-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-eat-media-type-05>.
[IANA-CWT]
IANA, "CBOR Web Token (CWT) Claims", , <https://www.iana.org/assignments/cwt/cwt.xhtml#claims-registry>.
[PSA-Cert-Guide]
PSA Certified, "PSA Certified Level 2 Step by Step Guide Version 1.1", , <https://www.psacertified.org/app/uploads/2020/07/JSADEN011-PSA_Certified_Level_2_Step-by-Step-1.1-20200403.pdf>.
[PSA-FF]
Arm, "Platform Security Architecture Firmware Framework 1.0 (PSA-FF)", , <https://developer.arm.com/documentation/den0063/a>.
[PSA-SM]
Arm, "Platform Security Model 1.1", , <https://www.psacertified.org/app/uploads/2021/12/JSADEN014_PSA_Certified_SM_V1.1_BET0.pdf>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3986]
Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, , <https://www.rfc-editor.org/rfc/rfc3986>.
[RFC4151]
Kindberg, T. and S. Hawke, "The 'tag' URI Scheme", RFC 4151, DOI 10.17487/RFC4151, , <https://www.rfc-editor.org/rfc/rfc4151>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC8392]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, , <https://www.rfc-editor.org/rfc/rfc8392>.
[RFC8610]
Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <https://www.rfc-editor.org/rfc/rfc8610>.
[RFC8949]
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, , <https://www.rfc-editor.org/rfc/rfc8949>.
[STD96]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <https://www.rfc-editor.org/rfc/rfc9052>.
[X509]
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, , <https://www.rfc-editor.org/rfc/rfc5280>.

12.2. Informative References

[COSE-X509]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Header Parameters for Carrying and Referencing X.509 Certificates", RFC 9360, DOI 10.17487/RFC9360, , <https://www.rfc-editor.org/rfc/rfc9360>.
[IANA-CoAP-Content-Formats]
IANA, "CoAP Content-Formats", , <https://www.iana.org/assignments/core-parameters>.
[IANA-HashFunctionTextualNames]
IANA, "Hash Function Textual Names", , <https://www.iana.org/assignments/hash-function-text-names>.
[IAT-VERIFIER]
Linaro, "iat-verifier", , <https://git.trustedfirmware.org/TF-M/tf-m-tools.git/tree/iat-verifier>.
[PSA]
Arm, "Platform Security Architecture Resources", , <https://developer.arm.com/architectures/security-architectures/platform-security-architecture/documentation>.
[PSA-API]
Arm, "PSA Attestation API 1.0.3", , <https://arm-software.github.io/psa-api/attestation/1.0/IHI0085-PSA_Certified_Attestation_API-1.0.3.pdf>.
[PSA-Endorsements]
Fossati, T., Deshpande, Y., and H. Birkholz, "Arm's Platform Security Architecture (PSA) Attestation Verifier Endorsements", Work in Progress, Internet-Draft, draft-fdb-rats-psa-endorsements-03, , <https://datatracker.ietf.org/doc/html/draft-fdb-rats-psa-endorsements-03>.
[PSACertified]
PSA Certified, "PSA Certified IoT Security Framework", , <https://psacertified.org>.
[RATS-AR4SI]
Voit, E., Birkholz, H., Hardjono, T., Fossati, T., and V. Scarlata, "Attestation Results for Secure Interactions", Work in Progress, Internet-Draft, draft-ietf-rats-ar4si-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-ar4si-05>.
[RATS-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-03, , <https://datatracker.ietf.org/doc/html/draft-ietf-rats-corim-03>.
[RFC9334]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, , <https://www.rfc-editor.org/rfc/rfc9334>.
[TF-M]
Linaro, "Trusted Firmware-M", , <https://www.trustedfirmware.org/projects/tf-m/>.
[TLS12-IoT]
Tschofenig, H., Ed. and T. Fossati, "Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things", RFC 7925, DOI 10.17487/RFC7925, , <https://www.rfc-editor.org/rfc/rfc7925>.
[TLS13-IoT]
Tschofenig, H., Fossati, T., and M. Richardson, "TLS/DTLS 1.3 Profiles for the Internet of Things", Work in Progress, Internet-Draft, draft-ietf-uta-tls13-iot-profile-08, , <https://datatracker.ietf.org/doc/html/draft-ietf-uta-tls13-iot-profile-08>.
[Veraison]
The Veraison Project, "Veraison psatoken package", , <https://github.com/veraison/psatoken>.
[Xclaim]
Lundblade, L., "Xclaim", , <https://github.com/laurencelundblade/xclaim>.

Appendix A. Examples

The following examples show PSA attestation tokens for an hypothetical system comprising a single measured software component. The attesting device is in a lifecycle state (Section 4.3.1) of SECURED. The attestation has been requested from a client residing in the SPE.

The example in Appendix A.1 illustrates the case where the IAK is an asymmetric key. A COSE Sign1 envelope is used to wrap the PSA claims-set.

Appendix A.2 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 [IAT-VERIFIER].

A.1. 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:p
sa#tfm",
  / psa-boot-seed /           2397: h'0000000000000000',
  / psa-software-components / 2399: [
    {
      / signer ID /         5: h'0404040404040404040404040404040
404040404040404040404040404040404',
      / measurement value / 2: h'0303030303030303030303030303030
303030303030303030303030303030303'
    }
  ]
}

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
612374666D19095D48000000000000000019095F81A205582004040404040404
0404040404040404040404040404040404040404040404040402582003030303
03030303030303030303030303030303030303030303030303030303',
  h'2F4C8152ED1691833EEBB6A182D2120E3D19220DF85B9AC51109113A423F
A024205CEDA0815968548DE4FBB6DC94B88C916F0D266E64CEA24183A84F977D
E475'
])

which has the following base16 encoding:
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A.2. 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:p
sa#tfm",
  / psa-boot-seed /           2397: h'0000000000000000',
  / psa-software-components / 2399: [
    {
      / signer ID /         5: h'0404040404040404040404040404040
404040404040404040404040404040404',
      / measurement value / 2: h'0303030303030303030303030303030
303030303030303030303030303030303'
    }
  ]
}

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
612374666D19095D48000000000000000019095F81A205582004040404040404
0404040404040404040404040404040404040404040404040402582003030303
03030303030303030303030303030303030303030303030303030303',
  h'9B9FAA23F25D630B620C40508C978FBDC46130A5898BA1E8014085B13E43
256E'
])

which has the following base16 encoding:
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Acknowledgments

Thanks to Carsten Bormann for help with the CDDL and Nicholas Wood for ideas and comments.

Contributors

Laurence Lundblade
Security Theory LLC
Tamas Ban
Arm Limited
Sergei Trofimov
Arm Limited

Authors' Addresses

Hannes Tschofenig
Simon Frost
Arm Limited
Mathias Brossard
Arm Limited
Adrian Shaw
HP Labs
Thomas Fossati
Linaro