v: 3
title: "Arm's Platform Security Architecture (PSA) Attestation Token" abbrev: "PSA Attestation Token" docname: draft-tschofenig-rats-psa-token-latest category: info submissionType: independent
ipr: trust200902 area: "" workgroup: "" keyword: Internet-Draft
stand_alone: yes
pi: rfcedstyle: yes toc: yes tocindent: yes sortrefs: yes symrefs: yes strict: yes comments: yes text-list-symbols: -o*+
author:
- name: Hannes Tschofenig organization: email: Hannes.Tschofenig@gmx.net
- name: Simon Frost organization: Arm Limited email: Simon.Frost@arm.com
- name: Mathias Brossard organization: Arm Limited email: Mathias.Brossard@arm.com
- name: Adrian Shaw organization: HP Labs email: adrianlshaw@acm.org
- name: Thomas Fossati organization: Linaro email: thomas.fossati@linaro.org
contributor:
- name: Laurence Lundblade organization: Security Theory LLC email: lgl@securitytheory.com
- name: Tamas Ban organization: Arm Limited email: Tamas.Ban@arm.com
- name: Sergei Trofimov organization: Arm Limited email: Sergei.Trofimov@arm.com
normative: PSA-SM: author: org: Arm title: Platform Security Model 1.1 target: https://www.psacertified.org/app/uploads/2021/12/JSADEN014_PSA_Certified_SM_V1.1_BET0.pdf date: 01. Dec. 2021 EAN-13: author: org: GS1 title: International Article Number - EAN/UPC barcodes target: https://www.gs1.org/standards/barcodes/ean-upc date: 2019 PSA-FF: author: org: Arm title: Platform Security Architecture Firmware Framework 1.0 (PSA-FF) target: https://developer.arm.com/documentation/den0063/a date: 20. Feb. 2019 PSA-Cert-Guide: author: org: PSA Certified title: PSA Certified Level 2 Step by Step Guide Version 1.1 target: https://www.psacertified.org/app/uploads/2020/07/JSADEN011-PSA_Certified_Level_2_Step-by-Step-1.1-20200403.pdf date: 2020 STD94: -: cbor =: RFC8949 STD96: -: cose =: RFC9052 COSE-ALGS: RFC9053 IANA-CWT: author: org: IANA title: CBOR Web Token (CWT) Claims target: https://www.iana.org/assignments/cwt/cwt.xhtml#claims-registry date: 2022 X509: RFC5280 EAT: I-D.ietf-rats-eat EAT-MEDIATYPES: I-D.ietf-rats-eat-media-type
informative: I-D.kdyxy-rats-tdx-eat-profile: I-D.mandyam-rats-qwestoken: TLS12-IoT: RFC7925 TLS13-IoT: I-D.draft-ietf-uta-tls13-iot-profile COSE-X509: RFC9360 RFC9334: IANA-CoAP-Content-Formats: author: org: IANA title: CoAP Content-Formats target: https://www.iana.org/assignments/core-parameters date: 2022 TF-M: author: org: Linaro title: Trusted Firmware-M target: https://www.trustedfirmware.org/projects/tf-m/ date: 2022 IAT-VERIFIER: author: org: Linaro title: iat-verifier target: https://git.trustedfirmware.org/TF-M/tf-m-tools.git/tree/iat-verifier date: 2023 Veraison: author: org: The Veraison Project title: Veraison psatoken package target: https://github.com/veraison/psatoken date: 2022 Xclaim: author: ins: L. Lundblade name: Laurence Lundblade title: Xclaim target: https://github.com/laurencelundblade/xclaim date: 2022 PSA: author: org: Arm title: Platform Security Architecture Resources target: https://developer.arm.com/architectures/security-architectures/platform-security-architecture/documentation date: 2022 PSACertified: author: org: PSA Certified title: PSA Certified IoT Security Framework target: https://psacertified.org date: 2022 PSA-API: author: org: Arm title: PSA Attestation API 1.0.3 target: https://arm-software.github.io/psa-api/attestation/1.0/IHI0085-PSA_Certified_Attestation_API-1.0.3.pdf date: 2022 PSA-Endorsements: I-D.fdb-rats-psa-endorsements RATS-CoRIM: I-D.ietf-rats-corim RATS-AR4SI: I-D.ietf-rats-ar4si PSA-OLD: I-D.tschofenig-rats-psa-token-07
entity: SELF: "RFCthis"
--- abstract
The Arm 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 profile of the 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 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.
--- middle
The Platform Security Architecture (PSA) {{PSA}} is a set of hardware and firmware specifications, backed by reference implementations and a security certification program {{PSACertified}}. 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 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 RATS Architecture {{RFC9334}}, 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 {{PSA-API}}. This output corresponds to Evidence in {{RFC9334}} and, as a design decision, the PSA attestation token is a profile of the Entity Attestation Token (EAT) {{EAT}}. Note that there are other profiles of EAT available, such as {{I-D.kdyxy-rats-tdx-eat-profile}} and {{I-D.mandyam-rats-qwestoken}}, for use with different use cases and by different attestation technologies.
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 {{PSA-SM}}.
As mentioned in the abstract, this memo documents a vendor extension to the RATS architecture, and is not a standard.
{::boilerplate bcp14}
The terms Attester, Relying Party, Verifier, Attestation Result, Target Environment, Attesting Environment 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.
{: vspace="0"} 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), "secure world", or "secure enclave".)
NSPE: : Non Secure Processing Environment, the security domain outside of the SPE, the Application domain, typically containing 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) {{!RFC8610}}.
{{fig-psa-attester}} outlines the structure of the PSA Attester according to the conceptual model described in {{Section 3.1 of RFC9334}}.
{::include art/psa-attester.ascii-art}
{: #fig-psa-attester title="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 Main Bootloader, executing at boot-time, 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 {{fig-psa-attester-boot}}.
{::include art/psa-boot.ascii-art}
{: #fig-psa-attester-boot title="PSA Attester Boot Phase" align="center" }
- The Initial Attestation Service (executing at run-time in SPE) answers requests coming from NSPE via the PSA attestation API {{PSA-API}}, collects and formats the claims from Main Boot State, and uses the Initial Attestation Key (IAK) to sign them and produce Evidence. See {{fig-psa-attester-runtime}}. {: #para-ias-intro}
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.
{::include art/psa-runtime.ascii-art}
{: #fig-psa-attester-runtime title="PSA Attester Run-time Phase" align="center" }
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 {{TF-M}}.
{: #sec-psa-claims }
This section describes the claims to be used in a PSA attestation token. A more comprehensive treatment of the EAT profile(s) defined by PSA is found in {{sec-profiles}}.
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:
{::include cddl/psa-common-types.cddl}
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 for PSA-originated claims.
{: #sec-nonce-claim}
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. Since the EAT nonce claim offers flexiblity for different
attestation technologies, this specifications 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.
{::include cddl/psa-nonce.cddl}
{: #sec-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.
{::include cddl/psa-client-id.cddl}
### Instance ID {: #sec-instance-id-claim}
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-byte unique identifier of the IAK. {{PSA-API}} provides implementation options for deriving the IAK unique identifier from the IAK itself.
This claim MUST be present in a PSA attestation token.
{::include cddl/psa-instance-id.cddl}
{: #sec-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 {{sec-instance-id-claim}}.
{::include cddl/psa-implementation-id.cddl}
{: #sec-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.
This claim MAY be present in a PSA attestation token.
{::include cddl/psa-certification-reference.cddl}
{: #sec-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:
- major[15:8] - PSA security lifecycle state, and
- minor[7:0] - IMPLEMENTATION DEFINED state.
The PSA lifecycle states are illustrated in {{fig-lifecycle-states}}. 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.
{::include art/psa-lifecycle.ascii-art}
{: #fig-lifecycle-states title="PSA Lifecycle States" }
The CDDL representation is shown below. {{tab-states-map}} provides the mappings between {{fig-lifecycle-states}} and the data model.
{::include cddl/psa-security-lifecycle.cddl}
psa-lifecycle-unknown-type is not shown in {{fig-lifecycle-states}}; it represents an invalid state that must not occur in a system.
| 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 |
| {: #tab-states-map title="Lifecycle States Mappings"} |
{: #sec-boot-seed }
The Boot Seed 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.
{::include cddl/psa-boot-seed.cddl}
{: #sec-sw-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 appraisal 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 Reference Values and provide an answer in form of an "high level" Attestation Result, which may or may not include the original Software Components claim.
{::include cddl/psa-software-components.cddl}
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 "-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.
#### 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.
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.
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 {{PSA-SM}}.
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" {{!IANA.named-information}}.
The following claims are part of the PSA token (and therefore still Evidence) but aim to help receivers, including relying parties, with the processing of the received attestation Evidence.
{: #sec-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.
{::include cddl/psa-verification-service-indicator.cddl}
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.
{: #sec-profile-definition-claim}
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 {{sec-tfm-profile}}.
Future profiles derived from the baseline PSA profile SHALL create their unique value, as described in {{sec-profile-uri-structure}}.
This claim MUST be present in a PSA attestation token.
See {{sec-backwards-compat}}, for considerations about backwards compatibility with previous versions of the PSA attestation token format.
{::include cddl/psa-profile.cddl}
{: #sec-profile-uri-structure}
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.
{: #sec-backwards-compat}
A previous version of this specification {{PSA-OLD}}, 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.
{{tab-claim-map}} provides the mappings between the deprecated and new claim keys.
| 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 |
| {: #tab-claim-map title="Claim Key Mappings"} |
The new profile introduces three further changes:
- the "Boot Seed" claim is now optional and of variable length (see {{sec-boot-seed}}),
- the "No Software Measurements" claim has been retired,
- the "Certification Reference" claim syntax changed from EAN-13 to EAN-13+5 (see {{sec-certification-reference}}).
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.
{: #sec-profiles}
This document defines a baseline with common requirements that all PSA profiles must satisfy. (Note that this does not apply to {{PSA-OLD}}.)
This document also defines a "TFM" profile ({{sec-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 EAT calls a "partial" and "full" profile, respectively. See {{Section 6.2 of EAT}} for further details regarding profiles.
### Token Encoding and Signing {: #sec-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 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 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 serialised COSE_Sign1 or COSE_Mac0 in the media type defined in {{sec-iana-media-types}} or the CoAP Content-Format defined in {{sec-iana-coap-content-format}}.
A PSA token is always directly signed by the PSA RoT. Therefore, a PSA claims-set ({{sec-psa-claims}}) is never carried in a Detached EAT bundle ({{Section 5 of EAT}}).
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.
{{tbl-baseline-profile}} presents a concise view of the requirements described in the preceding sections.
| 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 {{sec-psa-endorsements}} | | Freshness | nonce or epoch ID based | | Claims | Those defined in {{sec-psa-claims}}. As per general EAT rules, the receiver MUST NOT error out on claims it does not understand. | {: #tbl-baseline-profile title="Baseline Profile"}
{: #sec-tfm-profile}
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 different profile value should be used ({{sec-profile-uri-structure}}). This includes a restriction of the profile to a subset of the COSE Protection scheme requirements.
{{tbl-tfm-profile}} presents a concise view of the requirements.
The value of the eat_profile MUST be tag:psacertified.org,2023:psa#tfm.
| Issue | Profile Definition | | CBOR/JSON | See {{baseline-profile}} | | CBOR Encoding | See {{baseline-profile}} | | CBOR Encoding | See {{baseline-profile}} | | CBOR Serialization | See {{baseline-profile}} | | 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 {{baseline-profile}} | | Verification Key Identification | Claim-Based Key Identification ({{Appendix F.1.4 of EAT}}) using Instance ID | | Endorsements | See {{sec-psa-endorsements}} | | Freshness | See {{baseline-profile}} | | Claims | See {{baseline-profile}} | {: #tbl-tfm-profile title="TF-M Profile"}
{::include cddl/psa-attestation.cddl}
{: #sec-scalability}
IAKs ({{para-ias-intro}}) 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.)
Using certified public keys offers better scalability properties when compared to using raw public keys, namely:
- storage requirements for the Verifier are minimised - 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 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).
Constraints around network bandwidth and computing resources available to endpoints, such as network buffers, may dictate a reasonable split point.
To verify the token, the primary need is to check correct encoding and signing
as detailed in {{sec-token-encoding-and-signing}}.
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
{{sec-scalability}}.
If the IAK is a raw public key, 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
({{Section 6 of X509}}) up to a trusted CA MUST be successful before the key is
used to verify the token signature. This also includes revocation checking.
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:
- 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 {{PSACertified}} certificate.
## 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 {{tab-ar4si-map}} 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.
| Trustworthiness Vector claims | Related PSA claims |
|---|---|
configuration |
Software Components ({{sec-sw-components}}) |
executables |
ditto |
file-system |
N/A |
hardware |
Implementation ID ({{sec-implementation-id}}) |
instance-identity |
Instance ID ({{sec-instance-id-claim}}). The Security Lifecycle ({{sec-security-lifecycle}}) can also impact the derived identity. |
runtime-opaque |
Indirectly derived from executables, hardware, and instance-identity. The Security Lifecycle ({{sec-security-lifecycle}}) 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. |
| {: #tab-ar4si-map title="AR4SI Claims mappings"} |
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}}.
{: #sec-psa-endorsements}
{{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.
1 please remove this section before pubblication.
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.
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 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 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.
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}}.
### 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): {{sec-client-id}} of {{&SELF}}
### 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): {{sec-security-lifecycle}} of {{&SELF}}
### 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): {{sec-implementation-id}} of {{&SELF}}
### 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): {{sec-certification-reference}} of {{&SELF}}
### 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): {{sec-sw-components}} of {{&SELF}}
### 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): {{sec-verification-service-indicator}} of {{&SELF}}
{: #sec-iana-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 {{sec-backwards-compat}}).
{: #sec-iana-coap-content-format}
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+cwtmedia type with theeat_profileparameter equal totag:psacertified.org,2023:psa#tfm - Another for the
application/eat+cwtmedia type with theeat_profileparameter equal toPSA_IOT_PROFILE_1
The Content-Formats should be allocated from the Expert review range (0-255).
-
Media Type:
application/eat+cwt; eat_profile="tag:psacertified.org,2023:psa#tfm" -
Encoding: -
-
Id: [[To-be-assigned by IANA]]
-
Reference: {{&SELF}}
-
Media Type:
application/eat+cwt; eat_profile="PSA_IOT_PROFILE_1" -
Encoding: -
-
Id: [[To-be-assigned by IANA]]
-
Reference: {{&SELF}}
--- back
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 ({{sec-security-lifecycle}}) of SECURED. The attestation has been requested from a client residing in the SPE.
The example in {{ex-sign1}} illustrates the case where the IAK is an asymmetric key. A COSE Sign1 envelope is used to wrap the PSA claims-set.
{{ex-mac0}} 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}}.
{::include cddl/example/sign1-claims.diag}
The JWK representation of the IAK used for creating the COSE Sign1 signature over the PSA token is:
{::include cddl/example/tfm-es-iak.json}
The resulting COSE object is:
{::include cddl/example/psa-sign1.diag}
which has the following base16 encoding:
{::include cddl/example/psa-sign1.hex}
{::include cddl/example/mac0-claims.diag}
The JWK representation of the IAK used for creating the COSE Mac0 signature over the PSA token is:
{::include cddl/example/tfm-hs-iak.json}
The resulting COSE object is:
{::include cddl/example/psa-mac0.diag}
which has the following base16 encoding:
{::include cddl/example/psa-mac0.hex}
{:numbered="false"}
Thank you Carsten Bormann for help with the CDDL. Thanks to Nicholas Wood, Eliot Lear, Yaron Sheffer, Kathleen Moriarty and Ned Smith for ideas, comments and suggestions.
Footnotes
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RFC Editor: ↩