Internet DRAFT - draft-tschofenig-rats-psa-token
draft-tschofenig-rats-psa-token
Network Working Group H. Tschofenig
Internet-Draft
Intended status: Informational S. Frost
Expires: 24 August 2024 M. Brossard
Arm Limited
A. Shaw
HP Labs
T. Fossati
Linaro
21 February 2024
Arm's Platform Security Architecture (PSA) Attestation Token
draft-tschofenig-rats-psa-token-22
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.
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/.
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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 24 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. PSA Attester Model . . . . . . . . . . . . . . . . . . . . . 5
4. PSA Claims . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Caller Claims . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Nonce . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.2. Client ID . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Target Identification Claims . . . . . . . . . . . . . . 9
4.2.1. Instance ID . . . . . . . . . . . . . . . . . . . . 9
4.2.2. Implementation ID . . . . . . . . . . . . . . . . . . 10
4.2.3. Certification Reference . . . . . . . . . . . . . . . 10
4.3. Target State Claims . . . . . . . . . . . . . . . . . . . 11
4.3.1. Security Lifecycle . . . . . . . . . . . . . . . . . 11
4.3.2. Boot Seed . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Software Inventory Claims . . . . . . . . . . . . . . . . 14
4.4.1. Software Components . . . . . . . . . . . . . . . . . 14
4.5. Verification Claims . . . . . . . . . . . . . . . . . . . 16
4.5.1. Verification Service Indicator . . . . . . . . . . . 16
4.5.2. Profile Definition . . . . . . . . . . . . . . . . . 17
4.6. Backwards Compatibility Considerations . . . . . . . . . 18
5. Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5.1. Baseline Profile . . . . . . . . . . . . . . . . . . . . 20
5.1.1. Token Encoding and Signing . . . . . . . . . . . . . 20
5.1.2. Freshness Model . . . . . . . . . . . . . . . . . . . 21
5.1.3. Synopsis . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Profile TFM . . . . . . . . . . . . . . . . . . . . . . . 22
6. Collated CDDL . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Scalability Considerations . . . . . . . . . . . . . . . . . 26
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8. PSA Token Verification . . . . . . . . . . . . . . . . . . . 27
8.1. AR4SI Trustworthiness Claims Mappings . . . . . . . . . 28
8.2. Endorsements, Reference Values and Verification Key
Material . . . . . . . . . . . . . . . . . . . . . . . . 29
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 29
10. Security and Privacy Considerations . . . . . . . . . . . . . 29
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11.1. CBOR Web Token Claims Registration . . . . . . . . . . . 30
11.1.1. Client ID Claim . . . . . . . . . . . . . . . . . . 30
11.1.2. Security Lifecycle Claim . . . . . . . . . . . . . 30
11.1.3. Implementation ID Claim . . . . . . . . . . . . . . 30
11.1.4. Certification Reference Claim . . . . . . . . . . . 31
11.1.5. Software Components Claim . . . . . . . . . . . . . 31
11.1.6. Verification Service Indicator Claim . . . . . . . 31
11.2. Media Types . . . . . . . . . . . . . . . . . . . . . . 32
11.3. CoAP Content-Formats Registration . . . . . . . . . . . 32
11.3.1. Registry Contents . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 37
A.1. COSE Sign1 Token . . . . . . . . . . . . . . . . . . . . 37
A.2. COSE Mac0 Token . . . . . . . . . . . . . . . . . . . . . 39
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 41
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction
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.
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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.
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,
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.
RoT:
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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].
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].
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.----------.
| Verifier |
'----------'
^
|
PSA Token |
|
.--------------------------------------------------------|----------.
| .------------------------------------------------------|--------. |
| | Attesting Environment | | |
| | .------------. .-----. .------+------. | |
| | | Main | | Main | | Initial | | |
| | | Bootloader +--->| 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
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 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 Figure 2.
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i-th Target Main Boot Main Boot
Environment Loader State
| | |
.--------|-------------|-------------|----.
| loop i | | | |
| | measure | | |
| |o------------+ | |
| | | write | |
| | | measurement | |
| | +------------>| |
'--------|-------------|-------------|----'
| | |
Figure 2: PSA Attester Boot Phase
* 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 Figure 3.
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.
Initial
Main Boot Attestation
State Service Verifier
| | |
.--------|----------------|-----------|----.
| loop i | read | | |
| | measurement of | | |
| | i-th Target | | |
| | Environment | | |
| |<---------------+ | |
'--------|----------------|-----------|----'
| .---+ |
| sign | | |
| '-->| |
| | PSA Token |
| +---------->|
| | |
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Figure 3: PSA Attester Run-time Phase
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].
4. 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 Section 5.
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
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.
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. 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.
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* 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.
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* 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.
psa-instance-id-type = bytes .size 33
psa-instance-id = (
ueid-label => psa-instance-id-type
)
4.2.2. Implementation ID
The Implementation ID claim uniquely identifies the hardware assembly
of the immutable PSA RoT. A verification service uses this claim to
locate the details of the PSA RoT implementation from an Endorser or
manufacturer. Such details are used by a verification service to
determine the security properties or certification status of the PSA
RoT implementation.
The value and format of the ID is decided by the manufacturer or a
particular certification scheme. For example, the ID could take the
form of a product serial number, database ID, or other appropriate
identifier.
This claim MUST be present in a PSA attestation token.
Note that this identifies the PSA RoT implementation, not a
particular instance. To uniquely identify an instance, see the
Instance ID claim Section 4.2.1.
psa-implementation-id-type = bytes .size 32
psa-implementation-id = (
psa-implementation-id-key => psa-implementation-id-type
)
4.2.3. Certification Reference
The Certification Reference claim is used to link the class of chip
and PSA RoT of the attesting device to an associated entry in the PSA
Certification database. 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].
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Linking to the PSA Certification entry can still be achieved if this
claim is not present in the token by making an association at a
Verifier between the reference value and other token claim values -
for example, the Implementation ID.
This claim MAY be present in a PSA attestation token.
psa-certification-reference-type = text .regexp "[0-9]{13}-[0-9]{5}"
psa-certification-reference = (
? psa-certification-reference-key =>
psa-certification-reference-type
)
4.3. 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:
* major[15:8] - PSA security lifecycle state, and
* minor[7:0] - IMPLEMENTATION DEFINED state.
The PSA lifecycle states are illustrated in Figure 4. For PSA, a
Verifier can only trust reports from the PSA RoT when it is in
SECURED or NON_PSA_ROT_DEBUG major states.
This claim MUST be present in a PSA attestation token.
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.-------------------------.
| Device Assembly and Test |
'------------+------------'
| Device
| Lockdown
v
.----------------------.
| PSA RoT Provisioning |
'-----------+----------'
|
Provisioning | .------------------.
Lockdown | | |
v v |
.----------------. |
.-------------+ Secured +-------. |
| '-+--------------' | |
| | ^ Debug |
| Debug | | |
| | Recoverable | Recoverable
| v | v |
| .----------------+--. .----------+----.
| | (Non-Recoverable) | | Recoverable |
| | Non-PSA RoT Debug | | PSA RoT Debug |
| '---------+---------' '------+--------'
| | |
Terminate Non-Recoverable PSA RoT Compromised
| | |
| v |
| .----------------. |
'------------>| Decommissioned |<---------'
'----------------'
Figure 4: PSA Lifecycle States
The CDDL representation is shown below. Table 1 provides the
mappings between Figure 4 and the data model.
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psa-lifecycle-unknown-type = 0x0000..0x00ff
psa-lifecycle-assembly-and-test-type = 0x1000..0x10ff
psa-lifecycle-psa-rot-provisioning-type = 0x2000..0x20ff
psa-lifecycle-secured-type = 0x3000..0x30ff
psa-lifecycle-non-psa-rot-debug-type = 0x4000..0x40ff
psa-lifecycle-recoverable-psa-rot-debug-type = 0x5000..0x50ff
psa-lifecycle-decommissioned-type = 0x6000..0x60ff
psa-lifecycle-type =
psa-lifecycle-unknown-type /
psa-lifecycle-assembly-and-test-type /
psa-lifecycle-psa-rot-provisioning-type /
psa-lifecycle-secured-type /
psa-lifecycle-non-psa-rot-debug-type /
psa-lifecycle-recoverable-psa-rot-debug-type /
psa-lifecycle-decommissioned-type
psa-lifecycle = (
psa-lifecycle-key => psa-lifecycle-type
)
psa-lifecycle-unknown-type is not shown in Figure 4; it represents an
invalid state that must not occur in a system.
+==============================================+===================+
| 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 |
+----------------------------------------------+-------------------+
Table 1: Lifecycle States Mappings
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4.3.2. 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.
psa-boot-seed-type = bytes .size (8..32)
psa-boot-seed = (
boot-seed-label => 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 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.
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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 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.
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.
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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) 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].
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
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.
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
)
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It is assumed that the relying party is pre-configured with a list of
trusted verification services and that the contents of this hint can
be used to look up the correct one. Under no circumstances must the
relying party be tricked into contacting an unknown and untrusted
verification service since the returned Attestation Result cannot be
relied on.
Note: This hint requires the relying party to parse the content of
the PSA token. Since the relying party may not be in possession of a
trust anchor to verify the digital signature, it uses the hint in the
same way as it would treat any other information provided by an
external party, which includes attacker-provided data.
4.5.2. Profile Definition
The Profile Definition claim encodes the unique identifier that
corresponds to the EAT profile described by this document. This
allows a receiver to assign the intended semantics to the rest of the
claims found in the token.
The EAT eat_profile (claim key 265) is used.
The URI encoding MUST be used.
The value MUST be tag:psacertified.org,2023:psa#tfm for the profile
defined in Section 5.2.
Future profiles derived from the baseline PSA profile SHALL create
their unique value, as described in Section 4.5.2.1.
This claim MUST be present in a PSA attestation token.
See Section 4.6, for considerations about backwards compatibility
with previous versions of the PSA attestation token format.
psa-profile-type = "tag:psacertified.org,2023:psa#tfm"
psa-profile = (
profile-label => psa-profile-type
)
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].
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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 [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.
Table 2 provides the mappings between the deprecated and new claim
keys.
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+==============+=================+=================================+
| |PSA_IOT_PROFILE_1|tag:psacertified.org,2023:psa#tfm|
+==============+=================+=================================+
|Nonce |-75008 |10 (EAT nonce) |
+--------------+-----------------+---------------------------------+
|Instance ID |-75009 |256 (EAT euid) |
+--------------+-----------------+---------------------------------+
|Profile |-75000 |265 (EAT eat_profile) |
|Definition | | |
+--------------+-----------------+---------------------------------+
|Client ID |-75001 |2394 |
+--------------+-----------------+---------------------------------+
|Security |-75002 |2395 |
|Lifecycle | | |
+--------------+-----------------+---------------------------------+
|Implementation|-75003 |2396 |
|ID | | |
+--------------+-----------------+---------------------------------+
|Boot Seed |-75004 |268 (EAT bootseed) |
+--------------+-----------------+---------------------------------+
|Certification |-75005 |2398 |
|Reference | | |
+--------------+-----------------+---------------------------------+
|Software |-75006 |2399 |
|Components | | |
+--------------+-----------------+---------------------------------+
|Verification |-75010 |2400 |
|Service | | |
|Indicator | | |
+--------------+-----------------+---------------------------------+
Table 2: Claim Key Mappings
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
profile defined in this document (tag:psacertified.org,2023:psa#tfm),
at least for the time needed to their devices to upgrade.
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5. 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 (Section 5.2) 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.
5.1. Baseline Profile
5.1.1. Token Encoding and Signing
The PSA attestation token is encoded in CBOR [STD94] format. The
CBOR representation of a PSA token MUST be "valid" according to the
definition in Section 1.2 of [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 Section 11.2 or the CoAP
Content-Format defined in Section 11.3.
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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 3 presents a concise view of the requirements described in the
preceding sections.
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+==================+=============================================+
| 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 | Variant serialization MAY be used |
| Serialization | |
+------------------+---------------------------------------------+
| COSE Protection | COSE_Sign1 and/or COSE_Mac0 MUST be used |
+------------------+---------------------------------------------+
| Algorithms | [COSE-ALGS] SHOULD be used |
+------------------+---------------------------------------------+
| Detached EAT | Detached EAT bundles MUST NOT be sent |
| Bundle Usage | |
+------------------+---------------------------------------------+
| Verification Key | Any identification method listed in |
| Identification | Appendix F.1 of [EAT] |
+------------------+---------------------------------------------+
| Endorsements | See Section 8.2 |
+------------------+---------------------------------------------+
| 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. |
+------------------+---------------------------------------------+
Table 3: Baseline Profile
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 different 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 4 presents a concise view of the requirements.
The value of the eat_profile MUST be
tag:psacertified.org,2023:psa#tfm.
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+================+=============================================+
| Issue | Profile Definition |
+================+=============================================+
| CBOR/JSON | See Section 5.1 |
+----------------+---------------------------------------------+
| CBOR Encoding | See Section 5.1 |
+----------------+---------------------------------------------+
| CBOR Encoding | See Section 5.1 |
+----------------+---------------------------------------------+
| CBOR | See Section 5.1 |
| Serialization | |
+----------------+---------------------------------------------+
| COSE | COSE_Sign1 or COSE_Mac0 MUST be used |
| Protection | |
+----------------+---------------------------------------------+
| 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 | See Section 5.1 |
| Bundle Usage | |
+----------------+---------------------------------------------+
| Verification | Claim-Based Key Identification |
| Key | (Appendix F.1.4 of [EAT]) using Instance ID |
| Identification | |
+----------------+---------------------------------------------+
| Endorsements | See Section 8.2 |
+----------------+---------------------------------------------+
| Freshness | See Section 5.1 |
+----------------+---------------------------------------------+
| Claims | See Section 5.1 |
+----------------+---------------------------------------------+
Table 4: TF-M Profile
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
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psa-software-components
}
psa-client-id-key = 2394
psa-lifecycle-key = 2395
psa-implementation-id-key = 2396
psa-certification-reference-key = 2398
psa-software-components-key = 2399
psa-verification-service-indicator-key = 2400
nonce-label = 10
ueid-label = 256
boot-seed-label = 268
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 = (
boot-seed-label => 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 = (
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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
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psa-verification-service-indicator = (
? psa-verification-service-indicator-key =>
psa-verification-service-indicator-type
)
7. Scalability Considerations
IAKs (Section 3, Paragraph 7.1.1) 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.
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8. PSA Token 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 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.
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8.1. 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 5 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 | Related PSA claims |
| Vector 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. |
+-------------------+---------------------------------------------+
Table 5: 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].
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8.2. 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.
9. Implementation Status
// RFC Editor: 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.
10. 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 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.
11. IANA Considerations
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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
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* Claim Value Type(s): byte string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.2.2 of RFCthis
11.1.4. Certification Reference Claim
* Claim Name: psa-certification-reference
* Claim Description: PSA Certification Reference
* JWT Claim Name: N/A
* Claim Key: 2398
* Claim Value Type(s): text string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.2.3 of RFCthis
11.1.5. Software Components Claim
* Claim Name: psa-software-components
* Claim Description: PSA Software Components
* JWT Claim Name: N/A
* Claim Key: 2399
* Claim Value Type(s): array
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.4.1 of RFCthis
11.1.6. Verification Service Indicator Claim
* Claim Name: psa-verification-service-indicator
* Claim Description: PSA Verification Service Indicator
* JWT Claim Name: N/A
* Claim Key: 2400
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* 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
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12. References
12.1. Normative References
[COSE-ALGS]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
[EAN-13] GS1, "International Article Number - EAN/UPC barcodes",
2019, <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-25, 15
January 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-25>.
[EAT-MEDIATYPES]
Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media
Types", Work in Progress, Internet-Draft, draft-ietf-rats-
eat-media-type-05, 7 November 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
eat-media-type-05>.
[IANA-CWT] IANA, "CBOR Web Token (CWT) Claims", 2022,
<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", 2020,
<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)", February 2019,
<https://developer.arm.com/documentation/den0063/a>.
[PSA-SM] Arm, "Platform Security Model 1.1", December 2021,
<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, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC4151] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
RFC 4151, DOI 10.17487/RFC4151, October 2005,
<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,
May 2017, <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,
May 2018, <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,
June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
[STD94] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<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, August 2022,
<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, May 2008,
<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, February
2023, <https://www.rfc-editor.org/rfc/rfc9360>.
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[I-D.kdyxy-rats-tdx-eat-profile]
Kostal, G., Dittakavi, S., Yeluri, R., Xia, H., and J. Yu,
"EAT profile for Intel® Trust Domain Extensions (TDX)
attestation result", Work in Progress, Internet-Draft,
draft-kdyxy-rats-tdx-eat-profile-00, 19 October 2023,
<https://datatracker.ietf.org/doc/html/draft-kdyxy-rats-
tdx-eat-profile-00>.
[I-D.mandyam-rats-qwestoken]
Mandyam, G., Sekhar, V., and S. Mohammed, "The Qualcomm
Wireless Edge Services (QWES) Attestation Token", Work in
Progress, Internet-Draft, draft-mandyam-rats-qwestoken-00,
1 November 2019, <https://datatracker.ietf.org/doc/html/
draft-mandyam-rats-qwestoken-00>.
[IANA-CoAP-Content-Formats]
IANA, "CoAP Content-Formats", 2022,
<https://www.iana.org/assignments/core-parameters>.
[IANA-HashFunctionTextualNames]
IANA, "Hash Function Textual Names", 2022,
<https://www.iana.org/assignments/hash-function-text-
names>.
[IAT-VERIFIER]
Linaro, "iat-verifier", 2023,
<https://git.trustedfirmware.org/TF-M/tf-m-tools.git/tree/
iat-verifier>.
[PSA] Arm, "Platform Security Architecture Resources", 2022,
<https://developer.arm.com/architectures/security-
architectures/platform-security-architecture/
documentation>.
[PSA-API] Arm, "PSA Attestation API 1.0.3", 2022, <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, 10 September 2023,
<https://datatracker.ietf.org/doc/html/draft-fdb-rats-psa-
endorsements-03>.
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[PSA-OLD] Tschofenig, H., Frost, S., Brossard, M., Shaw, A. L., and
T. Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-07, 1 February 2021,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-07>.
[PSACertified]
PSA Certified, "PSA Certified IoT Security Framework",
2022, <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, 30 August 2023,
<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, 23
October 2023, <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, January
2023, <https://www.rfc-editor.org/rfc/rfc9334>.
[TF-M] Linaro, "Trusted Firmware-M", 2022,
<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, July 2016,
<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, 22 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-uta-
tls13-iot-profile-08>.
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[Veraison] The Veraison Project, "Veraison psatoken package", 2022,
<https://github.com/veraison/psatoken>.
[Xclaim] Lundblade, L., "Xclaim", 2022,
<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
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{
/ 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:
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18([
h'A10126',
{},
h'A81901005821010202020202020202020202020202020202020202020202
02020202020202020219095C5820000000000000000000000000000000000000
00000000000000000000000000000A5820010101010101010101010101010101
010101010101010101010101010101010119095A1A7FFFFFFF19095B19300019
010978217461673A7073616365727469666965642E6F72672C323032333A7073
612374666D19010C48000000000000000019095F81A305582004040404040404
0404040404040404040404040404040404040404040404040402582003030303
0303030303030303030303030303030303030303030303030303030301645052
6F54',
h'786E937A4C42667AF3847399319CA95C7E7DBABDC9B50FDB8DE3F6BFF4AB
82FF80C42140E2A488000219E3E10663193DA69C75F52B798EA10B2F7041A90E
8E5A'
])
which has the following base16 encoding: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A.2. COSE Mac0 Token
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{
/ 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:
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17([
h'A10105',
{},
h'A8190100582101C557BD4FADC83F756FCA2CD5EA2DCC8B82159BB4E7453D
6A744D4EECD6D0AC6019095C5820000000000000000000000000000000000000
00000000000000000000000000000A5820010101010101010101010101010101
010101010101010101010101010101010119095A1A7FFFFFFF19095B19300019
010978217461673A7073616365727469666965642E6F72672C323032333A7073
612374666D19010C48000000000000000019095F81A305582004040404040404
0404040404040404040404040404040404040404040404040402582003030303
0303030303030303030303030303030303030303030303030303030301645052
6F54',
h'CF88D330E7A5366A95CF744A4DBF0D50304D405EDD8B2530E243EDDBD317
7820'
])
which has the following base16 encoding: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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.
Contributors
Laurence Lundblade
Security Theory LLC
Email: lgl@securitytheory.com
Tamas Ban
Arm Limited
Email: Tamas.Ban@arm.com
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Sergei Trofimov
Arm Limited
Email: Sergei.Trofimov@arm.com
Authors' Addresses
Hannes Tschofenig
Email: Hannes.Tschofenig@gmx.net
Simon Frost
Arm Limited
Email: Simon.Frost@arm.com
Mathias Brossard
Arm Limited
Email: Mathias.Brossard@arm.com
Adrian Shaw
HP Labs
Email: adrianlshaw@acm.org
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org
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