Internet DRAFT - draft-tschofenig-rats-psa-token
draft-tschofenig-rats-psa-token
Remote ATtestation ProcedureS H. Tschofenig
Internet-Draft
Intended status: Informational S. Frost
Expires: 1 September 2023 M. Brossard
Arm Limited
A. Shaw
HP Labs
T. Fossati
Arm Limited
28 February 2023
Arm's Platform Security Architecture (PSA) Attestation Token
draft-tschofenig-rats-psa-token-11
Abstract
The Platform Security Architecture (PSA) is a family of hardware and
firmware security specifications, as well as open-source reference
implementations, to help device makers and chip manufacturers build
best-practice security into products. Devices that are PSA compliant
are able to produce attestation tokens as described in this memo,
which are the basis for a number of different protocols, including
secure provisioning and network access control. This document
specifies the PSA attestation token structure and semantics.
The PSA attestation token is a profiled Entity Attestation Token
(EAT).
This specification describes what claims are used in an attestation
token generated by PSA compliant systems, how these claims get
serialized to the wire, and how they are cryptographically protected.
Note to Readers
Source for this draft and an issue tracker can be found at
https://github.com/thomas-fossati/draft-psa-token
(https://github.com/thomas-fossati/draft-psa-token).
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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This Internet-Draft will expire on 1 September 2023.
Copyright Notice
Copyright (c) 2023 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
2.1. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 4
3. PSA Attester Model . . . . . . . . . . . . . . . . . . . . . 4
4. PSA Claims . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Caller Claims . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Nonce . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2. Client ID . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Target Identification Claims . . . . . . . . . . . . . . 7
4.2.1. Instance ID . . . . . . . . . . . . . . . . . . . . 7
4.2.2. Implementation ID . . . . . . . . . . . . . . . . . . 8
4.2.3. Certification Reference . . . . . . . . . . . . . . . 8
4.3. Target State Claims . . . . . . . . . . . . . . . . . . . 9
4.3.1. Security Lifecycle . . . . . . . . . . . . . . . . . 9
4.3.2. Boot Seed . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Software Inventory Claims . . . . . . . . . . . . . . . . 10
4.4.1. Software Components . . . . . . . . . . . . . . . . . 10
4.5. Verification Claims . . . . . . . . . . . . . . . . . . . 12
4.5.1. Verification Service Indicator . . . . . . . . . . . 12
4.5.2. Profile Definition . . . . . . . . . . . . . . . . . 13
5. Backwards Compatibility Considerations . . . . . . . . . . . 13
6. Token Encoding and Signing . . . . . . . . . . . . . . . . . 15
7. Freshness Model . . . . . . . . . . . . . . . . . . . . . . . 15
8. Collated CDDL . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
10. Security and Privacy Considerations . . . . . . . . . . . . . 18
11. Verification . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. AR4SI Trustworthiness Claims Mappings . . . . . . . . . 19
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11.2. Endorsements, Reference Values and Verification Key
Material . . . . . . . . . . . . . . . . . . . . . . . . 20
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
12.1. CBOR Web Token Claims Registration . . . . . . . . . . . 21
12.1.1. Client ID Claim . . . . . . . . . . . . . . . . . . 21
12.1.2. Security Lifecycle Claim . . . . . . . . . . . . . 21
12.1.3. Implementation ID Claim . . . . . . . . . . . . . . 21
12.1.4. Boot Seed Claim . . . . . . . . . . . . . . . . . . 22
12.1.5. Certification Reference Claim . . . . . . . . . . . 22
12.1.6. Software Components Claim . . . . . . . . . . . . . 22
12.1.7. Verification Service Indicator Claim . . . . . . . 23
12.2. Media Type Registration . . . . . . . . . . . . . . . . 23
12.3. CoAP Content-Formats Registration . . . . . . . . . . . 23
12.3.1. Registry Contents . . . . . . . . . . . . . . . . . 23
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
13.1. Normative References . . . . . . . . . . . . . . . . . . 24
13.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 27
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 28
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
Trusted execution environments are now present in many devices, which
provide a safe environment to place security sensitive code such as
cryptography, secure boot, secure storage, and other essential
security functions. These security functions are typically exposed
through a narrow and well-defined interface, and can be used by
operating system libraries and applications. Various APIs have been
developed by Arm as part of the Platform Security Architecture [PSA]
framework. This document focuses on the output provided by PSA's
Initial Attestation API. Since the tokens are also consumed by
services outside the device, there is an actual need to ensure
interoperability. Interoperability needs are addressed here by
describing the exact syntax and semantics of the attestation claims,
and defining the way these claims are encoded and cryptographically
protected.
Further details on concepts expressed below can be found in the PSA
Security Model documentation [PSA-SM].
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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.
2.1. Glossary
RoT:
Root of Trust, the minimal set of software, hardware and data that
has to be implicitly trusted in the platform - there is no
software or hardware at a deeper level that can verify that the
Root of Trust is authentic and unmodified. An example of RoT is
an initial bootloader in ROM, which contains cryptographic
functions and credentials, running on a specific hardware
platform.
SPE:
Secure Processing Environment, a platform's processing environment
for software that provides confidentiality and integrity for its
runtime state, from software and hardware, outside of the SPE.
Contains trusted code and trusted hardware. (Equivalent to
Trusted Execution Environment (TEE), or "secure world".)
NSPE:
Non Secure Processing Environment, the security domain outside of
the SPE, the Application domain, typically containing the
application firmware, operating systems, and general hardware.
(Equivalent to Rich Execution Environment (REE), or "normal
world".)
3. PSA Attester Model
Figure 1 outlines the structure of the PSA Attester according to the
conceptual model described in Section 3.1 of [RFC9334].
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.----------.
| Verifier |
'----------'
^
|
Evidence |
|
.--------------------------------------------------------|----------.
| .------------------------------------------------------|--------. |
| | Attesting Environment | | |
| | .------------. .-----. .------+------. | |
| | | Main | | Main | | Initial | | |
| | | Bootloader +-->| Boot o-----+ Attestation | | |
| | | | | State | | Service | | |
| | '-----+------' '-----' '-------------' | |
| '----------------------|----------------------------------------' |
| .------------+--------------+---------------. |
| .--------|-------------|--------------|----------------|--------. |
| | | | | | | |
| | .------o-----. .-----o-------. .----o--------. .-----o------. | |
| | | Updateable | | Application | | Application | | PSA RoT | | |
| | | PSA RoT | | RoT | | Loader | | Parameters | | |
| | '------------' '-------------' '-------------' '------------' | |
| | Target Environment | |
| '---------------------------------------------------------------' |
'-------------------------------------------------------------------'
Figure 1: PSA Attester
The PSA Attester is a relatively straightforward embodiment of the
RATS Attester with exactly one Attesting Environment and one Target
Environment.
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 loaded
software components, collects the relevant PSA RoT parameters, and
stores the recorded information in secure memory (Main Boot State)
from where the Initial Attestation Service will, when asked for a
platform attestation report, retrieve them.
* 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 the attestation
report.
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The Target Environment can be broken down into four macro "objects",
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.
CDDL [RFC8610] along with text descriptions is used to define each
claim independent of encoding. The following CDDL type(s) are reused
by different claims:
psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
4.1. Caller Claims
4.1.1. Nonce
The Nonce claim is used to carry the challenge provided by the caller
to demonstrate freshness of the generated token.
The EAT [I-D.ietf-rats-eat] nonce (claim key 10) is used. The
following constraints apply to the nonce-type:
* The length MUST be either 32, 48, or 64 bytes.
* Only a single nonce value is conveyed. Per [I-D.ietf-rats-eat]
the array notation is not used for encoding the nonce value.
This claim MUST be present in a PSA attestation token.
psa-nonce = (
nonce-label => psa-hash-type
)
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4.1.2. Client ID
The Client ID claim represents the security domain of the caller.
In PSA, a security domain is represented by a signed integer whereby
negative values represent callers from the NSPE and where positive
IDs represent callers from the SPE. The value 0 is not permitted.
For an example definition of client IDs, see the PSA Firmware
Framework [PSA-FF].
It is essential that this claim is checked in the verification
process to ensure that a security domain, i.e., an attestation
endpoint, cannot spoof a report from another security domain.
This claim MUST be present in a PSA attestation token.
psa-client-id-nspe-type = -2147483648...0
psa-client-id-spe-type = 1..2147483647
psa-client-id-type = psa-client-id-nspe-type / psa-client-id-spe-type
psa-client-id = (
psa-client-id-key => psa-client-id-type
)
4.2. Target Identification Claims
4.2.1. Instance ID
The Instance ID claim represents the unique identifier of the Initial
Attestation Key (IAK). The full definition is in [PSA-SM].
The EAT ueid (claim key 256) of type RAND is used. The following
constraints apply to the ueid-type:
* The length MUST be 33 bytes.
* The first byte MUST be 0x01 (RAND) followed by the 32-bytes key
hash.
This claim MUST be present in a PSA attestation token.
psa-instance-id-type = bytes .size 33
psa-instance-id = (
ueid-label => psa-instance-id-type
)
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4.2.2. Implementation ID
The Implementation ID claim uniquely identifies the implementation of
the immutable PSA RoT. A verification service uses this claim to
locate the details of the PSA RoT implementation from an Endorser or
manufacturer. Such details are used by a verification service to
determine the security properties or certification status of the PSA
RoT implementation.
The value and format of the ID is decided by the manufacturer or a
particular certification scheme. For example, the ID could take the
form of a product serial number, database ID, or other appropriate
identifier.
This claim MUST be present in a PSA attestation token.
Note that this identifies the PSA RoT implementation, not a
particular instance. To uniquely identify an instance, see the
Instance ID claim Section 4.2.1.
psa-implementation-id-type = bytes .size 32
psa-implementation-id = (
psa-implementation-id-key => psa-implementation-id-type
)
4.2.3. Certification Reference
The Certification Reference claim is used to link the class of chip
and PSA RoT of the attesting device to an associated entry in the PSA
Certification database. It MUST be represented as a string made of
nineteen numeric characters: a thirteen-digit [EAN-13], followed by a
dash "-", followed by the five-digit versioning information described
in [PSA-Cert-Guide].
Linking to the PSA Certification entry can still be achieved if this
claim is not present in the token by making an association at a
Verifier between the reference value and other token claim values -
for example, the Implementation ID.
psa-certification-reference-type = text .regexp "[0-9]{13}-[0-9]{5}"
psa-certification-reference = (
? psa-certification-reference-key =>
psa-certification-reference-type
)
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4.3. Target State Claims
4.3.1. Security Lifecycle
The Security Lifecycle claim represents the current lifecycle state
of the PSA RoT. The state is represented by an integer that is
divided to convey a major state and a minor state. A major state is
mandatory and defined by [PSA-SM]. A minor state is optional and
'IMPLEMENTATION DEFINED'. The PSA security lifecycle state and
implementation state are encoded as follows:
* version[15:8] - PSA security lifecycle state, and
* version[7:0] - IMPLEMENTATION DEFINED state.
The PSA lifecycle states are illustrated in Figure 2. For PSA, a
Verifier can only trust reports from the PSA RoT when it is in
SECURED or NON_PSA_ROT_DEBUG major states.
This claim MUST be present in a PSA attestation token.
.----------------------.
.--- Enrol ---+ Provisioning Lockdown |
| '-----------+----------'
| | .------------------.
| | | |
* v v |
.--------------. .---------. |
| Verifier | .---------+ Secured +-----------. |
'--------------' | '-+-------' | |
* | | ^ | |
| | v | v |
Blocklist | .------------+------. .----------+----.
| | | Non-PSA RoT Debug | | Recoverable |
| | '---------+---------' | PSA RoT Debug |
.-+-----------+-. | '------+--------'
| Terminate +------------+-------------------'
'------+--------'
| .----------------.
'------------>| Decommissioned |
'----------------'
Figure 2: PSA Lifecycle States
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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
)
4.3.2. Boot Seed
The Boot Seed claim represents a value created at system boot time
that will allow differentiation of reports from different boot
sessions.
This claim MAY be present in a PSA attestation token.
If present, it MUST be between 8 and 32 bytes.
psa-boot-seed-type = bytes .size (8..32)
psa-boot-seed = (
psa-boot-seed-key => psa-boot-seed-type
)
4.4. Software Inventory Claims
4.4.1. Software Components
The Software Components claim is a list of software components that
includes all the software (both code and configuration) loaded by the
PSA RoT. This claim MUST be included in attestation tokens produced
by an implementation conformant with [PSA-SM].
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Each entry in the Software Components list describes one software
component using the attributes described in the following
subsections. Unless explicitly stated, the presence of an attribute
is OPTIONAL.
Note that, as described in [RFC9334], a relying party will typically
see the result of the verification process from the Verifier in form
of an attestation result, rather than the PSA token from the
attesting endpoint. Therefore, a relying party is not expected to
understand the Software Components claim. Instead, it is for the
Verifier to check this claim against the available endorsements and
provide an answer in form of an "high level" attestation result,
which may or may not include the original Software Components claim.
psa-software-component = {
? &(measurement-type: 1) => text
&(measurement-value: 2) => psa-hash-type
? &(version: 4) => text
&(signer-id: 5) => psa-hash-type
? &(measurement-desc: 6) => text
}
psa-software-components = (
psa-software-components-key => [ + psa-software-component ]
)
4.4.1.1. Measurement Type
The Measurement Type attribute (key=1) is short string representing
the role of this software component.
The following measurement types MAY be used for code measurements:
* "BL": a Boot Loader
* "PRoT": a component of the PSA Root of Trust
* "ARoT": a component of the Application Root of Trust
* "App": a component of the NSPE application
* "TS": a component of a Trusted Subsystem
The same labels with a "-config" postfix (e.g., "PRoT-config") MAY be
used for configuration measurements.
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4.4.1.2. Measurement Value
The Measurement Value attribute (key=2) represents a hash of the
invariant software component in memory at startup time. The value
MUST be a cryptographic hash of 256 bits or stronger.
This attribute MUST be present in a PSA software component.
4.4.1.3. Version
The Version attribute (key=4) is the issued software version in the
form of a text string. The value of this attribute will correspond
to the entry in the original signed manifest of the component.
4.4.1.4. Signer ID
The Signer ID attribute (key=5) is the hash of a signing authority
public key for the software component. The value of this attribute
will correspond to the entry in the original manifest for the
component. This can be used by a Verifier to ensure the components
were signed by an expected trusted source.
This attribute MUST be present in a PSA software component to be
compliant with [PSA-SM].
4.4.1.5. Measurement Description
The Measurement Description attribute (key=6) contains a string
identifying the hash algorithm used to compute the corresponding
Measurement Value. The string SHOULD be encoded according to
[IANA-HashFunctionTextualNames].
4.5. Verification Claims
4.5.1. Verification Service Indicator
The Verification Service Indicator claim is a hint used by a relying
party to locate a verification service for the token. The value is a
text string that can be used to locate the service (typically, a URL
specifying the address of the verification service API). A Relying
Party may choose to ignore this claim in favor of other information.
psa-verification-service-indicator-type = text
psa-verification-service-indicator = (
? psa-verification-service-indicator-key =>
psa-verification-service-indicator-type
)
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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 profile (claim key 265) is used. The following constraints
apply to its type:
* The URI encoding MUST be used.
* The value MUST be http://arm.com/psa/2.0.0.
This claim MUST be present in a PSA attestation token.
See Section 5, for considerations about backwards compatibility with
previous versions of the PSA attestation token format.
psa-profile-type = "http://arm.com/psa/2.0.0"
psa-profile = (
profile-label => psa-profile-type
)
5. Backwards Compatibility Considerations
A previous version of this specification (identified by the
PSA_IOT_PROFILE_1 profile) used claim key values from the "private
use range" of the CWT Claims registry. These claim keys have now
been retired and their use is deprecated.
Table 1 provides the mappings between the deprecated and new claim
keys.
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+====================+===================+==========================+
| | PSA_IOT_PROFILE_1 | http://arm.com/psa/2.0.0 |
+====================+===================+==========================+
| 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 ID | -75003 | 2396 |
+--------------------+-------------------+--------------------------+
| Boot Seed | -75004 | 2397 |
+--------------------+-------------------+--------------------------+
| Certification | -75005 | 2398 |
| Reference | | |
+--------------------+-------------------+--------------------------+
| Software | -75006 | 2399 |
| Components | | |
+--------------------+-------------------+--------------------------+
| Verification | -75010 | 2400 |
| Service Indicator | | |
+--------------------+-------------------+--------------------------+
Table 1: Claim key mappings
The new profile introduces three further changes:
* the "Boot Seed" claim is now optional and variable length (see
Section 4.3.2),
* the "No Software Measurements" claim has been retired,
* the "Certification Reference" syntax changed from EAN-13 to EAN-
13+5 (see Section 4.2.3).
Unless compatibility with existing infrastructure is a concern,
emitters (e.g., devices that implement the PSA Attestation API)
SHOULD produce tokens with the claim keys specified in this document.
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To simplify the transition to the token format described in this
document it is RECOMMENDED that receivers (e.g., PSA Attestation
Verifiers) accept tokens encoded according to the old profile
(PSA_IOT_PROFILE_1) as well as to the new profile (http://arm.com/
psa/2.0.0), at least for the time needed to their clients to upgrade.
6. Token Encoding and Signing
The PSA attestation token is encoded in CBOR [RFC8949] format. Only
definite-length string, arrays, and maps are allowed.
Cryptographic protection is obtained by wrapping the psa-token map in
a COSE Web Token (CWT) [RFC8392]. For asymmetric key algorithms, the
signature structure MUST be COSE_Sign1. For symmetric key
algorithms, the signature structure MUST be COSE_Mac0.
Acknowledging the variety of markets, regulations and use cases in
which the PSA attestation token can be used, this specification does
not impose any strong requirement on the cryptographic algorithms
that need to be supported by Attesters and Verifiers. It is assumed
that the flexibility provided by the COSE format is sufficient to
deal with the level of cryptographic agility needed to adapt to
specific use cases. For interoperability considerations, it is
RECOMMENDED that commonly adopted algorithms are used, such as those
discussed in [COSE-ALGS]). It is expected that receivers (Verifiers
and Relying Parties) 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 12.2 or the CoAP
Content-Format defined in Section 12.3.
7. 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.
8. Collated CDDL
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psa-token = {
psa-nonce
psa-instance-id
psa-verification-service-indicator
psa-profile
psa-implementation-id
psa-client-id
psa-lifecycle
psa-certification-reference
? psa-boot-seed
psa-software-components
}
psa-client-id-key = 2394
psa-lifecycle-key = 2395
psa-implementation-id-key = 2396
psa-boot-seed-key = 2397
psa-certification-reference-key = 2398
psa-software-components-key = 2399
psa-verification-service-indicator-key = 2400
nonce-label = 10
ueid-label = 256
profile-label = 265
psa-hash-type = bytes .size 32 / bytes .size 48 / bytes .size 64
psa-boot-seed-type = bytes .size (8..32)
psa-boot-seed = (
psa-boot-seed-key => psa-boot-seed-type
)
psa-client-id-nspe-type = -2147483648...0
psa-client-id-spe-type = 1..2147483647
psa-client-id-type = psa-client-id-nspe-type / psa-client-id-spe-type
psa-client-id = (
psa-client-id-key => psa-client-id-type
)
psa-certification-reference-type = text .regexp "[0-9]{13}-[0-9]{5}"
psa-certification-reference = (
? psa-certification-reference-key =>
psa-certification-reference-type
)
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psa-implementation-id-type = bytes .size 32
psa-implementation-id = (
psa-implementation-id-key => psa-implementation-id-type
)
psa-instance-id-type = bytes .size 33
psa-instance-id = (
ueid-label => psa-instance-id-type
)
psa-nonce = (
nonce-label => psa-hash-type
)
psa-profile-type = "http://arm.com/psa/2.0.0"
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
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? &(measurement-desc: 6) => text
}
psa-software-components = (
psa-software-components-key => [ + psa-software-component ]
)
psa-verification-service-indicator-type = text
psa-verification-service-indicator = (
? psa-verification-service-indicator-key =>
psa-verification-service-indicator-type
)
9. Implementation Status
Implementations of this specification are provided by the Trusted
Firmware-M project [TF-M], the Veraison project [Veraison], and the
Xclaim [Xclaim] library. All three 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.
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.
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11. Verification
To verify the token, the primary need is to check correct encoding
and signing as detailed in Section 6. In particular, the Instance ID
claim is used (together with the kid in the COSE header, if present)
to assist in locating the public key used to verify the signature
covering the CWT token. The key used for verification is supplied to
the Verifier by an authorized Endorser along with the corresponding
Attester's Instance ID.
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.
11.1. 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.
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The contents of Table 2 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 2: 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].
11.2. 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.
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12. IANA Considerations
12.1. 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].
12.1.1. 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
12.1.2. 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
12.1.3. Implementation ID Claim
* Claim Name: psa-implementation-id
* Claim Description: PSA Implementation ID
* JWT Claim Name: N/A
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* Claim Key: 2396
* Claim Value Type(s): byte string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.2.2 of RFCthis
12.1.4. Boot Seed Claim
* Claim Name: psa-boot-seed
* Claim Description: PSA Boot Seed
* JWT Claim Name: N/A
* Claim Key: 2397
* Claim Value Type(s): byte string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.3.2 of RFCthis
12.1.5. Certification Reference Claim
* Claim Name: psa-certification-reference
* Claim Description: PSA Certification Reference
* JWT Claim Name: N/A
* Claim Key: 2398
* Claim Value Type(s): text string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.2.3 of RFCthis
12.1.6. Software Components Claim
* Claim Name: psa-software-components
* Claim Description: PSA Software Components
* JWT Claim Name: N/A
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* Claim Key: 2399
* Claim Value Type(s): array
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.4.1 of RFCthis
12.1.7. Verification Service Indicator Claim
* Claim Name: psa-verification-service-indicator
* Claim Description: PSA Verification Service Indicator
* JWT Claim Name: N/A
* Claim Key: 2400
* Claim Value Type(s): text string
* Change Controller: Hannes Tschofenig
* Specification Document(s): Section 4.5.1 of RFCthis
12.2. Media Type Registration
To indicate that the transmitted content is a PSA Attestation Token,
applications can use the application/eat-cwt media type defined in
[I-D.ietf-rats-eat-media-type] with the eat_profile parameter set to
http://arm.com/psa/2.0.0 (or PSA_IOT_PROFILE_1 if the token is
encoded according to the old profile, see Section 5).
12.3. 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 http://arm.com/psa/2.0.0
* Another for the application/eat-cwt media type with the
eat_profile parameter equal to PSA_IOT_PROFILE_1
12.3.1. Registry Contents
* Media Type: application/eat-cwt; eat_profile="http://arm.com/
psa/2.0.0"
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* 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
13. References
13.1. 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>.
[I-D.ietf-rats-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-19, 19
December 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-19>.
[I-D.ietf-rats-eat-media-type]
Lundblade, L., Birkholz, H., and T. Fossati, "EAT Media
Types", Work in Progress, Internet-Draft, draft-ietf-rats-
eat-media-type-01, 19 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
eat-media-type-01>.
[IANA-CWT] IANA, "CBOR Web Token (CWT) Claims", 2022,
<https://www.iana.org/assignments/cwt/cwt.xhtml#claims-
registry>.
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[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/-
/media/Files/pdf/PlatformSecurityArchitecture/Architect/
DEN0063-PSA_Firmware_Framework-1.0.0-2.pdf>.
[PSA-SM] Arm, "Platform Security Architecture Security Model 1.0
(PSA-SM)", February 2019, <https://developer.arm.com/-
/media/Files/pdf/PlatformSecurityArchitecture/Architect/
DEN0079_PSA_SM_ALPHA-03_RC01.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>.
[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>.
[RFC8949] 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>.
13.2. Informative References
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[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>.
[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", 2019,
<https://developer.arm.com/-
/media/Files/pdf/PlatformSecurityArchitecture/Implement/
IHI0085-PSA_Attestation_API-1.0.2.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-01, 11 May 2022,
<https://datatracker.ietf.org/doc/html/draft-fdb-rats-psa-
endorsements-01>.
[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-
03, 6 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
ar4si-03>.
[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-00, 6
September 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-corim-00>.
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[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/>.
[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. Example
The following example shows a PSA attestation token for an
hypothetical system comprising two measured software components (a
boot loader and a trusted RTOS). 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:
{
/ eat_profile / 265: "http://arm.com/psa/2.0.0",
/ psa-client-id / 2394: 2147483647,
/ psa-security-lifecycle / 2395: 12288,
/ psa-implementation-id / 2396: h'000000000000000000000000000
0000000000000000000000000000000000000',
/ psa-boot-seed / 2397: h'0000000000000000',
/ psa-certification-reference / 2398: "1234567890123-12345",
/ psa-software-components / 2399: [
{
/ measurement value / 2: h'0303030303030303030303030303030
303030303030303030303030303030303',
/ signer ID / 5: h'0404040404040404040404040404040
404040404040404040404040404040404'
}
],
/ nonce / 10: h'010101010101010101010101010101010101010101
0101010101010101010101',
/ ueid / 256: h'010202020202020202020202020202020202020202
020202020202020202020202',
/ psa-vsi / 2400: "https://veraison.example/v1/challenge-respo
nse"
}
The JWK representation of the IAK used for creating the COSE Sign1
signature over the PSA token is:
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{
"kty": "EC",
"crv": "P-256",
"x": "MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4",
"y": "4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM",
"d": "870MB6gfuTJ4HtUnUvYMyJpr5eUZNP4Bk43bVdj3eAE"
}
The resulting COSE object is:
18(
[
/ protected / h'A10126',
/ unprotected / {},
/ payload / h'AA1901097818687474703A2F2F61726D2E636F6D2F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',
/ signature / h'56F50D131FA83979AE064E76E70DC75C070B6D991A
EC08ADF9F41CAB7F1B7E2C47F67DACA8BB49E3119B7BAE77AEC6C89162713E0C
C6D0E7327831E67F32841A'
]
)
Acknowledgments
Thanks to Carsten Bormann for help with the CDDL and Nicholas Wood
for ideas and comments.
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
Arm Limited
Email: Thomas.Fossati@arm.com
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