Internet-Draft PSA Attestation Token March 2021
Tschofenig, et al. Expires 25 September 2021 [Page]
Workgroup:
RATS
Internet-Draft:
draft-tschofenig-rats-psa-token-08
Published:
Intended Status:
Informational
Expires:
Authors:
H. Tschofenig
Arm Limited
S. Frost
Arm Limited
M. Brossard
Arm Limited
A. Shaw
Arm Limited
T. Fossati
Arm Limited

Arm's Platform Security Architecture (PSA) Attestation Token

Abstract

The Platform Security Architecture (PSA) is a family of hardware and firmware security specifications, as well as open-source reference implementations, to help device makers and chip manufacturers build best-practice security into products. Devices that are PSA compliant 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.

Status of This Memo

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

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

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

This Internet-Draft will expire on 25 September 2021.

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].

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 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

3.1. Caller Claims

3.1.1. 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 = (
    10 => psa-hash-type
)

3.1.2. 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.

Note that the CDDL label used to be called arm_psa_partition_id.

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
)

3.2. Target Identification Claims

3.2.1. 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 11) 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 = (
    11 => psa-instance-id-type
)

3.2.2. Implementation ID

The Implementation ID claim uniquely identifies the underlying immutable PSA RoT. A verification service can use this claim to locate the details of the verification process. Such details include the implementation's origin and associated certification state. The full definition is in [PSA-SM].

This claim MUST be present in a PSA attestation token.

psa-implementation-id-type = bytes .size 32

psa-implementation-id = (
    psa-implementation-id-key => psa-implementation-id-type
)

3.2.3. 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 thirteen-digit [EAN-13].

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}"

psa-certification-reference = (
    ? psa-certification-reference-key =>
        psa-certification-reference-type
)

3.3. Target State Claims

3.3.1. 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 1. For PSA, a remote 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.

r T s r n i e R b t o e g b n P o r - o R e o d c e m e i L i R i o a m n c o a D s o c o e k V k e l i e l T N u f t v S a T r c d E v D e g e P s i u A o e n b d B u n o c r l P o A g l S n r r e n w D e i m i s i S
Figure 1: PSA Lifecycle States 
psa-lifecycle-unknown-type = 0x0000..0x00ff
psa-lifecycle-assembly-and-test-type = 0x1000..0x10ff
psa-lifecycle-psa-rot-provisioning-type = 0x2000..0x20ff
psa-lifecycle-secured-type = 0x3000..0x30ff
psa-lifecycle-non-psa-rot-debug-type = 0x4000..0x40ff
psa-lifecycle-recoverable-psa-rot-debug-type = 0x5000..0x50ff
psa-lifecycle-decommissioned-type = 0x6000..0x60ff

psa-lifecycle-type =
    psa-lifecycle-unknown-type /
    psa-lifecycle-assembly-and-test-type /
    psa-lifecycle-psa-rot-provisioning-type /
    psa-lifecycle-secured-type /
    psa-lifecycle-non-psa-rot-debug-type /
    psa-lifecycle-recoverable-psa-rot-debug-type /
    psa-lifecycle-decommissioned-type

psa-lifecycle = (
    psa-lifecycle-key => psa-lifecycle-type
)

3.3.2. Boot Seed

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

This claim MUST be present in a PSA attestation token.

psa-boot-seed-type = bytes .size 32

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

3.4. Software Inventory Claims

3.4.1. Software Components

The Software Components claim is a list of software components that includes all the software loaded by the PSA RoT. This claim SHALL be included in attestation tokens produced by an implementation conformant with [PSA-SM]. If the Software Components claim is present, then the No Software Measurement claim (Section 3.4.2) MUST NOT be present.

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 [I-D.ietf-rats-architecture], a relying party will typically see the result of the verification process from the Verifier in form of an attestation result, rather than the "naked" 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 = {
  ? 1 => text,          ; measurement type
    2 => psa-hash-type, ; measurement value
  ? 4 => text,          ; version
    5 => psa-hash-type, ; signer id
  ? 6 => text,          ; measurement description
}

psa-software-components = (
    psa-software-components-key => [ + psa-software-component ]
)
3.4.1.1. 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:

  • "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
3.4.1.2. 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.

3.4.1.3. 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.

3.4.1.4. 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].

3.4.1.5. Measurement Description

The Measurement Description attribute (key=6) is the description of the way in which the measurement value of the software component is computed. The value will be a text string containing an abbreviated description (or name) of the measurement method which can be used to lookup the details of the method in a profile document. This attribute will normally be excluded, unless there was an exception to the default measurement described in the profile for a specific component.

3.4.2. No Software Measurements

In the event that the implementation does not contain any software measurements then the Software Components claim Section 3.4.1 can be omitted but instead the token MUST include this claim to indicate this is a deliberate state. The value SHOULD be 1. This claim is intended for devices that are not compliant with [PSA-SM].

psa-no-sw-measurements-type = 1

psa-no-sw-measurement = (
    psa-no-sw-measurement-key => psa-no-sw-measurements-type
)

3.5. Verification Claims

3.5.1. Verification Service Indicator

The Verification Service Indicator claim is a hint used by a relying party to locate a validation service for the token. The value is a text string that can be used to locate the service or a URL specifying the address of the service. A verifier 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
)

3.5.2. 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 18) 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 4, 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 = (
    18 => psa-profile-type
)

4. Backwards Compatibility Considerations

Previous versions of this specification used different claim key values for the following claims:

  • Nonce (claim key -75008);
  • Instance ID (claim key -75009);
  • Profile Description (claim key -75000 and value PSA_IOT_PROFILE_1).

These claim keys have been retired and their use is deprecated.

Unless compatibility with existing infrastructure is a concern, emitters (e.g., devices that implement the PSA Attestation API) SHOULD produce tokens with their standard equivalent instead, as described in Section 3.1.1, Section 3.2.1 and Section 3.5.2 respectively.

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 (PROFILE_IOT_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.

5. Token Encoding and Signing

The PSA attestation token is encoded in CBOR [RFC7049] 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.

The CWT CBOR tag (61) is not used. An application that needs to exchange PSA attestation tokens can use the media type defined in Section 10.1 or the CoAP Content-Format defined in Section 10.2.

6. Freshness Model

The PSA Token supports the freshness models for attestation Evidence based on nonces and epoch handles (Section 10.2 and 10.3 of [I-D.ietf-rats-architecture]) 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.

7. Collated CDDL 

psa-token = {
    psa-nonce,
    psa-instance-id,
    psa-verification-service-indicator,
    psa-profile,
    psa-implementation-id,
    psa-client-id,
    psa-lifecycle,
    psa-certification-reference,
    psa-boot-seed,
    ( psa-software-components // psa-no-sw-measurement ),
}

psa-profile-key = -75000
psa-client-id-key = -75001
psa-lifecycle-key = -75002
psa-implementation-id-key = -75003
psa-boot-seed-key = -75004
psa-certification-reference-key = -75005
psa-software-components-key = -75006
psa-no-sw-measurement-key = -75007
psa-nonce-key = -75008
psa-instance-id-key = -75009
psa-verification-service-indicator-key = -75010

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

psa-boot-seed-type = bytes .size 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}"

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 = (
    11 => psa-instance-id-type
)

psa-no-sw-measurements-type = 1

psa-no-sw-measurement = (
    psa-no-sw-measurement-key => psa-no-sw-measurements-type
)

psa-nonce = (
    10 => psa-hash-type
)

psa-profile-type = "http://arm.com/psa/2.0.0"

psa-profile = (
    18 => 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 = {
  ? 1 => text,          ; measurement type
    2 => psa-hash-type, ; measurement value
  ? 4 => text,          ; version
    5 => psa-hash-type, ; signer id
  ? 6 => text,          ; measurement description
}

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
)

8. Security and Privacy Considerations

This specification re-uses the CWT and the EAT 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 based on use of public key cryptography as well as MAC authentication. The token MUST be signed following the structure of the COSE specification [RFC8152]. The COSE type MUST be COSE_Sign1 for public key signatures or COSE_Mac0 for MAC authentication. Note however that use of MAC authentication is NOT RECOMMENDED due to the associated infrastructure costs for key management and protocol complexities. It may also restrict the ability to interoperate with third parties.

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. Implementations that have privacy requirements must take appropriate measures to ensure that the token is only used to provision anonymous/pseudonym keys.

9. Verification

To verify the token, the primary need is to check correct formation and signing as for any CWT token. In addition though, the verifier can 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:

  • Instance ID - the value of the Instance ID can be used (together with the kid in the token COSE header, if present) to assist in locating the public key used to verify the token signature.
  • 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.

10. IANA Considerations

10.1. Media Type Registration

IANA is requested to register the "application/psa-attestation-token" media type [RFC2046] in the "Media Types" registry [IANA-MediaTypes] in the manner described in RFC 6838 [RFC6838], which can be used to indicate that the content is a PSA Attestation Token.

  • Type name: application
  • Subtype name: psa-attestation-token
  • Required parameters: n/a
  • Optional parameters: n/a
  • Encoding considerations: binary
  • Security considerations: See the Security Considerations section of [[this RFC]]
  • Interoperability considerations: n/a
  • Published specification: [[this RFC]]
  • Applications that use this media type: Attesters and Relying Parties sending PSA attestation tokens over HTTP(S), CoAP(S), and other transports.
  • Fragment identifier considerations: n/a

  • Additional information:

    • Magic number(s): n/a
    • File extension(s): n/a
    • Macintosh file type code(s): n/a
  • Person & email address to contact for further information: Hannes Tschofenig, Hannes.Tschofenig@arm.com
  • Intended usage: COMMON
  • Restrictions on usage: none
  • Author: Hannes Tschofenig, Hannes.Tschofenig@arm.com
  • Change controller: IESG
  • Provisional registration? No

10.2. CoAP Content-Formats Registration

IANA is requested to register the CoAP Content-Format ID for the "application/psa-attestation-token" media type in the "CoAP Content-Formats" registry [IANA-CoAP-Content-Formats].

10.2.1. Registry Contents

  • Media Type: application/psa-attestation-token
  • Encoding: -
  • Id: [[To-be-assigned by IANA]]
  • Reference: [[this RFC]]

11. References

11.1. Normative References

[EAN-13]
GS1, "International Article Number - EAN/UPC barcodes", , <https://www.gs1.org/standards/barcodes/ean-upc>.
[I-D.ietf-rats-eat]
Mandyam, G., Lundblade, L., Ballesteros, M., and J. O'Donoghue, "The Entity Attestation Token (EAT)", Work in Progress, Internet-Draft, draft-ietf-rats-eat-06, , <http://www.ietf.org/internet-drafts/draft-ietf-rats-eat-06.txt>.
[PSA-FF]
Arm, "Platform Security Architecture Firmware Framework 1.0 (PSA-FF)", , <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)", , <https://developer.arm.com/-/media/Files/pdf/PlatformSecurityArchitecture/Architect/DEN0079_PSA_SM_ALPHA-03_RC01.pdf>.
[RFC2046]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, DOI 10.17487/RFC2046, , <https://www.rfc-editor.org/info/rfc2046>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC6838]
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/info/rfc6838>.
[RFC7049]
Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, , <https://www.rfc-editor.org/info/rfc7049>.
[RFC8152]
Schaad, J., "CBOR Object Signing and Encryption (COSE)", RFC 8152, DOI 10.17487/RFC8152, , <https://www.rfc-editor.org/info/rfc8152>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8392]
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, , <https://www.rfc-editor.org/info/rfc8392>.
[RFC8610]
Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, , <https://www.rfc-editor.org/info/rfc8610>.

11.2. Informative References

[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote Attestation Procedures Architecture", Work in Progress, Internet-Draft, draft-ietf-rats-architecture-08, , <http://www.ietf.org/internet-drafts/draft-ietf-rats-architecture-08.txt>.
[IANA-CoAP-Content-Formats]
IANA, "CoAP Content-Formats", , <https://www.iana.org/assignments/core-parameters>.
[IANA-CWT]
IANA, "CBOR Web Token (CWT) Claims", , <https://www.iana.org/assignments/cwt/cwt.xhtml>.
[IANA-MediaTypes]
IANA, "Media Types", , <http://www.iana.org/assignments/media-types>.
[PSA]
Arm, "Platform Security Architecture Resources", , <https://developer.arm.com/architectures/security-architectures/platform-security-architecture/documentation>.
[TF-M]
Linaro, "Trusted Firmware-M", , <https://www.trustedfirmware.org/projects/tf-m/>.

Appendix A. Reference Implementation

A reference implementation is provided by the Trusted Firmware project [TF-M].

Appendix B. 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 3.3.1 of SECURED. The attestation has been requested from a client residing in the SPE:

{
  / profile /              18: "http://arm.com/psa/2.0.0",
  / psa-client-id /               -75001: 1,
  / psa-lifecycle /               -75002: 12288,
  / psa-implementation-id /       -75003: h'50515253545556575051
52535455565750515253545556575051525354555657',
  / psa-boot-seed /               -75004: h'DEADBEEFDEADBEEFDEAD
BEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEF',
  / psa-certification-reference / -75005: "1234567890123",
  / psa-software-components /     -75006: [
    {
      / measurement type /  1: "BL",
      / measurement value / 2: h'0001020400010204000102040001020
400010204000102040001020400010204',
      / signer ID /         5: h'519200FF519200FF519200FF519200F
F519200FF519200FF519200FF519200FF'
    },
    {
      / measurement type /  1: "PRoT",
      / measurement value / 2: h'0506070805060708050607080506070
805060708050607080506070805060708',
      / signer ID /         5: h'519200FF519200FF519200FF519200F
F519200FF519200FF519200FF519200FF'
    }
  ],
  / nonce /               10: h'00010203000102030001020300010203
00010203000102030001020300010203',
  / ueid /                11: h'01A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2
A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3',
  / psa-verification-service-indicator / -75010: "https://psa-ve
rifier.org"
}

The JWK representation of the IAK used for creating the COSE Sign1 signature over the PSA token is:

{
  "kty": "EC",
  "crv": "P-256",
  "x": "MKBCTNIcKUSDii11ySs3526iDZ8AiTo7Tu6KPAqv7D4",
  "y": "4Etl6SRW2YiLUrN5vfvVHuhp7x8PxltmWWlbbM4IFyM",
  "d": "870MB6gfuTJ4HtUnUvYMyJpr5eUZNP4Bk43bVdj3eAE",
  "use": "enc",
  "kid": "1"
}

The resulting COSE object is:

18(
  [
    / protected /   h'A10126',
    / unprotected / {},
    / payload /     h'AD127818687474703A2F2F61726D2E636F6D2F7073
612F322E302E303A000124F8013A000124F91930003A000124FA582050515253
545556575051525354555657505152535455565750515253545556573A000124
FB5820DEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDEADBEEFDE
ADBEEF3A000124FC6D313233343536373839303132333A000124FD82A3016242
4C02582000010204000102040001020400010204000102040001020400010204
00010204055820519200FF519200FF519200FF519200FF519200FF519200FF51
9200FF519200FFA3016450526F54025820050607080506070805060708050607
0805060708050607080506070805060708055820519200FF519200FF519200FF
519200FF519200FF519200FF519200FF519200FF0A5820000102030001020300
01020300010203000102030001020300010203000102030B582101A0A1A2A3A0
A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A3A0A1A2A33A00012501
781868747470733A2F2F7073612D76657269666965722E6F72673A00012500F6
3A000124F7F63A000124FFF6',
    / signature /   h'8C92FDC99CFDB0016F27008744B3730266342D2881
861DC9A3F89E02394DE7F906EE2D1A3C164A59D580CDD7DFA077290CBFB55069
C55A5D9A2AE17FA31D2108'
  ]
)

Contributors

We would like to thank the following colleagues for their contributions:

* Laurence Lundblade
  Security Theory LLC
  lgl@securitytheory.com

* Tamas Ban
  Arm Limited
  Tamas.Ban@arm.com

* Sergei Trofimov
  Arm Limited
  Sergei.Trofimov@arm.com

Acknowledgments

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

Authors' Addresses

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