Arm's Platform Security Architecture (PSA) Attestation Token
draft-tschofenig-rats-psa-token-05

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RATS                                                       H. Tschofenig
Internet-Draft                                                  S. Frost
Intended status: Informational                               M. Brossard
Expires: 7 September 2020                                        A. Shaw
                                                              T. Fossati
                                                             Arm Limited
                                                            6 March 2020

      Arm's Platform Security Architecture (PSA) Attestation Token
                   draft-tschofenig-rats-psa-token-05

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.

   At its core, the CWT (COSE Web Token) format is used and populated
   with a set of claims in a way similar to EAT (Entity Attestation
   Token).  This specification describes what claims are used by PSA
   compliant systems.

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/.

   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."

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   This Internet-Draft will expire on 7 September 2020.

Copyright Notice

   Copyright (c) 2020 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.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
     2.1.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  PSA Claims  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Caller Claims . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Auth Challenge  . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Client ID . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Target Identification Claims  . . . . . . . . . . . . . .   5
       3.2.1.  Instance ID . . . . . . . . . . . . . . . . . . . . .   5
       3.2.2.  Implementation ID . . . . . . . . . . . . . . . . . .   5
       3.2.3.  Hardware Version  . . . . . . . . . . . . . . . . . .   6
     3.3.  Target State Claims . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Security Lifecycle  . . . . . . . . . . . . . . . . .   6
       3.3.2.  Boot Seed . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Software Inventory Claims . . . . . . . . . . . . . . . .   8
       3.4.1.  Software Components . . . . . . . . . . . . . . . . .   8
       3.4.2.  No Software Measurements  . . . . . . . . . . . . . .  10
     3.5.  Verification Claims . . . . . . . . . . . . . . . . . . .  10
       3.5.1.  Verification Service Indicator  . . . . . . . . . . .  10
       3.5.2.  Profile Definition  . . . . . . . . . . . . . . . . .  11
   4.  Token Encoding and Signing  . . . . . . . . . . . . . . . . .  11
   5.  Collated CDDL . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security and Privacy Considerations . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Reference Implementation . . . . . . . . . . . . . .  16
   Appendix B.  Example  . . . . . . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  18

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   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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".)

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   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.  Auth Challenge

   The Auth Challenge claim is an input object from the caller.  For
   example, this can be a cryptographic nonce, a hash of locally
   attested data.  The length must be 32, 48, or 64 bytes.

   This claim MUST be present in a PSA attestation token.

   psa-nonce-claim = (
       arm_psa_nonce => psa-hash-type
   )

3.1.2.  Client ID

   The Client ID claim represents the Partition ID of the caller.  It is
   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 a definition of the Partition ID, 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.

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   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 = (
       arm_psa_partition_id => psa-client-id-type
   )

3.2.  Target Identification Claims

3.2.1.  Instance ID

   The Instance ID claim represents the unique identifier of the device
   instance.  It is a 32 bytes hash of the public key corresponding to
   the Initial Attestation Key (IAK).  If the IAK is a symmetric key
   then the Instance ID is a hash of the IAK itself.  It is encoded as a
   Universal Entity ID of type RAND [I-D.ietf-rats-eat], i.e.,
   prepending a 0x01 type byte to the key hash.  The full definition is
   in [PSA-SM].

   This claim MUST be present in a PSA attestation token.

   psa-instance-id-type = bytes .size 33

   psa-instance-id = (
       arm_psa_UEID => 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 = (
       arm_psa_implementation_id => psa-implementation-id-type
   )

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3.2.3.  Hardware Version

   The Hardware Version claim provides metadata linking the token to the
   GDSII that went to fabrication for this instance.  It can be used to
   link the class of chip and PSA RoT to the data on a certification
   website.  It MUST be represented as a thirteen-digit [EAN-13].

   psa-hardware-version-type = text .regexp "[0-9]{13}"

   psa-hardware-version = (
       ? arm_psa_hw_version => psa-hardware-version-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.

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                           .----------------------.
            .--- Enrol ---+ Provisioning Lockdown |
           |              '-----------+----------'
           |                          |   .------------------.
           |                          |  |                    |
           *                          v  v                    |
    .--------------.             .---------.                  |
   |    Verifier    |  .---------+ Secured +-----------.      |
    '--------------'   |         '-+-------'            |     |
           *           |           |     ^              |     |
           |           |           v     |              v     |
       Blacklist       |    .------------+------.  .----------+----.
           |           |    | Non-PSA RoT Debug |  | Recoverable   |
           |           |    '---------+---------'  | PSA RoT Debug |
         .-+-----------+-.            |            '------+--------'
        |    Terminate   +------------+-------------------'
        '------+--------'
               |              .----------------.
                '------------>| Decommissioned |
                              '----------------'

                       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 = (
       arm_psa_security_lifecycle => psa-lifecycle-type
   )

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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 = (
       arm_psa_boot_seed => 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.

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   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 = (
       arm_psa_sw_components => [ + 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.

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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 = (
       arm_psa_no_sw_measurements => 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.

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   psa-verification-service-indicator-type = text

   psa-verification-service-indicator = (
       ? arm_psa_origination => psa-verification-service-indicator-type
   )

3.5.2.  Profile Definition

   The Profile Definition claim contains the name of a document that
   describes the "profile" of the report.  The document name may include
   versioning.  The value for this specification MUST be
   PSA_IOT_PROFILE_1.

   psa-profile-type = "PSA_IOT_PROFILE_1"

   psa-profile = (
       ? arm_psa_profile_id => psa-profile-type
   )

4.  Token Encoding and Signing

   The report is encoded as a COSE Web Token (CWT) [RFC8392], similar to
   the Entity Attestation Token (EAT) [I-D.ietf-rats-eat].  The token
   consists of a series of claims declaring evidence as to the nature of
   the instance of hardware and software.  The claims are encoded in
   CBOR [RFC7049] format.  For asymmetric key algorithms, the signature
   structure MUST be COSE-Sign1.  For symmetric key algorithms, the
   structure MUST be COSE-Mac0.

5.  Collated CDDL

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

   arm_psa_profile_id = -75000
   arm_psa_partition_id = -75001
   arm_psa_security_lifecycle = -75002
   arm_psa_implementation_id = -75003

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   arm_psa_boot_seed = -75004
   arm_psa_hw_version = -75005
   arm_psa_sw_components = -75006
   arm_psa_no_sw_measurements = -75007
   arm_psa_nonce = -75008
   arm_psa_UEID = -75009
   arm_psa_origination = -75010

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

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

   psa-boot-seed = (
       arm_psa_boot_seed => 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 = (
       arm_psa_partition_id => psa-client-id-type
   )

   psa-hardware-version-type = text .regexp "[0-9]{13}"

   psa-hardware-version = (
       ? arm_psa_hw_version => psa-hardware-version-type
   )

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

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

   psa-instance-id-type = bytes .size 33

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

   psa-no-sw-measurements-type = 1

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

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   psa-nonce-claim = (
       arm_psa_nonce => psa-hash-type
   )

   psa-profile-type = "PSA_IOT_PROFILE_1"

   psa-profile = (
       ? arm_psa_profile_id => 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 = (
       arm_psa_security_lifecycle => 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 = (
       arm_psa_sw_components => [ + psa-software-component ]
   )

   psa-verification-service-indicator-type = text

   psa-verification-service-indicator = (
       ? arm_psa_origination => psa-verification-service-indicator-type
   )

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6.  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.

7.  IANA Considerations

   IANA is requested to allocate the claims defined in Section 3 to the
   CBOR Web Token (CWT) Claims registry [IANA-CWT].  The change
   controller are the authors and the reference is this document.

8.  References

8.1.  Normative References

   [EAN-13]   GS1, "International Article Number - EAN/UPC barcodes",
              2019, <https://www.gs1.org/standards/barcodes/ean-upc>.

   [PSA-FF]   Arm, "Platform Security Architecture Firmware Framework
              1.0 (PSA-FF)", February 2019,
              <https://pages.arm.com/psa-resources-ff.html>.

   [PSA-SM]   Arm, "Platform Security Architecture Security Model 1.0
              (PSA-SM)", February 2019,
              <https://pages.arm.com/psa-resources-sm.html>.

   [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/info/rfc2119>.

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   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <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,
              May 2017, <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,
              May 2018, <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,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

8.2.  Informative References

   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., and N. Smith,
              "Remote Attestation Procedures Architecture", Work in
              Progress, Internet-Draft, draft-ietf-rats-architecture-01,
              4 February 2020, <http://www.ietf.org/internet-drafts/
              draft-ietf-rats-architecture-01.txt>.

   [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-03, 20
              February 2020, <http://www.ietf.org/internet-drafts/draft-
              ietf-rats-eat-03.txt>.

   [IANA-CWT] IANA, "CBOR Web Token (CWT) Claims", 2020,
              <https://www.iana.org/assignments/cwt/cwt.xhtml>.

   [PSA]      Arm, "Platform Security Architecture Resources", 2019,
              <https://www.arm.com/why-arm/architecture/platform-
              security-architecture/psa-resources>.

   [TF-M]     Linaro, "Trusted Firmware", 2020,
              <https://www.trustedfirmware.org>.

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Appendix A.  Reference Implementation

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

Appendix B.  Example

   The following example shows an attestation token that was produced
   for a device that has a single-stage bootloader, and an RTOS with a
   device management client.  From a code point of view, the RTOS and
   the device management client form a single binary.

   EC key using curve P-256 with:

   *  x:
      0xdcf0d0f4bcd5e26a54ee36cad660d283d12abc5f7307de58689e77cd60452e75

   *  y:
      0x8cbadb5fe9f89a7107e5a2e8ea44ec1b09b7da2a1a82a0252a4c1c26ee1ed7cf

   *  d:
      0xc74670bcb7e85b3803efb428940492e73e3fe9d4f7b5a8ad5e480cbdbcb554c2

   Key using COSE format (base64-encoded):

     pSJYIIy621/p+JpxB+Wi6OpE7BsJt9oqGoKgJSpMHCbuHtfPI1ggx0ZwvLfoWzgD77Q
     olASS5z4/6dT3taitXkgMvby1VMIBAiFYINzw0PS81eJqVO42ytZg0oPRKrxfcwfeWG
     ied81gRS51IAE=

   Example of EAT token (base64-encoded):

     0oRDoQEmoFkCIqk6AAEk+1ggAAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8
     6AAEk+lggAAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh86AAEk/YSkAlggAA
     ECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8EZTMuMS40BVggAAECAwQFBgcIC
     QoLDA0ODxAREhMUFRYXGBkaGxwdHh8BYkJMpAJYIAABAgMEBQYHCAkKCwwNDg8QERIT
     FBUWFxgZGhscHR4fBGMxLjEFWCAAAQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRobHB0
     eHwFkUFJvVKQCWCAAAQIDBAUGBwgJCgsMDQ4PEBESExQVFhcYGRobHB0eHwRjMS4wBV
     ggAAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8BZEFSb1SkAlggAAECAwQFB
     gcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8EYzIuMgVYIAABAgMEBQYHCAkKCwwNDg8Q
     ERITFBUWFxgZGhscHR4fAWNBcHA6AAEk+RkwADoAAST/WCAAAQIDBAUGBwgJCgsMDQ4
     PEBESExQVFhcYGRobHB0eHzoAASUBbHBzYV92ZXJpZmllcjoAAST4IDoAASUAWCEBAA
     ECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh86AAEk93FQU0FfSW9UX1BST0ZJT
     EVfMVhAWIYFCO5+jMSOuoctu11pSlQrEyKtDVECPBlw30KfBlAcaDqVEIoMztCm6A4J
     ZvIr1j0cAFaXShG6My14d4f7Tw==

   Same token using extended CBOR diagnostic format:

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   18(
     [
     / protected / h'a10126' / {
         \ alg \ 1: -7 \ ECDSA 256 \
       } / ,
     / unprotected / {},
     / payload / h'a93a000124fb5820000102030405060708090a0b0c0d0e0f10111
     2131415161718191a1b1c1d1e1f3a000124fa5820000102030405060708090a0b0c
     0d0e0f101112131415161718191a1b1c1d1e1f3a000124fd84a4025820000102030
     405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f0465332e312e
     34055820000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1
     d1e1f0162424ca4025820000102030405060708090a0b0c0d0e0f10111213141516
     1718191a1b1c1d1e1f0463312e31055820000102030405060708090a0b0c0d0e0f1
     01112131415161718191a1b1c1d1e1f016450526f54a40258200001020304050607
     08090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f0463312e30055820000
     102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f016441
     526f54a4025820000102030405060708090a0b0c0d0e0f101112131415161718191
     a1b1c1d1e1f0463322e32055820000102030405060708090a0b0c0d0e0f10111213
     1415161718191a1b1c1d1e1f01634170703a000124f91930003a000124ff5820000
     102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f3a0001
     25016c7073615f76657269666965723a000124f8203a00012500582101000102030
     405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f3a000124f771
     5053415f496f545f50524f46494c455f
     31' / {
        / arm_psa_boot_seed / -75004: h'000102030405060708090a0b0c0d0e0f
        101112131415161718191a1b1c1d1e1f',
        / arm_psa_implementation_id / -75003: h'000102030405060708090a0b
        0c0d0e0f101112131415161718191a1b1c1d1e1f',
        / arm_psa_sw_components / -75006: [
             {
               / measurement / 2: h'000102030405060708090a0b0c0d0e0f1011
               12131415161718191a1b1c1d1e1f',
               / version / 4: "3.1.4",
               / signerID / 5: h'000102030405060708090a0b0c0d0e0f1011121
               31415161718191a1b1c1d1e1f',
               / type / 1: "BL"
             },
             {
               / measurement / 2: h'000102030405060708090a0b0c0d0e0f1011
               12131415161718191a1b1c1d1e1f',
               / version / 4: "1.1",
               / signerID / 5: h'000102030405060708090a0b0c0d0e0f1011121
               31415161718191a1b1c1d1e1f',
               / type / 1: "PRoT"
             },
             {
               / measurement / 2: h'000102030405060708090a0b0c0d0e0f1011
               12131415161718191a1b1c1d1e1f',

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               / version / 4: "1.0",
               / signerID / 5: h'000102030405060708090a0b0c0d0e0f1011121
               31415161718191a1b1c1d1e1f',
               / type / 1: "ARoT"
             },
             {
               / measurement / 2: h'000102030405060708090a0b0c0d0e0f1011
               12131415161718191a1b1c1d1e1f',
               / version / 4: "2.2",
               / signerID / 5: h'000102030405060708090a0b0c0d0e0f1011121
               31415161718191a1b1c1d1e1f',
               / type / 1: "App"
             }
           ],
         / arm_psa_security_lifecycle / -75002: 12288 / SECURED /,
         / arm_psa_nonce / -75008: h'000102030405060708090a0b0c0d0e0f101
         112131415161718191a1b1c1d1e1f',
         / arm_psa_origination / -75010: "psa_verifier",
         / arm_psa_partition_id / -75001: -1,
         / arm_psa_UEID / -75009: h'01000102030405060708090a0b0c0d0e0f10
         1112131415161718191a1b1c1d1e1f',
         / arm_psa_profile_id / -75000: "PSA_IoT_PROFILE_1"
       }),
       } / ,
     / signature / h'58860508ee7e8cc48eba872dbb5d694a542b1322ad0d51023c1
     970df429f06501c683a95108a0cced0a6e80e0966f22bd63d1c0056974a11ba332d
     787787fb4f'
     ]
   )

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

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Acknowledgments

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

Authors' Addresses

   Hannes Tschofenig
   Arm Limited

   Email: Hannes.Tschofenig@arm.com

   Simon Frost
   Arm Limited

   Email: Simon.Frost@arm.com

   Mathias Brossard
   Arm Limited

   Email: Mathias.Brossard@arm.com

   Adrian Shaw
   Arm Limited

   Email: Adrian.Shaw@arm.com

   Thomas Fossati
   Arm Limited

   Email: Thomas.Fossati@arm.com

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