RATS Working Group                                               E. Voit
Internet-Draft                                                     Cisco
Intended status: Standards Track                             H. Birkholz
Expires: 8 September 2022                                 Fraunhofer SIT
                                                             T. Hardjono
                                                                     MIT
                                                              T. Fossati
                                                             Arm Limited
                                                             V. Scarlata
                                                                   Intel
                                                            7 March 2022


              Attestation Results for Secure Interactions
                        draft-ietf-rats-ar4si-02

Abstract

   This document defines reusable Attestation Result information
   elements.  When these elements are offered to Relying Parties as
   Evidence, different aspects of Attester trustworthiness can be
   evaluated.  Additionally, where the Relying Party is interfacing with
   a heterogeneous mix of Attesting Environment and Verifier types,
   consistent policies can be applied to subsequent information exchange
   between each Attester and the Relying Party.

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
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   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 8 September 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Attestation Results for Secure Interactions . . . . . . . . .   5
     2.1.  Information driving a Relying Party Action  . . . . . . .   6
     2.2.  Non-repudiable Identity . . . . . . . . . . . . . . . . .   6
       2.2.1.  Attester and Attesting Environment  . . . . . . . . .   7
       2.2.2.  Verifier  . . . . . . . . . . . . . . . . . . . . . .  10
       2.2.3.  Communicating Identity  . . . . . . . . . . . . . . .  10
     2.3.  Trustworthiness Claims  . . . . . . . . . . . . . . . . .  11
       2.3.1.  Design Principles . . . . . . . . . . . . . . . . . .  11
       2.3.2.  Enumeration Encoding  . . . . . . . . . . . . . . . .  12
       2.3.3.  Assigning a Trustworthiness Claim value . . . . . . .  13
       2.3.4.  Specific Claims . . . . . . . . . . . . . . . . . . .  14
       2.3.5.  Trustworthiness Vector  . . . . . . . . . . . . . . .  18
       2.3.6.  Trustworthiness Vector for a type of Attesting
               Environment . . . . . . . . . . . . . . . . . . . . .  19
     2.4.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  19
   3.  Secure Interactions Models  . . . . . . . . . . . . . . . . .  20
     3.1.  Background-Check  . . . . . . . . . . . . . . . . . . . .  20
       3.1.1.  Verifier Retrieval  . . . . . . . . . . . . . . . . .  20
       3.1.2.  Co-resident Verifier  . . . . . . . . . . . . . . . .  20
     3.2.  Below Zero Trust  . . . . . . . . . . . . . . . . . . . .  21
     3.3.  Mutual Attestation  . . . . . . . . . . . . . . . . . . .  25
     3.4.  Transport Protocol Integration  . . . . . . . . . . . . .  26
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Appendix A.  Implementation Guidance  . . . . . . . . . . . . . .  28
     A.1.  Supplementing Trustworthiness Claims  . . . . . . . . . .  28
   Appendix B.  Supportable Trustworthiness Claims . . . . . . . . .  28
     B.1.  Supportable Trustworthiness Claims for HSM-based CC . . .  29
     B.2.  Supportable Trustworthiness Claims for process-based
           CC  . . . . . . . . . . . . . . . . . . . . . . . . . . .  31



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     B.3.  Supportable Trustworthiness Claims for VM-based CC  . . .  33
   Appendix C.  Some issues being worked . . . . . . . . . . . . . .  34
   Appendix D.  Contributors . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   The first paragraph of the May 2021 US Presidential Executive Order
   on Improving the Nation's Cybersecurity [US-Executive-Order] ends
   with the statement "the trust we place in our digital infrastructure
   should be proportional to how trustworthy and transparent that
   infrastructure is."  Later this order explores aspects of
   trustworthiness such as an auditable trust relationship, which it
   defines as an "agreed-upon relationship between two or more system
   elements that is governed by criteria for secure interaction,
   behavior, and outcomes."

   The Remote ATtestation procedureS (RATS) architecture
   [I-D.ietf-rats-architecture] provides a useful context for
   programmatically establishing and maintaining such auditable trust
   relationships.  Specifically, the architecture defines conceptual
   messages conveyed between architectural subsystems to support
   trustworthiness appraisal.  The RATS conceptual message used to
   convey evidence of trustworthiness is the Attestation Results.  The
   Attestation Results includes Verifier generated appraisals of an
   Attester including such information as the identity of the Attester,
   the security mechanisms employed on this Attester, and the Attester's
   current state of trustworthiness.

   Generated Attestation Results are ultimately conveyed to one or more
   Relying Parties.  Reception of an Attestation Result enables a
   Relying Party to determine what action to take with regards to an
   Attester.  Frequently, this action will be to choose whether to allow
   the Attester to securely interact with the Relying Party over some
   connection between the two.

   When determining whether to allow secure interactions with an
   Attester, a Relying Party is challenged with a number of difficult
   problems which it must be able to handle successfully.  These
   problems include:

   *  What Attestation Results (AR) might a Relying Party be willing to
      trust from a specific Verifier?

   *  What information does a Relying Party need before allowing
      interactions or choosing policies to apply to a connection?





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   *  What are the operating/environmental realities of the Attesting
      Environment where a Relying Party should only be able to associate
      a certain confidence regarding Attestation Results out of the
      Verifier?  (In other words, different types of Trusted Execution
      Environments (TEE) need not be treated as equivalent.)

   *  How to make direct comparisons where there is a heterogeneous mix
      of Attesting Environments and Verifier types.

   To address these problems, it is important that specific Attestation
   Result information elements are framed independently of Attesting
   Environment specific constraints.  If they are not, a Relying Party
   would be forced to adapt to the syntax and semantics of many vendor
   specific environments.  This is not a reasonable ask as there can be
   many types of Attesters interacting with or connecting to a Relying
   Party.

   The business need therefore is for common Attestation Result
   information element definitions.  With these definitions, consistent
   interaction or connectivity decisions can be made by a Relying Party
   where there is a heterogenous mix of Attesting Environment types and
   Verifier types.

   This document defines information elements for Attestation Results in
   a way which normalizes the trustworthiness assertions that can be
   made from a diverse set of Attesters.

1.1.  Requirements Notation

   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.

1.2.  Terminology

   The following terms are imported from [I-D.ietf-rats-architecture]:
   Appraisal Policy for Attestation Results, Attester, Attesting
   Environment, Claims, Evidence, Relying Party, Target Environment and
   Verifier.

   [I-D.ietf-rats-architecture] also describes topological patterns that
   illustrate the need for interoperable conceptual messages.  The two
   patterns called "background-check model" and "passport model" are
   imported from the RATS architecture and used in this document as a
   reference to the architectural concepts: Background-Check Model and
   Passport Model.



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   Newly defined terms for this document:

   AR-augmented Evidence:  a bundle of Evidence which includes at least
      the following:

      1.  Verifier signed Attestation Results.  These Attestation
          Results must include Identity Evidence for the Attester, a
          Trustworthiness Vector describing a Verifier's most recent
          appraisal of an Attester, and some Verifier Proof-of-Freshness
          (PoF).

      2.  A Relying Party PoF which is bound to the Attestation Results
          of (1) by the Attester's Attesting Environment signature.

      3.  Sufficient information to determine the elapsed interval
          between the Verifier PoF and Relying Party PoF.

   Identity Evidence:  Evidence which unambiguously identifies an
      identity.  Identity Evidence could take different forms, such as a
      certificate, or a signature which can be appraised to have only
      been generated by a specific private/public key pair.

   Trustworthiness Claim:  a specific quanta of trustworthiness which
      can be assigned by a Verifier based on its appraisal policy.

   Trustworthiness Tier:  a categorization of the levels of
      trustworthiness which may be assigned by a Verifier to a specific
      Trustworthiness Claim.  These enumerated categories are: Affirmed,
      Warning, Contraindicated, and None.

   Trustworthiness Vector:  a set of zero to many Trustworthiness Claims
      assigned during a single appraisal procedure by a Verifier using
      Evidence generated by an Attester.  The vector is included within
      Attestation Results.

2.  Attestation Results for Secure Interactions

   A Verifier generates the Attestation Results used by a Relying Party.
   When a Relying Party needs to determine whether to permit
   communications with an Attester, these Attestation Results must
   contain a specific set of information elements.  This section defines
   those information elements, and in some cases encodings for
   information elements.








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2.1.  Information driving a Relying Party Action

   When the action is a communication establishment attempt with an
   Attester, there is only a limited set of actions which a Relying
   Party might take.  These actions include:

   *  Allow or deny information exchange with the Attester.  When there
      is a deny, reasons should be returned to the Attester.

   *  Establish a transport connection between an Attester and a
      specific context within a Relying Party (e.g., a TEE, or Virtual
      Routing Function (VRF).)

   *  Apply policies on this connection (e.g., rate limits).

   There are three categories of information which must be conveyed to
   the Relying Party (which also is integrated with a Verifier) before
   it determines which of these actions to take.

   1.  Non-repudiable Identity Evidence - Evidence which undoubtably
       identifies one or more entities involved with a communication.

   2.  Trustworthiness Claims - Specifics a Verifier asserts with
       regards to its trustworthiness findings about an Attester.

   3.  Claim Freshness - Establishes the time of last update (or
       refresh) of Trustworthiness Claims.

   The following sections detail requirements for these three
   categories.

2.2.  Non-repudiable Identity

   Identity Evidence must be conveyed during the establishment of any
   trust-based relationship.  Specific use cases will define the minimum
   types of identities required by a particular Relying Party as it
   evaluates Attestation Results, and perhaps additional associated
   Evidence.  At a bare minimum, a Relying Party MUST start with the
   ability to verify the identity of a Verifier it chooses to trust.
   Attester identities may then be acquired through signed or encrypted
   communications with the Verifier identity and/or the pre-provisioning
   Attester public keys in the Attester.

   During the Remote Attestation process, the Verifier's identity must
   be established with a Relying Party, often via a Verifier signature
   across recent Attestation Results.  This Verifier identity could only
   have come from a key pair maintained by a trusted developer or
   operator of the Verifier.



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   Additionally, each set of Attestation Results must be provably and
   non-reputably bound to the identity of the original Attesting
   Environment which was evaluated by the Verifier.  This is
   accomplished via satisfying two requirements.  First the Verifier
   signed Attestation Results MUST include sufficient Identity Evidence
   to ensure that this Attesting Environment signature refers to the
   same Attesting Environment appraised by the Verifier.  Second, where
   the passport model is used as a subsystem, an Attesting Environment
   signature which spans the Verifier signature MUST also be included.
   As the Verifier signature already spans the Attester Identity as well
   as the Attestation Results, this restricts the viability of spoofing
   attacks.

   In a subset of use cases, these two pieces of Identity Evidence may
   be sufficient for a Relying Party to successfully meet the criteria
   for its Appraisal Policy for Attestation Results.  If the use case is
   a connection request, a Relying Party may simply then establish a
   transport session with an Attester after a successful appraisal.
   However an Appraisal Policy for Attestation Results will often be
   more nuanced, and the Relying Party may need additional information.
   Some Identity Evidence related policy questions which the Relying
   Party may consider include:

   *  Does the Relying Party only trust this Verifier to make
      Trustworthiness Claims on behalf a specific type of Attesting
      Environment?  Might a mix of Verifiers be necessary to cover all
      mandatory Trustworthiness Claims?

   *  Does the Relying Party only accept connections from a verified-
      authentic software build from a specific software developer?

   *  Does the Relying Party only accept connections from specific
      preconfigured list of Attesters?

   For any of these more nuanced appraisals, additional Identity
   Evidence or other policy related information must be conveyed or pre-
   provisioned during the formation of a trust context between the
   Relying Party, the Attester, the Attester's Attesting Environment,
   and the Verifier.

2.2.1.  Attester and Attesting Environment

   Per [I-D.ietf-rats-architecture] Figure 2, an Attester and a
   corresponding Attesting Environment might not share common code or
   even hardware boundaries.  Consequently, an Attester implementation
   needs to ensure that any Evidence which originates from outside the
   Attesting Environment MUST have been collected and delivered securely
   before any Attesting Environment signing may occur.  After the



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   Verifier performs its appraisal, it will include sufficient
   information in the Attestation Results to enable a Relying Party to
   have confidence that the Attester's trustworthiness is represented
   via Trustworthiness Claims signed by the appropriate Attesting
   Environment.

   This document recognizes three general categories of Attesters.

   1.  HSM-based: A Hardware Security Module (HSM) based cryptoprocessor
       which hashes one or more streams of security measurements from an
       Attester within the Attesting Environment.  Maintenance of this
       hash enables detection of an Attester which is lying about the
       set of security measurements taken.  An example of a HSM is a
       TPM2.0 [TPM2.0].

   2.  Process-based: An individual process which has its runtime memory
       encrypted by an Attesting Environment in a way that no other
       processes can read and decrypt that memory (e.g., [SGX] or
       [I-D.tschofenig-rats-psa-token].)

   3.  VM-based: An entire Guest VM (or a set of containers within a
       host) have been encrypted as a walled-garden unit by an Attesting
       Environment.  The result is that the host operating system cannot
       read and decrypt what is executing within that VM (e.g.,
       [SEV-SNP] or [TDX].)

   Each of these categories of Attesters above will be capable of
   generating Evidence which is protected using private keys /
   certificates which are not accessible outside of the corresponding
   Attesting Environment.  The owner of these secrets is the owner of
   the identity which is bound within the Attesting Environment.
   Effectively this means that for any Attester identity, there will
   exist a chain of trust ultimately bound to a hardware-based root of
   trust in the Attesting Environment.  It is upon this root of trust
   that unique, non-repudiable Attester identities may be founded.

   There are several types of Attester identities defined in this
   document.  This list is extensible:

   *  chip-vendor: the vendor of the hardware chip used for the
      Attesting Environment (e.g., a primary Endorsement Key from a TPM)

   *  chip-hardware: specific hardware with specific firmware from an
      'chip-vendor'







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   *  target-environment: a unique instance of a software build running
      in an Attester (e.g., MRENCLAVE [SGX], an Instance ID
      [I-D.tschofenig-rats-psa-token], an Identity Block [SEV-SNP], or a
      hash which represents a set of software loaded since boot (e.g.,
      TPM based integrity verification.))

   *  target-developer: the organizational unit responsible for a
      particular 'target-environment' (e.g., MRSIGNER [SGX])

   *  instance: a unique instantiated instance of an Attesting
      Environment running on 'chip-hardware' (e.g., an LDevID
      [IEEE802.1AR])

   Based on the category of the Attesting Environment, different types
   of identities might be exposed by an Attester.

    +========================+===============+===========+===========+
    | Attester Identity type | Process-based | VM-based  | HSM-based |
    +========================+===============+===========+===========+
    | chip-vendor            | Mandatory     | Mandatory | Mandatory |
    +------------------------+---------------+-----------+-----------+
    | chip-hardware          | Mandatory     | Mandatory | Mandatory |
    +------------------------+---------------+-----------+-----------+
    | target-environment     | Mandatory     | Mandatory | Optional  |
    +------------------------+---------------+-----------+-----------+
    | target-developer       | Mandatory     | Optional  | Optional  |
    +------------------------+---------------+-----------+-----------+
    | instance               | Optional      | Optional  | Optional  |
    +------------------------+---------------+-----------+-----------+

                                 Table 1

   It is expected that drafts subsequent to this specification will
   provide the definitions and value domains for specific identities,
   each of which falling within the Attester identity types listed
   above.  In some cases the actual unique identities might encoded as
   complex structures.  An example complex structure might be a 'target-
   environment' encoded as a Software Bill of Materials (SBOM).

   With the identity definitions and value domains, a Relying Party will
   have sufficient information to ensure that the Attester identities
   and Trustworthiness Claims asserted are actually capable of being
   supported by the underlying type of Attesting Environment.
   Consequently, the Relying Party SHOULD require Identity Evidence
   which indicates of the type of Attesting Environment when it
   considers its Appraisal Policy for Attestation Results.





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2.2.2.  Verifier

   For the Verifier identity, it is critical for a Relying Party to
   review the certificate and chain of trust for that Verifier.
   Additionally, the Relying Party must have confidence that the
   Trustworthiness Claims being relied upon from the Verifier considered
   the chain of trust for the Attesting Environment.

   There are two categorizations Verifier identities defined in this
   document.

   *  verifier build: a unique instance of a software build running as a
      Verifier.

   *  verifier developer: the organizational unit responsible for a
      particular 'verifier build'.

   Within each category, communicating the identity can be accomplished
   via a variety of objects and encodings.

2.2.3.  Communicating Identity

   Any of the above identities used by the Appraisal Policy for
   Attestation Results needed to be pre-established by the Relying Party
   before, or provided during, the exchange of Attestation Results.
   When provided during this exchange, the identity may be communicated
   either implicitly or explicitly.

   An example of explicit communication would be to include the
   following Identity Evidence directly within the Attestation Results:
   a unique identifier for an Attesting Environment, the name of a key
   which can be provably associated with that unique identifier, and the
   set of Attestation Results which are signed using that key.  As these
   Attestation Results are signed by the Verifier, it is the Verifier
   which is explicitly asserting the credentials it believes are
   trustworthy.

   An example of implicit communication would be to include Identity
   Evidence in the form of a signature which has been placed over the
   Attestation Results asserted by a Verifier.  It would be then up to
   the Relying Party's Appraisal Policy for Attestation Results to
   extract this signature and confirm that it only could have been
   generated by an Attesting Environment having access to a specific
   private key.  This implicit identity communication is only viable if
   the Attesting Environment's public key is already known by the
   Relying Party.





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   One final step in communicating identity is proving the freshness of
   the Attestation Results to the degree needed by the Relying Party.  A
   typical way to accomplish this is to include an element of freshness
   be embedded within a signed portion of the Attestation Results.  This
   element of freshness reduces the identity spoofing risks from a
   replay attack.  For more on this, see Section 2.4.

2.3.  Trustworthiness Claims

2.3.1.  Design Principles

   Trust is not absolute.  Trust is a belief in some aspect about an
   entity (in this case an Attester), and that this aspect is something
   which can be depended upon (in this case by a Relying Party.)  Within
   the context of Remote Attestation, believability of this aspect is
   facilitated by a Verifier.  This facilitation depends on the
   Verifier's ability to parse detailed Evidence from an Attester and
   then to assert conclusions about this aspect in a way interpretable
   by a Relying Party.

   Specific aspects for which a Verifier will assert trustworthiness are
   defined in this section.  These are known as Trustworthiness Claims.
   These claims have been designed to enable a common understanding
   between a broad array of Attesters, Verifiers, and Relying Parties.
   The following set of design principles have been applied in the
   Trustworthiness Claim definitions:

   1.  Expose a small number of Trustworthiness Claims.

       Reason: a plethora of similar Trustworthiness Claims will result
       in divergent choices made on which to support between different
       Verifiers.  This would place a lot of complexity in the Relying
       Party as it would be up to the Relying Party (and its policy
       language) to enable normalization across rich but incompatible
       Verifier object definitions.

   2.  Each Trustworthiness Claim enumerates only the specific states
       that could viably result in a different outcome after the Policy
       for Attestation Results has been applied.

       Reason: by explicitly disallowing the standardization of
       enumerated states which cannot easily be connected to a use case,
       we avoid forcing implementers from making incompatible guesses on
       what these states might mean.

   3.  Verifier and RP developers need explicit definitions of each
       state in order to accomplish the goals of (1) and (2).




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       Reason: without such guidance, the Verifier will append plenty of
       raw supporting info.  This relieves the Verifier of making the
       hard decisions.  Of course, this raw info will be mostly non-
       interpretable and therefore non-actionable by the Relying Party.

   4.  Support standards and non-standard extensibility for (1) and (2).

       Reason: standard types of Verifier generated Trustworthiness
       Claims should be vetted by the full RATS working group, rather
       than being maintained in a repository which doesn't follow the
       RFC process.  This will keep a tight lid on extensions which must
       be considered by the Relying Party's policy language.  Because
       this process takes time, non-standard extensions will be needed
       for implementation speed and flexibility.

   These design principles are important to keep the number of Verifier
   generated claims low, and to retain the complexity in the Verifier
   rather than the Relying Party.

2.3.2.  Enumeration Encoding

   Per design principle (2), each Trustworthiness Claim will only expose
   specific encoded values.  To simplify the processing of these
   enumerations by the Relying Party, the enumeration will be encoded as
   a single signed 8 bit integer.  These value assignments for this
   integer will be in four Trustworthiness Tiers which follow these
   guidelines:

   None: The Verifier makes no assertions regarding this aspect of
   trustworthiness.

   *  Value 0: The Evidence received is insufficient to make a
      conclusion.  Note: this should always be always treated
      equivalently by the Relying Party as no claim being made.  I.e.,
      the RP's Appraisal Policy for Attestation Results SHOULD NOT make
      any distinction between a Trustworthiness Claim with enumeration
      '0', and no Trustworthiness Claim being provided.

   *  Value 1: The Evidence received contains unexpected elements which
      the Verifier is unable to parse.  An example might be that the
      wrong type of Evidence has been delivered.

   *  Value -1: A verifier malfunction occurred during the Verifier's
      appraisal processing.

   Affirming: The Verifier affirms the Attester support for this aspect
   of trustworthiness.




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   *  Values 2 to 31: A standards enumerated reason for affirming.

   *  Values -2 to -32: A non-standard reason for affirming.

   Warning: The Verifier warns about this aspect of trustworthiness.

   *  Values 32 to 95: A standards enumerated reason for the warning.

   *  Values -33 to -96: A non-standard reason for the warning.

   Contraindicated: The Verifier asserts the Attester is explicitly
   untrustworthy in regard to this aspect.

   *  Values 96 to 127: A standards enumerated reason for the
      contraindication.

   *  Values -97 to -128: A non-standard reason for the
      contraindication.

   This enumerated encoding listed above will simplify the Appraisal
   Policy for Attestation Results.  Such a policies may be as simple as
   saying that a specific Verifier has recently asserted Trustworthiness
   Claims, all of which are Affirming.

2.3.3.  Assigning a Trustworthiness Claim value

   In order to simplify design, only a single encoded value is asserted
   by a Verifier for any Trustworthiness Claim within a using the
   following process.

   1.  If applicable, a Verifier MUST assign a standardized value from
       the Contraindicated tier.

   2.  Else if applicable, a Verifier MUST assign a non-standardized
       value from the Contraindicated tier.

   3.  Else if applicable, a Verifier MUST assign a standardized value
       from the Warning tier.

   4.  Else if applicable, a Verifier MUST assign a non-standardized
       value from the Warning tier.

   5.  Else if applicable, a Verifier MUST assign a standardized value
       from the Affirming tier.

   6.  Else if applicable, a Verifier MUST assign a non-standardized
       value from the Affirming tier.




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   7.  Else a Verifier MAY assign a 0 or -1.

2.3.4.  Specific Claims

   Following are the Trustworthiness Claims and their supported
   enumerations which may be asserted by a Verifier:

   configuration:  A Verifier has appraised an Attester's configuration,
      and is able to make conclusions regarding the exposure of known
      vulnerabilities

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  The configuration is a known and approved config.

      3:  The configuration includes or exposes no known
         vulnerabilities.

      32:  The configuration includes or exposes known vulnerabilities.

      96:  The configuration is unsupportable as it exposes unacceptable
         security vulnerabilities.

      99:  Cryptographic validation of the Evidence has failed.

   executables:  A Verifier has appraised and evaluated relevant runtime
      files, scripts, and/or other objects which have been loaded into
      the Target environment's memory.

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  Only a recognized genuine set of approved executables,
         scripts, files, and/or objects have been loaded during and
         after the boot process.

      3:  Only a recognized genuine set of approved executables have
         been loaded during the boot process.

      32:  Only a recognized genuine set of executables, scripts, files,




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         and/or objects have been loaded.  However the Verifier cannot
         vouch for a subset of these due to known bugs or other known
         vulnerabilities.

      33:  Runtime memory includes executables, scripts, files, and/or
         objects which are not recognized.

      96:  Runtime memory includes executables, scripts, files, and/or
         object which are contraindicated.

      99:  Cryptographic validation of the Evidence has failed.

   file-system:  A Verifier has evaluated a specific set of directories
      within the Attester's file system.  (Note: the Verifier may or may
      not indicate what these directory and expected files are via an
      unspecified management interface.)

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  Only a recognized set of approved files are found.

      32:  The file system includes unrecognized executables, scripts,
         or files.

      96:  The file system includes contraindicated executables,
         scripts, or files.

      99:  Cryptographic validation of the Evidence has failed.

   hardware:  A Verifier has appraised any Attester hardware and
      firmware which are able to expose fingerprints of their identity
      and running code.

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  An Attester has passed its hardware and/or firmware
         verifications needed to demonstrate that these are genuine/
         supported.





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      32: An Attester contains only genuine/supported hardware and/or
      firmware, but there are known security vulnerabilities.

      96:  Attester hardware and/or firmware is recognized, but its
         trustworthiness is contraindicated.

      97:  A Verifier does not recognize an Attester's hardware or
         firmware, but it should be recognized.

      99:  Cryptographic validation of the Evidence has failed.

   instance-identity:  A Verifier has appraised an Attesting
      Environment's unique identity based upon private key signed
      Evidence which can be correlated to a unique instantiated instance
      of the Attester.  (Note: this Trustworthiness Claim should only be
      generated if the Verifier actually expects to recognize the unique
      identity of the Attester.)

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  The Attesting Environment is recognized, and the associated
         instance of the Attester is not known to be compromised.

      96:  The Attesting Environment is recognized, and but its unique
         private key indicates a device which is not trustworthy.

      97:  The Attesting Environment is not recognized; however the
         Verifier believes it should be.

      99:  Cryptographic validation of the Evidence has failed.

   runtime-opaque:  A Verifier has appraised the visibility of Attester
      objects in memory from perspectives outside the Attester.

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  the Attester's executing Target Environment and Attesting






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         Environments are encrypted and within Trusted Execution
         Environment(s) opaque to the operating system, virtual machine
         manager, and peer applications.  (Note: This value corresponds
         to the protections asserted by O.RUNTIME_CONFIDENTIALITY from
         [GP-TEE-PP])

      32:  the Attester's executing Target Environment and Attesting
         Environments inaccessible from any other parallel application
         or Guest VM running on the Attester's physical device.  (Note
         that unlike "1" these environments are not encrypted in a way
         which restricts the Attester's root operator visibility.  See
         O.TA_ISOLATION from [GP-TEE-PP].)

      96:  The Verifier has concluded that in memory objects are
         unacceptably visible within the physical host that supports the
         Attester.

      99:  Cryptographic validation of the Evidence has failed.

   sourced-data:  A Verifier has evaluated of the integrity of data
      objects from external systems used by the Attester.

      0:  No assertion

      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  All essential Attester source data objects have been provided
         by other Attester(s) whose most recent appraisal(s) had both no
         Trustworthiness Claims of "0" where the current Trustworthiness
         Claim is "Affirming", as well as no "Warning" or
         "Contraindicated" Trustworthiness Claims.

      32:  Attester source data objects come from unattested sources, or
         attested sources with "Warning" type Trustworthiness Claims.

      96:  Attester source data objects come from contraindicated
         sources.

      99:  Cryptographic validation of the Evidence has failed.

   storage-opaque:  A Verifier has appraised that an Attester is capable
      of encrypting persistent storage.  (Note: Protections must meet
      the capabilities of [OMTP-ATE] Section 5, but need not be hardware
      tamper resistant.)

      0:  No assertion



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      1:  Verifer cannot parse unexpected Evidence.

      -1:  Verifier malfunction

      2:  the Attester encrypts all secrets in persistent storage via
         using keys which are never visible outside an HSM or the
         Trusted Execution Environment hardware.

      32:  the Attester encrypts all persistently stored secrets, but
         without using hardware backed keys

      96:  There are persistent secrets which are stored unencrypted in
         an Attester.

      99:  Cryptographic validation of the Evidence has failed.

   It is possible for additonal Trustworthiness Claims and enumerated
   values to be defined in subsequent documents.  At the same time, the
   standardized Trustworthiness Claim values listed above have been
   designed so there is no overlap within a Trustworthiness Tier.  As a
   result, it is possible to imagine a future where overlapping
   Trustworthiness Claims within a single Trustworthiness Tier may be
   defined.  Wherever possible, the Verifier SHOULD assign the best
   fitting standardized value.

   Where a Relying Party doesn't know how to handle a particular
   Trustworthiness Claim, it MAY choose an appropriate action based on
   the Trustworthiness Tier under which the enumerated value fits.

   It is up to the Verifier to publish the types of evaluations it
   performs when determining how Trustworthiness Claims are derived for
   a type of any particular type of Attester.  It is out of the scope of
   this document for the Verifier to provide proof or specific logic on
   how a particular Trustworthiness Claim which it is asserting was
   derived.

2.3.5.  Trustworthiness Vector

   Multiple Trustworthiness Claims may be asserted about an Attesting
   Environment at single point in time.  The set of Trustworthiness
   Claims inserted into an instance of Attestation Results by a Verifier
   is known as a Trustworthiness Vector.  The order of Claims in the
   vector is NOT meaningful.  A Trustworthiness Vector with no
   Trustworthiness Claims (i.e., a null Trustworthiness Vector) is a
   valid construct.  In this case, the Verifier is making no
   Trustworthiness Claims but is confirming that an appraisal has been
   made.




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2.3.6.  Trustworthiness Vector for a type of Attesting Environment

   Some Trustworthiness Claims are implicit based on the underlying type
   of Attesting Environment.  For example, a validated MRSIGNER identity
   can be present where the underlying [SGX] hardware is 'hw-authentic'.
   Where such implicit Trustworthiness Claims exist, they do not have to
   be explicitly included in the Trustworthiness Vector.  However, these
   implicit Trustworthiness Claims SHOULD be considered as being present
   by the Relying Party.  Another way of saying this is if a
   Trustworthiness Claim is automatically supported as a result of
   coming from a specific type of TEE, that claim need not be
   redundantly articulated.  Such implicit Trustworthiness Claims can be
   seen in the tables within Appendix B.2 and Appendix B.3.

   Additionally, there are some Trustworthiness Claims which cannot be
   adequately supported by an Attesting Environment.  For example, it
   would be difficult for an Attester that includes only a TPM (and no
   other TEE) from ever having a Verifier appraise support for 'runtime-
   opaque'.  As such, a Relying Party would be acting properly if it
   rejects any non-supportable Trustworthiness Claims asserted from a
   Verifier.

   As a result, the need for the ability to carry a specific
   Trustworthiness Claim will vary by the type of Attesting Environment.
   Example mappings can be seen in Appendix B.

2.4.  Freshness

   A Relying Party will care about the recentness of the Attestation
   Results, and the specific Trustworthiness Claims which are embedded.
   All freshness mechanisms of [I-D.ietf-rats-architecture], Section 10
   are supportable by this specification.

   Additionally, a Relying Party may track when a Verifier expires its
   confidence for the Trustworthiness Claims or the Trustworthiness
   Vector as a whole.  Mechanisms for such expiry are not defined within
   this document.

   There is a subset of secure interactions where the freshness of
   Trustworthiness Claims may need to be revisited asynchronously.  This
   subset is when trustworthiness depends on the continuous availability
   of a transport session between the Attester and Relying Party.  With
   such connectivity dependent Attestation Results, if there is a reboot
   which resets transport connectivity, all established Trustworthiness
   Claims should be cleared.  Subsequent connection re-establishment
   will allow fresh new Trustworthiness Claims to be delivered.





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3.  Secure Interactions Models

   There are multiple ways of providing a Trustworthiness Vector to a
   Relying Party.  This section describes two alternatives.

3.1.  Background-Check

3.1.1.  Verifier Retrieval

   It is possible to for a Relying Party to follow the Background-Check
   Model defined in Section 5.2 of [I-D.ietf-rats-architecture].  In
   this case, a Relying Party will receive Attestation Results
   containing the Trustworthiness Vector directly from a Verifier.
   These Attestation Results can then be used by the Relying Party in
   determining the appropriate treatment for interactions with the
   Attester.

   While applicable in some cases, the utilization of the Background-
   Check Model without modification has potential drawbacks in other
   cases.  These include:

   *  Verifier scale: if the Attester has many Relying Parties, a
      Verifier appraising that Attester could be frequently be queried
      based on the same Evidence.

   *  Information leak: Evidence which the Attester might consider
      private can be visible to the Relying Party.  Hiding that Evidence
      could devalue any resulting appraisal.

   *  Latency: a Relying Party will need to wait for the Verifier to
      return Attestation Results before proceeding with secure
      interactions with the Attester.

   An implementer should examine these potential drawbacks before
   selecting this alternative.

3.1.2.  Co-resident Verifier

   A simplified Background-Check Model may exist in a very specific
   case.
   This is where the Relying Party and Verifier functions are co-
   resident.  This model is appropriate when:

   *  Some hardware-based private key is used by an Attester while
      proving its identity as part of a mutually authenticated secure
      channel establishment with the Relying Party, and





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   *  this Attester identity is accepted as sufficient proof of Attester
      integrity.

   Effectively this means that detailed forensic capabilities of a
   robust Verifier are unnecessary because it is accepted that the code
   and operational behavior of the Attester cannot be manipulated after
   TEE initialization.

   An example of such a scenario may be when an SGX's MRENCLAVE and
   MRSIGNER values have been associated with a known QUOTE value.  And
   the code running within the TEE is not modifiable after launch.

3.2.  Below Zero Trust

   Zero Trust Architectures are referenced in [US-Executive-Order]
   eleven times.  However despite this high profile, there is an
   architectural gap with Zero Trust.  The credentials used for
   authentication and admission control can be manipulated on the
   endpoint.  Attestation can fill this gap through the generation of a
   compound credential called AR-augmented Evidence.
   This compound credential is rooted in the hardware based Attesting
   Environment of an endpoint, plus the trustworthiness of a Verifier.
   The overall solution is known as "Below Zero Trust" as the compound
   credential cannot be manipulated or spoofed by an administrator of an
   endpoint with root access.  This solution is not adversely impacted
   by the potential drawbacks with pure background-check described
   above.

   To kick-off the "Below Zero Trust" compound credential creation
   sequence, a Verifier evaluates an Attester and returns signed
   Attestation Results back to this original Attester no less frequently
   than a well-known interval.  This interval may also be asynchronous,
   based on the changing of certain Evidence as described in
   [I-D.ietf-rats-network-device-subscription].

   When a Relying Party is to receive information about the Attester's
   trustworthiness, the Attesting Environment assembles the minimal set
   of Evidence which can be used to confirm or refute whether the
   Attester remains in the state of trustworthiness represented by the
   AR.  To this Evidence, the Attesting Environment appends the
   signature from the most recent AR as well as a Relying Party Proof-
   of-Freshness.  The Attesting Environment then signs the combination.

   The Attester then assembles AR Augmented Evidence by taking the
   signed combination and appending the full AR.  The assembly now
   consists of two independent but semantically bound sets of signed
   Evidence.




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   The AR Augmented Evidence is then sent to the Relying Party.  The
   Relying Party then can appraise these semantically bound sets of
   signed Evidence by applying an Appraisal Policy for Attestation
   Results as described below.  This policy will consider both the AR as
   well as additional information about the Attester within the AR
   Augmented Evidence the when determining what action to take.

   This alternative combines the [I-D.ietf-rats-architecture] Sections
   5.1 Passport Model and Section 5.2 Background-Check Model.  Figure 1
   describes this flow of information.  The flows within this combined
   model are mapped to [I-D.ietf-rats-architecture] in the following
   way.  "Verifier A" below corresponds to the "Verifier" Figure 5
   within [I-D.ietf-rats-architecture].  And "Relying Party/Verifier B"
   below corresponds to the union of the "Relying Party" and "Verifier"
   boxes within Figure 6 of [I-D.ietf-rats-architecture].  This union is
   possible because Verifier B can be implemented as a simple, self-
   contained process.  The resulting combined process can appraise the
   AR-augmented Evidence to determine whether an Attester qualifies for
   secure interactions with the Relying Party.  The specific steps of
   this process are defined later in this section.

     .----------------.
     | Attester       |
     | .-------------.|
     | | Attesting   ||             .----------.    .---------------.
     | | Environment ||             | Verifier |    | Relying Party |
     | '-------------'|             |     A    |    |  / Verifier B |
     '----------------'             '----------'    '---------------'
           time(VG)                       |                 |
             |<------Verifier PoF-------time(NS)            |
             |                            |                 |
    time(EG)(1)------Evidence------------>|                 |
             |                          time(RG)            |
             |<------Attestation Results-(2)                |
             ~                            ~                 ~
           time(VG')?                     |                 |
             ~                            ~                 ~
             |<------Relying Party PoF-----------------(3)time(NS')
             |                            |                 |
   time(EG')(4)------AR-augmented Evidence----------------->|
             |                            |   time(RG',RA')(5)
                                                           (6)
                                                            ~
                                                         time(RX')

                         Figure 1: Below Zero Trust





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   The interaction model depicted above includes specific time related
   events from Appendix A of [I-D.ietf-rats-architecture].  With the
   identification of these time related events, time duration/interval
   tracking becomes possible.  Such duration/interval tracking can
   become important if the Relying Party cares if too much time has
   elapsed between the Verifier PoF and Relying Party PoF.  If too much
   time has elapsed, perhaps the Attestation Results themselves are no
   longer trustworthy.

   Note that while time intervals will often be relevant, there is a
   simplified case that does not require a Relying Party's PoF in step
   (3).  In this simplified case, the Relying Party trusts that the
   Attester cannot be meaningfully changed from the outside during any
   reportable interval.  Based on that assumption, and when this is the
   case then the step of the Relying Party PoF can be safely omitted.

   In all cases, appraisal policies define the conditions and
   prerequisites for when an Attester does qualify for secure
   interactions.  To qualify, an Attester has to be able to provide all
   of the mandatory affirming Trustworthiness Claims and identities
   needed by a Relying Party's Appraisal Policy for Attestation Results,
   and none of the disqualifying detracting Trustworthiness Claims.

   More details on each interaction step of Below Zero Trust are as
   follows.  The numbers used in this sequence match to the numbered
   steps in Figure 1:

   1.  An Attester sends Evidence which is provably fresh to Verifier A
       at time(EG).  Freshness from the perspective of Verifier A MAY be
       established with Verifier PoF such as a nonce.

   2.  Verifier A appraises (1), then sends the following items back to
       that Attester within Attestation Results:

       1.  the verified identity of the Attesting Environment,

       2.  the Verifier A appraised Trustworthiness Vector of an
           Attester,

       3.  a freshness proof associated with the Attestation Results,

       4.  a Verifier signature across (2.1) though (2.3).

   3.  At time(EG') a Relying Party PoF (such as a nonce) known to the
       Relying Party is sent to the Attester.

   4.  The Attester generates and sends AR-augmented Evidence to the
       Relying Party/Verifier B.  This AR-augmented Evidence includes:



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       1.  The Attestation Results from (2)

       2.  Any (optionally) new incremental Evidence from the Attesting
           Environment

       3.  Attestation Environment signature which spans a hash of the
           Attestation Results (such as the signature of (2.4)), the
           proof-of-freshness from (3), and (4.2).  Note: this construct
           allows the delta of time between (2.3) and (3) to be
           definitively calculated by the Relying Party.

   5.  On receipt of (4), the Relying Party applies its Appraisal Policy
       for Attestation Results.  At minimum, this appraisal policy
       process must include the following:

       1.  Verify that (4.3) includes the nonce from (3).

       2.  Use a local certificate to validate the signature (4.1).

       3.  Verify that the hash from (4.3) matches (4.1)

       4.  Use the identity of (2.1) to validate the signature of (4.3).

       5.  Failure of any steps (5.1) through (5.4) means the link does
           not meet minimum validation criteria, therefore appraise the
           link as having a null Verifier B Trustworthiness Vector.
           Jump to step (6.1).

       6.  When there is large or uncertain time gap between time(EG)
           and time(EG'), the link should be assigned a null Verifier B
           Trustworthiness Vector.  Jump to step (6.1).

       7.  Assemble the Verifier B Trustworthiness Vector

           1.  Copy Verifier A Trustworthiness Vector to Verifier B
               Trustworthiness Vector

           2.  Add implicit Trustworthiness Claims inherent to the type
               of TEE.

           3.  Prune any Trustworthiness Claims unsupportable by the
               Attesting Environment.

           4.  Prune any Trustworthiness Claims the Relying Party
               doesn't accept from this Verifier.






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   6.  The Relying Party takes action based on Verifier B's appraised
       Trustworthiness Vector, and applies the Appraisal Policy for
       Attestation Results.  Following is a reasonable process for such
       evaluation:

       1.  Prune any Trustworthiness Claims from the Trustworthiness
           Vector not used in the Appraisal Policy for Attestation
           Results.

       2.  Allow the information exchange from the Attester into a
           Relying Party context in the Appraisal Policy for Attestation
           Results where the Verifier B appraised Trustworthiness Vector
           includes all the mandatory Trustworthiness Claims are in the
           "Affirming" value range, and none of the disqualifying
           Trustworthiness Claims are in the "Contraindicated" value
           range.

       3.  Disallow any information exchange into a Relying Party
           context for which that Verifier B appraised Trustworthiness
           Vector is not qualified.

   As link layer protocols re-authenticate, steps (1) to (2) and steps
   (3) to (6) will independently refresh.  This allows the
   Trustworthiness of Attester to be continuously re-appraised.  There
   are only specific event triggers which will drive the refresh of
   Evidence generation (1), Attestation Result generation (2), or AR-
   augmented Evidence generation (4):

   *  life-cycle events, e.g. a change to an Authentication Secret of
      the Attester or an update of a software component.

   *  uptime-cycle events, e.g. a hard reset or a re-initialization of
      an Attester.

   *  authentication-cycle events, e.g. a link-layer interface reset
      could result in a new (4).

3.3.  Mutual Attestation

   In the interaction models described above, each device on either side
   of a secure interaction may require remote attestation of its peer.
   This process is known as mutual-attestation.  To support mutual-
   attestation, the interaction models listed above may be run
   independently on either side of the connection.







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3.4.  Transport Protocol Integration

   Either unidirectional attestation or mutual attestation may be
   supported within the protocol interactions needed for the
   establishment of a single transport session.  While this document
   does not mandate specific transport protocols, messages containing
   the Attestation Results and AR Augmented Evidence can be passed
   within an authentication framework such the EAP protocol [RFC5247]
   over TLS [RFC8446].

4.  Privacy Considerations

   Privacy Considerations Text

5.  Security Considerations

   Security Considerations Text

6.  IANA Considerations

   See Body.

7.  References

7.1.  Normative References

   [GP-TEE-PP]
              "Global Platform TEE Protection Profile v1.3", September
              2020, <https://globalplatform.org/specs-library/tee-
              protection-profile-v1-3/>.

   [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-
              15, 8 February 2022, <https://www.ietf.org/archive/id/
              draft-ietf-rats-architecture-15.txt>.

   [OMTP-ATE] "Open Mobile Terminal Platform - Advanced Trusted
              Environment", May 2009, <https://www.gsma.com/newsroom/wp-
              content/uploads/2012/03/
              omtpadvancedtrustedenvironmentomtptr1v11.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/info/rfc2119>.




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   [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>.

7.2.  Informative References

   [I-D.ietf-rats-network-device-subscription]
              Birkholz, H., Voit, E., and W. Pan, "Attestation Event
              Stream Subscription", Work in Progress, Internet-Draft,
              draft-ietf-rats-network-device-subscription-01, 7 March
              2022, <https://www.ietf.org/archive/id/draft-ietf-rats-
              network-device-subscription-01.txt>.

   [I-D.tschofenig-rats-psa-token]
              Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T.
              Fossati, "Arm's Platform Security Architecture (PSA)
              Attestation Token", Work in Progress, Internet-Draft,
              draft-tschofenig-rats-psa-token-09, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-tschofenig-rats-
              psa-token-09.txt>.

   [IEEE802.1AR]
              "802.1AR: Secure Device Identity", 2 August 2018,
              <https://ieeexplore.ieee.org/document/8423794>.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, DOI 10.17487/RFC5247, August 2008,
              <https://www.rfc-editor.org/info/rfc5247>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [SEV-SNP]  "AMD SEV-SNP: Stregthening VM Isolation with Integrity
              Protection and More", 2020,
              <https://www.amd.com/system/files/TechDocs/SEV-SNP-
              strengthening-vm-isolation-with-integrity-protection-and-
              more.pdf>.

   [SGX]      "Supporting Third Party Attestation for Intel SGX with
              Intel Data Center Attestation Primitives", 2017, <https://
              software.intel.com/content/dam/develop/external/us/en/
              documents/intel-sgx-support-for-third-party-attestation-
              801017.pdf>.






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   [TDX]      "Intel Trust Domain Extensions", 2020, <https://software.i
              ntel.com/content/dam/develop/external/us/en/documents/tdx-
              whitepaper-final9-17.pdf>.

   [TPM-ID]   "TPM Keys for Platform Identity for TPM 1.2", August 2015,
              <https://www.trustedcomputinggroup.org/wp-content/uploads/
              TPM_Keys_for_Platform_Identity_v1_0_r3_Final.pdf>.

   [TPM2.0]   "Trusted Platform Module Library - Part 1: Architecture",
              n.d., <https://trustedcomputinggroup.org/wp-
              content/uploads/TPM-Rev-2.0-Part-1-Architecture-
              01.07-2014-03-13.pdf>.

   [US-Executive-Order]
              "Executive Order on Improving the Nation's Cybersecurity",
              12 May 2021, <https://www.whitehouse.gov/briefing-room/
              presidential-actions/2021/05/12/executive-order-on-
              improving-the-nations-cybersecurity/>.

Appendix A.  Implementation Guidance

A.1.  Supplementing Trustworthiness Claims

   What has been encoded into each Trustworthiness Claim is the domain
   of integer values which is likely to drive a different programmatic
   decision in the Relying Party's Appraisal Policy for Attestation
   Results.  This will not be the only thing a Relying Party's
   Operations team might care to track for measurement or debugging
   purposes.

   There is also the opportunity for the Verifier to include
   supplementary Evidence beyond a set of asserted Trustworthiness
   Claims.  It is recommended that if supplementary Evidence is provided
   by the Verifier within the Attestation Results, that this
   supplementary Evidence includes a reference to a specific
   Trustworthiness Claim.  This will allow a deeper understanding of
   some of the reasoning behind the integer value assigned.

Appendix B.  Supportable Trustworthiness Claims

   The following is a table which shows what Claims are supportable by
   different Attesting Environment types.  Note that claims MAY BE
   implicit to an Attesting Environment type, and therefore do not have
   to be included in the Trustworthiness Vector to be considered as set
   by the Relying Party.






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B.1.  Supportable Trustworthiness Claims for HSM-based CC

   Following are Trustworthiness Claims which MAY be set for a HSM-based
   Confidential Computing Attester.  (Such as a TPM [TPM-ID].)















































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   +===================+===========+==================================+
   | Trustworthiness   | Required? | Appraisal Method                 |
   | Claim             |           |                                  |
   +===================+===========+==================================+
   | configuration     | Optional  | Verifier evaluation of Attester  |
   |                   |           | reveals no configuration lines   |
   |                   |           | which expose the Attester to     |
   |                   |           | known security vulnerabilities.  |
   |                   |           | This may be done with or without |
   |                   |           | the involvement of a TPM PCR.    |
   +-------------------+-----------+----------------------------------+
   | executables       | Yes       | Checks the TPM PCRs for the      |
   |                   |           | static operating system, and for |
   |                   |           | any tracked files subsequently   |
   |                   |           | loaded                           |
   +-------------------+-----------+----------------------------------+
   | file-system       | No        | Can be supported, but TPM        |
   |                   |           | tracking is unlikely             |
   +-------------------+-----------+----------------------------------+
   | hardware          | Yes       | If TPM PCR check ok from BIOS    |
   |                   |           | checks, through Master Boot      |
   |                   |           | Record configuration             |
   +-------------------+-----------+----------------------------------+
   | instance-identity | Optional  | Check IDevID                     |
   +-------------------+-----------+----------------------------------+
   | runtime-opaque    | n/a       | TPMs are not recommended to      |
   |                   |           | provide a sufficient technology  |
   |                   |           | base for this Trustworthiness    |
   |                   |           | Claim.                           |
   +-------------------+-----------+----------------------------------+
   | sourced-data      | n/a       | TPMs are not recommended to      |
   |                   |           | provide a sufficient technology  |
   |                   |           | base for this Trustworthiness    |
   |                   |           | Claim.                           |
   +-------------------+-----------+----------------------------------+
   | storage-opaque    | Minimal   | With a TPM, secure storage space |
   |                   |           | exists and is writeable by       |
   |                   |           | external applications.  But the  |
   |                   |           | space is so limited that it      |
   |                   |           | often is used just be used to    |
   |                   |           | store keys.                      |
   +-------------------+-----------+----------------------------------+

                                 Table 2

   Setting the Trustworthiness Claims may follow the following logic at
   the Verifier A within (2) of Figure 1:




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   Start: Evidence received starts the generation of a new
   Trustworthiness Vector.  (e.g.,  TPM Quote Received, log received,
   or appraisal timer expired)

   Step 0: set Trustworthiness Vector = Null

   Step 1: Is there sufficient fresh signed evidence to appraise?
     (yes) - No Action
     (no) -  Goto Step 6

   Step 2: Appraise Hardware Integrity PCRs
      if (hardware NOT "0") - push onto vector
      if (hardware NOT affirming or warning), go to Step 6

   Step 3: Appraise Attesting Environment identity
      if (instance-identity <> "0") - push onto vector

   Step 4: Appraise executable loaded and filesystem integrity
      if (executables NOT "0") - push onto vector
      if (executables NOT affirming or warning), go to Step 6

   Step 5: Appraise all remaining Trustworthiness Claims
           Independently and set as appropriate.

   Step 6: Assemble Attestation Results, and push to Attester

   End

B.2.  Supportable Trustworthiness Claims for process-based CC

   Following are Trustworthiness Claims which MAY be set for a process-
   based Confidential Computing based Attester.  (Such as a SGX Enclaves
   and TrustZone.)


















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   +===================+===========+==================================+
   | Trustworthiness   | Required? | Appraisal Method                 |
   | Claim             |           |                                  |
   +===================+===========+==================================+
   | instance-identity | Optional  | Internally available in TEE.     |
   |                   |           | But keys might not be known/     |
   |                   |           | exposed to the Relying Party by  |
   |                   |           | the Attesting Environment.       |
   +-------------------+-----------+----------------------------------+
   | configuration     | Optional  | If done, this is at the          |
   |                   |           | Application Layer.  Plus each    |
   |                   |           | process needs it own protection  |
   |                   |           | mechanism as the protection is   |
   |                   |           | limited to the process itself.   |
   +-------------------+-----------+----------------------------------+
   | executables       | Optional  | Internally available in TEE.     |
   |                   |           | But keys might not be known/     |
   |                   |           | exposed to the Relying Party by  |
   |                   |           | the Attesting Environment.       |
   +-------------------+-----------+----------------------------------+
   | file-system       | Optional  | Can be supported by application, |
   |                   |           | but process-based CC is not a    |
   |                   |           | sufficient technology base for   |
   |                   |           | this Trustworthiness Claim.      |
   +-------------------+-----------+----------------------------------+
   | hardware          | Implicit  | At least the TEE is protected    |
   |                   | in        | here.  Other elements of the     |
   |                   | signature | system outside of the TEE might  |
   |                   |           | need additional protections is   |
   |                   |           | used by the application process. |
   +-------------------+-----------+----------------------------------+
   | runtime-opaque    | Implicit  | From the TEE                     |
   |                   | in        |                                  |
   |                   | signature |                                  |
   +-------------------+-----------+----------------------------------+
   | storage-opaque    | Implicit  | Although the application must    |
   |                   | in        | assert that this function is     |
   |                   | signature | used by the code itself.         |
   +-------------------+-----------+----------------------------------+
   | sourced-data      | Optional  | Will need to be supported by     |
   |                   |           | application code                 |
   +-------------------+-----------+----------------------------------+

                                 Table 3







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B.3.  Supportable Trustworthiness Claims for VM-based CC

   Following are Trustworthiness Claims which MAY be set for a VM-based
   Confidential Computing based Attester.  (Such as SEV, TDX, ACCA, SEV-
   SNP.)

   +===================+===========+===================================+
   | Trustworthiness   | Required? | Appraisal Method                  |
   | Claim             |           |                                   |
   +===================+===========+===================================+
   | instance-identity | Optional  | Internally available in TEE.      |
   |                   |           | But keys might not be known/      |
   |                   |           | exposed to the Relying Party by   |
   |                   |           | the Attesting Environment.        |
   +-------------------+-----------+-----------------------------------+
   | configuration     | Optional  | Requires application              |
   |                   |           | integration.  Easier than with    |
   |                   |           | process-based solution, as the    |
   |                   |           | whole protected machine can be    |
   |                   |           | evaluated.                        |
   +-------------------+-----------+-----------------------------------+
   | executables       | Optional  | Internally available in TEE.      |
   |                   |           | But keys might not be known/      |
   |                   |           | exposed to the Relying Party by   |
   |                   |           | the Attesting Environment.        |
   +-------------------+-----------+-----------------------------------+
   | file-system       | Optional  | Can be supported by application   |
   +-------------------+-----------+-----------------------------------+
   | hardware          | Chip      | At least the TEE is protected     |
   |                   | dependent | here.  Other elements of the      |
   |                   |           | system outside of the TEE might   |
   |                   |           | need additional protections is    |
   |                   |           | used by the application process.  |
   +-------------------+-----------+-----------------------------------+
   | runtime-opaque    | Implicit  | From the TEE                      |
   |                   | in        |                                   |
   |                   | signature |                                   |
   +-------------------+-----------+-----------------------------------+
   | storage-opaque    | Chip      | Although the application must     |
   |                   | dependent | assert that this function is      |
   |                   |           | used by the code itself.          |
   +-------------------+-----------+-----------------------------------+
   | sourced-data      | Optional  | Will need to be supported by      |
   |                   |           | application code                  |
   +-------------------+-----------+-----------------------------------+

                                  Table 4




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Appendix C.  Some issues being worked

   It is possible for a cluster/hierarchy of Verifiers to have aggregate
   AR which are perhaps signed/endorsed by a lead Verifier.  What should
   be the Proof-of-Freshness or Verifier associated with any of the
   aggregate set of Trustworthiness Claims?

   There will need to be a subsequent document which documents how these
   objects which will be translated into a protocol on a wire (e.g.  EAP
   on TLS).  Some breakpoint between what is in this draft, and what is
   in specific drafts for wire encoding will need to be determined.
   Questions like architecting the cluster/hierarchy of Verifiers fall
   into this breakdown.

   For some Trustworthiness Claims, there could be value in identifying
   a specific Appraisal Policy for Attestation Results applied within
   the Attester.  One way this could be done would be a URI which
   identifies the policy used at Verifier A, and this URI would
   reference a specific Trustworthiness Claim.  As the URI also could
   encode the version of the software, it might also act as a mechanism
   to signal the Relying Party to refresh/re-evaluate its view of
   Verifier A.  Do we need this type of structure to be included here?
   Should it be in subsequent documents?

   Expand the variant of Figure 1 which requires no Relying Party PoF
   into its own picture.

   In what document (if any) do we attempt normalization of the identity
   claims between different types of TEE.  E.g., does MRSIGNER plus
   extra loaded software = the sum of TrustZone Signer IDs for loaded
   components?

Appendix D.  Contributors

   Guy Fedorkow

   Email: gfedorkow@juniper.net

   Dave Thaler

   Email: dthaler@microsoft.com

   Ned Smith

   Email: ned.smith@intel.com

   Lawrence Lundblade




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   Email: lgl@island-resort.com

Authors' Addresses

   Eric Voit
   Cisco Systems
   Email: evoit@cisco.com


   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany
   Email: henk.birkholz@sit.fraunhofer.de


   Thomas Hardjono
   MIT
   Email: hardjono@mit.edu


   Thomas Fossati
   Arm Limited
   Email: Thomas.Fossati@arm.com


   Vincent Scarlata
   Intel
   Email: vincent.r.scarlata@intel.com





















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