RATS Working Group                                           H. Birkholz
Internet-Draft                                                  M. Eckel
Intended status: Informational                            Fraunhofer SIT
Expires: 28 October 2021                                          W. Pan
                                                     Huawei Technologies
                                                                 E. Voit
                                                           26 April 2021

     Reference Interaction Models for Remote Attestation Procedures


   This document describes interaction models for remote attestation
   procedures (RATS).  Three conveying mechanisms -- Challenge/Response,
   Uni-Directional, and Streaming Remote Attestation -- are illustrated
   and defined.  Analogously, a general overview about the information
   elements typically used by corresponding conveyance protocols are

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 28 October 2021.

Copyright Notice

   Copyright (c) 2021 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

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Disambiguation  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope and Intent  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Essential Requirements  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Endorsement of Attesting Environments . . . . . . . . . .   5
   5.  Normative Prerequisites . . . . . . . . . . . . . . . . . . .   6
   6.  Generic Information Elements  . . . . . . . . . . . . . . . .   7
   7.  Interaction Models  . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Challenge/Response Remote Attestation . . . . . . . . . .   9
     7.2.  Uni-Directional Remote Attestation  . . . . . . . . . . .  11
     7.3.  Streaming Remote Attestation  . . . . . . . . . . . . . .  13
   8.  Additional Application-Specific Requirements  . . . . . . . .  15
     8.1.  Confidentiality . . . . . . . . . . . . . . . . . . . . .  15
     8.2.  Mutual Authentication . . . . . . . . . . . . . . . . . .  15
     8.3.  Hardware-Enforcement/Support  . . . . . . . . . . . . . .  15
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Implementer . . . . . . . . . . . . . . . . . . . . . . .  16
     9.2.  Implementation Name . . . . . . . . . . . . . . . . . . .  16
     9.3.  Implementation URL  . . . . . . . . . . . . . . . . . . .  16
     9.4.  Maturity  . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.5.  Coverage and Version Compatibility  . . . . . . . . . . .  16
     9.6.  License . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.7.  Implementation Dependencies . . . . . . . . . . . . . . .  17
     9.8.  Contact . . . . . . . . . . . . . . . . . . . . . . . . .  17
   10. Security and Privacy Considerations . . . . . . . . . . . . .  17
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  CDDL Specification for a simple CoAP Challenge/
           Response Interaction  . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   Remote ATtestation procedureS (RATS, [I-D.ietf-rats-architecture])
   are workflows composed of roles and interactions, in which Verifiers
   create Attestation Results about the trustworthiness of an Attester's
   system component characteristics.  The Verifier's assessment in the
   form of Attestation Results is created based on Attestation Policies
   and Evidence -- trustable and tamper-evident Claims Sets about an
   Attester's system component characteristics -- generated by an
   Attester.  The roles _Attester_ and _Verifier_, as well as the
   Conceptual Messages _Evidence_ and _Attestation Results_ are concepts
   defined by the RATS Architecture [I-D.ietf-rats-architecture].  This
   document defines interaction models that can be used in specific
   RATS-related solution documents.  The primary focus of this document
   is the conveyance of attestation Evidence.  The reference models
   defined can also be applied to the conveyance of other Conceptual
   Messages in RATS.  Specific goals of this document are to:

   1.) prevent inconsistencies in descriptions of interaction models in
   other documents (due to text cloning and evolution over time), and to
   2.) enable to highlight an exact delta/divergence between the core
   set of characteristics captured here in this document and variants of
   these interaction models used in other specifications or solutions.

   In summary, this document enables the specification and design of
   trustworthy and privacy preserving conveyance methods for attestation
   Evidence from an Attester to a Verifier.  While the conveyance of
   other Conceptual Messages is out-of-scope the methods described can
   also be applied to the conveyance of, for example, Endorsements or
   Attestation Results.

2.  Terminology

   This document uses the following set of terms, roles, and concepts as
   defined in [I-D.ietf-rats-architecture]: Attester, Verifier, Relying
   Party, Conceptual Message, Evidence, Endorsement, Attestation Result,
   Appraisal Policy, Attesting Environment, Target Environment

   A PKIX Certificate is an X.509v3 format certificate as specified by

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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2.1.  Disambiguation

   The term "Remote Attestation" is a common expression and often
   associated or connoted with certain properties.  The term "Remote" in
   this context does not necessarily refer to a remote entity in the
   scope of network topologies or the Internet.  It rather refers to
   decoupled systems or entities that exchange the payload of the
   Conceptual Message type called Evidence [I-D.ietf-rats-architecture].
   This conveyance can also be "Local", if the Verifier role is part of
   the same entity as the Attester role, e.g., separate system
   components of the same Composite Device (a single RATS entity).  Even
   if an entity takes on two or more different roles, the functions they
   provide typically reside in isolated environments that are components
   of the same entity.  Examples of such isolated environments include:
   a Trusted Execution Environment (TEE), Baseboard Management
   Controllers (BMCs), as well as other physical or logical
   protected/isolated/shielded Computing Environments (e.g. embedded
   Secure Elements (eSE) or Trusted Platform Modules (TPM)).  Readers of
   this document should be familiar with the concept of Layered
   Attestation as described in Section 3.1 Two Types of Environments of
   an Attester in [I-D.ietf-rats-architecture] and the definition of
   Attestation as described in

3.  Scope and Intent

   This document focuses on generic interaction models between Attesters
   and Verifiers in order to convey Evidence.  Complementary procedures,
   functions, or services that are required for a complete semantic
   binding of the concepts defined in [I-D.ietf-rats-architecture] are
   out-of-scope of this document.  Examples include: identity
   establishment, key distribution and enrollment, time synchronization,
   as well as certificate revocation.

   Furthermore, any processes and duties that go beyond carrying out
   remote attestation procedures are out-of-scope.

   For instance, using the results of a remote attestation procedure
   that are created by the Verifier, e.g., how to triggering remediation
   actions or recovery processes, as well as such remediation actions
   and recovery processes themselves, are also out-of-scope.

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   The interaction models illustrated in this document are intended to
   provide a stable basis and reference for other solutions documents
   inside or outside the IETF.  Solution documents of any kind can
   reference the interaction models in order to avoid text clones and to
   avoid the danger of subtle discrepancies.  Analogously, deviations
   from the generic model descriptions in this document can be
   illustrated in solutions documents to highlight distinct

4.  Essential Requirements

   In order to ensure appropriate conveyance of Evidence, there exist
   essential requirements which MUST be fulfilled:

   Integrity:  Information provided by an Attester MUST be integral.
      This may be achieved by means of a digital signature over
      Attestation Evidence.  The signature may be symmetric, such as an
      HMAC, or asymmetric, such as ECDSA.

   Authentication:  The information provided by the Attester MUST be
      authentic.  For that purpose, the Attester should authenticate
      itself to the Verifier.  This may be an implicit authentication by
      means of a digital signature over the Attestation Evidence, which
      does not require additional protocol steps, or may be achieved by
      using a confidential channel by means of encryption.

4.1.  Endorsement of Attesting Environments

   Via its Attesting Environments, an Attester only generates Evidence
   about its Target Environments.  After being appraised to be
   trustworthy, a Target Environment may become a new Attesting
   Environment in charge of generating Evidence for further Target
   Environments.  [I-D.ietf-rats-architecture] explains this as Layered
   Attestation.  Layered Attestation has to start with an initial
   Attesting Environment.  In essence, there cannot be turtles all the
   way down [turtles].  At this rock bottom of Layered Attestation, the
   Attesting Environments are always called Roots of Trust (RoT).  An
   Attester cannot generate Evidence about its own RoTs by design.  As a
   consequence, a Verifier requires trustable statements about this
   subset of Attesting Environments from a different source than the
   Attester itself.  The corresponding trustable statements are called
   Endorsements and originate from external, trustable entities that
   take on the role of an Endorser (e.g., supply chain entities).

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5.  Normative Prerequisites

   In order to ensure an appropriate conveyance of Evidence via
   interaction models in general, the following set of prerequisites
   MUST be in place to support the implementation of interaction models:

   Attester Identity:  A statement about a distinguishable Attester made
      by an Endorser without accompanying evidence about its validity,
      used as proof of identity.

      The provenance of Evidence with respect to a distinguishable
      Attesting Environment MUST be correct and unambiguous.

      An Attester Identity MAY be a unique identity, MAY be included in
      a zero-knowledge proof (ZKP), MAY be part of a group signature, or
      it MAY be a randomized DAA credential [DAA].

   Attestation Evidence Authenticity:  Attestation Evidence MUST be

      In order to provide proofs of authenticity, Attestation Evidence
      SHOULD be cryptographically associated with an identity document
      (e.g. an PKIX certificate or trusted key material, or a randomized
      DAA credential [DAA]), or SHOULD include a correct and unambiguous
      and stable reference to an accessible identity document.

   Authentication Secret:  An Authentication Secret MUST be available
      exclusively to an Attester's Attesting Environment.

      The Attester MUST protect Claims with that Authentication Secret,
      thereby proving the authenticity of the Claims included in
      Evidence.  The Authentication Secret MUST be established before
      RATS can take place.

   Evidence Freshness:  Evidence MUST include an indicator about its
      freshness that can be understood by a Verifier.  Analogously,
      interaction models MUST support the conveyance of proofs of
      freshness in a way that is useful to Verifiers and their appraisal

   Evidence Protection:  Evidence MUST be a set of well-formatted and
      well-protected Claims that an Attester can create and convey to a
      Verifier in a tamper-evident manner.

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6.  Generic Information Elements

   This section defines the information elements that are vital to all
   kinds interaction models.  Varying from solution to solution, generic
   information elements can be either included in the scope of protocol
   messages (instantiating Conceptual Messages) or can be included in
   additional protocol parameters or payload.  Ultimately, the following
   information elements are required by any kind of scalable remote
   attestation procedure using one or more of the interaction models

   Attester Identity ('attesterIdentity'):  _mandatory_

      A statement about a distinguishable Attester made by an Endorser
      without accompanying evidence about its validity - used as proof
      of identity.

   Authentication Secret IDs ('authSecIDs'):  _mandatory_

      A statement representing an identifier list that MUST be
      associated with corresponding Authentication Secrets used to
      protect Claims included in Evidence.

      Each Authentication Secret is uniquely associated with a
      distinguishable Attesting Environment.  Consequently, an
      Authentication Secret ID also identifies an Attesting Environment.

   Handle ('handle'):  _mandatory_

      A statement that is intended to uniquely distinguish received
      Evidence and/or determine the freshness of Evidence.

      A Verifier can also use a Handle as an indicator for authenticity
      or attestation provenance, as only Attesters and Verifiers that
      are intended to exchange Evidence should have knowledge of the
      corresponding Handles.  Examples include Nonces or signed

   Claims ('claims'):  _mandatory_

      Claims are assertions that represent characteristics of an
      Attester's Target Environment.

      Claims are part of a Conceptual Message and are, for example, used
      to appraise the integrity of Attesters via Verifiers.  The other
      information elements in this section can be expressed as Claims in
      any type of Conceptional Messages.

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   Event Logs ('eventLogs'):  _optional_

      Event Logs accompany Claims by providing event trails of security-
      critical events in a system.  The primary purpose of Event Logs is
      to support Claim reproducibility by providing information on how
      Claims originated.

   Reference Values ('refValues')  _mandatory_

      Reference Values as defined in [I-D.ietf-rats-architecture].  This
      specific type of Claims is used to appraise Claims incorporated in
      Evidence.  For example, Reference Values MAY be Reference
      Integrity Measurements (RIM) or assertions that are implicitly
      trusted because they are signed by a trusted authority (see
      Endorsements in [I-D.ietf-rats-architecture]).  Reference Values
      typically represent (trusted) Claim sets about an Attester's
      intended platform operational state.

   Claim Selection ('claimSelection'):  _optional_

      A (sub-)set of Claims which can be created by an Attester.

      Claim Selections act as filters to specify the exact set of Claims
      to be included in Evidence.  In a remote attestation process, a
      Verifier sends a Claim Selection, among other elements, to an
      Attester.  An Attester MAY decide whether or not to provide all
      requested Claims from a Claim Selection to the Verifier.

   Collected Claims ('collectedClaims'):  _mandatory_

      Collected Claims represent a (sub-)set of Claims created by an

      Collected Claims are gathered based on the Claims selected in the
      Claim Selection.  If a Verifier does not provide a Claim
      Selection, then all available Claims on the Attester are part of
      the Collected Claims.

   Evidence ('evidence'):  _mandatory_

      A set of Claims that consists of a list of Authentication Secret
      IDs that each identifies an Authentication Secret in a single
      Attesting Environment, the Attester Identity, Claims, and a
      Handle.  Attestation Evidence MUST cryptographically bind all of
      these information elements.  Evidence MUST be protected via an
      Authentication Secret.  The Authentication Secret MUST be trusted
      by the Verifier as authoritative.

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   Attestation Result ('attestationResult'):  _mandatory_

      An Attestation Result is produced by the Verifier as the output of
      the appraisal of Evidence.  Attestation Results include condensed
      assertions about integrity or other characteristics of the
      corresponding Attester that are processible by Relying Parties.

7.  Interaction Models

   The following subsections introduce and illustrate the interaction

   1.  Challenge/Response Remote Attestation

   2.  Uni-Directional Remote Attestation

   3.  Streaming Remote Attestation

   Each section starts with a sequence diagram illustrating the
   interactions between Attester and Verifier.  While the presented
   interaction models focus on the conveyance of Evidence, the intention
   of this document is in support of future work that applies the
   presented models to the conveyance of other Conceptual Messages,
   namely Attestation Results, Endorsements, Reference Values, or
   Appraisal Policies.

   All interaction models have a strong focus on the use of a handle to
   incorporate a type of proof of freshness and to prevent replay
   attacks.  The way these handles are processed is the most prominent
   difference between the three interaction models.

7.1.  Challenge/Response Remote Attestation

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.----------.                                                .----------.
| Attester |                                                | Verifier |
'----------'                                                '----------'
     |                                                            |
  generateClaims(attestingEnvironment)                            |
     | => claims, eventLogs                                       |
     |                                                            |
     | <-- requestAttestation(handle, authSecIDs, claimSelection) |
     |                                                            |
  collectClaims(claims, claimSelection)                           |
     | => collectedClaims                                         |
     |                                                            |
  generateEvidence(handle, authSecIDs, collectedClaims)           |
     | => evidence                                                |
     |                                                            |
     | evidence, eventLogs -------------------------------------> |
     |                                                            |
     |                appraiseEvidence(evidence, eventLogs, refValues)
     |                                       attestationResult <= |
     |                                                            |

   The Attester boots up and thereby produces claims about its boot
   state and its operational state.  Event Logs accompany the produced
   claims by providing an event trail of security-critical events in a
   system.  Claims are produced by all attesting Environments of an
   Attester system.

   The Challenge/Response remote attestation procedure is initiated by
   the Verifier by sending a remote attestation request to the Attester.
   A request includes a Handle, a list of Authentication Secret IDs, and
   a Claim Selection.

   In the Challenge/Response model, the handle is composed of qualifying
   data in the form of a practically infeasible to guess nonce, such as
   a cryptographically strong random number.  The Verifier-generated
   nonce is intended to guarantee Evidence freshness and to prevent
   replay attacks.

   The list of Authentication Secret IDs selects the attestation keys
   with which the Attester is requested to sign the Attestation
   Evidence.  Each selected key is uniquely associated with an Attesting
   Environment of the Attester.  As a result, a single Authentication
   Secret ID identifies a single Attesting Environment.
   Correspondingly, a particular set of Evidence originating from a
   particular Attesting Environment in a composite device can be
   requested via multiple Authentication Secret IDs.  Methods to acquire
   Authentication Secret IDs or mappings between Attesting Environments
   to Authentication Secret IDs are out-of-scope of this document.

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   The Attester collects Claims based on the Claim Selection.  With the
   Claim Selection the Verifier defines the set of Claims it requires.
   Correspondingly, collected Claims can be a subset of the produced
   Claims.  This could be all available Claims, depending on the Claim
   Selection.  If the Claim Selection is omitted, then by default all
   Claims that are known and available on the Attester MUST be used to
   create corresponding Evidence.  For example, when performing a boot
   integrity evaluation, a Verifier may only be requesting a particular
   subset of claims about the Attester, such as Evidence about BIOS/UEFI
   and firmware that the Attester booted up, and not include information
   about all currently running software.

   With the Handle, the Authentication Secret IDs, and the collected
   Claims, the Attester produces signed Evidence.  That is, it digitally
   signs the Handle and the collected Claims with a cryptographic secret
   identified by the Authentication Secret ID.  This is done once per
   Attesting Environment which is identified by the particular
   Authentication Secret ID.  The Attester communicates the signed
   Evidence as well as all accompanying Event Logs back to the Verifier.

   While it is crucial that Claims, the Handle, and the Attester
   Identity information MUST be cryptographically bound to the signature
   of Evidence, they MAY be presented obfuscated, encrypted, or
   cryptographically blinded.  For further reference see section
   Section 10.

   As soon as the Verifier receives the signed Evidence and Event Logs,
   it appraises the Evidence.  For this purpose, it validates the
   signature, the Attester Identity, and the Handle, and then appraises
   the Claims.  Appraisal procedures are application-specific and can be
   conducted via comparison of the Claims with corresponding Reference
   Values, such as Reference Integrity Measurements.  The final output
   of the Verifier are Attestation Results.  Attestation Results
   constitute new Claim Sets about the properties and characteristics of
   an Attester, which enables Relying Parties, for example, to assess an
   Attester's trustworthiness.

7.2.  Uni-Directional Remote Attestation

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.----------.                       .--------------------.   .----------.
| Attester |                       | Handle Distributor |   | Verifier |
'----------'                       '--------------------'   '----------'
     |                                       |                    |
     |                                    generateHandle()        |
     |                                       | => handle          |
     |                                       |                    |
     | <----------------------------- handle | handle ----------> |
     |                                       |                    |
  generateClaims(attestingEnvironment)       x                    |
     | => claims, eventLogs                                       |
     |                                                            |
  collectClaims(claims, claimSelection)                           |
     | => collectedClaims                                         |
     |                                                            |
  generateEvidence(handle, authSecIDs, collectedClaims)           |
     | => evidence                                                |
     |                                                            |
     | evidence, eventLogs -------------------------------------> |
     |                                                            |
     |                appraiseEvidence(evidence, eventLogs, refValues)
     |                                       attestationResult <= |
     ~                                                            ~
     |                                                            |
*    |                                                            |    *
* generateClaims(attestingEnvironment)                            |    *
*    | => claimsDelta, eventLogsDelta                             |    *
*    |                                                            |    *
* collectClaims(claimsDelta, claimSelection)                      |    *
*    | => collectedClaimsDelta                                    |    *
*    |                                                            |    *
* generateEvidence(handle, authSecIDs, collectedClaimsDelta)      |    *
*    | => evidence                                                |    *
*    |                                                            |    *
*    | evidence, eventLogsDelta --------------------------------> |    *
*    |                                                            |    *
*    |           appraiseEvidence(evidence, eventLogsDelta, refValues) *
*    |                                       attestationResult <= |    *
*    |                                                            |    *
     |                                                            |

   Uni-Directional Remote Attestation procedures can be initiated both
   by the Attester and by the Verifier.  Initiation by the Attester can
   result in unsolicited pushes of Evidence to the Verifier.  Initiation
   by the Verifier always results in solicited pushes to the Verifier.

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   The Uni-Directional model uses the same information elements as the
   Challenge/Response model.  In the sequence diagram above, the
   Attester initiates the conveyance of Evidence (comparable with a
   RESTful POST operation or the emission of a beacon).  While a request
   of Evidence from the Verifier would result in a sequence diagram more
   similar to the Challenge/Response model (comparable with a RESTful
   GET operation).  The specific manner how Handles are created and used
   always remains as the distinguishing quality of this model.

   In the Uni-Directional model, handles are composed of
   cryptographically signed trusted timestamps as shown in
   [I-D.birkholz-rats-tuda], potentially including other qualifying
   data.  The Handles are created by an external 3rd entity -- the
   Handle Distributor -- which includes a trustworthy source of time,
   and takes on the role of a Time Stamping Authority (TSA, as initially
   defined in [RFC3161]).  Timestamps created from local clocks
   (absolute clocks using a global timescale, as well as relative
   clocks, such as tick-counters) of Attesters and Verifiers MUST be
   cryptographically bound to fresh Handles received from the Handle
   Distributor.  This binding provides a proof of synchronization that
   MUST be included in all produced Evidence.  Correspondingly, conveyed
   Evidence in this model provides a proof that it was fresh at a
   certain point in time.

   While periodically pushing Evidence to the Verifier, the Attester
   only needs to generate and convey evidence generated from Claim
   values that have changed and new Event Logs entries since the
   previous conveyance.  This updates reflecting the differences are
   called "delta" in the sequence diagram above.

   Effectively, the Uni-Directional model allows for a series of
   Evidence to be pushed to multiple Verifiers simultaneously.  Methods
   to detect excessive time drift that would mandate a fresh Handle to
   be received by the Handle Distributor as well as timing of Handle
   distribution are out-of-scope of this document.

7.3.  Streaming Remote Attestation

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.----------.                                                .----------.
| Attester |                                                | Verifier |
'----------'                                                '----------'
     |                                                            |
  generateClaims(attestingEnvironment)                            |
     | => claims, eventLogs                                       |
     |                                                            |
     | <----------- subscribe(handle, authSecIDs, claimSelection) |
     | subscriptionResult --------------------------------------> |
     |                                                            |
  collectClaims(claims, claimSelection)                           |
     | => collectedClaims                                         |
     |                                                            |
  generateEvidence(handle, authSecIDs, collectedClaims)           |
     | => evidence                                                |
     |                                                            |
     | evidence, eventLogs -------------------------------------> |
     |                                                            |
     |                appraiseEvidence(evidence, eventLogs, refValues)
     |                                       attestationResult <= |
     ~                                                            ~
     |                                                            |
*    |                                                            |    *
* generateClaims(attestingEnvironment)                            |    *
*    | => claimsDelta, eventLogsDelta                             |    *
*    |                                                            |    *
* collectClaims(claimsDelta, claimSelection)                      |    *
*    | => collectedClaimsDelta                                    |    *
*    |                                                            |    *
* generateEvidence(handle, authSecIDs, collectedClaimsDelta)      |    *
*    | => evidence                                                |    *
*    |                                                            |    *
*    | evidence, eventLogsDelta --------------------------------> |    *
*    |                                                            |    *
*    |           appraiseEvidence(evidence, eventLogsDelta, refValues) *
*    |                                       attestationResult <= |    *
*    |                                                            |    *
     |                                                            |

   Streaming Remote Attestation procedures require the setup of
   subscription state.  Setting up subscription state between a Verifier
   and an Attester is conducted via a subscribe operation.  The
   subscribe operation is used to convey required Handles for producing
   Evidence.  Effectively, this allows for a series of Evidence to be
   pushed to a Verifier, similar to the Uni-Directional model.  While a
   Handle Distributor is not required in this model, it is also limited

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   to bi-lateral subscription relationships in which each Verifier has
   to create and provide its individual Handle.  Handles provided by a
   specific subscribing Verifier MUST be used in Evidence generation for
   that specific Verifier.  The Streaming model uses the same
   information elements as the Challenge/Response and the Uni-
   Directional model.  Methods to detect excessive time drift that would
   mandate a refreshed Handle to be conveyed via another subscribe
   operation are out-of-scope of this document.

8.  Additional Application-Specific Requirements

   Depending on the use cases covered, there can be additional
   requirements.  An exemplary subset is illustrated in this section.

8.1.  Confidentiality

   Confidentiality of exchanged attestation information may be
   desirable.  This requirement usually is present when communication
   takes place over insecure channels, such as the public Internet.  In
   such cases, TLS may be used as a suitable communication protocol
   which provides confidentiality protection.  In private networks, such
   as carrier management networks, it must be evaluated whether or not
   the transport medium is considered confidential.

8.2.  Mutual Authentication

   In particular use cases, mutual authentication may be desirable in
   such a way that a Verifier also needs to prove its identity to the
   Attester, instead of only the Attester proving its identity to the

8.3.  Hardware-Enforcement/Support

   Depending on given usage scenarios, hardware support for secure
   storage of cryptographic keys, crypto accelerators, as well as
   protected or isolated execution environments can be mandatory
   requirements.  Well-known technologies in support of these
   requirements are roots of trusts, such as Hardware Security Modules
   (HSM), Physically Unclonable Functions (PUFs), Shielded Secrets, or
   Trusted Executions Environments (TEEs).

9.  Implementation Status

   Note to RFC Editor: Please remove this section as well as references
   to [BCP205] before AUTH48.

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   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [BCP205].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [BCP205], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

9.1.  Implementer

   The open-source implementation was initiated and is maintained by the
   Fraunhofer Institute for Secure Information Technology - SIT.

9.2.  Implementation Name

   The open-source implementation is named "CHAllenge-Response based
   Remote Attestation" or in short: CHARRA.

9.3.  Implementation URL

   The open-source implementation project resource can be located via:

9.4.  Maturity

   The code's level of maturity is considered to be "prototype".

9.5.  Coverage and Version Compatibility

   The current version ('1bcb469') implements a challenge/response
   interaction model and is aligned with the exemplary specification of
   the CoAP FETCH bodies defined in Section Appendix A of this document.

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9.6.  License

   The CHARRA project and all corresponding code and data maintained on
   GitHub are provided under the BSD 3-Clause "New" or "Revised"

9.7.  Implementation Dependencies

   The implementation requires the use of the official Trusted Computing
   Group (TCG) open-source Trusted Software Stack (TSS) for the Trusted
   Platform Module (TPM) 2.0.  The corresponding code and data is also
   maintained on GitHub and the project resources can be located via:

   The implementation uses the Constrained Application Protocol
   [RFC7252] (http://coap.technology/) and the Concise Binary Object
   Representation [RFC7049] (https://cbor.io/).

9.8.  Contact

   Michael Eckel (michael.eckel@sit.fraunhofer.de)

10.  Security and Privacy Considerations

   In a remote attestation procedure the Verifier or the Attester MAY
   want to cryptographically blind several attributes.  For instance,
   information can be part of the signature after applying a one-way
   function (e. g. a hash function).

   There is also a possibility to scramble the Nonce or Attester
   Identity with other information that is known to both the Verifier
   and Attester.  A prominent example is the IP address of the Attester
   that usually is known by the Attester itself as well as the Verifier.
   This extra information can be used to scramble the Nonce in order to
   counter certain types of relay attacks.

11.  Acknowledgments

   Olaf Bergmann, Michael Richardson, and Ned Smith

12.  References

12.1.  Normative References

   [BCP205]   Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3161]  Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
              2001, <https://www.rfc-editor.org/info/rfc3161>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [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>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [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>.

   [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>.

12.2.  Informative References

   [DAA]      Brickell, E., Camenisch, J., and L. Chen, "Direct
              Anonymous Attestation", page 132-145, ACM Proceedings of
              the 11rd ACM conference on Computer and Communications
              Security, 2004.

              Fuchs, A., Birkholz, H., McDonald, I. E., and C. Bormann,
              "Time-Based Uni-Directional Attestation", Work in
              Progress, Internet-Draft, draft-birkholz-rats-tuda-04, 13
              January 2021, <https://www.ietf.org/archive/id/draft-

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              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              12, 23 April 2021, <https://www.ietf.org/archive/id/draft-

              Fedorkow, G., Voit, E., and J. Fitzgerald-McKay, "TPM-
              based Network Device Remote Integrity Verification", Work
              in Progress, Internet-Draft, draft-ietf-rats-tpm-based-
              network-device-attest-06, 7 December 2020,

   [turtles]  Rudnicki, R., "Turtles All the Way Down: Foundation,
              Edifice, and Ruin in Faulkner and McCarthy",
              DOI 10.1353/fau.2010.0002, The Faulkner Journal 25.2,
              2010, <https://doi.org/10.1353/fau.2010.0002>.

Appendix A.  CDDL Specification for a simple CoAP Challenge/Response

   The following CDDL specification is an exemplary proof-of-concept to
   illustrate a potential implementation of the Challenge/Response
   Interaction Model.  The transfer protocol used is CoAP using the
   FETCH operation.  The actual resource operated on can be empty.  Both
   the Challenge Message and the Response Message are exchanged via the
   FETCH operation and corresponding FETCH Request and FETCH Response

   In this example, evidence is created via the root-of-trust for
   reporting primitive operation "quote" that is provided by a TPM 2.0.

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   RAIM-Bodies = CoAP-FETCH-Body / CoAP-FETCH-Response-Body

   CoAP-FETCH-Body = [ hello: bool, ; if true, the AK-Cert is conveyed
                       nonce: bytes,
                       pcr-selection: [ + [ tcg-hash-alg-id: uint .size 2, ; TPM2_ALG_ID
                                            [ + pcr: uint .size 1 ],

   CoAP-FETCH-Response-Body = [ attestation-evidence: TPMS_ATTEST-quote,
                                tpm-native-signature: bytes,
                                ? ak-cert: bytes, ; attestation key certificate

   TPMS_ATTEST-quote = [ qualifiediSigner: uint .size 2, ;TPM2B_NAME
                         firmwareVersion: uint .size 8
                         quote-responses: [ * [ pcr: uint .size 1,
                                                + [ pcr-value: bytes,
                                                    ? hash-alg-id: uint .size 2,
                                            ? pcr-digest: bytes,

   TPMS_CLOCK_INFO = [ clock: uint .size 8,
                       resetCounter: uint .size 4,
                       restartCounter: uint .size 4,
                       save: bool,

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt

   Email: henk.birkholz@sit.fraunhofer.de

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   Michael Eckel
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt

   Email: michael.eckel@sit.fraunhofer.de

   Wei Pan
   Huawei Technologies

   Email: william.panwei@huawei.com

   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com

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