RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 D. Thaler
Expires: 25 October 2021                                       Microsoft
                                                           M. Richardson
                                                Sandelman Software Works
                                                                N. Smith
                                                                   Intel
                                                                  W. Pan
                                                     Huawei Technologies
                                                           23 April 2021


               Remote Attestation Procedures Architecture
                    draft-ietf-rats-architecture-12

Abstract

   In network protocol exchanges it is often useful for one end of a
   communication to know whether the other end is in an intended
   operating state.  This document provides an architectural overview of
   the entities involved that make such tests possible through the
   process of generating, conveying, and evaluating evidentiary claims.
   An attempt is made to provide for a model that is neutral toward
   processor architectures, the content of claims, and protocols.

Note to Readers

   Discussion of this document takes place on the RATS Working Group
   mailing list (rats@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/rats/
   (https://mailarchive.ietf.org/arch/browse/rats/).

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-rats-wg/architecture (https://github.com/
   ietf-rats-wg/architecture).

Status of This Memo

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

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





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

   This Internet-Draft will expire on 25 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Reference Use Cases . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Network Endpoint Assessment . . . . . . . . . . . . . . .   5
     2.2.  Confidential Machine Learning Model Protection  . . . . .   5
     2.3.  Confidential Data Protection  . . . . . . . . . . . . . .   6
     2.4.  Critical Infrastructure Control . . . . . . . . . . . . .   6
     2.5.  Trusted Execution Environment Provisioning  . . . . . . .   7
     2.6.  Hardware Watchdog . . . . . . . . . . . . . . . . . . . .   7
     2.7.  FIDO Biometric Authentication . . . . . . . . . . . . . .   7
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   8
     3.1.  Layered Attestation Environments  . . . . . . . . . . . .  12
     3.2.  Composite Device  . . . . . . . . . . . . . . . . . . . .  14
     3.3.  Implementation Considerations . . . . . . . . . . . . . .  16
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  Artifacts . . . . . . . . . . . . . . . . . . . . . . . .  18
   5.  Topological Patterns  . . . . . . . . . . . . . . . . . . . .  19
     5.1.  Passport Model  . . . . . . . . . . . . . . . . . . . . .  20
     5.2.  Background-Check Model  . . . . . . . . . . . . . . . . .  21
     5.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Roles and Entities  . . . . . . . . . . . . . . . . . . . . .  23
   7.  Trust Model . . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.1.  Relying Party . . . . . . . . . . . . . . . . . . . . . .  24
     7.2.  Attester  . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.3.  Relying Party Owner . . . . . . . . . . . . . . . . . . .  25



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     7.4.  Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.5.  Endorser, Reference Value Provider, and Verifier Owner  .  27
   8.  Conceptual Messages . . . . . . . . . . . . . . . . . . . . .  28
     8.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .  28
     8.2.  Endorsements  . . . . . . . . . . . . . . . . . . . . . .  28
     8.3.  Reference Values  . . . . . . . . . . . . . . . . . . . .  29
     8.4.  Attestation Results . . . . . . . . . . . . . . . . . . .  29
     8.5.  Appraisal Policies  . . . . . . . . . . . . . . . . . . .  30
   9.  Claims Encoding Formats . . . . . . . . . . . . . . . . . . .  31
   10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.1.  Explicit Timekeeping using Synchronized Clocks . . . . .  33
     10.2.  Implicit Timekeeping using Nonces  . . . . . . . . . . .  33
     10.3.  Implicit Timekeeping using Epoch IDs . . . . . . . . . .  33
     10.4.  Discussion . . . . . . . . . . . . . . . . . . . . . . .  35
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  35
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  36
     12.1.  Attester and Attestation Key Protection  . . . . . . . .  36
       12.1.1.  On-Device Attester and Key Protection  . . . . . . .  37
       12.1.2.  Attestation Key Provisioning Processes . . . . . . .  37
     12.2.  Integrity Protection . . . . . . . . . . . . . . . . . .  39
     12.3.  Epoch ID-based Attestation . . . . . . . . . . . . . . .  39
     12.4.  Trust Anchor Protection  . . . . . . . . . . . . . . . .  40
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  41
   15. Notable Contributions . . . . . . . . . . . . . . . . . . . .  41
   16. Appendix A: Time Considerations . . . . . . . . . . . . . . .  41
     16.1.  Example 1: Timestamp-based Passport Model Example  . . .  43
     16.2.  Example 2: Nonce-based Passport Model Example  . . . . .  44
     16.3.  Example 3: Epoch ID-based Passport Model Example . . . .  46
     16.4.  Example 4: Timestamp-based Background-Check Model
            Example  . . . . . . . . . . . . . . . . . . . . . . . .  47
     16.5.  Example 5: Nonce-based Background-Check Model Example  .  48
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     17.2.  Informative References . . . . . . . . . . . . . . . . .  49
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

   The question of how one system can know that another system can be
   trusted has found new interest and relevance in a world where trusted
   computing elements are maturing in processor architectures.








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   Systems that have been attested and verified to be in a good state
   (for some value of "good") can improve overall system posture.
   Conversely, systems that cannot be attested and verified to be in a
   good state can be taken out of service, or otherwise flagged for
   repair.

   For example:

   *  A bank back-end system might refuse to transact with another
      system that is not known to be in a good state.

   *  A healthcare system might refuse to transmit electronic healthcare
      records to a system that is not known to be in a good state.

   In Remote Attestation Procedures (RATS), one peer (the "Attester")
   produces believable information about itself - Evidence - to enable a
   remote peer (the "Relying Party") to decide whether to consider that
   Attester a trustworthy peer or not.  RATS are facilitated by an
   additional vital party, the Verifier.

   The Verifier appraises Evidence via appraisal policies and creates
   the Attestation Results to support Relying Parties in their decision
   process.  This document defines a flexible architecture consisting of
   attestation roles and their interactions via conceptual messages.
   Additionally, this document defines a universal set of terms that can
   be mapped to various existing and emerging Remote Attestation
   Procedures.  Common topological patterns and the sequence of data
   flows associated with them, such as the "Passport Model" and the
   "Background-Check Model", are illustrated.  The purpose is to define
   useful terminology for remote attestation and enable readers to map
   their solution architecture to the canonical attestation architecture
   provided here.  Having a common terminology that provides well-
   understood meanings for common themes such as roles, device
   composition, topological patterns, and appraisal procedures is vital
   for semantic interoperability across solutions and platforms
   involving multiple vendors and providers.

   Amongst other things, this document is about trust and
   trustworthiness.  Trust is a choice one makes about another system.
   Trustworthiness is a quality about the other system that can be used
   in making one's decision to trust it or not.  This is subtle
   difference and being familiar with the difference is crucial for
   using this document.  Additionally, the concepts of freshness and
   trust relationships with respect to RATS are elaborated on to enable
   implementers to choose appropriate solutions to compose their Remote
   Attestation Procedures.





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2.  Reference Use Cases

   This section covers a number of representative and generic use cases
   for remote attestation, independent of specific solutions.  The
   purpose is to provide motivation for various aspects of the
   architecture presented in this document.  Many other use cases exist,
   and this document does not intend to have a complete list, only to
   illustrate a set of use cases that collectively cover all the
   functionality required in the architecture.

   Each use case includes a description followed by an additional
   summary of the Attester and Relying Party roles derived from the use
   case.

2.1.  Network Endpoint Assessment

   Network operators want a trustworthy report that includes identity
   and version information about the hardware and software on the
   machines attached to their network, for purposes such as inventory,
   audit, anomaly detection, record maintenance and/or trending reports
   (logging).  The network operator may also want a policy by which full
   access is only granted to devices that meet some definition of
   hygiene, and so wants to get Claims about such information and verify
   its validity.  Remote attestation is desired to prevent vulnerable or
   compromised devices from getting access to the network and
   potentially harming others.

   Typically, solutions start with a specific component (called a root
   of trust) that is intended to provide trustworthy device identity and
   protected storage for measurements.  The system components perform a
   series of measurements that may be signed via functions provided by a
   root of trust, considered as Evidence about present system
   components, such as hardware, firmware, BIOS, software, etc.

   Attester:  A device desiring access to a network.

   Relying Party:  Network equipment such as a router, switch, or access
      point, responsible for admission of the device into the network.

2.2.  Confidential Machine Learning Model Protection

   A device manufacturer wants to protect its intellectual property.
   The intellectual property's scope primarily encompasses the machine
   learning (ML) model that is deployed in the devices purchased by its
   customers.  The protection goals include preventing attackers,
   potentially the customer themselves, from seeing the details of the
   model.




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   This typically works by having some protected environment in the
   device go through a remote attestation with some manufacturer service
   that can assess its trustworthiness.  If remote attestation succeeds,
   then the manufacturer service releases either the model, or a key to
   decrypt a model already deployed on the Attester in encrypted form,
   to the requester.

   Attester:  A device desiring to run an ML model.

   Relying Party:  A server or service holding ML models it desires to
      protect.

2.3.  Confidential Data Protection

   This is a generalization of the ML model use case above, where the
   data can be any highly confidential data, such as health data about
   customers, payroll data about employees, future business plans, etc.
   As part of the attestation procedure, an assessment is made against a
   set of policies to evaluate the state of the system that is
   requesting the confidential data.  Attestation is desired to prevent
   leaking data via compromised devices.

   Attester:  An entity desiring to retrieve confidential data.

   Relying Party:  An entity that holds confidential data for release to
      authorized entities.

2.4.  Critical Infrastructure Control

   Potentially harmful physical equipment (e.g., power grid, traffic
   control, hazardous chemical processing, etc.) is connected to a
   network in support of critical infrastructure.  The organization
   managing such infrastructure needs to ensure that only authorized
   code and users can control corresponding critical processes, and that
   these processes are protected from unauthorized manipulation or other
   threats.  When a protocol operation can affect a critical system
   component of the infrastructure, devices attached to that critical
   component require some assurances depending on the security context,
   including that: a requesting device or application has not been
   compromised, and the requesters and actors act on applicable
   policies.  As such, remote attestation can be used to only accept
   commands from requesters that are within policy.

   Attester:  A device or application wishing to control physical
      equipment.

   Relying Party:  A device or application connected to potentially




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      dangerous physical equipment (hazardous chemical processing,
      traffic control, power grid, etc.).

2.5.  Trusted Execution Environment Provisioning

   A Trusted Application Manager (TAM) server is responsible for
   managing the applications running in a Trusted Execution Environment
   (TEE) of a client device, as described in
   [I-D.ietf-teep-architecture].  To achieve its purpose, the TAM needs
   to assess the state of a TEE, or of applications in the TEE, of a
   client device.  The TEE conducts Remote Attestation Procedures with
   the TAM, which can then decide whether the TEE is already in
   compliance with the TAM's latest policy.  If not, the TAM has to
   uninstall, update, or install approved applications in the TEE to
   bring it back into compliance with the TAM's policy.

   Attester:  A device with a TEE capable of running trusted
      applications that can be updated.

   Relying Party:  A TAM.

2.6.  Hardware Watchdog

   There is a class of malware that holds a device hostage and does not
   allow it to reboot to prevent updates from being applied.  This can
   be a significant problem, because it allows a fleet of devices to be
   held hostage for ransom.

   A solution to this problem is a watchdog timer implemented in a
   protected environment such as a Trusted Platform Module (TPM), as
   described in [TCGarch] section 43.3.  If the watchdog does not
   receive regular, and fresh, Attestation Results as to the system's
   health, then it forces a reboot.

   Attester:  The device that should be protected from being held
      hostage for a long period of time.

   Relying Party:  A watchdog capable of triggering a procedure that
      resets a device into a known, good operational state.

2.7.  FIDO Biometric Authentication

   In the Fast IDentity Online (FIDO) protocol [WebAuthN], [CTAP], the
   device in the user's hand authenticates the human user, whether by
   biometrics (such as fingerprints), or by PIN and password.  FIDO
   authentication puts a large amount of trust in the device compared to
   typical password authentication because it is the device that
   verifies the biometric, PIN and password inputs from the user, not



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   the server.  For the Relying Party to know that the authentication is
   trustworthy, the Relying Party needs to know that the Authenticator
   part of the device is trustworthy.  The FIDO protocol employs remote
   attestation for this.

   The FIDO protocol supports several remote attestation protocols and a
   mechanism by which new ones can be registered and added.  Remote
   attestation defined by RATS is thus a candidate for use in the FIDO
   protocol.

   Other biometric authentication protocols such as the Chinese IFAA
   standard and WeChat Pay as well as Google Pay make use of remote
   attestation in one form or another.

   Attester:  Every FIDO Authenticator contains an Attester.

   Relying Party:  Any web site, mobile application back-end, or service
      that relies on authentication data based on biometric information.

3.  Architectural Overview

   Figure 1 depicts the data that flows between different roles,
   independent of protocol or use case.




























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    ************   *************    ************    *****************
    * Endorser *   * Reference *    * Verifier *    * Relying Party *
    ************   * Value     *    *  Owner   *    *  Owner        *
       |           * Provider  *    ************    *****************
       |           *************          |                 |
       |                  |               |                 |
       |Endorsements      |Reference      |Appraisal        |Appraisal
       |                  |Values         |Policy           |Policy for
       |                  |               |for              |Attestation
       .-----------.      |               |Evidence         |Results
                   |      |               |                 |
                   |      |               |                 |
                   v      v               v                 |
                 .---------------------------.              |
          .----->|          Verifier         |------.       |
          |      '---------------------------'      |       |
          |                                         |       |
          |                              Attestation|       |
          |                              Results    |       |
          | Evidence                                |       |
          |                                         |       |
          |                                         v       v
    .----------.                                .---------------.
    | Attester |                                | Relying Party |
    '----------'                                '---------------'

                      Figure 1: Conceptual Data Flow

   The text below summarizes the activities conducted by the roles
   illustrated in Figure 1.

   An Attester creates Evidence that is conveyed to a Verifier.

   A Verifier uses the Evidence, any Reference Values from Reference
   Value Providers, and any Endorsements from Endorsers, by applying an
   Appraisal Policy for Evidence to assess the trustworthiness of the
   Attester.  This procedure is called the appraisal of Evidence.

   Subsequently, the Verifier generates Attestation Results for use by
   Relying Parties.

   The Appraisal Policy for Evidence might be obtained from the Verifier
   Owner via some protocol mechanism, or might be configured into the
   Verifier by the Verifier Owner, or might be programmed into the
   Verifier, or might be obtained via some other mechanism.






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   A Relying Party uses Attestation Results by applying its own
   appraisal policy to make application-specific decisions, such as
   authorization decisions.  This procedure is called the appraisal of
   Attestation Results.

   The Appraisal Policy for Attestation Results might be obtained from
   the Relying Party Owner via some protocol mechanism, or might be
   configured into the Relying Party by the Relying Party Owner, or
   might be programmed into the Relying Party, or might be obtained via
   some other mechanism.

   See Section 8 for further discussion of the conceptual messages shown
   in Figure 1. ## Two Types of Environments of an Attester

   As shown in Figure 2, an Attester consists of at least one Attesting
   Environment and at least one Target Environment.  In some
   implementations, the Attesting and Target Environments might be
   combined.  Other implementations might have multiple Attesting and
   Target Environments, such as in the examples described in more detail
   in Section 3.1 and Section 3.2.  Other examples may exist.  All
   compositions of Attesting and Target Environments discussed in this
   architecture can be combined into more complex implementations.





























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                    .--------------------------------.
                    |                                |
                    |            Verifier            |
                    |                                |
                    '--------------------------------'
                                            ^
                                            |
                  .-------------------------|----------.
                  |                         |          |
                  |   .----------------.    |          |
                  |   | Target         |    |          |
                  |   | Environment    |    |          |
                  |   |                |    | Evidence |
                  |   '----------------'    |          |
                  |                   |     |          |
                  |                   |     |          |
                  |          Collect  |     |          |
                  |           Claims  |     |          |
                  |                   |     |          |
                  |                   v     |          |
                  |                 .-------------.    |
                  |                 | Attesting   |    |
                  |                 | Environment |    |
                  |                 |             |    |
                  |                 '-------------'    |
                  |               Attester             |
                  '------------------------------------'

                    Figure 2: Two Types of Environments

   Claims are collected from Target Environments.  That is, Attesting
   Environments collect the values and the information to be represented
   in Claims, by reading system registers and variables, calling into
   subsystems, taking measurements on code, memory, or other security
   related assets of the Target Environment.  Attesting Environments
   then format the Claims appropriately, and typically use key material
   and cryptographic functions, such as signing or cipher algorithms, to
   generate Evidence.  There is no limit to or requirement on the types
   of hardware or software environments that can be used to implement an
   Attesting Environment, for example: Trusted Execution Environments
   (TEEs), embedded Secure Elements (eSEs), Trusted Platform Modules
   (TPMs) [TCGarch], or BIOS firmware.

   An arbitrary execution environment may not, by default, be capable of
   Claims collection for a given Target Environment.  Execution
   environments that are designed specifically to be capable of Claims
   collection are referred to in this document as Attesting
   Environments.  For example, a TPM doesn't actively collect Claims



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   itself, it instead requires another component to feed various values
   to the TPM.  Thus, an Attesting Environment in such a case would be
   the combination of the TPM together with whatever component is
   feeding it the measurements.

3.1.  Layered Attestation Environments

   By definition, the Attester role generates Evidence.  An Attester may
   consist of one or more nested environments (layers).  The root layer
   of an Attester includes at least one root of trust.  In order to
   appraise Evidence generated by an Attester, the Verifier needs to
   trust the Attester's root of trust.  Trust in the Attester's root of
   trust can be established in various ways as discussed in Section 7.4.

   In layered attestation, a root of trust is the initial Attesting
   Environment.  Claims can be collected from or about each layer.  The
   corresponding Claims can be structured in a nested fashion that
   reflects the nesting of the Attester's layers.  Normally, Claims are
   not self-asserted, rather a previous layer acts as the Attesting
   Environment for the next layer.  Claims about a root of trust
   typically are asserted by an Endorser.

   The example device illustrated in Figure 3 includes (A) a BIOS stored
   in read-only memory, (B) a bootloader, and (C) an operating system
   kernel.


























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              .-------------.   Endorsement for ROM
              |  Endorser   |-----------------------.
              '-------------'                       |
                                                    v
              .-------------.   Reference      .----------.
              | Reference   |   Values for     |          |
              | Value       |----------------->| Verifier |
              | Provider(s) | ROM, bootloader, |          |
              '-------------'    and kernel    '----------'
                                                    ^
          .------------------------------------.    |
          |                                    |    |
          |   .---------------------------.    |    |
          |   | Kernel                    |    |    |
          |   |                           |    |    | Layered
          |   |   Target                  |    |    | Evidence
          |   | Environment               |    |    |   for
          |   '---------------------------'    |    | bootloader
          |           Collect |                |    |   and
          |           Claims  |                |    | kernel
          |   .---------------|-----------.    |    |
          |   | Bootloader    v           |    |    |
          |   |             .-----------. |    |    |
          |   |   Target    | Attesting | |    |    |
          |   | Environment |Environment|-----------'
          |   |             |           | |    |
          |   |             '-----------' |    |
          |   |                 ^         |    |
          |   '-----------------|---------'    |
          |          Collect |  | Evidence for |
          |          Claims  v  | bootloader   |
          |   .---------------------------.    |
          |   | ROM                       |    |
          |   |                           |    |
          |   |               Attesting   |    |
          |   |              Environment  |    |
          |   '---------------------------'    |
          |                                    |
          '------------------------------------'

                         Figure 3: Layered Attester










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   The first Attesting Environment, the read-only BIOS in this example,
   has to ensure the integrity of the bootloader (the first Target
   Environment).  There are potentially multiple kernels to boot, and
   the decision is up to the bootloader.  Only a bootloader with intact
   integrity will make an appropriate decision.  Therefore, the Claims
   relating to the integrity of the bootloader have to be measured
   securely.  At this stage of the boot-cycle of the device, the Claims
   collected typically cannot be composed into Evidence.

   After the boot sequence is started, the BIOS conducts the most
   important and defining feature of layered attestation, which is that
   the successfully measured bootloader now becomes (or contains) an
   Attesting Environment for the next layer.  This procedure in layered
   attestation is sometimes called "staging".  It is important that the
   bootloader not be able to alter any Claims about itself that were
   collected by the BIOS.  This can be ensured having those Claims be
   either signed by the BIOS or stored in a tamper-proof manner by the
   BIOS.

   Continuing with this example, the bootloader's Attesting Environment
   is now in charge of collecting Claims about the next Target
   Environment, which in this example is the kernel to be booted.  The
   final Evidence thus contains two sets of Claims: one set about the
   bootloader as measured and signed by the BIOS, plus a set of Claims
   about the kernel as measured and signed by the bootloader.

   This example could be extended further by making the kernel become
   another Attesting Environment for an application as another Target
   Environment.  This would result in a third set of Claims in the
   Evidence pertaining to that application.

   The essence of this example is a cascade of staged environments.
   Each environment has the responsibility of measuring the next
   environment before the next environment is started.  In general, the
   number of layers may vary by device or implementation, and an
   Attesting Environment might even have multiple Target Environments
   that it measures, rather than only one as shown by example in
   Figure 3.

3.2.  Composite Device

   A composite device is an entity composed of multiple sub-entities
   such that its trustworthiness has to be determined by the appraisal
   of all these sub-entities.

   Each sub-entity has at least one Attesting Environment collecting the
   Claims from at least one Target Environment, then this sub-entity
   generates Evidence about its trustworthiness.  Therefore, each sub-



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   entity can be called an Attester.  Among all the Attesters, there may
   be only some which have the ability to communicate with the Verifier
   while others do not.

   For example, a carrier-grade router consists of a chassis and
   multiple slots.  The trustworthiness of the router depends on all its
   slots' trustworthiness.  Each slot has an Attesting Environment, such
   as a TEE, collecting the Claims of its boot process, after which it
   generates Evidence from the Claims.

   Among these slots, only a "main" slot can communicate with the
   Verifier while other slots cannot.  But other slots can communicate
   with the main slot by the links between them inside the router.  So
   the main slot collects the Evidence of other slots, produces the
   final Evidence of the whole router and conveys the final Evidence to
   the Verifier.  Therefore the router is a composite device, each slot
   is an Attester, and the main slot is the lead Attester.

   Another example is a multi-chassis router composed of multiple single
   carrier-grade routers.  Multi-chassis router setups create redundancy
   groups that provide higher throughput by interconnecting multiple
   routers in these groups, which can be treated as one logical router
   for simpler management.  A multi-chassis router setup provides a
   management point that connects to the Verifier.  Typically one router
   in the group is designated as the main router.  Other routers in the
   multi-chassis setup are connected to the main router only via
   physical network links and are therefore managed and appraised via
   the main router's help.  In consequence, a multi-chassis router setup
   is a composite device, each router is an Attester, and the main
   router is the lead Attester.

   Figure 4 depicts the conceptual data flow for a composite device.



















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                      .-----------------------------.
                      |           Verifier          |
                      '-----------------------------'
                                      ^
                                      |
                                      | Evidence of
                                      | Composite Device
                                      |
   .----------------------------------|-------------------------------.
   | .--------------------------------|-----.      .------------.     |
   | |  Collect             .------------.  |      |            |     |
   | |  Claims   .--------->|  Attesting |<--------| Attester B |-.   |
   | |           |          |Environment |  |      '------------. |   |
   | |  .----------------.  |            |<----------| Attester C |-. |
   | |  |     Target     |  |            |  |        '------------' | |
   | |  | Environment(s) |  |            |<------------| ...        | |
   | |  |                |  '------------'  | Evidence '------------' |
   | |  '----------------'                  |    of                   |
   | |                                      | Attesters               |
   | | lead Attester A                      | (via Internal Links or  |
   | '--------------------------------------' Network Connections)    |
   |                                                                  |
   |                       Composite Device                           |
   '------------------------------------------------------------------'

                         Figure 4: Composite Device

   In a composite device, each Attester generates its own Evidence by
   its Attesting Environment(s) collecting the Claims from its Target
   Environment(s).  The lead Attester collects Evidence from other
   Attesters and conveys it to a Verifier.  Collection of Evidence from
   sub-entities may itself be a form of Claims collection that results
   in Evidence asserted by the lead Attester.  The lead Attester
   generates Evidence about the layout of the whole composite device,
   while sub-Attesters generate Evidence about their respective
   (sub-)modules.

   In this scenario, the trust model described in Section 7 can also be
   applied to an inside Verifier.

3.3.  Implementation Considerations

   An entity can take on multiple RATS roles (e.g., Attester, Verifier,
   Relying Party, etc.) at the same time.  Multiple entities can
   cooperate to implement a single RATS role as well.  In essence, the
   combination of roles and entities can be arbitrary.  For example, in
   the composite device scenario, the entity inside the lead Attester
   can also take on the role of a Verifier, and the outer entity of



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   Verifier can take on the role of a Relying Party.  After collecting
   the Evidence of other Attesters, this inside Verifier uses
   Endorsements and appraisal policies (obtained the same way as by any
   other Verifier) as part of the appraisal procedures that generate
   Attestation Results.  The inside Verifier then conveys the
   Attestation Results of other Attesters to the outside Verifier,
   whether in the same conveyance protocol as part of the Evidence or
   not.

4.  Terminology

   This document uses the following terms.

4.1.  Roles

   Attester:  A role performed by an entity (typically a device) whose
      Evidence must be appraised in order to infer the extent to which
      the Attester is considered trustworthy, such as when deciding
      whether it is authorized to perform some operation.

      Produces: Evidence

   Relying Party:  A role performed by an entity that depends on the
      validity of information about an Attester, for purposes of
      reliably applying application specific actions.  Compare /relying
      party/ in [RFC4949].

      Consumes: Attestation Results

   Verifier:  A role performed by an entity that appraises the validity
      of Evidence about an Attester and produces Attestation Results to
      be used by a Relying Party.

      Consumes: Evidence, Reference Values, Endorsements, Appraisal
      Policy for Evidence

      Produces: Attestation Results

   Relying Party Owner:  A role performed by an entity (typically an
      administrator), that is authorized to configure Appraisal Policy
      for Attestation Results in a Relying Party.

      Produces: Appraisal Policy for Attestation Results

   Verifier Owner:  A role performed by an entity (typically an
      administrator), that is authorized to configure Appraisal Policy
      for Evidence in a Verifier.




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      Produces: Appraisal Policy for Evidence

   Endorser:  A role performed by an entity (typically a manufacturer)
      whose Endorsements help Verifiers appraise the authenticity of
      Evidence.

      Produces: Endorsements

   Reference Value Provider:  A role performed by an entity (typically a
      manufacturer) whose Reference Values help Verifiers appraise
      Evidence to determine if acceptable known Claims have been
      recorded by the Attester.

      Produces: Reference Values

4.2.  Artifacts

   Claim:  A piece of asserted information, often in the form of a name/
      value pair.  Claims make up the usual structure of Evidence and
      other RATS artifacts.  Compare /claim/ in [RFC7519].

   Endorsement:  A secure statement that an Endorser vouches for the
      integrity of an Attester's various capabilities such as Claims
      collection and Evidence signing.

      Consumed By: Verifier

      Produced By: Endorser

   Evidence:  A set of Claims generated by an Attester to be appraised
      by a Verifier.  Evidence may include configuration data,
      measurements, telemetry, or inferences.

      Consumed By: Verifier

      Produced By: Attester

   Attestation Result:  The output generated by a Verifier, typically
      including information about an Attester, where the Verifier
      vouches for the validity of the results.

      Consumed By: Relying Party

      Produced By: Verifier

   Appraisal Policy for Evidence:  A set of rules that informs how a
      Verifier evaluates the validity of information about an Attester.
      Compare /security policy/ in [RFC4949].



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      Consumed By: Verifier

      Produced By: Verifier Owner

   Appraisal Policy for Attestation Results:  A set of rules that direct
      how a Relying Party uses the Attestation Results regarding an
      Attester generated by the Verifiers.  Compare /security policy/ in
      [RFC4949].

      Consumed by: Relying Party

      Produced by: Relying Party Owner

   Reference Values:  A set of values against which values of Claims can
      be compared as part of applying an Appraisal Policy for Evidence.
      Reference Values are sometimes referred to in other documents as
      known-good values, golden measurements, or nominal values,
      although those terms typically assume comparison for equality,
      whereas here Reference Values might be more general and be used in
      any sort of comparison.

      Consumed By: Verifier

      Produced By: Reference Value Provider

5.  Topological Patterns

   Figure 1 shows a data-flow diagram for communication between an
   Attester, a Verifier, and a Relying Party.  The Attester conveys its
   Evidence to the Verifier for appraisal, and the Relying Party
   receives the Attestation Result from the Verifier.  This section
   refines the data-flow diagram by describing two reference models, as
   well as one example composition thereof.  The discussion that follows
   is for illustrative purposes only and does not constrain the
   interactions between RATS roles to the presented patterns.
















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5.1.  Passport Model

   The passport model is so named because of its resemblance to how
   nations issue passports to their citizens.  The nature of the
   Evidence that an individual needs to provide to its local authority
   is specific to the country involved.  The citizen retains control of
   the resulting passport document and presents it to other entities
   when it needs to assert a citizenship or identity Claim, such as an
   airport immigration desk.  The passport is considered sufficient
   because it vouches for the citizenship and identity Claims, and it is
   issued by a trusted authority.  Thus, in this immigration desk
   analogy, the passport issuing agency is a Verifier, the passport is
   an Attestation Result, and the immigration desk is a Relying Party.

   In this model, an Attester conveys Evidence to a Verifier, which
   compares the Evidence against its appraisal policy.  The Verifier
   then gives back an Attestation Result.  If the Attestation Result was
   a successful one, the Attester can then present the Attestation
   Result (and possibly additional Claims) to a Relying Party, which
   then compares this information against its own appraisal policy.

   Three ways in which the process may fail include:

   *  First, the Verifier may not issue a positive Attestation Result
      due to the Evidence not passing the Appraisal Policy for Evidence.

   *  The second way in which the process may fail is when the
      Attestation Result is examined by the Relying Party, and based
      upon the Appraisal Policy for Attestation Results, the result does
      not pass the policy.

   *  The third way is when the Verifier is unreachable or unavailable.

   Since the resource access protocol between the Attester and Relying
   Party includes an Attestation Result, in this model the details of
   that protocol constrain the serialization format of the Attestation
   Result.  The format of the Evidence on the other hand is only
   constrained by the Attester-Verifier remote attestation protocol.
   This implies that interoperability and standardization is more
   relevant for Attestation Results than it is for Evidence.











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         +------------+
         |            | Compare Evidence
         |  Verifier  | against appraisal policy
         |            |
         +------------+
             ^    |
    Evidence |    | Attestation
             |    | Result
             |    v
         +------------+              +-------------+
         |            |------------->|             | Compare Attestation
         |  Attester  | Attestation  |   Relying   | Result against
         |            | Result       |    Party    | appraisal policy
         +------------+              +-------------+

                         Figure 5: Passport Model

5.2.  Background-Check Model

   The background-check model is so named because of the resemblance of
   how employers and volunteer organizations perform background checks.
   When a prospective employee provides Claims about education or
   previous experience, the employer will contact the respective
   institutions or former employers to validate the Claim.  Volunteer
   organizations often perform police background checks on volunteers in
   order to determine the volunteer's trustworthiness.  Thus, in this
   analogy, a prospective volunteer is an Attester, the organization is
   the Relying Party, and the organization that issues a report is a
   Verifier.

   In this model, an Attester conveys Evidence to a Relying Party, which
   simply passes it on to a Verifier.  The Verifier then compares the
   Evidence against its appraisal policy, and returns an Attestation
   Result to the Relying Party.  The Relying Party then compares the
   Attestation Result against its own appraisal policy.

   The resource access protocol between the Attester and Relying Party
   includes Evidence rather than an Attestation Result, but that
   Evidence is not processed by the Relying Party.  Since the Evidence
   is merely forwarded on to a trusted Verifier, any serialization
   format can be used for Evidence because the Relying Party does not
   need a parser for it.  The only requirement is that the Evidence can
   be _encapsulated in_ the format required by the resource access
   protocol between the Attester and Relying Party.

   However, like in the Passport model, an Attestation Result is still
   consumed by the Relying Party.  Code footprint and attack surface
   area can be minimized by using a serialization format for which the



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   Relying Party already needs a parser to support the protocol between
   the Attester and Relying Party, which may be an existing standard or
   widely deployed resource access protocol.  Such minimization is
   especially important if the Relying Party is a constrained node.

                                   +-------------+
                                   |             | Compare Evidence
                                   |   Verifier  | against appraisal
                                   |             | policy
                                   +-------------+
                                        ^   |
                               Evidence |   | Attestation
                                        |   | Result
                                        |   v
      +------------+               +-------------+
      |            |-------------->|             | Compare Attestation
      |  Attester  |   Evidence    |   Relying   | Result against
      |            |               |    Party    | appraisal policy
      +------------+               +-------------+

                      Figure 6: Background-Check Model

5.3.  Combinations

   One variation of the background-check model is where the Relying
   Party and the Verifier are on the same machine, performing both
   functions together.  In this case, there is no need for a protocol
   between the two.

   It is also worth pointing out that the choice of model depends on the
   use case, and that different Relying Parties may use different
   topological patterns.

   The same device may need to create Evidence for different Relying
   Parties and/or different use cases.  For instance, it would use one
   model to provide Evidence to a network infrastructure device to gain
   access to the network, and the other model to provide Evidence to a
   server holding confidential data to gain access to that data.  As
   such, both models may simultaneously be in use by the same device.

   Figure 7 shows another example of a combination where Relying Party 1
   uses the passport model, whereas Relying Party 2 uses an extension of
   the background-check model.  Specifically, in addition to the basic
   functionality shown in Figure 6, Relying Party 2 actually provides
   the Attestation Result back to the Attester, allowing the Attester to
   use it with other Relying Parties.  This is the model that the
   Trusted Application Manager plans to support in the TEEP architecture
   [I-D.ietf-teep-architecture].



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       +-------------+
       |             | Compare Evidence
       |   Verifier  | against appraisal policy
       |             |
       +-------------+
            ^   |
   Evidence |   | Attestation
            |   | Result
            |   v
       +-------------+
       |             | Compare
       |   Relying   | Attestation Result
       |   Party 2   | against appraisal policy
       +-------------+
            ^   |
   Evidence |   | Attestation
            |   | Result
            |   v
       +-------------+               +-------------+
       |             |-------------->|             | Compare Attestation
       |   Attester  |  Attestation  |   Relying   | Result against
       |             |     Result    |   Party 1   | appraisal policy
       +-------------+               +-------------+

                     Figure 7: Example Combination

6.  Roles and Entities

   An entity in the RATS architecture includes at least one of the roles
   defined in this document.

   An entity can aggregate more than one role into itself, such as being
   both a Verifier and a Relying Party, or being both a Reference Value
   Provider and an Endorser.  As such, any conceptual messages (see
   Section 8 for more discussion) originating from such roles might also
   be combined.  For example, Reference Values might be conveyed as part
   of an appraisal policy if the Verifier Owner and Reference Value
   Provider roles are combined.  Similarly, Reference Values might be
   conveyed as part of an Endorsement if the Endorser and Reference
   Value Provider roles are combined.

   Interactions between roles aggregated into the same entity do not
   necessarily use the Internet Protocol.  Such interactions might use a
   loopback device or other IP-based communication between separate
   environments, but they do not have to.  Alternative channels to
   convey conceptual messages include function calls, sockets, GPIO
   interfaces, local busses, or hypervisor calls.  This type of
   conveyance is typically found in composite devices.  Most



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   importantly, these conveyance methods are out-of-scope of RATS, but
   they are presumed to exist in order to convey conceptual messages
   appropriately between roles.

   In essence, an entity that combines more than one role creates and
   consumes the corresponding conceptual messages as defined in this
   document.

7.  Trust Model

7.1.  Relying Party

   This document covers scenarios for which a Relying Party trusts a
   Verifier that can appraise the trustworthiness of information about
   an Attester.  Such trust might come by the Relying Party trusting the
   Verifier (or its public key) directly, or might come by trusting an
   entity (e.g., a Certificate Authority) that is in the Verifier's
   certificate path.  Such trust is expressed by storing one or more
   "trust anchors" in a secure location known as a trust anchor store.

   As defined in [RFC6024], "A trust anchor represents an authoritative
   entity via a public key and associated data.  The public key is used
   to verify digital signatures, and the associated data is used to
   constrain the types of information for which the trust anchor is
   authoritative."  The trust anchor may be a certificate or it may be a
   raw public key along with additional data if necessary such as its
   public key algorithm and parameters.

   The Relying Party might implicitly trust a Verifier, such as in a
   Verifier/Relying Party combination where the Verifier and Relying
   Party roles are combined.  Or, for a stronger level of security, the
   Relying Party might require that the Verifier first provide
   information about itself that the Relying Party can use to assess the
   trustworthiness of the Verifier before accepting its Attestation
   Results.

   For example, one explicit way for a Relying Party "A" to establish
   such trust in a Verifier "B", would be for B to first act as an
   Attester where A acts as a combined Verifier/Relying Party.  If A
   then accepts B as trustworthy, it can choose to accept B as a
   Verifier for other Attesters.

   As another example, the Relying Party can establish trust in the
   Verifier by out of band establishment of key material, combined with
   a protocol like TLS to communicate.  There is an assumption that
   between the establishment of the trusted key material and the
   creation of the Evidence, that the Verifier has not been compromised.




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   Similarly, the Relying Party also needs to trust the Relying Party
   Owner for providing its Appraisal Policy for Attestation Results, and
   in some scenarios the Relying Party might even require that the
   Relying Party Owner go through a remote attestation procedure with it
   before the Relying Party will accept an updated policy.  This can be
   done similarly to how a Relying Party could establish trust in a
   Verifier as discussed above.

7.2.  Attester

   In some scenarios, Evidence might contain sensitive information such
   as Personally Identifiable Information (PII) or system identifiable
   information.  Thus, an Attester must trust entities to which it
   conveys Evidence, to not reveal sensitive data to unauthorized
   parties.  The Verifier might share this information with other
   authorized parties, according to a governing policy that address the
   handling of sensitive information (potentially included in Appraisal
   Policies for Evidence).  In the background-check model, this Evidence
   may also be revealed to Relying Party(s).

   When Evidence contains sensitive information, an Attester typically
   requires that a Verifier authenticates itself (e.g., at TLS session
   establishment) and might even request a remote attestation before the
   Attester sends the sensitive Evidence.  This can be done by having
   the Attester first act as a Verifier/Relying Party, and the Verifier
   act as its own Attester, as discussed above.

7.3.  Relying Party Owner

   The Relying Party Owner might also require that the Relying Party
   first act as an Attester, providing Evidence that the Owner can
   appraise, before the Owner would give the Relying Party an updated
   policy that might contain sensitive information.  In such a case,
   authentication or attestation in both directions might be needed, in
   which case typically one side's Evidence must be considered safe to
   share with an untrusted entity, in order to bootstrap the sequence.
   See Section 11 for more discussion.

7.4.  Verifier

   The Verifier trusts (or more specifically, the Verifier's security
   policy is written in a way that configures the Verifier to trust) a
   manufacturer, or the manufacturer's hardware, so as to be able to
   appraise the trustworthiness of that manufacturer's devices.  Such
   trust is expressed by storing one or more trust anchors in the
   Verifier's trust anchor store.





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   In a typical solution, a Verifier comes to trust an Attester
   indirectly by having an Endorser (such as a manufacturer) vouch for
   the Attester's ability to securely generate Evidence, in which case
   the Endorser's key material is stored in the Verifier's trust anchor
   store.

   In some solutions, a Verifier might be configured to directly trust
   an Attester by having the Verifier have the Attester's key material
   (rather than the Endorser's) in its trust anchor store.

   Such direct trust must first be established at the time of trust
   anchor store configuration either by checking with an Endorser at
   that time, or by conducting a security analysis of the specific
   device.  Having the Attester directly in the trust anchor store
   narrows the Verifier's trust to only specific devices rather than all
   devices the Endorser might vouch for, such as all devices
   manufactured by the same manufacturer in the case that the Endorser
   is a manufacturer.

   Such narrowing is often important since physical possession of a
   device can also be used to conduct a number of attacks, and so a
   device in a physically secure environment (such as one's own
   premises) may be considered trusted whereas devices owned by others
   would not be.  This often results in a desire to either have the
   owner run their own Endorser that would only endorse devices one
   owns, or to use Attesters directly in the trust anchor store.  When
   there are many Attesters owned, the use of an Endorser enables better
   scalability.

   That is, a Verifier might appraise the trustworthiness of an
   application component, operating system component, or service under
   the assumption that information provided about it by the lower-layer
   firmware or software is true.  A stronger level of assurance of
   security comes when information can be vouched for by hardware or by
   ROM code, especially if such hardware is physically resistant to
   hardware tampering.  In most cases, components that have to be
   vouched for via Endorsements because no Evidence is generated about
   them are referred to as roots of trust.

   The manufacturer having arranged for an Attesting Environment to be
   provisioned with key material with which to sign Evidence, the
   Verifier is then provided with some way of verifying the signature on
   the Evidence.  This may be in the form of an appropriate trust
   anchor, or the Verifier may be provided with a database of public
   keys (rather than certificates) or even carefully curated and secured
   lists of symmetric keys.





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   The nature of how the Verifier manages to validate the signatures
   produced by the Attester is critical to the secure operation of a
   remote attestation system, but is not the subject of standardization
   within this architecture.

   A conveyance protocol that provides authentication and integrity
   protection can be used to convey Evidence that is otherwise
   unprotected (e.g., not signed).  Appropriate conveyance of
   unprotected Evidence (e.g., [I-D.birkholz-rats-uccs]) relies on the
   following conveyance protocol's protection capabilities:

   1.  The key material used to authenticate and integrity protect the
       conveyance channel is trusted by the Verifier to speak for the
       Attesting Environment(s) that collected Claims about the Target
       Environment(s).

   2.  All unprotected Evidence that is conveyed is supplied exclusively
       by the Attesting Environment that has the key material that
       protects the conveyance channel

   3.  The root of trust protects both the conveyance channel key
       material and the Attesting Environment with equivalent strength
       protections.

   As illustrated in [I-D.birkholz-rats-uccs], an entity that receives
   unprotected Evidence via a trusted conveyance channel always takes on
   the responsibility of vouching for the Evidence's authenticity and
   freshness.  If protected Evidence is generated, the Attester's
   Attesting Environments take on that responsibility.  In cases where
   unprotected Evidence is processed by a Verifier, Relying Parties have
   to trust that the Verifier is capable of handling Evidence in a
   manner that preserves the Evidence's authenticity and freshness.
   Generating and conveying unprotected Evidence always creates
   significant risk and the benefits of that approach have to be
   carefully weighed against potential drawbacks.

   See Section 12 for discussion on security strength.

7.5.  Endorser, Reference Value Provider, and Verifier Owner

   In some scenarios, the Endorser, Reference Value Provider, and
   Verifier Owner may need to trust the Verifier before giving the
   Endorsement, Reference Values, or appraisal policy to it.  This can
   be done similarly to how a Relying Party might establish trust in a
   Verifier.






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   As discussed in Section 7.3, authentication or attestation in both
   directions might be needed, in which case typically one side's
   identity or Evidence must be considered safe to share with an
   untrusted entity, in order to bootstrap the sequence.  See Section 11
   for more discussion.

8.  Conceptual Messages

   Figure 1 illustrates the flow of a conceptual messages between
   various roles.  This section provides additional elaboration and
   implementation considerations.  It is the responsibility of protocol
   specifications to define the actual data format and semantics of any
   relevant conceptual messages.

8.1.  Evidence

   Evidence is a set of Claims about the target environment that reveal
   operational status, health, configuration or construction that have
   security relevance.  Evidence is appraised by a Verifier to establish
   its relevance, compliance, and timeliness.  Claims need to be
   collected in a manner that is reliable.  Evidence needs to be
   securely associated with the target environment so that the Verifier
   cannot be tricked into accepting Claims originating from a different
   environment (that may be more trustworthy).  Evidence also must be
   protected from man-in-the-middle attackers who may observe, change or
   misdirect Evidence as it travels from Attester to Verifier.  The
   timeliness of Evidence can be captured using Claims that pinpoint the
   time or interval when changes in operational status, health, and so
   forth occur.

8.2.  Endorsements

   An Endorsement is a secure statement that some entity (e.g., a
   manufacturer) vouches for the integrity of the device's signing
   capability.  For example, if the signing capability is in hardware,
   then an Endorsement might be a manufacturer certificate that signs a
   public key whose corresponding private key is only known inside the
   device's hardware.  Thus, when Evidence and such an Endorsement are
   used together, an appraisal procedure can be conducted based on
   appraisal policies that may not be specific to the device instance,
   but merely specific to the manufacturer providing the Endorsement.
   For example, an appraisal policy might simply check that devices from
   a given manufacturer have information matching a set of Reference
   Values, or an appraisal policy might have a set of more complex logic
   on how to appraise the validity of information.






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   However, while an appraisal policy that treats all devices from a
   given manufacturer the same may be appropriate for some use cases, it
   would be inappropriate to use such an appraisal policy as the sole
   means of authorization for use cases that wish to constrain _which_
   compliant devices are considered authorized for some purpose.  For
   example, an enterprise using remote attestation for Network Endpoint
   Assessment [RFC5209] may not wish to let every healthy laptop from
   the same manufacturer onto the network, but instead only want to let
   devices that it legally owns onto the network.  Thus, an Endorsement
   may be helpful information in authenticating information about a
   device, but is not necessarily sufficient to authorize access to
   resources which may need device-specific information such as a public
   key for the device or component or user on the device.

8.3.  Reference Values

   Reference Values used in appraisal procedures come from a Reference
   Value Provider and are then used by the Verifier to compare to
   Evidence.  Reference Values with matching Evidence produces
   acceptable Claims.  Additionally, appraisal policy may play a role in
   determining the acceptance of Claims.

8.4.  Attestation Results

   Attestation Results are the input used by the Relying Party to decide
   the extent to which it will trust a particular Attester, and allow it
   to access some data or perform some operation.

   Attestation Results may carry a boolean value indicating compliance
   or non-compliance with a Verifier's appraisal policy, or may carry a
   richer set of Claims about the Attester, against which the Relying
   Party applies its Appraisal Policy for Attestation Results.

   The quality of the Attestation Results depends upon the ability of
   the Verifier to evaluate the Attester.  Different Attesters have a
   different _Strength of Function_ [strengthoffunction], which results
   in the Attestation Results being qualitatively different in strength.

   An Attestation Result that indicates non-compliance can be used by an
   Attester (in the passport model) or a Relying Party (in the
   background-check model) to indicate that the Attester should not be
   treated as authorized and may be in need of remediation.  In some
   cases, it may even indicate that the Evidence itself cannot be
   authenticated as being correct.

   By default, the Relying Party does not believe the Attester to be
   compliant.  Upon receipt of an authentic Attestation Result and given
   the Appraisal Policy for Attestation Results is satisfied, the



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   Attester is allowed to perform the prescribed actions or access.  The
   simplest such appraisal policy might authorize granting the Attester
   full access or control over the resources guarded by the Relying
   Party.  A more complex appraisal policy might involve using the
   information provided in the Attestation Result to compare against
   expected values, or to apply complex analysis of other information
   contained in the Attestation Result.

   Thus, Attestation Results often need to include detailed information
   about the Attester, for use by Relying Parties, much like physical
   passports and drivers licenses include personal information such as
   name and date of birth.  Unlike Evidence, which is often very device-
   and vendor-specific, Attestation Results can be vendor-neutral, if
   the Verifier has a way to generate vendor-agnostic information based
   on the appraisal of vendor-specific information in Evidence.  This
   allows a Relying Party's appraisal policy to be simpler, potentially
   based on standard ways of expressing the information, while still
   allowing interoperability with heterogeneous devices.

   Finally, whereas Evidence is signed by the device (or indirectly by a
   manufacturer, if Endorsements are used), Attestation Results are
   signed by a Verifier, allowing a Relying Party to only need a trust
   relationship with one entity, rather than a larger set of entities,
   for purposes of its appraisal policy.

8.5.  Appraisal Policies

   The Verifier, when appraising Evidence, or the Relying Party, when
   appraising Attestation Results, checks the values of matched Claims
   against constraints specified in its appraisal policy.  Examples of
   such constraints checking include:

   *  comparison for equality against a Reference Value, or

   *  a check for being in a range bounded by Reference Values, or

   *  membership in a set of Reference Values, or

   *  a check against values in other Claims.

   Upon completing all appraisal policy constraints, the remaining
   Claims are accepted as input toward determining Attestation Results,
   when appraising Evidence, or as input to a Relying Party, when
   appraising Attestation Results.







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9.  Claims Encoding Formats

   The following diagram illustrates a relationship to which remote
   attestation is desired to be added:

      +-------------+               +------------+ Evaluate
      |             |-------------->|            | request
      |  Attester   |  Access some  |   Relying  | against
      |             |    resource   |    Party   | security
      +-------------+               +------------+ policy

                     Figure 8: Typical Resource Access

   In this diagram, the protocol between Attester and a Relying Party
   can be any new or existing protocol (e.g., HTTP(S), COAP(S), ROLIE
   [RFC8322], 802.1x, OPC UA [OPCUA], etc.), depending on the use case.

   Typically, such protocols already have mechanisms for passing
   security information for authentication and authorization purposes.
   Common formats include JWTs [RFC7519], CWTs [RFC8392], and X.509
   certificates.

   Retrofitting already deployed protocols with remote attestation
   requires adding RATS conceptual messages to the existing data flows.
   This must be done in a way that does not degrade the security
   properties of the systems involved and should use native extension
   mechanisms provided by the underlying protocol.  For example, if a
   TLS handshake is to be extended with remote attestation capabilities,
   attestation Evidence may be embedded in an ad-hoc X.509 certificate
   extension (e.g., [TCG-DICE]), or into a new TLS Certificate Type
   (e.g., [I-D.tschofenig-tls-cwt]).

   Especially for constrained nodes there is a desire to minimize the
   amount of parsing code needed in a Relying Party, in order to both
   minimize footprint and to minimize the attack surface.  While it
   would be possible to embed a CWT inside a JWT, or a JWT inside an
   X.509 extension, etc., there is a desire to encode the information
   natively in a format that is already supported by the Relying Party.

   This motivates having a common "information model" that describes the
   set of remote attestation related information in an encoding-agnostic
   way, and allowing multiple encoding formats (CWT, JWT, X.509, etc.)
   that encode the same information into the Claims format needed by the
   Relying Party.

   The following diagram illustrates that Evidence and Attestation
   Results might be expressed via multiple potential encoding formats,
   so that they can be conveyed by various existing protocols.  It also



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   motivates why the Verifier might also be responsible for accepting
   Evidence that encodes Claims in one format, while issuing Attestation
   Results that encode Claims in a different format.

                   Evidence           Attestation Results
   .--------------.   CWT                    CWT   .-------------------.
   |  Attester-A  |------------.      .----------->|  Relying Party V  |
   '--------------'            v      |            `-------------------'
   .--------------.   JWT   .------------.   JWT   .-------------------.
   |  Attester-B  |-------->|  Verifier  |-------->|  Relying Party W  |
   '--------------'         |            |         `-------------------'
   .--------------.  X.509  |            |  X.509  .-------------------.
   |  Attester-C  |-------->|            |-------->|  Relying Party X  |
   '--------------'         |            |         `-------------------'
   .--------------.   TPM   |            |   TPM   .-------------------.
   |  Attester-D  |-------->|            |-------->|  Relying Party Y  |
   '--------------'         '------------'         `-------------------'
   .--------------.  other     ^      |     other  .-------------------.
   |  Attester-E  |------------'      '----------->|  Relying Party Z  |
   '--------------'                                `-------------------'

      Figure 9: Multiple Attesters and Relying Parties with Different
                                  Formats

10.  Freshness

   A Verifier or Relying Party might need to learn the point in time
   (i.e., the "epoch") an Evidence or Attestation Result has been
   produced.  This is essential in deciding whether the included Claims
   and their values can be considered fresh, meaning they still reflect
   the latest state of the Attester, and that any Attestation Result was
   generated using the latest Appraisal Policy for Evidence.

   Freshness is assessed based on the Appraisal Policy for Evidence or
   Attestation Results that compares the estimated epoch against an
   "expiry" threshold defined locally to that policy.  There is,
   however, always a race condition possible in that the state of the
   Attester, and the appraisal policies might change immediately after
   the Evidence or Attestation Result was generated.  The goal is merely
   to narrow their recentness to something the Verifier (for Evidence)
   or Relying Party (for Attestation Result) is willing to accept.  Some
   flexibility on the freshness requirement is a key component for
   enabling caching and reuse of both Evidence and Attestation Results,
   which is especially valuable in cases where their computation uses a
   substantial part of the resource budget (e.g., energy in constrained
   devices).





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   There are three common approaches for determining the epoch of
   Evidence or an Attestation Result.

10.1.  Explicit Timekeeping using Synchronized Clocks

   The first approach is to rely on synchronized and trustworthy clocks,
   and include a signed timestamp (see [I-D.birkholz-rats-tuda]) along
   with the Claims in the Evidence or Attestation Result.  Timestamps
   can also be added on a per-Claim basis to distinguish the time of
   generation of Evidence or Attestation Result from the time that a
   specific Claim was generated.  The clock's trustworthiness can
   generally be established via Endorsements and typically requires
   additional Claims about the signer's time synchronization mechanism.

   In some use cases, however, a trustworthy clock might not be
   available.  For example, in many Trusted Execution Environments
   (TEEs) today, a clock is only available outside the TEE and so cannot
   be trusted by the TEE.

10.2.  Implicit Timekeeping using Nonces

   A second approach places the onus of timekeeping solely on the
   Verifier (for Evidence) or the Relying Party (for Attestation
   Results), and might be suitable, for example, in case the Attester
   does not have a trustworthy clock or time synchronization is
   otherwise impaired.  In this approach, a non-predictable nonce is
   sent by the appraising entity, and the nonce is then signed and
   included along with the Claims in the Evidence or Attestation Result.
   After checking that the sent and received nonces are the same, the
   appraising entity knows that the Claims were signed after the nonce
   was generated.  This allows associating a "rough" epoch to the
   Evidence or Attestation Result.  In this case the epoch is said to be
   rough because:

   *  The epoch applies to the entire Claim set instead of a more
      granular association, and

   *  The time between the creation of Claims and the collection of
      Claims is indistinguishable.

10.3.  Implicit Timekeeping using Epoch IDs

   A third approach relies on having epoch identifiers (or "IDs")
   periodically sent to both the sender and receiver of Evidence or
   Attestation Results by some "Epoch ID Distributor".






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   Epoch IDs are different from nonces as they can be used more than
   once and can even be used by more than one entity at the same time.
   Epoch IDs are different from timestamps as they do not have to convey
   information about a point in time, i.e., they are not necessarily
   monotonically increasing integers.

   Like the nonce approach, this allows associating a "rough" epoch
   without requiring a trustworthy clock or time synchronization in
   order to generate or appraise the freshness of Evidence or
   Attestation Results.  Only the Epoch ID Distributor requires access
   to a clock so it can periodically send new epoch IDs.

   The most recent epoch ID is included in the produced Evidence or
   Attestation Results, and the appraising entity can compare the epoch
   ID in received Evidence or Attestation Results against the latest
   epoch ID it received from the Epoch ID Distributor to determine if it
   is within the current epoch.  An actual solution also needs to take
   into account race conditions when transitioning to a new epoch, such
   as by using a counter signed by the Epoch ID Distributor as the epoch
   ID, or by including both the current and previous epoch IDs in
   messages and/or checks, by requiring retries in case of mismatching
   epoch IDs, or by buffering incoming messages that might be associated
   with a epoch ID that the receiver has not yet obtained.

   More generally, in order to prevent an appraising entity from
   generating false negatives (e.g., discarding Evidence that is deemed
   stale even if it is not), the appraising entity should keep an "epoch
   window" consisting of the most recently received epoch IDs.  The
   depth of such epoch window is directly proportional to the maximum
   network propagation delay between the first to receive the epoch ID
   and the last to receive the epoch ID, and it is inversely
   proportional to the epoch duration.  The appraising entity shall
   compare the epoch ID carried in the received Evidence or Attestation
   Result with the epoch IDs in its epoch window to find a suitable
   match.

   Whereas the nonce approach typically requires the appraising entity
   to keep state for each nonce generated, the epoch ID approach
   minimizes the state kept to be independent of the number of Attesters
   or Verifiers from which it expects to receive Evidence or Attestation
   Results, as long as all use the same Epoch ID Distributor.










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10.4.  Discussion

   Implicit and explicit timekeeping can be combined into hybrid
   mechanisms.  For example, if clocks exist and are considered
   trustworthy but are not synchronized, a nonce-based exchange may be
   used to determine the (relative) time offset between the involved
   peers, followed by any number of timestamp based exchanges.

   It is important to note that the actual values in Claims might have
   been generated long before the Claims are signed.  If so, it is the
   signer's responsibility to ensure that the values are still correct
   when they are signed.  For example, values generated at boot time
   might have been saved to secure storage until network connectivity is
   established to the remote Verifier and a nonce is obtained.

   A more detailed discussion with examples appears in Section 16.

   For a discussion on the security of epoch IDs see Section 12.3.

11.  Privacy Considerations

   The conveyance of Evidence and the resulting Attestation Results
   reveal a great deal of information about the internal state of a
   device as well as potentially any users of the device.  In many
   cases, the whole point of attestation procedures is to provide
   reliable information about the type of the device and the firmware/
   software that the device is running.  This information might be
   particularly interesting to many attackers.  For example, knowing
   that a device is running a weak version of firmware provides a way to
   aim attacks better.

   Many Claims in Evidence and Attestation Results are potentially
   Personally Identifying Information (PII) depending on the end-to-end
   use case of the remote attestation procedure.  Remote attestation
   that goes up to include containers and applications, e.g., a blood
   pressure monitor, may further reveal details about specific systems
   or users.

   In some cases, an attacker may be able to make inferences about the
   contents of Evidence from the resulting effects or timing of the
   processing.  For example, an attacker might be able to infer the
   value of specific Claims if it knew that only certain values were
   accepted by the Relying Party.

   Evidence and Attestation Results are expected to be integrity
   protected (i.e., either via signing or a secure channel) and
   optionally might be confidentiality protected via encryption.  If
   confidentiality protection via signing the conceptual messages is



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   omitted or unavailable, the protecting protocols that convey Evidence
   or Attestation Results are responsible for detailing what kinds of
   information are disclosed, and to whom they are exposed.

   As Evidence might contain sensitive or confidential information,
   Attesters are responsible for only sending such Evidence to trusted
   Verifiers.  Some Attesters might want a stronger level of assurance
   of the trustworthiness of a Verifier before sending Evidence to it.
   In such cases, an Attester can first act as a Relying Party and ask
   for the Verifier's own Attestation Result, and appraising it just as
   a Relying Party would appraise an Attestation Result for any other
   purpose.

   Another approach to deal with Evidence is to remove PII from the
   Evidence while still being able to verify that the Attester is one of
   a large set.  This approach is often called "Direct Anonymous
   Attestation".  See [CCC-DeepDive] section 6.2 for more discussion.

12.  Security Considerations

   This document provides an architecture for doing remote attestation.
   No specific wire protocol is documented here.  Without a specific
   proposal to compare against, it is impossible to know if the security
   threats listed below have been mitigated well.  The security
   considerations below should be read as being essentially requirements
   against realizations of the RATS Architecture.  Some threats apply to
   protocols, some are against implementations (code), and some threats
   are against physical infrastructure (such as factories).

12.1.  Attester and Attestation Key Protection

   Implementers need to pay close attention to the protection of the
   Attester and the manufacturing processes for provisioning attestation
   key material.  If either of these are compromised, intended levels of
   assurance for RATS are compromised because attackers can forge
   Evidence or manipulate the Attesting Environment.  For example, a
   Target Environment should not be able to tamper with the Attesting
   Environment that measures it, by isolating the two environments from
   each other in some way.

   Remote attestation applies to use cases with a range of security
   requirements, so the protections discussed here range from low to
   high security where low security may be limited to application or
   process isolation by the device's operating system, and high security
   may involve specialized hardware to defend against physical attacks
   on a chip.





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12.1.1.  On-Device Attester and Key Protection

   It is assumed that an Attesting Environment is sufficiently isolated
   from the Target Environment it collects Claims about and that it
   signs the resulting Claims set with an attestation key, so that the
   Target Environment cannot forge Evidence about itself.  Such an
   isolated environment might be provided by a process, a dedicated
   chip, a TEE, a virtual machine, or another secure mode of operation.
   The Attesting Environment must be protected from unauthorized
   modification to ensure it behaves correctly.  Confidentiality
   protection of the Attesting Environment's signing key is vital so it
   cannot be misused to forge Evidence.

   In many cases the user or owner of a device that includes the role of
   Attester must not be able to modify or extract keys from the
   Attesting Environments, to prevent creating forged Evidence.  Some
   common examples include the user of a mobile phone or FIDO
   authenticator.  An essential value-add provided by RATS is for the
   Relying Party to be able to trust the Attester even if the user or
   owner is not trusted.

   Measures for a minimally protected system might include process or
   application isolation provided by a high-level operating system, and
   restricted access to root or system privileges.  In contrast, For
   really simple single-use devices that don't use a protected mode
   operating system, like a Bluetooth speaker, the only factual
   isolation might be the sturdy housing of the device.

   Measures for a moderately protected system could include a special
   restricted operating environment, such as a TEE.  In this case, only
   security-oriented software has access to the Attester and key
   material.

   Measures for a highly protected system could include specialized
   hardware that is used to provide protection against chip decapping
   attacks, power supply and clock glitching, faulting injection and RF
   and power side channel attacks.

12.1.2.  Attestation Key Provisioning Processes

   Attestation key provisioning is the process that occurs in the
   factory or elsewhere to establish signing key material on the device
   and the validation key material off the device.  Sometimes this is
   procedure is referred to as personalization or customization.







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12.1.2.1.  Off-Device Key Generation

   One way to provision key material is to first generate it external to
   the device and then copy the key onto the device.  In this case,
   confidentiality protection of the generator, as well as for the path
   over which the key is provisioned, is necessary.  The manufacturer
   needs to take care to protect corresponding key material with
   measures appropriate for its value.

   The degree of protection afforded to this key material can vary by
   device, based upon considerations as to a cost/benefit evaluation of
   the intended function of the device.  The confidentiality protection
   is fundamentally based upon some amount of physical protection: while
   encryption is often used to provide confidentiality when a key is
   conveyed across a factory, where the attestation key is created or
   applied, it must be available in an unencrypted form.  The physical
   protection can therefore vary from situations where the key is
   unencrypted only within carefully controlled secure enclaves within
   silicon, to situations where an entire facility is considered secure,
   by the simple means of locked doors and limited access.

   The cryptography that is used to enable confidentiality protection of
   the attestation key comes with its own requirements to be secured.
   This results in recursive problems, as the key material used to
   provision attestation keys must again somehow have been provisioned
   securely beforehand (requiring an additional level of protection, and
   so on).

   So, this is why, in general, a combination of some physical security
   measures and some cryptographic measures is used to establish
   confidentiality protection.

12.1.2.2.  On-Device Key Generation

   When key material is generated within a device and the secret part of
   it never leaves the device, then the problem may lessen.  For public-
   key cryptography, it is, by definition, not necessary to maintain
   confidentiality of the public key: however integrity of the chain of
   custody of the public key is necessary in order to avoid attacks
   where an attacker is able get a key they control endorsed.

   To summarize: attestation key provisioning must ensure that only
   valid attestation key material is established in Attesters.








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12.2.  Integrity Protection

   Any solution that conveys information used for security purposes,
   whether such information is in the form of Evidence, Attestation
   Results, Endorsements, or appraisal policy must support end-to-end
   integrity protection and replay attack prevention, and often also
   needs to support additional security properties, including:

   *  end-to-end encryption,

   *  denial of service protection,

   *  authentication,

   *  auditing,

   *  fine grained access controls, and

   *  logging.

   Section 10 discusses ways in which freshness can be used in this
   architecture to protect against replay attacks.

   To assess the security provided by a particular appraisal policy, it
   is important to understand the strength of the root of trust, e.g.,
   whether it is mutable software, or firmware that is read-only after
   boot, or immutable hardware/ROM.

   It is also important that the appraisal policy was itself obtained
   securely.  If an attacker can configure appraisal policies for a
   Relying Party or for a Verifier, then integrity of the process is
   compromised.

   Security protections in RATS may be applied at different layers,
   whether by a conveyance protocol, or an information encoding format.
   This architecture expects conceptual messages (see Section 8) to be
   end-to-end protected based on the role interaction context.  For
   example, if an Attester produces Evidence that is relayed through
   some other entity that doesn't implement the Attester or the intended
   Verifier roles, then the relaying entity should not expect to have
   access to the Evidence.

12.3.  Epoch ID-based Attestation

   Epoch IDs, described in Section 10.3, can be tampered with, replayed,
   dropped, delayed, and reordered by an attacker.





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   An attacker could be either external or belong to the distribution
   group, for example, if one of the Attester entities have been
   compromised.

   An attacker who is able to tamper with epoch IDs can potentially lock
   all the participants in a certain epoch of choice for ever,
   effectively freezing time.  This is problematic since it destroys the
   ability to ascertain freshness of Evidence and Attestation Results.

   To mitigate this threat, the transport should be at least integrity
   protected and provide origin authentication.

   Selective dropping of epoch IDs is equivalent to pinning the victim
   node to a past epoch.  An attacker could drop epoch IDs to only some
   entities and not others, which will typically result in a denial of
   service due to the permanent staleness of the Attestation Result or
   Evidence.

   Delaying or reordering epoch IDs is equivalent to manipulating the
   victim's timeline at will.  This ability could be used by a malicious
   actor (e.g., a compromised router) to mount a confusion attack where,
   for example, a Verifier is tricked into accepting Evidence coming
   from a past epoch as fresh, while in the meantime the Attester has
   been compromised.

   Reordering and dropping attacks are mitigated if the transport
   provides the ability to detect reordering and drop.  However, the
   delay attack described above can't be thwarted in this manner.

12.4.  Trust Anchor Protection

   As noted in Section 7, Verifiers and Relying Parties have trust
   anchor stores that must be secured.  Specifically, a trust anchor
   store must resist modification against unauthorized insertion,
   deletion, and modification.

   If certificates are used as trust anchors, Verifiers and Relying
   Parties are also responsible for validating the entire certificate
   path up to the trust anchor, which includes checking for certificate
   revocation.  See Section 6 of [RFC5280] for details.

13.  IANA Considerations

   This document does not require any actions by IANA.







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14.  Acknowledgments

   Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
   Fitzgerald-McKay, Diego Lopez, Laurence Lundblade, Paul Rowe, Hannes
   Tschofenig, Frank Xia, and David Wooten.

15.  Notable Contributions

   Thomas Hardjono created initial versions of the terminology section
   in collaboration with Ned Smith.  Eric Voit provided the conceptual
   separation between Attestation Provision Flows and Attestation
   Evidence Flows.  Monty Wisemen created the content structure of the
   first three architecture drafts.  Carsten Bormann provided many of
   the motivational building blocks with respect to the Internet Threat
   Model.

16.  Appendix A: Time Considerations

   Section 10 discussed various issues and requirements around freshness
   of evidence, and summarized three approaches that might be used by
   different solutions to address them.  This appendix provides more
   details with examples to help illustrate potential approaches, to
   inform those creating specific solutions.

   The table below defines a number of relevant events, with an ID that
   is used in subsequent diagrams.  The times of said events might be
   defined in terms of an absolute clock time, such as the Coordinated
   Universal Time timescale, or might be defined relative to some other
   timestamp or timeticks counter, such as a clock resetting its epoch
   each time it is powered on.

   +====+============+=================================================+
   | ID | Event      | Explanation of event                            |
   +====+============+=================================================+
   | VG | Value      | A value to appear in a Claim was created.       |
   |    | generated  | In some cases, a value may have technically     |
   |    |            | existed before an Attester became aware of      |
   |    |            | it but the Attester might have no idea how      |
   |    |            | long it has had that value.  In such a          |
   |    |            | case, the Value created time is the time at     |
   |    |            | which the Claim containing the copy of the      |
   |    |            | value was created.                              |
   +----+------------+-------------------------------------------------+
   | NS | Nonce sent | A nonce not predictable to an Attester          |
   |    |            | (recentness & uniqueness) is sent to an         |
   |    |            | Attester.                                       |
   +----+------------+-------------------------------------------------+
   | NR | Nonce      | A nonce is relayed to an Attester by            |



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   |    | relayed    | another entity.                                 |
   +----+------------+-------------------------------------------------+
   | IR | Epoch ID   | An epoch ID is successfully received and        |
   |    | received   | processed by an entity.                         |
   +----+------------+-------------------------------------------------+
   | EG | Evidence   | An Attester creates Evidence from collected     |
   |    | generation | Claims.                                         |
   +----+------------+-------------------------------------------------+
   | ER | Evidence   | A Relying Party relays Evidence to a            |
   |    | relayed    | Verifier.                                       |
   +----+------------+-------------------------------------------------+
   | RG | Result     | A Verifier appraises Evidence and generates     |
   |    | generation | an Attestation Result.                          |
   +----+------------+-------------------------------------------------+
   | RR | Result     | A Relying Party relays an Attestation           |
   |    | relayed    | Result to a Relying Party.                      |
   +----+------------+-------------------------------------------------+
   | RA | Result     | The Relying Party appraises Attestation         |
   |    | appraised  | Results.                                        |
   +----+------------+-------------------------------------------------+
   | OP | Operation  | The Relying Party performs some operation       |
   |    | performed  | requested by the Attester via a resource        |
   |    |            | access protocol as depicted in Figure 8,        |
   |    |            | e.g., across a session created earlier at       |
   |    |            | time(RA).                                       |
   +----+------------+-------------------------------------------------+
   | RX | Result     | An Attestation Result should no longer be       |
   |    | expiry     | accepted, according to the Verifier that        |
   |    |            | generated it.                                   |
   +----+------------+-------------------------------------------------+

                                  Table 1

   Using the table above, a number of hypothetical examples of how a
   solution might be built are illustrated below.  This list is not
   intended to be complete, but is just representative enough to
   highlight various timing considerations.

   All times are relative to the local clocks, indicated by an "_a"
   (Attester), "_v" (Verifier), or "_r" (Relying Party) suffix.

   Times with an appended Prime (') indicate a second instance of the
   same event.

   How and if clocks are synchronized depends upon the model.






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   In the figures below, curly braces indicate containment.  For
   example, the notation Evidence{foo} indicates that 'foo' is contained
   in the Evidence and is thus covered by its signature.

16.1.  Example 1: Timestamp-based Passport Model Example

   The following example illustrates a hypothetical Passport Model
   solution that uses timestamps and requires roughly synchronized
   clocks between the Attester, Verifier, and Relying Party, which
   depends on using a secure clock synchronization mechanism.  As a
   result, the receiver of a conceptual message containing a timestamp
   can directly compare it to its own clock and timestamps.

      .----------.                     .----------.  .---------------.
      | Attester |                     | Verifier |  | Relying Party |
      '----------'                     '----------'  '---------------'
        time(VG_a)                           |               |
           |                                 |               |
           ~                                 ~               ~
           |                                 |               |
        time(EG_a)                           |               |
           |------Evidence{time(EG_a)}------>|               |
           |                              time(RG_v)         |
           |<-----Attestation Result---------|               |
           |      {time(RG_v),time(RX_v)}    |               |
           ~                                                 ~
           |                                                 |
           |----Attestation Result{time(RG_v),time(RX_v)}-->time(RA_r)
           |                                                 |
           ~                                                 ~
           |                                                 |
           |                                              time(OP_r)

   The Verifier can check whether the Evidence is fresh when appraising
   it at time(RG_v) by checking "time(RG_v) - time(EG_a) < Threshold",
   where the Verifier's threshold is large enough to account for the
   maximum permitted clock skew between the Verifier and the Attester.

   If time(VG_a) is also included in the Evidence along with the Claim
   value generated at that time, and the Verifier decides that it can
   trust the time(VG_a) value, the Verifier can also determine whether
   the Claim value is recent by checking "time(RG_v) - time(VG_a) <
   Threshold".  The threshold is decided by the Appraisal Policy for
   Evidence, and again needs to take into account the maximum permitted
   clock skew between the Verifier and the Attester.






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   The Relying Party can check whether the Attestation Result is fresh
   when appraising it at time(RA_r) by checking "time(RA_r) - time(RG_v)
   < Threshold", where the Relying Party's threshold is large enough to
   account for the maximum permitted clock skew between the Relying
   Party and the Verifier.  The result might then be used for some time
   (e.g., throughout the lifetime of a connection established at
   time(RA_r)).  The Relying Party must be careful, however, to not
   allow continued use beyond the period for which it deems the
   Attestation Result to remain fresh enough.  Thus, it might allow use
   (at time(OP_r)) as long as "time(OP_r) - time(RG_v) < Threshold".
   However, if the Attestation Result contains an expiry time time(RX_v)
   then it could explicitly check "time(OP_r) < time(RX_v)".

16.2.  Example 2: Nonce-based Passport Model Example

   The following example illustrates a hypothetical Passport Model
   solution that uses nonces instead of timestamps.  Compared to the
   timestamp-based example, it requires an extra round trip to retrieve
   a nonce, and requires that the Verifier and Relying Party track state
   to remember the nonce for some period of time.

   The advantage is that it does not require that any clocks are
   synchronized.  As a result, the receiver of a conceptual message
   containing a timestamp cannot directly compare it to its own clock or
   timestamps.  Thus we use a suffix ("a" for Attester, "v" for
   Verifier, and "r" for Relying Party) on the IDs below indicating
   which clock generated them, since times from different clocks cannot
   be compared.  Only the delta between two events from the sender can
   be used by the receiver.






















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      .----------.                     .----------.  .---------------.
      | Attester |                     | Verifier |  | Relying Party |
      '----------'                     '----------'  '---------------'
        time(VG_a)                           |               |
           |                                 |               |
           ~                                 ~               ~
           |                                 |               |
           |<--Nonce1---------------------time(NS_v)         |
        time(EG_a)                           |               |
           |---Evidence--------------------->|               |
           | {Nonce1, time(EG_a)-time(VG_a)} |               |
           |                              time(RG_v)         |
           |<--Attestation Result------------|               |
           |   {time(RX_v)-time(RG_v)}       |               |
           ~                                                 ~
           |                                                 |
           |<--Nonce2-------------------------------------time(NS_r)
        time(RR_a)                                           |
           |--[Attestation Result{time(RX_v)-time(RG_v)}, -->|time(RA_r)
           |        Nonce2, time(RR_a)-time(EG_a)]           |
           ~                                                 ~
           |                                                 |
           |                                              time(OP_r)

   In this example solution, the Verifier can check whether the Evidence
   is fresh at "time(RG_v)" by verifying that "time(RG_v)-time(NS_v) <
   Threshold".

   The Verifier cannot, however, simply rely on a Nonce to determine
   whether the value of a Claim is recent, since the Claim value might
   have been generated long before the nonce was sent by the Verifier.
   However, if the Verifier decides that the Attester can be trusted to
   correctly provide the delta "time(EG_a)-time(VG_a)", then it can
   determine recency by checking "time(RG_v)-time(NS_v) + time(EG_a)-
   time(VG_a) < Threshold".

   Similarly if, based on an Attestation Result from a Verifier it
   trusts, the Relying Party decides that the Attester can be trusted to
   correctly provide time deltas, then it can determine whether the
   Attestation Result is fresh by checking "time(OP_r)-time(NS_r) +
   time(RR_a)-time(EG_a) < Threshold".  Although the Nonce2 and
   "time(RR_a)-time(EG_a)" values cannot be inside the Attestation
   Result, they might be signed by the Attester such that the
   Attestation Result vouches for the Attester's signing capability.

   The Relying Party must still be careful, however, to not allow
   continued use beyond the period for which it deems the Attestation
   Result to remain valid.  Thus, if the Attestation Result sends a



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   validity lifetime in terms of "time(RX_v)-time(RG_v)", then the
   Relying Party can check "time(OP_r)-time(NS_r) < time(RX_v)-
   time(RG_v)".

16.3.  Example 3: Epoch ID-based Passport Model Example

   The example in Figure 10 illustrates a hypothetical Passport Model
   solution that uses epoch IDs instead of nonces or timestamps.

   The Epoch ID Distributor broadcasts epoch ID "I" which starts a new
   epoch "E" for a protocol participant upon reception at "time(IR)".

   The Attester generates Evidence incorporating epoch ID "I" and
   conveys it to the Verifier.

   The Verifier appraises that the received epoch ID "I" is "fresh"
   according to the definition provided in Section 10.3 whereby retries
   are required in the case of mismatching epoch IDs, and generates an
   Attestation Result.  The Attestation Result is conveyed to the
   Attester.

   After the transmission of epoch ID "I'" a new epoch "E'" is
   established when "I'" is received by each protocol participant.  The
   Attester relays the Attestation Result obtained during epoch "E"
   (associated with epoch ID "I") to the Relying Party using the epoch
   ID for the current epoch "I'".  If the Relying Party had not yet
   received "I'", then the Attestation Result would be rejected, but in
   this example, it is received.

   In the illustrated scenario, the epoch ID for relaying an Attestation
   Result to the Relying Party is current, while a previous epoch ID was
   used to generate Verifier evaluated evidence.  This indicates that at
   least one epoch transition has occurred, and the Attestation Results
   may only be as fresh as the previous epoch.  If the Relying Party
   remembers the previous epoch ID "I" during an epoch window as
   discussed in Section 10.3, and the message is received during that
   window, the Attestation Result is accepted as fresh, and otherwise it
   is rejected as stale.













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                     .-------------.
      .----------.   | Epoch ID    |   .----------.  .---------------.
      | Attester |   | Distributor |   | Verifier |  | Relying Party |
      '----------'   '-------------'   '----------'  '---------------'
        time(VG_a)          |                |               |
           |                |                |               |
           ~                ~                ~               ~
           |                |                |               |
        time(IR_a)<------I--+--I--------time(IR_v)----->time(IR_r)
           |                |                |               |
        time(EG_a)          |                |               |
           |---Evidence--------------------->|               |
           |   {I,time(EG_a)-time(VG_a)}     |               |
           |                |                |               |
           |                |           time(RG_v)           |
           |<--Attestation Result------------|               |
           |   {I,time(RX_v)-time(RG_v)}     |               |
           |                |                |               |
        time(IR'_a)<-----I'-+--I'-------time(IR'_v)---->time(IR'_r)
           |                |                |               |
           |---[Attestation Result--------------------->time(RA_r)
           |   {I,time(RX_v)-time(RG_v)},I'] |               |
           |                |                |               |
           ~                ~                ~               ~
           |                |                |               |
           |                |                |          time(OP_r)

                  Figure 10: Epoch ID-based Passport Model

16.4.  Example 4: Timestamp-based Background-Check Model Example

   The following example illustrates a hypothetical Background-Check
   Model solution that uses timestamps and requires roughly synchronized
   clocks between the Attester, Verifier, and Relying Party.

















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   .----------.         .---------------.                .----------.
   | Attester |         | Relying Party |                | Verifier |
   '----------'         '---------------'                '----------'
     time(VG_a)                 |                             |
           |                    |                             |
           ~                    ~                             ~
           |                    |                             |
     time(EG_a)                 |                             |
           |----Evidence------->|                             |
           |   {time(EG_a)} time(ER_r)--Evidence{time(EG_a)}->|
           |                    |                        time(RG_v)
           |                 time(RA_r)<-Attestation Result---|
           |                    |           {time(RX_v)}      |
           ~                    ~                             ~
           |                    |                             |
           |                 time(OP_r)                       |

   The time considerations in this example are equivalent to those
   discussed under Example 1 above.

16.5.  Example 5: Nonce-based Background-Check Model Example

   The following example illustrates a hypothetical Background-Check
   Model solution that uses nonces and thus does not require that any
   clocks are synchronized.  In this example solution, a nonce is
   generated by a Verifier at the request of a Relying Party, when the
   Relying Party needs to send one to an Attester.

   .----------.         .---------------.              .----------.
   | Attester |         | Relying Party |              | Verifier |
   '----------'         '---------------'              '----------'
     time(VG_a)                 |                           |
        |                       |                           |
        ~                       ~                           ~
        |                       |                           |
        |                       |<-------Nonce-----------time(NS_v)
        |<---Nonce-----------time(NR_r)                     |
     time(EG_a)                 |                           |
        |----Evidence{Nonce}--->|                           |
        |                    time(ER_r)--Evidence{Nonce}--->|
        |                       |                        time(RG_v)
        |                    time(RA_r)<-Attestation Result-|
        |                       |   {time(RX_v)-time(RG_v)} |
        ~                       ~                           ~
        |                       |                           |
        |                    time(OP_r)                     |





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   The Verifier can check whether the Evidence is fresh, and whether a
   Claim value is recent, the same as in Example 2 above.

   However, unlike in Example 2, the Relying Party can use the Nonce to
   determine whether the Attestation Result is fresh, by verifying that
   "time(OP_r)-time(NR_r) < Threshold".

   The Relying Party must still be careful, however, to not allow
   continued use beyond the period for which it deems the Attestation
   Result to remain valid.  Thus, if the Attestation Result sends a
   validity lifetime in terms of "time(RX_v)-time(RG_v)", then the
   Relying Party can check "time(OP_r)-time(ER_r) < time(RX_v)-
   time(RG_v)".

17.  References

17.1.  Normative References

   [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,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7519>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.

17.2.  Informative References

   [CCC-DeepDive]
              Confidential Computing Consortium, "Confidential Computing
              Deep Dive", n.d.,
              <https://confidentialcomputing.io/whitepaper-02-latest>.

   [CTAP]     FIDO Alliance, "Client to Authenticator Protocol", n.d.,
              <https://fidoalliance.org/specs/fido-v2.0-id-20180227/
              fido-client-to-authenticator-protocol-v2.0-id-
              20180227.html>.








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   [I-D.birkholz-rats-tuda]
              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://tools.ietf.org/html/draft-birkholz-rats-tuda-04>.

   [I-D.birkholz-rats-uccs]
              Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
              Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
              Work in Progress, Internet-Draft, draft-birkholz-rats-
              uccs-03, 8 March 2021,
              <https://tools.ietf.org/html/draft-birkholz-rats-uccs-03>.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-14, 22 February 2021,
              <https://tools.ietf.org/html/draft-ietf-teep-architecture-
              14>.

   [I-D.tschofenig-tls-cwt]
              Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens
              (CWTs) in Transport Layer Security (TLS) and Datagram
              Transport Layer Security (DTLS)", Work in Progress,
              Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020,
              <https://tools.ietf.org/html/draft-tschofenig-tls-cwt-02>.

   [OPCUA]    OPC Foundation, "OPC Unified Architecture Specification,
              Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
              November 2015, <https://opcfoundation.org/developer-tools/
              specifications-unified-architecture/part-2-security-
              model/>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4949>.

   [RFC5209]  Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
              Tardo, "Network Endpoint Assessment (NEA): Overview and
              Requirements", RFC 5209, DOI 10.17487/RFC5209, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5209>.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <https://www.rfc-editor.org/rfc/rfc6024>.




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   [RFC8322]  Field, J., Banghart, S., and D. Waltermire, "Resource-
              Oriented Lightweight Information Exchange (ROLIE)",
              RFC 8322, DOI 10.17487/RFC8322, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8322>.

   [strengthoffunction]
              NISC, "Strength of Function", n.d.,
              <https://csrc.nist.gov/glossary/term/
              strength_of_function>.

   [TCG-DICE] Trusted Computing Group, "DICE Certificate Profiles",
              n.d., <https://trustedcomputinggroup.org/wp-
              content/uploads/DICE-Certificate-Profiles-
              r01_3june2020-1.pdf>.

   [TCGarch]  Trusted Computing Group, "Trusted Platform Module Library
              - Part 1: Architecture", 8 November 2019,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>.

   [WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
              Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>.

Contributors

   Monty Wiseman

   Email: montywiseman32@gmail.com


   Liang Xia

   Email: frank.xialiang@huawei.com


   Laurence Lundblade

   Email: lgl@island-resort.com


   Eliot Lear

   Email: elear@cisco.com


   Jessica Fitzgerald-McKay





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   Sarah C. Helbe


   Andrew Guinn


   Peter Loscocco

   Email: pete.loscocco@gmail.com


   Eric Voit


   Thomas Fossati

   Email: thomas.fossati@arm.com


   Paul Rowe


   Carsten Bormann

   Email: cabo@tzi.org


   Giri Mandyam

   Email: mandyam@qti.qualcomm.com


   Kathleen Moriarty

   Email: kathleen.moriarty.ietf@gmail.com


   Guy Fedorkow

   Email: gfedorkow@juniper.net


   Simon Frost

   Email: Simon.Frost@arm.com






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Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany

   Email: henk.birkholz@sit.fraunhofer.de


   Dave Thaler
   Microsoft
   United States of America

   Email: dthaler@microsoft.com


   Michael Richardson
   Sandelman Software Works
   Canada

   Email: mcr+ietf@sandelman.ca


   Ned Smith
   Intel Corporation
   United States of America

   Email: ned.smith@intel.com


   Wei Pan
   Huawei Technologies

   Email: william.panwei@huawei.com















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