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Versions: (draft-thaler-rats-architecture) 00 01 02 05

RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 D. Thaler
Expires: 11 January 2021                                       Microsoft
                                                           M. Richardson
                                                Sandelman Software Works
                                                                N. Smith
                                                                   Intel
                                                                  W. Pan
                                                     Huawei Technologies
                                                            10 July 2020


               Remote Attestation Procedures Architecture
                    draft-ietf-rats-architecture-05

Abstract

   In network protocol exchanges, it is often the case that one entity
   (a Relying Party) requires evidence about a remote peer to assess the
   peer's trustworthiness, and a way to appraise such evidence.  The
   evidence is typically a set of claims about its software and hardware
   platform.  This document describes an architecture for such remote
   attestation procedures (RATS).

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
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   This Internet-Draft will expire on 11 January 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Reference Use Cases . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Network Endpoint Assessment . . . . . . . . . . . . . . .   5
     3.2.  Confidential Machine Learning (ML) Model Protection . . .   6
     3.3.  Confidential Data Retrieval . . . . . . . . . . . . . . .   6
     3.4.  Critical Infrastructure Control . . . . . . . . . . . . .   6
     3.5.  Trusted Execution Environment (TEE) Provisioning  . . . .   7
     3.6.  Hardware Watchdog . . . . . . . . . . . . . . . . . . . .   7
   4.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Appraisal Policies  . . . . . . . . . . . . . . . . . . .   9
     4.2.  Two Types of Environments of an Attester  . . . . . . . .   9
     4.3.  Layered Attestation Environments  . . . . . . . . . . . .  10
     4.4.  Composite Device  . . . . . . . . . . . . . . . . . . . .  12
   5.  Topological Models  . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Passport Model  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Background-Check Model  . . . . . . . . . . . . . . . . .  16
     5.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Roles and Entities  . . . . . . . . . . . . . . . . . . . . .  18
   7.  Trust Model . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Relying Party . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Attester  . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.3.  Relying Party Owner . . . . . . . . . . . . . . . . . . .  20
     7.4.  Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.5.  Endorser and Verifier Owner . . . . . . . . . . . . . . .  21



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   8.  Conceptual Messages . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.2.  Endorsements  . . . . . . . . . . . . . . . . . . . . . .  21
     8.3.  Attestation Results . . . . . . . . . . . . . . . . . . .  22
   9.  Claims Encoding Formats . . . . . . . . . . . . . . . . . . .  23
   10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . .  25
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  27
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  28
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   16. Appendix A: Time Considerations . . . . . . . . . . . . . . .  29
     16.1.  Example 1: Timestamp-based Passport Model Example  . . .  30
     16.2.  Example 2: Nonce-based Passport Model Example  . . . . .  32
     16.3.  Example 3: Timestamp-based Background-Check Model
            Example  . . . . . . . . . . . . . . . . . . . . . . . .  33
     16.4.  Example 4: Nonce-based Background-Check Model Example  .  33
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     17.2.  Informative References . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   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 documents 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 models and the 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 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 models, and
   appraisal is vital for semantic interoperability across solutions and
   platforms involving multiple vendors and providers.






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   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 in order to choose appropriate solutions to compose
   their Remote Attestation Procedures.

2.  Terminology

   This document uses the following terms.

   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]

   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]

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

   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

   Claim:  A piece of asserted information, often in the form of a name/
      value pair.  (Compare /claim/ in [RFC7519])

   Endorsement:  A secure statement that some entity (typically a
      manufacturer) vouches for the integrity of an Attester's signing
      capability

   Endorser:  An entity (typically a manufacturer) whose Endorsements
      help Verifiers appraise the authenticity of Evidence

   Evidence:  A set of information about an Attester that is to be
      appraised by a Verifier.  Evidence may include configuration data,
      measurements, telemetry, or inferences.

   Relying Party:  A role performed by an entity that depends on the




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      validity of information about an Attester, for purposes of
      reliably applying application specific actions.  Compare /relying
      party/ in [RFC4949]

   Relying Party Owner:  An entity (typically an administrator), that is
      authorized to configure Appraisal Policy for Attestation Results
      in a Relying Party

   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

   Verifier Owner:  An entity (typically an administrator), that is
      authorized to configure Appraisal Policy for Evidence in a
      Verifier

3.  Reference Use Cases

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

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

3.1.  Network Endpoint Assessment

   Network operators want a trustworthy report that includes identity
   and version of information of 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
   their 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 provides device identity and protected storage for
   measurements.  The system components perform a series of measurements
   that may be signed by the Root of Trust, considered as Evidence about
   the hardware, firmware, BIOS, software, etc. that is running.




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   Attester:  A device desiring access to a network

   Relying Party:  A network infrastructure device such as a router,
      switch, or access point

3.2.  Confidential Machine Learning (ML) Model Protection

   A device manufacturer wants to protect its intellectual property.
   This is primarily the ML model it developed and runs in the devices
   purchased by its customers.  The goals for the protection include
   preventing attackers, potentially the customer themselves, from
   seeing the details of the model.

   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 the Attester already has in encrypted form, to the
   requester.

   Attester:  A device desiring to run an ML model to do inferencing

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

3.3.  Confidential Data Retrieval

   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.
   An assessment of system state is made against a set of policies to
   evaluate the state of a system using attestations for the system
   requesting data.  Attestation is desired to prevent leaking data to
   compromised devices.

   Attester:  An entity desiring to retrieve confidential data

   Relying Party:  An entity that holds confidential data for retrieval
      by other entities

3.4.  Critical Infrastructure Control

   In this use case, potentially dangerous physical equipment (e.g.,
   power grid, traffic control, hazardous chemical processing, etc.) is
   connected to a network.  The organization managing such
   infrastructure needs to ensure that only authorized code and users
   can control such processes, and they are protected from malware or
   other adversaries.  When a protocol operation can affect some



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   critical system, the device attached to the critical equipment thus
   wants some assurance that the requester has not been compromised.  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
      dangerous physical equipment (hazardous chemical processing,
      traffic control, power grid, etc.)

3.5.  Trusted Execution Environment (TEE) Provisioning

   A "Trusted Application Manager (TAM)" server is responsible for
   managing the applications running in the TEE of a client device.  To
   do this, the TAM wants to assess the state of a TEE, or of
   applications in the TEE, of a client device.  The TEE conducts a
   remote attestation procedure with the TAM, which can then decide
   whether the TEE is already in compliance with the TAM's latest
   policy, or if the TAM needs 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 trusted execution environment capable of
      running trusted applications that can be updated

   Relying Party:  A Trusted Application Manager

3.6.  Hardware Watchdog

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

   In the case, the Relying Party is the watchdog timer in the TPM/
   secure enclave itself, as described in [TCGarch] section 43.3.  The
   Attestation Results are returned to the device, and provided to the
   enclave.

   If the watchdog does not receive regular, and fresh, Attestation
   Results as to the systems' health, then it forces a reboot.

   Attester:  The device that is desired to keep from being held hostage
      for a long period of time

   Relying Party:  A remote server that will securely grant the Attester



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      permission to continue operating (i.e., not reboot) for a period
      of time

4.  Architectural Overview

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

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

                       Figure 1: Conceptual Data Flow

   An Attester creates Evidence that is conveyed to a Verifier.

   The Verifier uses the Evidence, and any Endorsements from Endorsers,
   by applying an Evidence Appraisal Policy to assess the
   trustworthiness of the Attester, and generates Attestation Results
   for use by Relying Parties.  The Appraisal Policy for Evidence might
   be obtained from an Endorser along with the Endorsements, or might be
   obtained via some other mechanism such as being configured in the
   Verifier by an administrator.






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   The Relying Party uses Attestation Results by applying its own
   Appraisal Policy to make application-specific decisions such as
   authorization decisions.  The Appraisal Policy for Attestation
   Results might, for example, be configured in the Relying Party by an
   administrator.

4.1.  Appraisal Policies

   The Verifier, when appraising Evidence, or the Relying Party, when
   appraising Attestation Results, checks the values of some claims
   against constraints specified in its Appraisal Policy.  Such
   constraints might involve a 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, or any other test.

   Such reference values might be specified as part of the Appraisal
   Policy itself, or might be obtained from a separate source, such as
   an Endorsement, and then used by the Appraisal Policy.

   The actual data format and semantics of any reference values are
   specific to claims and implementations.  This architecture document
   does not define any general purpose format for them or general means
   for comparison.

4.2.  Two Types of Environments of an Attester

   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 4.3 and Section 4.4.
   Other examples may exist, and the examples discussed could even be
   combined into even more complex implementations.

   Claims are collected from Target Environments, as shown in Figure 2.
   That is, Attesting Environments collect the raw values and the
   information to be represented in claims, such as by doing some
   measurement of a Target Environment's code, memory, and/or registers.
   Attesting Environments then format the claims appropriately, and
   typically use key material and cryptographic functions, such as
   signing or cipher algorithms, to create Evidence.  Places that
   Attesting Environments can exist include Trusted Execution
   Environments (TEE), embedded Secure Elements (eSE), and BIOS
   firmware.  An execution environment may not, by default, be capable
   of claims collection for a given Target Environment.  Execution
   environments that are designed to be capable of claims collection are
   referred to in this document as Attesting Environments.



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

                    Figure 2: Two Types of Environments

4.3.  Layered Attestation Environments

   By definition, the Attester role creates Evidence.  An Attester may
   consist of one or more nested or staged environments, adding
   complexity to the architectural structure.  The unifying component is
   the Root of Trust and the nested, staged, or chained attestation
   Evidence produced.  The nested or chained structure includes Claims,
   collected by the Attester to aid in the assurance or believability of
   the attestation Evidence.

   Figure 3 depicts an example of a device that includes (A) a BIOS
   stored in read-only memory in this example, (B) an updatable
   bootloader, and (C) an operating system kernel.








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               .----------.                    .----------.
               |          |                    |          |
               | Endorser |------------------->| Verifier |
               |          |    Endorsements    |          |
               '----------'  for A, B, and C   '----------'
                                                     ^
           .------------------------------------.    |
           |                                    |    |
           |   .---------------------------.    |    |
           |   | Target                    |    |    | Layered
           |   | Environment               |    |    | Evidence
           |   | C                         |    |    |   for
           |   '---------------------------'    |    | B and C
           |           Collect |                |    |
           |           claims  |                |    |
           |   .---------------|-----------.    |    |
           |   | Target        v           |    |    |
           |   | Environment .-----------. |    |    |
           |   | B           | Attesting | |    |    |
           |   |             |Environment|-----------'
           |   |             |     B     | |    |
           |   |             '-----------' |    |
           |   |                     ^     |    |
           |   '---------------------|-----'    |
           |           Collect |     | Evidence |
           |           claims  v     |  for B   |
           |                 .-----------.      |
           |                 | Attesting |      |
           |                 |Environment|      |
           |                 |     A     |      |
           |                 '-----------'      |
           |                                    |
           '------------------------------------'

                         Figure 3: Layered Attester

   Attesting Environment A, the read-only BIOS in this example, has to
   ensure the integrity of the bootloader (Target Environment B).  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, these Claims 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 Target Environment B now becomes (or
   contains) an Attesting Environment for the next layer.  This



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   procedure in Layered Attestation is sometimes called "staging".  It
   is important that the new Attesting Environment B not be able to
   alter any Claims about its own Target Environment B.  This can be
   ensured having those Claims be either signed by Attesting Environment
   A or stored in an untamperable manner by Attesting Environment A.

   Continuing with this example, the bootloader's Attesting Environment
   B is now in charge of collecting Claims about Target Environment C,
   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 in Figure 3.

4.4.  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-
   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.














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   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.  The multi-chassis router provides higher
   throughput by interconnecting multiple routers and can be logically
   treated as one router for simpler management.  A multi-chassis router
   provides a management point that connects to the Verifier.  Other
   routers are only connected to the main router by the network cables,
   and therefore they are managed and appraised via this main router's
   help.  So, in this case, the multi-chassis router is the 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: Conceptual Data Flow for a Composite Device

   In the 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 the Evidence of all other
   Attesters and then generates the Evidence of the whole Composite
   Attester.

   An entity can take on multiple RATS roles (e.g., Attester, Verifier,
   Relying Party, etc.) at the same time.  The combination of roles can
   be arbitrary.  For example, in this Composite Device scenario, the
   entity inside the lead Attester can also take on the role of a
   Verifier, and the outside entity of 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 any other Verifier) in the verification
   process to generate Attestation Results.  The inside Verifier then
   conveys the Attestation Results of other Attesters, whether in the
   same conveyance protocol as the Evidence or not, to the outside
   Verifier.

   In this situation, the trust model described in Section 7 is also
   suitable for this inside Verifier.



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5.  Topological Models

   Figure 1 shows a basic model 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 gets the
   Attestation Result from the Verifier.  There are multiple other
   possible models.  This section includes some reference models.  This
   is not intended to be a restrictive list, and other variations may
   exist.

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 to a Relying Party, which then compares the Attestation Result
   against its own Appraisal Policy.

   There are three ways in which the process may fail.  First, the
   Verifier may refuse to issue the Attestation Result due to some error
   in processing, or some missing input to the Verifier.  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,
   the result does not pass the policy.  The third way is when the
   Verifier is unreachable.

   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.







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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 a former employer or government agency 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 and so the serialization format of the
   Attestation Result is still important.  If the Relying Party is a



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   constrained node whose purpose is to serve a given type resource
   using a standard resource access protocol, it already needs the
   parser(s) required by that existing protocol.  Hence, the ability to
   let the Relying Party obtain an Attestation Result in the same
   serialization format allows minimizing the code footprint and attack
   surface area of the Relying Party, especially 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 is generally
   up to the Relying Party.  The same device may need to create Evidence
   for different Relying Parties and/or different use cases.  For
   instance, it would provide Evidence to a network infrastructure
   device to gain access to the network, and 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.  These collapsed roles combine the duties of multiple
   roles.

   In these cases, interaction between these roles do not necessarily
   use the Internet Protocol.  They can be using 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 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.









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   For example, an entity that both connects to a wide-area network and
   to a system bus is taking on both the Attester and Verifier roles.
   As a system bus entity, a Verifier consumes Evidence from other
   devices connected to the system bus that implement Attester roles.
   As a wide-area network connected entity, it may implement an Attester
   role.  The entity, as a system bus Verifier, may choose to fully
   isolate its role as a wide-area network Attester.

   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

   The scope of this document is 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 chain.

   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.

   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.







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7.2.  Attester

   In some scenarios, Evidence might contain sensitive information such
   as Personally 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 rules that it controls.
   In the background-check model, this Evidence may also be revealed to
   Relying Party(s).

   In some cases where Evidence contains sensitive information, an
   Attester might even require that a Verifier first go through a remote
   attestation procedure with it before the Attester will send 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,
   mutual attestation 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.

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.  In
   solutions with weaker security, a Verifier might be configured to
   implicitly trust firmware or even software (e.g., a hypervisor).
   That is, it 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
   hypervisor or firmware 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.  The component that is implicitly trusted is
   often referred to as a Root of Trust.

   A conveyance protocol that provides authentication and integrity
   protection can be used to convey unprotected Evidence, assuming the
   following properties exists:




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   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.

7.5.  Endorser and Verifier Owner

   In some scenarios, the Endorser and Verifier Owner may need to trust
   the Verifier before giving the Endorsement and Appraisal Policy to
   it.  This can be done similarly to how a Relying Party might
   establish trust in a Verifier as discussed above, and in such a case,
   mutual attestation might even be needed as discussed in Section 7.3.

8.  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 evaluated 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



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   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 known-good
   reference values, or an Appraisal Policy might have a set of more
   complex logic on how to appraise the validity of information.

   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 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.  Attestation Results

   Attestation Results may indicate compliance or non-compliance with a
   Verifier's Appraisal Policy.  A 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.

   An Attestation Result that indicates compliance can be used by a
   Relying Party to make authorization decisions based on the Relying
   Party's Appraisal Policy.  The simplest such policy might be to
   simply authorize any party supplying a compliant Attestation Result
   signed by a trusted Verifier.  A more complex policy might also
   entail comparing information provided in the result against known-
   good reference values, or applying more complex logic on such
   information.












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   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.

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, etc.), depending on the use case.  Such
   protocols typically already have mechanisms for passing security
   information for purposes of authentication and authorization.  Common
   formats include JWTs [RFC7519], CWTs [RFC8392], and X.509
   certificates.














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   To enable remote attestation to be added to existing protocols,
   enabling a higher level of assurance against malware for example, it
   is important that information needed for appraising the Attester be
   usable with existing protocols that have constraints around what
   formats they can transport.  For example, OPC UA [OPCUA] (probably
   the most common protocol in industrial IoT environments) is defined
   to carry X.509 certificates and so security information must be
   embedded into an X.509 certificate to be passed in the protocol.
   Thus, remote attestation related information could be natively
   encoded in X.509 certificate extensions, or could be natively encoded
   in some other format (e.g., a CWT) which in turn is then encoded in
   an X.509 certificate extension.

   Especially for constrained nodes, however, 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
   area.  So 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 the format that is natural for 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 each have multiple possible encoding formats, so that
   they can be conveyed by various existing protocols.  It also
   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.


















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                   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 remote entity (Verifier or Relying Party) may 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 a policy defined by the consuming
   entity, Verifier or Relying Party, 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 Policy for Evidence, 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.  Freshness 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).

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





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   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 be added on a per-Claim basis, to distinguish the time of
   creation of Evidence or Attestation Result from the time that a
   specific Claim was generated.  The clock's trustworthiness typically
   requires additional Claims about the signer's time synchronization
   mechanism.

   A second approach places the onus of timekeeping solely on the
   appraising entity, i.e., 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 reliable clock or time
   synchronisation 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.

   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.  In
   another setup where all Roles (Attesters, Verifiers and Relying
   Parties) share the same broadcast channel, the nonce-based approach
   may be used to anchor all parties to the same (relative) timeline,
   without requiring synchronized clocks, by having a central entity
   emit nonces at regular intervals and have the "current" nonce
   included in the produced Evidence or Attestation Result.

   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.



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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 any users the device is associated with.  In many
   cases, the whole point of the Attestation process 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 Attestation Evidence and Attestation Results are
   potentially PII (Personally Identifying Information) depending on the
   end-to-end use case of the attestation.  Attestation that goes up to
   include containers and applications may further reveal details about
   a specific system or user.

   In some cases, an attacker may be able to make inferences about
   attestations from the results 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 data structures are expected to
   support integrity protection encoding (e.g., COSE, JOSE, X.509) and
   optionally might support confidentiality protection (e.g., COSE,
   JOSE).  Therefore, if confidentiality protection is omitted or
   unavailable, the protocols that convey Evidence or Attestation
   Results are responsible for detailing what kinds of information are
   disclosed, and to whom they are exposed.

   Furthermore, because Evidence might contain sensitive 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.

12.  Security Considerations

   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:



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   *  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.  As such, if Appraisal Policies for a Relying Party or for
   a Verifier can be configured via a network protocol, the ability to
   create Evidence about the integrity of the entity providing the
   Appraisal Policy needs to be considered.

   The security of conveyed information may be applied at different
   layers, whether by a conveyance protocol, or an information encoding
   format.  This architecture expects attestation messages (i.e.,
   Evidence, Attestation Results, Endorsements and Policies) are 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.

13.  IANA Considerations

   This document does not require any actions by IANA.

14.  Acknowledgments

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






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15.  Contributors

   Thomas Hardjono created older 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

   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 Coordinated
   Universal Time, or might be defined relative to some other timestamp
   or timeticks counter.

   +====+==============+===============================================+
   | ID | Event        | Explanation of event                          |
   +====+==============+===============================================+
   | VG | Value        | A value to appear in a Claim was              |
   |    | generation   | created.                                      |
   +----+--------------+-----------------------------------------------+
   | AA | Attester     | An Attesting Environment starts to            |
   |    | awareness    | be aware of a new/changed Claim               |
   |    |              | value.                                        |
   +----+--------------+-----------------------------------------------+
   | HD | Handle       | A centrally generated identifier for          |
   |    | distribution | time-bound recentness across a                |
   |    |              | domain of devices is successfully             |
   |    |              | distributed to Attesters.                     |
   +----+--------------+-----------------------------------------------+
   | 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          |
   |    | relayed      | another entity.                               |
   +----+--------------+-----------------------------------------------+
   | EG | Evidence     | An Attester creates Evidence from             |
   |    | generation   | collected Claims.                             |
   +----+--------------+-----------------------------------------------+
   | ER | Evidence     | A Relying Party relays Evidence to a          |
   |    | relayed      | Verifier.                                     |
   +----+--------------+-----------------------------------------------+
   | RG | Result       | A Verifier appraises Evidence and             |
   |    | generation   | generates an Attestation Result.              |



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   +----+--------------+-----------------------------------------------+
   | RR | Result       | A Relying Party relays an                     |
   |    | relayed      | Attestation Result to a Relying               |
   |    |              | Party.                                        |
   +----+--------------+-----------------------------------------------+
   | RA | Result       | The Relying Party appraises                   |
   |    | appraised    | Attestation Results.                          |
   +----+--------------+-----------------------------------------------+
   | OP | Operation    | The Relying Party performs some               |
   |    | performed    | operation requested by the Attester.          |
   |    |              | For example, acting upon some                 |
   |    |              | message just received across a                |
   |    |              | session created earlier at time(RA).          |
   +----+--------------+-----------------------------------------------+
   | RX | Result       | An Attestation Result should no               |
   |    | expiry       | longer be 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. a solution might be
   built.  This list is not intended to be complete, but is just
   representative enough to highlight various timing considerations.

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.



















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      .----------.                     .----------.  .---------------.
      | Attester |                     | Verifier |  | Relying Party |
      '----------'                     '----------'  '---------------'
        time(VG)                             |               |
           |                                 |               |
           ~                                 ~               ~
           |                                 |               |
        time(EG)                             |               |
           |------Evidence{time(EG)}-------->|               |
           |                              time(RG)           |
           |<-----Attestation Result---------|               |
           |      {time(RG),time(RX)}        |               |
           ~                                                 ~
           |                                                 |
           |------Attestation Result{time(RG),time(RX)}-->time(RA)
           |                                                 |
           ~                                                 ~
           |                                                 |
           |                                              time(OP)
           |                                                 |

   The Verifier can check whether the Evidence is fresh when appraising
   it at time(RG) by checking "time(RG) - time(EG) < 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) 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) value, the Verifier can also determine whether the
   claim value is recent by checking "time(RG) - time(VG) < Threshold",
   again where the threshold is large enough to account for the maximum
   permitted clock skew between the Verifier and the Attester.

   The Relying Party can check whether the Attestation Result is fresh
   when appraising it at time(RA) by checking "time(RA) - time(RG) <
   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)).  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)) as long as "time(OP) - time(RG) < Threshold".  However, if
   the Attestation Result contains an expiry time time(RX) then it could
   explicitly check "time(OP) < time(RX)".






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16.2.  Example 2: Nonce-based Passport Model Example

   The following example illustrates a hypothetical Passport Model
   solution that uses nonces and thus does not require that any clocks
   are synchronized.

      .----------.                     .----------.  .---------------.
      | Attester |                     | Verifier |  | Relying Party |
      '----------'                     '----------'  '---------------'
        time(VG)                             |               |
           |                                 |               |
           ~                                 ~               ~
           |                                 |               |
           |<---Nonce1--------------------time(NS)           |
        time(EG)                             |               |
           |----Evidence-------------------->|               |
           |     {Nonce1, time(EG)-time(VG)} |               |
           |                              time(RG)           |
           |<---Attestation Result-----------|               |
           |     {time(RX)-time(RG)}         |               |
           ~                                                 ~
           |                                                 |
           |<---Nonce2------------------------------------time(NS')
        time(RR)
           |----Attestation Result{time(RX)-time(RG)}---->time(RA)
           |    Nonce2, time(RR)-time(EG)                    |
           ~                                                 ~
           |                                                 |
           |                                              time(OP)

   In this example solution, the Verifier can check whether the Evidence
   is fresh at time(RG) by verifying that "time(RG) - time(NS) <
   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)-time(VG), then it can determine
   recency by checking "time(RG)-time(NS) + time(EG)-time(VG) <
   Threshold".










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   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) - time(NS') +
   time(RR)-time(EG) < Threshold".  Although the Nonce2 and time(RR)-
   time(EG) 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
   validity lifetime in terms of time(RX)-time(RG), then the Relying
   Party can check "time(OP) - time(NS') < time(RX)-time(RG)".

16.3.  Example 3: 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.

   .----------.         .---------------.              .----------.
   | Attester |         | Relying Party |              | Verifier |
   '----------'         '---------------'              '----------'
     time(VG)                   |                           |
           |                    |                           |
           ~                    ~                           ~
           |                    |                           |
     time(EG)                   |                           |
           |----Evidence------->|                           |
           |    {time(EG)}   time(ER)--Evidence{time(EG)}-->|
           |                    |                        time(RG)
           |                 time(RA)<-Attestation Result---|
           |                    |        {time(RX)}         |
           ~                    ~                           ~
           |                    |                           |
           |                 time(OP)                       |

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

16.4.  Example 4: 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.



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   .----------.         .---------------.              .----------.
   | Attester |         | Relying Party |              | Verifier |
   '----------'         '---------------'              '----------'
     time(VG)                   |                           |
        |                       |                           |
        ~                       ~                           ~
        |                       |                           |
        |                       |<-----Nonce-------------time(NS)
        |<---Nonce-----------time(NR)                       |
     time(EG)                   |                           |
        |----Evidence{Nonce}--->|                           |
        |                    time(ER)--Evidence{Nonce}----->|
        |                       |                        time(RG)
        |                    time(RA)<-Attestation Result---|
        |                       |      {time(RX)-time(RG)}  |
        ~                       ~                           ~
        |                       |                           |
        |                    time(OP)                       |

   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) - time(NR) < 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)-time(RG), then the Relying
   Party can check "time(OP) - time(ER) < time(RX)-time(RG)".

17.  References

17.1.  Normative References

   [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/info/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/info/rfc8392>.

17.2.  Informative References






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

   [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/info/rfc8322>.

   [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/>.

   [I-D.birkholz-rats-tuda]
              Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
              "Time-Based Uni-Directional Attestation", Work in
              Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9
              March 2020, <http://www.ietf.org/internet-drafts/draft-
              birkholz-rats-tuda-02.txt>.

   [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-11, 2 July 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-teep-
              architecture-11.txt>.

   [TCGarch]  Trusted Computing Group, "Trusted Platform Module Library
              - Part 1: Architecture", 7 July 2020,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>.

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany

   Email: henk.birkholz@sit.fraunhofer.de







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   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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