Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-02

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Document Type Active Internet-Draft (rats WG)
Authors Henk Birkholz  , Dave Thaler  , Michael Richardson  , Ned Smith  , Wei Pan 
Last updated 2020-03-07
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RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 D. Thaler
Expires: 8 September 2020                                      Microsoft
                                                           M. Richardson
                                                Sandelman Software Works
                                                                N. Smith
                                                                   Intel
                                                                  W. Pan
                                                     Huawei Technologies
                                                            7 March 2020

               Remote Attestation Procedures Architecture
                    draft-ietf-rats-architecture-02

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 September 2020.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Reference Use Cases . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Network Endpoint Assessment . . . . . . . . . . . . . . .   5
     3.2.  Confidential Machine Learning (ML) Model Protection . . .   5
     3.3.  Confidential Data Retrieval . . . . . . . . . . . . . . .   6
     3.4.  Critical Infrastructure Control . . . . . . . . . . . . .   6
     3.5.  Trusted Execution Environment (TEE) Provisioning  . . . .   6
     3.6.  Hardware Watchdog . . . . . . . . . . . . . . . . . . . .   7
   4.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   7
     4.1.  Two Types of Environments of an Attester  . . . . . . . .   9
     4.2.  Layered Attestation Procedures  . . . . . . . . . . . . .   9
     4.3.  Composite Device  . . . . . . . . . . . . . . . . . . . .  12
   5.  Topological Models  . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Passport Model  . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  Background-Check Model  . . . . . . . . . . . . . . . . .  15
     5.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Trust Model . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Conceptual Messages . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.2.  Endorsements  . . . . . . . . . . . . . . . . . . . . . .  19
     7.3.  Attestation Results . . . . . . . . . . . . . . . . . . .  19
   8.  Claims Encoding Formats . . . . . . . . . . . . . . . . . . .  20
   9.  Freshness . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23

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   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  24
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  24
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     15.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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 with corresponding
   roles and their interaction 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
   role compositions and data flows, such as the "Passport Model" and
   the "Background-Check Model" are illustrated to enable readers of
   this document to map their current and emerging solutions to the
   architecture provided and the corresponding terminology defined.  A
   common terminology that provides a well-understood semantic meaning
   to the concepts, roles, and models in this document is vital to
   create semantic interoperability between solutions and across
   different platforms.

   Amongst other things, this document is about trust and
   trustworthiness.  Trust is a decision being made.  Trustworthiness is
   a quality that is assessed via evidence created.  This is a 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 direct how a
      Verifier evaluates the validity of information about an Attester.
      Compare /security policy/ in [RFC4949]

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   Appraisal Policy for Attestation Result:  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:  An entity whose attributes must be appraised in order to
      determine whether the entity is considered trustworthy, such as
      when deciding whether the entity is authorized to perform some
      operation

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

   Endorser:  An entity that creates Endorsements that can be used to
      help to appraise the trustworthiness of Attesters

   Evidence:  A set of information about an Attester that is to be
      appraised by a Verifier

   Relying Party:  An entity that depends on the validity of information
      about another entity, typically for purposes of authorization.
      Compare /relying party/ in [RFC4949]

   Relying Party Owner:  An entity, such as an administrator, that is
      authorized to configure Appraisal Policy for Attestation Results
      in a Relying Party.

   Verifier:  An entity that appraises the validity of Evidence about an
      Attester

   Verifier Owner:  An entity, such as 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.

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   Each use case includes a description, and a summary of what an
   Attester and a Relying Party refer to in the use case.

3.1.  Network Endpoint Assessment

   Network operators want a trustworthy report of identity and version
   of information of the hardware and software on the machines attached
   to their network, for purposes such as inventory, auditing, and/or
   logging.  The network operator may also want a policy by which full
   access is only granted to devices that meet some definition of
   health, 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.  These components perform a series of measurements, and
   express this with Evidence as to the hardware and firmware/software
   that is running.

       FIXME from Henk: Measurements at early stages of
       Layered Attestation are NOT evidence yet.
       This text does not cover that yet

   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 in
   terms of the ML model it developed and that runs in the devices that
   its customers purchased, and it wants to prevent attackers,
   potentially including the customer themselves, from seeing the
   details of the model.

   This typically works by having some protected environment in the
   device attest to some manufacturer service.  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

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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.
   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
   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 attests to 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

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3.6.  Hardware Watchdog

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

   A hardware watchdog can be implemented by forcing a reboot unless
   remote attestation to a server succeeds within a periodic interval,
   and having the reboot do remediation by bringing a device into
   compliance, including installation of patches as needed.

   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
      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.

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                 ************   ************    *****************
                 * Endorser *   * Verifier *    * Relying Party *
                 ************   *  Owner   *    *  Owner        *
                       |        ************    *****************
                       |              |                 |
           Endorsements|              |                 |
                       |              |Appraisal        |
                       |              |Policy for       |
                       |              |Evidence         | Appraisal
                       |              |                 | Policy for
                       |              |                 | Attestation
                       |              |                 |  Result
                       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 Evidence Appraisal Policy 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.

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

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4.1.  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 4.2 and Section 4.3.
   Other examples may exist, and the examples discussed could even be
   combined into even more complex implementations.

   Claims are collected from Target Environments.  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.  Examples of environments that can be used as
   Attesting Environments include Trusted Execution Environments (TEE),
   embedded Secure Elements (eSE), or Hardware Security Modules (HSM).

4.2.  Layered Attestation Procedures

   By definition, the Attester role takes on the duty to create
   Evidence.  The fact that an Attester role is composed of several
   types of environments that can be nested or staged adds complexity to
   the architectural layout of how an Attester - in itself - is composed
   and therefore has to conduct the Claims collection in order to create
   believable Attestation Evidence.  The following example is intended
   to illustrate this composition:

   A very common example is elaborated on to illustrate Layered
   Attestation.

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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 2: Layered Attester

   The very first Attesting Environment has to ensure the integrity of
   the (U)EFI / BIOS / Firmware that initially boots up a composite
   device (e.g., a cell phone).

       Henk: we are looking for a better term than UEFI/BIOS/Firmware

   These Claims have to be measured securely.  At this stage of the
   boot-cycle of a composite device, the Claims collected typically
   cannot be composed into Evidence.

   The very first Attesting Environment in this example can be a
   hardware component that is a Static Code Root of Trust.  As in any

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   other scenario, this hardware component is the first Attesting
   Environment.  It collects a rather concise number of Claims about the
   Target Environment.  The Target Environment in this example is the
   (U)EFI / BIOS / Firmware After the boot sequence started, the Target
   Environment conducts the most important and defining feature of
   Layered Attestation: The successfully measured environment that is
   the (U)EFI / BIOS / Firmware now becomes the Attesting Environment.
   Analogously, the Attesting Environment hands off its duty to one of
   its Target Environments.  This procedure in Layered Attestation is
   called Staging.

   Now, the duties have been transferred and Layered Attestation takes
   place.  The initial Attesting Environment relinquishes its duties to
   the Target Environment.  It is important to note that the new
   Attesting Environment cannot alter the content about its own
   measurements.  If the Attesting Environment would be able to do that,
   Layered Attestation would become unfeasible.

   In this example the duty of being the Attesting Environment is now
   taken over by the (U)EFI / BIOS / Firmware that was the Attested
   Environment before.  This transfer of duty is the essential part of
   Layered Attestation.  The (U)EFI / BIOS / Firmware now is the
   Attesting Environment.  The next Target Environment is, in this
   example, a bootloader.  There are potentially multiple kernels to
   boot, the decision is up to the bootloader.  Only a bootloader with
   intact integrity will make an appropriate decision.  Therefore,
   Claims about the integrity of a bootloader are now collected by the
   freshly appointed Attesting Environment that is the (U)EFI / BIOS /
   Firmware.  Collected Claims have to be stored by the current
   Attesting Environment in a similar shielded and secured manner, so
   that the next Attesting Environment is not capable of altering the
   collection of claims stored.

   Continuing with this example, the bootloader is now in charge of
   collecting Claims about the next execution environment.  The next
   execution environment in this example is the kernel to be booted up.
   Analogously, the next transfer of duties in this Layered Attestation
   example occurs: The duty of being an Attesting Environment is
   transferred to a successfully measured kernel.  In this sequence, the
   kernel is now collecting additional Claims and is storing them in a
   secure and shielded manner.

       [Henk: we might have to define what successful
       means in this example and beyond]

   The essence of this example is a cascade of staged boot environments.
   Each environment (after the initial one that is a root-of-trust) has
   the duty of measuring its next environment before it is started.

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   Therefore, creating a layered boot sequence and correspondingly
   enabling Layered Attestation.

4.3.  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.

   For example, a carrier-grade router is 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 transiting 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.  Among these routers,
   there is only one main router 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 3 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 3: 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 verifies them using Endorsements and Appraisal
   Policies (obtained the same way as any other Verifier), to generate
   Attestation Results.  The inside Verifier then sends 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 6 is also
   suitable for this inside Verifier.

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5.  Topological Models

   There are multiple possible models for communication between an
   Attester, a Verifier, and a Relying Party.  This section includes
   some reference models, but this is not intended to be a restrictive
   list, and other variations may exist.

5.1.  Passport Model

   In this model, an Attester sends 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 resulting 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.

         +-------------+
         |             | Compare Evidence
         |   Verifier  | against Appraisal Policy
         |             |
         +-------------+
              ^    |
      Evidence|    |Attestation
              |    |  Result
              |    v
         +----------+              +---------+
         |          |------------->|         |Compare Attestation
         | Attester | Attestation  | Relying | Result against
         |          |    Result    |  Party  | Appraisal
         +----------+              +---------+  Policy

                          Figure 4: Passport Model

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   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.

5.2.  Background-Check Model

   In this model, an Attester sends 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
   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.

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                                  +-------------+
                                  |             | Compare Evidence
                                  |   Verifier  | against Appraisal
                                  |             | Policy
                                  +-------------+
                                       ^    |
                               Evidence|    |Attestation
                                       |    |  Result
                                       |    v
      +------------+               +-------------+
      |            |-------------->|             | Compare Attestation
      |   Attester |   Evidence    |   Relying   | Result against
      |            |               |    Party    | Appraisal Policy
      +------------+               +-------------+

                      Figure 5: 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.

5.3.  Combinations

   One variation of the background-check model is where the Relying
   Party and the Verifier on the same machine, and so 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, and the same device may need to create
   Evidence for different Relying Parties and different use cases (e.g.,
   a network infrastructure device to gain access to the network, and
   then a server holding confidential data to get access to that data).
   As such, both models may simultaneously be in use by the same device.

   Figure 6 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 5, 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

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   Trusted Application Manager plans to support in the TEEP architecture
   [I-D.ietf-teep-architecture].

         +-------------+
         |             | 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 6: Example Combination

6.  Trust Model

   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 the Verifying Relying Party
   combination).  Or, for a stronger level of security, the Relying
   Party might require that the Verifier itself provide information
   about itself that the Relying Party can use to assess the
   trustworthiness of the Verifier before accepting its Attestation
   Results.

   The Endorser and Verifier Owner may need to trust the Verifier before
   giving the Endorsement and Appraisal Policy to it.  Such trust can
   also be established directly or indirectly, implicitly or explicitly.
   One explicit way to establish such trust may be the Verifier first

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   acts as an Attester and creates Evidence about itself to be consumed
   by the Endorser and/or Verifier Owner as the Relying Parties.  If it
   is accepted as trustworthy, then they can provide Endorsements and
   Appraisal Policies that enable it to act as a 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, or 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 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.

   In some scenarios, Evidence might contain sensitive information such
   as Personally Identifiable Information.  Thus, an Attester must trust
   entities to which it sends Evidence, to not reveal sensitive data to
   unauthorized parties.  The Verifier might share this information with
   other authorized parties, according rules that it controls.  In the
   background-check model, this Evidence may also be revealed to Relying
   Party(s).

7.  Conceptual Messages

7.1.  Evidence

   Today, Evidence tends to be highly device-specific, since the
   information in the Evidence often includes vendor-specific
   information that is necessary to fully describe the manufacturer and
   model of the device including its security properties, the health of
   the device, and the level of confidence in the correctness of the
   information.  Evidence is typically signed by the device (whether by
   hardware, firmware, or software on the device), and its appraisal in
   isolation would require Appraisal Policy to be based on device-
   specific details (e.g., a device public key).

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7.2.  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 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.

7.3.  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.

8.  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 7: 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), 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.

   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

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   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.

                   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 8: Multiple Attesters and Relying Parties with Different
                                  Formats

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9.  Freshness

   It is important to prevent replay attacks where an attacker replays
   old Evidence or an old Attestation Result that is no longer correct.
   To do so, some mechanism of ensuring that the Evidence and
   Attestation Result are fresh, meaning that there is some degree of
   assurance that they still reflect the latest state of the Attester,
   and that any Attestation Result was generated using the latest
   Appraisal Policy for Evidence.  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 the time window to something the Verifier (for Evidence) or
   Relying Party (for an Attestation Result) is willing to accept.

   There are two common approaches to providing some assurance of
   freshness.  The first approach is that a nonce is generated by a
   remote entity (e.g., the Verifier for Evidence, or the Relying Party
   for an Attestation Result), and the nonce is then signed and included
   along with the claims in the Evidence or Attestation Result, so that
   the remote entity knows that the claims were signed after the nonce
   was generated.

   A second approach is to rely on synchronized clocks, and include a
   signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with
   the claims in the Evidence or Attestation Result, so that the remote
   entity knows that the claims were signed at that time, as long as it
   has some assurance that the timestamp is correct.  This typically
   requires additional claims about the signer's time synchronization
   mechanism in order to provide such assurance.

   In either approach, 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 might
   have been generated at boot, and then used in claims as long as the
   signer can guarantee that they cannot have changed since boot.

10.  Privacy Considerations

   The conveyance of Evidence and the resulting Attestation Results
   reveal a great deal of information about the internal state of a
   device.  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.

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   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.

11.  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, needs to support end-to-
   end integrity protection and replay attack prevention, and often also
   needs to support additional security protections.  For example,
   additional means of authentication, confidentiality, integrity,
   replay, denial of service and privacy protection are needed in many
   use cases.  Section 9 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.

12.  IANA Considerations

   This document does not require any actions by IANA.

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13.  Acknowledgments

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

14.  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.

15.  References

15.1.  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>.

15.2.  Informative References

   [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-01, 11
              September 2019, <http://www.ietf.org/internet-drafts/
              draft-birkholz-rats-tuda-01.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-06, 8 February 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-teep-
              architecture-06.txt>.

   [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/

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              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/info/rfc4949>.

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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