RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 D. Thaler
Expires: 22 November 2020                                      Microsoft
                                                           M. Richardson
                                                Sandelman Software Works
                                                                N. Smith
                                                                  W. Pan
                                                     Huawei Technologies
                                                             21 May 2020

               Remote Attestation Procedures Architecture


   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

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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 22 November 2020.

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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 . . .   5   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  . . . . . . . . . . . . . . . . . . .   7
     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  . . . . . . . . . . . . . . . . . . . . .  14  15
     5.1.  Passport Model  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Background-Check Model  . . . . . . . . . . . . . . . . .  16
     5.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Roles and Entities  . . . . . . . . . . . . . . . . . . . . .  18
   7.  Role Hosting and Composition  . . . . . . . . . . . . . . . .  19
   8.  Trust Model . . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  19
   8.  Conceptual Messages . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  20
     8.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.2.  Endorsements  . . . . . . . . . . . . . . . . . . . . . .  21
     8.3.  Attestation Results . . . . . . . . . . . . . . . . . . .  22

   9.  Claims Encoding Formats . . . . . . . . . . . . . . . . . . .  23
   11.  22
   10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  25
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  26
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  27
   16. Appendix A: Time Considerations . . . . . . . . . . . . . . .  27
     16.1.  Example 1: Timestamp-based Passport Model Example  . . .  29
     16.2.  Example 2: Nonce-based Passport Model Example  . . . . .  30
     16.3.  Example 3: Timestamp-based Background-Check Model
            Example  . . . . . . . . . . . . . . . . . . . . . . . .  31
     16.4.  Example 4: Nonce-based Background-Check Model Example  .  31
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     17.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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

   This documents defines a flexible architecture consisting of
   attestation with corresponding
   roles and their interactions 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
   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 enable readers of this document to
   map their solution architecture current and emerging solutions to the canonical attestation architecture provided here
   and to define useful the corresponding terminology for attestation.
   Having a defined.

   A common terminology that provides a well-understood meanings
   for common themes such as, semantic meaning
   to the concepts, roles, device composition, topological
   models and appraisal models in this document is vital for to
   create semantic interoperability across between solutions and platforms involving multiple vendors and providers. 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]

   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

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

   Attester:  An entity (typically a device), whose Evidence attributes must be appraised in order to infer the extent to which
      determine whether the Attester entity is considered trustworthy, such as
      when deciding whether it the entity is authorized to perform some

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

   Endorsement:  Statements  A secure statement that Endorsers make some entity (typically a
      manufacturer) that vouches for the design and implementation of
      the Attester.  Often this includes statements about the integrity of an Attester's signing

   Endorser:  An entity (typically a manufacturer) whose that creates Endorsements that can be used to
      help Verifiers to appraise the authenticity trustworthiness of Evidence Attesters

   Evidence:  A set of information that asserts the trustworthiness
      status of about an Attester, Attester that is to be
      appraised by a Verifier

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

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

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

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

   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.

   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

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

   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

   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

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.

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

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.  Attesting
   Environments are designed specifically with claims collection in

                    |                                |
                    |            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 takes on the duty to create
   Evidence.  The fact that an Attester role is composed of environments
   that can be nested or staged adds complexity to the architectural
   layout of how an Attester can be composed and therefore has to
   conduct the Claims collection in order to create believable
   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.

               .----------.                    .----------.
               |          |                    |          |
               | 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
   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, say, making the kernel
   become another Attesting Environment for an application as another
   Target Environment, resulting 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.

   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.  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

   Figure 4 depicts the conceptual data flow for a Composite Device.

                      |           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

5.  Topological Models

   Figure 1 shows a basic model for communication between an

   An entity can take on multiple RATS roles (e.g., Attester, a Verifier, and a
   Relying Party. Party, etc.) at the same time.  The Attester conveys its Evidence to 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 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.

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 Results from the Verifier.  There are multiple other
   possible models.  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 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 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 5: 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.

5.2.  Background-Check Model

   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
   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

   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 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

         |             | 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.  As a result, the entity can participate as
   a constituent of the RATS architecture.  Additionally, 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.

   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 also creates
   and consumes the corresponding conceptual messages as defined in this

7.  Role Hosting and Composition


   The RATS architecture includes the definition of Roles (e.g.
   Attester, Verifier, Relying Party, Endorser) and conceptual messages
   (e.g., Evidence, Attestation Results, Endorsements, Appraisal
   Policies) that captures canonical attestation behaviors, that are
   common to a broad range of attestation-enabled systems.  An entity
   that combines multiple Roles produces and consumes the associated
   Role Messages.

   The RATS architecture is not prescriptive about deployment
   configuration options of attestation-enabled systems, therefore the
   various Roles can be hosted on any participating entity.  This
   implies, for a given entity, that multiple Roles could be co-resident
   so that the duties of multiple roles could be performed
   simultaneously.  Nevertheless, the semantics of which Role Messages
   are inputs and outputs to a Role entity remains constant.  As a
   result, the entity can participate as a constituent of the RATS
   architecture while flexibly accommodating the needs of various
   deployment architectures.

   Interactions between Roles do not necessarily require use of Internet
   protocols.  They could, for example, use inter-process communication,
   local system buses, shared memory, hypervisors, IP-loopback devices
   or any communication path between the various environments that may
   exist on the entity that combines multiple Roles.

   The movement of Role Messages between locally hosted Roles is
   referred to as "local conveyance".  Most importantly, the definition
   of local conveyance methods is out-of-scope for the RATS

   The following paragraph elaborates on an exemplary usage scenario:

   In a Composite Device scenario, in addition to local entities that
   host the lead Attester and other subordinate Attesters, the Composite
   Device can host the Verifier role locally to appraise Evidence from
   one or more subordinate Attesters.  The local Verifier might convey
   local Attestation Results to a remote Relying party or the Relying
   Party role also could become local where an application-specific
   action is taken locally. to convey conceptual messages
   appropriately between roles.

   For example, a secure boot scenario
   prevents system software from loading if the firmware fails an entity that both connects to
   satisfy a local trustworthiness appraisal policy.

   In a multi-network scenario, a network node might bridge a wide-area
   network, local-area network, network and span various system buses.  In so
   doing, the bridge node might need
   to host multiple Roles depending a system bus is taking on both the type of behavior each connected domain expects.  For example, the
   node might be an Attester to and Verifier roles.
   As a wide-area network, system bus entity, a Verifier consumes Evidence from other
   devices connected to the
   local-area network, and system bus that implement Attester roles.
   As a Relying Party to components attached to wide-area network connected entity, it may implement an Attester
   role.  The entity, as a
   local system bus.

8. 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 also creates
   and consumes the corresponding conceptual messages as defined in this

7.  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

   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
   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.

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

   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

   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

   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 rules that it controls.  In
   the background-check model, this Evidence may also be revealed to
   Relying Party(s).


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
   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

   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), 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
   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 9: Multiple Attesters and Relying Parties with Different


10.  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.

   A more detailed discussion with examples appears in Section 17.

12. 16.

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.  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.

   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


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, 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 11 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,
   Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.


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


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 created      |
    |    | generation |                                               |
    | NS | Nonce sent | A random number not predictable to an         |
    |    |            | Attester is sent                              |
    | NR | Nonce      | The nonce is relayed to an Attester by        |
    |    | relayed    | enother entity                                |
    | EG | Evidence   | An Attester collects claims and generates     |
    |    | generation | Evidence                                      |
    | ER | Evidence   | A Relying Party relays Evidence to a Verifier |
    |    | relayed    |                                               |
    | RG | Result     | A Verifier appraises Evidence and generates   |
    |    | generation | an Attestation Result                         |
    | RR | Result     | A Relying Party relays an Attestation Result  |
    |    | relayed    | to a Relying Party                            |
    | RA | Result     | The Relying Party appraises Attestation       |
    |    | appraised  | Results                                       |
    | OP | Operation  | The Relying Party performs some operation     |
    |    | performed  | requested by the Attester.  For example,      |
    |    |            | acting upon some message just received across |
    |    |            | a session created earlier at time(RA).        |
    | RX | Result     | An Attestation Result should no longer be     |
    |    | expiry     | accepted, according to the Verifier that      |
    |    |            | generated it                                  |

                                  Table 1

   We now walk through a number of hypothetical examples of how a
   solution might be built.  This list is not intended to be complete,
   but is just representative enough to highlight various timing


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.

      .----------.                     .----------.  .---------------.
      | 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)".


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)}         |               |
           ~                                                 ~
           |                                                 |
           |----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) <

   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) <

   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.

   .----------.         .---------------.              .----------.
   | 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,

   [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

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [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/

              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-

              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-08, 4 April 2020,

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt

   Email: henk.birkholz@sit.fraunhofer.de

   Dave Thaler
   United States of America

   Email: dthaler@microsoft.com

   Michael Richardson
   Sandelman Software Works

   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