Skip to main content

Remote Attestation Procedures Architecture

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9334.
Authors Henk Birkholz , Dave Thaler , Michael Richardson , Ned Smith
Last updated 2020-02-04
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 9334 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD (None)
Send notices to (None)
RATS Working Group                                           H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Informational                                 D. Thaler
Expires: 7 August 2020                                         Microsoft
                                                           M. Richardson
                                                Sandelman Software Works
                                                                N. Smith
                                                         4 February 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 (, which is archived at

   Source for this draft and an issue tracker can be found at (

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

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

Birkholz, et al.          Expires 7 August 2020                 [Page 1]
Internet-Draft              RATS Arch & Terms              February 2020

   This Internet-Draft will expire on 7 August 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 (
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Reference Use Cases . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   4
     4.1.  Composite Attester  . . . . . . . . . . . . . . . . . . .   5
   5.  Topological Models  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Passport Model  . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Background-Check Model  . . . . . . . . . . . . . . . . .   8
     5.3.  Combinations  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Two Types of Environments of an Attester  . . . . . . . . . .  10
   7.  Trust Model . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Conceptual Messages . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  Endorsements  . . . . . . . . . . . . . . . . . . . . . .  12
     8.3.  Attestation Results . . . . . . . . . . . . . . . . . . .  13
   9.  Claims Encoding Formats . . . . . . . . . . . . . . . . . . .  13
   10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   16. Informative References  . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   <more text to be added here>

Birkholz, et al.          Expires 7 August 2020                 [Page 2]
Internet-Draft              RATS Arch & Terms              February 2020

   Remote Attestation, as used in this document, is a process by which
   one entity (the "Attester") provides evidence about its identity and
   state to another remote entity (the "Relying Party"), which then
   assesses the Attester's trustworthiness for the Relying Party's own

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 evaluation results about an
      Attester generated by the Verifiers.  Compare /security policy/ in

   *  Attestation Result: The evaluation results 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 evaluated in order to
      determine whether the entity is considered trustworthy, such as
      when deciding whether the entity is authorized to perform some

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

   *  Endorser: An entity that creates Endorsements that can be used to
      help evaluate trustworthiness of Attesters.

   *  Evidence: A set of information about an Attester that is to be
      evaluated 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 evaluates the validity of Evidence about
      an Attester.

Birkholz, et al.          Expires 7 August 2020                 [Page 3]
Internet-Draft              RATS Arch & Terms              February 2020

   *  Verifier Owner: An entity, such as an administrator, that is
      authorized to configure Appraisal Policy for Evidence in a

3.  Reference Use Cases

   <unclear if the WG wants this section in the arch doc>

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       |
                       |              |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

Birkholz, et al.          Expires 7 August 2020                 [Page 4]
Internet-Draft              RATS Arch & Terms              February 2020

   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.  Composite Attester

   A Composite Attester is an entity composed of multiple sub-entities
   such that its trustworthiness has to be determined by evaluating 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 these Attesters, there may be only some, which can
   be called Lead Attesters, that have the ability to communicate with
   the Verifier.  Other Attesters don't have this ability, but they are
   connected to the Lead Attesters via internal links or network
   connections, and they are evaluated via the Lead Attester's help.

   For example, a carrier-grade router is a composite device consisting
   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 TPM or 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 Attester, 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 simpler management
   by being logically treated as one router.  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 verified via this main router.  So, in
   this case, the multi-chassis router is the Composite Attester, each
   router is an Attester and the main router is the Lead Attester.

   Figure 2 depicts the conceptual data flow for a Composite Attester.

Birkholz, et al.          Expires 7 August 2020                 [Page 5]
Internet-Draft              RATS Arch & Terms              February 2020

                      |           Verifier          |
                                      | Composite
                                      | Evidence
   | .--------------------------------|-----.      .------------.     |
   | |                      .------------.  |      |            |     |
   | |                      |  Attesting |<--------| Attester B |-.   |
   | |                      |Environment |  |      '------------. |   |
   | |  .----------------.  |            |<----------| Attester C |-. |
   | |  |     Target     |  |            |  |        '------------' | |
   | |  | Environment(s) |  |            |<------------| ...        | |
   | |  |                |  '------------'  | Evidence '------------' |
   | |  |                |            ^     |    of                   |
   | |  |                |------------/     | Attesters               |
   | |  '----------------'  Collecting      | (via Internal Links or  |
   | |                      Claims          | Network Connections)    |
   | |                                      |                         |
   | | Lead Attester A                      |                         |
   | '--------------------------------------'                         |
   |                                                                  |
   |                    Device/Composite Device/Attester/TBD #33      |

          Figure 2: Conceptual Data Flow for a Composite Attester

   In the Composite Attester, 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

   The Lead Attester's Attesting Environment may or may not include its
   own Verifier.  One situation is that the Attesting Environment has no
   internal Verifier.  In this situation, the Lead Attesting Environment
   simply combines the various Evidences into the final Evidence that is
   sent off to the remote Verifier, which evaluates the Composite
   Attester's, including the Lead Attester's and other Attesters',

   The other situation is that the Lead Attesting Environment has an
   internal Verifier.  After collecting the Evidence of other Attesters,
   this Attesting Environment verifies them using Endorsements and
   Appraisal Policies (obtained the same way as any other Verifier), for

Birkholz, et al.          Expires 7 August 2020                 [Page 6]
Internet-Draft              RATS Arch & Terms              February 2020

   evaluating these Attesters' trustworthiness.  Then the Lead Attesting
   Environment combines the Attestation Results into the final Evidence
   of the whole Composite Attester which is sent off to the remote
   Verifier, which might treat the claims obtained from the local
   Attestation Results as if they were Evidence.

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.

   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 attestation protocol.

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

                          Figure 3: 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

Birkholz, et al.          Expires 7 August 2020                 [Page 7]
Internet-Draft              RATS Arch & Terms              February 2020

   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 security 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.

Birkholz, et al.          Expires 7 August 2020                 [Page 8]
Internet-Draft              RATS Arch & Terms              February 2020

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

                      Figure 4: 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 attest to
   different Relying Parties for 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 5 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 4, 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

Birkholz, et al.          Expires 7 August 2020                 [Page 9]
Internet-Draft              RATS Arch & Terms              February 2020

   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 5: Example Combination

6.  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.  One example
   is a set of components in a boot sequence (e.g., ROM, firmware, OS,
   and application) where a Target Environment is the Attesting
   Environment for the next environment in the boot sequence.

   Claims are collected from Target Environments.  That is, Attesting
   Environments collect the raw values and the information to be
   represented in claims.  Attesting Environments then format them
   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).

Birkholz, et al.          Expires 7 August 2020                [Page 10]
Internet-Draft              RATS Arch & Terms              February 2020

7.  Trust Model

   The scope of this document is scenarios for which a Relying Party
   trusts a Verifier that can evaluate 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 evaluate the
   trustworthiness of the Verifier before accepting its Attestation

   In solutions following the background-check model, the Attester is
   assumed to trust the Verifier (again, whether directly or indirectly
   via a Certificate Authority that it trusts), since the Attester
   relies on an Attestation Result it obtains from the Verifier, in
   order to access resources.

   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
   evaluate 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 evaluate 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.

8.  Conceptual Messages

Birkholz, et al.          Expires 7 August 2020                [Page 11]
Internet-Draft              RATS Arch & Terms              February 2020

8.1.  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 evaluating it in
   isolation would require Appraisal Policy to be based on device-
   specific details (e.g., a device public key).

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, evaluating them can be done against Appraisal Policy
   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 evaluate 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 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.

Birkholz, et al.          Expires 7 August 2020                [Page 12]
Internet-Draft              RATS Arch & Terms              February 2020

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 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
   evaluating 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 attestation
   is desired to be added:

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

                     Figure 6: Typical Resource Access

Birkholz, et al.          Expires 7 August 2020                [Page 13]
Internet-Draft              RATS Arch & Terms              February 2020

   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 attestation to be added to existing protocols, enabling a
   higher level of assurance against malware for example, it is
   important that information needed for evaluating 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, 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 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.

Birkholz, et al.          Expires 7 August 2020                [Page 14]
Internet-Draft              RATS Arch & Terms              February 2020

                       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 7: 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, may 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

Birkholz, et al.          Expires 7 August 2020                [Page 15]
Internet-Draft              RATS Arch & Terms              February 2020

   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.

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 is
   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.

   Protocols that convey Evidence or Attestation Results are responsible
   for detailing what kinds of information are disclosed, and to whom
   they are exposed.

12.  Security Considerations

   Any solution that conveys information used for security purposes,
   whether such information is in the form of Evidence, Attestation
   Results, or 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 10 discusses ways in which freshness can be used
   in this architecture to protect against replay attacks.

   To evaluate 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 Policy in a Relying Party or
   Verifier can be configured via a network protocol, the ability to
   attest to the health of the client providing the Appraisal Policy
   needs to be considered.

Birkholz, et al.          Expires 7 August 2020                [Page 16]
Internet-Draft              RATS Arch & Terms              February 2020

13.  IANA Considerations

   This document does not require any actions by IANA.

14.  Acknowledgments

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

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.  Informative References

              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, <

              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-teep-architecture-05, 12 December 2019,

   [OPCUA]    OPC Foundation, "OPC Unified Architecture Specification,
              Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
              November 2015, <

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

Birkholz, et al.          Expires 7 August 2020                [Page 17]
Internet-Draft              RATS Arch & Terms              February 2020

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <>.

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt


   Dave Thaler
   United States of America


   Michael Richardson
   Sandelman Software Works


   Ned Smith
   Intel Corporation
   United States of America


Birkholz, et al.          Expires 7 August 2020                [Page 18]