Architecture and Reference Terminology for Remote Attestation Procedures
draft-birkholz-rats-architecture-01

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Network Working Group                                        H. Birkholz
Internet-Draft                                            Fraunhofer SIT
Intended status: Standards Track                              M. Wiseman
Expires: September 13, 2019                           GE Global Research
                                                           H. Tschofenig
                                                                ARM Ltd.
                                                                N. Smith
                                                                   Intel
                                                          March 12, 2019

Architecture and Reference Terminology for Remote Attestation Procedures
                  draft-birkholz-rats-architecture-01

Abstract

   Remote ATtestation ProcedureS (RATS) architecture facilitates the
   attestation of device characteristics that, in general, are based on
   specific trustworthiness qualities intrinsic to a device or service.
   It includes trusted computing functionality provided by device
   hardware and software that allows trustworthiness qualities to be
   asserted and verified as part of, or pre-requisite to, the device's
   normal operation.  The RATS architecture maps corresponding
   attestation functions and capabilities to specific RATS Roles.  The
   goal is to enable an appropriate conveyance of evidence about device
   trustworthiness via network protocols.  RATS Roles provide the
   endpoint context for understanding the various interaction semantics
   of the attestation lifecycle.  The RATS architecture provides the
   building block concepts, semantics, syntax and framework for
   interoperable attestation while remaining hardware-agnostic.  This
   flexibility is intended to address a significant variety of use-cases
   and scenarios involving interoperable attestation.  Example usages
   include, but are not limited to: financial transactions, voting
   machines, critical safety systems, network equipment health, or
   trustworthy end-user device management.  Existing industry
   attestation efforts may be helpful toward informing RATS
   architecture.  Such as: Remote Integrity VERification (RIVER), the
   creation of Entity Attestation Tokens (EAT), software integrity
   Measurement And ATtestation (MAAT)

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

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   working documents as Internet-Drafts.  The list of current Internet-
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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  What is Remote Attestation  . . . . . . . . . . . . . . .   4
     1.2.  The purpose of RATS Architecture and Terminology  . . . .   4
     1.3.  Requirements notation . . . . . . . . . . . . . . . . . .   4
   2.  RATS Architecture . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Architectural Components  . . . . . . . . . . . . . . . . . .   5
     3.1.  RATS Roles  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  RATS Actors . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  RATS Duties . . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Attester Duties . . . . . . . . . . . . . . . . . . .   8
       4.1.2.  Verifier Duties . . . . . . . . . . . . . . . . . . .   8
       4.1.3.  Claimant Duties . . . . . . . . . . . . . . . . . . .   8
       4.1.4.  Relying Party Duties  . . . . . . . . . . . . . . . .   8
       4.1.5.  RATS Interactions . . . . . . . . . . . . . . . . . .   9
   5.  Application of RATS . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Trust and Trustworthiness . . . . . . . . . . . . . . . .  11
     5.2.  Claims and Evidence . . . . . . . . . . . . . . . . . . .  12
     5.3.  RATS Information Flows  . . . . . . . . . . . . . . . . .  12
   6.  Exemplary Composition of Roles  . . . . . . . . . . . . . . .  13
     6.1.  Conveyance of Trusted Claim Sets Validated by Signature .  13
     6.2.  Conveyance of Attestation Evidence Appraised by a

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           Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  The Scope of RATS . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  The Lying Endpoint Problem  . . . . . . . . . . . . . . .  15
       7.1.1.  How the RATS Architecture Addresses the Lying
               Endpoint Problem  . . . . . . . . . . . . . . . . . .  16
   8.  RATS Terminology  . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Computing Context . . . . . . . . . . . . . . . . . . . .  18
       8.1.1.  Characteristics of a Computing Context  . . . . . . .  19
       8.1.2.  Computing Context Semantic Relationships  . . . . . .  19
       8.1.3.  Computing Context Identity  . . . . . . . . . . . . .  21
     8.2.  Remote Attestation Concepts . . . . . . . . . . . . . . .  21
     8.3.  Core RATS Terminology . . . . . . . . . . . . . . . . . .  22
     8.4.  RATS Information Model Terminology  . . . . . . . . . . .  22
     8.5.  RATS Work-Flow Terminology  . . . . . . . . . . . . . . .  23
     8.6.  RATS Reference Use Cases  . . . . . . . . . . . . . . . .  24
       8.6.1.  Use Case A  . . . . . . . . . . . . . . . . . . . . .  24
       8.6.2.  Use Case B  . . . . . . . . . . . . . . . . . . . . .  24
     8.7.  RATS Reference Terminology  . . . . . . . . . . . . . . .  24
     8.8.  Interpretations of RFC4949 Terminology for Attestation  .  26
     8.9.  Building Block Vocabulary (Not in RFC4949)  . . . . . . .  28
   9.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  28
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  29
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   12. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  29
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     13.2.  Informative References . . . . . . . . . . . . . . . . .  29
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   In general, this document provides normative guidance how to use,
   create or adopt network protocols that facilitate remote attestation
   procedures.  The RATS Architecture anticipates broad deployment
   contexts that range from IoT to Cloud and Edge ecosystems.  The
   foundation of the RATS architecture is the specification of RATS
   Roles that can be chained via RATS Interactions and - as a result -
   may be composed into use-case specific Remote Attestation Procedures.
   RATS Actors establish an ecosystem neutral context where RATS Roles
   are hosted and where a variety of Remote Attestation Procedure
   interactions are defined independent of specific conveyance protocols
   or message formats.  In summary, the goal of the RATS Architecture is
   to enable interoperable interaction between the RATS Roles.  Hence,
   the RATS Architecture is designed to enable interoperability via
   well-defined semantics of the information model (attestation
   assertions/claims), associated with RATS Roles following a conveyance

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   model (RATS Interactions) that may be used to compose domain-specific
   remote attestation solutions.

1.1.  What is Remote Attestation

   Unfortunately, the term Attestation itself is an overloaded term.  In
   consequence, the term Remote Attestation covers a spectrum of
   meanings.  The common denominator encompasses the creation,
   conveyance, and appraisal of evidence pertaining to the
   trustworthiness characteristics of the creator of the evidence.  In
   essence, RATS are used to enable the assessment of the
   trustworthiness of a communication partner.

1.2.  The purpose of RATS Architecture and Terminology

   To consolidate the utilization of existing and emerging network
   protocols in the context of RATS, this document provides a detailed
   definition of Attestation Terminology that enables interoperability
   between different types pf RATS.  Specifically, this document
   illustrates and remediates the impedance mismatch of terms related to
   Remote Attestation Procedures used in different domains today.  As an
   additional contribution, new terms defined by this document provide a
   common basis that simplifies future work on RATS in the IETF and
   beyond.

1.3.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  RATS Architecture

   One of the goals of the RATS Architecture is to provide the building
   blocks - the roles defined by the RATS Architecture - to enable the
   composition of service-chains/hierarchies and work-flows that can
   create and appraise evidence about the trustworthiness of devices and
   services.

   The RATS Architecture is based on the use-cases defined in
   [I-D.richardson-rats-usecases].

   The RATS architecture specifies:

   o  The building blocks to create remote attestation procedures
      applicable Actors, Roles, Duties, and Interactions,

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   o  Mandatory and optional trust relationships between its Roles, that
      may assume a Root-of-Trust context,

   o  The interaction between Roles that reside on separate Actors and
      interact via network protocols,

   o  Protocol/message framing that allows for well-defined and opaque
      payloads,

   o  The means to prove, preserve and convey trust properties, such as
      identity, varacity, freshness, or provenance, and

   o  Primitives necessary for the construction of interoperable
      attestation payloads.

3.  Architectural Components

   The basic architectural components defined in this document are:

   o  RATS Roles

   o  RATS Actors

   o  RATS Duties

   o  RATS Interactions

   The following sub-section define and elaborate on these terms:

3.1.  RATS Roles

   A Role in the context of usage scenarios for remote attestation
   procedures is providing a service to other Roles.  Roles are building
   blocks that can be providers and consumers of information.  In the
   RATS architecture, devices or services can take on RATS roles.  They
   are composites of internal functions (RATS Duties) and external
   functions (RATS Interactions) that facilitate a required (sometimes
   optional) task in a remote attestation procedure.

   The base set of RATS roles is:

   Claimant:  The producer of trustworthiness assertions pertaining to
      an Attester; that may or not have a root-of-trust for measurement.

      It is not guaranteed that a Verifier Role can appraise the output
      of a Claimant via reference values (in contrast to the output of
      an Attester).

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      Examples of Claimant assertions include: * The hardware, firmware
      and software components of the Attester.  * The manufactuer of
      Attester components.  * The Attester's current configuration.  *
      The Attester's current location - e.g.  GPS coordinates.  * The
      method by which binding of an attester to an RTR.  * The
      identifier(s) available for identifying and authenticating the
      Attester - e.g.  Universal Entity ID (UEID).

      Typically, claimant role are taken on by RATS Actors that supply
      chain entities (SCE).  Various assertions (often represented as
      Claims or Trusted Claims Sets, e.g.  [I-D.mandyam-eat] or
      [I-D.tschofenig-rats-psa-token]).

   Attester:  The producer of attestation evidence that has a root of
      trust for reporting (RTR) and implements a conveyance protocol,
      authenticates using an attestation credential, consumes assertions
      about itself and presents it to a consumer of evidence (e.g. a
      relying party or a verifier).  Every output of an attester can be
      appraised via reference values.

   Authentication Checker:  The consumer of signed assertions such as
      trusted claim sets or attestation evidence that assesses the
      trustworthiness or other trust relationships of the information
      consumed via trusted third parties or external trust authorities,
      such as a privacy certificate authority.  In certain environments,
      an Authentication Checker can assess a system's trustworthiness
      via external trust anchors, implicitly.

   Verifier:  The consumer of attestation evidence that has a root of
      trust for verification (RTV), implements conveyance protocols,
      appraises attestation evidence against reference values or
      policies, and makes verification results available to relying
      parties.

   Relying Party:  The consumer and assessor of verifier or
      Authentication Checker results for the purpose of improved risk
      management, operational efficiency, security, privacy (natural or
      legal person) or safety.  The verifier and/or authentication
      checker roles and the relying party role may be tightly
      integrated.

4.  RATS Actors

   RATS Actors may be any entity, such as an user, organization,
   execution environment, device or service provider, that takes on
   (implements) one or more RATS Roles and performs RATS Duties and/or
   RATS Interactions.  RATS Interactions occur between RATS Actors.  The
   methods whereby RATS Actors are identified, discovered, and

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   connectivity established are out-of-scope for this architecture.  In
   contrast, if multiple RATS Roles reside on a single RATS Actor, the
   definition of RATS Interactions is out-of-scope of the RATS
   architecture, if no network protocols are required.

                +------------------+   +------------------+
                |      Actor 1     |   |      Actor 2     |
                |  +------------+  |   |  +------------+  |
                |  |            |  |   |  |            |  |
                |  |    Role 1  |<-|---|->|    Role 2  |  |
                |  |            |  |   |  |            |  |
                |  +------------+  |   |  +------------+  |
                |                  |   |                  |
                |  +-----+------+  |   |  +-----+------+  |
                |  |            |  |   |  |            |  |
                |  |    Role 2  |<-|---|->|    Role 3  |  |
                |  |            |  |   |  |            |  |
                |  +------------+  |   |  +------------+  |
                |                  |   |                  |
                +------------------+   +------------------+

                  Figure 1: RATS Actor-Role Interactions

   RATS Actors have the following properties: * Multiplicity - Multiple
   instances of RATS Actors that possess the same RATS Roles can exist.
   * Decomposability - A singleton RATS Actor possessing multiple RATS
   Roles can be separated into multiple RATS Actors.  RATS Interactions
   may occur between them.  * Composablility - RATS Actors possessing
   different RATS Roles can be combined into a singleton RATS Actor
   possessing the union of RATS Roles.  RATS Interactions between
   combined RATS Actors ceases.

   Interactions between RATS Roles belonging to the same RATS Actor are
   generally believed to be uninteresting.  Actor operations that apply
   resiliency, scaling, load balancing or replication are generally
   believed to be uninteresting.

4.1.  RATS Duties

   A RATS Role can take on one ore more duties.  RATS Duties are role-
   internal functions that do not require interaction with other RATS
   Roles.  In general, and RATS Duties are typically associated with a
   RATS Role.  The list presented in this document is exhaustive.  Also,
   there can be usage scenario where RATS Duties are associated with
   other RATS Roles than illustrated below:

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4.1.1.  Attester Duties

   o  Acquisition or collection of assertions about itself

   o  Provide or create proof that an assertion is bound to the Attester

   o  Create Evidence from assertion bundles via roots-of-trust

4.1.2.  Verifier Duties

   o  Acquisition and storage of assertion semantics

   o  Acquisition and storage of appraisal policies

   o  Verification of Attester Identity (attestation provenance)

   o  Comparing assertions or evidence with reference values according
      to appraisal policies

   o  Validate authentication information based on public keys,
      signatures, secrets that are shielded, or secrets that are access
      restricted via protection profiles

4.1.3.  Claimant Duties

   o  Hardens the device or service that implements the Attester role

   o  Provisions device identities and/or key material accessible to the
      Attester role

   o  Evaluates trustworthiness during manufacturing, supply chain and
      onboarding

   o  Produces trustworthiness assertions applicable to the Attestor
      role

   o  Embeds trustworthiness assertions about the Attester role in the
      device or service during manufacturing, supply chain or onboarding

4.1.4.  Relying Party Duties

   o  Evaluate assertions/evidence locally as far as possible

   o  Compare trust policies to attestation-results based on assertions
      or evidence

   o  Enforce policies or create input for risk engines

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4.1.5.  RATS Interactions

   The flow of information between RATS Roles located on RATS Actors
   compose individual remote attestation procedures.  The RATS
   Architecture provides a set of standard interactions between the RATS
   Roles defined in this document in order to enable this composability.
   In this section, common interactions between roles are specified.
   This list of interactions is not exhaustive, but provides the basis
   to create various standard RATS.

   Every RATS Interaction specified below is based on the information
   flow between two RATS Roles defined above.  Every RATS Interaction is
   conducted via an Interconnect between corresponding RATS Roles that
   RATS Actors take on.  If more than one RATS Role resides on the same
   RATS Actor, a network protocol might not be required.  If RATS Roles
   are collapsed into a singular RATS Actor in this way, the method of
   conveying information is out-of-scope of this document.  If network
   protocols are used to convey corresponding information between RATS
   Roles (collapsed on a singular RATS Actor or not), the definitions
   and requirements defined in this document apply.

   In essence, an Interconnect is an abstract "distance-less" channel
   between RATS Actors that can range from General Purpose Input Output
   (GPIO) interfaces to the Internet.

   Attester/Verifier:  The most basic RATS interaction is between the
      creator of evidence (Attester) and its complementary remote
      attestation service (Verifier).  In order to convey evidence (or
      assertions that are not accompanied by a proof of their validity)
      this RATS Interaction is required.

   Attester/Relying-Party:  A Relying Party typically requires external
      help to either validate authentication information or to appraise
      evidence presented by an Attester.  In most cases, a Relying Party
      requires a corresponding Verifier to process the assertions/
      evidence received.  In consequence, (a subset of) the information
      received by an Attester must be relayed securely to a Verifier.

   Relying-Party/Verifier:  Typically, trusted assertions or evidence
      are conveyed from an Attester to a Relying Party.  In an open
      ecosystem, such as the Internet, the appraisal of the evidence
      presented by an Attester provided in order to assess its
      trustworthiness requires a remote attestation service.  Hence,
      either the RATS roles of Verifier and Relying Party are collapsed
      and compose a single RATS Actor, or - if they reside on separate
      RATS Actors - a Relying Party requires appropriate configuration
      or a discovery/join/rendezvous service to initiate a RATS
      Interaction with an appropriate and trusted Verifier.

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      Attestation information originating from an Attester that is
      relayed via a Relying Party must be protected from replay or relay
      attacks, accordingly.  In a closed ecosystem, trustworthiness with
      respect to the Attester can be achieved via a simply query to the
      Verifier.  In an open ecosystem, the information conveyed in this
      interaction can include integrity measurements of every
      distinguishable software component that has been executed since
      its last boot cycle.

      In the scope of RATS, this interaction encompasses the largest
      variety of information conveyed.

   Claimant/Verifier:  The intended operational state an Attester is
      intended to be in, is defined by the supply chain entities that
      manufacture and maintain the Attestor.  In order to appraise
      trusted assertions or evidence conveyed by the Attester, every
      distinguishable system component the Attester is composed of can
      provide trusted assertions or evidence about its trustworthiness.
      A corresponding verifier that is tasked with assessing the
      trustworthiness of the Attester potentially requires a multitude
      of sources of reference values according to policies and the
      information provided.  As Relying Parties often have to discover
      an appropriate Verifier, a Verifier has to obtain and potentially
      store appropriate reference values in order to asses assertions or
      evidence about trustworthiness.

   Claimant/Attester:  To enable RATS, trustworthy assertions have to be
      embedded in an Attester by its manufactorer.  In some cases this
      involves various types of roots of trust.  In other cases shielded
      pre-master secrets in combination with key derivation functions
      (KDF) provide this binding of trusted information to an Attester.
      A supply chain entity can embed additional trusted assertions to
      an Attester.  These assertion can also be used to assert the
      trustworthiness on behalf of a separate RATS Actor or they can
      originate from an external entity (e.g. a security certification
      authority).

5.  Application of RATS

   Attester are typically composite devices (in the case of atomically
   integrated devices that would result in a composite device with one
   component) or services.  Services are software components - e.g. a
   daemon, a virtual network function (vnf) or a network security
   function (nsf) - that can reside on one or more Attester and are not
   necessarily bound to a specific set of hardware devices.

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   Relevant decision-factors that influence the composition of RATS
   Roles on RATS Actors, which result in specific work-flows are
   (amongst others):

   o  which RATS Role (or correspondingly, which RATS Actore that is
      taking on specific RATS roles) is triggering a Remote Attestation
      Procedure

   o  which entities are involved in a Remote Attestation Procedure
      (e.g. the Attester itself, trusted third parties, specific trust
      anchors, or other sources of assertions)

   o  the capabilities of the protocols used (e.g. challenge-response
      based, RESTful, or uni-directional)

   o  the security requirements and security capabilities of systems in
      a domain of application

   o  the risks and corresponding threats that are intended to be
      mitigated

5.1.  Trust and Trustworthiness

   [RFC4949] provides definitions that highlight the difference between
   a "trusted system" and a "trustworthy system".  The following
   definitions exclude the explicit specialization of concepts that are
   "environmental disruption" as well as "human user and operator
   errors".

   A trusted system in the context of RATS "operates as expected,
   according to design and policy, doing what is required and not doing
   other things" [RFC4949].  A trustworthy system is a system "that not
   only is trusted, but also warrants that trust because the system's
   behavior can be validated in some convincing way, such as through
   formal analysis or code review" [RFC4949].

   The goal of RATS is to convey information about system component
   characteristics, such as integrity or authenticity, that can be
   appraised in a convincing way.

   RATS require trust relationships with third parties that qualify
   assertions about, for example, origin of data, the manufacturer or
   the capabilities of a system, or the origination of attestation
   evidence (attestation provenance).  Without trusted authorities (e.g.
   a certificate authority) it is virtually impossible to assess the
   level of assurance (or resulting level of confidence,
   correspondingly) of information produced by RATS.  Trusting a system
   does not make it trustworthy.  Assessing trustworthiness requires the

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   conveyance of evidence that a system is a trustworthy system, which
   has to originate from the system itself and has to be convincing.  If
   the convincing information is not originating from the system itself,
   it comprises trusted claim sets and not evidence.  In essence, the
   attestation provenance of attestation evidence is the system that
   intends to present its trustworthiness in a believable manner.

   The essential basis for trust in the information created via RATS are
   roots of trust.

   Roots of trust are defined by the NIST special publication 800-164
   draft as "security primitives composed of hardware, firmware and/or
   software that provide a set of trusted, security-critical functions.
   They must always behave in an expected manner because their
   misbehavior cannot be detected.  As such, RoTs need to be secured by
   their design.  Hardware RoTs are preferred over software RoTs due to
   their immutability, smaller attack surface, and more reliable
   behavior."

   If the root of trust involved is a root of trust for measurement
   (RTM), the producer of information takes on the role of a asserter.
   An asserter can also make use of a root of trust for integrity (RTI)
   in order to increase the level of assurance in the assertions
   produced.  If the root of trust involved is a root of trust for
   reporting (RTR), the producer of information takes on the role of an
   attester.

5.2.  Claims and Evidence

   The RATS asserter role produces measurements about the system's
   characteristics in the form of signed (sometimes un-signed) claim
   sets in order to convey information.  A secret signing key is
   required for this procedure, which is typically stored in a shielded
   location that can be trusted, for example, via a root of trust for
   storage (RTS).

   The RATS attester role produces signed attestation evidence in order
   to convey information.  The secret key required for this procedure is
   stored in a shielded location that only allows access to that key, if
   a specific operational state of the system is met.  The trust with
   respect to this origination is based on a root of trust for
   reporting.

5.3.  RATS Information Flows

   There are six roles defined in the RATS architecture. iFigure 2
   provides a simplified overview of the RATS Roles defined above,

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   illustrating a general Interconnect in the center that facilitates
   all RATS Interactions.

          +------------+                     +------------------+
          |            |                     |                  |
          |  Attester  |                  +->|  Verifier        |
          |            |                  |  |                  |
          +------------+                  |  +------------------+
                ^                         |
                |                         |
                |     +----------------+  |
                +---->|                |<-+
                      |  Interconnect  |
                +---->|                |<-+
                |     +----------------+  |
                v                         |
          +------------+                  |  +------------------+
          |            |                  |  |                  |
          |  Claimant  |                  +->|  Relying Party   |
          |            |                     |                  |
          +------------+                     +------------------+

     Figure 2: Overall Relationships of Roles in the RATS Architecture

6.  Exemplary Composition of Roles

   In order to provide an intuitive understanding how the roles used in
   RATS can be composed into work-flows, this document provides a few
   example work-flows.  Boxes in the following examples that include
   more than one role are systems that take on more than one role.

6.1.  Conveyance of Trusted Claim Sets Validated by Signature

   If there is a trust relationship between a trusted third party that
   can assert that signed claims created by a claimant guarantee a
   trustworthy origination of claim, the work-flow depicted in Figure 3
   can facilitate a trust-based implicit remote attestation procedure.
   The information conveyed are signed claim sets that are trusted via
   an authoritative third party.  In this work-flow claim emission is
   triggered by the claimant.  Variations based on requests emitted by
   the relying party can be easily facilitated by the same set of roles.

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                                                  +-----------+
            +------------+  +----------------+    |           |
            |            |  |                |    |  Relying  |
            |  Claimant  |->|  Interconnect  |--->|  Party    |
            |            |  |                |    |           |
            +------------+  +----------------+    +-----------+

     Figure 3: Conveyance of Trusted Claim Sets Validated by Signature

6.2.  Conveyance of Attestation Evidence Appraised by a Verifier

   If there is trust in the root of trust for reporting based on the
   assertions of a trusted third party, the work-flow depicted in
   Figure 4 can facilitate an evidence-based explicit remote attestation
   procedure.  The information conveyed is signed attestation evidence
   that is created by the trusted verifier.  In this work-flow claims do
   not necessarily have to be signed and the work-flow is triggered by
   the attestor that aggregates claims from a root of trust of
   measurement.  Variations based on requests emitted by the verifier
   can be easily facilitated by the same set of roles.

    +------------------+                      +----------------------+
    |                  |                      |  +----------------+  |
    |  +------------+  |  +----------------+  |  |                |  |
    |  |            |  |  |                |  |  |                |  |
    |  |  Attester  |--+->|  Interconnect  |--+->|   Verifier     |  |
    |  |            |  |  |                |  |  |                |  |
    |  +------------+  |  +----------------+  |  +----------------+  |
    |        ^         |                      |          |           |
    |        |         |                      |          v           |
    |        |         |                      |   +-----------+      |
    |  +-----+------+  |                      |   |           |      |
    |  |            |  |                      |   |  Relying  |      |
    |  |  Claimant  |  |                      |   |  Party    |      |
    |  |            |  |                      |   |           |      |
    |  +------------+  |                      |   +-----------+      |
    |                  |                      |                      |
    +------------------+                      +----------------------+

   Figure 4: Conveyance of Attestation Evidence Appraised by a Verifier

7.  The Scope of RATS

   During its evolution, the term Remote Attestation has been used in
   multiple contexts and multiple scopes and in consequence accumulated
   various connotations with slightly different semantic meaning.

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   Correspondingly, Remote Attestation Procedures (RATS) are employed in
   various usage scenarios and different environments.

   In order to better understand and grasp the intent and meaning of
   specific RATS in the scope of the security area - including the
   requirements that are addressed by them - this document provides an
   overview of existing work, its background, and common terminology.
   As the contribution, from that state-of-the-art a set of terms that
   provides a stable basis for future work on RATS in the IETF is
   derived.

   In essence, a prerequisite for providing an adequate set of terms and
   definitions for the RATS architecute is a general understanding and a
   common definitions of "what" RATS can accomplish "how" RATS can to be
   used.

   Please note that this section is still missing various references and
   is considered "under construction".  The majority of definitions is
   still only originating from IETF work.  Future iterations will pull
   in more complementary definitions from other SDO (e.g.  Global
   Platform, TCG, etc.) and a general structure template to highlight
   semantic relationships and capable of resolving potential
   discrepancies will be introduced.  A section of context awareness
   will provide further insight on how Attestation procedures are vital
   to ongoing work in the IETF (e.g.  I2NSF & tokbind).  The definitions
   in the section about RATS are still self-describing in this version.
   Additional explanatory text will be added to provide more context and
   coherence.

7.1.  The Lying Endpoint Problem

   A very prominent goal of RATS is to address the "lying endpoint
   problem".  The lying endpoint problem is characterized as a condition
   of a Computing Context where the information or behavior embedded,
   created, relayed, stored, or emitted by the Computing Context is not
   "correct" according to expectations of the authorized system
   designers, operators and users.  There can be multiple reasons why
   these expectations are incorrect, either from malicious Activity,
   unanticipated conditions or accidental means.  The observed behavior,
   nevertheless, appears to be a compromised Computing Context.

   Attempts to "scrub" the data or "proxy" control elements implies the
   existence of a more fundamental trusted endpoint that is operating
   correctly.  Therefore, Remote Attestation - the technology designed
   to detect and mitigate the "lying endpoint problem" - must be trusted
   to behave correctly independent of other controls.

   Consequently, a "lying endpoint" cannot also be a "trusted system".

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   Remote Attestation procedures are intended to enable the consumer of
   information emitted by a Computing Context to assess the validity and
   integrity of the information transferred.  The approach is based, for
   example, on the assumption that if attestation evidence can be
   provided in order to prove the integrity of every software instance
   installed involved in the activity of creating the emitted
   information in question, the emitted information can be considered
   valid and integer.

   In contrast, such Evidence has to be impossible to create if the
   software instances used in a Computing Context are compromised.
   Attestation activities that are intended to create this Evidence
   therefore also provide guarantees about the validity of the Evidence
   they can create.

7.1.1.  How the RATS Architecture Addresses the Lying Endpoint Problem

   RATS imply the involvement of at least two players (roles) who seek
   to overcome the lying endpoint problem.  The Verifier wishes to
   consume application data supplied by a Computing Context.  But before
   application data is consumed, the Verifier obtains Attestation
   Evidence about the Computing Context to assess likelihood of poisoned
   data due to endpoint compromise or failure.  Remote Attestation
   argues that a systems's integrity characteristics should not be
   believed until rationale for believability is presented to the
   relying party seeking to interact with the system.

   An Interconnect defines an untrusted channel between subject and
   object wherein the rationale for believability is securely exchanged.
   The type of interconnect technology could vary widely, ranging from
   GPIO pins, to a PC peripheral IO bus, to the Internet, to a direct
   physical connection, to a wireless radio-receiver association, or to
   a world wide mesh of peers.  In other words, virtually every kind
   communication path could be used as the "Interconnect" in RATS.  In
   fact, a single party could take on all roles at the same time (e.g.
   Self Encrypting Devices).

   Attestation evidence can be thought of as the topics of the exchange
   that is created the operational primitives of a root of trust for
   reporting.  Evidence may be structured in an interoperable format
   called claims that may include references to the claimants which are
   asserting the claims.  RATS aims to define "interoperable Remote
   Attestation" such that evidence can be created and consumed by
   different ecosystem systems and can be securely exchanged by a broad
   set of network protocols.

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8.  RATS Terminology

   This document relies on terminology found in [RFC4949].  This
   document presumes the reader is familiar with the following terms.

   o  Cryptography

   o  Entity (System entity)

   o  Identity

   o  Object

   o  Principal

   o  Proof-of-possession protocol

   o  Security environment (Environment)

   o  Security perimeter

   o  Subject

   o  Subsystem

   o  System

   o  Target-of-Evaluation (TOE)

   o  Trusted Computing Base (TCB)

   o  Trusted Platform Module (TPM)

   o  Trusted (Trustworthy) system

   o  Verification

   Terminology defined by this document is preceded by a dollar sign ($)
   to distinguish it from terms defined elsewhere and as a way to
   disambiguate term definition from explanatory text.

   Terms defined by this document that are subsequently used by this
   document are distinguished by capitalizing the first letter of the
   term (e.g.  Term or First_word Second_word).

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8.1.  Computing Context

   This section introduces the term Computing Context in order to
   specialize the notions of environment and endpoint to terminology
   that has relevance to trusted computing.  Attestation is a discipline
   of trusted computing.

   A Computing Context could refer to a large variety of endpoints.
   Examples include but are not limited to: the compartmentalization of
   physical resources, the separation of software instances with
   different dependencies in dedicated containers, and the nesting of
   virtual components via hardware-based and software-based solutions.
   The number of approaches and techniques to construct an endpoint
   continuously changes with new innovation.  Hence, it isn't a goal of
   this document to define remote attestation for a fixed set of
   endpoints.  Rather, it attempts to define endpoints conceptually and
   rely on Claims management as a way to clarify the details and
   specific attributes of conceptual endpoints.

   Computing Contexts may be recursive in nature in that it could be
   composed of a system that is itself a composite of subsystems.  In
   consequence, a system may be composed of other systems that may be
   further composed of one or more Computing Contexts capable of taking
   on the RATS roles.  The scope and application of these roles can
   range from:

   o  Continuous mutual Attestation procedures of every subsystem inside
      a composite device, to

   o  Sporadic Remote Attestation of unknown parties via heterogeneous
      Interconnects.

   Analogously, the increasing number of features and functions that
   constitute components of a device start to blur the lines that are
   required to categorize each solution and approach precisely.  To
   address this increasingly challenging categorization, the term
   Computing Context defines the characteristics of the (sub)systems
   that can take on the role of an Attester and/or the role of a
   Verifier.  This approach is intended to provide a stable basis of
   definitions for future solutions that continuous to remain viable
   long-term.

   $ Computing Context :  An umbrella term that combines the scope of
      the definitions of endpoint [ref NEA], device [ref 1ar], and thing
      [ref t2trg], including hardware-based and software-based sub-
      contexts that constitute independent, isolated and distinguishable
      slices of a Computing Context created by compartmentalization

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      mechanisms, such as Trusted Execution Environments (TEE), Hardware
      Security Modules (HSM) or Virtual Network Function (VNF) contexts.

8.1.1.  Characteristics of a Computing Context

   While the semantic relationships highlighted above constitute the
   fundamental basis to provide a define Computing Context, the
   following list of object characteristics is intended to improve the
   application of the term and provide a better understanding of its
   meaning:

   $ Computing Context Characteristics:  A representation of the
      identity, composition, configuration and state of a Computing
      Context.

      Computing context characteristics provide the following: * An
      independent environment in regard to executing and running
      software, * An isolated control plane state (by potentially
      interacting with other Computing Contexts), * A dedicated
      management interface by which control plane behavior can be
      effected, * Unique identification towards reliable disambiguation
      within a given scope.

   Computing context characteristics do not necessarily include a
   network interface with associated network addresses (as required by
   the definition of an endpoint) - although it is very likely to have
   (access to) one.

   [Issue: This conclusion could be incorrect] In contrast, a container
   [ref docker, find a more general term here] context is not a
   distinguishable isolated slice of an information system and therefore
   is not an independent Computing Context. [more feedback on this
   statement is required as the capabilities of docker-like functions
   evolve continuously]

   Examples include: a smart phone, a nested virtual machine, a
   virtualized firewall function running distributed on a cluster of
   physical and virtual nodes, or a trust-zone.

8.1.2.  Computing Context Semantic Relationships

   Computing Contexts may relate to other Computing Contexts that are
   decomposable in a variety of ways.

   o  Singleton,

   o  Tuples (e.g. 2-tuple, n-tuple),

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   o  Nested,

   o  Clustered (homogeneous),

   o  Grouped (heterogenous).

   The scope of Computing Context encompasses a broad spectrum of
   systems including, but not limited to:

   o  An information system,

   o  An object,

   o  A composition of objects,

   o  A system component,

   o  A system sub-component,

   o  A composition of system sub-components,

   o  A system entity,

   o  A composition of system entities.

   A Computing Context may be realized in a variety of ways including,
   but not limited to:

   o  A process, thread or task as defined by an operating system,

   o  A privileged operating system task, interrupt handler or event
      handler,

   o  A virtual machine,

   o  A virtual machine monitor,

   o  A processor mode (e.g. system management mode),

   o  A co-processor,

   o  A peripheral device,

   o  A secure element,

   o  A trusted execution environment,

   o  A controller, sensor, actutor, switch, router or gateway,

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   o  An FPGA,

   o  An ASIC,

   o  A memory resource,

   o  A storage resource.

   Analogously, a computing sub-context is a decomposition of a
   Computing Context; a subsystem is a decomposition of a system; a sub-
   component is a decomposition of a component; and a peer node is a
   decomposition of a node cluster.

   A formal semantic relationship is therefore expressed using an
   information model that captures interactions, relationships, bindings
   and interfaces among systems, subsystems, system components, system
   entities or objects.

   [Issue: A tangible relationship to an information model is required
   here] An information model that richly captures Computing Context
   semantics is therefore believed to be relevant if not fundamental to
   Remote Attestation.

8.1.3.  Computing Context Identity

   The identity of a Computing Context implies there is a binding
   operation between an identifier and the Computing Context.

   $ Computing Context Identity:  Computing Context Identity provides
      the basis for associating attestation Evidence about a particular
      Computing Context to create believable knowledge about attestation
      provenance.

   Confidence in the identity assurance level [NIST SP-800-63-3] or the
   assurance levels for identity authentication [RFC4949] is a property
   of the identifier uniqueness properties and binding operation
   veracity.  Such properties impact the trustworthiness of associated
   attestation Evidence.

8.2.  Remote Attestation Concepts

   Attestation Evidence created by RATS is a form of telemetry about a
   computing environment that enables better security risk management
   through disclosure of security properties of the environment.
   Attestation may be performed locally (within the same computing
   environment) or remotely (between different computing environments).
   The exchange of attestation evidence can be formalized to include
   well-defined protocol, message syntax and semantics.

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8.3.  Core RATS Terminology

   $ Attestation:  The creation of evidence by the Attester based on
      measurements or other claimant output.

   A form of telemetry involving the delivery of Claims describing
   various security properties of a Computing Context by an Attester,
   such that the Claims can be used as Evidence toward convincing a
   Verifier regarding trustworthiness of the Computing Context.

   $ Conveyance:  The transfer of Evidence from the Attester to the
      Verifier.

   $ Verification:  The appraisal of Evidence by the Verifier who
      evaluates it against a reference policy.  See also RFC4949 [1].

   $ Remote Attestation:  A procedure involving Attestation, Conveyance
      and Verification.

8.4.  RATS Information Model Terminology

   Evidence conveyed to a Verifier by an Attester is structured to
   facilitate syntactic and semantic interoperability.  An information
   model defines the tag namespaces used to create tag-value pairs
   containing discrete bits of Evidence.

   $ Evidence:  A set of Measurements, quality metrics, quality
      procedures or assurance criteria about an Computing Context's
      behavioral, operational and intrinsic characteristics.

   $ Claim:  Structured Evidence asserted about a Computing Context.  It
      contains metadata that informs regarding the type, class,
      representation and semantics of Evidence information.  A Claim is
      represented as a name-value pair consisting of a Claim Name and a
      Claim Value [RFC7519].  In the context of SACM, a Claim is also
      specialized as an attribute-value pair that is intended to be
      related to a statement [I-D.ietf-sacm-terminology].

   $ Attestable Claim:  Structured Evidence including one or more Claims
      that are asserted by a Claimant (Note: an Attester role doubles as
      a Claimant role).  An Attestable Claim has the following
      structure:

   1.  A Claim or Claims.

   2.  A Claimant identity.

   3.  Proof of Claimant identity.

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   4.  Proof the Claimant intended to make these Claims.

   Note: Proofs of Claims assertions may be separated from the Claim
   itself.  For example, a secure transport over which Claims are
   conveyed where Claimant's signing key integrity protects the
   transport payload could be used as proof of Claim assertion.
   Alternatively, each Claim could be separately signed by a Claimant.

   $ Attested (Asserted) Claim:  An Attestable Claim where the proof
      elements are populated.

   $ Evidence (Claims) Creation:  Instantiation of Attested Claims by a
      Claimant.

   $ Evidence (Claims) Collection:  Assembling of Attested Claims by an
      Attester for the purpose of Conveyance.

   $ Verified (Valid) Claim:  An Attested Claim where the proof elements
      have been verified by a Verifier according to a policy that
      identifies trusted Claimants and/or trusted Evidence values.

8.5.  RATS Work-Flow Terminology

   This section introduces terms and definitions that are required to
   illustrate the scope and the granularity of RATS workflows in the
   domain of security automation.  Terms defined in the following
   sections will be based on this workflow-related definitions.

   In general, RATS are composed of iterative activities that can be
   conducted in intervals.  It is neither a generic set of actions nor
   simply a task, because the actual actions to be conducted by RATS can
   vary significantly depending on the protocols employed and types of
   Computing Contexts involved.

   $ Activity:  A sequence of actions conducted by Computing Contexts
      that compose a Remote Attestation procedure.  The actual
      composition of actions can vary, depending on the characteristics
      of the Computing Context they are conducted by/in and the
      protocols used to utilize an Interconnect.  A single Activity
      provides only a minimal amount of semantic context, e.g.defined by
      the Activity's requirements imposed upon the Computing Context, or
      via the set of actions it is composed of.  Example: The Conveyance
      of cryptographic Evidence or the appraisal of Evidence via
      imperative guidance.

   $ Task:  A unit of work to be done or undertaken.

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      In the scope of RATS, a task is a procedure to be conducted.
      Example: A Verifier can be tasked with the appraisal of Evidence
      originating from a specific type of Computing Contexts providing
      appropriate identities.

   $ Action:  The accomplishment of a thing usually over a period of
      time, in stages, or with the possibility of repetition.

      In the scope of RATS, an action is the execution of an operation
      or function in the scope of an Activity conducted by a Computing
      Context.  A single action provides no semantic context by itself,
      although it can limit potential semantic contexts of RATS to a
      specific scope.  Example: Signing an existing public key via a
      specific openssl library, transmitting data, or receiving data are
      actions.

   $ Procedure:  A series of actions that are done in a certain way or
      order.

      In the scope of RATS, a procedure is a composition of activities
      (sequences of actions) that is intended to create a well specified
      result with a well established semantic context.  Example: The
      activities of Attestation, Conveyance and Verification compose a
      Remote Attestation procedure.

8.6.  RATS Reference Use Cases

   A "lying endpoint" is not trustworthy.

   This document provides NNN prominent examples of use cases
   Attestation procedures are intended to address:

   o  Verification of the source integrity of a Computing Context via
      data integrity proofing of installed software instances that are
      executed, and

   o  Verification of the identity proofing of a Computing Context.

8.6.1.  Use Case A

8.6.2.  Use Case B

8.7.  RATS Reference Terminology

   $ Attestable Computing Context:  A Computing Context where a Claimant
      is able to create Claims, an Attester is able to Attest those
      Claims and a Verifier is able to verify the Claims.

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   $ Attestation Identity:  An identity that refers to an Attester.

   $ Attestation Identity Credential:  A credential used to authenticate
      an Attestation Identity.

   $ Attestation Identity Key (AIK):  An Attestation Identity Credential
      in the form of an asymmetric cryptographic key where the AIK
      private key is protected by a Computing Context with protection
      properties that are stronger than the Computing Context about
      which the AIK attests.  A root-of-trust Computing Context normally
      protects AIK private keys.

   $ Claimant Identity:  An identity that refers to an Claimant.

   $ Claimant Identity Credential:  A credential used to authenticate a
      Claimant Identity.

   $ Measurements / Integrity Measurements:  Metrics of Computing
      Context characteristics (i.e. composition, configuration and
      state) that affect the confidence in the trustworthiness of a
      Computing Context.  Digests of integrity Measurements can be
      stored in shielded locations (e.g. a PCR of a TPM).

   $ Reference Integrity Measurements:  Signed Measurements about a
      Computing Context's characteristics that are provided by a vendor
      or manufacturer and are intended to be used as declarative
      guidannce [I-D.ietf-sacm-terminology] (e.g. a signed CoSWID).

   $ Root-of-trust:  The Computing Context that protects the following
      where no other Computing Context is expected to provide its
      Attestation Evidence: + Attestation Evidence.  + AIKs.  + Code
      used during the collection and reporting of Attestation Evidence.

   $ Root-of-trust-for-measurement (RTM):  A trusted Computing Context
      where a Claimant creates integrity Measurements and other Evidence
      about a Computing Context where no other Computing Context is
      expected to provide its Attestation Evidence.

   $ Root-of-trust-for-reporting (RTR):  A trusted Computing Context
      where an Attester stages reporting of Claims where no other
      Computing Context is expected to provide its Attestation Evidence.

   $ Root-of-trust-for-storage (RTS):  A trusted Computing Context where
      a Claimaint or Attester stores Claims, Evidence, credentials or
      policies associated with Attestation where no other Computing
      Context is expected to provide its Attestation Evidence.

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   $ Trustworthy Computing Context:  A Computing Context that guarantees
      trustworthy behavior and/or composition (with respect to certain
      declarative guidance and a scope of confidence).  A trustworthy
      Computing Context is a trustworthy system.

   <NMS: is this necessary?> Trustworthy Statement:  Evidence conveyed
      by a Computing Context that is not necessarily trustworthy.
      [update with tamper related terms]

8.8.  Interpretations of RFC4949 Terminology for Attestation

   Assurance:  An attribute of an information system that provides
      grounds for having confidence that the system operates such that
      the system's security policy is enforced [RFC4949] (see Trusted
      System below).

      In common criteria, assurance is the basis for the metric level of
      assurance, which represents the "confidence that a system's
      principal security features are reliably implemented".

      The NIST Handbook [get ref from 4949] notes that the levels of
      assurance defined in Common Criteria represent "a degree of
      confidence, not a true measure of how secure the system actually
      is.  This distinction is necessary because it is extremely
      difficult-and in many cases, virtually impossible-to know exactly
      how secure a system is."

      Historically, assurance was well-defined in the Orange Book
      [http://csrc.nist.gov/publications/history/dod85.pdf] as
      "guaranteeing or providing confidence that the security policy has
      been implemented correctly and that the protection-relevant
      elements of the system do, indeed, accurately mediate and enforce
      the intent of that policy.  By extension, assurance must include a
      guarantee that the trusted portion of the system works only as
      intended."

   Confidence:  The definition of correctness integrity in [RFC4949]
      notes that "source integrity refers to confidence in data values".
      Hence, confidence in an Attestation procedure is referring to the
      degree of trustworthiness of an Attestation Activity that produces
      Evidence (Attester), of an Conveyance Activity that transfers
      Evidence (interconnect), and of a Verification Activity that
      appraises Evidence (Verifier), in respect to correctness
      integrity.

   Correctness:  The property of a system that is guaranteed as the
      result of formal Verification activities.

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   Correctness integrity:  The property that the information represented
      by data is accurate and consistent.

   Data Integrity:  (a) The property that data has not been changed,
      destroyed, or lost in an unauthorized or accidental manner.  (See:
      data integrity service.  Compare: correctness integrity, source
      integrity.)

      (b) The property that information has not been modified or
      destroyed in an unauthorized manner.

   Entity:  A principal, Subject, relying party or stake holder in an
      Attestation ecosystem.

   Identity:  The set of attributes that distinguishes a principal.

   Identifier:  The set of attributes that distinguishes an object.

   Identity Proofing:  A vetting process that verifies the information
      used to establish the identity of a system entity.

   (Information) System:  An organized assembly of computing and
      communication resources and procedures - i.e., equipment and
      services, together with their supporting infrastructure,
      facilities, and personnel - that create, collect, record, process,
      store, transport, retrieve, display, disseminate, control, or
      dispose of information to accomplish a specified set of functions.

   Object:  A system component that contains or receives information.

   Source Integrity:  The property that data is trustworthy (i.e.,
      worthy of reliance or trust), based on the trustworthiness of its
      sources and the trustworthiness of any procedures used for
      handling data in the system.

   Subject:  A Computing Context acting in accordance with the interests
      of a principal.

   Subsystem:  A collection of related system components that together
      perform a system function or deliver a system service.

   System Component:  An instance of a system resource that (a) forms a
      physical or logical part of the system, (b) has specified
      functions and interfaces, and (c) is extant (e.g., by policies or
      specifications) outside of other parts of the system.  (See:
      subsystem.)

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      An identifiable and self-contained part of a $Target-of-
      Evaluation.

   Token:  A data structure suitable for containing Claims.

   Trusted (Trustworthy) System:  A system that operates as expected,
      according to design and policy, doing what is required - despite
      environmental disruption, human user and operator errors, and
      attacks by hostile parties - and not doing other things.

   Verification:  (a) The process of examining information to establish
      the truth of a claimed fact or value.

      (b) The process of comparing two levels of system specification
      for proper correspondence, such as comparing a security model with
      a top-level specification, a top-level specification with source
      code, or source code with object code.

8.9.  Building Block Vocabulary (Not in RFC4949)

   [working title, pulled from various sources, vital]

   Attribute:  TBD

   Characteristic:  TBD

   Context:  TBD

   Endpoint:  TBD

   Environment:  TBD

   Manifest:  TBD

   Telemetry:  An automated communications process by which data,
      readings, Measurements and Evidence are collected at remote points
      and transmitted to receiving equipment for monitoring and
      analysis.  Derived from the Greek roots tele = remote, and metron
      = measure.

9.  IANA considerations

   This document will include requests to IANA:

   o  first item

   o  second item

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10.  Security Considerations

   There are always some.

11.  Acknowledgements

   Maybe.

12.  Change Log

   No changes yet.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

13.2.  Informative References

   [I-D.ietf-sacm-terminology]
              Birkholz, H., Lu, J., Strassner, J., Cam-Winget, N., and
              A. Montville, "Security Automation and Continuous
              Monitoring (SACM) Terminology", draft-ietf-sacm-
              terminology-16 (work in progress), December 2018.

   [I-D.mandyam-eat]
              Mandyam, G., Lundblade, L., Lundblade, L., Ballesteros,
              M., and J. O'Donoghue, "The Entity Attestation Token
              (EAT)", draft-mandyam-eat-01 (work in progress), November
              2018.

   [I-D.richardson-rats-usecases]
              Richardson, M., "Use cases for Remote Attestation common
              encodings", draft-richardson-rats-usecases-00 (work in
              progress), March 2019.

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   [I-D.tschofenig-rats-psa-token]
              Tschofenig, H., Frost, S., Brossard, M., and A. Shaw,
              "Arm's Platform Security Architecture (PSA) Attestation
              Token", draft-tschofenig-rats-psa-token-00 (work in
              progress), March 2019.

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

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

13.3.  URIs

   [1] https://tools.ietf.org/html/rfc4949

Authors' Addresses

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   Darmstadt  64295
   Germany

   Email: henk.birkholz@sit.fraunhofer.de

   Monty Wiseman
   GE Global Research
   USA

   Email: monty.wiseman@ge.com

   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   UK

   Email: hannes.tschofenig@gmx.net

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   Ned Smith
   Intel Corporation
   USA

   Email: ned.smith@intel.com

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