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Versions: 00

RATS Working Group                                      G. Fedorkow, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Informational                       J. Fitzgerald-McKay
Expires: January 1, 2020                        National Security Agency
                                                           June 30, 2019


                  Network Device Attestation Workflow
           draft-fedorkow-rats-network-device-attestation-00

Abstract

   This document describes a workflow for network device attestation.

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
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   This Internet-Draft will expire on January 1, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Problem Description . . . . . . . . . . . . . . . . . . .   4
     1.4.  Solution Requirements . . . . . . . . . . . . . . . . . .   6
     1.5.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       1.5.1.  Out of Scope  . . . . . . . . . . . . . . . . . . . .   8
       1.5.2.  Why Remote Integrity Verification?  . . . . . . . . .   8
       1.5.3.  Network Device Attestation Challenges . . . . . . . .   8
       1.5.4.  Why is OS Attestation Different?  . . . . . . . . . .   9
   2.  Solution Outline  . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  2.1 RIV Software Configuration Attestation using TPM  . .  10
     2.2.  RIV Keying  . . . . . . . . . . . . . . . . . . . . . . .  11
     2.3.  RIV Information Flow  . . . . . . . . . . . . . . . . . .  12
     2.4.  RIV Simplifying Assumptions . . . . . . . . . . . . . . .  13
       2.4.1.  DevID Alternatives  . . . . . . . . . . . . . . . . .  14
       2.4.2.  Additional Attestation of Platform Characteristics  .  14
       2.4.3.  Root of Trust for Measurement . . . . . . . . . . . .  15
       2.4.4.  Reference Integrity Measurements (RIMs) . . . . . . .  15
       2.4.5.  Attestation Logs  . . . . . . . . . . . . . . . . . .  16
   3.  Standards Components  . . . . . . . . . . . . . . . . . . . .  17
     3.1.  Reference Models  . . . . . . . . . . . . . . . . . . . .  17
       3.1.1.  IETF Reference Model for Challenge-Response Remote
               Attestation . . . . . . . . . . . . . . . . . . . . .  17
     3.2.  RIV Workflow  . . . . . . . . . . . . . . . . . . . . . .  18
     3.3.  Layering Model for Network Equipment Attester and
           Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  20
   4.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Implementation Notes  . . . . . . . . . . . . . . . . . .  22
     5.2.  Comparison with TCG PTS / IETF NEA  . . . . . . . . . . .  24
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   There are many components to consider in fielding a trusted computing
   device, from operating systems to applications.  Part of that is a
   trusted supply chain, where manufacturers can certify that the
   product they intended to build is actually the one that was installed
   at a customer's site.

   Attestation is defined here as the process of creating, conveying and
   appraising assertions about Platform trustworthiness characteristics,



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   including Roots of Trust, supply chain trust, identity, platform
   provenance, shielded locations, protected capabilities, software
   configuration, hardware configuration, platform composition,
   compliance to test suites, functional and assurance evaluations, etc.

   The supply chain itself has many elements, from validating suppliers
   of electronic components, to ensuring that shipping procedures
   protect against tampering through many stages of distribution and
   warehousing.  One element that helps maintain the integrity of the
   supply chain after manufacturing is Attestation.

   Within the Trusted Computing Group context, attestation is the
   process by which an independent Verifier can obtain cryptographic
   proof as to the identity of the device in question, evidence of the
   integrity of software loaded on that device when it started up, and
   then verify that what's there is what's supposed to be there.  For
   networking equipment, a verifier capability can be embedded in a
   Network Management Station (NMS), a posture collection server, or
   other network analytics tool (such as a software asset management
   solution, or a threat detection and mitigation tool, etc.).  While
   informally referred to as attestation, this document focuses on a
   subset defined here as Remote Integrity Verification (RIV).  RIV
   takes a network equipment centric perspective that includes a set of
   protocols and procedures for determining whether a particular device
   was launched with untampered software, starting from Roots of Trust.
   While there are many ways to accomplish attestation, RIV sets out a
   specific set of protocols and tools that work in environments
   commonly found in Networking Equipment.  RIV does not cover other
   platform characteristics that could be attested, although it does
   provide evidence of a secure infrastructure to increase the level of
   trust in other platform characteristics attested by other means.

   This profile outlines the RIV problem, and then identifies components
   that are necessary to get the complete attestation procedure working
   in a scalable solution using commercial products.

   This document focuses primarily on software integrity verification
   using the Trusted Platform Module (TPM) as a root of trust.

1.1.  Requirements Language

   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.





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1.2.  Goals

   The RIV attestation workflow outlined in this document is intended to
   meet the following high-level goals:

   o  Provable Device Identity - The ability to identify a device using
      a cryptographic identifier is a critical prerequisite to software
      inventory attestation.

   o  Software Inventory - A key goal is to identify the software
      release installed on the device, and to provide evidence of its
      integrity.

   o  Verification - Verification of software and configuration of the
      device shows that the software that's supposed to be installed on
      there actually has been launched, without unauthorized
      modification.

   This document itself is non-normative; the document does not define
   protocols, but rather identifies protocols that can be used together
   to achieve the goals above, and in some cases, highlights gaps in
   existing protocols.

1.3.  Problem Description

   RIV is a procedure that assures a network operator that the equipment
   on their network can be reliably identified, and that untampered
   software of a known version is installed on each endpoint.  In this
   context, endpoint might include the conventional endpoints like
   servers and laptops, but also network equipment itself, such as
   routers, switches and firewalls.

   RIV can be viewed as a link in a trusted supply chain, and includes
   three major processes:

   o  Creation of Evidence is the process whereby an endpoint generates
      cryptographic proof (evidence) of claims about platform
      properties.  In particular, the platform identity and its software
      configuration are of critical importance.












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        * Platform Identity refers to the mechanism assuring the
        attestation verifier (typically a network administrator)
        that the equipment on their network can be reliably identified,
        and that its manufacturer is certified by a trusted authority.
        This certification provides the verifier with assurance that
        the Root of Trust elements of the device were verified by the
        manufacturer before the device was shipped.

        * Software used to boot a platform can be described as a chain
        of measurements, starting from a Root of Trust for Measurement,
        that normally ends when the system software is loaded.
        Measurement records the identity, integrity and version of each
        software component registered with the TPM, so that the
        subsequent appraisal stage can determine whether the software
        installed is authentic and free of tampering. Clearly the second
        part of the problem, attesting the state of mutable components
        of a given device, is of little value without the first part,
        reliable identification of the device in question.  By the
        same token, unambiguous identity of a device is necessary,
        but is insufficient to assure the operator of the provenance
        of the device through the supply chain, or that the device is
        configured to behave properly.

   o  Conveyance of Evidence is the process of reliably transporting
      evidence from an endpoint to an appraiser/verifier, e.g. a
      management station.  The transport is typically carried out via a
      management network.  The channel must provide integrity and
      authenticity, and, in some use cases, may also require
      confidentiality.

   o  Appraisal of Evidence is the process of verifying the evidence
      received by a verifier/appraiser from a device, and using verified
      evidence to inform decision making.  In this context, verification
      means comparing the device characteristics reported as evidence
      with the configuration expected by the system administrator.  This
      step can work only when there is a way to express what should be
      there, often referred to as golden measurements, or Reference
      Integrity Measurements, representing the intended configured state
      of an endpoint.

   As a part of a trusted supply chain, RIV attestation provides two
   important benefits:

   o  Platform Identity is the mechanism providing trusted identity can
      reassure network managers that the specific devices they ordered
      from authorized manufacturers for attachment to their network are
      the ones that were installed, and that they continue to be present
      in their network.  As part of the mechanism for Platform Identity,



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      cryptographic proof of the identity of the manufacturer is also
      provided.

   o  Software Configuration is the mechanism that reports the state of
      mutable components on the device can assure network managers that
      they have known, untampered software configured to run in their
      network.

   An implementation of RIV requires three technologies

   1.  Identity: Platform identity can be based on IEEE 802.1AR Device
       Identity [IEEE-802-1AR], coupled with careful supply-chain
       management by the manufacturer.  The DevID certificate contains a
       statement by the manufacturer that establishes the provenance of
       the device as it left the factory.  Some applications with a
       more-complex post-manufacture supply chain (e.g.  Value Added
       Resellers), or with privacy concerns, may want to use an
       alternate mechanism for platform authentication based on TCG
       Platform Certificates [Platform-Certificates].

   2.  Platform Attestation provides evidence of configuration of
       software elements throughout the product lifecycle.  This form of
       attestation can be implemented with TPM PCR, Quote and log
       mechanisms, which provide an authenticated mechanism to report
       what software actually starts up on the device each time it
       reboots.  Note that the TPM requires separate keys for identity
       DevID) and attestation (PCR Quotes0) (see Section 2.2).

   3.  Reference Integrity Measurements must be conveyed from the
       software authority (often the manufacturer for embedded systems)
       to the system in which verification will take place

   Network operators benefit from a trustworthy attestation mechanism
   that provides assurance that their comprises authentic equipment, and
   has loaded software free of known vulnerabilities and unauthorized
   tampering.

1.4.  Solution Requirements

   An Attestation solution must meet a number of requirements to make it
   simple to deploy at scale.

   1.  Easy to Use - This solution should work "out of the box" as far
       as possible, that is, with the fewest possible steps needed at
       the end-user's site.  Eliminate complicated databases or
       provisioning steps that would have to be executed by the owner of
       a new device.  Network equipment is often required to "self-
       configure", to reliably reach out without manual intervention to



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       prove its identity and operating posture, then download its own
       configuration.  See [RFC8572] for an example of Secure Zero Touch
       Provisioning.

   2.  Multi-Vendor - This solution should identify standards-based
       interfaces that allow attestation to work with attestation-
       capable devices and verifiers supplied by different vendors in
       one network.

   3.  Scalable - The solution must not depend on choke points that
       limit the number of endpoints that could be evaluated in one
       network domain.

   4.  Extensible - A network equipment attestation solution needs to
       expand over time as new features are added.  The solution must
       allow new features to be added easily, providing for a smooth
       transition and allowing newer and older architectural components
       to continue to work together.  Further, a network equipment
       attestation solution and the specifications referenced here must
       define safe extensibility mechanisms that enable innovation
       without breaking interoperability.

   5.  Efficient - A network equipment attestation solution should, to
       the greatest extent feasible, continuously monitor the health and
       posture status of network devices.  Posture measurements should
       be updated in real-time as changes to device posture occur and
       should be published to remote integrity validators.  Validation
       reports should also be shared with their relying parties (for
       example, network administrators, or network analytics that rely
       on these reports for posture assessment) as soon as they are
       available.

1.5.  Scope

   This document includes a number of assumptions to limit the scope:

   o  This solution is for use in non-privacy-preserving applications
      (for example, networking, Industrial IoT), avoiding the need for a
      Privacy Certificate Authority for attestation keys

   o  This document applies primarily to "embedded" applications, where
      the device manufacturer ships the software image for the device.

   o  The approach outlined in this document assumes a physical TPM.







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1.5.1.  Out of Scope

   o  Run-Time Attestation: Run-time attestation of Linux or other
      multi-threaded operating system processes considerably expands the
      scope of the problem.  Many researchers are working on that
      problem, but this document defers the run-time attestation
      problem.

   o  Multi-Vendor Embedded Systems: Additional coordination would be
      needed for devices that themselves comprise hardware and software
      from multiple vendors, integrated by the end user.

   o  Processor Sleep Modes: Embedded equipment typically does not
      "sleep", so sleep and hibernate modes are not considered.

   o  Virtualization and Containerization: These technologies are
      increasingly used in embedded systems, but are not considered in
      this revision of the document.

1.5.2.  Why Remote Integrity Verification?

   Remote Integrity Verification can go a long way to solving the "Lying
   Endpoint" problem, in which malicious software on an endpoint may
   both subvert the intended function, and also prevent the endpoint
   from reporting its compromised status.  Man-in-the Middle attacks are
   also made more difficult through a strong focus on device identity

   Attestation data can be used for asset management, vulnerability and
   compliance assessment, plus configuration management.

1.5.3.  Network Device Attestation Challenges

   There have been demonstrations of attestation using TPMs for years,
   accompanied by compelling security reasons for adopting attestation.
   Despite this, the technology has not been widely adopted, in part,
   due to the difficulties in deploying TPM-based attestation.  Some of
   those difficulties are:

   o  Standardizing device identity.  Creating and using unique device
      identifiers is difficult, especially in a privacy-sensitive
      environment.  But attestation is of limited value if the operator
      is unable to determine which devices pass attestation validation
      tests, and which fail.  This problem is substantially simplified
      for infrastructure devices like network equipment, where identity
      can be explicitly coded using IEEE 802.1AR, but doing so relies on
      adoption of 802.1AR [IEEE-802-1AR] by manufacturers and hardware
      system integrators.




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   o  Standardizing attestation representations and conveyance.
      Interoperable remote attestation has a fundamental dependence on
      vendors agreeing to a limited set of network protocols for
      communicating attestation data.  Network device vendors will be
      slow to adopt the protocols necessary to implement remote
      attestation without a fully-realized plan for deployment.

   o  Interoperability.  Networking equipment operates in a
      fundamentally multi-vendor environment, putting additional
      emphasis on the need for standardized procedures and protocols.

   o  Attestation evidence is complex.  Operating systems used in larger
      embedded devices are often multi-threaded, so the order of
      completion for individual processes is non-deterministic.  While
      the hash of a specific component is stable, once extended into a
      PCR, the resulting values are dependent on the (non-deterministic)
      ordering of events, so there will never be a single known-good
      value for some PCRs.  Careful analysis of event logs can provide
      proof that the expected modules loaded, but it's much more
      complicated than simply comparing hashes.

   o  Software configurations can have seemingly infinite variability.
      This problem is nearly intractable on PC and Server equipment,
      where end users have unending needs for customization and new
      applications.  However, embedded systems, like networking
      equipment, are often simpler, in that there are fewer variations
      and releases, with vendors typically offering fewer options for
      mixing and matching.

   o  Software updates can be complex.  Even the most organized network
      operator may have many different releases in their network at any
      given time, with the result that there's never a single digest or
      fingerprint that indicates the software is "correct"; digests
      formed by hashing software modules on a device can only show the
      correct combination of versions for a specific device at a
      specific time.

   None of these issues are insurmountable, but together, they've made
   deployment of attestation a major challenge.  The intent of this
   document is to outline an attestation profile that's simple enough to
   deploy, while yielding enough security to be useful.

1.5.4.  Why is OS Attestation Different?

   Even in embedded systems, adding Attestation at the OS level (e.g.
   Linux IMA, Integrity Measurement Architecture [IMA]) increases the
   number of objects to be attested by one or two orders of magnitude,




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   involves software that's updated and changed frequently, and
   introduces processes that complete in unpredictable order.

   TCG and others (including the Linux community) are working on methods
   and procedures for attesting the operating system and application
   software, but standardization is still in process.

2.  Solution Outline

2.1.  2.1 RIV Software Configuration Attestation using TPM

   RIV Attestation is a process for determining the identity of software
   running on a specifically-identified device.  Remote Attestation is
   broken into two phases, shown in Figure 1:

   o  During system startup, measurements (i.e., hashes computed as
      fingerprints of files) are extended, or cryptographically folded,
      into the TPM.  Entries are also added to an informational log.
      The measurement process generally follows the Chain of Trust model
      used in Measured Boot, where each stage of the system measures the
      next one before launching it.

   o  Once the device is running and has operational network
      connectivity, a separate, trusted server (called a Verifier in
      this document) can interrogate the network device to retrieve the
      logs and a copy of the digests collected by hashing each software
      object, signed by an attestation private key known only to the
      TPM.

   The result is that the Verifier can verify the device's identity by
   checking the certificate corresponding to the TPM's attestation
   private key, and can validate the software that was launched by
   comparing digests in the log with known-good values, and verifying
   their correctness by comparing with the signed digests from the TPM.

   It should be noted that attestation and identity are inextricably
   linked; signed evidence that a particular version of software was
   loaded is of little value without cryptographic proof of the identity
   of the device producing the evidence.












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       +-------------------------------------------------------+
       | +--------+    +--------+   +--------+    +---------+  |
       | | BIOS   |--->| Loader |-->| Kernel |--->|Userland |  |
       | +--------+    +--------+   +--------+    +---------+  |
       |     |            |           |                        |
       |     |            |           |                        |
       |     +------------+-----------+-+                      |
       |                        Step 1  |                      |
       |                                V                      |
       |                            +--------+                 |
       |                            |  TPM   |                 |
       |                            +--------+                 |
       |   Router                       |                      |
       +--------------------------------|----------------------+
                                        |
                                        |  Step 2
                                        |    +-----------+
                                        +--->| Verifier  |
                                             +-----------+

       Reset---------------flow-of-time-during-boot--...------->

     In Step 1, measurements are "extended" into the TPM as processes
   start.  In Step 2, signed PCR digests are retreived from the TPM for
             offbox analysis after the system is operational.

                      Figure 1: TCG Attestation Model

2.2.  RIV Keying

   TPM 1.2 and TPM 2.0 have a variety of rules separating the functions
   of identity and attestation, allowing for use-cases where software
   configuration must be attested, but privacy must be maintained.

   To accommodate these rules in an environment where device privacy is
   not normally a requirement, the TCG Guidance for Securing Network
   Equipment [NetEq] suggests using separate keys for Identity (i.e.,
   DevID) and Attestation (i.e., signing a quote of the contents of the
   PCRs).

   In this case, the device manufacturer should provision an Initial
   Attestation Key (IAK) and x.509 certificate that parallels the
   IDevID, with the same device ID information as the IDevID certificate
   (i.e., the same Subject Name and Subject Alt Name, even though the
   key pairs are different).  This allows a quote from the device,
   signed by the IAK, to be linked directly to the device that provided
   it, by examining the corresponding IAK certificate.




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   Inclusion of an IAK by a vendor does not preclude a mechanism whereby
   an Administrator can define Local Attestation Keys (LAKs) if desired.

2.3.  RIV Information Flow

   RIV workflow for networking equipment is organized around a simple
   use-case, where a network operator wishes to verify the integrity of
   software installed in specific, fielded devices.  This use-case
   implies several components:

   1.  A Device (e.g. a router or other embedded device, also known as
       an Attester) somewhere and the network operator wants to examine
       its boot state.

   2.  A Verifier (which might be a network management station)
       somewhere separate from the Device that will retrieve the
       information and analyze it to pass judgement on the security
       posture of the device.

   3.  A Relying Party, which has access to the Verifier to request
       attestation and to act on results.

   4.  This document assumes that signed Reference Integrity
       Measurements (RIMs) (aka "golden measurements") can either be
       created by the device manufacturer and shipped along with the
       device as part of its software image, or alternatively, could be
       obtained a number of other ways (direct to the verifier from the
       manufacturer, from a third party, from the owner's observation of
       what's thought to be a "known good system", etc.).  Retrieving
       RIMs from the device itself allows attestation to be done in
       systems which may not have access to the public internet, or by
       other devices that are not management stations per-se (e.g., a
       peer device).  If reference measurements are obtained from
       multiple sources, the Verifier may need to evaluate the relative
       level of trust to be placed in each source in case of a
       discrepancy.

   These components are illustrated in Figure 2.

   A more-detailed taxonomy of terms is given in [I-D.birkholz-rats-
   architecture]










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   +---------------+        +-------------+        +---------+--------+
   |               |        | Attester    | Step 1 | Verifier|        |
   | Asserter      |        | (Device     |<-------| (Network| Relying|
   | (Device       |        | under       |------->| Mngmt   | Party  |
   | Manufacturer  |        | attestation)| Step 2 | Station)|        |
   | or other      |        |             |        |         |        |
   | authroity)    |        |             |        |         |        |
   +---------------+        +-------------+        +---------+--------+
          |                                             /\
          |                  Step 0                      |
          -----------------------------------------------


   In Step 0, The Asserter (the device manufacturer) provides a Software
    Image accompanied by Reference Integrity Measurements (RIMs) to the
    Attester (the device under attestation) signed by the asserter.  In
     Step 1, the Verifier (Network Management Station), on behalf of a
    Relying Party, requests Identity, Measurement Values (and possibly
     RIMs) from the Attester.  In Step 2, the Attester responds to the
        request by providing a DevID, Quotes (measured values), and
                 optionally RIMs, signed by the Attester.

        Figure 2: RIV Reference Configuration for Network Equipment

   See Section 3.1.1 for more narrowly defined terms related to
   Attestation

2.4.  RIV Simplifying Assumptions

   This document makes the following simplifying assumptions to reduce
   complexity:

   o  The product to be attested is shipped with an IEEE 802.1AR DevID
      and an Initial Attestation Key (IAK) with certificate.  The IAK
      cert contains the same identity information as the DevID
      (specifically, the same Subject Name and Subject Alt Name, signed
      by the manufacturer), but it's a type of key that can be used to
      sign a TPM Quote.  This convention is described in TCG Guidance
      for Securing Network Equipment [NetEq].  For network equipment,
      which is generally non-privacy-sensitive, shipping a device with
      both an IDevID and an IAK already provisioned substantially
      simplifies initial startup.  Privacy-sensitive applications may
      use the TCG Platform Certificate and additional procedures to
      install identity credentials on the platform after manufacture.
      (See Section 2.3.1 below for the Platform Certificate alternative)

   o  The product is equipped with a Root of Trust for Measurement, Root
      of Trust for Storage and Root of Trust for Reporting that is



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      capable of conforming to the TCG Trusted Attestation Protocol
      (TAP) Information Model [TAP].

   o  The vendor will ship Reference Integrity Measurements (i.e.,
      known-good measurements) in the form of signed CoSWID tags
      [I-D.ietf-sacm-coswid], [SWID], as described in TCG Reference
      Integrity Measurement Manifest [RIM].

2.4.1.  DevID Alternatives

   Some situations may have privacy-sensitive requirements that preclude
   shipping every device with an Initial Device ID installed.  In these
   cases, the IDevID can be installed remotely using the TCG Platform
   Certificate [Platform-Certificates].

   Some security-sensitive administrators may want to install their own
   identity credentials to certify platform identity and attestation
   results.  IEEE 802.1AR [IEEE-802-1AR] allows for both Initial Device
   Identity credentials, installed by the manufacturer, or Local Device
   Identity credentials installed by the administrator of the platform.
   TCG TPM 2.0 Keys documents [Platform-DevID-TPM-2.0] and
   [PC-Client-BIOS-TPM-2.0] specifies analogous Initial and Local
   Attestation Keys (IAK and LAK), and contains figures showing the
   relationship between IDevID, LDevID, IAK and LAK keys.

   Platform administrators are free to use any number of criteria to
   judge authenticity of a platform before installing local identity
   keys, as part of an on-boarding process.  The TCG TPM 2.0 Keys
   document [Platform-DevID-TPM-2.0] also outlines procedures for
   creating Local Attestation Keys and Local Device IDs (LDevIDs) rooted
   in the manufacturer's IDevID as a check to reduce the chances that
   counterfeit devices are installed in the network.

   Note that many networking devices are expected to self-configure (aka
   Zero Touch Provisioning).  Current standardized zero-touch mechanisms
   such as [RFC8572] assume that identity keys are already in place
   before network on-boarding can start.

2.4.2.  Additional Attestation of Platform Characteristics

   The Platform Attribute Credential [Platform-Certificates] can also be
   used to convey additional information about a platform from the
   manufacturer or other entities in the supply chain.  While outside
   the scope of RIV, the Platform Attribute Credential can deliver
   information such as lists of serial numbers for components embedded
   in a device or security assertions related to the platform, signed by
   the manufacturer, system integrator or value-added-reseller.




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2.4.3.  Root of Trust for Measurement

   The measurements needed for attestation require that the device being
   attested is equipped with a Root of Trust for Measurement, i.e., some
   trustworthy mechanism that can take the first measurement in the
   chain of trust required to attest that each stage of system startup
   is verified, and a Root of Trust for Reporting to report the results
   [Roots-of-Trust].

   While there are many complex aspects of a Root of Trust, two aspects
   that are important in the case of attestation are:

   o  The first measurement taken by the Root of Trust for Measurement,
      and stored in the Root of Trust for Storage, is presumed to be
      correct.

   o  There must not be a way to reset the RTM without re-entering the
      RTS code.

   The first measurement can't be checked by a code that's been
   previously checked by something further back up the chain (it's the
   first, after all); if that first measurement can be subverted, none
   of the remaining measurements can be trusted.  (See [NIST-SP-800-155]

2.4.4.  Reference Integrity Measurements (RIMs)

   Much of attestation focuses on collecting and transmitting 'evidence'
   in the form of PCR measurements and attestation logs.  But the
   critical part of the process is enabling the verifier to decide
   whether the measured hashes are "the right ones" or not.

   While it must be up to network administrators to decide what they
   want on their networks, the software supplier should supply the
   Reference Integrity Measurements, (aka Golden Measurements or "known
   good" hash digests) that may be used by a verifier to determine if
   evidence shows known good, known bad or unknown software
   configurations.

   In general, there are two kinds of reference measurements:

   1.  Measurements of early system startup (e.g., BIOS, boot loader, OS
       kernel) are essentially single threaded, and executed exactly
       once, in a known sequence, before any results could be reported.
       In this case, while the method for computing the hash and
       extending relevant PCRs may be complicated, the net result is
       that the software (more likely, firmware) vendor will have one
       known good PCR value that "should" be present in the PCR after




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       the box has booted.  In this case, the signed reference
       measurement simply lists the expected hash for the given version.

   2.  Measurements taken later in operation of the system, once an OS
       has started (for example, Linux IMA[16.]), may be more complex,
       with unpredictable "final" PCR values.  In this case, the
       Verifier must have enough information to reconstruct the expected
       PCR values from logs and signed reference measurements from a
       trusted authority.

   In both cases, the expected values can be expressed as signed CoSWID
   tags, but the SWID structure in the second case is somewhat more
   complex.  An example of how CoSWIDs could be incorporated into a
   reference manifest can be found in the IETF Internet-Draft "A SUIT
   Manifest Extension for Concise Software Identifiers"
   [I-D.birkholz-suit-coswid-manifest].

   The TCG has done exploratory work in defining formats for reference
   integrity manifests under the working title TCG Reference Integrity
   Manifest [RIM].

2.4.5.  Attestation Logs

   Quotes from a TPM can provide evidence of the state of a device at
   the time the quote was requested, but to make sense of the quote in
   most cases an event log of what software modules contributed which
   values to the quote during startup must also be provided.  The log
   needs not be secured, but it is essential that the logs contain
   enough information to exactly reconstruct the state of whatever went
   into the quote (e.g., PCR values).

   TCG has defined several event log formats:

   o  Legacy BIOS event log (TCG PC Client Specific Implementation
      Specification for Conventional BIOS,
      Section 11.3[PC-Client-BIOS-TPM-1.2])

   o  UEFI BIOS event log (TCG EFI Platform Specification for TPM Family
      1.1 or 1.2, Section 7 [EFI])

   o  Canonical Event Log [Canonical-Event-Log]

   It should be noted that a given device might use more than one event
   log format (e.g., a UEFI log during initial boot, switching to
   Canonical Log when the host OS launches).

   The TCG SNMP Attestation MIB [SNMP-Attestation-MIB] will support any
   record-oriented log format, including the three TCG-defined formats,



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   but it currently leaves figuring out which log(s) are in what format
   up to the Verifier.

3.  Standards Components

3.1.  Reference Models

3.1.1.  IETF Reference Model for Challenge-Response Remote Attestation

   Initial work at IETF defines remote attestation as follows:

   o  The Reference Interaction Model for Challenge-Response-based
      Remote Attestation is based on the standard roles defined in
      [I-D.birkholz-rats-architecture]:

   o

      *  Attester: The role that designates the subject of the remote
         attestation.  A system entity that is the provider of evidence
         takes on the role of an Attester.

      *  Verifier: The role that designates the system entity and that
         is the appraiser of evidence provided by the Attester.  A
         system entity that is the consumer of evidence takes on the
         role of a Verifier.

   The following diagram illustrates a common information flow between a
   Verifier and an Attester, specified in
   [I-D.birkholz-rats-reference-interaction-model]:

Attester                                                     Verifier
   |                                                               |
   | <------- requestAttestation(nonce, authSecID, claimSelection) |
   |                                                               |
collectAssertions(assertionsSelection)                             |
   | => assertions                                                 |
   |                                                               |
signAttestationEvidence(authSecID, assertions, nonce)              |
   | => signedAttestationEvidence                                  |
   |                                                               |
   | signedAttestationEvidence ----------------------------------> |
   |                                                               |
   | verifyAttestationEvidence(signedAttestationEvidence, refassertions)
   |                                          attestationResult <= |
   |                                                               |

                Figure 3: IETF Attestation Information Flow




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   The RIV approach outlined in this document aligns with the RATS
   reference model.

3.2.  RIV Workflow

   The overall flow for an attestation session is shown in Figure 4.  In
   this diagram:

   o  Step 0, positioning of the signed reference measurements, may
      happen in two ways:

   o  Step 0A below shows a verifier obtaining reference measurements
      directly from a software configuration authority, whether it's the
      vendor or another authority chosen by the system administrator.
      The reference measurements are signed by the Asserter (i.e., the
      software configuration authority).

   o  - Or - Step 0B, the reference measurements, signed by the
      Asserter, may be distributed as part of software installation,
      long before the attestation session begins.  Software installation
      is usually vendor-dependent, so there are no standards involved in
      this step.  However, the verifier can use the same protocol to
      obtain the reference measurements from the device as it would have
      used with an external reference authority

   o  In Step 1, the Verifier initiates an attestation session by
      opening a TLS session, validated using the DevID to prove that the
      connection is attesting the right box.

   o  In Step 2, measured values are retrieved from the Attester's TPM
      using a YANG or SNMP interface that implements the TCG TAP model
      (e.g.  YANG Module for Basic Challenge-Response-based Remote
      Attestation Procedures
      [I-D.birkholz-yang-basic-remote-attestation]).

   o  In Step 3, the Attester also delivers a copy of the signed
      reference measurements, using Software Inventory YANG module based
      on Software Identifiers [I-D.birkholz-yang-swid].













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   +---------------+        +-------------+        +---------+
   |               |        |             | Step 1 |         |
   |               |        | Attester    |<------>| Verifier|
   | Asserter      |        | (Device     |<------>| (Network|
   |(Configuration |------->| under       | Step 2 | Mngmt   |
   | Authority)    | Step 0A| attestation)|        | Station)|
   |               |        |             |------->|         |
   +---------------+        +-------------+ Step 3 +---------+
           |                                           /|\
           |                                            |
           ----------------------------------------------
                           Step 0B


   Either CoSWID-encoded reference measurements are signed by a trusted
    authority and retrieved directly prior to attestation (as shown in
   Step 0A), or CoSWID-encoded reference measurements are signed by the
       device manufacturer, installed on the device by a proprietary
    installer, and delivered during attestation (as shown in Step 0B).
   In Step 1, the Verifier initiates a connection for attestation.  The
   Attester's identity is validated using DevID with TLS.  In Step 2, a
   nonce, quotes (measured values) and measurement log are conveyed via
   TAP with a protocol-specific binding (e.g.  SNMP).  Logs are sent in
       the Canonical Log Format In Step 3, CoSWID-encoded reference
        measurements are retrieved from the Attester using the YANG
                       ([I-D.birkholz-yang-swid].  .

                Figure 4: RIV Protocol and Encoding Summary

   The following components are used:

   1.  TPM Keys are configured according to [Platform-DevID-TPM-2.0],
       [PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2]

   2.  Measurements of bootable modules are taken according to TCG PC
       Client [PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]

   3.  Device Identity is managed by IEEE 802.1AR certificates
       [IEEE-802-1AR], with keys protected by TPMs.

   4.  Quotes are retrieved according to TCG TAP Information Model [TAP]

   5.  Reference Integrity Measurements are encoded as CoSWID tags, as
       defined in the TCG RIM document [RIM], compatible with NIST IR
       8060 [NIST-IR-8060] and the IETF CoSWID draft
       [I-D.ietf-sacm-coswid].  Reference measurements are signed by the
       device manufacturer.




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3.3.  Layering Model for Network Equipment Attester and Verifier

   Retrieval of identity and attestation state uses one protocol stack,
   while retrieval of Reference Measurements uses a different set of
   protocols.  Figure 5 shows the components involved.

   +-----------------------+              +-------------------------+
   |                       |              |                         |
   |       Attester        |<-------------|        Verifier         |
   |       (Device)        |------------->|   (Management Station)  |
   |                       |      |       |                         |
   +-----------------------+      |       +-------------------------+
                                  |
              -------------------- --------------------
              |                                        |
   ---------------------------------- ---------------------------------
   |Reference Integrity Measurements| |         Attestation           |
   ---------------------------------- ---------------------------------

   ********************************************************************
   *         IETF Attestation Reference Interaction Diagram           *
   ********************************************************************

       .......................         .......................
       . Reference Integrity .         . TAPS (PTS2.0) Info  .
       .     Measurement     .         . Model and Canonical .
       .      Manifest       .         .     Log Format      .
       .......................         .......................

       *************************  .............. **********************
       * YANG SWID Module      *  . TCG        . * YANG Attestation   *
       * I-D.birkholz-yang-swid*  . Attestation. * Module             *
       *                       *  . MIB        . * I-D.birkholz-yang- *
       *                       *  .            . * basic-remote-      *
       *                       *  .            . * attestation        *
       *************************  .............. **********************

       *************************  ************ ************************
       * XML, JSON, CBOR (etc) *  *    UDP   * * XML, JSON, CBOR (etc)*
       *************************  ************ ************************

       *************************               ************************
       *   RESTCONF/NETCONF    *               *   RESTCONF/NETCONF   *
       ************************               *************************

       *************************               ************************
       *       TLS, SSH        *               *       TLS, SSH       *
       *************************               ************************



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    IETF documents are captured in boxes surrounded by asterisks.  TCG
       documements are shown in boxes surrounded by dots.  The IETF
      Attestation Reference Interaction Diagram, Reference Intefrity
   Measurment Manifest, TAPS Information Model and Canonical Log Format,
      and both YANG modules are works in progress.  Information Model
   layers describe abstract data objects that can be requested, and the
   corresponding response SNMP is still widely used, but the industry is
   transitioning to YANG, so in some cases, both will be required.  TLS
    Authentication with TPM has been shown to work; SSH authentication
        using TPM-protected keys is not as easily done [as of 2019]

                       Figure 5: RIV Protocol Stacks

4.  Conclusion

   TCG technologies can play an important part in the implementation of
   Remote Integrity Verification.  Standards for many of the components
   needed for implementation of RIV already exist:

   o  Platform identity can be based on IEEE 802.1AR Device identity,
      coupled with careful supply-chain management by the manufacturer.

   o  Complex supply chains can be certified using TCG Platform
      Certificates [Platform-Certificates]

   o  The TCG TAP mechanism can be used to retrieve attestation
      evidence.  Work is needed on a YANG model for this protocol.

   o  Reference Measurements must be conveyed from the software
      authority (e.g., the manufacturer) to the system in which
      verification will take place.  IETF CoSWID work forms the basis
      for this, but new work is needed to create an information model
      and YANG implementation.

   Gaps still exist for implementation in Network Equipment (as of May
   2019):

   o  Coordination of YANG model development with the IETF is still
      needed

   o  Specifications for management of signed Reference Integrity
      Measurements must still be completed

5.  Appendix







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5.1.  Implementation Notes

   Table 1 summarizes many of the actions needed to complete an
   Attestation system, with links to relevant documents.  While
   documents are controlled by a number of standards organizations, the
   implied actions required for implementation are all the
   responsibility of the manufacturer of the device, unless otherwise
   noted.

   +------------------------------------------------------------------+
   |             Component                           |  Controlling   |
   |                                                 | Specification  |
   --------------------------------------------------------------------
   | Make a Secure execution environment             |   TCG RoT      |
   |   o Attestation depends entirely on a secure    |   UEFI.org     |
   |     root of trust for measurement.              |                |
   |   o  Refer to TCG Root of Trust for Measurement.|                |
   |   o  NIST SP 800-193 also provides guidelines   |                |
   |      on Roots of Trust                          |                |
   --------------------------------------------------------------------
   | Get a TPM properly provisioned as described in  | TCG TPM DevID  |
   |   TCG documents.                                | TCG Platform   |
   |                                                 |   Certificate  |
   --------------------------------------------------------------------
   | Put a DevID or Platform Cert in the TPM         | TCG TPM DevID  |
   |    o Install an Initial Attestation Key at the  | TCG Platform   |
   |      same time so that Attestation can work out |   Certificate  |
   |      of the box                                 |-----------------
   |    o Equipment suppliers and owners may want to | IEEE 802.1AR   |
   |      implement Local Device ID as well as       |                |
   |      Initial Device ID                          |                |
   --------------------------------------------------------------------
   | Connect the TPM to the TLS stack                | Vendor TLS     |
   |    o  Use the DevID in the TPM to authenticate  | stack (This    |
   |       TAP connections, identifying the device   | action is      |
   |                                                 | simply         |
   |                                                 | configuring TLS|
   |                                                 | to use the     |
   |                                                 | DevID as its   |
   |                                                 | trust anchor.) |
   --------------------------------------------------------------------
   | Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID    |
   |    o  Add reference measurements into SWID tags | ISO/IEC 19770-2|
   |    o  Manufacturer should sign the SWID tags    | NIST IR 8060   |
   |    o  This should be covered in a new TCG       | TagVault SWID  |
   |       Reference Integrity Manifest document     |   Tag Signing  |
   |       -  IWG should define the literal SWID     |   Guidance     |
   |          format                                 |-----------------



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   |       -  IWG should evaluate whether IETF SUIT  | TCG RIM        |
   |          is a suitable manifest when multiple   |                |
   |          SWID tags are involved                 |                |
   |       -  There could be a proof-of-concept      |                |
   |          project to actually make sample SWID   |                |
   |          tags (a gap might appear in the        |                |
   |          process)                               |                |
   --------------------------------------------------------------------
   |  Package the SWID tags with a vendor software   | There is no    |
   |  release                                        | need to specify|
   |    o  A tag-generator plugin could help         | where the tags |
   |      (i.e., a plugin for common development     | are stored in a|
   |      environments.  NIST has something that     | vendor OS, as  |
   |      plugs into Maven Build Environment)        | long as there  |
   |                                                 | is a standards-|
   |                                                 | based mechanism|
   |                                                 | to retrieve    |
   |                                                 | them.          |
   |                                                 |-----------------
   |                                                 | TCG RIM        |
   --------------------------------------------------------------------
   |  BIOS SWIDs might be hard to manage on an OS    |                |
   |  disk-- maybe keep them in the BIOS flash?      | TCG RIM        |
   |  o  Maybe a UEFI Var?  Would its name have to be|                |
   |     specified by UEFI.org?                      |                |
   |  o  How big is a BIOS SWID tag?  Do we need to  |                |
   |     use a tag ID instead of an actual tag?      |                |
   |  o  Note that the presence of Option ROMs turns |                |
   |     the BIOS reference measurements into a      |                |
   |     multi-vendor interoperability problem       |                |
   --------------------------------------------------------------------
   |  Use PC Client measurement definitions as a     | TCG PC Client  |
   |  starting point to define the use of PCRs       | BIOS           |
   |  (although Windows  OS is rare on Networking    |-----------------
   |  Equipment)                                     | There have been|
   |                                                 | proposals for  |
   |                                                 | non-PC-Client  |
   |                                                 | allocation of  |
   |                                                 | PCRs, although |
   |                                                 | no specific    |
   |                                                 | document exists|
   |                                                 | yet.           |
   --------------------------------------------------------------------
   |  Use TAP to retrieve measurements               |                |
   |    o  Map TAP to SNMP                           | TCG SNMP MIB   |
   |    o  Map to YANG                               | YANG Module for|
   |    o  Complete Canonical Log Format             |   Basic        |
   |                                                 |   Attestation  |



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   |                                                 | TCG Canonical  |
   |                                                 |   Log Format   |
   --------------------------------------------------------------------
   | Posture Collection Server (as described in IETF |                |
   |  SACMs ECP) would have to request the           |                |
   |  attestation and analyze the result             |                |
   | The Management application might be broken down |                |
   |  to several more components:                    |                |
   |    o  A Posture Manager Server                  |                |
   |       which collects reports and stores them in |                |
   |       a database                                |                |
   |    o  One or more Analyzers that can look at the|                |
   |       results and figure out what it means.     |                |
   --------------------------------------------------------------------

                        Figure 6: Component Status

5.2.  Comparison with TCG PTS / IETF NEA

   Some components of an Attestation system have been implemented for
   end-user machines such as PCs and laptops.  Figure 7 shows the
   corresponding protocol stacks.





























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   +-----------------------+              +-------------------------+
   |                       |              |                         |
   |       Attester        |<-------------|        Verifier         |
   |       (Device)        |------------->|   (Management Station)  |
   |                       |      |       |                         |
   +-----------------------+      |       +-------------------------+
                                  |
              -------------------- --------------------
              |                                        |
   ---------------------------------- ---------------------------------
   |Reference Integrity Measurements| |         Attestation           |
   ---------------------------------- ---------------------------------

   --------------------------------------------------------------------
   |         IETF Attestation Reference Interaction Diagram           |
   -------------------------------------------------------------------

       .......................         --------------------------------
       . No Existing         .         |  TAPS (PTS2.0) Info Model and|
       . Reference Integrity .         |  Canonical Log Format        |
       . Measurement Manifest.         |                              |
       . Protocols Exist     .         --------------------------------
       .                     .
       .                     .        ---------------------- ----------
       .                     .        | YANG Attestation   | |IETF NEA|
       .                     .        | Module             | | Msg and|
       .                     .        | I-D.birkholz-yang- | | Attrib.|
       .                     .        | basic-remote-      | | for PA-|
       .                     .        | attestation        | | TNC    |
       .                     .        ---------------------- ----------
       .                     .        ---------------------- ----------
       .                     .        | XML, JSON, CBOR    | | PT-TLS |
       .                     .        ---------------------- | (for   |
       .                     .        ---------------------- |endpoint|
       .                     .        | NETCONF, RESTCONF, | |mcahines|
       .                     .        | COAP               | |        |
       .......................        ---------------------- ----------
       ----------------------------------------------------------------
       |                       TLS, SSH                               |
       ----------------------------------------------------------------

               Figure 7: Attestation for End User Computers

6.  IANA Considerations

   This memo includes no request to IANA.





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7.  Security Considerations

   TBD

8.  Informative References

   [Canonical-Event-Log]
              Trusted Computing Group, "DRAFT Canonical Event Log Format
              Version: 1.0, Revision: .12", October 2018.

   [EFI]      Trusted Computing Group, "TCG EFI Platform Specification
              for TPM Family 1.1 or 1.2, Specification Version 1.22,
              Revision 15", January 2014,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              EFI-Protocol-Specification-rev13-160330final.pdf>.

   [Firmware-Profile]
              Trusted Computing Group, "Trusted Computing Group, PC
              Client Specific Platform Firmware Profile Specification
              Family "2.0", Level 00 Revision 1.03 Version 51", June
              2019, <https://trustedcomputinggroup.org/pc-client-
              specific-platform-firmware-profile-specification/>.

   [I-D.birkholz-rats-architecture]
              Birkholz, H., Wiseman, M., Tschofenig, H., and N. Smith,
              "Architecture and Reference Terminology for Remote
              Attestation Procedures", draft-birkholz-rats-
              architecture-01 (work in progress), March 2019.

   [I-D.birkholz-rats-reference-interaction-model]
              Birkholz, H. and M. Eckel, "Reference Interaction Model
              for Challenge-Response-based Remote Attestation", draft-
              birkholz-rats-reference-interaction-model-00 (work in
              progress), March 2019.

   [I-D.birkholz-suit-coswid-manifest]
              Birkholz, H., "A SUIT Manifest Extension for Concise
              Software Identifiers", draft-birkholz-suit-coswid-
              manifest-00 (work in progress), July 2018.

   [I-D.birkholz-yang-basic-remote-attestation]
              Birkholz, H., Eckel, M., Bhandari, S., Sulzen, B., Voit,
              E., and G. Fedorkow, "YANG Module for Basic Challenge-
              Response-based Remote Attestation Procedures", draft-
              birkholz-yang-basic-remote-attestation-01 (work in
              progress), October 2018.





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   [I-D.birkholz-yang-swid]
              Birkholz, H., "Software Inventory YANG module based on
              Software Identifiers", draft-birkholz-yang-swid-02 (work
              in progress), October 2018.

   [I-D.ietf-sacm-coswid]
              Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
              Waltermire, "Concise Software Identification Tags", draft-
              ietf-sacm-coswid-11 (work in progress), June 2019.

   [IEEE-802-1AR]
              Seaman, M., "802.1AR-2018 - IEEE Standard for Local and
              Metropolitan Area Networks - Secure Device Identity, IEEE
              Computer Society", August 2018.

   [IMA]      and , "Integrity Measurement Architecture", June 2019,
              <https://sourceforge.net/p/linux-ima/wiki/Home/>.

   [NetEq]    Trusted Computing Group, "TCG Guidance for Securing
              Network Equipment", January 2018,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_Guidance_for_Securing_NetEq_1_0r29.pdf>.

   [NIST-IR-8060]
              National Institute for Standards and Technology,
              "Guidelines for the Creation of Interoperable Software
              Identification (SWID) Tags", April 2016,
              <https://nvlpubs.nist.gov/nistpubs/ir/2016/
              NIST.IR.8060.pdf>.

   [NIST-SP-800-155]
              National Institute for Standards and Technology, "BIOS
              Integrity Measurement Guidelines (Draft)", December 2011,
              <https://csrc.nist.gov/csrc/media/publications/sp/800-
              155/draft/documents/draft-sp800-155_dec2011.pdf>.

   [PC-Client-BIOS-TPM-1.2]
              Trusted Computing Group, "TCG PC Client Specific
              Implementation Specification for Conventional BIOS,
              Specification Version 1.21 Errata, Revision 1.00",
              February 2012, <https://www.trustedcomputinggroup.org/wp-
              content/uploads/TCG_PCClientImplementation_1-21_1_00.pdf>.









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   [PC-Client-BIOS-TPM-2.0]
              Trusted Computing Group, "PC Client Specific Platform
              Firmware Profile Specification Family "2.0", Level 00
              Revision 1.04", June 2019,
              <https://trustedcomputinggroup.org/pc-client-specific-
              platform-firmware-profile-specification>.

   [Platform-Certificates]
              Trusted Computing Group, "DRAFT: TCG Platform Attribute
              Credential Profile, Specification Version 1.0, Revision
              15, 07 December 2017", December 2017.

   [Platform-DevID-TPM-2.0]
              Trusted Computing Group, "DRAFT: TPM Keys for Platform
              DevID for TPM2, Specification Version 0.7, Revision 0",
              October 2018.

   [Platform-ID-TPM-1.2]
              Trusted Computing Group, "TPM Keys for Platform Identity
              for TPM 1.2, Specification Version 1.0, Revision 3",
              August 2015, <https://trustedcomputinggroup.org/wp-
              content/uploads/
              TPM_Keys_for_Platform_Identity_v1_0_r3_Final.pdf>.

   [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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

   [RFC8348]  Bierman, A., Bjorklund, M., Dong, J., and D. Romascanu, "A
              YANG Data Model for Hardware Management", RFC 8348,
              DOI 10.17487/RFC8348, March 2018,
              <https://www.rfc-editor.org/info/rfc8348>.

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,
              <https://www.rfc-editor.org/info/rfc8572>.





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   [RIM]      Trusted Computing Group, "DRAFT: TCG Reference Integrity
              Manifest", June 2019.

   [Roots-of-Trust]
              Trusted Computing Group, "TCG Roots of Trust
              Specification", October 2018,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_Roots_of_Trust_Specification_v0p20_PUBLIC_REVIEW.pdf>.

   [SNMP-Attestation-MIB]
              Trusted Computing Group, "DRAFT: SNMP MIB for TPM-Based
              Attestation, Specification Version 0.8, Revision 0.02",
              May 2018.

   [SWID]     The International Organization for Standardization/
              International Electrotechnical Commission, "Information
              Technology Software Asset Management Part 2: Software
              Identification Tag, ISO/IEC 19770-2", October 2015,
              <https://www.iso.org/standard/65666.html>.

   [TAP]      Trusted Computing Group, "DRAFT: TCG Trusted Attestation
              Protocol (TAP) Information Model for TPM Families 1.2 and
              2.0 and DICE Family 1.0, Version 1.0, Revision 0.29",
              October 2018.

Authors' Addresses

   Guy Fedorkow (editor)
   Juniper Networks, Inc.
   US

   Email: gfedorkow@juniper.net


   Jessica Fitzgerald-McKay
   National Security Agency
   US

   Email: jmfitz2@nsa.gov












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