RATS Working Group                                      G. Fedorkow, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Informational                                   E. Voit
Expires: March 22, 2021                              Cisco Systems, Inc.
                                                     J. Fitzgerald-McKay
                                                National Security Agency
                                                      September 18, 2020

         TPM-based Network Device Remote Integrity Verification


   This document describes a workflow for remote attestation of the
   integrity of firmware and software installed on network devices that
   contain Trusted Platform Modules [TPM1.2], [TPM2.0].

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Document Organization . . . . . . . . . . . . . . . . . .   4
     1.3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Description of Remote Integrity Verification (RIV)  . . .   5
     1.5.  Solution Requirements . . . . . . . . . . . . . . . . . .   7
     1.6.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       1.6.1.  Out of Scope  . . . . . . . . . . . . . . . . . . . .   8
   2.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  RIV Software Configuration Attestation using TPM  . . . .   9
       2.1.1.  What Does RIV Attest? . . . . . . . . . . . . . . . .  10
       2.1.2.  Notes on PCR Allocations  . . . . . . . . . . . . . .  12
     2.2.  RIV Keying  . . . . . . . . . . . . . . . . . . . . . . .  13
     2.3.  RIV Information Flow  . . . . . . . . . . . . . . . . . .  14
     2.4.  RIV Simplifying Assumptions . . . . . . . . . . . . . . .  16
       2.4.1.  Reference Integrity Manifests (RIMs)  . . . . . . . .  17
       2.4.2.  Attestation Logs  . . . . . . . . . . . . . . . . . .  18
   3.  Standards Components  . . . . . . . . . . . . . . . . . . . .  19
     3.1.  Prerequisites for RIV . . . . . . . . . . . . . . . . . .  19
       3.1.1.  Unique Device Identity  . . . . . . . . . . . . . . .  19
       3.1.2.  Keys  . . . . . . . . . . . . . . . . . . . . . . . .  19
       3.1.3.  Appraisal Policy for Evidence . . . . . . . . . . . .  19
     3.2.  Reference Model for Challenge-Response  . . . . . . . . .  20
       3.2.1.  Transport and Encoding  . . . . . . . . . . . . . . .  22
     3.3.  Centralized vs Peer-to-Peer . . . . . . . . . . . . . . .  23
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  24
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     5.1.  Keys Used in RIV  . . . . . . . . . . . . . . . . . . . .  25
     5.2.  Prevention of Spoofing and Man-in-the-Middle Attacks  . .  27
     5.3.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  28
     5.4.  Owner-Signed Keys . . . . . . . . . . . . . . . . . . . .  28
     5.5.  Other Trust Anchors . . . . . . . . . . . . . . . . . . .  29
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   8.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     8.1.  Using a TPM for Attestation . . . . . . . . . . . . . . .  30
     8.2.  Root of Trust for Measurement . . . . . . . . . . . . . .  32
     8.3.  Layering Model for Network Equipment Attester and
           Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  32
       8.3.1.  Why is OS Attestation Different?  . . . . . . . . . .  34
     8.4.  Implementation Notes  . . . . . . . . . . . . . . . . . .  34
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  36

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     9.2.  Informative References  . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

   There are many aspects to consider in fielding a trusted computing
   device, from operating systems to applications.  Mechanisms to prove
   that a device installed at a customer's site is authentic (i.e., not
   counterfeit) and has been configured with authorized software, all as
   part of a trusted supply chain, are just a few of the many aspects
   which need to be considered concurrently to have confidence that a
   device is truly trustworthy.

   A generic architecture for remote attestation has been defined in
   [I-D.ietf-rats-architecture].  Additionally, the use cases for
   remotely attesting networking devices are discussed within Section 6
   of [I-D.richardson-rats-usecases].  However, these documents do not
   provide sufficient guidance for network equipment vendors and
   operators to design, build, and deploy interoperable devices.

   The intent of this document is to provide such guidance.  It does
   this by outlining the Remote Integrity Verification (RIV) problem,
   and then identifies elements that are necessary to get the complete,
   scalable attestation procedure working with commercial networking
   products such as routers, switches and firewalls.  An underlying
   assumption will be the availability within the device of a Trusted
   Platform Module [TPM1.2], [TPM2.0] compliant cryptoprocessor to
   enable the trustworthy remote assessment of the device's software and

1.1.  Terminology

   A number of terms are reused from [I-D.ietf-rats-architecture].
   These include: Appraisal Policy for Attestation Results, Attestation
   Result, Attester, Evidence, Relying Party, Verifier, and Verifier

   Additionally, this document defines the following terms:

   Remote Attestation: the process of creating, conveying and appraising
   claims about device trustworthiness characteristics, including supply
   chain trust, identity, device provenance, software configuration,
   hardware configuration, device composition, compliance to test
   suites, functional and assurance evaluations, etc.

   This document uses the term Endorser to refer to the trusted
   authority for any signed object relating to the device, such as

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   certificates or reference measurement.  Typically, the manufacturer
   of an embedded device would be accepted as an Endorser.

   The goal of attestation is simply to assure an administrator that the
   software that was launched when the device was last started is an
   authentic and untampered-with copy of the software that the device
   vendor shipped.

   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, and 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 authentic 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
   device characteristics that could be attested (e.g., geographic
   location, connectivity; see [I-D.richardson-rats-usecases]), although
   it does provide evidence of a secure infrastructure to increase the
   level of trust in other device characteristics attested by other
   means (e.g., by Entity Attestation Tokens [I-D.ietf-rats-eat]).

1.2.  Document Organization

   The remainder of this document is organized into several sections:

   o  The remainder of this section covers goals and requirements, plus
      a top-level description of RIV.

   o  The Solution Overview section outlines how Remote Integrity
      Verification works.

   o  The Standards Components section links components of RIV to
      normative standards.

   o  Privacy and Security shows how specific features of RIV contribute
      to the trustworthiness of the Attestation Result.

   o  Supporting material is in an appendix at the end.

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1.3.  Goals

   Network operators benefit from a trustworthy attestation mechanism
   that provides assurance that their network comprises authentic
   equipment, and has loaded software free of known vulnerabilities and
   unauthorized tampering.  In line with the overall goal of assuring
   integrity, attestation can be used to assist in asset management,
   vulnerability and compliance assessment, plus configuration

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

   o  Provable Device Identity - This specification requires that an
      attesting device includes a cryptographic identifier unique to
      each device.  Effectively this means that the TPM must be so
      provisioned during the manufacturing cycle.

   o  Software Inventory - A key goal is to identify the software
      release(s) installed on the attesting device, and to provide
      evidence that the software stored within hasn't been altered
      without authorization.

   o  Verifiability - Verification of software and configuration of the
      device shows that the software that was authorized for
      installation by the administrator has actually been launched.

   In addition, RIV is designed to operate either in a centralized
   environment, such as with a central authority that manages and
   configures a number of network devices, or 'peer-to-peer', where
   network devices independently verify one another to establish a trust
   relationship.  (See Section 3.3 below, and also

1.4.  Description of Remote Integrity Verification (RIV)

   Attestation requires two interlocking services between the Attester
   network device and the Verifier:

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

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

   Using these two interlocking services, RIV is a component in a chain
   of procedures that can assure a network operator that the equipment
   in their network can be reliably identified, and that authentic
   software of a known version is installed on each device.  Equipment
   in the network includes devices that make up the network itself, such
   as routers, switches and firewalls.

   RIV includes several major processes:

   1.  Creation of Evidence is the process whereby an Attester generates
       cryptographic proof (Evidence) of claims about device properties.
       In particular, the device identity and its software configuration
       are both of critical importance.

   2.  Device Identification refers to the mechanism assuring the
       Relying Party (ultimately, a network administrator) of the
       identity of devices that make up their network, and that their
       manufacturers are known.

   3.  Software used to boot a device can be described as a chain of
       measurements, anchored at the start by a Root of Trust for
       Measurement, that normally ends when the system software is
       loaded.  A measurement signifies the identity, integrity and
       version of each software component registered with an attesting
       device's TPM [TPM1.2], [TPM2.0], so that the subsequent appraisal
       stage can determine if the software installed is authentic, up-
       to-date, and free of tampering.

   4.  Conveyance of Evidence reliably transports at least the minimum
       amount of Evidence from Attester to a Verifier to allow a
       management station to perform a meaningful appraisal in Step 5.
       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.

   5.  Finally, Appraisal of Evidence occurs.  As the Verifier and
       Relying Party roles are often combined within RIV, this is the
       process of verifying the Evidence received by a Verifier from the
       Attesting device, and using an Appraisal Policy to develop an
       Attestation Result, used to inform decision making.  In practice,
       this means comparing the device measurements reported as Evidence
       with the Attester configuration expected by the Verifier.
       Subsequently the Appraisal Policy for Attestation Results might

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       match what was found against Reference Integrity Measurements
       (aka Golden Measurements) which represent the intended configured
       state of the connected device.

   All implementations supporting this RIV specification require the
   support of the following three technologies:

   1.  Identity: Device identity MUST be based on IEEE 802.1AR Device
       Identity (DevID) [IEEE-802-1AR], coupled with careful supply-
       chain management by the manufacturer.  The DevID certificate
       contains a statement by the manufacturer that establishes the
       identity 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 different privacy concerns, may want to
       use alternative mechanisms for platform authentication (for
       example, TCG Platform Certificates [Platform-Certificates]).

   2.  Platform Attestation provides evidence of configuration of
       software elements present in the device.  This form of
       attestation can be implemented with TPM Platform Configuration
       Registers (PCRs), Quote and Log mechanisms, which provide
       cryptographically authenticated evidence to report what software
       was started on the device through the boot cycle.  Successful
       attestation requires an unbroken chain from a boot-time root of
       trust through all layers of software needed to bring the device
       to an operational state, in which each stage measures components
       of the next stage, updates the attestation log, and extends
       hashes into a PCR.  The TPM can then report the hashes of all the
       measured hashes as signed evidence called a Quote (see
       Section 8.1 for an overview of TPM operation, or [TPM1.2] and
       [TPM2.0] for many more details).

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

1.5.  Solution Requirements

   Remote Integrity Verification must address the "Lying Endpoint"
   problem, in which malicious software on an endpoint may subvert the
   intended function, and also prevent the endpoint from reporting its
   compromised status.  (See Section 5 for further Security

   RIV attestation is designed to be simple to deploy at scale.  RIV
   should work "out of the box" as far as possible, that is, with the
   fewest possible provisioning steps or configuration databases needed

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   at the end-user's site, as network equipment is often required to
   "self-configure", to reliably reach out without manual intervention
   to prove its identity and operating posture, then download its own
   configuration.  See [RFC8572] for an example of Secure Zero Touch

1.6.  Scope

   Remote Attestation is a very general problem that could apply to most
   network-connected computing devices.  However, this document includes
   several assumptions that limit the scope to Network Equipment (e.g.,
   routers, switches and firewalls):

   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
      [AIK-Enrollment] or TCG Platform Certificates

   o  This document assumes network protocols that are common in
      networking equipment such as YANG [RFC7950] and NETCONF [RFC6241],
      but not generally used in other applications.

   o  The approach outlined in this document mandates the use of a
      compliant TPM [TPM1.2], [TPM2.0].

1.6.1.  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

   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: Network equipment typically does not
      "sleep", so sleep and hibernate modes are not considered.
      Although out of scope for RIV, Trusted Computing Group
      specifications do encompass sleep and hibernate states.

   o  Virtualization and Containerization: In a non-virtualized system,
      the host OS is responsible for measuring each User Space file or
      process, but that't the end of the boot process.  For virtualized
      systems, the host OS must verify the hypervisor, which then
      manages its own chain of trust through the virtual machine.

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      Virtualization and containerization technologies are increasingly
      used in Network equipment, but are not considered in this revision
      of the document.

2.  Solution Overview

2.1.  RIV Software Configuration Attestation using TPM

   RIV Attestation is a process which can be used to determine 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, each distinct software object is
      "measured".  Its identity, hash (i.e., cryptographic digest) and
      version information are recorded in a log.  Hashes are also
      extended, or cryptographically folded, into the TPM, in a way that
      can be used to validate the log entries.  The measurement process
      generally follows the Chain of Trust model used in Measured Boot,
      where each stage of the system measures the next one, and extends
      its measurement into the TPM, before launching it.

   o  Once the device is running and has operational network
      connectivity, a separate, trusted Verifier will 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 Subject Field and signature of certificate containing
   the TPM's attestation public key, and can validate the software that
   was launched by verifying the correctness of the logs by comparing
   with the signed digests from the TPM, and comparing digests in the
   log with known-good values.

   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 Attester producing the Evidence.

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


                      Figure 1: RIV Attestation Model

   In Step 1, measurements are "extended", or hashed, into the TPM as
   processes start, with the result that the PCR ends up containing a
   hash of all the measured hashes.  In Step 2, signed PCR digests are
   retrieved from the TPM for off-box analysis after the system is

2.1.1.  What Does RIV Attest?

   TPM attestation is strongly focused on Platform Configuration
   Registers (PCRs), but those registers are only vehicles for
   certifying accompanying Evidence, conveyed in log entries.  It is the
   hashes in log entries that are extended into PCRs, where the final
   PCR values can be retrieved in the form of a structured called a
   Quote, signed by an Attestation key known only to the TPM.  The use
   of multiple PCRs serves only to provide some independence between
   different classes of object, so that one class of objects can be
   updated without changing the extended hash for other classes.
   Although PCRs can be used for any purpose, this section outlines the
   objects within the scope of this document which may be extended into
   the TPM.

   In general, assignment of measurements to PCRs is a policy choice
   made by the device manufacturer, selected to independently attest
   three classes of object:

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   o  Code, (i.e., instructions) to be executed by a CPU.

   o  Configuration - Many devices offer numerous options controlled by
      non-volatile configuration variables which can impact the device's
      security posture.  These settings may have vendor defaults, but
      often can be changed by administrators, who may want to verify via
      attestation that the settings they intend are still in place.

   o  Credentials - Administrators may wish to verify via attestation
      that keys (and other credentials) outside the Root of Trust have
      not been subject to unauthorized tampering.  (By definition, keys
      inside the root of trust can't be verified independently).

   The TCG PC Client Platform Firmware Profile Specification
   [PC-Client-BIOS-TPM-2.0] gives considerable detail on what is to be
   measured during the boot phase of platform startup using a UEFI BOIS
   (www.uefi.org), but the goal is simply to measure every bit of code
   executed in the process of starting the device, along with any
   configuration information related to security posture, leaving no gap
   for unmeasured code to remain undetected and subvert the chain.

   For devices using a UEFI BIOS, [PC-Client-BIOS-TPM-2.0] gives
   detailed normative requirements for PCR usage.  But for other
   platform architectures, the table in Figure 2 gives guidance for PCR
   assignment that generalizes the specific details of

   By convention, most PCRs are assigned in pairs, which the even-
   numbered PCR used to measure executable code, and the odd-numbered
   PCR used to measure whatever data and configuration are associated
   with that code.  It is important to note that each PCR may contain
   results from dozens (or even thousands) of individual measurements.

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   |                                            |    Assigned PCR #   |
   | Function                                   | Code | Configuration|
   | Firmware Static Root of Trust, (i.e.,      |  0   |    1         |
   | initial boot firmware and drivers)         |      |              |
   | Drivers and initialization for optional    |  2   |    3         |
   | or add-in devices                          |      |              |
   | OS Loader code and configuration, (i.e.,   |  4   |    5         |
   | the code launched by firmware) to load an  |      |              |
   | operating system kernel. These PCRs record |      |              |
   | each boot attempt, and an identifier for   |      |              |
   | where the loader was found                 |      |              |
   | Vendor Specific Measurements during boot   |  6   |    6         |
   | Secure Boot Policy.  This PCR records keys |      |    7         |
   | and configuration used to validate the OS  |      |              |
   | loader                                     |      |              |
   | Measurements made by the OS Loader         |  8   |    9         |
   | (e.g GRUB2 for Linux)                      |      |              |
   | Measurements made by OS (e.g., Linux IMA)  |  10  |    10        |

                        Figure 2: Attested Objects

2.1.2.  Notes on PCR Allocations

   It is important to recognize that PCR[0] is critical.  The first
   measurement into PCR[0] taken by the Root of Trust for Measurement,
   is critical to establishing the chain of trust for all subsequent
   measurements.  If the PCR[0] measurement cannot be trusted, the
   validity of the entire chain is put into question.

   Distinctions Between PCR[0], PCR[2], PCR[4] and PCR[8] are summarized

   o  PCR[0] typically represents a consistent view of the Host Platform
      between boot cycles, allowing Attestation and Sealed Storage
      policies to be defined using the less changeable components of the
      transitive trust chain.  This PCR typically provides a consistent
      view of the platform regardless of user selected options.

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   o  PCR[2] is intended to represent a "user configurable" environment
      where the user has the ability to alter the components that are
      measured into PCR[2].  This is typically done by adding adapter
      cards, etc., into user-accessible PCI or other slots.  In UEFI
      systems these devices may be configured by Option ROMs measured
      into PCR[2] and executed by the BIOS.

   o  PCR[4] is intended to represent the software that manages the
      transition between the platform's Pre-Operating System Start and
      the state of a system with the Operating System present.  This
      PCR, along with PCR[5], identifies the initial operating system
      loader (e.g., GRUB for Linux).

   o  PCR[8] is used by the OS loader to record measurements of the
      various components of the operating system.

   Although the TCG PC Client document specifies the use of the first
   eight PCRs very carefully to ensure interoperability among multiple
   UEFI BIOS vendors, it should be noted that embedded software vendors
   may have considerably more flexibility.  Verifiers typically need to
   know which log entries are consequential and which are not (possibly
   controlled by local policies) but the Verifier may not need to know
   what each log entry means or why it was assigned to a particular PCR.
   Designers must recognize that some PCRs may cover log entries that a
   particular Verifier considers critical and other log entries that are
   not considered important, so differing PCR values may not on their
   own constitute a check for authenticity.

   Designers may allocate particular events to specific PCRs in order to
   achieve a particular objective with Local Attestation, (e.g.,
   allowing a procedure to execute only if a given PCR is in a given
   state).  It may also be important to designers to consider whether
   streaming notification of PCR updates is required (see
   [I-D.birkholz-rats-network-device-subscription]).  Specific log
   entries can only be validated if the Verifier receives every log
   entry affecting the relevant PCR, so (for example) a designer might
   want to separate rare, high-value events such as configuration
   changes, from high-volume, routine measurements such as IMA [IMA]

2.2.  RIV Keying

   RIV attestation relies on two keys:

   o  An identity key is required to certify the identity of the
      Attester itself.  RIV specifies the use of an IEEE 802.1AR Device
      Identity (DevID) [IEEE-802-1AR], signed by the device
      manufacturer, containing the device serial number.

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   o  An Attestation Key is required to sign the Quote generated by the
      TPM to report evidence of software configuration.

   In TPM application, the Attestation key MUST be protected by the TPM,
   and the DevID SHOULD be as well.  Depending on other TPM
   configuration procedures, the two keys are likely be different; some
   of the considerations are outlined in TCG Guidance for Securing
   Network Equipment [NetEq].

   TCG Guidance for Securing Network Equipment specifies further
   conventions for these keys:

   o  When separate Identity and Attestation keys are used, the
      Attestation Key (AK) and its X.509 certificate should parallel the
      DevID, with the same device ID information as the DevID
      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 an AK, to be linked directly to the
      device that provided it, by examining the corresponding AK

   o  Network devices that are expected to use secure zero touch
      provisioning as specified in [RFC8572]) MUST be shipped by the
      manufacturer with pre-provisioned keys (Initial DevID and AK,
      called IDevID and IAK).  Inclusion of an IDevID and IAK by a
      vendor does not preclude a mechanism whereby an Administrator can
      define Local Identity and Attestation Keys (LDevID and LAK) if
      desired.  IDevID and IAK certificates MUST both be signed by the
      Endorser (typically the device manufacturer).

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.  The Attesting Device, which the network operator wants to

   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 judgment on the security
       posture of the device.

   3.  A Relying Party, which can act on Attestation Results.
       Interaction between the Relying Party and the Verifier is
       considered out of scope for RIV.

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   4.  Signed Reference Integrity Manifests (RIMs), containing Reference
       Integrity 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 several 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 that may not have
       access to the public internet, or by other devices that are not
       management stations per se (e.g., a peer device; see
       Section 3.1.3).  If Reference Integrity 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

   These components are illustrated in Figure 3.

   A more-detailed taxonomy of terms is given in

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

        Figure 3: RIV Reference Configuration for Network Equipment

   In Step 0, The Endorser (the device manufacturer or other authority)
   provides a software image to the Attester (the device under
   attestation), and makes one or more Reference Integrity Manifests
   (RIMs) signed by the Endorser, available to the Verifier (see
   Section 3.1.3 for "in-band" and "out of band" ways to make this
   happen).  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, signed
   by the Attester), and optionally RIMs.

   To achieve interoperability, the following standards components
   SHOULD be used:

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

   2.  For devices using UEFI and Linux, measurements of firmware and
       bootable modules SHOULD be taken according to TCG PC Client
       [PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]

   3.  Device Identity MUST be managed as specified in IEEE 802.1AR
       Device Identity certificates [IEEE-802-1AR], with keys protected
       by TPMs.

   4.  Attestation logs SHOULD be formatted according to the Canonical
       Event Log format [Canonical-Event-Log], although other
       specialized formats may be used.  UEFI-based systems MAY use the
       TCG UEFI BIOS event log [EFI-TPM]).

   5.  Quotes are retrieved from the TPM according to the TCG TAP
       Information Model [TAP].  While the TAP IM gives a protocol-
       independent description of the data elements involved, it's
       important to note that quotes from the TPM are signed inside the
       TPM, so MUST be retrieved in a way that does not invalidate the
       signature, as specified in [I-D.ietf-rats-yang-tpm-charra], to
       preserve the trust model.  (See Section 5 Security

   6.  Reference Integrity Measurements SHOULD be 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].  See Section 2.4.1.

2.4.  RIV Simplifying Assumptions

   This document makes the following simplifying assumptions to reduce

   o  The product to be attested MUST be shipped with an IEEE 802.1AR
      Device Identity 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

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      procedures to install identity credentials into the device after

   o  The product MUST be equipped with a Root of Trust for Measurement,
      Root of Trust for Storage and Root of Trust for Reporting (as
      defined in [SP800-155]) that are capable of conforming to the TCG
      Trusted Attestation Protocol (TAP) Information Model [TAP].

   o  The authorized software supplier MUST make available 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 Information Model

2.4.1.  Reference Integrity Manifests (RIMs)

   [I-D.ietf-rats-yang-tpm-charra] 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 measurements 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 that may be used by a Verifier to
   determine if evidence shows known good, known bad or unknown software

   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 relevant
       PCRs after the box has booted.  In this case, the signed
       reference measurement could simply list the expected hashes for
       the given version.  However, a RIM that contains the intermediate
       hashes can be useful in debugging cases where the expected final
       hash is not the one reported.

   2.  Measurements taken later in operation of the system, once an OS
       has started (for example, Linux IMA[IMA]), 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.

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   In both cases, the expected values can be expressed as signed SWID or
   CoSWID tags, but the SWID structure in the second case is somewhat
   more complex, as reconstruction of the extended hash in a PCR may
   involve thousands of files and other objects.

   The TCG has published an information model defining elements of
   reference integrity manifests under the title TCG Reference Integrity
   Manifest Information Model [RIM].  This information model outlines
   how SWID tags should be structured to allow attestation, and defines
   "bundles" of SWID tags that may be needed to describe a complete
   software release.  The RIM contains metadata relating to the software
   release it belongs to, plus hashes for each individual file or other
   object that could be attested.

   TCG has also published the PC Client Reference Integrity Measurement
   specification [PC-Client-RIM], which focuses on a SWID-compatible
   format suitable for expressing expected measurement values in the
   specific case of a UEFI-compatible BIOS, where the SWID focus on
   files and file systems is not a direct fit.  While the PC Client RIM
   is not directly applicable to network equipment, many vendors do use
   a conventional UEFI BIOS to launch their network OS.

2.4.2.  Attestation Logs

   Quotes from a TPM can provide evidence of the state of a device up to
   the time the evidence was recorded, but to make sense of the quote in
   most cases an event log that identifies which software modules
   contributed which values to the quote during startup MUST also be
   provided.  The log MUST contain enough information to demonstrate its
   integrity by allowing exact reconstruction of the digest conveyed in
   the signed quote (i.e., PCR values).

   There are multiple event log formats which may be supported as viable
   formats of Evidence between the Attester and Verifier:

   o  Event log exports from [I-D.ietf-rats-yang-tpm-charra]

   o  IMA Event log file exports [IMA]

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

   o  TCG Canonical Event Log [Canonical-Event-Log]

   Devices which use UEFI BIOS and Linux SHOULD use TCG Canonical Event
   Log [Canonical-Event-Log] and TCG UEFI BIOS event log [EFI-TPM])

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3.  Standards Components

3.1.  Prerequisites for RIV

   The Reference Interaction Model for Challenge-Response-based Remote
   Attestation is based on the standard roles defined in
   [I-D.ietf-rats-architecture].  However additional prerequisites have
   been established to allow for interoperable RIV use case
   implementations.  These prerequisites are intended to provide
   sufficient context information so that the Verifier can acquire and
   evaluate Attester measurements.

3.1.1.  Unique Device Identity

   A secure Device Identity (DevID) in the form of an IEEE 802.1AR DevID
   certificate [IEEE-802-1AR] MUST be provisioned in the Attester's

3.1.2.  Keys

   The Attestation Identity Key (AIK) and certificate MUST also be
   provisioned on the Attester according to [Platform-DevID-TPM-2.0],
   [PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2].

   The Attester's TPM Keys MUST be associated with the DevID on the
   Verifier (see [Platform-DevID-TPM-2.0] and Section 5 Security
   Considerations, below).

3.1.3.  Appraisal Policy for Evidence

   The Verifier MUST obtain trustworthy Endorsements in the form of
   reference measurements (e.g., Known Good Values, encoded as CoSWID
   tags [I-D.birkholz-yang-swid]).  These reference measurements will
   eventually be compared to signed PCR Evidence acquired from an
   Attester's TPM using Attestation Policies chosen by the administrator
   or owner of the device.

   This document does not specify the format or contents for the
   Appraisal Policy for Evidence, but Endorsements may be acquired in
   one of two ways:

   1.  a Verifier may obtain reference measurements directly from an
       Endorser chosen by the Verifier administrator (e.g., through a
       web site).

   2.  Signed reference measurements may be distributed by the Endorser
       to the Attester, as part of a software update.  From there, the
       reference measurement may be acquired by the Verifier.

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   In either case, the Verifier Owner MUST select the source of trusted
   endorsements through the Appraisal Policy for Evidence.

   *************         .-------------.         .-----------.
   * Endorser  *         | Attester    |         | Verifier/ |
   *           *         |             |         | Relying   |
   *(Device    *----2--->| (Network    |----2--->| Party     |
   * config    *         |  Device)    |         |(Ntwk Mgmt |
   * Authority)*         |             |         |  Station) |
   *************         '-------------'         '-----------'
           |                                           ^
           |                                           |

           Figure 4: Appraisal Policy for Evidence Prerequisites

   In either case the Endorsements must be generated, acquired and
   delivered in a secure way, including reference measurements of
   firmware and bootable modules taken according to TCG PC Client
   [PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA].  Endorsementa MUST be
   encoded as SWID or CoSWID tags, signed by the device manufacturer, as
   defined in the TCG RIM document [RIM], compatible with NIST IR 8060
   [NIST-IR-8060] or the IETF CoSWID draft [I-D.ietf-sacm-coswid].

3.2.  Reference Model for Challenge-Response

   Once the prerequisites for RIV are met, a Verifier is able to acquire
   Evidence from an Attester.  The following diagram illustrates a RIV
   information flow between a Verifier and an Attester, derived from
   Section 8.1 of [I-D.birkholz-rats-reference-interaction-model].
   Event times shown correspond to the time types described within
   Appendix A of [I-D.ietf-rats-architecture]:

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.----------.                                .--------------------------.
| Attester |                                | Relying Party / Verifier |
'----------'                                '--------------------------'
   time(VG)                                                       |
valueGeneration(targetEnvironment)                                |
     | => claims                                                  |
     |                                                            |
     | <--------------requestEvidence(nonce, PcrSelection)-----time(NS)
     |                                                            |
   time(EG)                                                       |
evidenceGeneration(nonce, PcrSelection, collectedClaims)          |
     | => SignedPcrEvidence(nonce, PcrSelection)                  |
     | => LogEvidence(collectedClaims)                            |
     |                                                            |
     | returnSignedPcrEvidence----------------------------------> |
     | returnLogEvidence----------------------------------------> |
     |                                                            |
     |                                                       time(RG,RA)
     |         evidenceAppraisal(SignedPcrEvidence, eventLog, refClaims)
     |                                       attestationResult <= |
     ~                                                            ~
     |                                                          time(RX)

                Figure 5: IETF Attestation Information Flow

   o  Step 1 (time(VG)): One or more Attesting Network Device PCRs are
      extended with measurements.  RIV provides no direct link between
      the time at which the event takes place and the time that it's
      attested, although streaming attestation as in
      [I-D.birkholz-rats-network-device-subscription] could.

   o  Step 2 (time(NS)): The Verifier generates a unique nonce ("number
      used once"), and makes a request attestation data for one or more
      PCRs from an Attester.  For interoperability, this MUST be
      accomplished via a YANG [RFC7950] interface that implements the
      TCG TAP model (e.g., YANG Module for Basic Challenge-Response-
      based Remote Attestation Procedures

   o  Step 3 (time(EG)): On the Attester, measured values are retrieved
      from the Attester's TPM.  This requested PCR evidence is signed by
      the Attestation Identity Key (AIK) associated with the DevID.
      Quotes are retrieved according to TCG TAP Information Model [TAP].
      While the TAP IM gives a protocol-independent description of the
      data elements involved, it's important to note that quotes from
      the TPM are signed inside the TPM, so must be retrieved in a way
      that does not invalidate the signature, as specified in
      [I-D.ietf-rats-yang-tpm-charra], to preserve the trust model.

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      (See Section 5 Security Considerations).  At the same time, the
      Attester collects log evidence showing what values have been
      extended into that PCR.

   o  Step 4: Collected Evidence is passed from the Attester to the

   o  Step 5 (time(RG,RA)): The Verifier reviews the Evidence and takes
      action as needed.  As the interaction between Relying Party and
      Verifier is out of scope for RIV, this can happen in one step.

      *  If the signed PCR values do not match the set of log entries
         which have extended a particular PCR, the device SHOULD NOT be

      *  If the log entries that the Verifier considers important do not
         match known good values, the device SHOULD NOT be trusted.  We
         note that the process of collecting and analyzing the log can
         be omitted if the value in the relevant PCR is already a known-
         good value.

      *  If the set of log entries are not seen as acceptable by the
         Appraisal Policy for Evidence, the device SHOULD NOT be

      *  If the AIK signature is not correct, or freshness such as that
         provided by the nonce is not included in the response, the
         device SHOULD NOT be trusted.

      *  If time(RG)-time(NS) is greater than the threshold in the
         Appraisal Policy for Evidence, the Evidence is considered stale
         and SHOULD NOT be trusted.

3.2.1.  Transport and Encoding

   Network Management systems may retrieve signed PCR based Evidence as
   shown in Figure 5, and can be accomplished via NETCONF or RESTCONF,
   with XML, JSON, or CBOR encoded Evidence.

   Implementations that use NETCONF MUST do so over a TLS or SSH secure
   tunnel.  Implementations that use RESTCONF transport MAY do so over a
   TLS or SSH secure tunnel.

   Retrieval of Log Evidence SHOULD be via log interfaces on the network
   device.  (For example, see [I-D.ietf-rats-yang-tpm-charra]).

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3.3.  Centralized vs Peer-to-Peer

   Figure 5 above assumes that the Verifier is implicitly trusted, while
   the Attesting device is not.  In a Peer-to-Peer application such as
   two routers negotiating a trust relationship
   [I-D.voit-rats-trusted-path-routing], the two peers can each ask the
   other to prove software integrity.  In this application, the
   information flow is the same, but each side plays a role both as an
   Attester and a Verifier.  Each device issues a challenge, and each
   device responds to the other's challenge, as shown in Figure 6.
   Peer-to-peer challenges, particularly if used to establish a trust
   relationship between routers, require devices to carry their own
   signed reference measurements (RIMs).  Devices may also have to carry
   Appraisal Policy for Evidence for each possible peer device so that
   each device has everything needed for attestation, without having to
   resort to a central authority.

 +---------------+                             +---------------+
 |               |                             |               |
 | Endorser A    |                             | Endorser B    |
 | Firmware      |                             | Firmware      |
 | Configuration |                             | Configuration |
 | Authority     |                             | Authority     |
 |               |                             |               |
 +---------------+                             +---------------+
        |                                              |
        |       +-------------+        +------------+  |
        |       |             | Step 1 |            |  |   \
        |       | Attester    |<------>| Verifier   |  |   |
        |       |             |<------>|            |  |   |  Router B
        +------>|             | Step 2 |            |  |   |- Challenges
         Step 0A|             |        |            |  |   |  Router A
                |             |------->|            |  |   |
                |- Router A --| Step 3 |- Router B -|  |   /
                |             |        |            |  |
                |             |        |            |  |
                |             | Step 1 |            |  |   \
                | Verifier    |<------>| Attester   |<-+   |  Router A
                |             |<------>|            |      |- Challenges
                |             | Step 2 |            |      |  Router B
                |             |        |            |      |
                |             |<-------|            |      |
                +-------------+ Step 3 +------------+      /

            Figure 6: Peer-to-Peer Attestation Information Flow

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   In this application, each device may need to be equipped with signed
   RIMs to act as an Attester, and also an Appraisal Policy for Evidence
   and a selection of trusted X.509 root certificates, to allow the
   device to act as a Verifier.  An existing link layer protocol such as
   802.1x [IEEE-802.1x] or 802.1AE [IEEE-802.1ae], with Evidence being
   enclosed over a variant of EAP [RFC3748] or LLDP [LLDP] are suitable
   methods for such an exchange.

4.  Privacy Considerations

   Networking Equipment, such as routers, switches and firewalls, has a
   key role to play in guarding the privacy of individuals using the

   o  Packets passing through the device must not be sent to
      unauthorized destinations.  For example:

      *  Routers often act as Policy Enforcement Points, where
         individual subscribers may be checked for authorization to
         access a network.  Subscriber login information must not be
         released to unauthorized parties.

      *  Networking Equipment is often called upon to block access to
         protected resources from unauthorized users.

   o  Routing information, such as the identity of a router's peers,
      must not be leaked to unauthorized neighbors.

   o  If configured, encryption and decryption of traffic must be
      carried out reliably, while protecting keys and credentials.

   Functions that protect privacy are implemented as part of each layer
   of hardware and software that makes up the networking device.  In
   light of these requirements for protecting the privacy of users of
   the network, the Network Equipment must identify itself, and its boot
   configuration and measured device state (for example, PCR values), to
   the Equipment's Administrator, so there's no uncertainty as to what
   function each device and configuration is configured to carry out.
   This allows the administrator to ensure that the network provides
   individual and peer privacy guarantees.

   RIV specifically addresses the collection of information from
   enterprise network devices by authorized agents of the enterprise.
   As such, privacy is a fundamental concern for those deploying this
   solution, given EU GDPR, California CCPA, and many other privacy
   regulations.  The enterprise SHOULD implement and enforce their duty
   of care.

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   See [NetEq] for more context on privacy in networking devices.

5.  Security Considerations

   Attestation Results from the RIV procedure are subject to a number of

   o  Keys may be compromised.

   o  A counterfeit device may attempt to impersonate (spoof) a known
      authentic device.

   o  Man-in-the-middle attacks may be used by a counterfeit device to
      attempt to deliver responses that originate in an actual authentic

   o  Replay attacks may be attempted by a compromised device.

5.1.  Keys Used in RIV

   Trustworthiness of RIV attestation depends strongly on the validity
   of keys used for identity and attestation reports.  RIV takes full
   advantage of TPM capabilities to ensure that results can be trusted.

   Two sets of keys are relevant to RIV attestation:

   o  A DevID key is used to certify the identity of the device in which
      the TPM is installed.

   o  An Attestation Key (AK) key signs attestation reports (called
      'quotes' in TCG documents), used to provide evidence for integrity
      of the software on the device.

   TPM practices usually require that these keys be different, as a way
   of ensuring that a general-purpose signing key cannot be used to
   spoof an attestation quote.

   In each case, the private half of the key is known only to the TPM,
   and cannot be retrieved externally, even by a trusted party.  To
   ensure that's the case, specification-compliant private/public key-
   pairs are generated inside the TPM, where they're never exposed, and
   cannot be extracted (See [Platform-DevID-TPM-2.0]).

   Keeping keys safe is just part of attestation security; knowing which
   keys are bound to the device in question is just as important.

   While there are many ways to manage keys in a TPM (see
   [Platform-DevID-TPM-2.0]), RIV includes support for "zero touch"

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   provisioning (also known as zero-touch onboarding) of fielded devices
   (e.g., Secure ZTP, [RFC8572]), where keys which have predictable
   trust properties are provisioned by the device vendor.

   Device identity in RIV is based on IEEE 802.1AR Device Identity
   (DevID).  This specification provides several elements:

   o  A DevID requires a unique key pair for each device, accompanied by
      an X.509 certificate,

   o  The private portion of the DevID key is to be stored in the
      device, in a manner that provides confidentiality (Section 6.2.5

   The X.509 certificate contains several components:

   o  The public part of the unique DevID key assigned to that device
      allows a challenge of identity.

   o  An identifying string that's unique to the manufacturer of the
      device.  This is normally the serial number of the unit, which
      might also be printed on a label on the device.

   o  The certificate must be signed by a key traceable to the
      manufacturer's root key.

   With these elements, the device's manufacturer and serial number can
   be identified by analyzing the DevID certificate plus the chain of
   intermediate certificates leading back to the manufacturer's root
   certificate.  As is conventional in TLS or SSH connections, a nonce
   must be signed by the device in response to a challenge, proving
   possession of its DevID private key.

   RIV uses the DevID to validate a TLS or SSH connection to the device
   as the attestation session begins.  Security of this process derives
   from TLS or SSH security, with the DevID providing proof that the
   session terminates on the intended device.  See [RFC8446], [RFC4253].

   Evidence of software integrity is delivered in the form of a quote
   signed by the TPM itself.  Because the contents of the quote are
   signed inside the TPM, any external modification (including
   reformatting to a different data format) after measurements have been
   taken will be detected as tampering.  An unbroken chain of trust is
   essential to ensuring that blocks of code that are taking
   measurements have been verified before execution (see Figure 1.

   Requiring results of attestation of the operating software to be
   signed by a key known only to the TPM also removes the need to trust

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   the device's operating software (beyond the first measurement; see
   below); any changes to the quote, generated and signed by the TPM
   itself, made by malicious device software, or in the path back to the
   Verifier, will invalidate the signature on the quote.

   A critical feature of the YANG model described in
   [I-D.ietf-rats-yang-tpm-charra] is the ability to carry TPM data
   structures in their native format, without requiring any changes to
   the structures as they were signed and delivered by the TPM.  While
   alternate methods of conveying TPM quotes could compress out
   redundant information, or add an additional layer of signing using
   external keys, the implementation MUST preserve the TPM signing, so
   that tampering anywhere in the path between the TPM itself and the
   Verifier can be detected.

5.2.  Prevention of Spoofing and Man-in-the-Middle Attacks

   Prevention of spoofing attacks against attestation systems is also
   important.  There are two cases to consider:

   o  The entire device could be spoofed, that is, when the Verifier
      goes to verify a specific device, it might be redirected to a
      different device.  Use of the 802.1AR Device Identity (DevID) in
      the TPM ensures that the Verifier's TLS or SSH session is in fact
      terminating on the right device.

   o  A compromised device could respond with a spoofed Attestation
      Result, that is, a compromised OS could return a fabricated quote.

   Protection against spoofed quotes from a device with valid identity
   is a bit more complex.  An identity key must be available to sign any
   kind of nonce or hash offered by the Verifier, and consequently,
   could be used to sign a fabricated quote.  To block a spoofed
   Attestation Result, the quote generated inside the TPM must be signed
   by a key that's different from the DevID, called an Attestation Key

   Given separate Attestation and DevID keys, the binding between the AK
   and the same device must also be proven to prevent a man-in-the-
   middle attack (e.g., the 'Asokan Attack' [RFC6813]).

   This is accomplished in RIV through use of an AK certificate with the
   same elements as the DevID (i.e., same manufacturer's serial number,
   signed by the same manufacturer's key), but containing the device's
   unique AK public key instead of the DevID public key.

   [Editor's Note: does this require an OID that says the key is known
   by the CA to be an Attestation key?]

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   These two keys and certificates are used together:

   o  The DevID is used to validate a TLS connection terminating on the
      device with a known serial number.

   o  The AK is used to sign attestation quotes, providing proof that
      the attestation evidence comes from the same device.

5.3.  Replay Attacks

   Replay attacks, where results of a previous attestation are submitted
   in response to subsequent requests, are usually prevented by
   inclusion of a nonce in the request to the TPM for a quote.  Each
   request from the Verifier includes a new random number (a nonce).
   The resulting quote signed by the TPM contains the same nonce,
   allowing the Verifier to determine freshness, (i.e., that the
   resulting quote was generated in response to the Verifier's specific
   request).  Time-Based Uni-directional Attestation
   [I-D.birkholz-rats-tuda] provides an alternate mechanism to verify
   freshness without requiring a request/response cycle.

5.4.  Owner-Signed Keys

   Although device manufacturers MUST pre-provision devices with easily
   verified DevID and AK certificates, use of those credentials is not
   mandatory.  IEEE 802.1AR incorporates the idea of an Initial Device
   ID (IDevID), provisioned by the manufacturer, and a Local Device ID
   (LDevID) provisioned by the owner of the device.  RIV and
   [Platform-DevID-TPM-2.0] extends that concept by defining an Initial
   Attestation Key (IAK) and Local Attestation Key (LAK) with the same

   Device owners can use any method to provision the Local credentials.

   o  The TCG document [Platform-DevID-TPM-2.0] shows how the initial
      Attestation keys can be used to certify LDevID and LAK keys.  Use
      of the LDevID and LAK allows the device owner to use a uniform
      identity structure across device types from multiple manufacturers
      (in the same way that an "Asset Tag" is used by many enterprises
      to identify devices they own).  The TCG document
      [Provisioning-TPM-2.0] also contains guidance on provisioning
      identity keys in TPM 2.0.

   o  Device owners, however, can use any other mechanism they want to
      assure themselves that Local identity certificates are inserted
      into the intended device, including physical inspection and
      programming in a secure location, if they prefer to avoid placing
      trust in the manufacturer-provided keys.

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   Clearly, Local keys can't be used for secure Zero Touch provisioning;
   installation of the Local keys can only be done by some process that
   runs before the device is installed for network operation.

   On the other end of the device life cycle, provision should be made
   to wipe Local keys when a device is decommissioned, to indicate that
   the device is no longer owned by the enterprise.  The manufacturer's
   Initial identity keys must be preserved, as they contain no
   information that's not already printed on the device's serial number

5.5.  Other Trust Anchors

   In addition to trustworthy provisioning of keys, RIV depends on other
   trust anchors.  (See [SP800-155] for definitions of Roots of Trust.)

   o  Secure identity depends on mechanisms to prevent per-device secret
      keys from being compromised.  The TPM provides this capability as
      a Root of Trust for Storage.

   o  Attestation depends on an unbroken chain of measurements, starting
      from the very first measurement.  That first measurement is made
      by code called the Root of Trust for Measurement, typically done
      by trusted firmware stored in boot flash.  Mechanisms for
      maintaining the trustworthiness of the RTM are out of scope for
      RIV, but could include immutable firmware, signed updates, or a
      vendor-specific hardware verification technique.  See Section 8.1
      for background on TPM practices.

   o  The device owner SHOULD provide some level of physical defense for
      the device.  If a TPM that has already been programmed with an
      authentic DevID is stolen and inserted into a counterfeit device,
      attestation of that counterfeit device may become
      indistinguishable from an authentic device.

   RIV also depends on reliable reference measurements, as expressed by
   the RIM [RIM].  The definition of trust procedures for RIMs is out of
   scope for RIV, and the device owner is free to use any policy to
   validate a set of reference measurements.  RIMs may be conveyed out-
   of-band or in-band, as part of the attestation process (see
   Section 3.1.3).  But for embedded devices, where software is usually
   shipped as a self-contained package, RIMs signed by the manufacturer
   and delivered in-band may be more convenient for the device owner.

   The validity of RIV attestation results is also influenced by
   procedures used to create reference measurements:

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   o  While the RIM itself is signed, supply-chains SHOULD be carefully
      scrutinized to ensure that the values are not subject to
      unexpected manipulation prior to signing.  Insider-attacks against
      code bases and build chains are particularly hard to spot.

   o  Designers SHOULD guard against hash collision attacks.  Reference
      Integrity Measurements often give hashes for large objects of
      indeterminate size; if one of the measured objects can be replaced
      with an implant engineered to produce the same hash, RIV will be
      unable to detect the substitution.  TPM1.2 uses SHA-1 hashes only,
      which have been shown to be susceptible to collision attack.
      TPM2.0 will produce quotes with SHA-256, which so far has resisted
      such attacks.  Consequently RIV implementations SHOULD use TPM2.0.

6.  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 Integrity 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.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Appendix

8.1.  Using a TPM for Attestation

   The Trusted Platform Module and surrounding ecosystem provide three
   interlocking capabilities to enable secure collection of evidence
   from a remote device, Platform Configuration Registers (PCRs), a
   Quote mechanism, and a standardized Event Log.

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   Each TPM has at least between eight and twenty-four PCRs (depending
   on the profile and vendor choices), each one large enough to hold one
   hash value (SHA-1, SHA-256, and other hash algorithms can be used,
   depending on TPM version).  PCRs can't be accessed directly from
   outside the chip, but the TPM interface provides a way to "extend" a
   new security measurement hash into any PCR, a process by which the
   existing value in the PCR is hashed with the new security measurement
   hash, and the result placed back into the same PCR.  The result is a
   composite fingerprint of all the security measurements extended into
   each PCR since the system was reset.

   Every time a PCR is extended, an entry should be added to the
   corresponding Event Log.  Logs contain the security measurement hash
   plus informative fields offering hints as to what event it was that
   generated the security measurement.
   The Event Log itself is protected against accidental manipulation,
   but it is implicitly tamper-evident - any verification process can
   read the security measurement hash from the log events, compute the
   composite value and compare that to what ended up in the PCR.  If
   there's a discrepancy, the logs do not provide an accurate view of
   what was placed into the PCR.

   In a conventional TPM Attestation environment, the first measurement
   must be made and extended into the TPM by trusted device code (called
   the Root of Trust for Measurement, RTM).  That first measurement
   should cover the segment of code that is run immediately after the
   RTM, which then measures the next code segment before running it, and
   so on, forming an unbroken chain of trust.  See [TCGRoT] for more on
   Mutable vs Immutable roots of trust.

   The TPM provides another mechanism called a Quote that can read the
   current value of the PCRs and package them into a data structure
   signed by an Attestation Key (which is private key that is known only
   to the TPM).

   The Verifier uses the Quote and Log together.  The Quote, containing
   the composite hash of the complete sequence of security measurement
   hashes, is used to verify the integrity of the Event Log.  Each hash
   in the validated Quote can then be compared to corresponding expected
   values in the set of Reference Integrity Measurements to validate
   overall system integrity.

   A summary of information exchanged in obtaining quotes from TPM1.2
   and TPM2.0 can be found in [TAP], Section 4.  Detailed information
   about PCRs and Quote data structures can be found in [TPM1.2],
   [TPM2.0].  Recommended log formats include [PC-Client-BIOS-TPM-2.0]
   and [Canonical-Event-Log].

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8.2.  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, that is,
   some trustworthy mechanism that can compute the first measurement in
   the chain of trust required to attest that each stage of system
   startup is verified, a Root of Trust for Storage (i.e., the TPM PCRs)
   to record the results, and a Root of Trust for Reporting to report
   the results [TCGRoT], [SP800-155].

   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 computed by the Root of Trust for
      Measurement, and stored in the TPM's Root of Trust for Storage, is
      presumed to be correct.

   o  There must not be a way to reset the Root of Trust for Storage
      without re-entering the Root of Trust for Measurement code.

   The first measurement must be computed by code that is implicitly
   trusted; if that first measurement can be subverted, none of the
   remaining measurements can be trusted.  (See [NIST-SP-800-155])

8.3.  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.

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

   *         IETF Attestation Reference Interaction Diagram           *

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

       *************************  .............. **********************
       * YANG SWID Module      *  . TCG        . * YANG Attestation   *
       * I-D.birkholz-yang-swid*  . Attestation. * Module             *
       *                       *  . MIB        . * I-D.ietf-rats-     *
       *                       *  .            . * yang-tpm-charra    *
       *************************  .............. **********************

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

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

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

                       Figure 7: RIV Protocol Stacks

   IETF documents are captured in boxes surrounded by asterisks.  TCG
   documents are shown in boxes surrounded by dots.

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8.3.1.  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,
   involves software that's updated and changed frequently, and
   introduces processes that begin in an 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.

8.4.  Implementation Notes

   Figure 8 summarizes many of the actions needed to complete an
   Attestation system, with links to relevant documents.  While
   documents are controlled by several standards organizations, the
   implied actions required for implementation are all the
   responsibility of the manufacturer of the device, unless otherwise
   noted.  It should be noted that, while the YANG model is RECOMMENDED
   for attestation, this table identifies an optional SNMP MIB as well,

   |             Component                           |  Controlling   |
   |                                                 | Specification  |
   | Make a Secure execution environment             |   TCG RoT      |
   |   o Attestation depends on a secure root of     |   UEFI.org     |
   |     trust for measurement outside the TPM, as   |                |
   |     well as roots for storage and reporting     |                |
   |     inside the TPM.                             |                |
   |   o  Refer to TCG Root of Trust for Measurement.|                |
   |   o  NIST SP 800-193 also provides guidelines   |                |
   |      on Roots of Trust                          |                |
   | Provision the TPM 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                          |                |

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   | 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  The TCG RIM-IM identifies further         |                |
   |       procedures to create signed RIM           |                |
   |       documents that provide the necessary      |                |
   |       reference information                     |                |
   |  Package the SWID tags with a vendor software   | Retrieve tags  |
   |  release                                        | with           |
   |    o  A tag-generator plugin such      | draft-birkholz-yang-swid|
   |     as [SWID-Gen] can be used                   |----------------|
   |                                                 | TCG PC Client  |
   |                                                 | RIM            |
   |  Use PC Client measurement definitions          | TCG PC Client  |
   |  to define the use of PCRs                      | BIOS           |
   |  (although Windows  OS is rare on Networking    |                |
   |  Equipment, UEFI BIOS is not)                   |                |
   |  Use TAP to retrieve measurements               |                |
   |    o  Map TAP to SNMP                           | TCG SNMP MIB   |
   |    o  Map to YANG                               | YANG Module for|
   |  Use Canonical Log Format                       |   Basic        |
   |                                                 |   Attestation  |
   |                                                 | TCG Canonical  |
   |                                                 |   Log Format   |
   | Posture Collection Server (as described in IETF |                |
   |  SACMs ECP) should 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.     |                |

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                        Figure 8: Component Status

9.  References

9.1.  Normative References

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

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

              Birkholz, H., Eckel, M., Bhandari, S., Sulzen, B., Voit,
              E., Xia, L., Laffey, T., and G. Fedorkow, "A YANG Data
              Model for Challenge-Response-based Remote Attestation
              Procedures using TPMs", draft-ietf-rats-yang-tpm-charra-02
              (work in progress), June 2020.

              Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
              Waltermire, "Concise Software Identification Tags", draft-
              ietf-sacm-coswid-15 (work in progress), May 2020.

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

              Trusted Computing Group, "TCG PC Client Specific
              Implementation Specification for Conventional BIOS,
              Specification Version 1.21 Errata, Revision 1.00",
              February 2012,

              Trusted Computing Group, "PC Client Specific Platform
              Firmware Profile Specification Family "2.0", Level 00
              Revision 1.04", June 2019,

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              Trusted Computing Group, "DRAFT: TCG PC Client Reference
              Integrity Manifest Specification, v.09", December 2019,

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

              Trusted Computing Group, "TPM Keys for Platform Identity
              for TPM 1.2, Specification Version 1.0, Revision 3",
              August 2015, <https://trustedcomputinggroup.org/resource/

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,

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

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   [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,

   [TAP]      Trusted Computing Group, "TCG Trusted Attestation Protocol
              (TAP) Information Model for TPM Families 1.2 and 2.0 and
              DICE Family 1.0, Version 1.0, Revision 0.36", October
              2018, <https://trustedcomputinggroup.org/resource/tcg-tap-

9.2.  Informative References

              Trusted Computing Group, "TCG Infrastructure Working Group
              - A CMC Profile for AIK Certificate Enrollment Version
              1.0, Revision 7", March 2011,

              Trusted Computing Group, "SNMP MIB for TPM-Based
              Attestation, Version 0.8Revision 0.02", May 2018,

   [EFI-TPM]  Trusted Computing Group, "TCG EFI Platform Specification
              for TPM Family 1.1 or 1.2, Specification Version 1.22,
              Revision 15", January 2014,

              Birkholz, H., Voit, E., and W. Pan, "Attestation Event
              Stream Subscription", draft-birkholz-rats-network-device-
              subscription-00 (work in progress), June 2020.

              Birkholz, H., Eckel, M., Newton, C., and L. Chen,
              "Reference Interaction Models for Remote Attestation
              Procedures", draft-birkholz-rats-reference-interaction-
              model-03 (work in progress), July 2020.

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              Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
              "Time-Based Uni-Directional Attestation", draft-birkholz-
              rats-tuda-03 (work in progress), July 2020.

              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture",
              draft-ietf-rats-architecture-06 (work in progress),
              September 2020.

              Mandyam, G., Lundblade, L., Ballesteros, M., and J.
              O'Donoghue, "The Entity Attestation Token (EAT)", draft-
              ietf-rats-eat-04 (work in progress), August 2020.

              Richardson, M., Wallace, C., and W. Pan, "Use cases for
              Remote Attestation common encodings", draft-richardson-
              rats-usecases-07 (work in progress), March 2020.

              Voit, E., "Trusted Path Routing", draft-voit-rats-trusted-
              path-routing-02 (work in progress), June 2020.

              Seaman, M., "802.1AE MAC Security (MACsec)", 2018,

              IEEE Computer Society, "802.1X-2020 - IEEE Standard for
              Local and Metropolitan Area Networks--Port-Based Network
              Access Control", February 2020,

   [IMA]      and , "Integrity Measurement Architecture", June 2019,

   [LLDP]     IEEE Computer Society, "802.1AB-2016 - IEEE Standard for
              Local and metropolitan area networks - Station and Media
              Access Control Connectivity Discovery", March 2016,

   [NetEq]    Trusted Computing Group, "TCG Guidance for Securing
              Network Equipment, Version 1.0, Revision 29", January
              2018, <https://trustedcomputinggroup.org/resource/tcg-

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              National Institute for Standards and Technology,
              "Guidelines for the Creation of Interoperable Software
              Identification (SWID) Tags", April 2016,

              National Institute for Standards and Technology, "BIOS
              Integrity Measurement Guidelines (Draft)", December 2011,

              Trusted Computing Group, "TCG Platform Attribute
              Credential Profile, Specification Version 1.0, Revision
              16", January 2018,

              Trusted Computing Group, "TCG TPM v2.0 Provisioning
              Guidance, Version 1.0, Revision 1.0", March 2015,

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,

   [RFC6813]  Salowey, J. and S. Hanna, "The Network Endpoint Assessment
              (NEA) Asokan Attack Analysis", RFC 6813,
              DOI 10.17487/RFC6813, December 2012,

              National Institute of Standards and Technology, "BIOS
              Integrity Measurement Guidelines (Draft)", December 2011,

              Labs64, Munich, Germany, "SoftWare IDentification (SWID)
              Tags Generator (Maven Plugin)", n.d.,

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   [TCGRoT]   Trusted Computing Group, "DRAFT: TCG Roots of Trust
              Specification", October 2018,

   [TPM1.2]   Trusted Computing Group, "TPM Main Specification Level 2
              Version 1.2, Revision 116", March 2011,

   [TPM2.0]   Trusted Computing Group, "Trusted Platform Module Library
              Specification, Family "2.0", Level 00, Revision 01.59",
              November 2019,

Authors' Addresses

   Guy Fedorkow (editor)
   Juniper Networks, Inc.

   Email: gfedorkow@juniper.net

   Eric Voit
   Cisco Systems, Inc.

   Email: evoit@cisco.com

   Jessica Fitzgerald-McKay
   National Security Agency

   Email: jmfitz2@nsa.gov

Fedorkow, et al.         Expires March 22, 2021                [Page 41]