TPM-based Network Device Remote Integrity Verification
draft-ietf-rats-tpm-based-network-device-attest-09

Document Type Active Internet-Draft (rats WG)
Authors Guy Fedorkow  , Eric Voit  , Jessica Fitzgerald-McKay 
Last updated 2021-11-18
Replaces draft-fedorkow-rats-network-device-attestation
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RATS Working Group                                    G.C. Fedorkow, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Informational                                   E. Voit
Expires: 22 May 2022                                               Cisco
                                                     J. Fitzgerald-McKay
                                                National Security Agency
                                                        18 November 2021

         TPM-based Network Device Remote Integrity Verification
           draft-ietf-rats-tpm-based-network-device-attest-09

Abstract

   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], as defined by
   the Trusted Computing Group (TCG).

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 22 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Document Organization . . . . . . . . . . . . . . . . . .   5
     1.4.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.5.  Description of Remote Integrity Verification (RIV)  . . .   6
     1.6.  Solution Requirements . . . . . . . . . . . . . . . . . .   8
     1.7.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   9
       1.7.1.  Out of Scope  . . . . . . . . . . . . . . . . . . . .   9
   2.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  RIV Software Configuration Attestation using TPM  . . . .  10
       2.1.1.  What Does RIV Attest? . . . . . . . . . . . . . . . .  11
       2.1.2.  Notes on PCR Allocations  . . . . . . . . . . . . . .  13
     2.2.  RIV Keying  . . . . . . . . . . . . . . . . . . . . . . .  15
     2.3.  RIV Information Flow  . . . . . . . . . . . . . . . . . .  16
     2.4.  RIV Simplifying Assumptions . . . . . . . . . . . . . . .  18
       2.4.1.  Reference Integrity Manifests (RIMs)  . . . . . . . .  18
       2.4.2.  Attestation Logs  . . . . . . . . . . . . . . . . . .  20
   3.  Standards Components  . . . . . . . . . . . . . . . . . . . .  20
     3.1.  Prerequisites for RIV . . . . . . . . . . . . . . . . . .  20
       3.1.1.  Unique Device Identity  . . . . . . . . . . . . . . .  20
       3.1.2.  Keys  . . . . . . . . . . . . . . . . . . . . . . . .  21
       3.1.3.  Appraisal Policy for Evidence . . . . . . . . . . . .  21
     3.2.  Reference Model for Challenge-Response  . . . . . . . . .  21
       3.2.1.  Transport and Encoding  . . . . . . . . . . . . . . .  23
     3.3.  Centralized vs Peer-to-Peer . . . . . . . . . . . . . . .  24
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  25
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     5.1.  Keys Used in RIV  . . . . . . . . . . . . . . . . . . . .  26
     5.2.  Prevention of Spoofing and Person-in-the-Middle
           Attacks . . . . . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  29
     5.4.  Owner-Signed Keys . . . . . . . . . . . . . . . . . . . .  29
     5.5.  Other Factors for Trustworthy Operation . . . . . . . . .  30
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  31
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32

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   9.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  Using a TPM for Attestation . . . . . . . . . . . . . . .  32
     9.2.  Root of Trust for Measurement . . . . . . . . . . . . . .  34
     9.3.  Layering Model for Network Equipment Attester and
           Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  34
     9.4.  Implementation Notes  . . . . . . . . . . . . . . . . . .  36
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     10.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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
   hardware.

1.1.  Requirements notation

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

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1.2.  Terminology

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

   Additionally, this document defines the following term:

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

   The goal of attestation is simply to assure an administrator or
   auditor that the device configuration and software that was launched
   when the device was last started is authentic and untampered-with.
   The determination of software authenticity is not prescribed in this
   document, but it's typically taken to mean a software image generated
   by an authority trusted by the administrator, such as the device
   manufacturer.

   Within the Trusted Computing Group (TCG) context, the scope of
   attestation is typically narrowed to describe 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 matches the intended configuration.  For network
   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 specific
   subset of attestation tasks, 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 network 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]).

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   In line with [I-D.ietf-rats-architecture] definitions, this document
   uses the term Endorser to refer to the role that signs identity and
   attestation certificates used by the Attester, while Reference Values
   are signed by a Reference Value Provider.  Typically, the
   manufacturer of an network device would be accepted as both the
   Endorser and Reference Value Provider, although the choice is
   ultimately up to the Verifier Owner.

1.3.  Document Organization

   The remainder of this document is organized into several sections:

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

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

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

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

   *  Supporting material is in an appendix at the end.

1.4.  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
   management.

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

   *  Provable Device Identity - This specification requires that an
      Attester (i.e., the attesting device) includes a cryptographic
      identifier unique to each device.  Effectively this means that the
      device's TPM must be so provisioned during the manufacturing
      cycle.

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

   *  Verifiability - Verification of software and configuration of the
      device shows that the software that the administrator authorized
      for use was actually 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)

1.5.  Description of Remote Integrity Verification (RIV)

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

   *  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
      provided.

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

   Using these two interlocking mechanisms, 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.

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   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 (see Section 9.2), each measuring the next stage and
   recording the result in tamper-resistant storage, normally ending
   when the system software is fully loaded.  A measurement signifies
   the identity, integrity and version of each software component
   registered with an Attester's TPM [TPM1.2], [TPM2.0], so that a
   subsequent verification stage can determine if the software installed
   is authentic, up-to-date, and free of tampering.

   RIV includes several major processes, split between the Attester and
   Verifier:

   1.  Generation 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.  Conveyance of Evidence reliably transports the collected Evidence
       from Attester to a Verifier to allow a management station to
       perform a meaningful appraisal in Step 4.  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.

   4.  Finally, Appraisal of Evidence occurs.  This is the process of
       verifying the Evidence received by a Verifier from the Attester,
       and using an Appraisal Policy to develop an Attestation Result,
       used to inform decision making.  In practice, this means
       comparing the Attester's measurements reported as Evidence with
       the device configuration expected by the Verifier.  Subsequently
       the Appraisal Policy for Evidence might match Evidence found
       against Reference Values (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 in RIV is based on IEEE 802.1AR Device
       Identity (DevID) [IEEE-802-1AR], coupled with careful supply-
       chain management by the manufacturer.  The Initial DevID (IDevID)
       certificate contains a statement by the manufacturer that
       establishes the identity of the device as it left the factory.

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       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], or post-manufacture installation of
       Local Device ID (LDevID)).

   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 computes the hash of
       components of the next stage, then updates the attestation log
       and the TPM.  The TPM can then report the hashes of all the
       measured hashes as signed evidence called a Quote (see
       Section 9.1 for an overview of TPM operation, or [TPM1.2] and
       [TPM2.0] for many more details).

   3.  Signed Reference Values (aka Reference Integrity Measurements)
       must be conveyed from the Reference Value Provider (the entity
       accepted as the software authority, often the manufacturer of the
       network device) to the Verifier.

1.6.  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
   Considerations.)

   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
   at the end-user's site.  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, a process which precludes pre-installation
   configuration.  See [RFC8572] for an example of Secure Zero Touch
   Provisioning.

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1.7.  Scope

   The need for assurance of software integrity, addressed by 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):

   *  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 [AK-Enrollment]
      or TCG Platform Certificates [Platform-Certificates].

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

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

1.7.1.  Out of Scope

   *  Run-Time Attestation: The Linux Integrity Measurement Architecture
      [IMA] attests each process launched after a device is started (and
      is in scope for RIV), but continuous run-time attestation of Linux
      or other multi-threaded operating system processes after they've
      started considerably expands the scope of the problem.  Many
      researchers are working on that problem, but this document defers
      the problem of continuous, in-memory run-time attestation.

   *  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.  Although out
      of scope for this document, these issues are accommodated in
      [I-D.ietf-rats-architecture].

   *  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.

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   *  Virtualization and Containerization: In a non-virtualized system,
      the host OS is responsible for measuring each User Space file or
      process, but that's 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.
      Virtualization and containerization technologies are increasingly
      used in network equipment, but are not considered in this
      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.
   The Remote Attestation steps of Section 1.5 are broken into two
   phases, shown in Figure 1:

   *  During system startup, or boot phase, each distinct software
      object is "measured" by the Attester.  The object's identity, hash
      (i.e., cryptographic digest) and version information are recorded
      in a log.  Hashes are also extended into the TPM (see Section 9.1
      for more on 'extending hashes'), in a way that can be used to
      validate the log entries.  The measurement process generally
      follows the layered 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.  See
      [I-D.ietf-rats-architecture], section "Layered Attestation
      Environments," for an architectural definition of this model.

   *  Once the device is running and has operational network
      connectivity, verification can take place.  A separate Verifier,
      running in its own trusted environment, 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 secured by, but never released by, the
      TPM.  The YANG model described in [I-D.ietf-rats-yang-tpm-charra]
      facilitates this operation.

   The result is that the Verifier can verify the device's identity by
   checking the subjectName and signature of the 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 Reference Values.

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   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.

       +-------------------------------------------------------+
       | +--------+    +--------+   +--------+    +---------+  |
       | | BIOS   |--->| Loader |-->| Kernel |--->|Userland |  |
       | +--------+    +--------+   +--------+    +---------+  |
       |     |            |           |                        |
       |     |            |           |                        |
       |     +------------+-----------+-+                      |
       |                    Boot Phase  |                      |
       |                                V                      |
       |                            +--------+                 |
       |                            |  TPM   |                 |
       |                            +--------+                 |
       |   Router                       |                      |
       +--------------------------------|----------------------+
                                        |
                                        |  Verification Phase
                                        |    +-----------+
                                        +--->| Verifier  |
                                             +-----------+

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

                  Figure 1: Layered RIV Attestation Model

   In the Boot phase, measurements are "extended", or hashed, into the
   TPM as processes start, with the result that the TPM ends up
   containing hashes of all the measured hashes.  Later, once the system
   is operational, during the Verification phase, signed digests are
   retrieved from the TPM for off-box analysis.

2.1.1.  What Does RIV Attest?

   TPM attestation is 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 structure 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.

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

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

   *  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 operational state of the settings match their
      intended state.

   *  Credentials - Administrators may wish to verify via attestation
      that public keys (and other credentials) outside the Root of Trust
      have not been subject to unauthorized tampering.  (By definition,
      keys protecting 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 BIOS
   (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, potentially subverting the
   chain.

   For devices using a UEFI BIOS, [PC-Client-BIOS-TPM-2.0] and
   [PC-Client-EFI-TPM-1.2] give detailed normative requirements for PCR
   usage.  For other platform architectures, where TCG normative
   requirements currently do not exist, the table in Figure 2 gives non-
   normative guidance for PCR assignment that generalizes the specific
   details of [PC-Client-BIOS-TPM-2.0].

   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] is taken by the Root of Trust for
   Measurement, code which, by definition, cannot be verified by
   measurement.  This measurement establishes 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
   below:

   *  PCR[0] typically represents a consistent view of rarely-changed
      Host Platform boot components, allowing Attestation 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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   *  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 UEFI BIOS.

   *  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).

   *  PCR[8] is used by the OS loader (e.g.  GRUB) 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.  For example, in a UEFI
   system, some administrators may consider booting an image from a
   removable drive, something recorded in a PCR, to be a security
   violation, while others might consider that operation an authorized
   recovery procedure.

   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, or releasing a particular decryption
   key, 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]
   logs.

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2.2.  RIV Keying

   RIV attestation relies on two credentials:

   *  An identity key pair and matching certificate 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.  This requirement goes slightly beyond 802.1AR; see
      Section 2.4 for notes.

   *  An Attestation key pair and matching certificate is required to
      sign the Quote generated by the TPM to report evidence of software
      configuration.

   In a TPM application, both the Attestation private key and the DevID
   private key MUST be protected by the TPM.  Depending on other TPM
   configuration procedures, the two keys are likely be different; some
   of the considerations are outlined in TCG "TPM 2.0 Keys for Device
   Identity and Attestation" [Platform-DevID-TPM-2.0].

   The TCG TPM 2.0 Keys document [Platform-DevID-TPM-2.0] specifies
   further conventions for these keys:

   *  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 (that is, the same subjectName and subjectAltName (if
      present), 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
      certificate.  If the subjectName in the AK certificate doesn't
      match the corresponding DevID certificate, or they're signed by
      differing authorities the Verifier may signal the detection of an
      Asokan-style person-in-the-middle attack (see Section 5.2).

   *  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 Initial
      AK, called IDevID and IAK).  IDevID and IAK certificates MUST both
      be signed by the Endorser (typically the device manufacturer).
      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.

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2.3.  RIV Information Flow

   RIV workflow for network equipment is organized around a simple use
   case where a network operator wishes to verify the integrity of
   software installed in specific, fielded devices.  A normative
   taxonomy of terms is given in [I-D.ietf-rats-architecture], but as a
   reminder, this use case implies several roles and objects:

   1.  The Attester, the device which the network operator wants to
       examine.

   2.  A Verifier (which might be a network management station)
       somewhere separate from the Device that will retrieve the signed
       evidence and measurement logs, and analyze them 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.

   4.  Signed Reference Integrity Manifests (RIMs), containing Reference
       Values, 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 Values 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 3.

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

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        Figure 3: RIV Reference Configuration for Network Equipment

   *  In Step 0, The Reference Value Provider (the device manufacturer
      or other authority) makes one or more Reference Integrity
      Manifests (RIMs), corresponding to the software image expected to
      be found on the device, signed by the Reference Value Provider,
      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.

   Use of the following standards components allows for
   interoperability:

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

   2.  For devices using UEFI and Linux, measurements of firmware and
       bootable modules MUST be taken according to TCG PC Client
       [PC-Client-EFI-TPM-1.2] or [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 from Linux-based systems MUST be formatted
       according to the Canonical Event Log format
       [Canonical-Event-Log].  UEFI-based systems MUST use the TCG UEFI
       BIOS event log [PC-Client-EFI-TPM-1.2] for TPM1.2 systems, and
       TCG PC Client Platform Firmware Profile [PC-Client-BIOS-TPM-2.0]
       for TPM2.0.

   5.  Quotes MUST be retrieved from the TPM according to TCG TAP
       Information Model [TAP] and the CHARRA YANG model
       [I-D.ietf-rats-yang-tpm-charra].  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, and MUST be retrieved in a way that does not invalidate
       the signature, to preserve the trust model.  The
       [I-D.ietf-rats-yang-tpm-charra] can be used for this purpose.
       (See Section 5 Security Considerations).

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   6.  Reference Values MUST be encoded as defined in the TCG RIM
       document [RIM], typically using SWID [SWID], [NIST-IR-8060] or
       CoSWID tags [I-D.ietf-sacm-coswid].

2.4.  RIV Simplifying Assumptions

   This document makes the following simplifying assumptions to reduce
   complexity:

   *  The product to be attested MUST be shipped by the equipment vendor
      with both an IEEE 802.1AR Device Identity and an Initial
      Attestation Key (IAK) with certificate in place.  The IAK
      certificate MUST contain the same identity information as the
      DevID (specifically, the same subjectName and subjectAltName (if
      used), signed by the manufacturer), but it's a type of key that
      can be used to sign a TPM Quote, but not other objects (i.e., it's
      marked as a TCG "Restricted" key; this convention is described in
      "TPM 2.0 Keys for Device Identity and Attestation"
      [Platform-DevID-TPM-2.0]).  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.

   *  IEEE 802.1AR does not require a product serial number as part of
      the subjectName, but RIV-compliant devices MUST include their
      serial numbers in the DevID/IAK certificates to simplify tracking
      logistics for network equipment users.  All other optional 802.1AR
      fields remain optional in RIV

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

   *  The authorized software supplier MUST make available Reference
      Values in the form of signed SWID or CoSWID tags.

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.

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   While it must be up to network administrators to decide what they
   want on their networks, the software supplier should supply the
   Reference Values, in signed Reference Integrity Manifests, 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 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.

   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.

   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.

   Many network equipment vendors use a UEFI BIOS to launch their
   network operating system.  These vendors may want to also use the TCG
   PC Client Reference Integrity Measurement specification
   [PC-Client-RIM], which focuses specifically on a SWID-compatible
   format suitable for expressing measurement values expected from a
   UEFI BIOS.

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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 (that is, calculating the hash of all the hashes in
   the log should produce the same values as contained in the PCRs; if
   they don't match, the log may have been tampered with.  See
   Section 9.1).

   There are multiple event log formats which may be supported as viable
   formats of Evidence between the Attester and Verifier, but to
   simplify interoperability, RIV focuses on just three:

   *  TCG UEFI BIOS event log for TPM 2.0 (TCG PC Client Platform
      Firmware Profile) [PC-Client-BIOS-TPM-2.0]

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

   *  TCG Canonical Event Log [Canonical-Event-Log]

3.  Standards Components

3.1.  Prerequisites for RIV

   The Reference Interaction Model for Challenge-Response-based Remote
   Attestation ([I-D.birkholz-rats-reference-interaction-model]) 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 measurements collected by the
   Attester.

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
   TPMs.

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3.1.2.  Keys

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

   It MUST be possible for the Verifier to determine that the Attester's
   Attestation keys are resident in the same TPM as its DevID keys (see
   Section 2.2 and Section 5 Security Considerations).

3.1.3.  Appraisal Policy for Evidence

   As noted in Section 2.3, the Verifier may obtain Reference Values
   from several sources.  In addition, administrators may make
   authorized, site-specific changes (e.g. keys in key databases) that
   could impact attestation results.  As such, there could be conflicts,
   omissions or ambiguities between some Reference Values and collected
   Evidence.

   The Verifier MUST have an Appraisal Policy for Evidence to evaluate
   the significance of any discrepancies between different reference
   sources, or between reference values and evidence from logs and
   quotes.  While there must be an Appraisal Policy, this document does
   not specify the format or mechanism to convey the intended policy,
   nor does RIV specify mechanisms by which the results of applying the
   policy are communicated to the Relying Party.

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 7.1 of [I-D.birkholz-rats-reference-interaction-model].  In
   this diagram, each event with its input and output parameters is
   shown as "Event(input-params)=>(outputs)".  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)                                                      |
   generateClaims(attestingEnvironment)                            |
      | => claims, eventLogs                                       |
      |                                                            |
      |                                                        time(NS)
      | <-- requestAttestation(handle, authSecIDs, claimSelection) |
      |                                                            |
    time(EG)                                                       |
   collectClaims(claims, claimSelection)                           |
      | => collectedClaims                                         |
      |                                                            |
   generateEvidence(handle, authSecIDs, collectedClaims)           |
      | => evidence                                                |
      |                                                    time(RG,RA)
      | evidence, eventLogs -------------------------------------> |
      |                                                            |
      |               appraiseEvidence(evidence, eventLogs, refValues)
      |                                       attestationResult <= |
      |                                                            |
      ~                                                            ~
      |                                                       time(RX)

                Figure 4: IETF Attestation Information Flow

   *  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.

   *  Step 2 (time(NS)): The Verifier generates a unique random nonce
      ("number used once"), and makes a request for one or more PCRs
      from an Attester.  For interoperability, this MUST be accomplished
      via an interface that implements the YANG Data Model for
      Challenge-Response-based Remote Attestation Procedures using TPMs
      [I-D.ietf-rats-yang-tpm-charra].  TPM1.2 and TPM2.0 both allow
      nonces as large as the operative digest size (i.e., 20 or 32
      bytes; see [TPM1.2] Part 2, Section 5.5 and [TPM2.0] Part 2,
      Section 10.4.4).

   *  Step 3 (time(EG)): On the Attester, measured values are retrieved
      from the Attester's TPM.  This requested PCR evidence, along with
      the Verifier's nonce, called a Quote, is signed by the Attestation
      Key (AK) associated with the DevID.  Quotes are retrieved
      according to CHARRA YANG model [I-D.ietf-rats-yang-tpm-charra].

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      At the same time, the Attester collects log evidence showing the
      values have been extended into that PCR.  Section 9.1 gives more
      detail on how this works, including references to the structure
      and contents of quotes in TPM documents.

   *  Step 4: Collected Evidence is passed from the Attester to the
      Verifier

   *  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 be described as one
      step.

      -  If the signature covering TPM Evidence is not correct, the
         device SHOULD NOT be trusted.

      -  If the nonce in the response doesn't match the Verifier's
         nonce, the response may be a replay, and device SHOULD NOT be
         trusted.

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

      -  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
         trusted.

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

3.2.1.  Transport and Encoding

   Network Management systems MUST retrieve signed PCR based Evidence
   using [I-D.ietf-rats-yang-tpm-charra] with NETCONF or RESTCONF.

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

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   Log Evidence MUST be retrieved via log interfaces specified in
   [I-D.ietf-rats-yang-tpm-charra].

3.3.  Centralized vs Peer-to-Peer

   Figure 4 above assumes that the Verifier is trusted, while the
   Attester is not.  In a Peer-to-Peer application such as two routers
   negotiating a trust relationship, 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 5.
   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 remote attestation, without
   having to resort to a central authority.

   +---------------+                            +---------------+
   | RefVal        |                            | RefVal        |
   | Provider A    |                            | Provider 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 5: 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

   Network equipment, such as routers, switches and firewalls, has a key
   role to play in guarding the privacy of individuals using the
   network.  Network equipment generally adheres to several rules to
   protect privacy:

   *  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.

      -  Network equipment is often called upon to block access to
         protected resources from unauthorized users.

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

   *  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.
   Attestation is a component that allows the administrator to ensure
   that the network provides individual and peer privacy guarantees,
   even though the device itself may not have a right to keep its
   identity secret.

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

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   While attestation information from network devices is not likely to
   contain privacy-sensitive content regarding network users,
   administrators may want to keep attestation records confidential to
   avoid disclosing versions of software loaded on the device,
   information which could facilitate attacks against known
   vulnerabilities.

5.  Security Considerations

   Attestation Evidence from the RIV procedure are subject to a number
   of attacks:

   *  Keys may be compromised.

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

   *  Person-in-the-middle attacks may be used by a compromised device
      to attempt to deliver responses that originate in an authentic
      device.

   *  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 evidence can be trusted.

   Two sets of key-pairs are relevant to RIV attestation:

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

   *  An Attestation Key-pair (AK) key is used to certify attestation
      Evidence (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 are never exposed, and
   cannot be extracted (See [Platform-DevID-TPM-2.0]).

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   Keeping keys safe is a critical enabler of trustworthiness, but it's
   just part of attestation security; knowing which keys are bound to
   the device in question is just as important in an environment where
   private keys are never exposed.

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

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

   *  The private portion of the DevID key is to be stored in the
      device, in a manner that provides confidentiality (Section 6.2.5
      [IEEE-802-1AR])

   The X.509 certificate contains several components:

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

   *  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.

   *  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 random
   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

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   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 measurements of the operating software to be signed by a
   key known only to the TPM also removes the need to trust the device's
   operating software (beyond the first measurement in the RTM; 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 Person-in-the-Middle Attacks

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

   *  The entire device could be spoofed.  If the Verifier goes to
      appraise a specific Attester, it might be redirected to a
      different Attester.  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.

   *  A device with a compromised OS could return a fabricated quote
      providing spoofed attestation Evidence.

   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
   (AK).

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

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   This is accomplished in RIV through use of an AK certificate with the
   same elements as the DevID (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.

   The TCG document TPM 2.0 Keys for Device Identity and Attestation
   [Platform-DevID-TPM-2.0] specifies OIDs for Attestation Certificates
   that allow the CA to mark a key as specifically known to be an
   Attestation key.

   These two key-pairs and certificates are used together:

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

   *  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 random 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 if zero-touch provisioning such as
   described in [RFC8572] is to be supported, 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
   properties.

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

   *  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

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      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).  TCG document
      [Provisioning-TPM-2.0] also contains guidance on provisioning
      Initial and Local identity keys in TPM 2.0.

   *  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.

   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
   plate.

5.5.  Other Factors for Trustworthy Operation

   In addition to trustworthy provisioning of keys, RIV depends on a
   number of other factors for trustworthy operation.

   *  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.

   *  Attestation depends on an unbroken chain of measurements, starting
      from the very first measurement.  See Section 9.1 for background
      on TPM practices.

   *  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 9.2 for background on roots of trust.

   *  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.

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   RIV also depends on reliable Reference Values, 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 network 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 Values:

   *  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.

   *  Designers SHOULD guard against hash collision attacks.  Reference
      Integrity Manifests 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:

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

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

   *  The TCG TAP mechanism couple with [I-D.ietf-rats-yang-tpm-charra]
      can be used to retrieve attestation evidence.

   *  Reference Values must be conveyed from the software authority
      (e.g., the manufacturer) in Reference Integrity Manifests, to the
      system in which verification will take place.  IETF and TCG SWID
      and CoSWID work [I-D.ietf-sacm-coswid], [RIM])) forms the basis
      for this function.

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7.  IANA Considerations

   This memo includes no request to IANA.

8.  Acknowledgements

   The authors wish to thank numerous reviewers for generous assistance,
   including William Bellingrath, Mark Baushke, Ned Smith, Henk
   Birkholz, Tom Laffey, Dave Thaler, Wei Pan, Michael Eckel, Thomas
   Hardjono, Bill Sulzen, Willard (Monty) Wiseman, Kathleen Moriarty,
   Nancy Cam-Winget and Shwetha Bhandari

9.  Appendix

9.1.  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.

   Each TPM has at least eight and at most 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 comprising the hash 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 which event 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 no discrepancy, the logs do provide
   an accurate view of what was placed into the PCR.

   Note that the composite hash-of-hashes recorded in PCRs is order-
   dependent, resulting in different PCR values for different ordering
   of the same set of events (e.g.  Event A followed by Event B yields a
   different PCR value than B followed by A).  For single-threaded code,
   where both the events and their order are fixed, a Verifier may
   validate a single PCR value, and use the log only to diagnose a

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   mismatch from Reference Values.  However, operating system code is
   usually non-deterministic, meaning that there may never be a single
   "known good" PCR value.  In this case, the Verifier may have to
   verify that the log is correct, and then analyze each item in the log
   to determine if it represents an authorized event.

   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, along with the Verifier's
   nonce, into a TPM-specific data structure signed by an Attestation
   private key, known only to the TPM.

   As noted above in Section 5 Security Considerations, it's important
   to note that the Quote data structure is signed inside the TPM.  The
   trust model is preserved by retrieving the Quote in a way that does
   not invalidate the signature, as specified in
   [I-D.ietf-rats-yang-tpm-charra].  The structure of the command and
   response for a quote, including its signature, as generated by the
   TPM, can be seen in [TPM1.2] Part 3, Section 16.5, and [TPM2.0]
   Section 18.4.2.

   The Verifier uses the Quote and Log together.  The Quote contains the
   composite hash of the complete sequence of security measurement
   hashes, signed by the TPM's private Attestation Key.  The Log
   contains a record of each measurement extended into the TPM's PCRs.
   By computing the composite hash of all the measurements, the Verifier
   can verify the integrity of the Event Log, even though the Event Log
   itself is not signed.  Each hash in the validated Event Log can then
   be compared to corresponding expected values in the set of Reference
   Values 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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9.2.  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], [SP800-193].

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

   *  The first measurement computed by the Root of Trust for
      Measurement, and stored in the TPM's Root of Trust for Storage,
      must be assumed to be correct.

   *  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 [SP800-155])

   It's important to note that the trustworthiness of the RTM code
   cannot be assured by the TPM or TPM supplier - code or procedures
   external to the TPM must guarantee the security of the RTM.

9.3.  Layering Model for Network Equipment Attester and Verifier

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

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

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

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

       *************************               **********************
       * YANG SWID Module      *               * YANG Attestation   *
       * I-D.ietf-sacm-coswid  *               * Module             *
       *                       *               * 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 6: 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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9.4.  Implementation Notes

   Figure 7 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.

   As noted, SWID tags can be generated many ways, but one possible tool
   is [SWID-Gen]

   +------------------------------------------------------------------+
   |             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       |[Platform-DevID-TPM-2.0]|
   |   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 client    |
   |                                               | certificate)     |
   --------------------------------------------------------------------
   | Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID    |
   |    o  Add reference measurements into SWID tags | ISO/IEC 19770-2|

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   |    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          | I-D.ietf-sacm-coswid|
   |     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 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.     |                |
   --------------------------------------------------------------------

                         Figure 7: Component Status

10.  References

10.1.  Normative References

   [Canonical-Event-Log]
              Trusted Computing Group, "DRAFT Canonical Event Log Format
              Version: 1.0, Revision: .30", December 2020,
              <https://www.trustedcomputinggroup.org/wp-content/uploads/
              TCG_IWG_CEL_v1_r0p30_13feb2021.pdf>.

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   [I-D.ietf-rats-yang-tpm-charra]
              Birkholz, H., Eckel, M., Bhandari, S., Voit, E., Sulzen,
              B., (Frank), L. X., Laffey, T., and G. C. Fedorkow, "A
              YANG Data Model for Challenge-Response-based Remote
              Attestation Procedures using TPMs", Work in Progress,
              Internet-Draft, draft-ietf-rats-yang-tpm-charra-11, 26
              August 2021, <https://www.ietf.org/archive/id/draft-ietf-
              rats-yang-tpm-charra-11.txt>.

   [I-D.ietf-sacm-coswid]
              Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
              Waltermire, "Concise Software Identification Tags", Work
              in Progress, Internet-Draft, draft-ietf-sacm-coswid-19, 20
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              sacm-coswid-19.txt>.

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

   [PC-Client-BIOS-TPM-2.0]
              Trusted Computing Group, "PC Client Specific Platform
              Firmware Profile Specification Family "2.0", Level 00
              Revision 1.05", May 2021,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_PCClient_PFP_r1p05_v23_pub.pdf>.

   [PC-Client-EFI-TPM-1.2]
              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/resource/tcg-efi-
              platform-specification/>.

   [PC-Client-RIM]
              Trusted Computing Group, "TCG PC Client Reference
              Integrity Manifest Specification, v1.04", December 2019,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_PC_Client_RIM_r1p04_pub.pdf>.

   [Platform-DevID-TPM-2.0]
              Trusted Computing Group, "TPM 2.0 Keys for Device Identity
              and Attestation, Specification Version 1.0, Revision 2",

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              September 2020,
              <https://trustedcomputinggroup.org/resource/tpm-2-0-keys-
              for-device-identity-and-attestation/>.

   [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/resource/
              tpm-keys-for-platform-identity-for-tpm-1-2-2/>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [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>.

   [RIM]      Trusted Computing Group, "TCG Reference Integrity Manifest
              (RIM) Information Model, v1.0, r0.16", June 2019,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_RIM_Model_v1p01_r0p16_pub.pdf>.

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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,
              <https://www.iso.org/standard/65666.html>.

   [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-
              information-model/>.

10.2.  Informative References

   [AK-Enrollment]
              Trusted Computing Group, "TCG Infrastructure Working Group
              - A CMC Profile for AIK Certificate Enrollment Version
              1.0, Revision 7", March 2011,
              <https://trustedcomputinggroup.org/resource/tcg-
              infrastructure-working-group-a-cmc-profile-for-aik-
              certificate-enrollment/>.

   [I-D.birkholz-rats-network-device-subscription]
              Birkholz, H., Voit, E., and W. Pan, "Attestation Event
              Stream Subscription", Work in Progress, Internet-Draft,
              draft-birkholz-rats-network-device-subscription-03, 17
              August 2021, <https://www.ietf.org/archive/id/draft-
              birkholz-rats-network-device-subscription-03.txt>.

   [I-D.birkholz-rats-reference-interaction-model]
              Birkholz, H., Eckel, M., Newton, C., and L. Chen,
              "Reference Interaction Models for Remote Attestation
              Procedures", Work in Progress, Internet-Draft, draft-
              birkholz-rats-reference-interaction-model-03, 7 July 2020,
              <https://www.ietf.org/archive/id/draft-birkholz-rats-
              reference-interaction-model-03.txt>.

   [I-D.birkholz-rats-tuda]
              Fuchs, A., Birkholz, H., McDonald, I. E., and C. Bormann,
              "Time-Based Uni-Directional Attestation", Work in
              Progress, Internet-Draft, draft-birkholz-rats-tuda-05, 12
              July 2021, <https://www.ietf.org/archive/id/draft-
              birkholz-rats-tuda-05.txt>.

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   [I-D.ietf-rats-architecture]
              Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote Attestation Procedures Architecture", Work
              in Progress, Internet-Draft, draft-ietf-rats-architecture-
              13, 8 November 2021, <https://www.ietf.org/archive/id/
              draft-ietf-rats-architecture-13.txt>.

   [I-D.ietf-rats-eat]
              Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
              Attestation Token (EAT)", Work in Progress, Internet-
              Draft, draft-ietf-rats-eat-11, 24 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-rats-eat-
              11.txt>.

   [I-D.richardson-rats-usecases]
              Richardson, M., Wallace, C., and W. Pan, "Use cases for
              Remote Attestation common encodings", Work in Progress,
              Internet-Draft, draft-richardson-rats-usecases-08, 2
              November 2020, <https://www.ietf.org/archive/id/draft-
              richardson-rats-usecases-08.txt>.

   [IEEE-802.1AE]
              Seaman, M., "802.1AE MAC Security (MACsec)", 2018,
              <https://1.ieee802.org/security/802-1ae/>.

   [IEEE-802.1X]
              IEEE Computer Society, "802.1X-2020 - IEEE Standard for
              Local and Metropolitan Area Networks--Port-Based Network
              Access Control", February 2020,
              <https://standards.ieee.org/standard/802_1X-2020.html>.

   [LLDP]     IEEE Computer Society, "802.1AB-2016 - IEEE Standard for
              Local and metropolitan area networks - Station and Media
              Access Control Connectivity Discovery", March 2016,
              <https://standards.ieee.org/standard/802_1AB-2016.html>.

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

   [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>.

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   [Platform-Certificates]
              Trusted Computing Group, "TCG Platform Attribute
              Credential Profile, Specification Version 1.0, Revision
              16", January 2018,
              <https://trustedcomputinggroup.org/resource/tcg-platform-
              attribute-credential-profile/>.

   [Provisioning-TPM-2.0]
              Trusted Computing Group, "TCG TPM v2.0 Provisioning
              Guidance, Version 1.0, Revision 1.0", March 2015,
              <https://trustedcomputinggroup.org/wp-content/uploads/TCG-
              TPM-v2.0-Provisioning-Guidance-Published-v1r1.pdf>.

   [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,
              <https://www.rfc-editor.org/info/rfc3748>.

   [RFC6813]  Salowey, J. and S. Hanna, "The Network Endpoint Assessment
              (NEA) Asokan Attack Analysis", RFC 6813,
              DOI 10.17487/RFC6813, December 2012,
              <https://www.rfc-editor.org/info/rfc6813>.

   [SP800-155]
              National Institute of 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>.

   [SP800-193]
              National Institute for Standards and Technology, "NIST
              Special Publication 800-193: Platform Firmware Resiliency
              Guidelines", April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-193.pdf>.

   [SWID-Gen] Labs64, Munich, Germany, "SoftWare IDentification (SWID)
              Tags Generator (Maven Plugin)", n.d.,
              <https://github.com/Labs64/swid-maven-plugin>.

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

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   [TPM1.2]   Trusted Computing Group, "TPM Main Specification Level 2
              Version 1.2, Revision 116", March 2011,
              <https://trustedcomputinggroup.org/resource/tpm-main-
              specification/>.

   [TPM2.0]   Trusted Computing Group, "Trusted Platform Module Library
              Specification, Family "2.0", Level 00, Revision 01.59",
              November 2019,
              <https://trustedcomputinggroup.org/resource/tpm-library-
              specification/>.

Authors' Addresses

   Guy Fedorkow (editor)
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts 01886
   United States of America

   Email: gfedorkow@juniper.net

   Eric Voit
   Cisco Systems

   Email: evoit@cisco.com

   Jessica Fitzgerald-McKay
   National Security Agency
   9800 Savage Road
   Ft. Meade, Maryland 20755
   United States of America

   Email: jmfitz2@nsa.gov

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