draft-fedorkow-rats-network-device-attestation-02.txt

RATS Working Group                                      G. Fedorkow, Ed.
Internet-Draft                                    Juniper Networks, Inc.
Intended status: Informational                       J. Fitzgerald-McKay
Expires: December 3, 2019                       National Security Agency
                                                            June 1, 2019


              Network Device Remote Integrity Verification
           draft-fedorkow-rats-network-device-attestation-02

Abstract

   This document describes a workflow for remote attestation of
   integrity of network devices.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Description of Remote Integrity Verification (RIV)  . . .   5
     1.4.  Solution Requirements . . . . . . . . . . . . . . . . . .   6
     1.5.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       1.5.1.  Out of Scope  . . . . . . . . . . . . . . . . . . . .   8
       1.5.2.  Why Remote Integrity Verification?  . . . . . . . . .   8
       1.5.3.  Network Device Attestation Challenges . . . . . . . .   9
       1.5.4.  Why is OS Attestation Different?  . . . . . . . . . .  10
   2.  Solution Outline  . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  RIV Software Configuration Attestation using TPM  . . . .  10
       2.1.1.  What Does RIV Attest? . . . . . . . . . . . . . . . .  11
     2.2.  RIV Keying  . . . . . . . . . . . . . . . . . . . . . . .  13
     2.3.  RIV Information Flow  . . . . . . . . . . . . . . . . . .  14
     2.4.  RIV Simplifying Assumptions . . . . . . . . . . . . . . .  15
       2.4.1.  DevID Alternatives  . . . . . . . . . . . . . . . . .  16
       2.4.2.  Additional Attestation of Platform Characteristics  .  16
       2.4.3.  Root of Trust for Measurement . . . . . . . . . . . .  17
       2.4.4.  Reference Integrity Manifests (RIMs)  . . . . . . . .  17
       2.4.5.  Attestation Logs  . . . . . . . . . . . . . . . . . .  18
   3.  Standards Components  . . . . . . . . . . . . . . . . . . . .  19
     3.1.  Reference Models  . . . . . . . . . . . . . . . . . . . .  19
       3.1.1.  IETF Reference Model for Challenge-Response Remote
               Attestation . . . . . . . . . . . . . . . . . . . . .  19
     3.2.  RIV Workflow  . . . . . . . . . . . . . . . . . . . . . .  20
     3.3.  Layering Model for Network Equipment Attester and
           Verifier  . . . . . . . . . . . . . . . . . . . . . . . .  22
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  24
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  29
   7.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     7.1.  Implementation Notes  . . . . . . . . . . . . . . . . . .  29
     7.2.  Comparison with TCG PTS / IETF NEA  . . . . . . . . . . .  31
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

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, is one of those aspects that's easily
   overlooked.



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   Attestation is defined here as the process of creating, conveying and
   appraising assertions about Platform trustworthiness characteristics,
   including supply chain trust, identity, platform provenance, software
   configuration, hardware configuration, platform composition,
   compliance to test suites, functional and assurance evaluations, etc.

   The supply chain itself has many elements, from validating suppliers
   of electronic components, to ensuring that shipping procedures
   protect against tampering through many stages of distribution and
   warehousing.  One element that helps maintain the integrity of the
   supply chain after manufacturing is Attestation, by assuring an
   administrator that the software that was launched when the device was
   started is the same as the software that the device vendor initially
   shipped.

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

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

   This document focuses primarily on software integrity verification
   using the Trusted Platform Module (TPM) to ensure a trustworthy
   result.

   The integrity of attestation information must be protected by means
   of cryptographic techniques, to assure its validity.  It's important
   to note that TCG technologies are available to use either symmetric
   key encryption with shared keys, or public key cryptography using



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   private/public key pairs.  The two techniques can each produce secure
   results, but do require different provisioning mechanisms.  The RIV
   document currently focuses on asymmetric keying approaches only;
   future work might include techniques for attestation using symmetric
   keys.

1.1.  Requirements Language

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

1.2.  Goals

   Attestation requires two interlocking services on the device:

   o  Platform 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 Platform Identity,
      cryptographic proof of the identity of the manufacturer is also
      provided.

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

   As a part of a trusted supply chain, the RIV attestation workflow
   outlined in this document is intended to meet the following high-
   level goals:

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

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

   o  Verifiability - Verification of software and configuration of the
      device shows that the software that's supposed to be installed on
      there actually has been launched.  Verification against reference
      manifests signed by the supplier of the software provides
      assurance that the software is free of unauthorized modification.




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1.3.  Description of Remote Integrity Verification (RIV)

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

   RIV can be viewed as a link in a trusted supply chain that ensures
   that devices launch software without unauthorized modification, and
   includes three major processes:

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

   o  Platform Identity refers to the mechanism assuring the attestation
      relying party (typically a network administrator) of the identity
      of devices that make up their network, and that their
      manufacturers are known.

   o  Software used to boot a platform can be described as a chain of
      measurements, started by a Root of Trust for Measurement, that
      normally ends when the system software is loaded.  A measurement
      signifies the identity, integrity and version of each software
      component registered with the TPM, so that the subsequent
      appraisal stage can determine if the software installed is
      authentic, up-to-date, and free of tampering.

   Clearly the second part of the problem, attesting the state of
   mutable components of a given device, is of little value without
   reliable identification of the device in question.  By the same
   token, unambiguous identity of a device is necessary, but is
   insufficient to assure the operator of the provenance of the device
   through the supply chain, or that the device is configured to behave
   properly.

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

   2.  Appraisal of Evidence is the process of verifying the evidence
       received by a verifier/appraiser from a device, and using



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       verified evidence to inform decision making.  In this context,
       verification means comparing the device measurements reported as
       evidence with the configuration expected by the system
       administrator.  This step can work only when there is a way to
       express what should be there, often referred to as golden
       measurements, or Reference Integrity Measurements, representing
       the intended configured state of the connected device.

   An implementation of RIV requires three technologies

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

   2.  Platform Attestation provides evidence of configuration of
       software elements throughout the product lifecycle.  This form of
       attestation can be implemented with TPM PCR, Quote and log
       mechanisms, which provide an authenticated mechanism to report
       what software actually starts up on the device each time it
       reboots.

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

   Service Providers 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.

1.4.  Solution Requirements

   The RIV attestation solution must meet a number of requirements to
   make it simple to deploy at scale.

   1.  Easy to Use - This solution should work "out of the box" as far
       as possible, that is, with the fewest possible steps needed at
       the end-user's site.  Eliminate complicated databases or
       provisioning steps that would have to be executed by the owner of
       a new device.  Network equipment is often required to "self-



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       configure", to reliably reach out without manual intervention to
       prove its identity and operating posture, then download its own
       configuration.  See [RFC8572] for an example of Secure Zero Touch
       Provisioning.

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

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

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

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

1.5.  Scope

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

   o  This solution is for use in non-privacy-preserving applications
      (for example, networking, Industrial IoT), avoiding the need for a
      Privacy Certificate Authority for attestation keys
      [AIK-Enrollment] or TCG Platform Certificates
      [Platform-Certificates]

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



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   o  This document assumes network protocols that are common in
      networking equipment, but not generally used in other
      applications.  Other applications (such as Industrial IoT) would
      replace YANG/Netconf with a protocol suite more commonly used in
      that environment.  Similarly, the same information flow would be
      used in Network Endpoint Assessment [RFC5209], mapped to NEA
      protocols.

   o  The approach outlined in this document assumes the use of TPM 1.2
      or TPM 2.0.  Other roots of trust could be used with the same
      information flow, although different data structures would likely
      be called for.

1.5.1.  Out of Scope

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

   o  Multi-Vendor Embedded Systems: Additional coordination would be
      needed for devices that themselves comprise hardware and software
      from multiple vendors, integrated by the end user.  Although
      currently out of scope for RIV, Trusted Computing Group
      specifications do encompass attestation of multi-vendor endpoint
      computing devices.

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

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

1.5.2.  Why Remote Integrity Verification?

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

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




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1.5.3.  Network Device Attestation Challenges

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

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

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

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

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

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



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   o  Standards-based mechanisms to encode and distribute Reference
      Integrity Measurements, (i.e., expected values) are still in
      development.

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

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

1.5.4.  Why is OS Attestation Different?

   Even in embedded systems, adding Attestation at the OS level (e.g.
   Linux IMA, Integrity Measurement Architecture [IMA]) increases the
   number of objects to be attested by one or two orders of magnitude,
   involves software that's updated and changed frequently, and
   introduces processes that begin in unpredictable order.

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

2.  Solution Outline

2.1.  RIV Software Configuration Attestation using TPM

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

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

   o  Once the device is running and has operational network
      connectivity, a separate, trusted server (called a Verifier in
      this document) can interrogate the network device to retrieve the



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      logs and a copy of the digests collected by hashing each software
      object, signed by an attestation private key known only to the
      TPM.

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

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

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

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

                      Figure 1: RIV Attestation Model

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

2.1.1.  What Does RIV Attest?

   TPM attestation is strongly focused around Platform Configuration
   Registers (PCRs), but those registers are only vehicles for
   certifying attestation evidence.  The evidence itself is conveyed in



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   log files (xref) which give the name and hash of each object to be
   attested.  These hashes are also extended into PCRs, where they can
   be retrieved in the form of a quote signed by a key known only to the
   TPM (xref).  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 that are conventionally attested with a
   TPM.

   In general, PCRs are organized to independently attest three classes
   of object:

   o  Code, i.e., instructions to be executed by a CPU.  To maintain a
      chain of trust, it's important to measure every bit of code that's
      run outside the root of trust.

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

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

   The TCG PC Client Platform Firmware Profile Specification
   [PC-Client-BIOS-TPM-2.0] gives considerable detail on what is to be
   measured during the boot phase of a platform boot using a UEFI BOIS
   (www.uefi.org), but the goal is simply to measure every bit of code
   executed in the process of starting the device, along with any
   configuration information related to security posture.  Table XX
   summarizes the functions that are measured, and how they are
   allocated to PCRs.  It's important to note that each PCR may contain
   results from dozens (or even thousands) of individual measurements.














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   +------------------------------------------------------------------+
   |                                            |   Allocated PCR #   |
   | Function                                   | Code | Configuration|
   --------------------------------------------------------------------
   | BIOS Static Root of Trust, plus embedded   |  0   |    1         |
   | Option ROMs and drivers                    |      |              |
   --------------------------------------------------------------------
   | Pluggable Option ROMs to initialize and    |  2   |    3         |
   | configure add-in devices                   |      |              |
   --------------------------------------------------------------------
   | Boot Manager code and configuration (UEFI  |  4   |    5         |
   | uses a separate module to implement        |      |              |
   | policies for selecting among a variety of  |      |              |
   | potential boot devices).  This PCR records |      |              |
   | boot attempts, and identifies what         |      |              |
   | resources were used to boot the OS.        |      |              |
   --------------------------------------------------------------------
   | Vendor Specific Measurements               |  6   |    6         |
   --------------------------------------------------------------------
   | Secure Boot Policy.  This PCR records keys |      |    7         |
   | and configuration used to validate the OS  |      |              |
   | loader                                     |      |              |
   --------------------------------------------------------------------
   | OS Loader (e.g GRUB2 for Linux)            |  8   |    9         |
   --------------------------------------------------------------------
   | Reserved for OS (e.g. Linux IMA)           |  10  |    10        |
   +------------------------------------------------------------------+

                        Figure 2: Attested Objects

2.2.  RIV Keying

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

   To accommodate these rules, enforced inside the TPM, in an
   environment where device privacy is not normally a requirement, the
   TCG Guidance for Securing Network Equipment [NetEq] suggests using
   separate keys for Identity (i.e., DevID) and Attestation (i.e.,
   signing a quote of the contents of the PCRs), but provisioning an
   Initial Attestation Key (IAK) and x.509 certificate that parallels
   the IDevID, with the same device ID information as the IDevID
   certificate (i.e., the same Subject Name and Subject Alt Name, even
   though the key pairs are different).  This allows a quote from the
   device, signed by the IAK, to be linked directly to the device that
   provided it, by examining the corresponding IAK certificate.




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

2.3.  RIV Information Flow

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

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

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

   3.  A Relying Party, which has access to the Verifier to request
       attestation and to act on results.  Interaction between the
       Relying Party and the Verifier is considered out of scope for
       RIV.

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

   These components are illustrated in Figure 2.

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








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


        Figure 3: RIV Reference Configuration for Network Equipment

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

   See [I-D.birkholz-rats-reference-interaction-model] for more narrowly
   defined terms related to Attestation

2.4.  RIV Simplifying Assumptions

   This document makes the following simplifying assumptions to reduce
   complexity:

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

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



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

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

2.4.1.  DevID Alternatives

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

   Some administrators may want to install their own identity
   credentials to certify device identity and attestation results.  IEEE
   802.1AR [IEEE-802-1AR] allows for both Initial Device Identity
   credentials, installed by the manufacturer, (analogous to a physical
   serial number plate), or Local Device Identity credentials installed
   by the administrator of the device (analogous to the physical Asset
   Tag used by many enterprises to identify their property).  TCG TPM
   2.0 Keys documents [Platform-DevID-TPM-2.0] and
   [PC-Client-BIOS-TPM-2.0] specifies parallel Initial and Local
   Attestation Keys (IAK and LAK), and contains figures showing the
   relationship between IDevID, LDevID, IAK and LAK keys.

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

   Note that many networking devices are expected to self-configure (aka
   Zero Touch Provisioning).  Current standardized zero-touch mechanisms
   such as [RFC8572] assume that identity keys are already in place
   before network on-boarding can start, and as such, are compatible
   with IDevID and IAK keys installed by the manufacturer, but not with
   LDevID and LAK keys, which would have to be installed by the
   administrator.

2.4.2.  Additional Attestation of Platform Characteristics

   The Platform Attribute Credential [Platform-Certificates] can also be
   used to convey additional information about a platform from the
   manufacturer or other entities in the supply chain.  While outside



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   the scope of RIV, the Platform Attribute Credential can deliver
   information such as lists of serial numbers for components embedded
   in a device or security assertions related to the platform, signed by
   the manufacturer, system integrator or value-added-reseller.

2.4.3.  Root of Trust for Measurement

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

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

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

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

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

2.4.4.  Reference Integrity Manifests (RIMs)

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

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

   In general, there are two kinds of reference measurements:

   1.  Measurements of early system startup (e.g., BIOS, boot loader, OS
       kernel) are essentially single threaded, and executed exactly
       once, in a known sequence, before any results could be reported.
       In this case, while the method for computing the hash and



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       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.

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

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

2.4.5.  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 of what 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 (e.g., PCR values).




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   TCG has defined several event log formats:

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

   o  Canonical Event Log [Canonical-Event-Log]

   o  There is also a Legacy BIOS event log, although this document is
      less relevant as UEFI has largely replaced the Legacy BIOS (TCG PC
      Client Specific Implementation Specification for Conventional
      BIOS, Section 11.3[PC-Client-BIOS-TPM-1.2])

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

   The TCG SNMP Attestation MIB [SNMP-Attestation-MIB] will support any
   record-oriented log format, including the three TCG-defined formats,
   but it currently leaves figuring out which log(s) are in what format
   up to the Verifier.

3.  Standards Components

3.1.  Reference Models

3.1.1.  IETF Reference Model for Challenge-Response Remote Attestation

   Initial work at IETF defines remote attestation as follows:

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

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

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

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






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

                Figure 4: IETF Attestation Information Flow

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

3.2.  RIV Workflow

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

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

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

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

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





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   o  In Step 2, measured values are retrieved from the Attester's TPM
      using a YANG [RFC8348] or SNMP [RFC3413] interface that implements
      the TCG TAP model (e.g.  YANG Module for Basic Challenge-Response-
      based Remote Attestation Procedures
      [I-D.birkholz-yang-basic-remote-attestation]).

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

   These steps yield enough information for the Verifier to verify
   measurements against reference values.  Of course, in all cases, the
   signatures protecting quotes and RIMs must be checked before the
   contents are used.

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


                Figure 5: RIV Protocol and Encoding Summary

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

   The following components are used:

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



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

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

   4.  Attestation logs are formatted according to the Canonical Event
       Log format [Canonical-Event-Log]

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

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

3.3.  Layering Model for Network Equipment Attester and Verifier

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























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

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

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

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

       *************************  ************ ************************
       * 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.  The IETF



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

4.  Privacy Considerations

   Networking Equipment such as routers, switches and firewalls has a
   key role to play in guarding the privacy of individuals using the
   network: * Packets passing through the device must not be sent to
   unauthorized destinations.  For example * Routers often act as Policy
   Enforcement Points, where individual subscribers may be checked for
   authorization to access a network.  Subscriber login information must
   not be released to unauthorized parties.  * Networking Equipment is
   often called upon to block access to protected resources, or 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 .
   This allows the administrator to ensure that the network provides
   individual and peer privacy guarantees.

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

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

5.  Security Considerations

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

   o  Keys may be compromised




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

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

   o  Replay attacks may be attempted by a compromised device

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

   Two sets of keys are relevant to RIV attestation

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

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

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

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

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

   While there are many ways to manage keys in a TPM (See
   [Platform-DevID-TPM-2.0]), RIV includes support for "zero touch"
   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 DevID.  This
   specification provides several elements

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





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

   The x.509 certificate contains several components

   o  The public part of the unique DevID key assigned to that device

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

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

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

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

   Evidence of software integrity is delivered in the form of a quote
   signed by the TPM itself.  Because the contents of the quote are
   signed inside the TPM, any external modification (including
   reformatting to a different data format) will be detected as
   tampering.

   A critical feature of the YANG model described in
   [I-D.birkholz-yang-basic-remote-attestation] 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 add an
   additional layer of signing using external keys, the important part
   is to preserve the TPM signing, so that tampering anywhere in the
   path between the TPM itself and the Verifier can be detected.

   Prevention of spoofing attacks against attestation systems is also
   important.  There are two cases to consider: * The entire device
   could be spoofed, that is, when the Verifier goes to verify a
   specific device, it might be redirected to a different device.  Use
   of the 802.1AR identity in the TPM ensures that the Verifier's TLS
   session is in fact terminating on the right device.  * A compromised



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   device could respond with a spoofed attestation result, that is, a
   compromised OS could return a fabricated quote.

   Protection against spoofed quotes from a device with valid identity
   is a bit more complex.  An identity key must be available to sign any
   kind of nonce or hash offered by the verifier, and consequently,
   could be used to sign a fabricated quote.  To block spoofed
   attestation result, the quote generated inside the TPM must by 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 man-in-the-
   middle attack (e.g. the 'Asokan Attack' [RFC6813]).

   This is accomplished in RIV through use of an AK certificate with the
   same elements as the DevID (i.e., same manufacturer's serial number,
   signed by the same manufacturer's key), but containing the device's
   unique AK public key instead of the DevID public key.  [this will
   require an OID that says the key is known by the CA to be an
   Attestation key]

   These two keys and certificates are used together:

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

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

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

   Requiring results of attestation of the operating software to be
   signed by a key known only to the TPM also removes the need to trust
   the device's operating software (beyond the first measurement; see
   below); any changes to the quote, generated and signed by the TPM
   itself, made by malicious device software, or in the path back to the
   verifier, will invalidate the signature on the quote.




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   Although RIV recommends that device manufacturers pre-provision
   devices with easily-verified DevID and AK certs, 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
   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.

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

   o  But device owners 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 configured 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.

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

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

   o  Attestation depends on an unbroken chain of measurements, starting
      from the very first measurement.  That first measurement is made
      by code called the Root of Trust for Measurement, typically done
      by trusted firmware stored in boot flash.  Mechanisms for
      maintaining the trustworthiness of the RTM are out of scope for




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      RIV, but could include immutable firmware, signed updates, or a
      vendor-specific hardware verification technique.

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

   RIV also depends on reliable reference measurements, as expressed by
   the RIM [RIM].  The definition of trust procedures for RIMs is out of
   scope for RIV, and the device owner is free to use any policy to
   validate a set of reference measurements.  RIMs may be conveyed out-
   of-band or in-band, as part of the attestation process (see
   Section 3.2).  But for embedded devices, where software is usually
   shipped as a self-contained package, RIMs signed by the manufacturer
   and delivered in-band may be more convenient for the device owner.

6.  Conclusion

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

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

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

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

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

7.  Appendix

7.1.  Implementation Notes

   Table 1 summarizes many of the actions needed to complete an
   Attestation system, with links to relevant documents.  While
   documents are controlled by several standards organizations, the
   implied actions required for implementation are all the




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   responsibility of the manufacturer of the device, unless otherwise
   noted.

  +------------------------------------------------------------------+
  |             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 amd reporting     |                |
  |     inside the TPM.                             |                |
  |   o  Refer to TCG Root of Trust for Measurement.|                |
  |   o  NIST SP 800-193 also provides guidelines   |                |
  |      on Roots of Trust                          |                |
  --------------------------------------------------------------------
  | Provision the TPM as described in               | TCG TPM DevID  |
  |   TCG documents.                                | TCG Platform   |
  |                                                 |   Certificate  |
  --------------------------------------------------------------------
  | Put a DevID or Platform Cert in the TPM         | TCG TPM DevID  |
  |    o Install an Initial Attestation Key at the  | TCG Platform   |
  |      same time so that Attestation can work out |   Certificate  |
  |      of the box                                 |-----------------
  |    o Equipment suppliers and owners may want to | IEEE 802.1AR   |
  |      implement Local Device ID as well as       |                |
  |      Initial Device ID                          |                |
  --------------------------------------------------------------------
  | Connect the TPM to the TLS stack                | Vendor TLS     |
  |    o  Use the DevID in the TPM to authenticate  | stack (This    |
  |       TAP connections, identifying the device   | action is      |
  |                                                 | simply         |
  |                                                 | configuring TLS|
  |                                                 | to use the     |
  |                                                 | DevID as its   |
  |                                                 | trust anchor.) |
  --------------------------------------------------------------------
  | Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID    |
  |    o  Add reference measurements into SWID tags | ISO/IEC 19770-2|
  |    o  Manufacturer should sign the SWID tags    | NIST IR 8060   |
  |    o  The TCG RIM-IM identifies further         |                |
  |       procedures to create signed RIM           |                |
  |       documents that provide the necessary      |                |
  |       reference information                     |                |
  --------------------------------------------------------------------
  |  Package the SWID tags with a vendor software   | Retrieve tags  |
  |  release                                        | with           |
  |    o  A tag-generator plugin such      | {{I-D.birkholz-yang-swid}}|



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  |     as https://github.com/Labs64/swid-maven-plugin               |
  |     can be used                                 |----------------|
  |                                                 | TCG PC Client  |
  |                                                 | RIM            |
  --------------------------------------------------------------------
  |  Use PC Client measurement definitions          | TCG PC Client  |
  |  to define the use of PCRs                      | BIOS           |
  |  (although Windows  OS is rare on Networking    |                |
  |  Equipment, UEFI BIOS is not)                   |                |
  --------------------------------------------------------------------
  |  Use TAP to retrieve measurements               |                |
  |    o  Map TAP to SNMP                           | TCG SNMP MIB   |
  |    o  Map to YANG                               | YANG Module for|
  |  Use Canonical Log Format                       |   Basic        |
  |                                                 |   Attestation  |
  |                                                 | TCG Canonical  |
  |                                                 |   Log Format   |
  --------------------------------------------------------------------
  | Posture Collection Server (as described in IETF |                |
  |  SACMs ECP) should request the                  |                |
  |  attestation and analyze the result             |                |
  | The Management application might be broken down |                |
  |  to several more components:                    |                |
  |    o  A Posture Manager Server                  |                |
  |       which collects reports and stores them in |                |
  |       a database                                |                |
  |    o  One or more Analyzers that can look at the|                |
  |       results and figure out what it means.     |                |
  --------------------------------------------------------------------

                        Figure 7: Component Status

7.2.  Comparison with TCG PTS / IETF NEA

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














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

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

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

               Figure 8: Attestation for End User Computers

8.  IANA Considerations

   This memo includes no request to IANA.





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9.  Informative References

   [AIK-Enrollment]
              Trusted Computing Group, "TCG Infrastructure Working
              GroupA CMC Profile for AIK Certificate Enrollment Version
              1.0, Revision 7", March 2011,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              IWG_CMC_Profile_Cert_Enrollment_v1_r7.pdf>.

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

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

   [GloPlaRoT]
              GlobalPlatform Technology, "Root of Trust Definitions and
              Requirements Version 1.1", June 2018,
              <https://globalplatform.org/specs-library/globalplatform-
              root-of-trust-definitions-and-requirements/>.

   [I-D.birkholz-rats-architecture]
              Birkholz, H., Wiseman, M., Tschofenig, H., Smith, N., and
              M. Richardson, "Remote Attestation Procedures
              Architecture", draft-birkholz-rats-architecture-03 (work
              in progress), November 2019.

   [I-D.birkholz-rats-reference-interaction-model]
              Birkholz, H. and M. Eckel, "Reference Interaction Models
              for Remote Attestation Procedures", draft-birkholz-rats-
              reference-interaction-model-02 (work in progress), January
              2020.

   [I-D.birkholz-rats-tuda]
              Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
              "Time-Based Uni-Directional Attestation", draft-birkholz-
              rats-tuda-01 (work in progress), September 2019.

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



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

   [I-D.ietf-sacm-coswid]
              Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
              Waltermire, "Concise Software Identification Tags", draft-
              ietf-sacm-coswid-13 (work in progress), November 2019.

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

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

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

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

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

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









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

   [PC-Client-RIM]
              Trusted Computing Group, "DRAFT: TCG PC Client Reference
              Integrity Manifest Specification, v.09", December 2019,
              <https://trustedcomputinggroup.org/xx>.

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

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

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

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

   [RFC3413]  Levi, D., Meyer, P., and B. Stewart, "Simple Network
              Management Protocol (SNMP) Applications", STD 62,
              RFC 3413, DOI 10.17487/RFC3413, December 2002,
              <https://www.rfc-editor.org/info/rfc3413>.

   [RFC5209]  Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
              Tardo, "Network Endpoint Assessment (NEA): Overview and
              Requirements", RFC 5209, DOI 10.17487/RFC5209, June 2008,
              <https://www.rfc-editor.org/info/rfc5209>.







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   [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>.

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

   [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, "DRAFT: TCG Reference Integrity
              Manifest Information Model", June 2019,
              <https://trustedcomputinggroup.org/wp-content/uploads/
              TCG_RIM_Model_v1-r13_2feb20.pdf>.

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

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

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

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







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Authors' Addresses

   Guy Fedorkow (editor)
   Juniper Networks, Inc.
   US

   Email: gfedorkow@juniper.net


   Jessica Fitzgerald-McKay
   National Security Agency
   US

   Email: jmfitz2@nsa.gov





































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