RATS Working Group G. Fedorkow, Ed.
Internet-Draft Juniper Networks, Inc.
Intended status: Informational E. Voit
Expires: March 22, 2021 Cisco Systems, Inc.
J. Fitzgerald-McKay
National Security Agency
September 18, 2020
TPM-based Network Device Remote Integrity Verification
draft-ietf-rats-tpm-based-network-device-attest-04
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].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Organization . . . . . . . . . . . . . . . . . . 4
1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Description of Remote Integrity Verification (RIV) . . . 5
1.5. Solution Requirements . . . . . . . . . . . . . . . . . . 7
1.6. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.6.1. Out of Scope . . . . . . . . . . . . . . . . . . . . 8
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 9
2.1. RIV Software Configuration Attestation using TPM . . . . 9
2.1.1. What Does RIV Attest? . . . . . . . . . . . . . . . . 10
2.1.2. Notes on PCR Allocations . . . . . . . . . . . . . . 12
2.2. RIV Keying . . . . . . . . . . . . . . . . . . . . . . . 13
2.3. RIV Information Flow . . . . . . . . . . . . . . . . . . 14
2.4. RIV Simplifying Assumptions . . . . . . . . . . . . . . . 16
2.4.1. Reference Integrity Manifests (RIMs) . . . . . . . . 17
2.4.2. Attestation Logs . . . . . . . . . . . . . . . . . . 18
3. Standards Components . . . . . . . . . . . . . . . . . . . . 19
3.1. Prerequisites for RIV . . . . . . . . . . . . . . . . . . 19
3.1.1. Unique Device Identity . . . . . . . . . . . . . . . 19
3.1.2. Keys . . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.3. Appraisal Policy for Evidence . . . . . . . . . . . . 19
3.2. Reference Model for Challenge-Response . . . . . . . . . 20
3.2.1. Transport and Encoding . . . . . . . . . . . . . . . 22
3.3. Centralized vs Peer-to-Peer . . . . . . . . . . . . . . . 23
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 24
5. Security Considerations . . . . . . . . . . . . . . . . . . . 25
5.1. Keys Used in RIV . . . . . . . . . . . . . . . . . . . . 25
5.2. Prevention of Spoofing and Man-in-the-Middle Attacks . . 27
5.3. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 28
5.4. Owner-Signed Keys . . . . . . . . . . . . . . . . . . . . 28
5.5. Other Trust Anchors . . . . . . . . . . . . . . . . . . . 29
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 30
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
8. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Using a TPM for Attestation . . . . . . . . . . . . . . . 30
8.2. Root of Trust for Measurement . . . . . . . . . . . . . . 32
8.3. Layering Model for Network Equipment Attester and
Verifier . . . . . . . . . . . . . . . . . . . . . . . . 32
8.3.1. Why is OS Attestation Different? . . . . . . . . . . 34
8.4. Implementation Notes . . . . . . . . . . . . . . . . . . 34
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.1. Normative References . . . . . . . . . . . . . . . . . . 36
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9.2. Informative References . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
There are many aspects to consider in fielding a trusted computing
device, from operating systems to applications. Mechanisms to prove
that a device installed at a customer's site is authentic (i.e., not
counterfeit) and has been configured with authorized software, all as
part of a trusted supply chain, are just a few of the many aspects
which need to be considered concurrently to have confidence that a
device is truly trustworthy.
A generic architecture for remote attestation has been defined in
[I-D.ietf-rats-architecture]. Additionally, the use cases for
remotely attesting networking devices are discussed within Section 6
of [I-D.richardson-rats-usecases]. However, these documents do not
provide sufficient guidance for network equipment vendors and
operators to design, build, and deploy interoperable devices.
The intent of this document is to provide such guidance. It does
this by outlining the Remote Integrity Verification (RIV) problem,
and then identifies elements that are necessary to get the complete,
scalable attestation procedure working with commercial networking
products such as routers, switches and firewalls. An underlying
assumption will be the availability within the device of a Trusted
Platform Module [TPM1.2], [TPM2.0] compliant cryptoprocessor to
enable the trustworthy remote assessment of the device's software and
hardware.
1.1. Terminology
A number of terms are reused from [I-D.ietf-rats-architecture].
These include: Appraisal Policy for Attestation Results, Attestation
Result, Attester, Evidence, Relying Party, Verifier, and Verifier
Owner.
Additionally, this document defines the following terms:
Remote Attestation: the process of creating, conveying and appraising
claims about device trustworthiness characteristics, including supply
chain trust, identity, device provenance, software configuration,
hardware configuration, device composition, compliance to test
suites, functional and assurance evaluations, etc.
This document uses the term Endorser to refer to the trusted
authority for any signed object relating to the device, such as
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certificates or reference measurement. Typically, the manufacturer
of an embedded device would be accepted as an Endorser.
The goal of attestation is simply to assure an administrator that the
software that was launched when the device was last started is an
authentic and untampered-with copy of the software that the device
vendor shipped.
Within the Trusted Computing Group context, attestation is the
process by which an independent Verifier can obtain cryptographic
proof as to the identity of the device in question, and evidence of
the integrity of software loaded on that device when it started up,
and then verify that what's there is what's supposed to be there.
For networking equipment, a Verifier capability can be embedded in a
Network Management Station (NMS), a posture collection server, or
other network analytics tool (such as a software asset management
solution, or a threat detection and mitigation tool, etc.). While
informally referred to as attestation, this document focuses on a
subset defined here as Remote Integrity Verification (RIV). RIV
takes a network equipment centric perspective that includes a set of
protocols and procedures for determining whether a particular device
was launched with authentic software, starting from Roots of Trust.
While there are many ways to accomplish attestation, RIV sets out a
specific set of protocols and tools that work in environments
commonly found in Networking Equipment. RIV does not cover other
device characteristics that could be attested (e.g., geographic
location, connectivity; see [I-D.richardson-rats-usecases]), although
it does provide evidence of a secure infrastructure to increase the
level of trust in other device characteristics attested by other
means (e.g., by Entity Attestation Tokens [I-D.ietf-rats-eat]).
1.2. Document Organization
The remainder of this document is organized into several sections:
o The remainder of this section covers goals and requirements, plus
a top-level description of RIV.
o The Solution Overview section outlines how Remote Integrity
Verification works.
o The Standards Components section links components of RIV to
normative standards.
o Privacy and Security shows how specific features of RIV contribute
to the trustworthiness of the Attestation Result.
o Supporting material is in an appendix at the end.
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1.3. Goals
Network operators benefit from a trustworthy attestation mechanism
that provides assurance that their network comprises authentic
equipment, and has loaded software free of known vulnerabilities and
unauthorized tampering. In line with the overall goal of assuring
integrity, attestation can be used to assist in asset management,
vulnerability and compliance assessment, plus configuration
management.
The RIV attestation workflow outlined in this document is intended to
meet the following high-level goals:
o Provable Device Identity - This specification requires that an
attesting device includes a cryptographic identifier unique to
each device. Effectively this means that the TPM must be so
provisioned during the manufacturing cycle.
o Software Inventory - A key goal is to identify the software
release(s) installed on the attesting device, and to provide
evidence that the software stored within hasn't been altered
without authorization.
o Verifiability - Verification of software and configuration of the
device shows that the software that was authorized for
installation by the administrator has actually been launched.
In addition, RIV is designed to operate either in a centralized
environment, such as with a central authority that manages and
configures a number of network devices, or 'peer-to-peer', where
network devices independently verify one another to establish a trust
relationship. (See Section 3.3 below, and also
[I-D.voit-rats-trusted-path-routing])
1.4. Description of Remote Integrity Verification (RIV)
Attestation requires two interlocking services between the Attester
network device and the Verifier:
o Device Identity, the mechanism providing trusted identity, can
reassure network managers that the specific devices they ordered
from authorized manufacturers for attachment to their network are
those that were installed, and that they continue to be present in
their network. As part of the mechanism for Device Identity,
cryptographic proof of the identity of the manufacturer is also
provided.
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o Software Measurement is the mechanism that reports the state of
mutable software components on the device, and can assure network
managers that they have known, authentic software configured to
run in their network.
Using these two interlocking services, RIV is a component in a chain
of procedures that can assure a network operator that the equipment
in their network can be reliably identified, and that authentic
software of a known version is installed on each device. Equipment
in the network includes devices that make up the network itself, such
as routers, switches and firewalls.
RIV includes several major processes:
1. Creation of Evidence is the process whereby an Attester generates
cryptographic proof (Evidence) of claims about device properties.
In particular, the device identity and its software configuration
are both of critical importance.
2. Device Identification refers to the mechanism assuring the
Relying Party (ultimately, a network administrator) of the
identity of devices that make up their network, and that their
manufacturers are known.
3. Software used to boot a device can be described as a chain of
measurements, anchored at the start by a Root of Trust for
Measurement, that normally ends when the system software is
loaded. A measurement signifies the identity, integrity and
version of each software component registered with an attesting
device's TPM [TPM1.2], [TPM2.0], so that the subsequent appraisal
stage can determine if the software installed is authentic, up-
to-date, and free of tampering.
4. Conveyance of Evidence reliably transports at least the minimum
amount of Evidence from Attester to a Verifier to allow a
management station to perform a meaningful appraisal in Step 5.
The transport is typically carried out via a management network.
The channel must provide integrity and authenticity, and, in some
use cases, may also require confidentiality.
5. Finally, Appraisal of Evidence occurs. As the Verifier and
Relying Party roles are often combined within RIV, this is the
process of verifying the Evidence received by a Verifier from the
Attesting device, and using an Appraisal Policy to develop an
Attestation Result, used to inform decision making. In practice,
this means comparing the device measurements reported as Evidence
with the Attester configuration expected by the Verifier.
Subsequently the Appraisal Policy for Attestation Results might
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match what was found against Reference Integrity Measurements
(aka Golden Measurements) which represent the intended configured
state of the connected device.
All implementations supporting this RIV specification require the
support of the following three technologies:
1. Identity: Device identity MUST be based on IEEE 802.1AR Device
Identity (DevID) [IEEE-802-1AR], coupled with careful supply-
chain management by the manufacturer. The DevID certificate
contains a statement by the manufacturer that establishes the
identity of the device as it left the factory. Some applications
with a more-complex post-manufacture supply chain (e.g., Value
Added Resellers), or with different privacy concerns, may want to
use alternative mechanisms for platform authentication (for
example, TCG Platform Certificates [Platform-Certificates]).
2. Platform Attestation provides evidence of configuration of
software elements present in the device. This form of
attestation can be implemented with TPM Platform Configuration
Registers (PCRs), Quote and Log mechanisms, which provide
cryptographically authenticated evidence to report what software
was started on the device through the boot cycle. Successful
attestation requires an unbroken chain from a boot-time root of
trust through all layers of software needed to bring the device
to an operational state, in which each stage measures components
of the next stage, updates the attestation log, and extends
hashes into a PCR. The TPM can then report the hashes of all the
measured hashes as signed evidence called a Quote (see
Section 8.1 for an overview of TPM operation, or [TPM1.2] and
[TPM2.0] for many more details).
3. Reference Integrity Measurements must be conveyed from the
Endorser (the entity accepted as the software authority, often
the manufacturer for embedded systems) to the system in which
verification will take place.
1.5. Solution Requirements
Remote Integrity Verification must address the "Lying Endpoint"
problem, in which malicious software on an endpoint may subvert the
intended function, and also prevent the endpoint from reporting its
compromised status. (See Section 5 for further Security
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
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at the end-user's site, as network equipment is often required to
"self-configure", to reliably reach out without manual intervention
to prove its identity and operating posture, then download its own
configuration. See [RFC8572] for an example of Secure Zero Touch
Provisioning.
1.6. Scope
Remote Attestation is a very general problem that could apply to most
network-connected computing devices. However, this document includes
several assumptions that limit the scope to Network Equipment (e.g.,
routers, switches and firewalls):
o This solution is for use in non-privacy-preserving applications
(for example, networking, Industrial IoT), avoiding the need for a
Privacy Certificate Authority for attestation keys
[AIK-Enrollment] or TCG Platform Certificates
[Platform-Certificates]
o This document assumes network protocols that are common in
networking equipment such as YANG [RFC7950] and NETCONF [RFC6241],
but not generally used in other applications.
o The approach outlined in this document mandates the use of a
compliant TPM [TPM1.2], [TPM2.0].
1.6.1. Out of Scope
o Run-Time Attestation: Run-time attestation of Linux or other
multi-threaded operating system processes considerably expands the
scope of the problem. Many researchers are working on that
problem, but this document defers the run-time attestation
problem.
o Multi-Vendor Embedded Systems: Additional coordination would be
needed for devices that themselves comprise hardware and software
from multiple vendors, integrated by the end user.
o Processor Sleep Modes: Network equipment typically does not
"sleep", so sleep and hibernate modes are not considered.
Although out of scope for RIV, Trusted Computing Group
specifications do encompass sleep and hibernate states.
o Virtualization and Containerization: In a non-virtualized system,
the host OS is responsible for measuring each User Space file or
process, but that't the end of the boot process. For virtualized
systems, the host OS must verify the hypervisor, which then
manages its own chain of trust through the virtual machine.
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Virtualization and containerization technologies are increasingly
used in Network equipment, but are not considered in this revision
of the document.
2. Solution Overview
2.1. RIV Software Configuration Attestation using TPM
RIV Attestation is a process which can be used to determine the
identity of software running on a specifically-identified device.
Remote Attestation is broken into two phases, shown in Figure 1:
o During system startup, each distinct software object is
"measured". Its identity, hash (i.e., cryptographic digest) and
version information are recorded in a log. Hashes are also
extended, or cryptographically folded, into the TPM, in a way that
can be used to validate the log entries. The measurement process
generally follows the Chain of Trust model used in Measured Boot,
where each stage of the system measures the next one, and extends
its measurement into the TPM, before launching it.
o Once the device is running and has operational network
connectivity, a separate, trusted Verifier will interrogate the
network device to retrieve the logs and a copy of the digests
collected by hashing each software object, signed by an
attestation private key known only to the TPM.
The result is that the Verifier can verify the device's identity by
checking the Subject Field and signature of certificate containing
the TPM's attestation public key, and can validate the software that
was launched by verifying the correctness of the logs by comparing
with the signed digests from the TPM, and comparing digests in the
log with known-good values.
It should be noted that attestation and identity are inextricably
linked; signed Evidence that a particular version of software was
loaded is of little value without cryptographic proof of the identity
of the Attester producing the Evidence.
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+-------------------------------------------------------+
| +--------+ +--------+ +--------+ +---------+ |
| | BIOS |--->| Loader |-->| Kernel |--->|Userland | |
| +--------+ +--------+ +--------+ +---------+ |
| | | | |
| | | | |
| +------------+-----------+-+ |
| Step 1 | |
| V |
| +--------+ |
| | TPM | |
| +--------+ |
| Router | |
+--------------------------------|----------------------+
|
| Step 2
| +-----------+
+--->| Verifier |
+-----------+
Reset---------------flow-of-time-during-boot--...------->
Figure 1: RIV Attestation Model
In Step 1, measurements are "extended", or hashed, into the TPM as
processes start, with the result that the PCR ends up containing a
hash of all the measured hashes. In Step 2, signed PCR digests are
retrieved from the TPM for off-box analysis after the system is
operational.
2.1.1. What Does RIV Attest?
TPM attestation is strongly focused on Platform Configuration
Registers (PCRs), but those registers are only vehicles for
certifying accompanying Evidence, conveyed in log entries. It is the
hashes in log entries that are extended into PCRs, where the final
PCR values can be retrieved in the form of a structured called a
Quote, signed by an Attestation key known only to the TPM. The use
of multiple PCRs serves only to provide some independence between
different classes of object, so that one class of objects can be
updated without changing the extended hash for other classes.
Although PCRs can be used for any purpose, this section outlines the
objects within the scope of this document which may be extended into
the TPM.
In general, assignment of measurements to PCRs is a policy choice
made by the device manufacturer, selected to independently attest
three classes of object:
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o Code, (i.e., instructions) to be executed by a CPU.
o Configuration - Many devices offer numerous options controlled by
non-volatile configuration variables which can impact the device's
security posture. These settings may have vendor defaults, but
often can be changed by administrators, who may want to verify via
attestation that the settings they intend are still in place.
o Credentials - Administrators may wish to verify via attestation
that keys (and other credentials) outside the Root of Trust have
not been subject to unauthorized tampering. (By definition, keys
inside the root of trust can't be verified independently).
The TCG PC Client Platform Firmware Profile Specification
[PC-Client-BIOS-TPM-2.0] gives considerable detail on what is to be
measured during the boot phase of platform startup using a UEFI BOIS
(www.uefi.org), but the goal is simply to measure every bit of code
executed in the process of starting the device, along with any
configuration information related to security posture, leaving no gap
for unmeasured code to remain undetected and subvert the chain.
For devices using a UEFI BIOS, [PC-Client-BIOS-TPM-2.0] gives
detailed normative requirements for PCR usage. But for other
platform architectures, the table in Figure 2 gives guidance for PCR
assignment that generalizes the specific details of
[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] taken by the Root of Trust for Measurement,
is critical to establishing the chain of trust for all subsequent
measurements. If the PCR[0] measurement cannot be trusted, the
validity of the entire chain is put into question.
Distinctions Between PCR[0], PCR[2], PCR[4] and PCR[8] are summarized
below:
o PCR[0] typically represents a consistent view of the Host Platform
between boot cycles, allowing Attestation and Sealed Storage
policies to be defined using the less changeable components of the
transitive trust chain. This PCR typically provides a consistent
view of the platform regardless of user selected options.
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o PCR[2] is intended to represent a "user configurable" environment
where the user has the ability to alter the components that are
measured into PCR[2]. This is typically done by adding adapter
cards, etc., into user-accessible PCI or other slots. In UEFI
systems these devices may be configured by Option ROMs measured
into PCR[2] and executed by the BIOS.
o PCR[4] is intended to represent the software that manages the
transition between the platform's Pre-Operating System Start and
the state of a system with the Operating System present. This
PCR, along with PCR[5], identifies the initial operating system
loader (e.g., GRUB for Linux).
o PCR[8] is used by the OS loader to record measurements of the
various components of the operating system.
Although the TCG PC Client document specifies the use of the first
eight PCRs very carefully to ensure interoperability among multiple
UEFI BIOS vendors, it should be noted that embedded software vendors
may have considerably more flexibility. Verifiers typically need to
know which log entries are consequential and which are not (possibly
controlled by local policies) but the Verifier may not need to know
what each log entry means or why it was assigned to a particular PCR.
Designers must recognize that some PCRs may cover log entries that a
particular Verifier considers critical and other log entries that are
not considered important, so differing PCR values may not on their
own constitute a check for authenticity.
Designers may allocate particular events to specific PCRs in order to
achieve a particular objective with Local Attestation, (e.g.,
allowing a procedure to execute only if a given PCR is in a given
state). It may also be important to designers to consider whether
streaming notification of PCR updates is required (see
[I-D.birkholz-rats-network-device-subscription]). Specific log
entries can only be validated if the Verifier receives every log
entry affecting the relevant PCR, so (for example) a designer might
want to separate rare, high-value events such as configuration
changes, from high-volume, routine measurements such as IMA [IMA]
logs.
2.2. RIV Keying
RIV attestation relies on two keys:
o An identity key is required to certify the identity of the
Attester itself. RIV specifies the use of an IEEE 802.1AR Device
Identity (DevID) [IEEE-802-1AR], signed by the device
manufacturer, containing the device serial number.
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o An Attestation Key is required to sign the Quote generated by the
TPM to report evidence of software configuration.
In TPM application, the Attestation key MUST be protected by the TPM,
and the DevID SHOULD be as well. Depending on other TPM
configuration procedures, the two keys are likely be different; some
of the considerations are outlined in TCG Guidance for Securing
Network Equipment [NetEq].
TCG Guidance for Securing Network Equipment specifies further
conventions for these keys:
o When separate Identity and Attestation keys are used, the
Attestation Key (AK) and its X.509 certificate should parallel the
DevID, with the same device ID information as the DevID
certificate (i.e., the same Subject Name and Subject Alt Name,
even though the key pairs are different). This allows a quote
from the device, signed by an AK, to be linked directly to the
device that provided it, by examining the corresponding AK
certificate.
o Network devices that are expected to use secure zero touch
provisioning as specified in [RFC8572]) MUST be shipped by the
manufacturer with pre-provisioned keys (Initial DevID and AK,
called IDevID and IAK). Inclusion of an IDevID and IAK by a
vendor does not preclude a mechanism whereby an Administrator can
define Local Identity and Attestation Keys (LDevID and LAK) if
desired. IDevID and IAK certificates MUST both be signed by the
Endorser (typically the device manufacturer).
2.3. RIV Information Flow
RIV workflow for networking equipment is organized around a simple
use case where a network operator wishes to verify the integrity of
software installed in specific, fielded devices. This use case
implies several components:
1. The Attesting Device, which the network operator wants to
examine.
2. A Verifier (which might be a network management station)
somewhere separate from the Device that will retrieve the
information and analyze it to pass judgment on the security
posture of the device.
3. A Relying Party, which can act on Attestation Results.
Interaction between the Relying Party and the Verifier is
considered out of scope for RIV.
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4. Signed Reference Integrity Manifests (RIMs), containing Reference
Integrity Measurements, can either be created by the device
manufacturer and shipped along with the device as part of its
software image, or alternatively, could be obtained several other
ways (direct to the Verifier from the manufacturer, from a third
party, from the owner's observation of what's thought to be a
"known good system", etc.). Retrieving RIMs from the device
itself allows attestation to be done in systems that may not have
access to the public internet, or by other devices that are not
management stations per se (e.g., a peer device; see
Section 3.1.3). If Reference Integrity Measurements are obtained
from multiple sources, the Verifier may need to evaluate the
relative level of trust to be placed in each source in case of a
discrepancy.
These components are illustrated in Figure 3.
A more-detailed taxonomy of terms is given in
[I-D.ietf-rats-architecture]
+---------------+ +-------------+ +---------+--------+
| | | Attester | Step 1 | Verifier| |
| Endorser | | (Device |<-------| (Network| Relying|
| (Device | | under |------->| Mngmt | Party |
| Manufacturer | | attestation)| Step 2 | Station)| |
| or other | | | | | |
| authority) | | | | | |
+---------------+ +-------------+ +---------+--------+
| /\
| Step 0 |
-----------------------------------------------
Figure 3: RIV Reference Configuration for Network Equipment
In Step 0, The Endorser (the device manufacturer or other authority)
provides a software image to the Attester (the device under
attestation), and makes one or more Reference Integrity Manifests
(RIMs) signed by the Endorser, available to the Verifier (see
Section 3.1.3 for "in-band" and "out of band" ways to make this
happen). In Step 1, the Verifier (Network Management Station), on
behalf of a Relying Party, requests Identity, Measurement Values, and
possibly RIMs, from the Attester. In Step 2, the Attester responds
to the request by providing a DevID, quotes (measured values, signed
by the Attester), and optionally RIMs.
To achieve interoperability, the following standards components
SHOULD be used:
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1. TPM Keys MUST be configured according to
[Platform-DevID-TPM-2.0], [PC-Client-BIOS-TPM-1.2], or
[Platform-ID-TPM-1.2].
2. For devices using UEFI and Linux, measurements of firmware and
bootable modules SHOULD be taken according to TCG PC Client
[PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]
3. Device Identity MUST be managed as specified in IEEE 802.1AR
Device Identity certificates [IEEE-802-1AR], with keys protected
by TPMs.
4. Attestation logs SHOULD be formatted according to the Canonical
Event Log format [Canonical-Event-Log], although other
specialized formats may be used. UEFI-based systems MAY use the
TCG UEFI BIOS event log [EFI-TPM]).
5. Quotes are retrieved from the TPM according to the TCG TAP
Information Model [TAP]. While the TAP IM gives a protocol-
independent description of the data elements involved, it's
important to note that quotes from the TPM are signed inside the
TPM, so MUST be retrieved in a way that does not invalidate the
signature, as specified in [I-D.ietf-rats-yang-tpm-charra], to
preserve the trust model. (See Section 5 Security
Considerations).
6. Reference Integrity Measurements SHOULD be encoded as CoSWID
tags, as defined in the TCG RIM document [RIM], compatible with
NIST IR 8060 [NIST-IR-8060] and the IETF CoSWID draft
[I-D.ietf-sacm-coswid]. See Section 2.4.1.
2.4. RIV Simplifying Assumptions
This document makes the following simplifying assumptions to reduce
complexity:
o The product to be attested MUST be shipped with an IEEE 802.1AR
Device Identity and an Initial Attestation Key (IAK) with
certificate. The IAK cert contains the same identity information
as the DevID (specifically, the same Subject Name and Subject Alt
Name, signed by the manufacturer), but it's a type of key that can
be used to sign a TPM Quote. This convention is described in TCG
Guidance for Securing Network Equipment [NetEq]. For network
equipment, which is generally non-privacy-sensitive, shipping a
device with both an IDevID and an IAK already provisioned
substantially simplifies initial startup. Privacy-sensitive
applications may use the TCG Platform Certificate and additional
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procedures to install identity credentials into the device after
manufacture.
o The product MUST be equipped with a Root of Trust for Measurement,
Root of Trust for Storage and Root of Trust for Reporting (as
defined in [SP800-155]) that are capable of conforming to the TCG
Trusted Attestation Protocol (TAP) Information Model [TAP].
o The authorized software supplier MUST make available Reference
Integrity Measurements (i.e., known-good measurements) in the form
of signed CoSWID tags [I-D.ietf-sacm-coswid], [SWID], as described
in TCG Reference Integrity Measurement Manifest Information Model
[RIM].
2.4.1. Reference Integrity Manifests (RIMs)
[I-D.ietf-rats-yang-tpm-charra] focuses on collecting and
transmitting evidence in the form of PCR measurements and attestation
logs. But the critical part of the process is enabling the Verifier
to decide whether the measurements are "the right ones" or not.
While it must be up to network administrators to decide what they
want on their networks, the software supplier should supply the
Reference Integrity Measurements that may be used by a Verifier to
determine if evidence shows known good, known bad or unknown software
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.
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In both cases, the expected values can be expressed as signed SWID or
CoSWID tags, but the SWID structure in the second case is somewhat
more complex, as reconstruction of the extended hash in a PCR may
involve thousands of files and other objects.
The TCG has published an information model defining elements of
reference integrity manifests under the title TCG Reference Integrity
Manifest Information Model [RIM]. This information model outlines
how SWID tags should be structured to allow attestation, and defines
"bundles" of SWID tags that may be needed to describe a complete
software release. The RIM contains metadata relating to the software
release it belongs to, plus hashes for each individual file or other
object that could be attested.
TCG has also published the PC Client Reference Integrity Measurement
specification [PC-Client-RIM], which focuses on a SWID-compatible
format suitable for expressing expected measurement values in the
specific case of a UEFI-compatible BIOS, where the SWID focus on
files and file systems is not a direct fit. While the PC Client RIM
is not directly applicable to network equipment, many vendors do use
a conventional UEFI BIOS to launch their network OS.
2.4.2. Attestation Logs
Quotes from a TPM can provide evidence of the state of a device up to
the time the evidence was recorded, but to make sense of the quote in
most cases an event log that identifies which software modules
contributed which values to the quote during startup MUST also be
provided. The log MUST contain enough information to demonstrate its
integrity by allowing exact reconstruction of the digest conveyed in
the signed quote (i.e., PCR values).
There are multiple event log formats which may be supported as viable
formats of Evidence between the Attester and Verifier:
o Event log exports from [I-D.ietf-rats-yang-tpm-charra]
o IMA Event log file exports [IMA]
o TCG UEFI BIOS event log (TCG EFI Platform Specification for TPM
Family 1.1 or 1.2, Section 7) [EFI-TPM])
o TCG Canonical Event Log [Canonical-Event-Log]
Devices which use UEFI BIOS and Linux SHOULD use TCG Canonical Event
Log [Canonical-Event-Log] and TCG UEFI BIOS event log [EFI-TPM])
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3. Standards Components
3.1. Prerequisites for RIV
The Reference Interaction Model for Challenge-Response-based Remote
Attestation is based on the standard roles defined in
[I-D.ietf-rats-architecture]. However additional prerequisites have
been established to allow for interoperable RIV use case
implementations. These prerequisites are intended to provide
sufficient context information so that the Verifier can acquire and
evaluate Attester measurements.
3.1.1. Unique Device Identity
A secure Device Identity (DevID) in the form of an IEEE 802.1AR DevID
certificate [IEEE-802-1AR] MUST be provisioned in the Attester's
TPMs.
3.1.2. Keys
The Attestation Identity Key (AIK) and certificate MUST also be
provisioned on the Attester according to [Platform-DevID-TPM-2.0],
[PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2].
The Attester's TPM Keys MUST be associated with the DevID on the
Verifier (see [Platform-DevID-TPM-2.0] and Section 5 Security
Considerations, below).
3.1.3. Appraisal Policy for Evidence
The Verifier MUST obtain trustworthy Endorsements in the form of
reference measurements (e.g., Known Good Values, encoded as CoSWID
tags [I-D.birkholz-yang-swid]). These reference measurements will
eventually be compared to signed PCR Evidence acquired from an
Attester's TPM using Attestation Policies chosen by the administrator
or owner of the device.
This document does not specify the format or contents for the
Appraisal Policy for Evidence, but Endorsements may be acquired in
one of two ways:
1. a Verifier may obtain reference measurements directly from an
Endorser chosen by the Verifier administrator (e.g., through a
web site).
2. Signed reference measurements may be distributed by the Endorser
to the Attester, as part of a software update. From there, the
reference measurement may be acquired by the Verifier.
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In either case, the Verifier Owner MUST select the source of trusted
endorsements through the Appraisal Policy for Evidence.
************* .-------------. .-----------.
* Endorser * | Attester | | Verifier/ |
* * | | | Relying |
*(Device *----2--->| (Network |----2--->| Party |
* config * | Device) | |(Ntwk Mgmt |
* Authority)* | | | Station) |
************* '-------------' '-----------'
| ^
| |
'----------------1--------------------------'
Figure 4: Appraisal Policy for Evidence Prerequisites
In either case the Endorsements must be generated, acquired and
delivered in a secure way, including reference measurements of
firmware and bootable modules taken according to TCG PC Client
[PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]. Endorsementa MUST be
encoded as SWID or CoSWID tags, signed by the device manufacturer, as
defined in the TCG RIM document [RIM], compatible with NIST IR 8060
[NIST-IR-8060] or the IETF CoSWID draft [I-D.ietf-sacm-coswid].
3.2. Reference Model for Challenge-Response
Once the prerequisites for RIV are met, a Verifier is able to acquire
Evidence from an Attester. The following diagram illustrates a RIV
information flow between a Verifier and an Attester, derived from
Section 8.1 of [I-D.birkholz-rats-reference-interaction-model].
Event times shown correspond to the time types described within
Appendix A of [I-D.ietf-rats-architecture]:
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.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(VG) |
valueGeneration(targetEnvironment) |
| => claims |
| |
| <--------------requestEvidence(nonce, PcrSelection)-----time(NS)
| |
time(EG) |
evidenceGeneration(nonce, PcrSelection, collectedClaims) |
| => SignedPcrEvidence(nonce, PcrSelection) |
| => LogEvidence(collectedClaims) |
| |
| returnSignedPcrEvidence----------------------------------> |
| returnLogEvidence----------------------------------------> |
| |
| time(RG,RA)
| evidenceAppraisal(SignedPcrEvidence, eventLog, refClaims)
| attestationResult <= |
~ ~
| time(RX)
Figure 5: IETF Attestation Information Flow
o Step 1 (time(VG)): One or more Attesting Network Device PCRs are
extended with measurements. RIV provides no direct link between
the time at which the event takes place and the time that it's
attested, although streaming attestation as in
[I-D.birkholz-rats-network-device-subscription] could.
o Step 2 (time(NS)): The Verifier generates a unique nonce ("number
used once"), and makes a request attestation data for one or more
PCRs from an Attester. For interoperability, this MUST be
accomplished via a YANG [RFC7950] interface that implements the
TCG TAP model (e.g., YANG Module for Basic Challenge-Response-
based Remote Attestation Procedures
[I-D.ietf-rats-yang-tpm-charra]).
o Step 3 (time(EG)): On the Attester, measured values are retrieved
from the Attester's TPM. This requested PCR evidence is signed by
the Attestation Identity Key (AIK) associated with the DevID.
Quotes are retrieved according to TCG TAP Information Model [TAP].
While the TAP IM gives a protocol-independent description of the
data elements involved, it's important to note that quotes from
the TPM are signed inside the TPM, so must be retrieved in a way
that does not invalidate the signature, as specified in
[I-D.ietf-rats-yang-tpm-charra], to preserve the trust model.
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(See Section 5 Security Considerations). At the same time, the
Attester collects log evidence showing what values have been
extended into that PCR.
o Step 4: Collected Evidence is passed from the Attester to the
Verifier
o Step 5 (time(RG,RA)): The Verifier reviews the Evidence and takes
action as needed. As the interaction between Relying Party and
Verifier is out of scope for RIV, this can happen in one step.
* If the signed PCR values do not match the set of log entries
which have extended a particular PCR, the device SHOULD NOT be
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 the AIK signature is not correct, or freshness such as that
provided by the nonce is not included in the response, the
device SHOULD NOT be trusted.
* If time(RG)-time(NS) is greater than the threshold in the
Appraisal Policy for Evidence, the Evidence is considered stale
and SHOULD NOT be trusted.
3.2.1. Transport and Encoding
Network Management systems may retrieve signed PCR based Evidence as
shown in Figure 5, and can be accomplished via NETCONF or RESTCONF,
with XML, JSON, or CBOR encoded Evidence.
Implementations that use NETCONF MUST do so over a TLS or SSH secure
tunnel. Implementations that use RESTCONF transport MAY do so over a
TLS or SSH secure tunnel.
Retrieval of Log Evidence SHOULD be via log interfaces on the network
device. (For example, see [I-D.ietf-rats-yang-tpm-charra]).
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3.3. Centralized vs Peer-to-Peer
Figure 5 above assumes that the Verifier is implicitly trusted, while
the Attesting device is not. In a Peer-to-Peer application such as
two routers negotiating a trust relationship
[I-D.voit-rats-trusted-path-routing], the two peers can each ask the
other to prove software integrity. In this application, the
information flow is the same, but each side plays a role both as an
Attester and a Verifier. Each device issues a challenge, and each
device responds to the other's challenge, as shown in Figure 6.
Peer-to-peer challenges, particularly if used to establish a trust
relationship between routers, require devices to carry their own
signed reference measurements (RIMs). Devices may also have to carry
Appraisal Policy for Evidence for each possible peer device so that
each device has everything needed for attestation, without having to
resort to a central authority.
+---------------+ +---------------+
| | | |
| Endorser A | | Endorser B |
| Firmware | | Firmware |
| Configuration | | Configuration |
| Authority | | Authority |
| | | |
+---------------+ +---------------+
| |
| +-------------+ +------------+ |
| | | Step 1 | | | \
| | Attester |<------>| Verifier | | |
| | |<------>| | | | Router B
+------>| | Step 2 | | | |- Challenges
Step 0A| | | | | | Router A
| |------->| | | |
|- Router A --| Step 3 |- Router B -| | /
| | | | |
| | | | |
| | Step 1 | | | \
| Verifier |<------>| Attester |<-+ | Router A
| |<------>| | |- Challenges
| | Step 2 | | | Router B
| | | | |
| |<-------| | |
+-------------+ Step 3 +------------+ /
Figure 6: Peer-to-Peer Attestation Information Flow
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In this application, each device may need to be equipped with signed
RIMs to act as an Attester, and also an Appraisal Policy for Evidence
and a selection of trusted X.509 root certificates, to allow the
device to act as a Verifier. An existing link layer protocol such as
802.1x [IEEE-802.1x] or 802.1AE [IEEE-802.1ae], with Evidence being
enclosed over a variant of EAP [RFC3748] or LLDP [LLDP] are suitable
methods for such an exchange.
4. Privacy Considerations
Networking Equipment, such as routers, switches and firewalls, has a
key role to play in guarding the privacy of individuals using the
network:
o Packets passing through the device must not be sent to
unauthorized destinations. For example:
* Routers often act as Policy Enforcement Points, where
individual subscribers may be checked for authorization to
access a network. Subscriber login information must not be
released to unauthorized parties.
* Networking Equipment is often called upon to block access to
protected resources from unauthorized users.
o Routing information, such as the identity of a router's peers,
must not be leaked to unauthorized neighbors.
o If configured, encryption and decryption of traffic must be
carried out reliably, while protecting keys and credentials.
Functions that protect privacy are implemented as part of each layer
of hardware and software that makes up the networking device. In
light of these requirements for protecting the privacy of users of
the network, the Network Equipment must identify itself, and its boot
configuration and measured device state (for example, PCR values), to
the Equipment's Administrator, so there's no uncertainty as to what
function each device and configuration is configured to carry out.
This allows the administrator to ensure that the network provides
individual and peer privacy guarantees.
RIV specifically addresses the collection of information from
enterprise network devices by authorized agents of the enterprise.
As such, privacy is a fundamental concern for those deploying this
solution, given EU GDPR, California CCPA, and many other privacy
regulations. The enterprise SHOULD implement and enforce their duty
of care.
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See [NetEq] for more context on privacy in networking devices.
5. Security Considerations
Attestation Results from the RIV procedure are subject to a number of
attacks:
o Keys may be compromised.
o A counterfeit device may attempt to impersonate (spoof) a known
authentic device.
o Man-in-the-middle attacks may be used by a counterfeit device to
attempt to deliver responses that originate in an actual authentic
device.
o Replay attacks may be attempted by a compromised device.
5.1. Keys Used in RIV
Trustworthiness of RIV attestation depends strongly on the validity
of keys used for identity and attestation reports. RIV takes full
advantage of TPM capabilities to ensure that results can be trusted.
Two sets of keys are relevant to RIV attestation:
o A DevID key is used to certify the identity of the device in which
the TPM is installed.
o An Attestation Key (AK) key signs attestation reports (called
'quotes' in TCG documents), used to provide evidence for integrity
of the software on the device.
TPM practices usually require that these keys be different, as a way
of ensuring that a general-purpose signing key cannot be used to
spoof an attestation quote.
In each case, the private half of the key is known only to the TPM,
and cannot be retrieved externally, even by a trusted party. To
ensure that's the case, specification-compliant private/public key-
pairs are generated inside the TPM, where they're never exposed, and
cannot be extracted (See [Platform-DevID-TPM-2.0]).
Keeping keys safe is just part of attestation security; knowing which
keys are bound to the device in question is just as important.
While there are many ways to manage keys in a TPM (see
[Platform-DevID-TPM-2.0]), RIV includes support for "zero touch"
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provisioning (also known as zero-touch onboarding) of fielded devices
(e.g., Secure ZTP, [RFC8572]), where keys which have predictable
trust properties are provisioned by the device vendor.
Device identity in RIV is based on IEEE 802.1AR Device Identity
(DevID). This specification provides several elements:
o A DevID requires a unique key pair for each device, accompanied by
an X.509 certificate,
o The private portion of the DevID key is to be stored in the
device, in a manner that provides confidentiality (Section 6.2.5
[IEEE-802-1AR])
The X.509 certificate contains several components:
o The public part of the unique DevID key assigned to that device
allows a challenge of identity.
o An identifying string that's unique to the manufacturer of the
device. This is normally the serial number of the unit, which
might also be printed on a label on the device.
o The certificate must be signed by a key traceable to the
manufacturer's root key.
With these elements, the device's manufacturer and serial number can
be identified by analyzing the DevID certificate plus the chain of
intermediate certificates leading back to the manufacturer's root
certificate. As is conventional in TLS or SSH connections, a nonce
must be signed by the device in response to a challenge, proving
possession of its DevID private key.
RIV uses the DevID to validate a TLS or SSH connection to the device
as the attestation session begins. Security of this process derives
from TLS or SSH security, with the DevID providing proof that the
session terminates on the intended device. See [RFC8446], [RFC4253].
Evidence of software integrity is delivered in the form of a quote
signed by the TPM itself. Because the contents of the quote are
signed inside the TPM, any external modification (including
reformatting to a different data format) after measurements have been
taken will be detected as tampering. An unbroken chain of trust is
essential to ensuring that blocks of code that are taking
measurements have been verified before execution (see Figure 1.
Requiring results of attestation of the operating software to be
signed by a key known only to the TPM also removes the need to trust
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the device's operating software (beyond the first measurement; see
below); any changes to the quote, generated and signed by the TPM
itself, made by malicious device software, or in the path back to the
Verifier, will invalidate the signature on the quote.
A critical feature of the YANG model described in
[I-D.ietf-rats-yang-tpm-charra] is the ability to carry TPM data
structures in their native format, without requiring any changes to
the structures as they were signed and delivered by the TPM. While
alternate methods of conveying TPM quotes could compress out
redundant information, or add an additional layer of signing using
external keys, the implementation MUST preserve the TPM signing, so
that tampering anywhere in the path between the TPM itself and the
Verifier can be detected.
5.2. Prevention of Spoofing and Man-in-the-Middle Attacks
Prevention of spoofing attacks against attestation systems is also
important. There are two cases to consider:
o The entire device could be spoofed, that is, when the Verifier
goes to verify a specific device, it might be redirected to a
different device. Use of the 802.1AR Device Identity (DevID) in
the TPM ensures that the Verifier's TLS or SSH session is in fact
terminating on the right device.
o A compromised device could respond with a spoofed Attestation
Result, that is, a compromised OS could return a fabricated quote.
Protection against spoofed quotes from a device with valid identity
is a bit more complex. An identity key must be available to sign any
kind of nonce or hash offered by the Verifier, and consequently,
could be used to sign a fabricated quote. To block a spoofed
Attestation Result, the quote generated inside the TPM must be signed
by a key that's different from the DevID, called an Attestation Key
(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.
[Editor's Note: does this require an OID that says the key is known
by the CA to be an Attestation key?]
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These two keys and certificates are used together:
o The DevID is used to validate a TLS connection terminating on the
device with a known serial number.
o The AK is used to sign attestation quotes, providing proof that
the attestation evidence comes from the same device.
5.3. Replay Attacks
Replay attacks, where results of a previous attestation are submitted
in response to subsequent requests, are usually prevented by
inclusion of a nonce in the request to the TPM for a quote. Each
request from the Verifier includes a new random number (a nonce).
The resulting quote signed by the TPM contains the same nonce,
allowing the Verifier to determine freshness, (i.e., that the
resulting quote was generated in response to the Verifier's specific
request). Time-Based Uni-directional Attestation
[I-D.birkholz-rats-tuda] provides an alternate mechanism to verify
freshness without requiring a request/response cycle.
5.4. Owner-Signed Keys
Although device manufacturers MUST pre-provision devices with easily
verified DevID and AK certificates, use of those credentials is not
mandatory. IEEE 802.1AR incorporates the idea of an Initial Device
ID (IDevID), provisioned by the manufacturer, and a Local Device ID
(LDevID) provisioned by the owner of the device. RIV and
[Platform-DevID-TPM-2.0] extends that concept by defining an Initial
Attestation Key (IAK) and Local Attestation Key (LAK) with the same
properties.
Device owners can use any method to provision the Local credentials.
o The TCG document [Platform-DevID-TPM-2.0] shows how the initial
Attestation keys can be used to certify LDevID and LAK keys. Use
of the LDevID and LAK allows the device owner to use a uniform
identity structure across device types from multiple manufacturers
(in the same way that an "Asset Tag" is used by many enterprises
to identify devices they own). The TCG document
[Provisioning-TPM-2.0] also contains guidance on provisioning
identity keys in TPM 2.0.
o Device owners, however, can use any other mechanism they want to
assure themselves that Local identity certificates are inserted
into the intended device, including physical inspection and
programming in a secure location, if they prefer to avoid placing
trust in the manufacturer-provided keys.
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Clearly, Local keys can't be used for secure Zero Touch provisioning;
installation of the Local keys can only be done by some process that
runs before the device is installed for network operation.
On the other end of the device life cycle, provision should be made
to wipe Local keys when a device is decommissioned, to indicate that
the device is no longer owned by the enterprise. The manufacturer's
Initial identity keys must be preserved, as they contain no
information that's not already printed on the device's serial number
plate.
5.5. Other Trust Anchors
In addition to trustworthy provisioning of keys, RIV depends on other
trust anchors. (See [SP800-155] for definitions of Roots of Trust.)
o Secure identity depends on mechanisms to prevent per-device secret
keys from being compromised. The TPM provides this capability as
a Root of Trust for Storage.
o Attestation depends on an unbroken chain of measurements, starting
from the very first measurement. That first measurement is made
by code called the Root of Trust for Measurement, typically done
by trusted firmware stored in boot flash. Mechanisms for
maintaining the trustworthiness of the RTM are out of scope for
RIV, but could include immutable firmware, signed updates, or a
vendor-specific hardware verification technique. See Section 8.1
for background on TPM practices.
o The device owner SHOULD provide some level of physical defense for
the device. If a TPM that has already been programmed with an
authentic DevID is stolen and inserted into a counterfeit device,
attestation of that counterfeit device may become
indistinguishable from an authentic device.
RIV also depends on reliable reference measurements, as expressed by
the RIM [RIM]. The definition of trust procedures for RIMs is out of
scope for RIV, and the device owner is free to use any policy to
validate a set of reference measurements. RIMs may be conveyed out-
of-band or in-band, as part of the attestation process (see
Section 3.1.3). But for embedded devices, where software is usually
shipped as a self-contained package, RIMs signed by the manufacturer
and delivered in-band may be more convenient for the device owner.
The validity of RIV attestation results is also influenced by
procedures used to create reference measurements:
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o While the RIM itself is signed, supply-chains SHOULD be carefully
scrutinized to ensure that the values are not subject to
unexpected manipulation prior to signing. Insider-attacks against
code bases and build chains are particularly hard to spot.
o Designers SHOULD guard against hash collision attacks. Reference
Integrity Measurements often give hashes for large objects of
indeterminate size; if one of the measured objects can be replaced
with an implant engineered to produce the same hash, RIV will be
unable to detect the substitution. TPM1.2 uses SHA-1 hashes only,
which have been shown to be susceptible to collision attack.
TPM2.0 will produce quotes with SHA-256, which so far has resisted
such attacks. Consequently RIV implementations SHOULD use TPM2.0.
6. Conclusion
TCG technologies can play an important part in the implementation of
Remote Integrity Verification. Standards for many of the components
needed for implementation of RIV already exist:
o Platform identity can be based on IEEE 802.1AR Device Identity,
coupled with careful supply-chain management by the manufacturer.
o Complex supply chains can be certified using TCG Platform
Certificates [Platform-Certificates].
o The TCG TAP mechanism can be used to retrieve attestation
evidence. Work is needed on a YANG model for this protocol.
o Reference Integrity Measurements must be conveyed from the
software authority (e.g., the manufacturer) to the system in which
verification will take place. IETF CoSWID work forms the basis
for this, but new work is needed to create an information model
and YANG implementation.
7. IANA Considerations
This memo includes no request to IANA.
8. Appendix
8.1. Using a TPM for Attestation
The Trusted Platform Module and surrounding ecosystem provide three
interlocking capabilities to enable secure collection of evidence
from a remote device, Platform Configuration Registers (PCRs), a
Quote mechanism, and a standardized Event Log.
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Each TPM has at least between eight and twenty-four PCRs (depending
on the profile and vendor choices), each one large enough to hold one
hash value (SHA-1, SHA-256, and other hash algorithms can be used,
depending on TPM version). PCRs can't be accessed directly from
outside the chip, but the TPM interface provides a way to "extend" a
new security measurement hash into any PCR, a process by which the
existing value in the PCR is hashed with the new security measurement
hash, and the result placed back into the same PCR. The result is a
composite fingerprint of all the security measurements extended into
each PCR since the system was reset.
Every time a PCR is extended, an entry should be added to the
corresponding Event Log. Logs contain the security measurement hash
plus informative fields offering hints as to what event it was that
generated the security measurement.
The Event Log itself is protected against accidental manipulation,
but it is implicitly tamper-evident - any verification process can
read the security measurement hash from the log events, compute the
composite value and compare that to what ended up in the PCR. If
there's a discrepancy, the logs do not provide an accurate view of
what was placed into the PCR.
In a conventional TPM Attestation environment, the first measurement
must be made and extended into the TPM by trusted device code (called
the Root of Trust for Measurement, RTM). That first measurement
should cover the segment of code that is run immediately after the
RTM, which then measures the next code segment before running it, and
so on, forming an unbroken chain of trust. See [TCGRoT] for more on
Mutable vs Immutable roots of trust.
The TPM provides another mechanism called a Quote that can read the
current value of the PCRs and package them into a data structure
signed by an Attestation Key (which is private key that is known only
to the TPM).
The Verifier uses the Quote and Log together. The Quote, containing
the composite hash of the complete sequence of security measurement
hashes, is used to verify the integrity of the Event Log. Each hash
in the validated Quote can then be compared to corresponding expected
values in the set of Reference Integrity Measurements to validate
overall system integrity.
A summary of information exchanged in obtaining quotes from TPM1.2
and TPM2.0 can be found in [TAP], Section 4. Detailed information
about PCRs and Quote data structures can be found in [TPM1.2],
[TPM2.0]. Recommended log formats include [PC-Client-BIOS-TPM-2.0]
and [Canonical-Event-Log].
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8.2. Root of Trust for Measurement
The measurements needed for attestation require that the device being
attested is equipped with a Root of Trust for Measurement, that is,
some trustworthy mechanism that can compute the first measurement in
the chain of trust required to attest that each stage of system
startup is verified, a Root of Trust for Storage (i.e., the TPM PCRs)
to record the results, and a Root of Trust for Reporting to report
the results [TCGRoT], [SP800-155].
While there are many complex aspects of a Root of Trust, two aspects
that are important in the case of attestation are:
o The first measurement computed by the Root of Trust for
Measurement, and stored in the TPM's Root of Trust for Storage, is
presumed to be correct.
o There must not be a way to reset the Root of Trust for Storage
without re-entering the Root of Trust for Measurement code.
The first measurement must be computed by code that is implicitly
trusted; if that first measurement can be subverted, none of the
remaining measurements can be trusted. (See [NIST-SP-800-155])
8.3. Layering Model for Network Equipment Attester and Verifier
Retrieval of identity and attestation state uses one protocol stack,
while retrieval of Reference Measurements uses a different set of
protocols. Figure 5 shows the components involved.
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+-----------------------+ +-------------------------+
| | | |
| Attester |<-------------| Verifier |
| (Device) |------------->| (Management Station) |
| | | | |
+-----------------------+ | +-------------------------+
|
-------------------- --------------------
| |
---------------------------------- ---------------------------------
|Reference Integrity Measurements| | Attestation |
---------------------------------- ---------------------------------
********************************************************************
* IETF Attestation Reference Interaction Diagram *
********************************************************************
....................... .......................
. Reference Integrity . . TAP (PTS2.0) Info .
. Manifest . . Model and Canonical .
. . . Log Format .
....................... .......................
************************* .............. **********************
* YANG SWID Module * . TCG . * YANG Attestation *
* I-D.birkholz-yang-swid* . Attestation. * Module *
* * . MIB . * I-D.ietf-rats- *
* * . . * yang-tpm-charra *
************************* .............. **********************
************************* ************ ************************
* XML, JSON, CBOR (etc) * * UDP * * XML, JSON, CBOR (etc)*
************************* ************ ************************
************************* ************************
* RESTCONF/NETCONF * * RESTCONF/NETCONF *
************************ *************************
************************* ************************
* TLS, SSH * * TLS, SSH *
************************* ************************
Figure 7: RIV Protocol Stacks
IETF documents are captured in boxes surrounded by asterisks. TCG
documents are shown in boxes surrounded by dots.
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8.3.1. Why is OS Attestation Different?
Even in embedded systems, adding Attestation at the OS level (e.g.,
Linux IMA, Integrity Measurement Architecture [IMA]) increases the
number of objects to be attested by one or two orders of magnitude,
involves software that's updated and changed frequently, and
introduces processes that begin in an unpredictable order.
TCG and others (including the Linux community) are working on methods
and procedures for attesting the operating system and application
software, but standardization is still in process.
8.4. Implementation Notes
Figure 8 summarizes many of the actions needed to complete an
Attestation system, with links to relevant documents. While
documents are controlled by several standards organizations, the
implied actions required for implementation are all the
responsibility of the manufacturer of the device, unless otherwise
noted. It should be noted that, while the YANG model is RECOMMENDED
for attestation, this table identifies an optional SNMP MIB as well,
[Attest-MIB].
+------------------------------------------------------------------+
| Component | Controlling |
| | Specification |
--------------------------------------------------------------------
| Make a Secure execution environment | TCG RoT |
| o Attestation depends on a secure root of | UEFI.org |
| trust for measurement outside the TPM, as | |
| well as roots for storage and reporting | |
| inside the TPM. | |
| o Refer to TCG Root of Trust for Measurement.| |
| o NIST SP 800-193 also provides guidelines | |
| on Roots of Trust | |
--------------------------------------------------------------------
| Provision the TPM as described in | TCG TPM DevID |
| TCG documents. | TCG Platform |
| | Certificate |
--------------------------------------------------------------------
| Put a DevID or Platform Cert in the TPM | TCG TPM DevID |
| o Install an Initial Attestation Key at the | TCG Platform |
| same time so that Attestation can work out | Certificate |
| of the box |-----------------
| o Equipment suppliers and owners may want to | IEEE 802.1AR |
| implement Local Device ID as well as | |
| Initial Device ID | |
--------------------------------------------------------------------
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| Connect the TPM to the TLS stack | Vendor TLS |
| o Use the DevID in the TPM to authenticate | stack (This |
| TAP connections, identifying the device | action is |
| | simply |
| | configuring TLS|
| | to use the |
| | DevID as its |
| | trust anchor.) |
--------------------------------------------------------------------
| Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID |
| o Add reference measurements into SWID tags | ISO/IEC 19770-2|
| o Manufacturer should sign the SWID tags | NIST IR 8060 |
| o The TCG RIM-IM identifies further | |
| procedures to create signed RIM | |
| documents that provide the necessary | |
| reference information | |
--------------------------------------------------------------------
| Package the SWID tags with a vendor software | Retrieve tags |
| release | with |
| o A tag-generator plugin such | draft-birkholz-yang-swid|
| as [SWID-Gen] can be used |----------------|
| | TCG PC Client |
| | RIM |
--------------------------------------------------------------------
| Use PC Client measurement definitions | TCG PC Client |
| to define the use of PCRs | BIOS |
| (although Windows OS is rare on Networking | |
| Equipment, UEFI BIOS is not) | |
--------------------------------------------------------------------
| Use TAP to retrieve measurements | |
| o Map TAP to SNMP | TCG SNMP MIB |
| o Map to YANG | YANG Module for|
| Use Canonical Log Format | Basic |
| | Attestation |
| | TCG Canonical |
| | Log Format |
--------------------------------------------------------------------
| Posture Collection Server (as described in IETF | |
| SACMs ECP) should request the | |
| attestation and analyze the result | |
| The Management application might be broken down | |
| to several more components: | |
| o A Posture Manager Server | |
| which collects reports and stores them in | |
| a database | |
| o One or more Analyzers that can look at the| |
| results and figure out what it means. | |
--------------------------------------------------------------------
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Figure 8: Component Status
9. References
9.1. Normative References
[Canonical-Event-Log]
Trusted Computing Group, "DRAFT Canonical Event Log Format
Version: 1.0, Revision: .12", October 2018.
[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-rats-yang-tpm-charra]
Birkholz, H., Eckel, M., Bhandari, S., Sulzen, B., Voit,
E., Xia, L., Laffey, T., and G. Fedorkow, "A YANG Data
Model for Challenge-Response-based Remote Attestation
Procedures using TPMs", draft-ietf-rats-yang-tpm-charra-02
(work in progress), June 2020.
[I-D.ietf-sacm-coswid]
Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags", draft-
ietf-sacm-coswid-15 (work in progress), May 2020.
[IEEE-802-1AR]
Seaman, M., "802.1AR-2018 - IEEE Standard for Local and
Metropolitan Area Networks - Secure Device Identity, IEEE
Computer Society", August 2018.
[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://trustedcomputinggroup.org/resource/pc-client-
work-group-specific-implementation-specification-for-
conventional-bios/>.
[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>.
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[PC-Client-RIM]
Trusted Computing Group, "DRAFT: TCG PC Client Reference
Integrity Manifest Specification, v.09", December 2019,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_PC_Client_RIM_r0p15_15june2020.pdf>.
[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/resource/
tpm-keys-for-platform-identity-for-tpm-1-2-2/>.
[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>.
[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>.
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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/>.
9.2. Informative References
[AIK-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/>.
[Attest-MIB]
Trusted Computing Group, "SNMP MIB for TPM-Based
Attestation, Version 0.8Revision 0.02", May 2018,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_SNMP_MIB_for_TPM-
Based_Attestation_v0.8r2_PUBLIC_REVIEW.pdf>.
[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/resource/tcg-efi-
platform-specification/>.
[I-D.birkholz-rats-network-device-subscription]
Birkholz, H., Voit, E., and W. Pan, "Attestation Event
Stream Subscription", draft-birkholz-rats-network-device-
subscription-00 (work in progress), June 2020.
[I-D.birkholz-rats-reference-interaction-model]
Birkholz, H., Eckel, M., Newton, C., and L. Chen,
"Reference Interaction Models for Remote Attestation
Procedures", draft-birkholz-rats-reference-interaction-
model-03 (work in progress), July 2020.
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[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
rats-tuda-03 (work in progress), July 2020.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-06 (work in progress),
September 2020.
[I-D.ietf-rats-eat]
Mandyam, G., Lundblade, L., Ballesteros, M., and J.
O'Donoghue, "The Entity Attestation Token (EAT)", draft-
ietf-rats-eat-04 (work in progress), August 2020.
[I-D.richardson-rats-usecases]
Richardson, M., Wallace, C., and W. Pan, "Use cases for
Remote Attestation common encodings", draft-richardson-
rats-usecases-07 (work in progress), March 2020.
[I-D.voit-rats-trusted-path-routing]
Voit, E., "Trusted Path Routing", draft-voit-rats-trusted-
path-routing-02 (work in progress), June 2020.
[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>.
[IMA] and , "Integrity Measurement Architecture", June 2019,
<https://sourceforge.net/p/linux-ima/wiki/Home/>.
[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/>.
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[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>.
[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>.
[SWID-Gen]
Labs64, Munich, Germany, "SoftWare IDentification (SWID)
Tags Generator (Maven Plugin)", n.d.,
<https://github.com/Labs64/swid-maven-plugin>.
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[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>.
[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.
US
Email: gfedorkow@juniper.net
Eric Voit
Cisco Systems, Inc.
US
Email: evoit@cisco.com
Jessica Fitzgerald-McKay
National Security Agency
US
Email: jmfitz2@nsa.gov
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