RATS Working Group
Internet Engineering Task Force (IETF) G. C. Fedorkow, Ed.
Internet-Draft
Request for Comments: 9683 Juniper Networks, Inc.
Intended status:
Category: Informational E. Voit
Expires: 23 September 2022
ISSN: 2070-1721 Cisco
J. Fitzgerald-McKay
National Security Agency
22 March 2022
TPM-based Network Device
November 2024
Remote Integrity Verification
draft-ietf-rats-tpm-based-network-device-attest-14 of Network Devices Containing Trusted
Platform Modules
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], (TPMs), as defined by the Trusted
Computing Group (TCG)), (TCG), or equivalent hardware implementations that
include the protected capabilities, as provided by TPMs.
Status of This Memo
This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
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Internet-Drafts are draft the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents valid
approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of six months RFC 7841.
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This Internet-Draft will expire on 23 September 2022.
https://www.rfc-editor.org/info/rfc9683.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3 Notation
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Document Organization . . . . . . . . . . . . . . . . . . 5
1.4. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5. Description of Remote Integrity Verification (RIV) . . . 6
1.6. Solution Requirements . . . . . . . . . . . . . . . . . . 8
1.7. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.7.1. Out of Scope . . . . . . . . . . . . . . . . . . . . 9
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 9
2.1. RIV Software Configuration Attestation using Using TPM . . . . 9
2.1.1. What Does RIV Attest? . . . . . . . . . . . . . . . . 11
2.1.2. Notes on PCR Allocations . . . . . . . . . . . . . . 13
2.2. RIV Keying . . . . . . . . . . . . . . . . . . . . . . . 15
2.3. RIV Information Flow . . . . . . . . . . . . . . . . . . 16
2.4. RIV Simplifying Assumptions . . . . . . . . . . . . . . . 18
2.4.1. Reference Integrity Manifests (RIMs) . . . . . . . . 18
2.4.2. Attestation Logs . . . . . . . . . . . . . . . . . . 20
3. Standards Components . . . . . . . . . . . . . . . . . . . . 20
3.1. Prerequisites for RIV . . . . . . . . . . . . . . . . . . 20
3.1.1. Unique Device Identity . . . . . . . . . . . . . . . 20
3.1.2. Keys . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1.3. Appraisal Policy for Evidence . . . . . . . . . . . . 21
3.2. Reference Model for Challenge-Response . . . . . . . . . 21
3.2.1. Transport and Encoding . . . . . . . . . . . . . . . 23
3.3. Centralized vs vs. Peer-to-Peer . . . . . . . . . . . . . . . 24
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
5. Security Considerations . . . . . . . . . . . . . . . . . . . 26
5.1. Keys Used in RIV . . . . . . . . . . . . . . . . . . . . 26
5.2. Prevention of Spoofing and Person-in-the-Middle Attacks . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.3. Replay Attacks . . . . . . . . . . . . . . . . . . . . . 29
5.4. Owner-Signed Keys . . . . . . . . . . . . . . . . . . . . 30
5.5. Other Factors for Trustworthy Operation . . . . . . . . . 30
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1.
8.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2.
8.2. Informative References . . . . . . . . . . . . . . . . . 41
A.
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 32 A. Supporting Materials
A.1. Using a TPM for Attestation . . . . . . . . . . . . . . . 32
A.2. Root of Trust for Measurement . . . . . . . . . . . . . . 34 (RTM)
A.3. Layering Model for Network Equipment Attester and Verifier . . . . . . . . . . . . . . . . . . . . . . . . 35
A.4. Implementation Notes . . . . . . . . . . . . . . . . . . 37
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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
that 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].
[RFC9334]. Additionally, use cases for remotely attesting networking
devices are discussed within Section 6 5 of
[I-D.richardson-rats-usecases]. [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, problem and
then identifies elements that are by identifying the necessary elements to get the complete,
scalable attestation procedure working with commercial networking
products such as routers, switches switches, and firewalls. An underlying
assumption will be is the availability within the device of a cryptoprocessor
that is compatible with the Trusted Platform Module [TPM1.2], [TPM2.0] compatible cryptoprocessor specifications
[TPM-1.2] [TPM-2.0] to enable the trustworthy trustworthy, remote assessment of
the device's software and hardware.
1.1. Requirements notation Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
A number of terms are reused from [I-D.ietf-rats-architecture]. [RFC9334]. These include: include Appraisal
Policy for Evidence, Attestation Result, Attester, Evidence,
Reference Value, Relying Party, Verifier, and Verifier Owner.
Additionally, this document defines the following term:
Attestation: the The process of generating, conveying conveying, and appraising
claims, backed by evidence, about device trustworthiness
characteristics, including supply chain trust, identity, device
provenance, software configuration, device composition, compliance
to test suites, functional and assurance evaluations, etc.
The goal of attestation is simply to assure an administrator or
auditor that the device device's configuration and software that was launched were authentic
and unmodified when the device was last started is authentic and untampered-with. started. The determination of
software authenticity is not prescribed in this document, but it's
typically taken to mean a software image generated by an authority
trusted by the administrator, such as the device manufacturer.
Within the context of the Trusted Computing Group (TCG) context, (TCG), the scope of
attestation is typically narrowed to describe the process by which an
independent Verifier can obtain cryptographic proof as to the
identity of the device in question, and evidence of the integrity of the
device's software loaded on that device when it started up, was loaded upon startup, and then verify verification that what's there
the current configuration matches the intended configuration. For
network equipment, a Verifier capability can be embedded in a Network
Management Station (NMS), Station, 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 This document focuses
on a specific subset of attestation tasks, defined here as Remote
Integrity Verification (RIV). (RIV), and informally referred to as
attestation. RIV in this document takes a network-equipment-
centric network-equipment-centric
perspective that includes a set of protocols and procedures for
determining whether a particular device was launched with authentic
software, starting from Roots of Trust. While there are many ways to
accomplish attestation, RIV sets out a specific set of protocols and
tools that work in environments commonly found in network equipment.
RIV does not cover other device characteristics that could be
attested (e.g., geographic location, location or connectivity; see
[I-D.richardson-rats-usecases]),
[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]). [RATS-EAT]).
In line with [I-D.ietf-rats-architecture] definitions, definitions found in [RFC9334], this document uses the
term Endorser to refer to the role that signs identity and
attestation certificates used by the Attester, while Reference Values
are signed by a Reference Value Provider. Typically, the
manufacturer of a network device would be accepted as both the
Endorser and Reference Value Provider, although the choice is
ultimately up to the Verifier Owner.
1.3. Document Organization
The remainder of this document is organized into several sections:
* The remainder of this section covers goals and requirements, plus
a top-level description of RIV.
* The Solution Overview section (Section 2) outlines how Remote Integrity
Verification RIV works.
* The Standards Components section (Section 3) links components of
RIV to normative standards.
* The Privacy and Security Considerations sections (Sections 4 and
5) shows how specific features of RIV contribute to the
trustworthiness of the Attestation Result.
* Supporting material is in an appendix at the end. (Appendix A).
1.4. Goals
Network operators benefit from a trustworthy attestation mechanism
that provides assurance that their network comprises authentic
equipment,
equipment and has loaded software free of known vulnerabilities and
unauthorized tampering. In line with the overall goal of assuring
integrity, attestation can be used to assist in asset management,
vulnerability and compliance assessment, plus configuration
management.
The RIV attestation workflow outlined in this document is intended to
meet the following high-level goals:
* Provable Device Identity - This specification requires that an
Attester (i.e., the attesting device) includes a cryptographic
identifier unique to each device. Effectively Effectively, this means that
the device's TPM must be so provisioned with this during the
manufacturing cycle.
* Software Inventory - A key goal is Key goals are to identify the software
release(s) installed on the Attester, Attester and to provide evidence that
the software stored within hasn't been altered without
authorization.
* Verifiability - Verification of the device's software and
configuration of the
device shows that the software that the administrator
authorized for use was actually launched.
In addition, RIV is designed to operate either in a centralized
environment, such as with a central authority that manages and
configures a number of network devices, or 'peer-to-peer', "peer-to-peer", where
network devices independently verify one another to establish a trust
relationship. (See Section 3.3 below) 3.3.)
1.5. Description of Remote Integrity Verification (RIV)
Attestation requires two interlocking mechanisms between the Attester
network device and the Verifier:
* Device Identity, Identity is the mechanism providing that provides trusted identity,
which can reassure network managers that the specific devices they
ordered from authorized manufacturers for attachment to their
network are those that were installed, installed and that they continue to be
present in their network. As part of the mechanism for Device
Identity, cryptographic proof of the manufacturer's identity of the manufacturer is
also provided.
* Software Measurement is the mechanism that reports the state of
mutable software components on the device, device and that can assure
administrators that they have known, authentic software configured
to run in their network.
Using
By using these two interlocking mechanisms, RIV RIV, which is a component
in a chain of procedures that procedures, can assure a network operator that the
equipment in their network can be reliably identified, 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 switches, and firewalls.
Software used to boot a device can be identified by a chain of
measurements, anchored at the start by a Root of Trust for
Measurement (RTM) (see Section 9.2), Appendix A.2). An attestation function
embedded in each measuring stage, verified by the previous stage, measures the
next stage and
recording records the result in tamper-resistant storage, normally ending
when the system software is fully loaded. storage. A
measurement signifies the identity, integrity integrity, and version of each
software component registered with an Attester's TPM [TPM1.2], [TPM2.0], [TPM-1.2]
[TPM-2.0] so that a subsequent verification stage can determine if
the software installed is authentic, up-to-date, and free of
tampering.
RIV includes several major processes, which are split between the
Attester and Verifier:
1. Generation of Evidence is the process whereby an Attester
generates cryptographic proof (Evidence) of claims about device
properties. In particular, the device identity and its software
configuration are both of critical importance.
2. Device Identification refers to the mechanism assuring the
Relying Party (ultimately, a network administrator) of the
identity
identities of devices devices, and the identities of their manufacturers,
that make up their network, and that their
manufacturers are known. network.
3. Conveyance of Evidence reliably transports the collected Evidence
from the Attester to a Verifier to allow a management station to
perform a meaningful appraisal in Step 4. The transport is
typically carried out via a management network. While Although not
required for reliable attestation, an encrypted channel may be
used to provide integrity, authenticity, or confidentiality once
attestation is complete. It should be noted that critical
attestation evidence from the TPM is signed by a key known only
to TPM, and is not dependent on encyption encryption carried out as part of
a reliable transport.
4. Finally, Appraisal appraisal of Evidence evidence occurs. This is the process of
verifying the Evidence received by a Verifier from the Attester, Attester
and using an Appraisal Policy to develop an Attestation Result,
which is used to inform decision-making. In practice, this means
comparing the Attester's measurements reported as Evidence with
the device configuration expected by the Verifier. Subsequently,
the Appraisal Policy for Evidence might match Evidence found
against Reference Values (aka Golden Measurements), which
represent the intended configured state of the connected device.
All implementations supporting this RIV specification require the
support of the following three technologies:
1. Identity: Device identity in RIV is based on IEEE 802.1AR Device Identity
(DevID) defined by IEEE Std 802.1AR [IEEE-802-1AR], coupled with
careful supply-
chain supply-chain management by the manufacturer. The Initial
DevID (IDevID) certificate contains a statement by the
manufacturer that establishes the identity of the device as it
left the factory. Some applications with a more-complex post-manufacture more complex post-
manufacture supply chain (e.g., Value Added Resellers), value added resellers), or with
different privacy concerns, may want to use alternative
mechanisms for platform authentication (for example, TCG Platform
Certificates
[Platform-Certificates], [PLATFORM-CERTS] or post-manufacture installation of
Local Device ID DevID (LDevID)).
2. Platform Attestation 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), (PCRs) and 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 Root of
trust
Trust through all layers of software needed to bring the device
to an operational state, in which each stage computes the hash of
components of the next stage, then updates the attestation log
and the TPM. The TPM can then report the hashes of all the
measured hashes as signed evidence called a Quote (see
Section 9.1
Appendix A.1 for an overview of TPM operation, operation or [TPM1.2] [TPM-1.2] and
[TPM2.0]
[TPM-2.0] for many more details).
3. Signed Reference Values (aka Reference Integrity Measurements) reference integrity measurements)
must be conveyed from the Reference Value Provider (the entity
accepted as the software authority, often the manufacturer of the
network device) to the Verifier.
1.6. Solution Requirements
Remote Integrity Verification
RIV must address the "Lying Endpoint" problem, in which malicious
software on an endpoint may subvert the intended function, function and also
prevent the endpoint from reporting its compromised status. (See
Section 5 for further Security Considerations.)
RIV attestation is designed to be simple to deploy at scale. RIV
should work "out of the box" as far as possible, that is, with the
fewest possible provisioning steps or configuration databases needed
at the end-user's end user's site. Network equipment is often required to
"self-configure", to reliably reach out without manual intervention
to prove its identity and operating posture, then download its own
configuration, a process which precludes pre-installation
configuration. See [RFC8572] for an example of Secure Zero Touch
Provisioning.
Provisioning (SZTP).
1.7. Scope
The need for assurance of software integrity, addressed by Remote
Attestation, is a very general problem that could apply to most
network-connected computing devices. However, this document includes
several assumptions that limit the scope to network equipment (e.g.,
routers, switches switches, and firewalls):
* This solution is for use in non-privacy-preserving applications
(for example, networking, Industrial IoT), avoiding networking or industrial Internet of Things (IoT)
applications), which avoids the need for a Privacy Certificate Certification
Authority (also called an Attestation CA) for
attestation keys [AK-Enrollment] Attestation Keys
(AKs) [AIK-ENROLL] or TCG Platform Certificates
[Platform-Certificates]. [PLATFORM-CERTS].
* This document assumes network protocols that are common in network
equipment such as YANG [RFC7950] and NETCONF Network Configuration
Protocol (NETCONF) [RFC6241], but not generally used in other
applications.
* The approach outlined in this document mandates the use of a TPM
[TPM1.2], [TPM2.0],
[TPM-1.2] [TPM-2.0] or a compatible cryptoprocessor.
1.7.1. Out of Scope
*
Run-Time Attestation: The Linux Integrity Measurement Architecture
[IMA] attests each process launched after a device is started (and
is in scope for RIV in general), but continuous run-time
attestation of Linux or other multi-threaded operating system
processes after the OS has started considerably expands the scope
of the problem. Many researchers are working on that problem, but
this document defers the problem of continuous, in-memory run-time
attestation.
*
Multi-Vendor Embedded Systems: Additional coordination would be
needed for devices that themselves comprise hardware and software
from multiple vendors, vendors and are integrated by the end user.
Although out of scope for this document, these issues are
accommodated in
[I-D.ietf-rats-architecture].
* [RFC9334].
Processor Sleep Modes: Network equipment typically does not "sleep",
so sleep and hibernate modes are not considered. Although out of
scope for RIV in this document, Trusted Computing
Group TCG specifications do encompass
sleep and hibernate states, which could be incorporated into
remote attestation for network equipment in the future, given a
compelling need.
*
Virtualization and Containerization: In a non-virtualized system,
the host OS is responsible for measuring each User Space user-space file or
process throughout the operational lifetime of the system. For
virtualized systems, the host OS must verify the hypervisor, but
then the hypervisor must manage its own chain of trust through the
virtual machine. Virtualization and containerization technologies
are increasingly used in network equipment, but are not considered
in this document.
2. Solution Overview
2.1. RIV Software Configuration Attestation using Using TPM
RIV Attestation is a process which that can be used to determine the
identity of software running on a specifically-identified specifically identified device.
The Remote Attestation steps of Section 1.5 are broken split into two
phases, phases
as shown in Figure 1:
* During system startup, or boot phase, Boot Phase, each distinct software
object is "measured" by the Attester. The object's identity, hash
(i.e., cryptographic digest) digest), and version information are recorded
in a log. Hashes are also extended into the TPM (see Section 9.1 Appendix A.1
for more on 'extending hashes'), extending hashes) in a way that can be used to
validate the log entries. The measurement process generally
follows the layered chain-of-trust model used in Measured Boot,
where each stage of the system measures the next one, and extends
its measurement into the TPM, before launching it. See
[I-D.ietf-rats-architecture], section
Section 3.2 of [RFC9334], "Layered Attestation
Environments," Environments", for
an architectural definition of this model.
* Once the device is running and has operational network
connectivity, verification can take place. A separate Verifier,
running in its own trusted environment, will interrogate the
network device to retrieve the logs and a copy of the digests
collected by hashing each software object, signed by an
attestation private key secured by, but never released by, the
TPM. The YANG model described in [I-D.ietf-rats-yang-tpm-charra] [RFC9684] facilitates this
operation.
The result is that the Verifier can verify the device's identity by
checking the subject[RFC5280] subject [RFC5280] and signature of the certificate
containing the TPM's attestation public key, and key. The Verifier can validate then
verify the
software that was launched log's correctness by verifying accumulating all the correctness of hashes in the logs
by
log and comparing with that to the signed digests from the TPM, and TPM. From
there, the Verifier can validate the launched software by comparing
the digests in the log with Reference 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.
+-------------------------------------------------------+
| +---------+ +--------+ +--------+ +---------+ |
| |UEFI BIOS|--->| Loader |-->| Kernel |--->|Userland | |
| +---------+ +--------+ +--------+ +---------+ |
| | | | |
| | | | |
| +------------+-----------+-+ |
| Boot Phase | |
| V |
| +--------+ |
| | TPM | |
| +--------+ |
| Router | |
+--------------------------------|----------------------+
|
| Verification Phase
| +-----------+
+--->| Verifier |
+-----------+
Reset---------------flow-of-time-during-boot...--------->
Figure 1: Layered RIV Attestation Model
In the Boot phase, Phase, measurements are "extended", or hashed, into the
TPM as processes start, with the which result that in the TPM ends up containing hashes of
all the measured hashes. Later, once the system is operational, during the Verification phase,
signed digests are retrieved from the TPM during the Verification
Phase for off-box analysis.
2.1.1. What Does RIV Attest?
TPM attestation is focused on Platform Configuration Registers
(PCRs), PCRs, but those registers are only
vehicles for certifying accompanying Evidence, Evidence conveyed in log
entries. It is the hashes in log entries that are extended into
PCRs, where the final PCR values can be retrieved in the form of a
structure called a Quote, which is signed by an Attestation key AK known only to the
TPM. The use of multiple PCRs serves only to provide some
independence between different classes of
object, 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 that 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:
* Code, (i.e., instructions)
Code: Instructions to be executed by a CPU.
* Configuration -
Configuration: Many devices offer numerous options controlled by
non-volatile configuration variables which that can impact the device's
security posture. These settings may have vendor defaults, but
often can be changed by administrators, who may want to verify via
attestation that the operational state of the settings match their
intended state.
* Credentials -
Credentials: Administrators may wish to verify via attestation that
public keys and credentials outside the Root of Trust have not
been subject to unauthorized tampering. (By definition, keys
protecting the root Root of trust Trust can't be verified independently.)
The TCG "TCG PC Client Specific Platform Firmware Profile Specification
[PC-Client-BIOS-TPM-2.0] gives considerable detail on Specification"
[PC-CLIENT-BIOS-TPM-2.0] details what is to be measured during the boot phase
Boot Phase of platform startup using a UEFI Unified Extensible Firmware
Interface (UEFI) BIOS
(www.uefi.org), (<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, undetected and
potentially subverting the chain.
For devices using a UEFI BIOS, [PC-Client-BIOS-TPM-2.0] [PC-CLIENT-BIOS-TPM-2.0] and
[PC-Client-EFI-TPM-1.2]
[PC-CLIENT-EFI-TPM-1.2] give detailed normative requirements for PCR
usage. For other platform architectures, where TCG normative
requirements currently do not exist, the table in Figure 2 Table 1 gives non-
normative non-normative
guidance for PCR assignment that generalizes the specific details of [PC-Client-BIOS-TPM-2.0].
[PC-CLIENT-BIOS-TPM-2.0].
By convention, most PCRs are assigned in pairs, which with the even-
numbered PCR used to measure executable code, 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.
+------------------------------------------------------------------+
+===========================================+======================+
| | Assigned PCR # |
+===========================================+======+===============+
| Function | Code | Configuration|
-------------------------------------------------------------------- Configuration |
+===========================================+======+===============+
| Firmware Static Root of Trust, Trust (i.e., | 0 | 1 |
| initial boot firmware and drivers) | | |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| Drivers and initialization for optional | 2 | 3 |
| or add-in devices | | |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| OS Loader loader code and configuration, configuration (i.e., | 4 | 5 |
| the code launched by firmware) to load an | | |
| operating system kernel. These PCRs record | | |
| record each boot attempt, and an identifier for | | |
| identifier for where the loader was found | | |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| Vendor Specific Measurements Vendor-specific measurements during boot | 6 | 6 |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| Secure Boot Policy. This PCR records keys | | 7 |
| keys and configuration used to validate the OS | | |
| the OS loader | | |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| Measurements made by the OS Loader loader (e.g., | 8 | 9 |
| (e.g. GRUB2 for Linux) | | |
--------------------------------------------------------------------
+-------------------------------------------+------+---------------+
| Measurements made by OS (e.g., Linux IMA) | 10 | 10 |
+------------------------------------------------------------------+
Figure 2:
+-------------------------------------------+------+---------------+
Table 1: Attested Objects
2.1.2. Notes on PCR Allocations
It is important to recognize that PCR[0] is critical. The first
measurement into PCR[0] is taken by the Root of Trust for
Measurement, which is code which, that, by definition, cannot be verified by
measurement. This measurement establishes the chain of trust for all
subsequent measurements. If the PCR[0] measurement cannot be
trusted, the validity of the entire chain is put called into question.
Distinctions Between between PCR[0], PCR[2], PCR[4] PCR[4], and PCR[8] are
summarized below:
*
PCR[0] typically represents a consistent view of rarely-changed
Host Platform rarely changed boot components, allowing
components of the host platform, which allows Attestation policies
to be defined using the less changeable components of the
transitive trust chain. This PCR typically provides a consistent
view of the platform regardless of user selected user-selected options.
*
PCR[2] is intended to represent a "user configurable" "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 Peripheral Component
Interconnect (PCI) or other slots. In UEFI
systems systems, these devices
may be configured by Option ROMs measured into PCR[2] and executed
by the UEFI BIOS.
*
PCR[4] is intended to represent the software that manages the
transition between the platform's Pre-Operating System pre-OS start and the state of a
system with the Operating System OS present. This PCR, along with PCR[5],
identifies the initial operating system OS loader (e.g., GRUB for Linux).
*
PCR[8] is used by the OS loader (e.g. (e.g., GRUB) to record measurements
of the various components of the operating system.
Although the TCG PC Client document [PC-CLIENT-BIOS-TPM-2.0] 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) policies), but the Verifier may not need to know
what each log entry means or why it was assigned to a particular PCR.
Designers must recognize that some PCRs may cover log entries that a
particular Verifier considers critical and other log entries that are
not considered important, so differing PCR values may not on their
own constitute a check for authenticity. For example, in a UEFI
system, some administrators may consider booting an image from a
removable drive, something recorded in a PCR, to be a security
violation, while others might consider that operation to be an
authorized recovery procedure.
Designers may allocate particular events to specific PCRs in order to
achieve a particular objective with local attestation, attestation (e.g., allowing
a procedure to execute, or releasing a particular decryption key,
only if a given PCR is in a given state). It may also be important
to designers to consider whether streaming notification of PCR
updates is required (see
[I-D.birkholz-rats-network-device-subscription]). [RATS-NET-DEV-SUB]). 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 events, such as configuration changes, from
high-volume, routine measurements such as IMA [IMA]
logs. logs [IMA].
2.2. RIV Keying
RIV attestation relies on two credentials:
* An identity key pair and matching certificate is required to
certify the identity of the Attester itself. RIV specifies the
use of an IEEE 802.1AR Device Identity (DevID) [IEEE-802-1AR], DevID [IEEE-802-1AR] that is signed by the
device manufacturer, containing manufacturer and contains the device serial number. This
requirement goes slightly beyond 802.1AR; see Section 2.4 for
notes.
* An Attestation key pair and matching certificate is required to
sign the Quote generated by the TPM to report evidence of software
configuration.
In a TPM application, both the Attestation private key and the DevID
private key MUST be protected by the TPM. Depending on other TPM
configuration procedures, the two keys are likely to be different;
some of the considerations are outlined in TCG the "TPM 2.0 Keys for
Device Identity and Attestation" [Platform-DevID-TPM-2.0]. document [PLATFORM-DEVID-TPM-2.0].
The TCG TPM "TPM 2.0 Keys for Device Identity and Attestation" document [Platform-DevID-TPM-2.0]
[PLATFORM-DEVID-TPM-2.0] specifies further conventions for these
keys:
* When separate Identity and Attestation keys are used, the
Attestation Key (AK) AK and
its X.509 certificate should parallel the DevID, with the same
unique device identification as the DevID certificate (that is,
the same subject and subjectAltName (if present), even though the
key pairs are different). This allows a
quote from By examining the device, corresponding AK
certificate, the Verifier can directly link a device's quote,
which was signed by an AK, to be linked directly to the device that provided it, by examining the corresponding AK
certificate. it. If the
subject in the AK certificate doesn't match the corresponding
DevID certificate, or if they're signed by
differing authorities different authorities,
the Verifier may signal the detection of an Asokan-style person-in-the-middle person-
in-the-middle attack (see Section 5.2).
* Network devices that are expected to use secure zero touch
provisioning SZTP as specified in
[RFC8572] MUST be shipped by the manufacturer with pre-provisioned
keys (Initial DevID and Initial AK, called IDevID and IAK). IAK,
respectively). IDevID and IAK certificates MUST both be signed by
the Endorser (typically the device manufacturer). Inclusion of an
IDevID and IAK by a vendor does not preclude a mechanism whereby
an administrator can define Local Identity LDevID and Local Attestation Keys (LDevID and LAK)
(LAK) if desired.
2.3. RIV Information Flow
RIV workflow for network equipment is organized around a simple use
case where a network operator wishes to verify the integrity of
software installed in specific, fielded devices. A normative
taxonomy of terms is given in [I-D.ietf-rats-architecture], [RFC9334], but as a reminder, this use
case implies several roles and objects:
1.
Attester: The Attester, the device which that the network operator wants to examine.
2. A Verifier (which
Verifier: Which might be a network management station)
somewhere Network Management Station and is
somewhat separate from the Device that will retrieve the signed
evidence and measurement logs, and analyze them to pass judgment
on the security posture of the device.
3. A
Relying Party, which can Party: Can act on Attestation Results. Interaction between
the Relying Party and the Verifier is considered out of scope for
RIV.
4.
Signed Reference Integrity Manifests (RIMs), containing (RIMs): Contains Reference
Values,
Values. RIMs 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 concept 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, Internet, or by other
devices that are not management stations per se (e.g., a peer
device; see Section 3.1.3). If Reference Values are obtained from
multiple sources, the Verifier may need to evaluate the relative
level of trust to be placed in each source in case of a
discrepancy.
These components are illustrated in Figure 3. 2.
+----------------+ +-------------+ +---------+--------+
|Reference Value | | Attester | Step 1 | Verifier| |
|Provider | | (Device |<-------| (Network| Relying|
|(Device | | under |------->| Mngmt Mgmt | Party |
|Manufacturer | | attestation)| Step 2 | Station)| |
|or other | | | | | |
|authority) | | | | | |
+----------------+ +-------------+ +---------+--------+
| /\
| Step 0 |
-----------------------------------------------
Figure 3: 2: RIV Reference Configuration for Network Equipment
* In
Step 0, 0: The Reference Value Provider (the device manufacturer or
other authority) makes one or more Reference Integrity
Manifests (RIMs), corresponding RIMs, which correspond to
the software image expected to be found on the device, device and
are signed by the Reference Value Provider, available to the Verifier (see
Verifier. (See Section 3.1.3 for "in-band" and "out of
band" ways to make this happen).
* In happen.)
Step 1, the Verifier (Network Management Station), on 1: On behalf of a Relying Party, the Verifier (Network
Management Station) requests Identity, DevID, Measurement Values, and
possibly RIMs, RIMs from the Attester.
* In
Step 2, the 2: The Attester responds to the request by providing a DevID,
quotes (measured values, values that are signed by the Attester),
and optionally RIMs.
Use
The use of the following standards components allows for
interoperability:
1. TPM Keys keys MUST be configured according to
[Platform-DevID-TPM-2.0], [PLATFORM-DEVID-TPM-2.0]
or [Platform-ID-TPM-1.2]. [PLATFORM-ID-TPM-1.2].
2. For devices using UEFI and Linux, measurements of firmware and
bootable modules MUST be taken according to TCG "TCG EFI Platform
Specification" [PC-CLIENT-EFI-TPM-1.2] or "TCG PC Client
[PC-Client-EFI-TPM-1.2] or [PC-Client-BIOS-TPM-2.0], Specific
Platform Firmware Profile Specification"
[PC-CLIENT-BIOS-TPM-2.0], and Linux IMA [IMA].
3. Device Identity DevID MUST be managed as DevID certificates as specified in IEEE
Std 802.1AR
Device Identity certificates [IEEE-802-1AR], with keys protected by TPMs.
4. Attestation logs from Linux-based systems MUST be formatted
according to the Canonical "Canonical Event Log format
[Canonical-Event-Log]. Format" [CEL]. UEFI-based
systems MUST use the TCG UEFI BIOS event log [PC-Client-EFI-TPM-1.2]
[PC-CLIENT-EFI-TPM-1.2] for TPM1.2 systems, TPM 1.2 systems and
TCG the "TCG PC
Client Specific Platform Firmware Profile [PC-Client-BIOS-TPM-2.0] Profile"
[PC-CLIENT-BIOS-TPM-2.0] for TPM2.0. TPM 2.0 systems.
5. Quotes MUST be retrieved from the TPM according to the TCG TAP
Trusted Attestation Protocol Information Model (TAP IM) [TAP] and
the CHARRA Challenge-Response-based Remote Attestation (CHARRA) YANG
model
[I-D.ietf-rats-yang-tpm-charra]. [RFC9684]. 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, TPM and MUST be
retrieved in a way that does not invalidate the signature, to
preserve the trust model. The
[I-D.ietf-rats-yang-tpm-charra] CHARRA YANG model [RFC9684] is
used for this purpose. (See Section 5 5, Security Considerations).
6. Reference Values MUST be encoded as defined in the TCG RIM
document [RIM], typically using SWID [SWID], Software Identification (SWID)
tags [SWID] [NIST-IR-8060] or
CoSWID Concise SWID (CoSWID) tags [I-D.ietf-sacm-coswid].
[RFC9393].
2.4. RIV Simplifying Assumptions
This document makes the following simplifying assumptions to reduce
complexity:
* The product to be attested MUST be shipped by the equipment vendor
with both an a DevID as specified by IEEE Std 802.1AR Device Identity and an Initial
Attestation Key (IAK), IAK,
with certificates in place. The IAK certificate must contain the
same identity information as the DevID (specifically, the same
subject and subjectAltName (if used), signed by the manufacturer).
The IAK is a type of key that can be used to sign a TPM Quote, but
not other objects (i.e., it's marked as a TCG "Restricted" key;
this convention is described in "TPM 2.0 Keys for Device Identity
and Attestation"
[Platform-DevID-TPM-2.0]). [PLATFORM-DEVID-TPM-2.0]). For network
equipment, which is generally non-privacy-sensitive, not privacy sensitive, shipping a
device with both an IDevID and an IAK already provisioned
substantially simplifies initial startup.
* IEEE Std 802.1AR does not require a product serial number as part
of the subject, but RIV-compliant devices MUST include their
serial numbers in the DevID/IAK certificates to simplify tracking
logistics for network equipment users. All other optional 802.1AR
fields remain optional in RIV.
It should be noted that 802.1AR the use of X.509 certificate fields as
specified by IEEE Std 802.1AR is not identical to those descsribed that described
in [RFC6125] [RFC9525] for representation of application service identity.
* The product MUST be equipped with an RTM, a Root of Trust for Measurement
(RTM), Root of Trust for Storage
Storage, and a Root of Trust for Reporting (as defined in [SP800-155])
[SP800-155]), which together are capable of conforming to the TCG Trusted Attestation Protocol Information Model
TAP IM [TAP].
* The authorized software supplier MUST make available Reference
Values in the form of signed SWID or CoSWID tags.
2.4.1. Reference Integrity Manifests (RIMs)
[I-D.ietf-rats-yang-tpm-charra]
[RFC9684] 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 Values, in signed Reference Integrity Manifests, RIMs, that may be used by a Verifier to
determine if evidence shows known good, known bad 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, single threaded and executed exactly
once, in a known sequence, before any results could can be reported.
In this case, while the method for computing the hash and
extending relevant PCRs may be complicated, the net result is
that the software (more likely, firmware) vendor will have one
known good PCR value that "should" be present in the relevant
PCRs after the box has booted. In this case, the signed
reference measurement could simply list the expected hashes for
the given version. However, a RIM that contains the intermediate
hashes can be useful in debugging cases where the expected final
hash is not the one reported.
2. Measurements taken later in operation of the system, once an OS
has started (for example, Linux IMA [IMA]), may be more complex,
with unpredictable "final" PCR values. In this case, the
Verifier must have enough information to reconstruct the expected
PCR values from logs and signed reference measurements from a
trusted authority.
In both cases, the expected values can be expressed as signed SWID or
CoSWID tags, but the SWID structure in the second case is somewhat
more complex, as reconstruction of the extended hash in a PCR may
involve thousands of files and other objects.
TCG has published an information model defining elements of Reference
Integrity Manifests RIMs
under the title TCG "TCG Reference Integrity Manifest (RIM) Information Model
Model" [RIM]. This information model outlines how SWID tags should
be structured to allow attestation, and it defines "bundles" of SWID
tags that may be needed to describe a complete software release. The
RIM contains metadata relating to the software release it belongs to,
plus hashes for each individual file or other object that could be
attested.
Many network equipment vendors use a UEFI BIOS to launch their
network operating system. These vendors may want to also use the TCG
"TCG PC Client Reference Integrity Measurement specification
[PC-Client-RIM], Manifest Specification"
[PC-CLIENT-RIM], which focuses specifically on a SWID-compatible
format suitable for expressing measurement values expected from a
UEFI BIOS.
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 recorded. However, to make sense of the
quote in cases where several events are extended into one PCR PCR, an
event log that identifies which software modules contributed which
values to the quote during startup must also be provided. When
required, the log MUST contain enough information to demonstrate its
integrity by allowing exact reconstruction of the digest conveyed in
the signed quote (that is, calculating the hash of all the hashes in
the log should produce the same values as contained in the PCRs; if
they don't match, the log may have been tampered with. See Section 9.1).
Appendix A.1).
There are multiple event log formats which that may be supported as viable
formats of Evidence between the Attester and Verifier, but Verifier; however, to
simplify interoperability, RIV focuses on just three:
*
1. TCG UEFI BIOS event log for TPM 2.0 (TCG ("TCG PC Client Specific
Platform Firmware Profile) [PC-Client-BIOS-TPM-2.0]
* Profile Specification")
[PC-CLIENT-BIOS-TPM-2.0]
2. TCG UEFI BIOS event log for TPM 1.2 (TCG ("TCG EFI Platform
Specification
Specification" for TPM Family 1.1 or 1.2, Section 7)
[PC-Client-EFI-TPM-1.2]
*
[PC-CLIENT-EFI-TPM-1.2]
3. TCG Canonical "Canonical Event Log [Canonical-Event-Log] Format" [CEL]
3. Standards Components
3.1. Prerequisites for RIV
The Reference Interaction Model for Challenge-Response-based Remote
Attestation ([I-D.birkholz-rats-reference-interaction-model]) ([RATS-INTERACTION-MODELS]) is based on the standard
roles defined in [I-D.ietf-rats-architecture]. [RFC9334]. However, additional prerequisites have
been established to allow for interoperable implementations of RIV
use case implementations. cases. These prerequisites are intended to provide sufficient
context information so that the Verifier can acquire and evaluate
measurements collected by the Attester.
3.1.1. Unique Device Identity
A secure Device Identity (DevID) DevID in the form of an IEEE 802.1AR a DevID certificate as specified by IEEE Std
802.1AR [IEEE-802-1AR] must be provisioned in the Attester's TPMs.
3.1.2. Keys
The Attestation Key (AK) AK and certificate must also be provisioned on the Attester
according to [Platform-DevID-TPM-2.0], [PLATFORM-DEVID-TPM-2.0] or
[Platform-ID-TPM-1.2]. [PLATFORM-ID-TPM-1.2].
It MUST be possible for the Verifier to determine that the Attester's
Attestation keys
AKs are resident in the same TPM as its DevID keys (see Section 2.2
and Section 5 5, Security Considerations).
3.1.3. Appraisal Policy for Evidence
As noted in Section 2.3, the Verifier may obtain Reference Values
from several sources. In addition, administrators may make
authorized, site-specific changes (e.g. (e.g., keys in key databases) that
could impact attestation results. As such, there could be conflicts,
omissions
omissions, or ambiguities between some Reference Values and collected
Evidence.
The Verifier MUST have an Appraisal Policy for Evidence to evaluate
the significance of any discrepancies between different reference
sources, or between reference values Reference Values and evidence from logs and
quotes. While there must be an Appraisal Policy, this document does
not specify the format or mechanism to convey the intended policy,
nor does RIV specify mechanisms by which the results of applying the
policy are communicated to the Relying Party.
3.2. Reference Model for Challenge-Response
Once the prerequisites for RIV are met, a Verifier is able to acquire
Evidence from an Attester. The following diagram Figure 3 illustrates a RIV information
flow between a Verifier and an Attester, derived from Section 7.1 of [I-D.birkholz-rats-reference-interaction-model].
[RATS-INTERACTION-MODELS]. In this diagram, each event with its
input and output parameters is shown as "Event(input-params)=>(outputs)". Event "Event(input-
params)=>(outputs)". The event times shown correspond to the time
types described within Appendix A of
[I-D.ietf-rats-architecture]: [RFC9334]:
.----------. .-----------------------.
| Attester | | Relying Party/Verifier |
'----------' '------------------------'
time(VG) |
generateClaims(attestingEnvironment) |
| => claims, eventLogs |
| |
| time(NS)
| <-- requestAttestation(handle, authSecIDs, claimSelection) |
| |
time(EG) |
collectClaims(claims, claimSelection) |
| => collectedClaims |
| |
generateEvidence(handle, authSecIDs, collectedClaims) |
| => evidence |
| time(RG,RA)
| evidence, eventLogs -------------------------------------> |
| |
| appraiseEvidence(evidence, eventLogs, refValues)
| attestationResult <= |
| |
~ ~
| time(RX)
Figure 4: 3: IETF Attestation Information Flow
*
Step 1 (time(VG)): One or more Attesting Network Device 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 described in
[I-D.birkholz-rats-network-device-subscription]
[RATS-NET-DEV-SUB] could.
*
Step 2 (time(NS)): The Verifier generates a unique random nonce
("number used once"), once") and makes a request for one or more PCRs from
an Attester. For interoperability, this must be accomplished as
specified in the "A YANG Data Model for Challenge-Response-based Challenge-Response-Based
Remote Attestation (CHARRA) Procedures using TPMs
[I-D.ietf-rats-yang-tpm-charra]. TPM1.2 Using Trusted Platform
Modules (TPMs)" [RFC9684]. Both TPM 1.2 and TPM2.0 both TPM 2.0 allow nonces
as large as the operative digest size (i.e., 20 or 32 bytes; see [TPM1.2]
[TPM-1.2] Part 2, Section 5.5 5.5, and [TPM2.0] [TPM-2.0] Part 2,
Section 10.4.4).
*
Step 3 (time(EG)): On the Attester, measured values are retrieved
from the Attester's TPM. This requested PCR evidence, evidence along with
the Verifier's nonce, nonce is called a Quote, Quote and is signed by the Attestation
Key (AK) AK
associated with the DevID. Quotes are retrieved according to the
CHARRA YANG model [I-D.ietf-rats-yang-tpm-charra]. [RFC9684]. At the same time, the Attester
collects log evidence showing the values have been extended into
that PCR. Section 9.1 Appendix A.1 gives more detail on how this works, including works and
includes references to the structure and contents of quotes in TPM
documents.
*
Step 4: Collected The collected Evidence is passed from the Attester to the
Verifier
*
Verifier.
Step 5 (time(RG,RA)): The Verifier reviews the Evidence and takes
action as needed. As the interaction between Relying Party and
Verifier is out of scope for RIV, this can be described as one
step.
-
* If the signature covering TPM Evidence is not correct, the
device SHOULD NOT be trusted.
-
* If the nonce in the response doesn't match the Verifier's
nonce, the response may be a replay, and the device SHOULD NOT
be trusted.
-
* If the signed PCR values do not match the set of log entries
which
that have extended a particular PCR, the device SHOULD NOT be
trusted.
-
* If the log entries that the Verifier considers important do not
match known good values, the device SHOULD NOT be trusted. We
note that the process of collecting and analyzing the log can
be omitted if the value in the relevant PCR is already a known-
good value.
-
* If the set of log entries are not seen as acceptable by the
Appraisal Policy for Evidence, the device SHOULD NOT be
trusted.
-
* If time(RG)-time(NS) is greater than the Appraisal Policy for
Evidence's threshold for assessing freshness, the Evidence is
considered stale and SHOULD NOT be trusted.
3.2.1. Transport and Encoding
Network Management systems may retrieve signed PCR based PCR-based Evidence
using NETCONF or RESTCONF with [I-D.ietf-rats-yang-tpm-charra]. [RFC9684]. In either case, implementatations
implementations must do so using a secure tunnel.
Log Evidence MUST be retrieved via log interfaces specified in
[I-D.ietf-rats-yang-tpm-charra].
[RFC9684].
3.3. Centralized vs vs. Peer-to-Peer
Figure 4 above 3 assumes that the Verifier is trusted, while the Attester is
not. In a Peer-to-Peer peer-to-peer application such as two routers negotiating a
trust relationship, the two peers can each ask the other to prove
software integrity. In this application, the information flow is the
same, but each side plays a role both as an Attester and a Verifier.
Each device issues a challenge, and each device responds to the
other's challenge, as shown in Figure 5. 4. 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 an appraisal
policy for Evidence evidence for each possible peer device so that each device
has everything needed for remote attestation, without having to
resort to a central authority.
+---------------+ +---------------+
| RefVal | | RefVal |
| Provider A | | Provider B |
| Firmware | | Firmware |
| Configuration | | Configuration |
| Authority | | Authority |
| | | |
+---------------+ +---------------+
| |
| |Step 0B
| +------------+ +------------+ |
| | | Step 1 | | | \
| | Attester |<------>| Verifier | | |
| | |<------>| | | | Router B
+------>| | Step 2 | | | |- Challenges
Step 0A| | | | | | Router A
| |------->| | | |
|- Router A -| Step 3 |- Router B -| | /
| | | | |
| | | | |
| | Step 1 | | | \
| Verifier |<------>| Attester |<-+ | Router A
| |<------>| | |- Challenges
| | Step 2 | | | Router B
| | | | |
| |<-------| | |
+------------+ Step 3 +------------+ /
Figure 5: 4: Peer-to-Peer Attestation Information Flow
In this application, each device may need to be equipped with signed
RIMs to act as an Attester, and also to allow each device to act as a
Verifier, each may need to be equipped with an Appraisal Policy for
Evidence and a selection of trusted X.509 root certificates, to allow the
device to act as a Verifier. certificates also. 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 the
Extensible Authentication Protocol (EAP) [RFC3748] or LLDP [LLDP] Link Layer
Discovery Protocol (LLDP) [LLDP], are suitable methods for such an
exchange. Details of peer-to-peer operation are out of scope for
this document.
4. Privacy Considerations
Network equipment, such as routers, switches switches, and firewalls, has a
key role to play in guarding the privacy of individuals using the
network. Network equipment generally adheres to several rules to
protect privacy:
* Packets passing through the device must not be sent to
unauthorized destinations. For example:
- Routers often act as Policy Enforcement Points, where
individual subscribers may be checked for authorization to
access a network. Subscriber login information must not be
released to unauthorized parties.
- Network equipment is often called upon to block access to
protected resources from unauthorized users.
* Routing information, such as the identity of a router's peers,
must not be leaked to unauthorized neighbors.
* If configured, encryption and decryption of traffic must be
carried out reliably, while protecting keys and credentials.
Functions that protect privacy are implemented as part of each layer
of hardware and software that makes up the networking device. In
light of these requirements for protecting the privacy of users of
the network, the network equipment must identify itself, and its boot
configuration and measured device state (for example, PCR values), to
the equipment's administrator, administrator so there's no uncertainty as to what about the
device's function each device and configuration is configured to carry out. configuration. Attestation is a component that
allows the administrator to ensure that the network provides
individual and peer privacy guarantees, even though the device itself
may not have a right to keep its identity secret.
See [NetEq] [NET-EQ] for more context on privacy in networking devices.
While attestation information from network devices is not likely to
contain privacy-sensitive content regarding network users,
administrators may want to keep attestation records confidential to
avoid disclosing versions of software loaded on the device,
information which is
information that could facilitate attacks against known
vulnerabilities.
5. Security Considerations
Specifications such as TLS [RFC8446] (TLS) and YANG [RFC7950] (YANG) contain
considerable advice on keeping network-connected systems secure.
This section outlines specific risks and mitigations related to
attestation.
Attestation Evidence obtained by the RIV procedure is subject to a
number of attacks:
* Keys may be compromised.
* A counterfeit device may attempt to impersonate (spoof) a known
authentic device.
* Person-in-the-middle attacks may be used by a compromised device
to attempt to deliver responses that originate in an authentic
device.
* Replay attacks may be attempted by a compromised device.
5.1. Keys Used in RIV
Trustworthiness of RIV attestation depends strongly on the validity
of keys used for identity and attestation reports. RIV takes full
advantage of TPM capabilities to ensure that evidence can be trusted.
Two sets of key-pairs key pairs are relevant to RIV attestation:
* A DevID key-pair key pair is used to certify the identity of the device in
which the TPM is installed.
* An Attestation Key-pair (AK) AK key pair is used to certify attestation Evidence (called 'quotes' in TCG documents), used (i.e.,
quotes) and to provide evidence for integrity of the software on the device
software.
TPM practices usually require that these keys be different, as a way
of ensuring different to ensure
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, TPM
and cannot be retrieved externally, even by a trusted party. To
ensure that's the case, specification-compliant private/public key- key
pairs are generated inside the TPM, where they are never exposed, exposed and
cannot be extracted (See [Platform-DevID-TPM-2.0]). (see [PLATFORM-DEVID-TPM-2.0]).
Keeping keys safe is a critical enabler of trustworthiness, but it's
just part of attestation security; knowing which keys are bound to
the device in question is just as important in an environment where
private keys are never exposed.
While there are many ways to manage keys in a TPM (see
[Platform-DevID-TPM-2.0]),
[PLATFORM-DEVID-TPM-2.0]), RIV includes support for "zero touch"
provisioning (also known as zero-touch zero touch onboarding) of fielded devices
(e.g., Secure ZTP, SZTP [RFC8572]), where keys which that have predictable trust
properties are provisioned by the device vendor.
Device identity in RIV is based on DevID defined by IEEE 802.1AR Device Identity
(DevID). Std 802.1AR.
This specification provides several elements:
* A DevID requires a unique key pair for each device, accompanied by
an X.509 certificate, certificate.
* The private portion of the DevID key is to be stored in the
device, in a manner that provides confidentiality (Section 6.2.5
[IEEE-802-1AR])
of [IEEE-802-1AR]).
The X.509 certificate contains several components:
* The public part of the unique DevID key assigned to that device
allows a challenge of identity.
* An identifying string that's unique to the manufacturer of the
device. This is normally the serial number of the unit, which
might also be printed on a label on the device.
* The certificate must be signed by a key traceable to the
manufacturer's root key.
With these elements, the device's manufacturer and serial number can
be identified by analyzing the DevID certificate plus the chain of
intermediate certificates leading back to the manufacturer's root
certificate. As is conventional in TLS or SSH connections, a random
nonce must be signed by the device in response to a challenge,
proving possession of its DevID private key.
RIV uses the DevID to validate a TLS or SSH connection to the device
as the attestation session begins. Security of this process derives
from TLS or SSH security, with the DevID, containing which contains a device
serial number, providing proof that the session terminates on the
intended device. See [RFC8446], [RFC8446] [RFC4253].
Evidence of software integrity is delivered in the form of a quote
that is signed by the TPM itself, itself and accompanied by an IAK
certificate containing the same identity information as the DevID.
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 for ensuring that
blocks of code that are taking measurements have been verified before
execution (see Figure 1).
Requiring measurements of the operating software to be signed by a
key known only to the TPM also removes the need to trust the device's
operating software (beyond the first measurement in the RTM; see
below);
below). If malicious software makes any changes to a quote in the quote, generated and signed by the TPM
itself, made by malicious
device software, itself, or in the path back to the Verifier, will invalidate the signature on
the quote. quote will be invalidated.
A critical feature of the YANG model described in
[I-D.ietf-rats-yang-tpm-charra] [RFC9684] is the
ability to carry TPM data structures in their TCG-defined 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 reduce redundant information, or add another layer of
signing using external keys, the implementation MUST preserve the TPM signing,
signing so that tampering anywhere in the path between the TPM itself
and the Verifier can be detected.
5.2. Prevention of Spoofing and Person-in-the-Middle Attacks
Prevention of spoofing attacks against attestation systems is also
important. There are several cases to consider:
* The entire device could be spoofed. If the Verifier goes to
appraise a specific Attester, it might be redirected to a
different Attester.
* A compromised device could have a valid DevID, but substitute a
quote from a knonwn-good device, known-good device instead of returning its own, as
described in [RFC6813].
* A device with a compromised OS could return a fabricated quote
providing spoofed attestation Evidence.
Use of the 802.1AR Device Identity (DevID) DevID in the TPM provides protection against the
case of a spoofed device, device by ensuring that the Verifier's TLS or SSH
session is in fact terminating on the right device.
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 key, known as an AK, that's different from the DevID, called an Attestation Key
(AK). DevID.
Given separate Attestation and DevID keys, the binding between the AK
and the same device must also be proven to prevent a person-in-the-
middle attack (e.g., the 'Asokan Attack' "Asokan Attack" [RFC6813]).
This is accomplished in RIV through use of an AK certificate with the
same elements as the DevID (same manufacturer's serial number, number and
signed by the same manufacturer's key), but containing the device's
unique AK public key instead of the DevID public key. This binding
between DevID and AK certificates is critical to reliable
attestation.
The TCG document TPM "TPM 2.0 Keys for Device Identity and Attestation
[Platform-DevID-TPM-2.0] Attestation"
[PLATFORM-DEVID-TPM-2.0] specifies OIDs for Attestation Certificates
that allow the CA to mark a key as specifically known to be an
Attestation key. AK.
These two key-pairs key pairs and certificates are used together:
* The DevID is used to validate a TLS connection terminating on the
device with a known serial number.
* The AK is used to sign attestation quotes, providing which provides proof
that the attestation evidence comes from the same device.
5.3. Replay Attacks
Replay attacks, where the results of a previous attestation are
submitted in response to subsequent requests, are usually prevented
by the inclusion of a random nonce in the request to the TPM for a
quote. Each request from the Verifier includes a new random number
(a nonce). The resulting quote signed by the TPM contains the same
nonce, allowing which allows the Verifier to determine freshness, 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] "Time-Based Uni-Directional Attestation"
[RATS-TUDA] provides an alternate mechanism to verify freshness
without requiring a request/response cycle.
5.4. Owner-Signed Keys
Although device manufacturers must pre-provision devices with easily
verified DevID and AK certificates if zero-touch provisioning SZTP such as described in
[RFC8572] is to be supported, use of those credentials is not
mandatory. IEEE Std 802.1AR incorporates the idea of an Initial
Device ID (IDevID), IDevID,
which is provisioned by the manufacturer, and a Local
Device ID (LDevID) LDevID, which is
provisioned by the owner of the device. RIV and
[Platform-DevID-TPM-2.0] extends
[PLATFORM-DEVID-TPM-2.0] extend that concept by defining an Initial
Attestation Key (IAK) IAK and Local Attestation Key (LAK)
LAK with the same properties.
Device owners can use any method to provision the Local local credentials.
* The TCG document [Platform-DevID-TPM-2.0] [PLATFORM-DEVID-TPM-2.0] shows how the initial
Attestation keys IAKs can
be used to certify LDevID and LAK keys. Use The 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] [PROV-TPM-2.0] also contains
guidance on provisioning
Local local identity keys in TPM 2.0. Owners
should follow the same practice of binding Local DevID LDevID and Local AK LAK as the
manufacturer would for IDevID and IAK. See Section Section 2.2.
* Device owners, however, can use any other mechanism they want want,
including physical inspection and programming in a secure
location, to assure themselves that local identity certificates
are inserted into the intended device, including physical inspection and
programming in a secure location, device if they prefer to avoid
placing trust in the manufacturer-provided keys.
Clearly, local keys can't be used for secure Zero Touch provisioning; SZTP; installation of the local
keys can only be done by some process that runs before the device is
installed for network operation, or by using procedures such as those
outlined in Bootstrapping Remote Secure Key Infrastructure (BRSKI)
[RFC8995].
On the other end of the device life cycle, lifecycle, provision should be made to
wipe local keys when a device is decommissioned, decommissioned to indicate that the
device is no longer owned by the enterprise. The manufacturer's
Initial
initial identity keys must be preserved, as they contain no
information that's not already printed on the device's serial number
plate.
5.5. Other Factors for Trustworthy Operation
In addition to the trustworthy provisioning of keys, RIV depends on a
number of other factors for trustworthy operation.
* Secure identity depends on mechanisms to prevent per-device secret
keys from being compromised. The TPM provides this capability as
a Root of Trust for Storage.
* Attestation depends on an unbroken chain of measurements, starting
from the very first measurement. See Section 9.1 Appendix A.1 for background
on TPM practices.
* That first measurement is made by code called the Root of Trust
for Measurement, RTM, typically
done by trusted firmware stored in boot flash. Mechanisms for
maintaining the trustworthiness of the RTM are out of scope for
RIV, but could include immutable firmware, signed updates, or a
vendor-specific hardware verification technique. See Section 9.2 Appendix A.2
for background on roots Roots of trust. Trust.
* The device owner SHOULD provide some level of physical defense for
the device. If a TPM that has already been programmed with an
authentic DevID is stolen and is inserted into a counterfeit
device, attestation of that counterfeit device may become
indistinguishable from an authentic device.
RIV also depends on reliable Reference Values, as expressed by the
RIM [RIM]. The definition of trust procedures for RIMs is out of
scope for RIV, and the device owner is free to use any policy to
validate a set of reference measurements. It should also be noted
that, while RIV can provide a reliable indication that a known
software package is in use by the device, device and that the package has not
been tampered, tampered with, it is the device owner's responsibility to
determine that it's the correct package for the application.
RIMs may be conveyed either out-of-band or in-band, in-band as part of the
attestation process (see Section 3.1.3). But However, for network
devices, where software is usually shipped as a self-contained
package, RIMs signed by the manufacturer and delivered in-band may be
more convenient for the device owner.
The validity of RIV attestation results is also influenced by
procedures used to create Reference Values:
* While the RIM itself is signed, supply-chains supply chains SHOULD be carefully
scrutinized to ensure that the values are not subject to
unexpected manipulation prior to signing. Insider-attacks Insider attacks against
code bases and build chains are particularly hard to spot.
* Designers SHOULD guard against hash collision attacks. Reference
Integrity Manifests RIMs often
give hashes for large objects of indeterminate size; if 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 TPM 1.2 only uses SHA-1 hashes only, hashes, which have been
shown to be susceptible to collision attack.
TPM2.0 TPM 2.0 will produce
quotes with SHA-256, which so far has resisted such attacks.
Consequently, RIV implementations SHOULD use
TPM2.0. TPM 2.0.
6. IANA Considerations
This document has no IANA actions.
7. Conclusion
TCG technologies can play an important part in the implementation of
Remote Integrity Verification.
RIV. Standards for many of the components needed for implementation
of RIV already exist:
* Platform identity can be based on IEEE 802.1AR Device Identity, DevID, coupled with
careful supply-chain management by the manufacturer.
* Complex supply chains can be certified using TCG Platform
Certificates [Platform-Certificates]. [PLATFORM-CERTS].
* The TCG TAP mechanism coupled with [I-D.ietf-rats-yang-tpm-charra] [RFC9684] can be used to
retrieve attestation evidence.
* Reference Values must be conveyed from the software authority
(e.g., the manufacturer) in Reference Integrity Manifests, RIMs to the system in which
verification will take place. IETF and TCG SWID and CoSWID work ([I-D.ietf-sacm-coswid],
([RFC9393] [RIM]) forms the basis for this function.
10.
8. References
10.1.
8.1. Normative References
[Canonical-Event-Log]
[CEL] Trusted Computing Group, "Canonical Event Log Format Format",
Version 1.0 1.0, Revision .41, 0.41, February 25, 2022", December
2020, <https://trustedcomputinggroup.org/resource/
canonical-event-log-format/>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
15, 8 February 2022, <https://www.ietf.org/archive/id/
draft-ietf-rats-architecture-15.txt>.
[I-D.ietf-rats-yang-tpm-charra]
Birkholz, H., Eckel, M., Bhandari, S., Voit, E., Sulzen,
B., (Frank), L. X., Laffey, T., and G. C. Fedorkow, "A
YANG Data Model for Challenge-Response-based Remote
Attestation Procedures using TPMs", Work in Progress,
March 2022, <https://www.ietf.org/archive/id/draft-ietf-
rats-yang-tpm-charra-18.txt>.
[I-D.ietf-sacm-coswid]
Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags", Work
in Progress, Internet-Draft, draft-ietf-sacm-coswid-21, 7
March 2022, <https://www.ietf.org/archive/id/draft-ietf-
sacm-coswid-21.txt>.
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_IWG_CEL_v1_r0p41_pub.pdf>.
[IEEE-802-1AR]
Seaman, M., "802.1AR-2018 - IEEE
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Secure Device Identity, Identity", IEEE
Computer Society", Std 802.1AR-2018,
DOI 10.1109/IEEESTD.2018.8423794, August 2018. 2018,
<https://doi.org/10.1109/IEEESTD.2018.8423794>.
[IMA] dsafford, kds_etu, mzohar, reinersailer, and serge_hallyn,
"Integrity Measurement Architecture", June 2019,
<https://sourceforge.net/p/linux-ima/wiki/Home/>.
[PC-Client-BIOS-TPM-2.0] The kernel development community, "dm-ima", Linux Kernel
6.11, 15 September 2024,
<https://www.kernel.org/doc/html/v6.11/admin-guide/device-
mapper/dm-ima.html>. The latest version can be found at
https://docs.kernel.org/admin-guide/device-mapper/dm-
ima.html.
[PC-CLIENT-BIOS-TPM-2.0]
Trusted Computing Group, "PC "TCG PC Client Specific Platform
Firmware Profile Specification Specification", Family "2.0", Level 00
Revision 1.05 00,
Version 1.05, Revision 23, May 7, 2021", May 2021,
<https://trustedcomputinggroup.org/resource/pc-client-
specific-platform-firmware-profile-specification/>.
[PC-Client-EFI-TPM-1.2]
[PC-CLIENT-EFI-TPM-1.2]
Trusted Computing Group, "TCG EFI Platform Specification
for Specification",
For TPM Family 1.1 or 1.2, Specification Version 1.22, Revision 15", 15,
January 2014,
<https://trustedcomputinggroup.org/resource/tcg-efi-
platform-specification/>.
[PC-Client-RIM] <https://trustedcomputinggroup.org/resource/
tcg-efi-platform-specification/>.
[PC-CLIENT-RIM]
Trusted Computing Group, "TCG PC Client Reference
Integrity Manifest Specification, v1.04, Nov 4, 2020",
December 2019,
<https://trustedcomputinggroup.org/resource/tcg-pc-client-
reference-integrity-manifest-specification/>.
[Platform-DevID-TPM-2.0] Specification", Version 1.04, November
2020, <https://trustedcomputinggroup.org/resource/tcg-pc-
client-reference-integrity-manifest-specification/>.
[PLATFORM-DEVID-TPM-2.0]
Trusted Computing Group, "TPM 2.0 Keys for Device Identity
and Attestation, Specification Attestation", Version 1.0, 1.00, Revision 2",
September 2020, 12, October 2021,
<https://trustedcomputinggroup.org/resource/tpm-2-0-keys-
for-device-identity-and-attestation/>.
[Platform-ID-TPM-1.2]
[PLATFORM-ID-TPM-1.2]
Trusted Computing Group, "TPM "TCG Infrastructure WG TPM Keys
for Platform Identity for TPM 1.2, 1.2", Specification Version
1.0, Revision 3", 3, August 2015, <https://trustedcomputinggroup.org/resource/
tpm-keys-for-platform-identity-for-tpm-1-2-2/>.
<https://trustedcomputinggroup.org/resource/tpm-keys-for-
platform-identity-for-tpm-1-2-2/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC9334] Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote ATtestation procedureS (RATS)
Architecture", RFC 9334, DOI 10.17487/RFC9334, January
2023, <https://www.rfc-editor.org/info/rfc9334>.
[RFC9393] Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags",
RFC 9393, DOI 10.17487/RFC9393, June 2023,
<https://www.rfc-editor.org/info/rfc9393>.
[RFC9684] Birkholz, H., Eckel, M., Bhandari, S., Voit, E., Sulzen,
B., Xia, L., Laffey, T., and G. Fedorkow, "A YANG Data
Model for Challenge-Response-Based Remote Attestation
(CHARRA) Procedures Using Trusted Platform Modules
(TPMs)", RFC 9684, DOI 10.17487/RFC9684, October 2024,
<https://www.rfc-editor.org/info/rfc9684>.
[RIM] Trusted Computing Group, "TCG Reference Integrity Manifest
(RIM) Information Model, v1.0, Model", Version 1.01, Revision 0.16, Nov 12,
2020", June 2019,
November 2020,
<https://trustedcomputinggroup.org/resource/tcg-reference-
integrity-manifest-rim-information-model/>.
[SWID] The International Organization for Standardization/
International Electrotechnical Commission, ISO/IEC, "Information
Technology Software Asset Management technology - IT asset management -
Part 2: Software
Identification Tag, ISO/IEC 19770-2", identification tag", ISO/
IEC 19770-2:2015, 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, 1.0", Version 1.0, Revision 0.36", 0.36, October
2018, <https://trustedcomputinggroup.org/resource/tcg-tap-
information-model/>.
10.2. <https://trustedcomputinggroup.org/wp-
content/uploads/
TNC_TAP_Information_Model_v1.00_r0.36-FINAL.pdf>.
8.2. Informative References
[AK-Enrollment]
[AIK-ENROLL]
Trusted Computing Group, "TCG Infrastructure Working Group
-
A CMC Profile for AIK Certificate Enrollment Enrollment", Version
1.0, Revision 7", 7, March 2011,
<https://trustedcomputinggroup.org/resource/tcg-
infrastructure-working-group-a-cmc-profile-for-aik-
certificate-enrollment/>.
[I-D.birkholz-rats-network-device-subscription]
Birkholz, H., Voit, E., and W. Pan, "Attestation Event
Stream Subscription", Work in Progress, Internet-Draft,
draft-birkholz-rats-network-device-subscription-03, 17
August 2021, <https://www.ietf.org/archive/id/draft-
birkholz-rats-network-device-subscription-03.txt>.
[I-D.birkholz-rats-reference-interaction-model]
Birkholz, H., Eckel, M., Newton, C., and L. Chen,
"Reference Interaction Models
[IEEE-802.1AE]
IEEE, "IEEE Standard for Remote Attestation
Procedures", Work in Progress, Internet-Draft, draft-
birkholz-rats-reference-interaction-model-03, 7 July 2020,
<https://www.ietf.org/archive/id/draft-birkholz-rats-
reference-interaction-model-03.txt>.
[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I. E., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-06, 12
January 2022, <https://www.ietf.org/archive/id/draft-
birkholz-rats-tuda-06.txt>.
[I-D.ietf-rats-eat]
Lundblade, L., Mandyam, G., and J. O'Donoghue, "The Entity
Attestation Token (EAT)", Work in Progress, Internet-
Draft, draft-ietf-rats-eat-12, 24 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-rats-eat-
12.txt>.
[I-D.richardson-rats-usecases]
Richardson, M., Wallace, C., Local and W. Pan, "Use cases for
Remote Attestation common encodings", Work in Progress,
November 2020, <https://www.ietf.org/archive/id/draft-
richardson-rats-usecases-08.txt>.
[IEEE-802.1AE]
Seaman, M., "802.1AE MAC Security (MACsec)", 2018,
<https://1.ieee802.org/security/802-1ae/>.
[IEEE-802.1X]
IEEE Computer Society, "802.1X-2020 metropolitan area
networks - Media Access Control (MAC) Security", IEEE Std
802.1AE-2018, DOI 10.1109/IEEESTD.2018.8585421, 2018,
<https://doi.org/10.1109/IEEESTD.2018.8585421>.
[IEEE-802.1X]
IEEE, "IEEE Standard for Local and Metropolitan Area Networks--Port-Based
Networks - Port-Based Network Access Control", IEEE Std
802.1X-2020, DOI 10.1109/IEEESTD.2020.9018454, February
2020,
<https://standards.ieee.org/standard/802_1X-2020.html>. <https://doi.org/10.1109/IEEESTD.2020.9018454>.
[LLDP] IEEE Computer Society, "802.1AB-2016 - IEEE IEEE, "IEEE Standard for Local and metropolitan area
networks - Station and Media Access Control Connectivity
Discovery", IEEE Std 802.1AB-2016,
DOI 10.1109/IEEESTD.2016.7433915, March 2016,
<https://standards.ieee.org/standard/802_1AB-2016.html>.
[NetEq]
<https://doi.org/10.1109/IEEESTD.2016.7433915>.
[NET-EQ] Trusted Computing Group, "TCG Guidance for Securing
Network Equipment, Equipment Using TCG Technology", Version 1.0,
Revision 29", 29, January 2018, <https://trustedcomputinggroup.org/resource/tcg-
guidance-securing-network-equipment/>.
<https://trustedcomputinggroup.org/resource/tcg-guidance-
securing-network-equipment/>.
[NIST-IR-8060]
National Institute for Standards
Waltermire, D., Cheikes, B. A., Feldman, L., and Technology, G. Witte,
"Guidelines for the Creation of Interoperable Software
Identification (SWID) Tags", NIST NISTIR 8060,
DOI 10.6028/NIST.IR.8060, April 2016,
<https://nvlpubs.nist.gov/nistpubs/ir/2016/
NIST.IR.8060.pdf>.
[Platform-Certificates]
[PLATFORM-CERTS]
Trusted Computing Group, "TCG Platform Attribute
Credential Profile, Profile", Specification Version 1.0, Revision
16",
16, January 2018,
<https://trustedcomputinggroup.org/resource/tcg-platform-
attribute-credential-profile/>.
[Provisioning-TPM-2.0]
[PROV-TPM-2.0]
Trusted Computing Group, "TCG TPM v2.0 Provisioning
Guidance,
Guidance", Version 1.0, Revision 1.0", 1.0, March 2015, 2017,
<https://trustedcomputinggroup.org/wp-content/uploads/TCG-
TPM-v2.0-Provisioning-Guidance-Published-v1r1.pdf>.
[RATS-EAT] Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
Wallace, "The Entity Attestation Token (EAT)", Work in
Progress, Internet-Draft, draft-ietf-rats-eat-31, 6
September 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-eat-31>.
[RATS-INTERACTION-MODELS]
Birkholz, H., Eckel, M., Pan, W., and E. Voit, "Reference
Interaction Models for Remote Attestation Procedures",
Work in Progress, Internet-Draft, draft-ietf-rats-
reference-interaction-models-11, 22 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
reference-interaction-models-11>.
[RATS-NET-DEV-SUB]
Birkholz, H., Voit, E., and W. Pan, "Attestation Event
Stream Subscription", Work in Progress, Internet-Draft,
draft-ietf-rats-network-device-subscription-05, 7 July
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
rats-network-device-subscription-05>.
[RATS-TUDA]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-07, 10
July 2022, <https://datatracker.ietf.org/doc/html/draft-
birkholz-rats-tuda-07>.
[RATS-USECASES]
Richardson, M., Wallace, C., and W. Pan, "Use cases for
Remote Attestation common encodings", Work in Progress,
November 2020, <https://datatracker.ietf.org/doc/html/
draft-richardson-rats-usecases-08>.
[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>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[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>.
[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>.
[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>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
[SP800-155]
National Institute of Standards
[RFC9525] Saint-Andre, P. and Technology, R. Salz, "Service Identity in TLS",
RFC 9525, DOI 10.17487/RFC9525, November 2023,
<https://www.rfc-editor.org/info/rfc9525>.
[SP800-155]
NIST, "BIOS Integrity Measurement Guidelines (Draft)",
NIST SP 800-155 (Draft), December 2011,
<https://csrc.nist.gov/csrc/media/publications/sp/800-
155/draft/documents/draft-sp800-155_dec2011.pdf>.
<https://csrc.nist.gov/files/pubs/sp/800/155/ipd/docs/
draft-sp800-155_dec2011.pdf>.
[SP800-193]
National Institute for Standards and Technology, "NIST
Special Publication 800-193: Platform
NIST, "Platform Firmware Resiliency Guidelines", April NIST
SP 800-193, DOI 10.6028/NIST.SP.800-193, May 2018,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-193.pdf>.
[SWID-Gen]
[SWID-GEN] Labs64, Munich, Germany, "SoftWare IDentification (SWID) Tags Generator
(Maven Plugin)", n.d.,
<https://github.com/Labs64/swid-maven-plugin>.
[TCGRoT]
[TCG-RT] Trusted Computing Group, "DRAFT: TCG "TCG Roots of Trust
Specification", October (Draft), Family "1.0", Level 00, Revision
0.20, July 2018,
<https://trustedcomputinggroup.org/wp-content/uploads/ <https://trustedcomputinggroup.org/wp-
content/uploads/
TCG_Roots_of_Trust_Specification_v0p20_PUBLIC_REVIEW.pdf>.
[TPM1.2]
[TPM-1.2] Trusted Computing Group, "TPM 1.2 Main Specification Specification",
Level 2 2, Version 1.2, Revision 116", 116, March 2011,
<https://trustedcomputinggroup.org/resource/tpm-main-
specification/>.
[TPM2.0]
[TPM-2.0] Trusted Computing Group, "Trusted Platform Module Library
Specification,
Library", Family "2.0", Level 00, Revision 01.59",
November 2019,
<https://trustedcomputinggroup.org/resource/tpm-library-
specification/>.
A. 01.83, March
2024, <https://trustedcomputinggroup.org/resource/tpm-
library-specification/>.
Appendix A. Supporting Materials
A.1. Using a TPM for Attestation
The Trusted Platform Module TPM and surrounding ecosystem provide three interlocking
capabilities to enable secure collection of evidence from a remote device, Platform Configuration Registers (PCRs),
device: PCRs, a Quote mechanism, and a standardized Event Log.
Each TPM has at least eight and at most twenty-four PCRs (depending
on the profile and vendor choices), each one large enough to hold one
hash value (SHA-1, SHA-256, and other hash algorithms can be used,
depending on TPM version). PCRs can't be accessed directly from
outside the chip, but the TPM interface provides a way to "extend" a
new security measurement hash into any PCR, a process by which the
existing value in the PCR is hashed with the new security measurement
hash, and the result placed back into the same PCR. The result is a
composite fingerprint comprising the hash of all the security
measurements extended into each PCR since the system was reset.
Every time a PCR is extended, an entry should be added to the
corresponding Event Log. Logs contain the security measurement hash
plus informative fields offering hints as to which event generated
the security measurement. The Event Log itself is protected against
accidental manipulation, but it is implicitly tamper-evident - any tamper-evident: Any
verification process can read the security measurement hash from the
log events, compute the composite value value, and compare that to what
ended up is
in the PCR. If there's no discrepancy, the logs do provide an
accurate view of what was placed into the PCR.
Note that the composite hash-of-hashes recorded in PCRs is order-
dependent, resulting in different PCR values for different ordering
of the same set of events (e.g. (e.g., Event A followed by Event B yields a
different PCR value than B followed by A). For single-threaded code,
where both the events and their order are fixed, a Verifier may
validate a single PCR value, and use the log only to diagnose a
mismatch from Reference Values. However, operating system code is
usually non-deterministic, nondeterministic, meaning that there may never be a single
"known good" PCR value. In this case, the Verifier may have to
verify that the log is correct, and then analyze each item in the log
to determine if it represents an authorized event.
In a conventional TPM Attestation environment, the first measurement
must be made and extended into the TPM by trusted device code (called
the Root of Trust for Measurement, RTM). That first measurement should cover the segment of code
that is run immediately after the RTM, which then measures the next
code segment before running it, and so on, forming an unbroken chain
of trust. See [TCGRoT] [TCG-RT] for more on Mutable vs vs. Immutable roots Roots of trust.
Trust.
The TPM provides another mechanism called a Quote that can read the
current value of the PCRs and package them, along with the Verifier's
nonce, into a TPM-specific data structure signed by an Attestation
private key, known only to the TPM.
As noted above in Section 5 Security Considerations, it's
It's important to note that the Quote data structure is signed inside
the TPM. TPM (see Section 5, Security Considerations). The trust model is
preserved by retrieving the Quote in a way that does not invalidate
the signature, as specified in
[I-D.ietf-rats-yang-tpm-charra]. [RFC9684]. The structure of the
command and response for a quote, including its signature, as
generated by the TPM, can be seen in [TPM1.2] Part 3, Section 16.5, of
[TPM-1.2] and [TPM2.0] Section 18.4.2. 18.4.2 of [TPM-2.0].
The Verifier uses the Quote and Log together. The Quote contains the
composite hash of the complete sequence of security measurement
hashes, signed by the TPM's private Attestation Key. AK. The Log contains a record of
each measurement extended into the TPM's PCRs. By computing the
composite hash of all the measurements, the Verifier can verify the
integrity of the Event Log, even though the Event Log itself is not
signed. Each hash in the validated Event Log can then be compared to
corresponding expected values in the set of Reference Values to
validate overall system integrity.
A summary of information exchanged in obtaining quotes from TPM1.2 TPM 1.2
and TPM2.0 TPM 2.0 can be found in [TAP], Section 4. Detailed information
about PCRs and Quote data structures can be found in [TPM1.2],
[TPM2.0]. [TPM-1.2],
[TPM-2.0]. Recommended log formats include [PC-Client-BIOS-TPM-2.0], [PC-CLIENT-BIOS-TPM-2.0],
and [Canonical-Event-Log]. [CEL].
A.2. Root of Trust for Measurement (RTM)
The measurements needed for attestation require that the device being
attested is equipped with a Root of Trust for Measurement, an RTM, 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.
While there are many complex aspects of Roots of Trust ( [TCGRoT],
[SP800-155], ([TCG-RT]
[SP800-155] [SP800-193]), two aspects that are important in the case
of attestation are:
* The first measurement computed by the Root of Trust for
Measurement, RTM and stored in the TPM's
Root of Trust for Storage, Storage must be assumed to be correct.
* There must not be a way to reset the Root of Trust for Storage
without re-entering the Root of Trust for Measurement 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 [SP800-155]) [SP800-155].)
It's important to note that the trustworthiness of the RTM code
cannot be assured by the TPM or TPM supplier - -- code or procedures
external to the TPM must guarantee the security of the RTM.
A.3. Layering Model for Network Equipment Attester and Verifier
Retrieval of identity and attestation state uses one protocol stack,
while retrieval of Reference Values uses a different set of
protocols. Figure 5 shows the components involved.
+-----------------------+ +-------------------------+
| | | |
| Attester |<-------------| Verifier |
| (Device) |------------->| (Management Station) |
| | | | |
+-----------------------+ | +-------------------------+
|
-------------------- --------------------
| |
------------------------------- ---------------------------------
|Reference
| Reference Values | | Attestation |
------------------------------- ---------------------------------
********************************************************************
* IETF Remote Attestation Reference Interaction Diagram Conceptual Data Flow *
* RFC9334, Figure 1 *
********************************************************************
......................... .........................
. Reference Integrity . . TAP (PTS2.0) Info .
. Manifest . . Model and Canonical .
. . . Log Format .
......................... .........................
************************* *************************
* YANG SWID Module * * YANG Attestation *
* I-D.ietf-sacm-coswid RFC9393 * * Module *
* * * I-D.ietf-rats- RFC9684 *
* * * yang-tpm-charra *
************************* *************************
************************* *************************
* XML, JSON, CBOR (etc) CBOR, etc. * * XML, JSON, CBOR (etc) CBOR, etc. *
************************* *************************
************************* *************************
* RESTCONF/NETCONF * * RESTCONF/NETCONF *
************************* *************************
************************* *************************
* TLS, SSH * * TLS, SSH *
************************* *************************
Figure 6: 5: RIV Protocol Stacks
IETF documents are captured in boxes surrounded by asterisks. TCG
documents are shown in boxes surrounded by dots.
A.4. Implementation Notes
Figure 7
Table 2 summarizes many of the actions needed to complete an
Attestation system, with links to relevant documents. While
documents are controlled by several standards organizations, the
implied actions required for implementation are all the
responsibility of the manufacturer of the device, unless otherwise
noted.
As noted, SWID tags can be generated many ways, but one possible tool
is [SWID-Gen]
+------------------------------------------------------------------+ [SWID-GEN].
+========================================+==========================+
| Component | Controlling |
| | Specification |
--------------------------------------------------------------------
+========================================+==========================+
| Make a Secure execution environment environment: | [TCG-RT] |
| | TCG RoT |
| o * Attestation depends on a secure root of | UEFI.org <www.uefi.org> |
| trust for measurement RTM outside the TPM, as well as | |
| well as roots Roots for storage Storage and reporting Reporting | |
| inside the TPM. | |
| o | |
| * Refer to TCG Root "TCG Roots of Trust for Measurement.| | | o NIST SP 800-193
| Specification" [TCG-RT]. | |
| | |
| * [SP800-193] also provides guidelines | |
| guidelines on Roots of Trust Trust. | |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Provision the TPM as described in |[Platform-DevID-TPM-2.0]| the | [PLATFORM-DEVID-TPM-2.0] |
| TCG documents. | TCG Platform |
| | Certificate [PLATFORM-CERTS] |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Put a DevID or Platform Cert Certificate | [PLATFORM-DEVID-TPM-2.0] |
| in the TPM TPM: | | TCG TPM DevID
| | o [PLATFORM-CERTS] |
| * Install an Initial Attestation Key IAK 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 | [IEEE-802-1AR] |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Connect the TPM to the TLS stack stack: | Vendor TLS stack (This |
| | action configures TLS to |
| o * Use the DevID in the TPM to authenticate | stack (This |
| TAP connections, identifying the device | action is |
| | configuring TLS|
| | to use the DevID as its |
| authenticate TAP connections, | as its client certificate) |
| identifying the device | certificate) |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Make CoSWID tags for BIOS/Loader/Kernel objects BIOS/Loader/ | IETF CoSWID [RFC9393] |
| Kernel objects: | |
| | [SWID] |
| o * Add reference measurements into | |
| SWID tags tags. | [NIST-IR-8060] |
| | | ISO/IEC 19770-2|
| o * Manufacturer should sign the SWID tags | NIST IR 8060 |
| o tags. | |
| | |
| * The TCG RIM-IM [RIM] identifies further | |
| further procedures to create signed RIM | |
| signed RIM documents that provide | |
| the necessary reference | |
| reference information information. | |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Package the SWID tags with a vendor software | Retrieve tags with |
| release software release: | with [RFC9393]. |
| o +--------------------------+
| * A tag-generator plugin such | I-D.ietf-sacm-coswid|
| as [SWID-Gen] can be used |----------------|
| | TCG PC Client | [PC-CLIENT-RIM] | | RIM |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Use PC Client measurement definitions | TCG PC Client [PC-CLIENT-BIOS-TPM-2.0] |
| to define the use of PCRs (although | BIOS |
| (although Windows OS is rare on Networking | |
| Equipment, UEFI BIOS is not) not). | |
--------------------------------------------------------------------
+----------------------------------------+--------------------------+
| Use TAP to retrieve measurements measurements: | [RFC9684] |
| o Map to YANG | YANG Module for| | Use Canonical Log Format
| Basic * Map to YANG. | [CEL] |
| Attestation | |
| TCG * Use Canonical |
| | Log Format Format. |
-------------------------------------------------------------------- | Posture Collection Server
+----------------------------------------+--------------------------+
| A Verifier (as described in IETF | |
| SACMs ECP) [RFC9334], Section 3) should request the | |
| the attestation and analyze the result | |
| result. The Management Verifier application | |
| might be broken down | |
| to several more | |
| components: | |
| o | |
| * A Posture Manager Server that | |
| which collects reports and stores them in | |
| in a database database. | |
| o | |
| * One or more Analyzers that can look at the| | |
| look at the results and figure out | |
| what it means. | |
--------------------------------------------------------------------
Figure 7:
+----------------------------------------+--------------------------+
Table 2: Component Status
Acknowledgements
The authors wish to thank numerous reviewers for generous assistance,
including William Bellingrath, Mark Baushke, Ned Smith, Henk
Birkholz, Tom Laffey, Dave Thaler, Wei Pan, Michael Eckel, Thomas
Hardjono, Bill Sulzen, Willard (Monty) Wiseman, Kathleen Moriarty,
Nancy Cam-Winget Cam-Winget, and Shwetha Bhandari Bhandari.
Authors' Addresses
Guy C. Fedorkow (editor)
Juniper Networks, Inc.
10 Technology Park Drive
Westford, Massachusetts 01886
United States of America
Email: gfedorkow@juniper.net
Eric Voit
Cisco Systems
Email: evoit@cisco.com
Jessica Fitzgerald-McKay
National Security Agency
9800 Savage Road
Ft. Meade, Maryland 20755
United States of America
Email: jmfitz2@nsa.gov