Internet DRAFT - draft-fedorkow-rats-network-device-attestation
draft-fedorkow-rats-network-device-attestation
RATS Working Group G. Fedorkow, Ed.
Internet-Draft Juniper Networks, Inc.
Intended status: Informational E. Voit
Expires: October 18, 2020 Cisco Systems, Inc.
J. Fitzgerald-McKay
National Security Agency
April 16, 2020
TPM-based Network Device Remote Integrity Verification
draft-fedorkow-rats-network-device-attestation-05
Abstract
This document describes a workflow for remote attestation of
integrity of network devices.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Document Organization . . . . . . . . . . . . . . . . . . 4
1.3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Description of Remote Integrity Verification (RIV) . . . 5
1.5. Solution Requirements . . . . . . . . . . . . . . . . . . 7
1.6. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6.1. Out of Scope . . . . . . . . . . . . . . . . . . . . 8
2. Solution Outline . . . . . . . . . . . . . . . . . . . . . . 8
2.1. RIV Software Configuration Attestation using TPM . . . . 8
2.1.1. What Does RIV Attest? . . . . . . . . . . . . . . . . 9
2.2. RIV Keying . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. RIV Information Flow . . . . . . . . . . . . . . . . . . 12
2.4. RIV Simplifying Assumptions . . . . . . . . . . . . . . . 14
2.4.1. Reference Integrity Manifests (RIMs) . . . . . . . . 15
2.4.2. Attestation Logs . . . . . . . . . . . . . . . . . . 16
3. Standards Components . . . . . . . . . . . . . . . . . . . . 16
3.1. Prerequisites for RIV . . . . . . . . . . . . . . . . . . 16
3.1.1. Unique Device Identity . . . . . . . . . . . . . . . 17
3.1.2. Keys . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.3. Appraisal Policy for Evidence . . . . . . . . . . . . 17
3.2. Reference Model for Challenge-Response . . . . . . . . . 18
3.2.1. Transport and Encoding . . . . . . . . . . . . . . . 19
3.3. Centralized vs Peer-to-Peer . . . . . . . . . . . . . . . 20
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
5. Security Considerations . . . . . . . . . . . . . . . . . . . 22
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 26
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
8. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Layering Model for Network Equipment Attester and
Verifier . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1.1. Why is OS Attestation Different? . . . . . . . . . . 29
8.2. Implementation Notes . . . . . . . . . . . . . . . . . . 29
8.3. Root of Trust for Measurement . . . . . . . . . . . . . . 31
9. Informative References . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
There are many aspects to consider in fielding a trusted computing
device, from operating systems to applications. Mechanisms to prove
that a device installed at a customer's site is authentic (i.e., not
counterfeit) and has been configured with authorized software, all as
part of a trusted supply chain, are just a few of the many aspects
which need to be considered concurrently to have confidence that a
device is truly trustworthy.
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A generic architecture for remote attestation has been defined in
[I-D.ietf-rats-architecture]. Additionally, the use case for
remotely attesting networking devices is within Section 6 of
[I-D.richardson-rats-usecases]. However, two these documents do not
provide sufficient guidance for equipment vendors and network
operators and to design, build, and deploy interoperable platforms.
The intent of this document is to provide such guidance. It does
this by outlining the Remote Integrity Verification (RIV) problem,
and then identifies elements that are necessary to get the complete,
scalable attestation procedure working with commercial networking
products such as Routers and Switches. An underlying assumption will
be the availability within the device of a Trusted Platform Module
(TPM) compliant cryptoprocessor to enable the remote trustworthy
assessment of the device's software and hardware.
1.1. Terminology
A number of terms are reused from [I-D.ietf-rats-architecture].
These include: Appraisal Policy for Attestation Result, Attestation
Result, Attester, Endorser, Evidence, Relying Party, Verifier,
Verifier Owner.
Additionally, this document defines the following terms:
Attestation: the process of creating, conveying and appraising
assertions about Platform trustworthiness characteristics, including
supply chain trust, identity, platform provenance, software
configuration, hardware configuration, platform composition,
compliance to test suites, functional and assurance evaluations, etc.
The goal of attestation is simply to assure an administrator that the
software that was launched when the device was last started is the
same as the software that the device vendor initially shipped.
Within the Trusted Computing Group context, attestation is the
process by which an independent Verifier can obtain cryptographic
proof as to the identity of the device in question, evidence of the
integrity of software loaded on that device when it started up, and
then verify that what's there is what's supposed to be there. For
networking equipment, a verifier capability can be embedded in a
Network Management Station (NMS), a posture collection server, or
other network analytics tool (such as a software asset management
solution, or a threat detection and mitigation tool, etc.). While
informally referred to as attestation, this document focuses on a
subset defined here as Remote Integrity Verification (RIV). RIV
takes a network equipment centric perspective that includes a set of
protocols and procedures for determining whether a particular device
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was launched with untampered software, starting from Roots of Trust.
While there are many ways to accomplish attestation, RIV sets out a
specific set of protocols and tools that work in environments
commonly found in Networking Equipment. RIV does not cover other
platform characteristics that could be attested, although it does
provide evidence of a secure infrastructure to increase the level of
trust in other platform characteristics attested by other means
(e.g., by Entity Attestation Tokens [I-D.ietf-rats-eat].
1.2. Document Organization
The remainder of this document is organized into several sections:
o The remainder of this section covers goals and requirements, plus
a top-level overview
o The Solution Overview section outlines how RIV works
o The Standards Components section links components of RIV to
normative standards.
o Supporting material is in an appendix at the end.
1.3. Goals
Network operators benefit from a trustworthy attestation mechanism
that provides assurance that their network comprises authentic
equipment, and has loaded software free of known vulnerabilities and
unauthorized tampering. In line with the overall goal of assuring
integrity, attestation can be used for asset management,
vulnerability and compliance assessment, plus configuration
management.
As a part of a trusted supply chain, the RIV attestation workflow
outlined in this document is intended to meet the following high-
level goals:
o Provable Device Identity - This specification requires that an
attesting device includes a cryptographic identifier unique to
each device. Effectively this means that the TPM or equivalent
cryptoprocessor must be so provisioned during the manufacturing
cycle.
o Software Inventory - A key goal is to identify the software
release installed on the attesting device, and to provide evidence
that the software stored within hasn't been altered
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o Verifiability - Verification of software and configuration of the
device shows that the software that was authorized for
installation by the administrator has actually has been launched.
In addition, RIV is designed to operate in a centralized environment,
such as with a central authority that manages and configures a number
of network devices, or 'peer-to-peer', where network devices
independently verify one another to establish a trust relationship.
(See Section 3.3 below, and also
[I-D.voit-rats-trusted-path-routing])
1.4. Description of Remote Integrity Verification (RIV)
Attestation requires two interlocking services between the Attester
network device and the Verifier::
o Platform Identity, the mechanism providing trusted identity, can
reassure network managers that the specific devices they ordered
from authorized manufacturers for attachment to their network are
those that were installed, and that they continue to be present in
their network. As part of the mechanism for Platform Identity,
cryptographic proof of the identity of the manufacturer is also
provided.
o Software Measurement is the mechanism that reports the state of
mutable software components on the device, and can assure network
managers that they have known, untampered software configured to
run in their network.
Using these two interlocking services, RIV provides a procedure that
assures a network operator that the equipment in their network can be
reliably identified, and that untampered software of a known version
is installed on each endpoint. Equipment in the network includes
devices that make up the network itself, such as routers, switches
and firewalls.
RIV includes several major processes:
1. Creation of Evidence is the process whereby an Attester generates
cryptographic proof (Evidence) of claims about platform
properties. In particular, the platform identity and its
software configuration are both of critical importance
2. Platform Identification refers to the mechanism assuring the
Relying Party (ultimately, a network administrator) of the
identity of devices that make up their network, and that their
manufacturers are known.
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3. Software used to boot a platform can be described as a chain of
measurements, started by a Root of Trust for Measurement, that
normally ends when the system software is loaded. A measurement
signifies the identity, integrity and version of each software
component registered with an attesting device's TPM, so that the
subsequent appraisal stage can determine if the software
installed is authentic, up-to-date, and free of tampering.
4. Conveyance of Evidence reliably transports at least the minimum
amount of Evidence from Attester to a Verifier to allow a
management station to perform a meaningful appraisal in Step 5.
The transport is typically carried out via a management network.
The channel must provide integrity and authenticity, and, in some
use cases, may also require confidentiality.
5. Finally, Appraisal of Evidence occurs. As the Verifier and
Relaying Party roles are often combined within RIV, this is the
process of verifying the Evidence received by a Verifier from the
Attesting device, and using an Appraisal Policy to develop an
Attestation Result, used to inform decision making. In practice,
this means comparing the device measurements reported as Evidence
with the Attester configuration expected by the Verifier.
Subsequently the Appraisal Policy for Attestation Results might
match what was found against Reference Integrity Measurements
(aka Golden Measurements) which representing the intended
configured state of the connected device.
All implementations supporting this RIV specification require the
support of the following three technologies : 1. Identity: Platform
identity can be based on IEEE 802.1AR Device Identity [IEEE-802-1AR],
coupled with careful supply-chain management by the manufacturer.
The DevID certificate contains a statement by the manufacturer that
establishes the identity of the device as it left the factory. Some
applications with a more-complex post-manufacture supply chain (e.g.
Value Added Resellers), or with different privacy concerns, may want
to use alternate mechanisms for platform authentication (for example,
TCG Platform Certificates [Platform-Certificates]).
1. Platform Attestation provides evidence of configuration of
software elements present in the device. This form of
attestation can be implemented with TPM PCR, Quote and Log
mechanisms, which provide an authenticated mechanism to report
what software was started on the device through the boot cycle.
Successful attestation requires an unbroken chain from a boot-
time root of trust through all layers of software needed to bring
the device to an operational state.
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2. Reference Integrity Measurements must be conveyed from the
Endorser (the entity accepted as the software authority, often
the manufacturer for embedded systems) to the system in which
verification will take place
1.5. Solution Requirements
Remote Integrity Verification must address the "Lying Endpoint"
problem, in which malicious software on an endpoint may subvert the
intended function, and also prevent the endpoint from reporting its
compromised status. (See Section 5 for further Security
Considerations)
RIV attestation is designed to be simple to deploy at scale. RIV
should work "out of the box" as far as possible, that is, with the
fewest possible provisioning steps or configuration databases needed
at the end-user's site, as network equipment is often required to
"self-configure", to reliably reach out without manual intervention
to prove its identity and operating posture, then download its own
configuration. See [RFC8572] for an example of Secure Zero Touch
Provisioning.
1.6. Scope
Remote Attestation is a very general problem that could apply to most
network-connected computing devices. However, this document includes
several assumptions that limit the scope to Network Equipment (e.g.
routers, switches and firewalls):
o This solution is for use in non-privacy-preserving applications
(for example, networking, Industrial IoT), avoiding the need for a
Privacy Certificate Authority for attestation keys
[AIK-Enrollment] or TCG Platform Certificates
[Platform-Certificates]
o This document assumes network protocols that are common in
networking equipment such as YANG [RFC7950] and NETCONF [RFC6241],
but not generally used in other applications.
o The approach outlined in this document mandates the use of TPM 1.2
or TPM 2.0. Other roots of trust could be used with the same
information flow, although different data structures would likely
be called for.
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1.6.1. Out of Scope
o Run-Time Attestation: Run-time attestation of Linux or other
multi-threaded operating system processes considerably expands the
scope of the problem. Many researchers are working on that
problem, but this document defers the run-time attestation
problem.
o Multi-Vendor Embedded Systems: Additional coordination would be
needed for devices that themselves comprise hardware and software
from multiple vendors, integrated by the end user.
o Processor Sleep Modes: Network equipment typically does not
"sleep", so sleep and hibernate modes are not considered.
Although out of scope for RIV, Trusted Computing Group
specifications do encompass sleep and hibernate states.
o Virtualization and Containerization: These technologies are
increasingly used in Network equipment, but are not considered in
this revision of the document.
2. Solution Outline
2.1. RIV Software Configuration Attestation using TPM
RIV Attestation is a process which can be used to determine the
identity of software running on a specifically-identified device.
Remote Attestation is broken into two phases, shown in Figure 1:
o During system startup, each distinct software object is
"measured". Its identity, hash (i.e. cryptographic digest) and
version information is recorded in a log. Hashes are also
extended, or cryptographically folded, into the TPM, in a way that
can be used to validate the log entries. The measurement process
generally follows the Chain of Trust model used in Measured Boot,
where each stage of the system measures the next one, and extends
its measurement into the TPM, before launching it.
o Once the device is running and has operational network
connectivity, a separate, trusted Verifier will interrogate the
network device to retrieve the logs and a copy of the digests
collected by hashing each software object, signed by an
attestation private key known only to the TPM.
The result is that the Verifier can verify the device's identity by
checking the certificate containing the TPM's attestation public key,
and can validate the software that was launched by comparing digests
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in the log with known-good values, and verifying their correctness by
comparing with the signed digests from the TPM.
It should be noted that attestation and identity are inextricably
linked; signed Evidence that a particular version of software was
loaded is of little value without cryptographic proof of the identity
of the Attester producing the Evidence.
+-------------------------------------------------------+
| +--------+ +--------+ +--------+ +---------+ |
| | BIOS |--->| Loader |-->| Kernel |--->|Userland | |
| +--------+ +--------+ +--------+ +---------+ |
| | | | |
| | | | |
| +------------+-----------+-+ |
| Step 1 | |
| V |
| +--------+ |
| | TPM | |
| +--------+ |
| Router | |
+--------------------------------|----------------------+
|
| Step 2
| +-----------+
+--->| Verifier |
+-----------+
Reset---------------flow-of-time-during-boot--...------->
Figure 1: RIV Attestation Model
In Step 1, measurements are "extended" into the TPM as processes
start. In Step 2, signed PCR digests are retrieved from the TPM for
off-box analysis after the system is operational.
2.1.1. What Does RIV Attest?
TPM attestation is strongly focused around Platform Configuration
Registers (PCRs), but those registers are only vehicles for
certifying accompanying Evidence, conveyed in log entries. It is the
hashes in log entries are extended into PCRs, where they can be
retrieved in the form of a Quote signed by a key known only to the
TPM (xref). The use of multiple PCRs serves only to provide some
independence between different classes of object, so that one class
of objects can be updated without changing the extended hash for
other classes. Although PCRs can be used for any purpose, this
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section outlines the objects within the scope of this document which
may be extended into the TPM.
In general, PCRs are organized to independently attest three classes
of object:
o Code, i.e., instructions to be executed by a CPU.
o Configuration - Many devices offer numerous options controlled by
non-volatile configuration variables which can impact the device's
security posture. These settings may have vendor defaults, but
often can be changed by administrators, who may want to verify via
attestation that the settings they intend are still in place.
o Credentials - Administrators may wish to verify via attestation
that keys (and other credentials) outside the Root of Trust have
not be subject to unauthorized tampering. (By definition, keys
inside the root of trust can't be verified independently)
The TCG PC Client Platform Firmware Profile Specification
[PC-Client-BIOS-TPM-2.0] gives considerable detail on what is to be
measured during the boot phase of a platform boot using a UEFI BOIS
(www.uefi.org), but the goal is simply to measure every bit of code
executed in the process of starting the device, along with any
configuration information related to security posture. Table XX
summarizes the functions that are measured, and how this document
recommends they be allocated to PCRs. It's important to note that
each PCR may contain results from dozens (or even thousands) of
individual measurements.
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+------------------------------------------------------------------+
| | Allocated PCR # |
| Function | Code | Configuration|
--------------------------------------------------------------------
| BIOS Static Root of Trust, plus embedded | 0 | 1 |
| Option ROMs and drivers | | |
--------------------------------------------------------------------
| Pluggable Option ROMs to initialize and | 2 | 3 |
| configure add-in devices | | |
--------------------------------------------------------------------
| Boot Manager code and configuration (UEFI | 4 | 5 |
| uses a separate module to implement | | |
| policies for selecting among a variety of | | |
| potential boot devices). This PCR records | | |
| boot attempts, and identifies what | | |
| resources were used to boot the OS. | | |
--------------------------------------------------------------------
| Vendor Specific Measurements during boot | 6 | 6 |
--------------------------------------------------------------------
| Secure Boot Policy. This PCR records keys | | 7 |
| and configuration used to validate the OS | | |
| loader | | |
--------------------------------------------------------------------
| OS Loader (e.g GRUB2 for Linux) | 8 | 9 |
--------------------------------------------------------------------
| Reserved for OS (e.g. Linux IMA) | 10 | 10 |
+------------------------------------------------------------------+
Figure 2: Attested Objects
2.2. RIV Keying
RIV attestation relies on two keys:
o An identity key is required to certify the identity of the
Attester itself. RIV specifies the use of an IEEE 802.1AR DevID
[IEEE-802-1AR], signed by the device manufacturer, containing the
device serial number.
o An Attestation Key is required to sign the Quote generated by the
TPM to report evidence of software configuration.
In TPM application, the Attestation key must be protected by the TPM,
and the DevID should be as well. Depending on other TPM
configuration procedures, the two keys may be different. Some of the
considerations are outlined in TCG Guidance for Securing Network
Equipment [NetEq].
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TCG Guidance for Securing Network Equipment specifies further
conventions for these keys:
o When separate Identity and Attestation keys are used, the
Attestation Key (AK) and its x.509 certificate should parallel the
DevID, with the same device ID information as the DevID
certificate (i.e., the same Subject Name and Subject Alt Name,
even though the key pairs are different). This allows a quote
from the device, signed by an AK, to be linked directly to the
device that provided it, by examining the corresponding AK
certificate.
o Network devices that are expected to use secure zero touch
provisioning as specified in [RFC8572]) must be shipped by the
manufacturer with pre-provisioned keys (Initial DevID and AK,
called IDevID and IAK). Inclusion of an DevID and IAK by a vendor
does not preclude a mechanism whereby an Administrator can define
Local Identity and Attestation Keys (LDevID and LAK) if desired.
2.3. RIV Information Flow
RIV workflow for networking equipment is organized around a simple
use-case, where a network operator wishes to verify the integrity of
software installed in specific, fielded devices. This use-case
implies several components:
1. The Attesting Device, which the network operator wants to
examine.
2. A Verifier (which might be a network management station)
somewhere separate from the Device that will retrieve the
information and analyze it to pass judgment on the security
posture of the device.
3. A Relying Party, which can act on Attestation results.
Interaction between the Relying Party and the Verifier is
considered out of scope for RIV.
4. Signed Reference Integrity Manifests (RIMs), containing Reference
Integrity Measurements, can either be created by the device
manufacturer and shipped along with the device as part of its
software image, or alternatively, could be obtained several other
ways (direct to the Verifier from the manufacturer, from a third
party, from the owner's observation of what's thought to be a
"known good system", etc.). Retrieving RIMs from the device
itself allows attestation to be done in systems which may not
have access to the public internet, or by other devices that are
not management stations per-se (e.g., a peer device; See
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Section 3.1.3). If reference measurements are obtained from
multiple sources, the Verifier may need to evaluate the relative
level of trust to be placed in each source in case of a
discrepancy.
These components are illustrated in Figure 2.
A more-detailed taxonomy of terms is given in
[I-D.ietf-rats-architecture]
+---------------+ +-------------+ +---------+--------+
| | | Attester | Step 1 | Verifier| |
| Endorser | | (Device |<-------| (Network| Relying|
| (Device | | under |------->| Mngmt | Party |
| Manufacturer | | attestation)| Step 2 | Station)| |
| or other | | | | | |
| authority) | | | | | |
+---------------+ +-------------+ +---------+--------+
| /\
| Step 0 |
-----------------------------------------------
Figure 3: RIV Reference Configuration for Network Equipment
In Step 0, The Asserter (the device manufacturer) provides a Software
Image accompanied by one or more Reference Integrity Manifests (RIMs)
to the Attester (the device under attestation) signed by the
asserter. In Step 1, the Verifier (Network Management Station), on
behalf of a Relying Party, requests Identity, Measurement Values (and
possibly RIMs) from the Attester. In Step 2, the Attester responds
to the request by providing a DevID, quotes (measured values), and
optionally RIMs, signed by the Attester.
The following standards components may be used:
1. TPM Keys are configured according to [Platform-DevID-TPM-2.0],
[PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2]
2. Measurements of firmware and bootable modules may be taken
according to TCG PC Client [PC-Client-BIOS-TPM-2.0] and Linux IMA
[IMA]
3. Device Identity is managed by IEEE 802.1AR certificates
[IEEE-802-1AR], with keys protected by TPMs.
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4. Attestation logs may be formatted according to the Canonical
Event Log format [Canonical-Event-Log], although other
specialized formats may be used.
5. Quotes are retrieved from the TPM according to TCG TAP
Information Model [TAP]. While the TAP IM gives a protocol-
independent description of the data elements involved, it's
important to note that quotes from the TPM are signed inside the
TPM, so must be retrieved in a way that does not invalidate the
signature, as specified in [I-D.ietf-rats-yang-tpm-charra], to
preserve the trust model. (See Section 5 Security
Considerations).
6. Reference Integrity Measurements may be encoded as CoSWID tags,
as defined in the TCG RIM document [RIM], compatible with NIST IR
8060 [NIST-IR-8060] and the IETF CoSWID draft
[I-D.ietf-sacm-coswid]. See Section 2.4.1.
2.4. RIV Simplifying Assumptions
This document makes the following simplifying assumptions to reduce
complexity:
o The product to be attested is shipped with an IEEE 802.1AR DevID
and an Initial Attestation Key (IAK) with certificate. The IAK
cert contains the same identity information as the DevID
(specifically, the same Subject Name and Subject Alt Name, signed
by the manufacturer), but it's a type of key that can be used to
sign a TPM Quote. This convention is described in TCG Guidance
for Securing Network Equipment [NetEq]. For network equipment,
which is generally non-privacy-sensitive, shipping a device with
both an IDevID and an IAK already provisioned substantially
simplifies initial startup. Privacy-sensitive applications may
use the TCG Platform Certificate and additional procedures to
install identity credentials on the platform after manufacture.
o The product is equipped with a Root of Trust for Measurement, Root
of Trust for Storage and Root of Trust for Reporting (as defined
in [GloPlaRoT]) that are capable of conforming to the TCG Trusted
Attestation Protocol (TAP) Information Model [TAP].
o The vendor will ship Reference Integrity Measurements (i.e.,
known-good measurements) in the form of signed CoSWID tags
[I-D.ietf-sacm-coswid], [SWID], as described in TCG Reference
Integrity Measurement Manifest Information Model [RIM].
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2.4.1. Reference Integrity Manifests (RIMs)
[I-D.ietf-rats-yang-tpm-charra] focuses on collecting and
transmitting evidence in the form of PCR measurements and attestation
logs. But the critical part of the process is enabling the verifier
to decide whether the measurements are "the right ones" or not.
While it must be up to network administrators to decide what they
want on their networks, the software supplier should supply the
Reference Integrity Measurements that may be used by a verifier to
determine if evidence shows known good, known bad or unknown software
configurations.
In general, there are two kinds of reference measurements:
1. Measurements of early system startup (e.g., BIOS, boot loader, OS
kernel) are essentially single threaded, and executed exactly
once, in a known sequence, before any results could be reported.
In this case, while the method for computing the hash and
extending relevant PCRs may be complicated, the net result is
that the software (more likely, firmware) vendor will have one
known good PCR value that "should" be present in the relevant
PCRs after the box has booted. In this case, the signed
reference measurement could simply list the expected hashes for
the given version. However, a RIM that contains the intermediate
hashes can be useful in debugging cases where the expected final
hash is not the one reported.
2. Measurements taken later in operation of the system, once an OS
has started (for example, Linux IMA[IMA]), may be more complex,
with unpredictable "final" PCR values. In this case, the
Verifier must have enough information to reconstruct the expected
PCR values from logs and signed reference measurements from a
trusted authority.
In both cases, the expected values can be expressed as signed SWID or
CoSWID tags, but the SWID structure in the second case is somewhat
more complex, as reconstruction of the extended hash in a PCR may
involve thousands of files and other objects.
The TCG has published an information model defining elements of
reference integrity manifests under the title TCG Reference Integrity
Manifest Information Model [RIM]. This information model outlines
how SWID tags should be structured to allow attestation, and defines
"bundles" of SWID tags that may be needed to describe a complete
software release. The RIM contains some metadata relating to the
software release it belongs to, plus hashes for each individual file
or other object that could be attested.
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TCG has also published the PC Client Reference Integrity Measurement
specification [PC-Client-RIM], which focuses on a SWID-compatible
format suitable for expressing expected measurement values in the
specific case of a UEFI-compatible BIOS, where the SWID focus on
files and file systems is not a direct fit. While the PC Client RIM
is not directly applicable to network equipment, many vendors do use
a conventional UEFI BIOS to launch their network OS.
2.4.2. Attestation Logs
Quotes from a TPM can provide evidence of the state of a device up to
the time the evidence was recorded, but to make sense of the quote in
most cases an event log that identifies which software modules
contributed which values to the quote during startup must also be
provided. The log must contain enough information to demonstrate its
integrity by allowing exact reconstruction of the digest conveyed in
the signed quote (i.e., PCR values).
There are multiple event log formats which may be supported as viable
formats of Evidence between the Attester and Verifier:
o Event log exports from [I-D.ietf-rats-yang-tpm-charra]
o IMA Event log file exports [IMA]
o TCG UEFI BIOS event log (TCG EFI Platform Specification for TPM
Family 1.1 or 1.2, Section 7 [EFI-TPM])
o TCG Canonical Event Log [Canonical-Event-Log]
o Legacy BIOS event log, although this document is less relevant as
UEFI has largely replaced the Legacy BIOS (TCG PC Client Specific
Implementation Specification for Conventional BIOS,
Section 11.3[PC-Client-BIOS-TPM-1.2])
3. Standards Components
3.1. Prerequisites for RIV
The Reference Interaction Model for Challenge-Response-based Remote
Attestation is based on the standard roles defined in
[I-D.ietf-rats-architecture]. However additional prerequisites must
be established to allow for interoperable RIV use case
implementations. These prerequisites are intended to provide
sufficient context information so that the Verifier can acquire and
evaluate Attester measurements.
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3.1.1. Unique Device Identity
A Secure device Identity (DevID) in the form of an IEEE 802.1AR
certificate [IEEE-802-1AR] must be provisioned in the Attester's
TPMs.
3.1.2. Keys
The Attestation Identity Key (AIK) and certificate must also be
provisioned on the Attester according to [Platform-DevID-TPM-2.0],
[PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2].
The Attester's TPM Keys must be associated with the DevID on the
Verifier (see Section 5 Security Considerations).
3.1.3. Appraisal Policy for Evidence
The Verifier must obtain the Appraisal Policy for Evidence. This
policy may be in the form of reference measurements (e.g., Known Good
Values, CoSWID tags [I-D.birkholz-yang-swid]). These reference
measurements will eventually be compared to signed PCR Evidence
acquired from an Attester's TPM.
This document does not specify the format or contents for the
Appraisal Policy for Evidence. But acquiring this policy may happen
in one of two ways:
1. a Verifier obtains reference measurements directly from a
Verifier Owner / device configuration authority chosen by the
network administrator.
2. Signed reference measurements may be distributed by the Verifier
Owner to the Attester. From there, the reference measurement may
be acquired by the Verifier.
************* .-------------. .-----------.
* Verifier * | Attester | | Verifier/ |
* Owner * | | | Relying |
*(Device *----2--->| (Network |----2--->| Party |
* config * | Device) | |(Ntwk Mgmt |
* Authority)* | | | Station) |
************* '-------------' '-----------'
| ^
| |
'----------------1--------------------------'
Figure 4: Appraisal Policy for Evidence Prerequisites
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In either case the Appraisal Policy for Evidence must be generated,
acquired and delivered in a secure way. This includes reference
measurements of:
o firmware and bootable modules taken according to TCG PC Client
[PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]
o encoded CoSWID tags signed by the device manufacturer, are as
defined in the TCG RIM document [RIM], compatible with NIST IR
8060 [NIST-IR-8060] and the IETF CoSWID draft
[I-D.ietf-sacm-coswid].
3.2. Reference Model for Challenge-Response
Once the prerequisites for RIV are met, a Verifier may acquire
Evidence from an Attester. The following diagram illustrates a RIV
information flow between a Verifier and an Attester. Event times
shown correspond to the time types described within Appendix A of
[I-D.ietf-rats-architecture]:
.----------. .--------------------------.
| Attester | | Relying Party / Verifier |
'----------' '--------------------------'
time(vg) |
|<-- requestAttestation(nonce,PcrSelection)-------time(ns)
| |
time(eg) LogEvidence(assertionsSelection) |
| SignedPcrEvidence(PCR, nonce) |
| |
|-----------LogEvidence,SignedPcrEvidence---------->|
| |
| verifyAttestationEvidence time(rg,ra)
| ~
time(rx)
Figure 5: IETF Attestation Information Flow
o time(vg): One or more Attesting Network Device PCRs are extended
with measurements.
o time(ns): The Verifier generates a nonce, and makes a request
attestation data for one or more PCRs from an Attester. This can
be accomplished via a YANG [RFC7950] interface that implements the
TCG TAP model (e.g. YANG Module for Basic Challenge-Response-
based Remote Attestation Procedures
[I-D.ietf-rats-yang-tpm-charra]).
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o time(eg): On the Attester, measured values are retrieved from the
Attester's TPM. This requested PCR evidence is signed by the
Attestation Identity Key (AIK) associated with the DevID. Quotes
are retrieved according to TCG TAP Information Model [TAP]. While
the TAP IM gives a protocol-independent description of the data
elements involved, it's important to note that quotes from the TPM
are signed inside the TPM, so must be retrieved in a way that does
not invalidate the signature, as specified in
[I-D.ietf-rats-yang-tpm-charra], to preserve the trust model.
(See Section 5 Security Considerations).
* At the same time, for any PCRs where known good values might
not be known by the Verifier, the Attester collects log
evidence showing what values have been extended into that PCR.
Attestation logs are formatted according to the Canonical Event
Log format [Canonical-Event-Log].
o Collected Evidence is passed from the Attester to the Verifier
o time(rg,ra): The Verifier reviews the Evidence and takes action as
needed. As the Relying Party and Verifier are assumed co-
resident, this can happen in one step.
* If the signed PCR values do not match either KGVs, or the set
of log entries which have extended a particular PCR, the device
should not be trusted.
* If the set of log entries are not seen as acceptable by the
Appraisal Policy for Evidence, the device should not be
trusted.
* If the AIK signature is not correct, or freshness such as that
provided by the nonce is not included in the response, the
device should not be trusted.
o time(rx): At some point after the verification of Evidence, the
Attester can no longer be considered Attested as trustworthy.
3.2.1. Transport and Encoding
Network Management systems may retrieve signed PCR based Evidence as
shown in Figure 5, and can be accomplished via:
o XML, JSON, or CBOR encoded Evidence, using
o RESTCONF or NETCONF transport, over a
o TLS or SSH secure tunnel
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Retrieval of Log Evidence will be via log interfaces on the network
device. (For example, see [I-D.ietf-rats-yang-tpm-charra]).
3.3. Centralized vs Peer-to-Peer
Figure 5 above assumes that the Verifier is implicitly trusted, while
the Attesting device is not. In a Peer-to-Peer application such as
two routers negotiating a trust relationship
[I-D.voit-rats-trusted-path-routing], the two peers can each ask the
other to prove software integrity. In this application, the
information flow is the same, but each side plays a role both as an
Attester and a Verifier. Each device issues a challenge, and each
device responds to the other's challenge, as shown in Figure 6.
Peer-to-peer challenges, particularly if used to establish a trust
relationship between routers, require devices to carry their own
signed reference measurements (RIMs) so that each device has
everything needed for attestation, without having to resort to a
central authority.
+---------------+ +---------------+
| | | |
| Asserter A | | Asserter B |
| Firmware | | Firmware |
| Configuration | | Configuration |
| Authority | | Authority |
| | | |
+---------------+ +---------------+
| |
| +-------------+ +------------+ |
| | | Step 1 | | | \
| | Attester |<------>| Verifier | | |
| | |<------>| | | | Router B
+------>| | Step 2 | | | |- Challenges
Step 0A| | | | | | Router A
| |------->| | | |
|- Router A --| Step 3 |- Router B -| | /
| | | | |
| | | | |
| | Step 1 | | | \
| Verifier |<------>| Attester |<-+ | Router A
| |<------>| | |- Challenges
| | Step 2 | | | Router B
| | | | |
| |<-------| | |
+-------------+ Step 3 +------------+ /
Figure 6: Peer-to-Peer Attestation Information Flow
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In this application, each device may need to be equipped with signed
RIMs to act as an Attester, and also a selection of trusted x.509
root certificates to allow the device to act as a Verifier. An
existing link layer protocol such as 802.1x [IEEE-802.1x] or 802.1AE
[IEEE-802.1ae], with Evidence being enclosed over a variant of EAP
[RFC3748] or LLDP [LLDP] are suitable methods for such an exchange.
4. Privacy Considerations
Networking Equipment such as routers, switches and firewalls has a
key role to play in guarding the privacy of individuals using the
network:
o Packets passing through the device must not be sent to
unauthorized destinations. For example
o 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.
o Networking Equipment is often called upon to block access to
protected resources from unauthorized users.
o Routing information, such as the identity of a router's peers,
must not be leaked to unauthorized neighbors.
o If configured, encryption and decryption of traffic must be
carried out reliably, while protecting keys and credentials.
Functions that protect privacy are implemented as part of each layer
of hardware and software that makes up the networking device. In
light of these requirements for protecting the privacy of users of
the network, the Network Equipment must identify itself, and its boot
configuration and measured device state (for example, PCR values), to
the Equipment's Administrator, so there's no uncertainty as to what
function each device and configuration is configured to carry out.
This allows the administrator to ensure that the network provides
individual and peer privacy guarantees.
RIV specifically addresses the collection information from enterprise
network devices by an enterprise network. As such, privacy is a
fundamental concern for those deploying this solution, given EU GDPR,
California CCPA, and many other privacy regulations. The enterprise
should implement and enforce their duty of care.
See [NetEq] for more context on privacy in networking devices
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5. Security Considerations
Attestation results from the RIV procedure are subject to a number of
attacks:
o Keys may be compromised
o A counterfeit device may attempt to impersonate (spoof) a known
authentic device
o Man-in-the-middle attacks may be used by a counterfeit device to
attempt to deliver responses that originate in an actual authentic
device
o Replay attacks may be attempted by a compromised device
Trustworthiness of RIV attestation depends strongly on the validity
of keys used for identity and attestation reports. RIV takes full
advantage of TPM capabilities to ensure that results can be trusted.
Two sets of keys are relevant to RIV attestation
o A DevID key is used to certify the identity of the device in which
the TPM is installed.
o An Attestation Key (AK) key signs attestation reports, (called
'quotes' in TCG documents), used to provide evidence for integrity
of the software on the device.
TPM practices usually require that these keys be different, as a way
of ensuring that a general-purpose signing key cannot be used to
spoof an attestation quote.
In each case, the private half of the key is known only to the TPM,
and cannot be retrieved externally, even by a trusted party. To
ensure that's the case, specification-compliant private/public key-
pairs are generated inside the TPM, where they're never exposed, and
cannot be extracted (See [Platform-DevID-TPM-2.0]).
Keeping keys safe is just part of attestation security; knowing which
keys are bound to the device in question is just as important.
While there are many ways to manage keys in a TPM (See
[Platform-DevID-TPM-2.0]), RIV includes support for "zero touch"
provisioning (also known as zero-touch onboarding) of fielded devices
(e.g. Secure ZTP, [RFC8572]), where keys which have predictable
trust properties are provisioned by the device vendor.
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Device identity in RIV is based on IEEE 802.1AR DevID. This
specification provides several elements
o A DevID requires a unique key pair for each device, accompanied by
an x.509 certificate
o The private portion of the DevID key is to be stored in the
device, in a manner that provides confidentiality (Section 6.2.5
[IEEE-802-1AR])
The x.509 certificate contains several components
o The public part of the unique DevID key assigned to that device
o An identifying string that's unique to the manufacturer of the
device. This is normally the serial number of the unit, which
might also be printed on label on the device.
o The certificate must be signed by a key traceable to the
manufacturer's root key.
With these elements, the device's manufacturer and serial number can
be identified by analyzing the DevID certificate plus the chain of
intermediate certs leading back to the manufacturer's root
certificate. As is conventional in TLS connections, a nonce must be
signed by the device in response to a challenge, proving possession
of its DevID private key.
RIV uses the DevID to validate a TLS connection to the device as the
attestation session begins. Security of this process derives from
TLS security, with the DevID providing proof that the TLS session
terminates on the intended device. [RFC8446].
Evidence of software integrity is delivered in the form of a quote
signed by the TPM itself. Because the contents of the quote are
signed inside the TPM, any external modification (including
reformatting to a different data format) will be detected as
tampering.
A critical feature of the YANG model described in
[I-D.ietf-rats-yang-tpm-charra] is the ability to carry TPM data
structures in their native format, without requiring any changes to
the structures as they were signed and delivered by the TPM. While
alternate methods of conveying TPM quotes could compress out
redundant information, or add an additional layer of signing using
external keys, the important part is to preserve the TPM signing, so
that tampering anywhere in the path between the TPM itself and the
Verifier can be detected.
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Prevention of spoofing attacks against attestation systems is also
important. There are two cases to consider:
o The entire device could be spoofed, that is, when the Verifier
goes to verify a specific device, it might be redirected to a
different device. Use of the 802.1AR identity in the TPM ensures
that the Verifier's TLS session is in fact terminating on the
right device.
o A compromised device could respond with a spoofed attestation
result, that is, a compromised OS could return a fabricated quote.
Protection against spoofed quotes from a device with valid identity
is a bit more complex. An identity key must be available to sign any
kind of nonce or hash offered by the verifier, and consequently,
could be used to sign a fabricated quote. To block spoofed
attestation result, the quote generated inside the TPM must by signed
by a key that's different from the DevID, called an Attestation Key
(AK).
Given separate Attestation and DevID keys, the binding between the AK
and the same device must also be proven to prevent a man-in-the-
middle attack (e.g. the 'Asokan Attack' [RFC6813]).
This is accomplished in RIV through use of an AK certificate with the
same elements as the DevID (i.e., same manufacturer's serial number,
signed by the same manufacturer's key), but containing the device's
unique AK public key instead of the DevID public key. [this will
require an OID that says the key is known by the CA to be an
Attestation key]
These two keys and certificates are used together:
o The DevID is used to validate a TLS connection terminating on the
device with a known serial number.
o The AK is used to sign attestation quotes, providing proof that
the attestation evidence comes from the same device.
Replay attacks, where results of a previous attestation are submitted
in response to subsequent requests, are usually prevented by
inclusion of a nonce in the request to the TPM for a quote. Each
request from the Verifier includes a new random number (a nonce).
The resulting quote signed by the TPM contains the same nonce,
allowing the verifier to determine freshness, i.e., that the
resulting quote was generated in response to the verifier's specific
request. Time-Based Uni-directional Attestation
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[I-D.birkholz-rats-tuda] provides an alternate mechanism to verify
freshness without requiring a request/response cycle.
Requiring results of attestation of the operating software to be
signed by a key known only to the TPM also removes the need to trust
the device's operating software (beyond the first measurement; see
below); any changes to the quote, generated and signed by the TPM
itself, made by malicious device software, or in the path back to the
verifier, will invalidate the signature on the quote.
Although RIV recommends that device manufacturers pre-provision
devices with easily-verified DevID and AK certs, use of those
credentials is not mandatory. IEEE 802.1AR incorporates the idea of
an Initial Device ID (IDevID), provisioned by the manufacturer, and a
Local Device ID (LDevID) provisioned by the owner of the device. RIV
extends that concept by defining an Initial Attestation Key (IAK) and
Local Attestation Key (LAK) with the same properties.
Device owners can use any method to provision the Local credentials.
o TCG document [Platform-DevID-TPM-2.0] shows how the initial
Attestation keys can be used to certify LDevID and LAK keys. Use
of the LDevID and LAK allows the device owner to use a uniform
identity structure across device types from multiple manufacturers
(in the same way that an "Asset Tag" is used by many enterprises
use to identify devices they own). TCG doc [Provisioning-TPM-2.0]
also contains guidance on provisioning identity keys in TPM 2.0.
o But device owners can use any other mechanism they want to assure
themselves that Local identity certificates are inserted into the
intended device, including physical inspection and programming in
a secure location, if they prefer to avoid placing trust in the
manufacturer-provided keys.
Clearly, Local keys can't be used for secure Zero Touch provisioning;
installation of the Local keys can only be done by some process that
runs before the device is configured for network operation.
On the other end of the device life cycle, provision should be made
to wipe Local keys when a device is decommissioned, to indicate that
the device is no longer owned by the enterprise. The manufacturer's
Initial identity keys must be preserved, as they contain no
information that's not already printed on the device's serial number
plate.
In addition to trustworthy provisioning of keys, RIV depends on other
trust anchors. (See [GloPlaRoT] for definitions of Roots of Trust.)
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o Secure identity depends on mechanisms to prevent per-device secret
keys from being compromised. The TPM provides this capability as
a Root of Trust for Storage
o Attestation depends on an unbroken chain of measurements, starting
from the very first measurement. That first measurement is made
by code called the Root of Trust for Measurement, typically done
by trusted firmware stored in boot flash. Mechanisms for
maintaining the trustworthiness of the RTM are out of scope for
RIV, but could include immutable firmware, signed updates, or a
vendor-specific hardware verification technique.
o RIV assumes some level of physical defense for the device. If a
TPM that has already been programmed with an authentic DevID is
stolen and inserted into a counterfeit device, attestation of that
counterfeit device may become indistinguishable from an authentic
device.
RIV also depends on reliable reference measurements, as expressed by
the RIM [RIM]. The definition of trust procedures for RIMs is out of
scope for RIV, and the device owner is free to use any policy to
validate a set of reference measurements. RIMs may be conveyed out-
of-band or in-band, as part of the attestation process (see
Section 3.1.3). But for embedded devices, where software is usually
shipped as a self-contained package, RIMs signed by the manufacturer
and delivered in-band may be more convenient for the device owner.
6. Conclusion
TCG technologies can play an important part in the implementation of
Remote Integrity Verification. Standards for many of the components
needed for implementation of RIV already exist:
o Platform identity can be based on IEEE 802.1AR Device identity,
coupled with careful supply-chain management by the manufacturer.
o Complex supply chains can be certified using TCG Platform
Certificates [Platform-Certificates]
o The TCG TAP mechanism can be used to retrieve attestation
evidence. Work is needed on a YANG model for this protocol.
o Reference Measurements must be conveyed from the software
authority (e.g., the manufacturer) to the system in which
verification will take place. IETF CoSWID work forms the basis
for this, but new work is needed to create an information model
and YANG implementation.
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7. IANA Considerations
This memo includes no request to IANA.
8. Appendix
8.1. Layering Model for Network Equipment Attester and Verifier
Retrieval of identity and attestation state uses one protocol stack,
while retrieval of Reference Measurements uses a different set of
protocols. Figure 5 shows the components involved.
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+-----------------------+ +-------------------------+
| | | |
| Attester |<-------------| Verifier |
| (Device) |------------->| (Management Station) |
| | | | |
+-----------------------+ | +-------------------------+
|
-------------------- --------------------
| |
---------------------------------- ---------------------------------
|Reference Integrity Measurements| | Attestation |
---------------------------------- ---------------------------------
********************************************************************
* IETF Attestation Reference Interaction Diagram *
********************************************************************
....................... .......................
. Reference Integrity . . TAP (PTS2.0) Info .
. Manifest . . Model and Canonical .
. . . Log Format .
....................... .......................
************************* .............. **********************
* YANG SWID Module * . TCG . * YANG Attestation *
* I-D.birkholz-yang-swid* . Attestation. * Module *
* * . MIB . * I-D.ietf-rats- *
* * . . * yang-tpm-charra *
************************* .............. **********************
************************* ************ ************************
* XML, JSON, CBOR (etc) * * UDP * * XML, JSON, CBOR (etc)*
************************* ************ ************************
************************* ************************
* RESTCONF/NETCONF * * RESTCONF/NETCONF *
************************ *************************
************************* ************************
* TLS, SSH * * TLS, SSH *
************************* ************************
Figure 7: RIV Protocol Stacks
IETF documents are captured in boxes surrounded by asterisks. TCG
documents are shown in boxes surrounded by dots. The IETF
Attestation Reference Interaction Diagram, Reference Integrity
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Manifest, TAP Information Model and Canonical Log Format, and both
YANG modules are works in progress. Information Model layers
describe abstract data objects that can be requested, and the
corresponding response SNMP is still widely used, but the industry is
transitioning to YANG, so in some cases, both will be required. TLS
Authentication with TPM has been shown to work; SSH authentication
using TPM-protected keys is not as easily done [as of 2019]
8.1.1. Why is OS Attestation Different?
Even in embedded systems, adding Attestation at the OS level (e.g.
Linux IMA, Integrity Measurement Architecture [IMA]) increases the
number of objects to be attested by one or two orders of magnitude,
involves software that's updated and changed frequently, and
introduces processes that begin in unpredictable order.
TCG and others (including the Linux community) are working on methods
and procedures for attesting the operating system and application
software, but standardization is still in process.
8.2. Implementation Notes
Table 1 summarizes many of the actions needed to complete an
Attestation system, with links to relevant documents. While
documents are controlled by several standards organizations, the
implied actions required for implementation are all the
responsibility of the manufacturer of the device, unless otherwise
noted.
+------------------------------------------------------------------+
| Component | Controlling |
| | Specification |
--------------------------------------------------------------------
| Make a Secure execution environment | TCG RoT |
| o Attestation depends on a secure root of | UEFI.org |
| trust for measurement outside the TPM, as | |
| well as roots for storage amd reporting | |
| inside the TPM. | |
| o Refer to TCG Root of Trust for Measurement.| |
| o NIST SP 800-193 also provides guidelines | |
| on Roots of Trust | |
--------------------------------------------------------------------
| Provision the TPM as described in | TCG TPM DevID |
| TCG documents. | TCG Platform |
| | Certificate |
--------------------------------------------------------------------
| Put a DevID or Platform Cert in the TPM | TCG TPM DevID |
| o Install an Initial Attestation Key at the | TCG Platform |
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| same time so that Attestation can work out | Certificate |
| of the box |-----------------
| o Equipment suppliers and owners may want to | IEEE 802.1AR |
| implement Local Device ID as well as | |
| Initial Device ID | |
--------------------------------------------------------------------
| Connect the TPM to the TLS stack | Vendor TLS |
| o Use the DevID in the TPM to authenticate | stack (This |
| TAP connections, identifying the device | action is |
| | simply |
| | configuring TLS|
| | to use the |
| | DevID as its |
| | trust anchor.) |
--------------------------------------------------------------------
| Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID |
| o Add reference measurements into SWID tags | ISO/IEC 19770-2|
| o Manufacturer should sign the SWID tags | NIST IR 8060 |
| o The TCG RIM-IM identifies further | |
| procedures to create signed RIM | |
| documents that provide the necessary | |
| reference information | |
--------------------------------------------------------------------
| Package the SWID tags with a vendor software | Retrieve tags |
| release | with |
| o A tag-generator plugin such | {{I-D.birkholz-yang-swid}}|
| as https://github.com/Labs64/swid-maven-plugin |
| can be used |----------------|
| | TCG PC Client |
| | RIM |
--------------------------------------------------------------------
| Use PC Client measurement definitions | TCG PC Client |
| to define the use of PCRs | BIOS |
| (although Windows OS is rare on Networking | |
| Equipment, UEFI BIOS is not) | |
--------------------------------------------------------------------
| Use TAP to retrieve measurements | |
| o Map TAP to SNMP | TCG SNMP MIB |
| o Map to YANG | YANG Module for|
| Use Canonical Log Format | Basic |
| | Attestation |
| | TCG Canonical |
| | Log Format |
--------------------------------------------------------------------
| Posture Collection Server (as described in IETF | |
| SACMs ECP) should request the | |
| attestation and analyze the result | |
| The Management application might be broken down | |
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| to several more components: | |
| o A Posture Manager Server | |
| which collects reports and stores them in | |
| a database | |
| o One or more Analyzers that can look at the| |
| results and figure out what it means. | |
--------------------------------------------------------------------
Figure 8: Component Status
8.3. Root of Trust for Measurement
The measurements needed for attestation require that the device being
attested is equipped with a Root of Trust for Measurement, i.e., some
trustworthy mechanism that can compute the first measurement in the
chain of trust required to attest that each stage of system startup
is verified, and a Root of Trust for Reporting to report the results
[TCGRoT], [GloPlaRoT].
While there are many complex aspects of a Root of Trust, two aspects
that are important in the case of attestation are:
o The first measurement computed by the Root of Trust for
Measurement, and stored in the TPM's Root of Trust for Storage, is
presumed to be correct.
o There must not be a way to reset the RTS without re-entering the
RTM code.
The first measurement must be computed by code that is implicitly
trusted; if that first measurement can be subverted, none of the
remaining measurements can be trusted. (See [NIST-SP-800-155])
9. Informative References
[AIK-Enrollment]
Trusted Computing Group, "TCG Infrastructure Working
GroupA CMC Profile for AIK Certificate Enrollment Version
1.0, Revision 7", March 2011,
<https://trustedcomputinggroup.org/wp-content/uploads/
IWG_CMC_Profile_Cert_Enrollment_v1_r7.pdf>.
[Canonical-Event-Log]
Trusted Computing Group, "DRAFT Canonical Event Log Format
Version: 1.0, Revision: .12", October 2018.
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[EFI-TPM] Trusted Computing Group, "TCG EFI Platform Specification
for TPM Family 1.1 or 1.2, Specification Version 1.22,
Revision 15", January 2014,
<https://trustedcomputinggroup.org/wp-content/uploads/EFI-
Protocol-Specification-rev13-160330final.pdf>.
[GloPlaRoT]
GlobalPlatform Technology, "Root of Trust Definitions and
Requirements Version 1.1", June 2018,
<https://globalplatform.org/specs-library/globalplatform-
root-of-trust-definitions-and-requirements/>.
[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
rats-tuda-02 (work in progress), March 2020.
[I-D.birkholz-yang-swid]
Birkholz, H., "Software Inventory YANG module based on
Software Identifiers", draft-birkholz-yang-swid-02 (work
in progress), October 2018.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-02 (work in progress), March
2020.
[I-D.ietf-rats-eat]
Mandyam, G., Lundblade, L., Ballesteros, M., and J.
O'Donoghue, "The Entity Attestation Token (EAT)", draft-
ietf-rats-eat-03 (work in progress), February 2020.
[I-D.ietf-rats-yang-tpm-charra]
Birkholz, H., Eckel, M., Bhandari, S., Sulzen, B., Voit,
E., Xia, L., Laffey, T., and G. Fedorkow, "A YANG Data
Model for Challenge-Response-based Remote Attestation
Procedures using TPMs", draft-ietf-rats-yang-tpm-charra-01
(work in progress), March 2020.
[I-D.ietf-sacm-coswid]
Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags", draft-
ietf-sacm-coswid-13 (work in progress), November 2019.
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[I-D.richardson-rats-usecases]
Richardson, M., Wallace, C., and W. Pan, "Use cases for
Remote Attestation common encodings", draft-richardson-
rats-usecases-07 (work in progress), March 2020.
[I-D.voit-rats-trusted-path-routing]
Voit, E., "Trusted Path Routing using Remote Attestation",
draft-voit-rats-trusted-path-routing-01 (work in
progress), March 2020.
[IEEE-802-1AR]
Seaman, M., "802.1AR-2018 - IEEE Standard for Local and
Metropolitan Area Networks - Secure Device Identity, IEEE
Computer Society", August 2018.
[IEEE-802.1ae]
Seaman, M., "802.1AE MAC Security (MACsec)", 2018,
<https://1.ieee802.org/security/802-1ae/>.
[IEEE-802.1x]
IEEE Computer Society, "802.1X-2020 - IEEE Standard for
Local and Metropolitan Area Networks--Port-Based Network
Access Control", February 2020,
<https://standards.ieee.org/standard/802_1X-2020.html>.
[IMA] and , "Integrity Measurement Architecture", June 2019,
<https://sourceforge.net/p/linux-ima/wiki/Home/>.
[LLDP] IEEE Computer Society, "802.1AB-2016 - IEEE Standard for
Local and metropolitan area networks - Station and Media
Access Control Connectivity Discovery", March 2016,
<https://standards.ieee.org/standard/802_1AB-2016.html>.
[NetEq] Trusted Computing Group, "TCG Guidance for Securing
Network Equipment", January 2018,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_Guidance_for_Securing_NetEq_1_0r29.pdf>.
[NIST-IR-8060]
National Institute for Standards and Technology,
"Guidelines for the Creation of Interoperable Software
Identification (SWID) Tags", April 2016,
<https://nvlpubs.nist.gov/nistpubs/ir/2016/
NIST.IR.8060.pdf>.
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[NIST-SP-800-155]
National Institute for Standards and Technology, "BIOS
Integrity Measurement Guidelines (Draft)", December 2011,
<https://csrc.nist.gov/csrc/media/publications/sp/800-
155/draft/documents/draft-sp800-155_dec2011.pdf>.
[PC-Client-BIOS-TPM-1.2]
Trusted Computing Group, "TCG PC Client Specific
Implementation Specification for Conventional BIOS,
Specification Version 1.21 Errata, Revision 1.00",
February 2012, <https://www.trustedcomputinggroup.org/wp-
content/uploads/TCG_PCClientImplementation_1-21_1_00.pdf>.
[PC-Client-BIOS-TPM-2.0]
Trusted Computing Group, "PC Client Specific Platform
Firmware Profile Specification Family "2.0", Level 00
Revision 1.04", June 2019,
<https://trustedcomputinggroup.org/pc-client-specific-
platform-firmware-profile-specification>.
[PC-Client-RIM]
Trusted Computing Group, "DRAFT: TCG PC Client Reference
Integrity Manifest Specification, v.09", December 2019,
<https://trustedcomputinggroup.org/xx>.
[Platform-Certificates]
Trusted Computing Group, "DRAFT: TCG Platform Attribute
Credential Profile, Specification Version 1.0, Revision
15, 07 December 2017", December 2017.
[Platform-DevID-TPM-2.0]
Trusted Computing Group, "DRAFT: TPM Keys for Platform
DevID for TPM2, Specification Version 0.7, Revision 0",
October 2018.
[Platform-ID-TPM-1.2]
Trusted Computing Group, "TPM Keys for Platform Identity
for TPM 1.2, Specification Version 1.0, Revision 3",
August 2015, <https://trustedcomputinggroup.org/wp-
content/uploads/
TPM_Keys_for_Platform_Identity_v1_0_r3_Final.pdf>.
[Provisioning-TPM-2.0]
Trusted Computing Group, "TCG TPM v2.0 Provisioning
Guidance", March 2015, <https://trustedcomputinggroup.org/
wp-content/uploads/TCG-TPM-v2.0-Provisioning-Guidance-
Published-v1r1.pdf>.
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[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>.
[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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/info/rfc8572>.
[RIM] Trusted Computing Group, "DRAFT: TCG Reference Integrity
Manifest Information Model", June 2019,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_RIM_Model_v1-r13_2feb20.pdf>.
[SWID] The International Organization for Standardization/
International Electrotechnical Commission, "Information
Technology Software Asset Management Part 2: Software
Identification Tag, ISO/IEC 19770-2", October 2015,
<https://www.iso.org/standard/65666.html>.
[TAP] Trusted Computing Group, "DRAFT: TCG Trusted Attestation
Protocol (TAP) Information Model for TPM Families 1.2 and
2.0 and DICE Family 1.0, Version 1.0, Revision 0.29",
October 2018.
[TCGRoT] Trusted Computing Group, "TCG Roots of Trust
Specification", October 2018,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_Roots_of_Trust_Specification_v0p20_PUBLIC_REVIEW.pdf>.
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Authors' Addresses
Guy Fedorkow (editor)
Juniper Networks, Inc.
US
Email: gfedorkow@juniper.net
Eric Voit
Cisco Systems, Inc.
US
Email: evoit@cisco.com
Jessica Fitzgerald-McKay
National Security Agency
US
Email: jmfitz2@nsa.gov
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