Network Device Attestation Workflow
draft-fedorkow-rats-network-device-attestation-00
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| Authors | Guy Fedorkow , Jessica Fitzgerald-McKay | ||
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draft-fedorkow-rats-network-device-attestation-00
RATS Working Group G. Fedorkow, Ed.
Internet-Draft Juniper Networks, Inc.
Intended status: Informational J. Fitzgerald-McKay
Expires: January 1, 2020 National Security Agency
June 30, 2019
Network Device Attestation Workflow
draft-fedorkow-rats-network-device-attestation-00
Abstract
This document describes a workflow for network device attestation.
Status of This Memo
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This Internet-Draft will expire on January 1, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Problem Description . . . . . . . . . . . . . . . . . . . 4
1.4. Solution Requirements . . . . . . . . . . . . . . . . . . 6
1.5. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1. Out of Scope . . . . . . . . . . . . . . . . . . . . 8
1.5.2. Why Remote Integrity Verification? . . . . . . . . . 8
1.5.3. Network Device Attestation Challenges . . . . . . . . 8
1.5.4. Why is OS Attestation Different? . . . . . . . . . . 9
2. Solution Outline . . . . . . . . . . . . . . . . . . . . . . 10
2.1. 2.1 RIV Software Configuration Attestation using TPM . . 10
2.2. RIV Keying . . . . . . . . . . . . . . . . . . . . . . . 11
2.3. RIV Information Flow . . . . . . . . . . . . . . . . . . 12
2.4. RIV Simplifying Assumptions . . . . . . . . . . . . . . . 13
2.4.1. DevID Alternatives . . . . . . . . . . . . . . . . . 14
2.4.2. Additional Attestation of Platform Characteristics . 14
2.4.3. Root of Trust for Measurement . . . . . . . . . . . . 15
2.4.4. Reference Integrity Measurements (RIMs) . . . . . . . 15
2.4.5. Attestation Logs . . . . . . . . . . . . . . . . . . 16
3. Standards Components . . . . . . . . . . . . . . . . . . . . 17
3.1. Reference Models . . . . . . . . . . . . . . . . . . . . 17
3.1.1. IETF Reference Model for Challenge-Response Remote
Attestation . . . . . . . . . . . . . . . . . . . . . 17
3.2. RIV Workflow . . . . . . . . . . . . . . . . . . . . . . 18
3.3. Layering Model for Network Equipment Attester and
Verifier . . . . . . . . . . . . . . . . . . . . . . . . 20
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 21
5. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Implementation Notes . . . . . . . . . . . . . . . . . . 22
5.2. Comparison with TCG PTS / IETF NEA . . . . . . . . . . . 24
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. Informative References . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
There are many components to consider in fielding a trusted computing
device, from operating systems to applications. Part of that is a
trusted supply chain, where manufacturers can certify that the
product they intended to build is actually the one that was installed
at a customer's site.
Attestation is defined here as the process of creating, conveying and
appraising assertions about Platform trustworthiness characteristics,
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including Roots of Trust, supply chain trust, identity, platform
provenance, shielded locations, protected capabilities, software
configuration, hardware configuration, platform composition,
compliance to test suites, functional and assurance evaluations, etc.
The supply chain itself has many elements, from validating suppliers
of electronic components, to ensuring that shipping procedures
protect against tampering through many stages of distribution and
warehousing. One element that helps maintain the integrity of the
supply chain after manufacturing is Attestation.
Within the Trusted Computing Group context, attestation is the
process by which an independent Verifier can obtain cryptographic
proof as to the identity of the device in question, evidence of the
integrity of software loaded on that device when it started up, and
then verify that what's there is what's supposed to be there. For
networking equipment, a verifier capability can be embedded in a
Network Management Station (NMS), a posture collection server, or
other network analytics tool (such as a software asset management
solution, or a threat detection and mitigation tool, etc.). While
informally referred to as attestation, this document focuses on a
subset defined here as Remote Integrity Verification (RIV). RIV
takes a network equipment centric perspective that includes a set of
protocols and procedures for determining whether a particular device
was launched with untampered software, starting from Roots of Trust.
While there are many ways to accomplish attestation, RIV sets out a
specific set of protocols and tools that work in environments
commonly found in Networking Equipment. RIV does not cover other
platform characteristics that could be attested, although it does
provide evidence of a secure infrastructure to increase the level of
trust in other platform characteristics attested by other means.
This profile outlines the RIV problem, and then identifies components
that are necessary to get the complete attestation procedure working
in a scalable solution using commercial products.
This document focuses primarily on software integrity verification
using the Trusted Platform Module (TPM) as a root of trust.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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1.2. Goals
The RIV attestation workflow outlined in this document is intended to
meet the following high-level goals:
o Provable Device Identity - The ability to identify a device using
a cryptographic identifier is a critical prerequisite to software
inventory attestation.
o Software Inventory - A key goal is to identify the software
release installed on the device, and to provide evidence of its
integrity.
o Verification - Verification of software and configuration of the
device shows that the software that's supposed to be installed on
there actually has been launched, without unauthorized
modification.
This document itself is non-normative; the document does not define
protocols, but rather identifies protocols that can be used together
to achieve the goals above, and in some cases, highlights gaps in
existing protocols.
1.3. Problem Description
RIV is a procedure that assures a network operator that the equipment
on their network can be reliably identified, and that untampered
software of a known version is installed on each endpoint. In this
context, endpoint might include the conventional endpoints like
servers and laptops, but also network equipment itself, such as
routers, switches and firewalls.
RIV can be viewed as a link in a trusted supply chain, and includes
three major processes:
o Creation of Evidence is the process whereby an endpoint generates
cryptographic proof (evidence) of claims about platform
properties. In particular, the platform identity and its software
configuration are of critical importance.
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* Platform Identity refers to the mechanism assuring the
attestation verifier (typically a network administrator)
that the equipment on their network can be reliably identified,
and that its manufacturer is certified by a trusted authority.
This certification provides the verifier with assurance that
the Root of Trust elements of the device were verified by the
manufacturer before the device was shipped.
* Software used to boot a platform can be described as a chain
of measurements, starting from a Root of Trust for Measurement,
that normally ends when the system software is loaded.
Measurement records the identity, integrity and version of each
software component registered with the TPM, so that the
subsequent appraisal stage can determine whether the software
installed is authentic and free of tampering. Clearly the second
part of the problem, attesting the state of mutable components
of a given device, is of little value without the first part,
reliable identification of the device in question. By the
same token, unambiguous identity of a device is necessary,
but is insufficient to assure the operator of the provenance
of the device through the supply chain, or that the device is
configured to behave properly.
o Conveyance of Evidence is the process of reliably transporting
evidence from an endpoint to an appraiser/verifier, e.g. a
management station. The transport is typically carried out via a
management network. The channel must provide integrity and
authenticity, and, in some use cases, may also require
confidentiality.
o Appraisal of Evidence is the process of verifying the evidence
received by a verifier/appraiser from a device, and using verified
evidence to inform decision making. In this context, verification
means comparing the device characteristics reported as evidence
with the configuration expected by the system administrator. This
step can work only when there is a way to express what should be
there, often referred to as golden measurements, or Reference
Integrity Measurements, representing the intended configured state
of an endpoint.
As a part of a trusted supply chain, RIV attestation provides two
important benefits:
o Platform Identity is the mechanism providing trusted identity can
reassure network managers that the specific devices they ordered
from authorized manufacturers for attachment to their network are
the ones that were installed, and that they continue to be present
in their network. As part of the mechanism for Platform Identity,
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cryptographic proof of the identity of the manufacturer is also
provided.
o Software Configuration is the mechanism that reports the state of
mutable components on the device can assure network managers that
they have known, untampered software configured to run in their
network.
An implementation of RIV requires three technologies
1. Identity: Platform identity can be based on IEEE 802.1AR Device
Identity [IEEE-802-1AR], coupled with careful supply-chain
management by the manufacturer. The DevID certificate contains a
statement by the manufacturer that establishes the provenance of
the device as it left the factory. Some applications with a
more-complex post-manufacture supply chain (e.g. Value Added
Resellers), or with privacy concerns, may want to use an
alternate mechanism for platform authentication based on TCG
Platform Certificates [Platform-Certificates].
2. Platform Attestation provides evidence of configuration of
software elements throughout the product lifecycle. This form of
attestation can be implemented with TPM PCR, Quote and log
mechanisms, which provide an authenticated mechanism to report
what software actually starts up on the device each time it
reboots. Note that the TPM requires separate keys for identity
DevID) and attestation (PCR Quotes0) (see Section 2.2).
3. Reference Integrity Measurements must be conveyed from the
software authority (often the manufacturer for embedded systems)
to the system in which verification will take place
Network operators benefit from a trustworthy attestation mechanism
that provides assurance that their comprises authentic equipment, and
has loaded software free of known vulnerabilities and unauthorized
tampering.
1.4. Solution Requirements
An Attestation solution must meet a number of requirements to make it
simple to deploy at scale.
1. Easy to Use - This solution should work "out of the box" as far
as possible, that is, with the fewest possible steps needed at
the end-user's site. Eliminate complicated databases or
provisioning steps that would have to be executed by the owner of
a new device. Network equipment is often required to "self-
configure", to reliably reach out without manual intervention to
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prove its identity and operating posture, then download its own
configuration. See [RFC8572] for an example of Secure Zero Touch
Provisioning.
2. Multi-Vendor - This solution should identify standards-based
interfaces that allow attestation to work with attestation-
capable devices and verifiers supplied by different vendors in
one network.
3. Scalable - The solution must not depend on choke points that
limit the number of endpoints that could be evaluated in one
network domain.
4. Extensible - A network equipment attestation solution needs to
expand over time as new features are added. The solution must
allow new features to be added easily, providing for a smooth
transition and allowing newer and older architectural components
to continue to work together. Further, a network equipment
attestation solution and the specifications referenced here must
define safe extensibility mechanisms that enable innovation
without breaking interoperability.
5. Efficient - A network equipment attestation solution should, to
the greatest extent feasible, continuously monitor the health and
posture status of network devices. Posture measurements should
be updated in real-time as changes to device posture occur and
should be published to remote integrity validators. Validation
reports should also be shared with their relying parties (for
example, network administrators, or network analytics that rely
on these reports for posture assessment) as soon as they are
available.
1.5. Scope
This document includes a number of assumptions to limit the scope:
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
o This document applies primarily to "embedded" applications, where
the device manufacturer ships the software image for the device.
o The approach outlined in this document assumes a physical TPM.
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1.5.1. Out of Scope
o Run-Time Attestation: Run-time attestation of Linux or other
multi-threaded operating system processes considerably expands the
scope of the problem. Many researchers are working on that
problem, but this document defers the run-time attestation
problem.
o Multi-Vendor Embedded Systems: Additional coordination would be
needed for devices that themselves comprise hardware and software
from multiple vendors, integrated by the end user.
o Processor Sleep Modes: Embedded equipment typically does not
"sleep", so sleep and hibernate modes are not considered.
o Virtualization and Containerization: These technologies are
increasingly used in embedded systems, but are not considered in
this revision of the document.
1.5.2. Why Remote Integrity Verification?
Remote Integrity Verification can go a long way to solving the "Lying
Endpoint" problem, in which malicious software on an endpoint may
both subvert the intended function, and also prevent the endpoint
from reporting its compromised status. Man-in-the Middle attacks are
also made more difficult through a strong focus on device identity
Attestation data can be used for asset management, vulnerability and
compliance assessment, plus configuration management.
1.5.3. Network Device Attestation Challenges
There have been demonstrations of attestation using TPMs for years,
accompanied by compelling security reasons for adopting attestation.
Despite this, the technology has not been widely adopted, in part,
due to the difficulties in deploying TPM-based attestation. Some of
those difficulties are:
o Standardizing device identity. Creating and using unique device
identifiers is difficult, especially in a privacy-sensitive
environment. But attestation is of limited value if the operator
is unable to determine which devices pass attestation validation
tests, and which fail. This problem is substantially simplified
for infrastructure devices like network equipment, where identity
can be explicitly coded using IEEE 802.1AR, but doing so relies on
adoption of 802.1AR [IEEE-802-1AR] by manufacturers and hardware
system integrators.
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o Standardizing attestation representations and conveyance.
Interoperable remote attestation has a fundamental dependence on
vendors agreeing to a limited set of network protocols for
communicating attestation data. Network device vendors will be
slow to adopt the protocols necessary to implement remote
attestation without a fully-realized plan for deployment.
o Interoperability. Networking equipment operates in a
fundamentally multi-vendor environment, putting additional
emphasis on the need for standardized procedures and protocols.
o Attestation evidence is complex. Operating systems used in larger
embedded devices are often multi-threaded, so the order of
completion for individual processes is non-deterministic. While
the hash of a specific component is stable, once extended into a
PCR, the resulting values are dependent on the (non-deterministic)
ordering of events, so there will never be a single known-good
value for some PCRs. Careful analysis of event logs can provide
proof that the expected modules loaded, but it's much more
complicated than simply comparing hashes.
o Software configurations can have seemingly infinite variability.
This problem is nearly intractable on PC and Server equipment,
where end users have unending needs for customization and new
applications. However, embedded systems, like networking
equipment, are often simpler, in that there are fewer variations
and releases, with vendors typically offering fewer options for
mixing and matching.
o Software updates can be complex. Even the most organized network
operator may have many different releases in their network at any
given time, with the result that there's never a single digest or
fingerprint that indicates the software is "correct"; digests
formed by hashing software modules on a device can only show the
correct combination of versions for a specific device at a
specific time.
None of these issues are insurmountable, but together, they've made
deployment of attestation a major challenge. The intent of this
document is to outline an attestation profile that's simple enough to
deploy, while yielding enough security to be useful.
1.5.4. Why is OS Attestation Different?
Even in embedded systems, adding Attestation at the OS level (e.g.
Linux IMA, Integrity Measurement Architecture [IMA]) increases the
number of objects to be attested by one or two orders of magnitude,
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involves software that's updated and changed frequently, and
introduces processes that complete in unpredictable order.
TCG and others (including the Linux community) are working on methods
and procedures for attesting the operating system and application
software, but standardization is still in process.
2. Solution Outline
2.1. 2.1 RIV Software Configuration Attestation using TPM
RIV Attestation is a process for determining the identity of software
running on a specifically-identified device. Remote Attestation is
broken into two phases, shown in Figure 1:
o During system startup, measurements (i.e., hashes computed as
fingerprints of files) are extended, or cryptographically folded,
into the TPM. Entries are also added to an informational log.
The measurement process generally follows the Chain of Trust model
used in Measured Boot, where each stage of the system measures the
next one before launching it.
o Once the device is running and has operational network
connectivity, a separate, trusted server (called a Verifier in
this document) can interrogate the network device to retrieve the
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 corresponding to the TPM's attestation
private key, and can validate the software that was launched by
comparing digests in the log with known-good values, and verifying
their correctness by comparing with the signed digests from the TPM.
It should be noted that attestation and identity are inextricably
linked; signed evidence that a particular version of software was
loaded is of little value without cryptographic proof of the identity
of the device producing the evidence.
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+-------------------------------------------------------+
| +--------+ +--------+ +--------+ +---------+ |
| | BIOS |--->| Loader |-->| Kernel |--->|Userland | |
| +--------+ +--------+ +--------+ +---------+ |
| | | | |
| | | | |
| +------------+-----------+-+ |
| Step 1 | |
| V |
| +--------+ |
| | TPM | |
| +--------+ |
| Router | |
+--------------------------------|----------------------+
|
| Step 2
| +-----------+
+--->| Verifier |
+-----------+
Reset---------------flow-of-time-during-boot--...------->
In Step 1, measurements are "extended" into the TPM as processes
start. In Step 2, signed PCR digests are retreived from the TPM for
offbox analysis after the system is operational.
Figure 1: TCG Attestation Model
2.2. RIV Keying
TPM 1.2 and TPM 2.0 have a variety of rules separating the functions
of identity and attestation, allowing for use-cases where software
configuration must be attested, but privacy must be maintained.
To accommodate these rules in an environment where device privacy is
not normally a requirement, the TCG Guidance for Securing Network
Equipment [NetEq] suggests using separate keys for Identity (i.e.,
DevID) and Attestation (i.e., signing a quote of the contents of the
PCRs).
In this case, the device manufacturer should provision an Initial
Attestation Key (IAK) and x.509 certificate that parallels the
IDevID, with the same device ID information as the IDevID certificate
(i.e., the same Subject Name and Subject Alt Name, even though the
key pairs are different). This allows a quote from the device,
signed by the IAK, to be linked directly to the device that provided
it, by examining the corresponding IAK certificate.
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Inclusion of an IAK by a vendor does not preclude a mechanism whereby
an Administrator can define Local Attestation Keys (LAKs) if desired.
2.3. RIV Information Flow
RIV workflow for networking equipment is organized around a simple
use-case, where a network operator wishes to verify the integrity of
software installed in specific, fielded devices. This use-case
implies several components:
1. A Device (e.g. a router or other embedded device, also known as
an Attester) somewhere and the network operator wants to examine
its boot state.
2. A Verifier (which might be a network management station)
somewhere separate from the Device that will retrieve the
information and analyze it to pass judgement on the security
posture of the device.
3. A Relying Party, which has access to the Verifier to request
attestation and to act on results.
4. This document assumes that signed Reference Integrity
Measurements (RIMs) (aka "golden measurements") can either be
created by the device manufacturer and shipped along with the
device as part of its software image, or alternatively, could be
obtained a number of other ways (direct to the verifier from the
manufacturer, from a third party, from the owner's observation of
what's thought to be a "known good system", etc.). Retrieving
RIMs from the device itself allows attestation to be done in
systems which may not have access to the public internet, or by
other devices that are not management stations per-se (e.g., a
peer device). If reference measurements are obtained from
multiple sources, the Verifier may need to evaluate the relative
level of trust to be placed in each source in case of a
discrepancy.
These components are illustrated in Figure 2.
A more-detailed taxonomy of terms is given in [I-D.birkholz-rats-
architecture]
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+---------------+ +-------------+ +---------+--------+
| | | Attester | Step 1 | Verifier| |
| Asserter | | (Device |<-------| (Network| Relying|
| (Device | | under |------->| Mngmt | Party |
| Manufacturer | | attestation)| Step 2 | Station)| |
| or other | | | | | |
| authroity) | | | | | |
+---------------+ +-------------+ +---------+--------+
| /\
| Step 0 |
-----------------------------------------------
In Step 0, The Asserter (the device manufacturer) provides a Software
Image accompanied by Reference Integrity Measurements (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.
Figure 2: RIV Reference Configuration for Network Equipment
See Section 3.1.1 for more narrowly defined terms related to
Attestation
2.4. RIV Simplifying Assumptions
This document makes the following simplifying assumptions to reduce
complexity:
o The product to be attested is shipped with an IEEE 802.1AR DevID
and an Initial Attestation Key (IAK) with certificate. The IAK
cert contains the same identity information as the DevID
(specifically, the same Subject Name and Subject Alt Name, signed
by the manufacturer), but it's a type of key that can be used to
sign a TPM Quote. This convention is described in TCG Guidance
for Securing Network Equipment [NetEq]. For network equipment,
which is generally non-privacy-sensitive, shipping a device with
both an IDevID and an IAK already provisioned substantially
simplifies initial startup. Privacy-sensitive applications may
use the TCG Platform Certificate and additional procedures to
install identity credentials on the platform after manufacture.
(See Section 2.3.1 below for the Platform Certificate alternative)
o The product is equipped with a Root of Trust for Measurement, Root
of Trust for Storage and Root of Trust for Reporting that is
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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 [RIM].
2.4.1. DevID Alternatives
Some situations may have privacy-sensitive requirements that preclude
shipping every device with an Initial Device ID installed. In these
cases, the IDevID can be installed remotely using the TCG Platform
Certificate [Platform-Certificates].
Some security-sensitive administrators may want to install their own
identity credentials to certify platform identity and attestation
results. IEEE 802.1AR [IEEE-802-1AR] allows for both Initial Device
Identity credentials, installed by the manufacturer, or Local Device
Identity credentials installed by the administrator of the platform.
TCG TPM 2.0 Keys documents [Platform-DevID-TPM-2.0] and
[PC-Client-BIOS-TPM-2.0] specifies analogous Initial and Local
Attestation Keys (IAK and LAK), and contains figures showing the
relationship between IDevID, LDevID, IAK and LAK keys.
Platform administrators are free to use any number of criteria to
judge authenticity of a platform before installing local identity
keys, as part of an on-boarding process. The TCG TPM 2.0 Keys
document [Platform-DevID-TPM-2.0] also outlines procedures for
creating Local Attestation Keys and Local Device IDs (LDevIDs) rooted
in the manufacturer's IDevID as a check to reduce the chances that
counterfeit devices are installed in the network.
Note that many networking devices are expected to self-configure (aka
Zero Touch Provisioning). Current standardized zero-touch mechanisms
such as [RFC8572] assume that identity keys are already in place
before network on-boarding can start.
2.4.2. Additional Attestation of Platform Characteristics
The Platform Attribute Credential [Platform-Certificates] can also be
used to convey additional information about a platform from the
manufacturer or other entities in the supply chain. While outside
the scope of RIV, the Platform Attribute Credential can deliver
information such as lists of serial numbers for components embedded
in a device or security assertions related to the platform, signed by
the manufacturer, system integrator or value-added-reseller.
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2.4.3. Root of Trust for Measurement
The measurements needed for attestation require that the device being
attested is equipped with a Root of Trust for Measurement, i.e., some
trustworthy mechanism that can take 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
[Roots-of-Trust].
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 taken by the Root of Trust for Measurement,
and stored in the Root of Trust for Storage, is presumed to be
correct.
o There must not be a way to reset the RTM without re-entering the
RTS code.
The first measurement can't be checked by a code that's been
previously checked by something further back up the chain (it's the
first, after all); if that first measurement can be subverted, none
of the remaining measurements can be trusted. (See [NIST-SP-800-155]
2.4.4. Reference Integrity Measurements (RIMs)
Much of attestation focuses on collecting and transmitting 'evidence'
in the form of PCR measurements and attestation logs. But the
critical part of the process is enabling the verifier to decide
whether the measured hashes are "the right ones" or not.
While it must be up to network administrators to decide what they
want on their networks, the software supplier should supply the
Reference Integrity Measurements, (aka Golden Measurements or "known
good" hash digests) that may be used by a verifier to determine if
evidence shows known good, known bad or unknown software
configurations.
In general, there are two kinds of reference measurements:
1. Measurements of early system startup (e.g., BIOS, boot loader, OS
kernel) are essentially single threaded, and executed exactly
once, in a known sequence, before any results could be reported.
In this case, while the method for computing the hash and
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 PCR after
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the box has booted. In this case, the signed reference
measurement simply lists the expected hash for the given version.
2. Measurements taken later in operation of the system, once an OS
has started (for example, Linux IMA[16.]), 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 CoSWID
tags, but the SWID structure in the second case is somewhat more
complex. An example of how CoSWIDs could be incorporated into a
reference manifest can be found in the IETF Internet-Draft "A SUIT
Manifest Extension for Concise Software Identifiers"
[I-D.birkholz-suit-coswid-manifest].
The TCG has done exploratory work in defining formats for reference
integrity manifests under the working title TCG Reference Integrity
Manifest [RIM].
2.4.5. Attestation Logs
Quotes from a TPM can provide evidence of the state of a device at
the time the quote was requested, but to make sense of the quote in
most cases an event log of what software modules contributed which
values to the quote during startup must also be provided. The log
needs not be secured, but it is essential that the logs contain
enough information to exactly reconstruct the state of whatever went
into the quote (e.g., PCR values).
TCG has defined several event log formats:
o Legacy BIOS event log (TCG PC Client Specific Implementation
Specification for Conventional BIOS,
Section 11.3[PC-Client-BIOS-TPM-1.2])
o UEFI BIOS event log (TCG EFI Platform Specification for TPM Family
1.1 or 1.2, Section 7 [EFI])
o Canonical Event Log [Canonical-Event-Log]
It should be noted that a given device might use more than one event
log format (e.g., a UEFI log during initial boot, switching to
Canonical Log when the host OS launches).
The TCG SNMP Attestation MIB [SNMP-Attestation-MIB] will support any
record-oriented log format, including the three TCG-defined formats,
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but it currently leaves figuring out which log(s) are in what format
up to the Verifier.
3. Standards Components
3.1. Reference Models
3.1.1. IETF Reference Model for Challenge-Response Remote Attestation
Initial work at IETF defines remote attestation as follows:
o The Reference Interaction Model for Challenge-Response-based
Remote Attestation is based on the standard roles defined in
[I-D.birkholz-rats-architecture]:
o
* Attester: The role that designates the subject of the remote
attestation. A system entity that is the provider of evidence
takes on the role of an Attester.
* Verifier: The role that designates the system entity and that
is the appraiser of evidence provided by the Attester. A
system entity that is the consumer of evidence takes on the
role of a Verifier.
The following diagram illustrates a common information flow between a
Verifier and an Attester, specified in
[I-D.birkholz-rats-reference-interaction-model]:
Attester Verifier
| |
| <------- requestAttestation(nonce, authSecID, claimSelection) |
| |
collectAssertions(assertionsSelection) |
| => assertions |
| |
signAttestationEvidence(authSecID, assertions, nonce) |
| => signedAttestationEvidence |
| |
| signedAttestationEvidence ----------------------------------> |
| |
| verifyAttestationEvidence(signedAttestationEvidence, refassertions)
| attestationResult <= |
| |
Figure 3: IETF Attestation Information Flow
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The RIV approach outlined in this document aligns with the RATS
reference model.
3.2. RIV Workflow
The overall flow for an attestation session is shown in Figure 4. In
this diagram:
o Step 0, positioning of the signed reference measurements, may
happen in two ways:
o Step 0A below shows a verifier obtaining reference measurements
directly from a software configuration authority, whether it's the
vendor or another authority chosen by the system administrator.
The reference measurements are signed by the Asserter (i.e., the
software configuration authority).
o - Or - Step 0B, the reference measurements, signed by the
Asserter, may be distributed as part of software installation,
long before the attestation session begins. Software installation
is usually vendor-dependent, so there are no standards involved in
this step. However, the verifier can use the same protocol to
obtain the reference measurements from the device as it would have
used with an external reference authority
o In Step 1, the Verifier initiates an attestation session by
opening a TLS session, validated using the DevID to prove that the
connection is attesting the right box.
o In Step 2, measured values are retrieved from the Attester's TPM
using a YANG or SNMP interface that implements the TCG TAP model
(e.g. YANG Module for Basic Challenge-Response-based Remote
Attestation Procedures
[I-D.birkholz-yang-basic-remote-attestation]).
o In Step 3, the Attester also delivers a copy of the signed
reference measurements, using Software Inventory YANG module based
on Software Identifiers [I-D.birkholz-yang-swid].
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+---------------+ +-------------+ +---------+
| | | | Step 1 | |
| | | Attester |<------>| Verifier|
| Asserter | | (Device |<------>| (Network|
|(Configuration |------->| under | Step 2 | Mngmt |
| Authority) | Step 0A| attestation)| | Station)|
| | | |------->| |
+---------------+ +-------------+ Step 3 +---------+
| /|\
| |
----------------------------------------------
Step 0B
Either CoSWID-encoded reference measurements are signed by a trusted
authority and retrieved directly prior to attestation (as shown in
Step 0A), or CoSWID-encoded reference measurements are signed by the
device manufacturer, installed on the device by a proprietary
installer, and delivered during attestation (as shown in Step 0B).
In Step 1, the Verifier initiates a connection for attestation. The
Attester's identity is validated using DevID with TLS. In Step 2, a
nonce, quotes (measured values) and measurement log are conveyed via
TAP with a protocol-specific binding (e.g. SNMP). Logs are sent in
the Canonical Log Format In Step 3, CoSWID-encoded reference
measurements are retrieved from the Attester using the YANG
([I-D.birkholz-yang-swid]. .
Figure 4: RIV Protocol and Encoding Summary
The following components are used:
1. TPM Keys are configured according to [Platform-DevID-TPM-2.0],
[PC-Client-BIOS-TPM-1.2], or [Platform-ID-TPM-1.2]
2. Measurements of bootable modules are taken according to TCG PC
Client [PC-Client-BIOS-TPM-2.0] and Linux IMA [IMA]
3. Device Identity is managed by IEEE 802.1AR certificates
[IEEE-802-1AR], with keys protected by TPMs.
4. Quotes are retrieved according to TCG TAP Information Model [TAP]
5. Reference Integrity Measurements are encoded as CoSWID tags, as
defined in the TCG RIM document [RIM], compatible with NIST IR
8060 [NIST-IR-8060] and the IETF CoSWID draft
[I-D.ietf-sacm-coswid]. Reference measurements are signed by the
device manufacturer.
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3.3. Layering Model for Network Equipment Attester and Verifier
Retrieval of identity and attestation state uses one protocol stack,
while retrieval of Reference Measurements uses a different set of
protocols. Figure 5 shows the components involved.
+-----------------------+ +-------------------------+
| | | |
| Attester |<-------------| Verifier |
| (Device) |------------->| (Management Station) |
| | | | |
+-----------------------+ | +-------------------------+
|
-------------------- --------------------
| |
---------------------------------- ---------------------------------
|Reference Integrity Measurements| | Attestation |
---------------------------------- ---------------------------------
********************************************************************
* IETF Attestation Reference Interaction Diagram *
********************************************************************
....................... .......................
. Reference Integrity . . TAPS (PTS2.0) Info .
. Measurement . . Model and Canonical .
. Manifest . . Log Format .
....................... .......................
************************* .............. **********************
* YANG SWID Module * . TCG . * YANG Attestation *
* I-D.birkholz-yang-swid* . Attestation. * Module *
* * . MIB . * I-D.birkholz-yang- *
* * . . * basic-remote- *
* * . . * attestation *
************************* .............. **********************
************************* ************ ************************
* XML, JSON, CBOR (etc) * * UDP * * XML, JSON, CBOR (etc)*
************************* ************ ************************
************************* ************************
* RESTCONF/NETCONF * * RESTCONF/NETCONF *
************************ *************************
************************* ************************
* TLS, SSH * * TLS, SSH *
************************* ************************
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IETF documents are captured in boxes surrounded by asterisks. TCG
documements are shown in boxes surrounded by dots. The IETF
Attestation Reference Interaction Diagram, Reference Intefrity
Measurment Manifest, TAPS 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]
Figure 5: RIV Protocol Stacks
4. 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.
Gaps still exist for implementation in Network Equipment (as of May
2019):
o Coordination of YANG model development with the IETF is still
needed
o Specifications for management of signed Reference Integrity
Measurements must still be completed
5. Appendix
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5.1. Implementation Notes
Table 1 summarizes many of the actions needed to complete an
Attestation system, with links to relevant documents. While
documents are controlled by a number of 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 entirely on a secure | UEFI.org |
| root of trust for measurement. | |
| o Refer to TCG Root of Trust for Measurement.| |
| o NIST SP 800-193 also provides guidelines | |
| on Roots of Trust | |
--------------------------------------------------------------------
| Get a TPM properly provisioned as described in | TCG TPM DevID |
| TCG documents. | TCG Platform |
| | Certificate |
--------------------------------------------------------------------
| Put a DevID or Platform Cert in the TPM | TCG TPM DevID |
| o Install an Initial Attestation Key at the | TCG Platform |
| same time so that Attestation can work out | Certificate |
| of the box |-----------------
| o Equipment suppliers and owners may want to | IEEE 802.1AR |
| implement Local Device ID as well as | |
| Initial Device ID | |
--------------------------------------------------------------------
| Connect the TPM to the TLS stack | Vendor TLS |
| o Use the DevID in the TPM to authenticate | stack (This |
| TAP connections, identifying the device | action is |
| | simply |
| | configuring TLS|
| | to use the |
| | DevID as its |
| | trust anchor.) |
--------------------------------------------------------------------
| Make CoSWID tags for BIOS/LoaderLKernel objects | IETF CoSWID |
| o Add reference measurements into SWID tags | ISO/IEC 19770-2|
| o Manufacturer should sign the SWID tags | NIST IR 8060 |
| o This should be covered in a new TCG | TagVault SWID |
| Reference Integrity Manifest document | Tag Signing |
| - IWG should define the literal SWID | Guidance |
| format |-----------------
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| - IWG should evaluate whether IETF SUIT | TCG RIM |
| is a suitable manifest when multiple | |
| SWID tags are involved | |
| - There could be a proof-of-concept | |
| project to actually make sample SWID | |
| tags (a gap might appear in the | |
| process) | |
--------------------------------------------------------------------
| Package the SWID tags with a vendor software | There is no |
| release | need to specify|
| o A tag-generator plugin could help | where the tags |
| (i.e., a plugin for common development | are stored in a|
| environments. NIST has something that | vendor OS, as |
| plugs into Maven Build Environment) | long as there |
| | is a standards-|
| | based mechanism|
| | to retrieve |
| | them. |
| |-----------------
| | TCG RIM |
--------------------------------------------------------------------
| BIOS SWIDs might be hard to manage on an OS | |
| disk-- maybe keep them in the BIOS flash? | TCG RIM |
| o Maybe a UEFI Var? Would its name have to be| |
| specified by UEFI.org? | |
| o How big is a BIOS SWID tag? Do we need to | |
| use a tag ID instead of an actual tag? | |
| o Note that the presence of Option ROMs turns | |
| the BIOS reference measurements into a | |
| multi-vendor interoperability problem | |
--------------------------------------------------------------------
| Use PC Client measurement definitions as a | TCG PC Client |
| starting point to define the use of PCRs | BIOS |
| (although Windows OS is rare on Networking |-----------------
| Equipment) | There have been|
| | proposals for |
| | non-PC-Client |
| | allocation of |
| | PCRs, although |
| | no specific |
| | document exists|
| | yet. |
--------------------------------------------------------------------
| Use TAP to retrieve measurements | |
| o Map TAP to SNMP | TCG SNMP MIB |
| o Map to YANG | YANG Module for|
| o Complete Canonical Log Format | Basic |
| | Attestation |
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| | TCG Canonical |
| | Log Format |
--------------------------------------------------------------------
| Posture Collection Server (as described in IETF | |
| SACMs ECP) would have to request the | |
| attestation and analyze the result | |
| The Management application might be broken down | |
| to several more components: | |
| o A Posture Manager Server | |
| which collects reports and stores them in | |
| a database | |
| o One or more Analyzers that can look at the| |
| results and figure out what it means. | |
--------------------------------------------------------------------
Figure 6: Component Status
5.2. Comparison with TCG PTS / IETF NEA
Some components of an Attestation system have been implemented for
end-user machines such as PCs and laptops. Figure 7 shows the
corresponding protocol stacks.
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+-----------------------+ +-------------------------+
| | | |
| Attester |<-------------| Verifier |
| (Device) |------------->| (Management Station) |
| | | | |
+-----------------------+ | +-------------------------+
|
-------------------- --------------------
| |
---------------------------------- ---------------------------------
|Reference Integrity Measurements| | Attestation |
---------------------------------- ---------------------------------
--------------------------------------------------------------------
| IETF Attestation Reference Interaction Diagram |
-------------------------------------------------------------------
....................... --------------------------------
. No Existing . | TAPS (PTS2.0) Info Model and|
. Reference Integrity . | Canonical Log Format |
. Measurement Manifest. | |
. Protocols Exist . --------------------------------
. .
. . ---------------------- ----------
. . | YANG Attestation | |IETF NEA|
. . | Module | | Msg and|
. . | I-D.birkholz-yang- | | Attrib.|
. . | basic-remote- | | for PA-|
. . | attestation | | TNC |
. . ---------------------- ----------
. . ---------------------- ----------
. . | XML, JSON, CBOR | | PT-TLS |
. . ---------------------- | (for |
. . ---------------------- |endpoint|
. . | NETCONF, RESTCONF, | |mcahines|
. . | COAP | | |
....................... ---------------------- ----------
----------------------------------------------------------------
| TLS, SSH |
----------------------------------------------------------------
Figure 7: Attestation for End User Computers
6. IANA Considerations
This memo includes no request to IANA.
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7. Security Considerations
TBD
8. Informative References
[Canonical-Event-Log]
Trusted Computing Group, "DRAFT Canonical Event Log Format
Version: 1.0, Revision: .12", October 2018.
[EFI] 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>.
[Firmware-Profile]
Trusted Computing Group, "Trusted Computing Group, PC
Client Specific Platform Firmware Profile Specification
Family "2.0", Level 00 Revision 1.03 Version 51", June
2019, <https://trustedcomputinggroup.org/pc-client-
specific-platform-firmware-profile-specification/>.
[I-D.birkholz-rats-architecture]
Birkholz, H., Wiseman, M., Tschofenig, H., and N. Smith,
"Architecture and Reference Terminology for Remote
Attestation Procedures", draft-birkholz-rats-
architecture-01 (work in progress), March 2019.
[I-D.birkholz-rats-reference-interaction-model]
Birkholz, H. and M. Eckel, "Reference Interaction Model
for Challenge-Response-based Remote Attestation", draft-
birkholz-rats-reference-interaction-model-00 (work in
progress), March 2019.
[I-D.birkholz-suit-coswid-manifest]
Birkholz, H., "A SUIT Manifest Extension for Concise
Software Identifiers", draft-birkholz-suit-coswid-
manifest-00 (work in progress), July 2018.
[I-D.birkholz-yang-basic-remote-attestation]
Birkholz, H., Eckel, M., Bhandari, S., Sulzen, B., Voit,
E., and G. Fedorkow, "YANG Module for Basic Challenge-
Response-based Remote Attestation Procedures", draft-
birkholz-yang-basic-remote-attestation-01 (work in
progress), October 2018.
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[I-D.birkholz-yang-swid]
Birkholz, H., "Software Inventory YANG module based on
Software Identifiers", draft-birkholz-yang-swid-02 (work
in progress), October 2018.
[I-D.ietf-sacm-coswid]
Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags", draft-
ietf-sacm-coswid-11 (work in progress), June 2019.
[IEEE-802-1AR]
Seaman, M., "802.1AR-2018 - IEEE Standard for Local and
Metropolitan Area Networks - Secure Device Identity, IEEE
Computer Society", August 2018.
[IMA] and , "Integrity Measurement Architecture", June 2019,
<https://sourceforge.net/p/linux-ima/wiki/Home/>.
[NetEq] Trusted Computing Group, "TCG Guidance for Securing
Network Equipment", January 2018,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_Guidance_for_Securing_NetEq_1_0r29.pdf>.
[NIST-IR-8060]
National Institute for Standards and Technology,
"Guidelines for the Creation of Interoperable Software
Identification (SWID) Tags", April 2016,
<https://nvlpubs.nist.gov/nistpubs/ir/2016/
NIST.IR.8060.pdf>.
[NIST-SP-800-155]
National Institute for Standards and Technology, "BIOS
Integrity Measurement Guidelines (Draft)", December 2011,
<https://csrc.nist.gov/csrc/media/publications/sp/800-
155/draft/documents/draft-sp800-155_dec2011.pdf>.
[PC-Client-BIOS-TPM-1.2]
Trusted Computing Group, "TCG PC Client Specific
Implementation Specification for Conventional BIOS,
Specification Version 1.21 Errata, Revision 1.00",
February 2012, <https://www.trustedcomputinggroup.org/wp-
content/uploads/TCG_PCClientImplementation_1-21_1_00.pdf>.
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[PC-Client-BIOS-TPM-2.0]
Trusted Computing Group, "PC Client Specific Platform
Firmware Profile Specification Family "2.0", Level 00
Revision 1.04", June 2019,
<https://trustedcomputinggroup.org/pc-client-specific-
platform-firmware-profile-specification>.
[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>.
[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>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[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>.
[RFC8348] Bierman, A., Bjorklund, M., Dong, J., and D. Romascanu, "A
YANG Data Model for Hardware Management", RFC 8348,
DOI 10.17487/RFC8348, March 2018,
<https://www.rfc-editor.org/info/rfc8348>.
[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>.
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[RIM] Trusted Computing Group, "DRAFT: TCG Reference Integrity
Manifest", June 2019.
[Roots-of-Trust]
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>.
[SNMP-Attestation-MIB]
Trusted Computing Group, "DRAFT: SNMP MIB for TPM-Based
Attestation, Specification Version 0.8, Revision 0.02",
May 2018.
[SWID] The International Organization for Standardization/
International Electrotechnical Commission, "Information
Technology Software Asset Management Part 2: Software
Identification Tag, ISO/IEC 19770-2", October 2015,
<https://www.iso.org/standard/65666.html>.
[TAP] Trusted Computing Group, "DRAFT: TCG Trusted Attestation
Protocol (TAP) Information Model for TPM Families 1.2 and
2.0 and DICE Family 1.0, Version 1.0, Revision 0.29",
October 2018.
Authors' Addresses
Guy Fedorkow (editor)
Juniper Networks, Inc.
US
Email: gfedorkow@juniper.net
Jessica Fitzgerald-McKay
National Security Agency
US
Email: jmfitz2@nsa.gov
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