Internet Engineering Task Force (IETF) H. Birkholz
Request for Comments: 9334 Fraunhofer SIT
Category: Informational D. Thaler
ISSN: 2070-1721 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
W. Pan
Huawei
January 2023
Remote ATtestation procedureS (RATS) Architecture
Abstract
In network protocol exchanges, it is often useful for one end of a
communication to know whether the other end is in an intended
operating state. This document provides an architectural overview of
the entities involved that make such tests possible through the
process of generating, conveying, and evaluating evidentiary Claims.
It provides a model that is neutral toward processor architectures,
the content of Claims, and protocols.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9334.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Reference Use Cases
2.1. Network Endpoint Assessment
2.2. Confidential Machine Learning Model Protection
2.3. Confidential Data Protection
2.4. Critical Infrastructure Control
2.5. Trusted Execution Environment Provisioning
2.6. Hardware Watchdog
2.7. FIDO Biometric Authentication
3. Architectural Overview
3.1. Two Types of Environments of an Attester
3.2. Layered Attestation Environments
3.3. Composite Device
3.4. Implementation Considerations
4. Terminology
4.1. Roles
4.2. Artifacts
5. Topological Patterns
5.1. Passport Model
5.2. Background-Check Model
5.3. Combinations
6. Roles and Entities
7. Trust Model
7.1. Relying Party
7.2. Attester
7.3. Relying Party Owner
7.4. Verifier
7.5. Endorser, Reference Value Provider, and Verifier Owner
8. Conceptual Messages
8.1. Evidence
8.2. Endorsements
8.3. Reference Values
8.4. Attestation Results
8.5. Appraisal Policies
9. Claims Encoding Formats
10. Freshness
10.1. Explicit Timekeeping Using Synchronized Clocks
10.2. Implicit Timekeeping Using Nonces
10.3. Implicit Timekeeping Using Epoch IDs
10.4. Discussion
11. Privacy Considerations
12. Security Considerations
12.1. Attester and Attestation Key Protection
12.1.1. On-Device Attester and Key Protection
12.1.2. Attestation Key Provisioning Processes
12.2. Conceptual Message Protection
12.3. Attestation Based on Epoch ID
12.4. Trust Anchor Protection
13. IANA Considerations
14. References
14.1. Normative References
14.2. Informative References
Appendix A. Time Considerations
A.1. Example 1: Timestamp-Based Passport Model
A.2. Example 2: Nonce-Based Passport Model
A.3. Example 3: Passport Model Based on Epoch ID
A.4. Example 4: Timestamp-Based Background-Check Model
A.5. Example 5: Nonce-Based Background-Check Model
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
The question of how one system can know that another system can be
trusted has found new interest and relevance in a world where trusted
computing elements are maturing in processor architectures.
Systems that have been attested and verified to be in a good state
(for some value of "good") can improve overall system posture.
Conversely, systems that cannot be attested and verified to be in a
good state can be given reduced access or privileges, taken out of
service, or otherwise flagged for repair.
For example:
* A bank backend system might refuse to transact with another system
that is not known to be in a good state.
* A healthcare system might refuse to transmit electronic healthcare
records to a system that is not known to be in a good state.
In Remote ATtestation procedureS (RATS), one peer (the "Attester")
produces believable information about itself ("Evidence") to enable a
remote peer (the "Relying Party") to decide whether or not to
consider that Attester a trustworthy peer. Remote attestation
procedures are facilitated by an additional vital party (the
"Verifier").
The Verifier appraises Evidence via appraisal policies and creates
the Attestation Results to support Relying Parties in their decision
process. This document defines a flexible architecture consisting of
attestation roles and their interactions via conceptual messages.
Additionally, this document defines a universal set of terms that can
be mapped to various existing and emerging remote attestation
procedures. Common topological patterns and the sequence of data
flows associated with them, such as the "Passport Model" and the
"Background-Check Model", are illustrated. The purpose is to define
useful terminology for remote attestation and enable readers to map
their solution architecture to the canonical attestation architecture
provided here. Having a common terminology that provides well-
understood meanings for common themes, such as roles, device
composition, topological patterns, and appraisal procedures, is vital
for semantic interoperability across solutions and platforms
involving multiple vendors and providers.
Amongst other things, this document is about trust and
trustworthiness. Trust is a choice one makes about another system.
Trustworthiness is a quality about the other system that can be used
in making one's decision to trust it or not. This is a subtle
difference; being familiar with the difference is crucial for using
this document. Additionally, the concepts of freshness and trust
relationships are specified to enable implementers to choose
appropriate solutions to compose their remote attestation procedures.
2. Reference Use Cases
This section covers a number of representative and generic use cases
for remote attestation, independent of specific solutions. The
purpose is to provide motivation for various aspects of the
architecture presented in this document. Many other use cases exist;
this document does not contain a complete list. It only illustrates
a set of use cases that collectively cover all the functionality
required in the architecture.
Each use case includes a description followed by an additional
summary of the Attester and Relying Party roles derived from the use
case.
2.1. Network Endpoint Assessment
Network operators want trustworthy reports that include identity and
version information about the hardware and software on the machines
attached to their network. Examples of reports include purposes
(such as inventory summaries), audit results, and anomaly
notifications (which typically include the maintenance of log records
or trend reports). The network operator may also want a policy by
which full access is only granted to devices that meet some
definition of hygiene, and so wants to get Claims about such
information and verify its validity. Remote attestation is desired
to prevent vulnerable or compromised devices from getting access to
the network and potentially harming others.
Typically, a solution starts with a specific component (sometimes
referred to as a "root of trust") that often provides a trustworthy
device identity and performs a series of operations that enables
trustworthiness appraisals for other components. Such components
perform operations that help determine the trustworthiness of yet
other components by collecting, protecting, or signing measurements.
Measurements that have been signed by such components are comprised
of Evidence that either supports or refutes a claim of
trustworthiness when evaluated. Measurements can describe a variety
of attributes of system components, such as hardware, firmware, BIOS,
software, etc., and how they are hardened.
Attester: A device desiring access to a network.
Relying Party: Network equipment (such as a router, switch, or
access point) that is responsible for admission of the device into
the network.
2.2. Confidential Machine Learning Model Protection
A device manufacturer wants to protect its intellectual property.
The intellectual property's scope primarily encompasses the machine
learning (ML) model that is deployed in the devices purchased by its
customers. The protection goals include preventing attackers,
potentially the customer themselves, from seeing the details of the
model.
Typically, this works by having some protected environment in the
device go through a remote attestation with some manufacturer service
that can assess its trustworthiness. If remote attestation succeeds,
then the manufacturer service releases either the model or a key to
decrypt a model already deployed on the Attester in encrypted form to
the requester.
Attester: A device desiring to run an ML model.
Relying Party: A server or service holding ML models it desires to
protect.
2.3. Confidential Data Protection
This is a generalization of the ML model use case above where the
data can be any highly confidential data, such as health data about
customers, payroll data about employees, future business plans, etc.
As part of the attestation procedure, an assessment is made against a
set of policies to evaluate the state of the system that is
requesting the confidential data. Attestation is desired to prevent
leaking data via compromised devices.
Attester: An entity desiring to retrieve confidential data.
Relying Party: An entity that holds confidential data for release to
authorized entities.
2.4. Critical Infrastructure Control
Potentially harmful physical equipment (e.g., power grid, traffic
control, hazardous chemical processing, etc.) is connected to a
network in support of critical infrastructure. The organization
managing such infrastructure needs to ensure that only authorized
code and users can control corresponding critical processes, and that
these processes are protected from unauthorized manipulation or other
threats. When a protocol operation can affect a critical system
component of the infrastructure, devices attached to that critical
component require some assurances depending on the security context,
including assurances that a requesting device or application has not
been compromised and the requesters and actors act on applicable
policies. As such, remote attestation can be used to only accept
commands from requesters that are within policy.
Attester: A device or application wishing to control physical
equipment.
Relying Party: A device or application connected to potentially
dangerous physical equipment (hazardous chemical processing,
traffic control, power grid, etc.).
2.5. Trusted Execution Environment Provisioning
A Trusted Application Manager (TAM) server is responsible for
managing the applications running in a Trusted Execution Environment
(TEE) of a client device, as described in [TEEP-ARCH]. To achieve
its purpose, the TAM needs to assess the state of a TEE or
applications in the TEE of a client device. The TEE conducts remote
attestation procedures with the TAM, which can then decide whether
the TEE is already in compliance with the TAM's latest policy. If
not, the TAM has to uninstall, update, or install approved
applications in the TEE to bring it back into compliance with the
TAM's policy.
Attester: A device with a TEE capable of running trusted
applications that can be updated.
Relying Party: A TAM.
2.6. Hardware Watchdog
There is a class of malware that holds a device hostage and does not
allow it to reboot to prevent updates from being applied. This can
be a significant problem because it allows a fleet of devices to be
held hostage for ransom.
A solution to this problem is a watchdog timer implemented in a
protected environment, such as a Trusted Platform Module (TPM), as
described in Section 43.3 of [TCGarch]. If the watchdog does not
receive regular and fresh Attestation Results regarding the system's
health, then it forces a reboot.
Attester: The device that should be protected from being held
hostage for a long period of time.
Relying Party: A watchdog capable of triggering a procedure that
resets a device into a known, good operational state.
2.7. FIDO Biometric Authentication
In the Fast IDentity Online (FIDO) protocol [WebAuthN] [CTAP], the
device in the user's hand authenticates the human user, whether by
biometrics (such as fingerprints) or by PIN and password. FIDO
authentication puts a large amount of trust in the device compared to
typical password authentication because it is the device that
verifies the biometric, PIN, and password inputs from the user, not
the server. For the Relying Party to know that the authentication is
trustworthy, the Relying Party needs to know that the Authenticator
part of the device is trustworthy. The FIDO protocol employs remote
attestation for this.
The FIDO protocol supports several remote attestation protocols and a
mechanism by which new ones can be registered and added; thus, remote
attestation defined by the RATS architecture is a candidate for use
in the FIDO protocol.
Attester: FIDO Authenticator.
Relying Party: Any website, mobile application backend, or service
that relies on authentication data based on biometric information.
3. Architectural Overview
Figure 1 depicts the data that flows between different roles,
independent of protocol or use case.
.--------. .---------. .--------. .-------------.
| Endorser | | Reference | | Verifier | | Relying Party |
'-+------' | Value | | Owner | | Owner |
| | Provider | '---+----' '----+--------'
| '-----+---' | |
| | | |
| Endorsements | Reference | Appraisal | Appraisal
| | Values | Policy for | Policy for
| | | Evidence | Attestation
'-----------. | | | Results
| | | |
v v v |
.-------------------------. |
.------>| Verifier +-----. |
| '-------------------------' | |
| | |
| Evidence Attestation | |
| Results | |
| | |
| v v
.-----+----. .---------------.
| Attester | | Relying Party |
'----------' '---------------'
Figure 1: Conceptual Data Flow
The text below summarizes the activities conducted by the roles
illustrated in Figure 1. Roles are assigned to entities. Entities
are often system components [RFC4949], such as devices. As the term
"device" is typically more intuitive than the term "entity" or
"system component", device is often used as an illustrative synonym
throughout this document.
The Attester role is assigned to entities that create Evidence that
is conveyed to a Verifier.
The Verifier role is assigned to entities that use the Evidence, any
Reference Values from Reference Value Providers, and any Endorsements
from Endorsers by applying an Appraisal Policy for Evidence to assess
the trustworthiness of the Attester. This procedure is called the
"appraisal of Evidence".
Subsequently, the Verifier role generates Attestation Results for use
by Relying Parties.
The Appraisal Policy for Evidence might be obtained from the Verifier
Owner via some protocol mechanism, configured into the Verifier by
the Verifier Owner, programmed into the Verifier, or obtained via
some other mechanism.
The Relying Party role is assigned to an entity that uses Attestation
Results by applying its own appraisal policy to make application-
specific decisions, such as authorization decisions. This procedure
is called the "appraisal of Attestation Results".
The Appraisal Policy for Attestation Results might be obtained from
the Relying Party Owner via some protocol mechanism, configured into
the Relying Party by the Relying Party Owner, programmed into the
Relying Party, or obtained via some other mechanism.
See Section 8 for further discussion of the conceptual messages shown
in Figure 1. Section 4 provides a more complete definition of all
RATS roles.
3.1. Two Types of Environments of an Attester
As shown in Figure 2, an Attester consists of at least one Attesting
Environment and at least one Target Environment co-located in one
entity. In some implementations, the Attesting and Target
Environments might be combined into one environment. Other
implementations might have multiple Attesting and Target
Environments, such as in the examples described in more detail in
Sections 3.2 and 3.3. Other examples may exist. All compositions of
Attesting and Target Environments discussed in this architecture can
be combined into more complex implementations.
.--------------------------------.
| |
| Verifier |
| |
'--------------------------------'
^
|
.-------------------------|----------.
| | |
| .----------------. | |
| | Target | | |
| | Environment | | |
| | | | Evidence |
| '--------------+-' | |
| | | |
| | | |
| Collect | | |
| Claims | | |
| | | |
| v | |
| .-------+-----. |
| | Attesting | |
| | Environment | |
| | | |
| '-------------' |
| Attester |
'------------------------------------'
Figure 2: Two Types of Environments within an Attester
Claims are collected from Target Environments. That is, Attesting
Environments collect the values and the information to be represented
in Claims by reading system registers and variables, calling into
subsystems, and taking measurements on code, memory, or other
relevant assets of the Target Environment. Attesting Environments
then format the Claims appropriately; typically, they use key
material and cryptographic functions, such as signing or cipher
algorithms, to generate Evidence. There is no limit or requirement
on the types of hardware or software environments that can be used to
implement an Attesting Environment. For example, TEEs, embedded
Secure Elements (eSEs), TPMs [TCGarch], or BIOS firmware.
An arbitrary execution environment may not, by default, be capable of
Claims collection for a given Target Environment. Execution
environments that are designed specifically to be capable of Claims
collection are referred to in this document as "Attesting
Environments". For example, a TPM doesn't actively collect Claims
itself. Instead, it requires another component to feed various
values to the TPM. Thus, an Attesting Environment in such a case
would be the combination of the TPM together with whatever component
is feeding it the measurements.
3.2. Layered Attestation Environments
By definition, the Attester role generates Evidence. An Attester may
consist of one or more nested environments (layers). The bottom
layer of an Attester has an Attesting Environment that is typically
designed to be immutable or difficult to modify by malicious code.
In order to appraise Evidence generated by an Attester, the Verifier
needs to trust various layers, including the bottom Attesting
Environment. Trust in the Attester's layers, including the bottom
layer, can be established in various ways, as discussed in
Section 7.4.
In layered attestation, Claims can be collected from or about each
layer beginning with an initial layer. The corresponding Claims can
be structured in a nested fashion that reflects the nesting of the
Attester's layers. Normally, Claims are not self-asserted. Rather,
a previous layer acts as the Attesting Environment for the next
layer. Claims about an initial layer are typically asserted by an
Endorser.
The example device illustrated in Figure 3 includes (A) a BIOS stored
in read-only memory, (B) a bootloader, and (C) an operating system
kernel.
.-------------. Endorsement for ROM
| Endorser +-----------------------.
'-------------' |
v
.-------------. Reference .----------.
| Reference | Values for | |
| Value +----------------->| Verifier |
| Provider(s) | ROM, bootloader, | |
'-------------' and kernel '----------'
^
.------------------------------------. |
| | |
| .---------------------------. | |
| | Kernel(C) | | |
| | | | | Layered
| | Target | | | Evidence
| | Environment | | | for
| '---------------+-----------' | | bootloader
| Collect | | | and
| Claims | | | kernel
| .---------------|-----------. | |
| | Bootloader(B) v | | |
| | .-----------. | | |
| | Target | Attesting | | | |
| | Environment |Environment+-----------'
| | | | | |
| | '-----------' | |
| | ^ | |
| '--------------+--|---------' |
| Collect | | Evidence for |
| Claims v | bootloader |
| .-----------------+---------. |
| | ROM(A) | |
| | | |
| | Attesting | |
| | Environment | |
| '---------------------------' |
| |
'------------------------------------'
Figure 3: Layered Attester
The first Attesting Environment (the ROM in this example) has to
ensure the integrity of the bootloader (the first Target
Environment). There are potentially multiple kernels to boot; the
decision is up to the bootloader. Only a bootloader with intact
integrity will make an appropriate decision. Therefore, the Claims
relating to the integrity of the bootloader have to be measured
securely. At this stage of the boot cycle of the device, the Claims
collected typically cannot be composed into Evidence.
After the boot sequence is started, the BIOS conducts the most
important and defining feature of layered attestation: the
successfully measured bootloader now becomes (or contains) an
Attesting Environment for the next layer. This procedure in layered
attestation is sometimes called "staging". It is important that the
bootloader not be able to alter any Claims about itself that were
collected by the BIOS. This can be ensured having those Claims be
either signed by the BIOS or stored in a tamper-proof manner by the
BIOS.
Continuing with this example, the bootloader's Attesting Environment
is now in charge of collecting Claims about the next Target
Environment. In this example, it is the kernel to be booted. The
final Evidence thus contains two sets of Claims: one set about the
bootloader as measured and signed by the BIOS and another set of
Claims about the kernel as measured and signed by the bootloader.
This example could be extended further by making the kernel become
another Attesting Environment for an application as another Target
Environment. This would result in a third set of Claims in the
Evidence pertaining to that application.
The essence of this example is a cascade of staged environments.
Each environment has the responsibility of measuring the next
environment before the next environment is started. In general, the
number of layers may vary by device or implementation, and an
Attesting Environment might even have multiple Target Environments
that it measures, rather than only one as shown by example in
Figure 3.
3.3. Composite Device
A composite device is an entity composed of multiple sub-entities
such that its trustworthiness has to be determined by the appraisal
of all these sub-entities.
Each sub-entity has at least one Attesting Environment collecting the
Claims from at least one Target Environment. Then, this sub-entity
generates Evidence about its trustworthiness; therefore, each sub-
entity can be called an "Attester". Among all the Attesters, there
may be only some that have the ability to communicate with the
Verifier while others do not.
For example, a carrier-grade router consists of a chassis and
multiple slots. The trustworthiness of the router depends on all its
slots' trustworthiness. Each slot has an Attesting Environment, such
as a TEE, collecting the Claims of its boot process, after which it
generates Evidence from the Claims.
Among these slots, only a "main" slot can communicate with the
Verifier while other slots cannot. However, other slots can
communicate with the main slot by the links between them inside the
router. The main slot collects the Evidence of other slots, produces
the final Evidence of the whole router, and conveys the final
Evidence to the Verifier. Therefore, the router is a composite
device, each slot is an Attester, and the main slot is the lead
Attester.
Another example is a multi-chassis router composed of multiple single
carrier-grade routers. Multi-chassis router setups create redundancy
groups that provide higher throughput by interconnecting multiple
routers in these groups, which can be treated as one logical router
for simpler management. A multi-chassis router setup provides a
management point that connects to the Verifier. Typically, one
router in the group is designated as the main router. Other routers
in the multi-chassis setup are connected to the main router only via
physical network links; therefore, they are managed and appraised via
the main router's help. Consequently, a multi-chassis router setup
is a composite device, each router is an Attester, and the main
router is the lead Attester.
Figure 4 depicts the conceptual data flow for a composite device.
.-----------------------------.
| Verifier |
'-----------------------------'
^
|
| Evidence of
| Composite Device
|
.----------------------------------|-------------------------------.
| .--------------------------------|-----. .------------. |
| | Collect .---------+--. | | | |
| | Claims .--------->| Attesting |<--------+ Attester B +-. |
| | | |Environment | | '-+----------' | |
| | .--------+-------. | |<----------+ Attester C +-. |
| | | Target | | | | '-+----------' | |
| | | Environment(s) | | |<------------+ ... | |
| | | | '------------' | Evidence '------------' |
| | '----------------' | of |
| | | Attesters |
| | lead Attester A | (via Internal Links or |
| '--------------------------------------' Network Connections) |
| |
| Composite Device |
'------------------------------------------------------------------'
Figure 4: Composite Device
In a composite device, each Attester generates its own Evidence by
its Attesting Environment(s) collecting the Claims from its Target
Environment(s). The lead Attester collects Evidence from other
Attesters and conveys it to a Verifier. Collection of Evidence from
sub-entities may itself be a form of Claims collection that results
in Evidence asserted by the lead Attester. The lead Attester
generates Evidence about the layout of the whole composite device,
while sub-Attesters generate Evidence about their respective
(sub-)modules.
In this scenario, the trust model described in Section 7 can also be
applied to an inside Verifier.
3.4. Implementation Considerations
An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. Multiple entities can
cooperate to implement a single RATS role as well. In essence, the
combination of roles and entities can be arbitrary. For example, in
the composite device scenario, the entity inside the lead Attester
can also take on the role of a Verifier and the outer entity of
Verifier can take on the role of a Relying Party. After collecting
the Evidence of other Attesters, this inside Verifier uses
Endorsements and appraisal policies (obtained the same way as by any
other Verifier) as part of the appraisal procedures that generate
Attestation Results. The inside Verifier then conveys the
Attestation Results of other Attesters to the outside Verifier,
whether in the same conveyance protocol as part of the Evidence or
not.
As explained in Section 4, there are a variety of roles in the RATS
architecture; they are defined by a unique combination of artifacts
they produce and consume. Conversely, artifacts are also defined by
the roles that produce or consume them. To produce an artifact means
that a given role introduces it into the RATS architecture. To
consume an artifact means that a given role has responsibility for
processing it in the RATS architecture. Roles also have the ability
to perform additional actions, such as caching or forwarding
artifacts as opaque data. As depicted in Section 5, these additional
actions can be performed by several roles.
4. Terminology
[RFC4949] has defined a number of terms that are also used in this
document. Some of the terms are close to, but not exactly the same.
Where the terms are similar, they are noted below with references.
As explained in Section 2.6 of [RFC4949], when this document says
"Compare:", the terminology used in this document differs
significantly from the definition in the reference.
This document uses the terms in the subsections that follow.
4.1. Roles
Attester: A role performed by an entity (typically a device) whose
Evidence must be appraised in order to infer the extent to which
the Attester is considered trustworthy, such as when deciding
whether it is authorized to perform some operation.
Produces: Evidence
Relying Party: A role performed by an entity that depends on the
validity of information about an Attester for purposes of reliably
applying application-specific actions. Compare: relying party
[RFC4949].
Consumes: Attestation Results, Appraisal Policy for Attestation
Results
Verifier: A role performed by an entity that appraises the validity
of Evidence about an Attester and produces Attestation Results to
be used by a Relying Party.
Consumes: Evidence, Reference Values, Endorsements, Appraisal
Policy for Evidence
Produces: Attestation Results
Relying Party Owner: A role performed by an entity (typically an
administrator) that is authorized to configure an Appraisal Policy
for Attestation Results in a Relying Party.
Produces: Appraisal Policy for Attestation Results
Verifier Owner: A role performed by an entity (typically an
administrator) that is authorized to configure an Appraisal Policy
for Evidence in a Verifier.
Produces: Appraisal Policy for Evidence
Endorser: A role performed by an entity (typically a manufacturer)
whose Endorsements may help Verifiers appraise the authenticity of
Evidence and infer further capabilities of the Attester.
Produces: Endorsements
Reference Value Provider: A role performed by an entity (typically a
manufacturer) whose Reference Values help Verifiers appraise
Evidence to determine if acceptable known Claims have been
recorded by the Attester.
Produces: Reference Values
4.2. Artifacts
Claim: A piece of asserted information, often in the form of a name/
value pair. Claims make up the usual structure of Evidence and
other RATS conceptual messages. Compare: claim [RFC7519].
Endorsement: A secure statement that an Endorser vouches for the
integrity of an Attester's various capabilities, such as Claims
collection and Evidence signing.
Consumed By: Verifier
Produced By: Endorser
Evidence: A set of Claims generated by an Attester to be appraised
by a Verifier. Evidence may include configuration data,
measurements, telemetry, or inferences.
Consumed By: Verifier
Produced By: Attester
Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier
vouches for the validity of the results.
Consumed By: Relying Party
Produced By: Verifier
Appraisal Policy for Evidence: A set of rules that a Verifier uses
to evaluate the validity of information about an Attester.
Compare: security policy [RFC4949].
Consumed By: Verifier
Produced By: Verifier Owner
Appraisal Policy for Attestation Results: A set of rules that direct
how a Relying Party uses the Attestation Results regarding an
Attester generated by the Verifiers. Compare: security policy
[RFC4949].
Consumed by: Relying Party
Produced by: Relying Party Owner
Reference Values: A set of values against which values of Claims can
be compared as part of applying an Appraisal Policy for Evidence.
Reference Values are sometimes referred to in other documents as
"known-good values", "golden measurements", or "nominal values".
These terms typically assume comparison for equality, whereas
here, Reference Values might be more general and be used in any
sort of comparison.
Consumed By: Verifier
Produced By: Reference Value Provider
5. Topological Patterns
Figure 1 shows a data flow diagram for communication between an
Attester, a Verifier, and a Relying Party. The Attester conveys its
Evidence to the Verifier for appraisal and the Relying Party receives
the Attestation Result from the Verifier. This section refines the
data-flow diagram by describing two reference models, as well as one
example composition thereof. The discussion that follows is for
illustrative purposes only and does not constrain the interactions
between RATS roles to the presented models.
5.1. Passport Model
The Passport Model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity Claim, such as at
an airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity Claims and it is
issued by a trusted authority.
Thus, in this immigration desk analogy, the citizen is the Attester,
the passport-issuing agency is a Verifier, and the passport
application and identifying information (e.g., birth certificate) is
the Evidence. The passport is an Attestation Result and the
immigration desk is a Relying Party.
In this model, an Attester conveys Evidence to a Verifier that
compares the Evidence against its appraisal policy. The Verifier
then gives back an Attestation Result that the Attester treats as
opaque data.
The Attester does not consume the Attestation Result, but it might
cache it. The Attester can then present the Attestation Result (and
possibly additional Claims) to a Relying Party, which then compares
this information against its own appraisal policy. The Attester may
also present the same Attestation Result to other Relying Parties.
There are three ways in which the process may fail:
* First, the Verifier may not issue a positive Attestation Result
due to the Evidence not passing the Appraisal Policy for Evidence.
* The second way in which the process may fail is when the
Attestation Result is examined by the Relying Party, and based
upon the Appraisal Policy for Attestation Results, the result does
not comply with the policy.
* The third way is when the Verifier is unreachable or unavailable.
As with any other information needed by the Relying Party to make an
authorization decision, an Attestation Result can be carried in a
resource access protocol between the Attester and Relying Party. In
this model, the details of the resource access protocol constrain the
serialization format of the Attestation Result. On the other hand,
the format of the Evidence is only constrained by the Attester-
Verifier remote attestation protocol. This implies that
interoperability and standardization is more relevant for Attestation
Results than it is for Evidence.
.------------.
| | Compare Evidence
| Verifier | against appraisal policy
| |
'--------+---'
^ |
Evidence | | Attestation
| | Result
| v
.---+--------. .-------------.
| +------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party | appraisal policy
'------------' '-------------'
Figure 5: Passport Model
5.2. Background-Check Model
The Background-Check Model is so named because of the resemblance of
how employers and volunteer organizations perform background checks.
When a prospective employee provides Claims about education or
previous experience, the employer will contact the respective
institutions or former employers to validate the Claim. Volunteer
organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and the organization that issues a report is a
Verifier.
In this model, an Attester conveys Evidence to a Relying Party, which
treats it as opaque and simply forwards it on to a Verifier. The
Verifier compares the Evidence against its appraisal policy and
returns an Attestation Result to the Relying Party. The Relying
Party then compares the Attestation Result against its own appraisal
policy.
The resource access protocol between the Attester and Relying Party
includes Evidence rather than an Attestation Result, but that
Evidence is not processed by the Relying Party.
Since the Evidence is merely forwarded on to a trusted Verifier, any
serialization format can be used for Evidence because the Relying
Party does not need a parser for it. The only requirement is that
the Evidence can be _encapsulated_ in the format required by the
resource access protocol between the Attester and Relying Party.
However, as seen in the Passport Model, an Attestation Result is
still consumed by the Relying Party. Code footprint and attack
surface area can be minimized by using a serialization format for
which the Relying Party already needs a parser to support the
protocol between the Attester and Relying Party, which may be an
existing standard or widely deployed resource access protocol. Such
minimization is especially important if the Relying Party is a
constrained node.
.-------------.
| | Compare Evidence
| Verifier | against appraisal
| | policy
'--------+----'
^ |
Evidence | | Attestation
| | Result
| v
.------------. .----|--------.
| +-------------->|---' | Compare Attestation
| Attester | Evidence | Relying | Result against
| | | Party | appraisal policy
'------------' '-------------'
Figure 6: Background-Check Model
5.3. Combinations
One variation of the Background-Check Model is where the Relying
Party and the Verifier are on the same machine, performing both
functions together. In this case, there is no need for a protocol
between the two.
It is also worth pointing out that the choice of model depends on the
use case and that different Relying Parties may use different
topological patterns.
The same device may need to create Evidence for different Relying
Parties and/or different use cases. For instance, it would use one
model to provide Evidence to a network infrastructure device to gain
access to the network and the other model to provide Evidence to a
server holding confidential data to gain access to that data. As
such, both models may simultaneously be in use by the same device.
Figure 7 shows another example of a combination where Relying Party 1
uses the Passport Model, whereas Relying Party 2 uses an extension of
the Background-Check Model. Specifically, in addition to the basic
functionality shown in Figure 6, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the TAM
plans to support in the TEEP architecture [TEEP-ARCH].
.-------------.
| | Compare Evidence
| Verifier | against appraisal policy
| |
'--------+----'
^ |
Evidence | | Attestation
| | Result
| v
.----+--------.
| | Compare
| Relying | Attestation Result
| Party 2 | against appraisal policy
'--------+----'
^ |
Evidence | | Attestation
| | Result
| v
.----+--------. .-------------.
| +-------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party 1 | appraisal policy
'-------------' '-------------'
Figure 7: Example Combination
6. Roles and Entities
An entity in the RATS architecture includes at least one of the roles
defined in this document.
An entity can aggregate more than one role into itself, such as being
both a Verifier and a Relying Party or being both a Reference Value
Provider and an Endorser. As such, any conceptual messages (see
Section 8 for more discussion) originating from such roles might also
be combined. For example, Reference Values might be conveyed as part
of an appraisal policy if the Verifier Owner and Reference Value
Provider roles are combined. Similarly, Reference Values might be
conveyed as part of an Endorsement if the Endorser and Reference
Value Provider roles are combined.
Interactions between roles aggregated into the same entity do not
necessarily use the Internet Protocol. Such interactions might use a
loopback device or other IP-based communication between separate
environments, but they do not have to. Alternative channels to
convey conceptual messages include function calls, sockets, General-
Purpose Input/Output (GPIO) interfaces, local buses, or hypervisor
calls. This type of conveyance is typically found in composite
devices. Most importantly, these conveyance methods are out of scope
of the RATS architecture, but they are presumed to exist in order to
convey conceptual messages appropriately between roles.
In essence, an entity that combines more than one role creates and
consumes the corresponding conceptual messages as defined in this
document.
7. Trust Model
7.1. Relying Party
This document covers scenarios for which a Relying Party trusts a
Verifier that can appraise the trustworthiness of information about
an Attester. Such trust is expressed by storing one or more "trust
anchors" in a secure location known as a "trust anchor store".
As defined in [RFC6024]:
| A trust anchor represents an authoritative entity via a public key
| and associated data. The public key is used to verify digital
| signatures, and the associated data is used to constrain the types
| of information for which the trust anchor is authoritative.
The trust anchor may be a certificate or it may be a raw public key
along with additional data if necessary, such as its public key
algorithm and parameters. In the context of this document, a trust
anchor may also be a symmetric key, as in [TCG-DICE-SIBDA], or the
symmetric mode described in [RATS-PSA-TOKEN].
Thus, trusting a Verifier might be expressed by having the Relying
Party store the Verifier's key or certificate in its trust anchor
store. It might also be expressed by storing the public key or
certificate of an entity (e.g., a Certificate Authority) that is in
the Verifier's certificate path. For example, the Relying Party can
verify that the Verifier is an expected one by out-of-band
establishment of key material combined with a protocol like TLS to
communicate. There is an assumption that the Verifier has not been
compromised between the establishment of the trusted key material and
the creation of the Evidence.
For a stronger level of security, the Relying Party might require
that the Verifier first provide information about itself that the
Relying Party can use to assess the trustworthiness of the Verifier
before accepting its Attestation Results. Such a process would
provide a stronger level of confidence in the correctness of the
information provided, such as a belief that the authentic Verifier
has not been compromised by malware.
For example, one explicit way for a Relying Party "A" to establish
such confidence in the correctness of a Verifier "B" would be for B
to first act as an Attester where A acts as a combined Verifier/
Relying Party. If A then accepts B as trustworthy, it can choose to
accept B as a Verifier for other Attesters.
Similarly, the Relying Party also needs to trust the Relying Party
Owner for providing its Appraisal Policy for Attestation Results,
and, in some scenarios, the Relying Party might even require that the
Relying Party Owner go through a remote attestation procedure with it
before the Relying Party will accept an updated policy. This can be
done in a manner similar to how a Relying Party could establish trust
in a Verifier as discussed above, i.e., verifying credentials against
a trust anchor store and optionally requiring Attestation Results
from the Relying Party Owner.
7.2. Attester
In some scenarios, Evidence might contain sensitive information, such
as Personally Identifiable Information (PII) or system identifiable
information. Thus, an Attester must trust the entities to which it
conveys Evidence to not reveal sensitive data to unauthorized
parties. The Verifier might share this information with other
authorized parties according to a governing policy that addresses the
handling of sensitive information (potentially included in Appraisal
Policies for Evidence). In the Background-Check Model, this Evidence
may also be revealed to Relying Parties.
When Evidence contains sensitive information, an Attester typically
requires that a Verifier authenticates itself (e.g., at TLS session
establishment) and might even request a remote attestation before the
Attester sends the sensitive Evidence. This can be done by having
the Attester first act as a Verifier/Relying Party and the Verifier
act as its own Attester, as discussed above.
7.3. Relying Party Owner
The Relying Party Owner might also require that the Relying Party
first act as an Attester by providing Evidence that the Owner can
appraise before the Owner would give the Relying Party an updated
policy that might contain sensitive information. In such a case,
authentication or attestation in both directions might be needed.
Typically, one side's Evidence must be considered safe to share with
an untrusted entity in order to bootstrap the sequence. See
Section 11 for more discussion.
7.4. Verifier
The Verifier trusts (or more specifically, the Verifier's security
policy is written in a way that configures the Verifier to trust) a
manufacturer or the manufacturer's hardware so as to be able to
appraise the trustworthiness of that manufacturer's devices. Such
trust is expressed by storing one or more trust anchors in the
Verifier's trust anchor store.
In a typical solution, a Verifier comes to trust an Attester
indirectly by having an Endorser (such as a manufacturer) vouch for
the Attester's ability to securely generate Evidence through
Endorsements (see Section 8.2). Endorsements might describe the ways
in which the Attester resists attacks, protects secrets, and measures
Target Environments. Consequently, the Endorser's key material is
stored in the Verifier's trust anchor store so that Endorsements can
be authenticated and used in the Verifier's appraisal process.
In some solutions, a Verifier might be configured to directly trust
an Attester by having the Verifier possess the Attester's key
material (rather than the Endorser's) in its trust anchor store.
Such direct trust must first be established at the time of trust
anchor store configuration either by checking with an Endorser at
that time or by conducting a security analysis of the specific
device. Having the Attester directly in the trust anchor store
narrows the Verifier's trust to only specific devices rather than all
devices the Endorser might vouch for, such as all devices
manufactured by the same manufacturer in the case that the Endorser
is a manufacturer.
Such narrowing is often important since physical possession of a
device can also be used to conduct a number of attacks, and so a
device in a physically secure environment (such as one's own
premises) may be considered trusted, whereas devices owned by others
would not be. This often results in a desire either to have the
owner run their own Endorser that would only endorse devices one owns
or to use Attesters directly in the trust anchor store. When there
are many Attesters owned, the use of an Endorser enables better
scalability.
That is, a Verifier might appraise the trustworthiness of an
application component, operating system component, or service under
the assumption that information provided about it by the lower-layer
firmware or software is true. A stronger level of assurance of
security comes when information can be vouched for by hardware or by
ROM code, especially if such hardware is physically resistant to
hardware tampering. In most cases, components that have to be
vouched for via Endorsements (because no Evidence is generated about
them) are referred to as "roots of trust".
The manufacturer having arranged for an Attesting Environment to be
provisioned with key material with which to sign Evidence, the
Verifier is then provided with some way of verifying the signature on
the Evidence. This may be in the form of an appropriate trust anchor
or the Verifier may be provided with a database of public keys
(rather than certificates) or even carefully curated and secured
lists of symmetric keys.
The nature of how the Verifier manages to validate the signatures
produced by the Attester is critical to the secure operation of a
remote attestation system but is not the subject of standardization
within this architecture.
A conveyance protocol that provides authentication and integrity
protection can be used to convey Evidence that is otherwise
unprotected (e.g., not signed). Appropriate conveyance of
unprotected Evidence (e.g., [RATS-UCCS]) relies on the following
conveyance protocol's protection capabilities:
1. The key material used to authenticate and integrity protect the
conveyance channel is trusted by the Verifier to speak for the
Attesting Environment(s) that collected Claims about the Target
Environment(s).
2. All unprotected Evidence that is conveyed is supplied exclusively
by the Attesting Environment that has the key material that
protects the conveyance channel.
3. A trusted environment protects the conveyance channel's key
material, which may depend on other Attesting Environments with
equivalent strength protections.
As illustrated in [RATS-UCCS], an entity that receives unprotected
Evidence via a trusted conveyance channel always takes on the
responsibility of vouching for the Evidence's authenticity and
freshness. If protected Evidence is generated, the Attester's
Attesting Environments take on that responsibility. In cases where
unprotected Evidence is processed by a Verifier, Relying Parties have
to trust that the Verifier is capable of handling Evidence in a
manner that preserves the Evidence's authenticity and freshness.
Generating and conveying unprotected Evidence always creates
significant risk and the benefits of that approach have to be
carefully weighed against potential drawbacks.
See Section 12 for discussion on security strength.
7.5. Endorser, Reference Value Provider, and Verifier Owner
In some scenarios, the Endorser, Reference Value Provider, and
Verifier Owner may need to trust the Verifier before giving the
Endorsement, Reference Values, or appraisal policy to it. This can
be done in a similar manner to how a Relying Party might establish
trust in a Verifier.
As discussed in Section 7.3, authentication or attestation in both
directions might be needed. Typically, one side's identity or
Evidence in this case must be considered safe to share with an
untrusted entity in order to bootstrap the sequence. See Section 11
for more discussion.
8. Conceptual Messages
Figure 1 illustrates the flow of conceptual messages between various
roles. This section provides additional elaboration and
implementation considerations. It is the responsibility of protocol
specifications to define the actual data format and semantics of any
relevant conceptual messages.
8.1. Evidence
Evidence is a set of Claims about the Target Environment that reveal
operational status, health, configuration, or construction that have
security relevance. Evidence is appraised by a Verifier to establish
its relevance, compliance, and timeliness. Claims need to be
collected in a manner that is reliable such that a Target Environment
cannot lie to the Attesting Environment about its trustworthiness
properties. Evidence needs to be securely associated with the Target
Environment so that the Verifier cannot be tricked into accepting
Claims originating from a different environment (that may be more
trustworthy). Evidence also must be protected from an active on-path
attacker who may observe, change, or misdirect Evidence as it travels
from the Attester to the Verifier. The timeliness of Evidence can be
captured using Claims that pinpoint the time or interval when changes
in operational status, health, and so forth occur.
8.2. Endorsements
An Endorsement is a secure statement that some entity (e.g., a
manufacturer) vouches for the integrity of the device's various
capabilities, such as Claims collection, signing, launching code,
transitioning to other environments, storing secrets, and more. For
example, if the device's signing capability is in hardware, then an
Endorsement might be a manufacturer certificate that signs a public
key whose corresponding private key is only known inside the device's
hardware. Thus, when Evidence and such an Endorsement are used
together, an appraisal procedure can be conducted based on appraisal
policies that may not be specific to the device instance but are
merely specific to the manufacturer providing the Endorsement. For
example, an appraisal policy might simply check that devices from a
given manufacturer have information matching a set of Reference
Values. An appraisal policy might also have a set of more complex
logic on how to appraise the validity of information.
However, while an appraisal policy that treats all devices from a
given manufacturer the same may be appropriate for some use cases, it
would be inappropriate to use such an appraisal policy as the sole
means of authorization for use cases that wish to constrain _which_
compliant devices are considered authorized for some purpose. For
example, an enterprise using remote attestation for Network Endpoint
Assessment (NEA) [RFC5209] may not wish to let every healthy laptop
from the same manufacturer onto the network. Instead, it may only
want to let devices that it legally owns onto the network. Thus, an
Endorsement may be helpful information in authenticating information
about a device, but is not necessarily sufficient to authorize access
to resources that may need device-specific information, such as a
public key for the device or component or user on the device.
8.3. Reference Values
Reference Values used in appraisal procedures come from a Reference
Value Provider and are then used by the Verifier to compare to
Evidence. Reference Values with matching Evidence produce acceptable
Claims. Additionally, an appraisal policy may play a role in
determining the acceptance of Claims.
8.4. Attestation Results
Attestation Results are the input used by the Relying Party to decide
the extent to which it will trust a particular Attester and allow it
to access some data or perform some operation.
Attestation Results may carry a boolean value indicating compliance
or non-compliance with a Verifier's appraisal policy or may carry a
richer set of Claims about the Attester, against which the Relying
Party applies its Appraisal Policy for Attestation Results.
The quality of the Attestation Results depends upon the ability of
the Verifier to evaluate the Attester. Different Attesters have a
different _Strength of Function_ [strengthoffunction], which results
in the Attestation Results being qualitatively different in strength.
An Attestation Result that indicates non-compliance can be used by an
Attester (in the Passport Model) or a Relying Party (in the
Background-Check Model) to indicate that the Attester should not be
treated as authorized and may be in need of remediation. In some
cases, it may even indicate that the Evidence itself cannot be
authenticated as being correct.
By default, the Relying Party does not believe the Attester to be
compliant. Upon receipt of an authentic Attestation Result and given
the Appraisal Policy for Attestation Results is satisfied, the
Attester is allowed to perform the prescribed actions or access. The
simplest such appraisal policy might authorize granting the Attester
full access or control over the resources guarded by the Relying
Party. A more complex appraisal policy might involve using the
information provided in the Attestation Result to compare against
expected values or to apply complex analysis of other information
contained in the Attestation Result.
Thus, Attestation Results can contain detailed information about an
Attester, which can include privacy sensitive information as
discussed in Section 11. Unlike Evidence, which is often very
device- and vendor-specific, Attestation Results can be vendor-
neutral, if the Verifier has a way to generate vendor-agnostic
information based on the appraisal of vendor-specific information in
Evidence. This allows a Relying Party's appraisal policy to be
simpler, potentially based on standard ways of expressing the
information, while still allowing interoperability with heterogeneous
devices.
Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity rather than a larger set of entities for
purposes of its appraisal policy.
8.5. Appraisal Policies
The Verifier (when appraising Evidence) or the Relying Party (when
appraising Attestation Results) checks the values of matched Claims
against constraints specified in its appraisal policy. Examples of
such constraints checking include the following:
* Comparison for equality against a Reference Value.
* A check for being in a range bounded by Reference Values.
* Membership in a set of Reference Values.
* A check against values in other Claims.
Upon completing all appraisal policy constraints, the remaining
Claims are accepted as input toward determining Attestation Results
(when appraising Evidence) or as input to a Relying Party (when
appraising Attestation Results).
9. Claims Encoding Formats
Figure 8 illustrates a relationship to which remote attestation is
desired to be added:
.-------------. .------------. Evaluate
| +-------------->| | request
| Attester | Access some | Relying | against
| | resource | Party | security
'-------------' '------------' policy
Figure 8: Typical Resource Access
In this diagram, the protocol between the Attester and a Relying
Party can be any new or existing protocol (e.g., HTTP(S), CoAP(S),
Resource-Oriented Lightweight Information Exchange (ROLIE) [RFC8322],
802.1x, OPC UA [OPCUA], etc.) depending on the use case.
Typically, such protocols already have mechanisms for passing
security information for authentication and authorization purposes.
Common formats include JSON Web Tokens (JWTs) [RFC7519], CWTs
[RFC8392], and X.509 certificates.
Retrofitting already-deployed protocols with remote attestation
requires adding RATS conceptual messages to the existing data flows.
This must be done in a way that does not degrade the security
properties of the systems involved and should use extension
mechanisms provided by the underlying protocol. For example, if a
TLS handshake is to be extended with remote attestation capabilities,
attestation Evidence may be embedded in an ad hoc X.509 certificate
extension (e.g., [TCG-DICE]) or into a new TLS Certificate Type
(e.g., [TLS-CWT]).
Especially for constrained nodes, there is a desire to minimize the
amount of parsing code needed in a Relying Party in order to both
minimize footprint and the attack surface. While it would be
possible to embed a CWT inside a JWT, or a JWT inside an X.509
extension, etc., there is a desire to encode the information in a
format that is already supported by the Relying Party.
This motivates having a common "information model" that describes the
set of remote attestation related information in an encoding-agnostic
way and allows multiple encoding formats (CWT, JWT, X.509, etc.) that
encode the same information into the Claims format needed by the
Relying Party.
Figure 9 illustrates that Evidence and Attestation Results might be
expressed via multiple potential encoding formats so that they can be
conveyed by various existing protocols. It also motivates why the
Verifier might also be responsible for accepting Evidence that
encodes Claims in one format while issuing Attestation Results that
encode Claims in a different format.
Evidence Attestation Results
.--------------. CWT CWT .-------------------.
| Attester-A +-----------. .---------->| Relying Party V |
'--------------' | | `-------------------'
v |
.--------------. JWT .---------+--. JWT .-------------------.
| Attester-B +-------->| +-------->| Relying Party W |
'--------------' | | `-------------------'
| |
.--------------. X.509 | | X.509 .-------------------.
| Attester-C +-------->| Verifier +-------->| Relying Party X |
'--------------' | | `-------------------'
| |
.--------------. TPM | | TPM .-------------------.
| Attester-D +-------->| +-------->| Relying Party Y |
'--------------' '---------+--' `-------------------'
^ |
.--------------. other | | other .-------------------.
| Attester-E +-----------' '---------->| Relying Party Z |
'--------------' `-------------------'
Figure 9: Multiple Attesters and Relying Parties with Different
Formats
10. Freshness
A Verifier or Relying Party might need to learn the point in time
(i.e., the "epoch") an Evidence or Attestation Result has been
produced. This is essential in deciding whether the included Claims
can be considered fresh, meaning they still reflect the latest state
of the Attester, and that any Attestation Result was generated using
the latest Appraisal Policy for Evidence, Endorsements, and Reference
Values.
This section provides a number of details. However, it does not
define any protocol formats and the interactions shown are abstract.
This section is intended for those creating protocols and solutions
to understand the options available to ensure freshness. The way in
which freshness is provisioned in a protocol is an architectural
decision. Provisioning of freshness has an impact on the number of
needed round trips in a protocol; therefore, it must be made very
early in the design. Different decisions will have significant
impacts on resulting interoperability, which is why this section goes
into sufficient detail such that choices in freshness will be
compatible across interacting protocols, such as depicted in
Figure 9.
Freshness is assessed based on the Appraisal Policy for Evidence or
Attestation Results that compares the estimated epoch against an
"expiry" threshold defined locally to that policy. There is,
however, always a race condition possible in that the state of the
Attester and the appraisal policies might change immediately after
the Evidence or Attestation Result was generated. The goal is merely
to narrow their recentness to something the Verifier (for Evidence)
or Relying Party (for Attestation Result) is willing to accept. Some
flexibility on the freshness requirement is a key component for
enabling caching and reuse of both Evidence and Attestation Results,
which is especially valuable in cases where their computation uses a
substantial part of the resource budget (e.g., energy in constrained
devices).
There are three common approaches for determining the epoch of
Evidence or an Attestation Result.
10.1. Explicit Timekeeping Using Synchronized Clocks
The first approach is to rely on synchronized and trustworthy clocks
and include a signed timestamp (see [RATS-TUDA]) along with the
Claims in the Evidence or Attestation Result. Timestamps can also be
added on a per-Claim basis to distinguish the time of generation of
Evidence or Attestation Result from the time that a specific Claim
was generated. The clock's trustworthiness can generally be
established via Endorsements and typically requires additional Claims
about the signer's time synchronization mechanism.
However, a trustworthy clock might not be available in some use
cases. For example, in many TEEs today, a clock is only available
outside the TEE; thus, it cannot be trusted by the TEE.
10.2. Implicit Timekeeping Using Nonces
A second approach places the onus of timekeeping solely on the
Verifier (for Evidence) or the Relying Party (for Attestation
Results). For example, this approach might be suitable in case the
Attester does not have a trustworthy clock or time synchronization is
otherwise impaired. In this approach, an unpredictable nonce is sent
by the appraising entity and the nonce is then signed and included
along with the Claims in the Evidence or Attestation Result. After
checking that the sent and received nonces are the same, the
appraising entity knows that the Claims were signed after the nonce
was generated. This allows associating a "rough" epoch to the
Evidence or Attestation Result. In this case, the epoch is said to
be rough because:
* The epoch applies to the entire Claim set instead of a more
granular association, and
* The time between the creation of Claims and the collection of
Claims is indistinguishable.
10.3. Implicit Timekeeping Using Epoch IDs
A third approach relies on having epoch identifiers (IDs)
periodically sent to both the sender and receiver of Evidence or
Attestation Results by some "epoch ID distributor".
Epoch IDs are different from nonces as they can be used more than
once and can even be used by more than one entity at the same time.
Epoch IDs are different from timestamps as they do not have to convey
information about a point in time, i.e., they are not necessarily
monotonically increasing integers.
Like the nonce approach, this allows associating a "rough" epoch
without requiring a trustworthy clock or time synchronization in
order to generate or appraise the freshness of Evidence or
Attestation Results. Only the epoch ID distributor requires access
to a clock so it can periodically send new epoch IDs.
The most recent epoch ID is included in the produced Evidence or
Attestation Results, and the appraising entity can compare the epoch
ID in received Evidence or Attestation Results against the latest
epoch ID it received from the epoch ID distributor to determine if it
is within the current epoch. An actual solution also needs to take
into account race conditions when transitioning to a new epoch, such
as by using a counter signed by the epoch ID distributor as the epoch
ID, by including both the current and previous epoch IDs in messages
and/or checks by requiring retries in case of mismatching epoch IDs,
or by buffering incoming messages that might be associated with an
epoch ID that the receiver has not yet obtained.
More generally, in order to prevent an appraising entity from
generating false negatives (e.g., discarding Evidence that is deemed
stale even if it is not), the appraising entity should keep an "epoch
window" consisting of the most recently received epoch IDs. The
depth of such epoch window is directly proportional to the maximum
network propagation delay between the first to receive the epoch ID
and the last to receive the epoch ID and it is inversely proportional
to the epoch duration. The appraising entity shall compare the epoch
ID carried in the received Evidence or Attestation Result with the
epoch IDs in its epoch window to find a suitable match.
Whereas the nonce approach typically requires the appraising entity
to keep state for each nonce generated, the epoch ID approach
minimizes the state kept to be independent of the number of Attesters
or Verifiers from which it expects to receive Evidence or Attestation
Results as long as all use the same epoch ID distributor.
10.4. Discussion
Implicit and explicit timekeeping can be combined into hybrid
mechanisms. For example, if clocks exist within the Attesting
Environment and are considered trustworthy (tamper-proof) but are not
synchronized, a nonce-based exchange may be used to determine the
(relative) time offset between the involved peers followed by any
number of timestamp based exchanges.
It is important to note that the actual values in Claims might have
been generated long before the Claims are signed. If so, it is the
signer's responsibility to ensure that the values are still fresh
when they are signed. For example, values generated at boot time
might have been saved to secure storage until network connectivity is
established to the remote Verifier and a nonce is obtained.
A more detailed discussion with examples appears in Appendix A.
For a discussion on the security of epoch IDs see Section 12.3.
11. Privacy Considerations
The conveyance of Evidence and the resulting Attestation Results
reveal a great deal of information about the internal state of a
device as well as potentially any users of the device.
In many cases, the whole point of attestation procedures is to
provide reliable information about the type of the device and the
firmware/software that the device is running.
This information might be particularly interesting to many attackers.
For example, knowing that a device is running a weak version of
firmware provides a way to aim attacks better.
In some circumstances, if an attacker can become aware of
Endorsements, Reference Values, or appraisal policies, it could
potentially provide an attacker with insight into defensive
mitigations. It is recommended that attention be paid to
confidentiality of such information.
Additionally, many Evidence, Attestation Results, and appraisal
policies potentially contain Personally Identifying Information (PII)
depending on the end-to-end use case of the remote attestation
procedure. Remote attestation that includes containers and
applications, e.g., a blood pressure monitor, may further reveal
details about specific systems or users.
In some cases, an attacker may be able to make inferences about the
contents of Evidence from the resulting effects or timing of the
processing. For example, an attacker might be able to infer the
value of specific Claims if it knew that only certain values were
accepted by the Relying Party.
Conceptual messages (see Section 8) carrying sensitive or
confidential information are expected to be integrity protected
(i.e., either via signing or a secure channel) and optionally might
be confidentiality protected via encryption. If there isn't
confidentiality protection of conceptual messages themselves, the
underlying conveyance protocol should provide these protections.
As Evidence might contain sensitive or confidential information,
Attesters are responsible for only sending such Evidence to trusted
Verifiers. Some Attesters might want a stronger level of assurance
of the trustworthiness of a Verifier before sending Evidence to it.
In such cases, an Attester can first act as a Relying Party and ask
for the Verifier's own Attestation Result. Appraising it just as a
Relying Party would appraise an Attestation Result for any other
purpose.
Another approach to deal with Evidence is to remove PII from the
Evidence while still being able to verify that the Attester is one of
a large set. This approach is often called "Direct Anonymous
Attestation". See Section 6.2 of [CCC-DeepDive] and [RATS-DAA] for
more discussion.
12. Security Considerations
This document provides an architecture for doing remote attestation.
No specific wire protocol is documented here. Without a specific
proposal to compare against, it is impossible to know if the security
threats listed below have been mitigated well.
The security considerations below should be read as being,
essentially, requirements against realizations of the RATS
architecture. Some threats apply to protocols and some are against
implementations (code) and physical infrastructure (such as
factories).
The fundamental purpose of the RATS architecture is to allow a
Relying Party to establish a basis for trusting the Attester.
12.1. Attester and Attestation Key Protection
Implementers need to pay close attention to the protection of the
Attester and the manufacturing processes for provisioning attestation
key material. If either of these are compromised, intended levels of
assurance for remote attestation procedures are compromised because
attackers can forge Evidence or manipulate the Attesting Environment.
For example, a Target Environment should not be able to tamper with
the Attesting Environment that measures it by isolating the two
environments from each other in some way.
Remote attestation applies to use cases with a range of security
requirements. The protections discussed here range from low to high
security: low security may be limited to application or process
isolation by the device's operating system and high security may
involve specialized hardware to defend against physical attacks on a
chip.
12.1.1. On-Device Attester and Key Protection
It is assumed that an Attesting Environment is sufficiently isolated
from the Target Environment it collects Claims about and that it
signs the resulting Claims set with an attestation key so that the
Target Environment cannot forge Evidence about itself. Such an
isolated environment might be provided by a process, a dedicated
chip, a TEE, a virtual machine, or another secure mode of operation.
The Attesting Environment must be protected from unauthorized
modification to ensure it behaves correctly. Confidentiality
protection of the Attesting Environment's signing key is vital so it
cannot be misused to forge Evidence.
In many cases, the user or owner of a device that includes the role
of Attester must not be able to modify or extract keys from the
Attesting Environments to prevent creating forged Evidence. Some
common examples include the user of a mobile phone or FIDO
authenticator.
Measures for a minimally protected system might include process or
application isolation provided by a high-level operating system and
restricted access to root or system privileges. In contrast, for
really simple single-use devices that don't use a protected mode
operating system (like a Bluetooth speaker), the only factual
isolation might be the sturdy housing of the device.
Measures for a moderately protected system could include a special
restricted operating environment, such as a TEE. In this case, only
security-oriented software has access to the Attester and key
material.
Measures for a highly protected system could include specialized
hardware that is used to provide protection against chip decapping
attacks, power supply and clock glitching, faulting injection and RF,
and power side channel attacks.
12.1.2. Attestation Key Provisioning Processes
Attestation key provisioning is the process that occurs in the
factory or elsewhere to establish signing key material on the device
and the validation key material off the device. Sometimes, this
procedure is referred to as "personalization" or "customization".
When generating keys off-device in the factory or in the device, the
use of a cryptographically strong sequence ([RFC4086], Section 6.2)
needs consideration.
12.1.2.1. Off-Device Key Generation
One way to provision key material is to first generate it external to
the device and then copy the key onto the device. In this case,
confidentiality protection of the generator and the path over which
the key is provisioned is necessary. The manufacturer needs to take
care to protect corresponding key material with measures appropriate
for its value.
The degree of protection afforded to this key material can vary by
the intended function of the device and the specific practices of the
device manufacturer or integrator. The confidentiality protection is
fundamentally based upon some amount of physical protection. While
encryption is often used to provide confidentiality when a key is
conveyed across a factory where the attestation key is created or
applied, it must be available in an unencrypted form. The physical
protection can therefore vary from situations where the key is
unencrypted only within carefully controlled secure enclaves within
silicon to situations where an entire facility is considered secure
by the simple means of locked doors and limited access.
The cryptography that is used to enable confidentiality protection of
the attestation key comes with its own requirements to be secured.
This results in recursive problems, as the key material used to
provision attestation keys must again somehow have been provisioned
securely beforehand (requiring an additional level of protection and
so on).
Commonly, a combination of some physical security measures and some
cryptographic measures are used to establish confidentiality
protection.
12.1.2.2. On-Device Key Generation
When key material is generated within a device and the secret part of
it never leaves the device, the problem may lessen. For public-key
cryptography, it is not necessary to maintain confidentiality of the
public key. However, integrity of the chain of custody of the public
key is necessary in order to avoid attacks where an attacker is able
to get a key endorsed that the attacker controls.
To summarize, attestation key provisioning must ensure that only
valid attestation key material is established in Attesters.
12.2. Conceptual Message Protection
Any solution that conveys information in any conceptual message (see
Section 8) must support end-to-end integrity protection and replay
attack prevention. It often also needs to support additional
security properties, including:
* end-to-end encryption,
* denial-of-service protection,
* authentication,
* auditing,
* fine-grained access controls, and
* logging.
Section 10 discusses ways in which freshness can be used in this
architecture to protect against replay attacks.
To assess the security provided by a particular appraisal policy, it
is important to understand the strength of the root of trust, e.g.,
whether it is mutable software or firmware that is read-only after
boot or immutable hardware/ROM.
It is also important that the appraisal policy was obtained securely
itself. If an attacker can configure or modify appraisal policies
and Endorsements or Reference Values for a Relying Party or a
Verifier, then integrity of the process is compromised.
Security protections in the RATS architecture may be applied at
different layers, whether by a conveyance protocol or an information
encoding format. This architecture expects conceptual messages to be
end-to-end protected based on the role interaction context. For
example, if an Attester produces Evidence that is relayed through
some other entity that doesn't implement the Attester or the intended
Verifier roles, then the relaying entity should not expect to have
access to the Evidence.
The RATS architecture allows for an entity to function in multiple
roles (Section 6) and for composite devices (Section 3.3).
Implementers need to evaluate their designs to ensure that the
assumed security properties of the individual components and roles
still hold despite the lack of separation and that emergent risk is
not introduced. The specifics of this evaluation will depend on the
implementation and the use case; hence, they are out of scope for
this document. Isolation mechanisms in software or hardware that
separate Attesting Environments and Target Environments (Section 3.1)
can support an implementer's evaluation and resulting design
decisions.
12.3. Attestation Based on Epoch ID
Epoch IDs, described in Section 10.3, can be tampered with, replayed,
dropped, delayed, and reordered by an attacker.
An attacker could either be external or belong to the distribution
group (for example, if one of the Attester entities have been
compromised).
An attacker who is able to tamper with epoch IDs can potentially lock
all the participants in a certain epoch of choice forever,
effectively freezing time. This is problematic since it destroys the
ability to ascertain freshness of Evidence and Attestation Results.
To mitigate this threat, the transport should be at least integrity
protected and provide origin authentication.
Selective dropping of epoch IDs is equivalent to pinning the victim
node to a past epoch. An attacker could drop epoch IDs to only some
entities and not others, which will typically result in a denial of
service due to the permanent staleness of the Attestation Result or
Evidence.
Delaying or reordering epoch IDs is equivalent to manipulating the
victim's timeline at will. This ability could be used by a malicious
actor (e.g., a compromised router) to mount a confusion attack. For
example, a Verifier can be tricked into accepting Evidence coming
from a past epoch as fresh, while, in the meantime, the Attester has
been compromised.
Reordering and dropping attacks are mitigated if the transport
provides the ability to detect reordering and drop. However, the
delay attack described above can't be thwarted in this manner.
12.4. Trust Anchor Protection
As noted in Section 7, Verifiers and Relying Parties have trust
anchor stores that must be secured. [RFC6024] contains more
discussion of trust anchor store requirements for protecting public
keys. Section 6 of [NIST-800-57-p1] contains a comprehensive
treatment of the topic, including the protection of symmetric key
material. Specifically, a trust anchor store must resist
modification against unauthorized insertion, deletion, and
modification. Additionally, if the trust anchor is a symmetric key,
the trust anchor store must not allow unauthorized read.
If certificates are used as trust anchors, Verifiers and Relying
Parties are also responsible for validating the entire certificate
path up to the trust anchor, which includes checking for certificate
revocation. For an example of such a procedure, see Section 6 of
[RFC5280].
13. IANA Considerations
This document has no IANA actions.
14. References
14.1. Normative References
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
14.2. Informative References
[CCC-DeepDive]
Confidential Computing Consortium, "A Technical Analysis
of Confidential Computing", Version 1.3, November 2022,
<https://confidentialcomputing.io/white-papers-reports>.
[CTAP] FIDO Alliance, "Client to Authenticator Protocol (CTAP)",
February 2018, <https://fidoalliance.org/specs/fido-v2.0-
id-20180227/fido-client-to-authenticator-protocol-v2.0-id-
20180227.html>.
[NIST-800-57-p1]
Barker, E., "Recommendation for Key Management: Part 1 -
General", DOI 10.6028/NIST.SP.800-57pt1r5, May 2020,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-57pt1r5.pdf>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 ,
November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security-
model/>.
[RATS-DAA] Birkholz, H., Newton, C., Chen, L., and D. Thaler, "Direct
Anonymous Attestation for the Remote Attestation
Procedures Architecture", Work in Progress, Internet-
Draft, draft-ietf-rats-daa-02, 7 September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-rats-
daa-02>.
[RATS-PSA-TOKEN]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T.
Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-10, 6 September 2022,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
rats-psa-token-10>.
[RATS-TUDA]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-07, 10
July 2022, <https://datatracker.ietf.org/doc/html/draft-
birkholz-rats-tuda-07>.
[RATS-UCCS]
Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
Work in Progress, Internet-Draft, draft-ietf-rats-uccs-04,
11 January 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-rats-uccs-04>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC5209] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
Tardo, "Network Endpoint Assessment (NEA): Overview and
Requirements", RFC 5209, DOI 10.17487/RFC5209, June 2008,
<https://www.rfc-editor.org/info/rfc5209>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>.
[RFC8322] Field, J., Banghart, S., and D. Waltermire, "Resource-
Oriented Lightweight Information Exchange (ROLIE)",
RFC 8322, DOI 10.17487/RFC8322, February 2018,
<https://www.rfc-editor.org/info/rfc8322>.
[strengthoffunction]
NIST, "Strength of Function",
<https://csrc.nist.gov/glossary/term/
strength_of_function>.
[TCG-DICE] Trusted Computing Group, "DICE Attestation Architecture",
Version 1.00, Revision 0.23, March 2021,
<https://trustedcomputinggroup.org/wp-content/uploads/
DICE-Attestation-Architecture-r23-final.pdf>.
[TCG-DICE-SIBDA]
Trusted Computing Group, "Symmetric Identity Based Device
Attestation", Version 1.0, Revision 0.95, January 2020,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_DICE_SymIDAttest_v1_r0p95_pub-1.pdf>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library,
Part 1: Architecture", November 2019,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>.
[TEEP-ARCH]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-19, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-teep-
architecture-19>.
[TLS-CWT] Tschofenig, H. and M. Brossard, "Using CBOR Web Tokens
(CWTs) in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS)", Work in Progress,
Internet-Draft, draft-tschofenig-tls-cwt-02, 13 July 2020,
<https://datatracker.ietf.org/doc/html/draft-tschofenig-
tls-cwt-02>.
[WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
Credentials Level 1", March 2019,
<https://www.w3.org/TR/webauthn-1/>.
Appendix A. Time Considerations
Section 10 discussed various issues and requirements around freshness
of Evidence and summarized three approaches that might be used by
different solutions to address them. This appendix provides more
details with examples to help illustrate potential approaches and
inform those creating specific solutions.
The table below defines a number of relevant events with an ID that
is used in subsequent diagrams. The times of said events might be
defined in terms of an absolute clock time, such as the Coordinated
Universal Time timescale, or might be defined relative to some other
timestamp or timeticks counter, such as a clock resetting its epoch
each time it is powered on.
+====+============+=================================================+
| ID | Event | Explanation of event |
+====+============+=================================================+
| VG | Value | A value to appear in a Claim was created. |
| | generated | In some cases, a value may have technically |
| | | existed before an Attester became aware of |
| | | it, but the Attester might have no idea how |
| | | long it has had that value. In such a |
| | | case, the value created time is the time at |
| | | which the Claim containing the copy of the |
| | | value was created. |
+----+------------+-------------------------------------------------+
| NS | Nonce sent | A nonce not predictable to an Attester |
| | | (recentness & uniqueness) is sent to an |
| | | Attester. |
+----+------------+-------------------------------------------------+
| NR | Nonce | A nonce is relayed to an Attester by |
| | relayed | another entity. |
+----+------------+-------------------------------------------------+
| IR | Epoch ID | An epoch ID is successfully received and |
| | received | processed by an entity. |
+----+------------+-------------------------------------------------+
| EG | Evidence | An Attester creates Evidence from collected |
| | generation | Claims. |
+----+------------+-------------------------------------------------+
| ER | Evidence | A Relying Party relays Evidence to a |
| | relayed | Verifier. |
+----+------------+-------------------------------------------------+
| RG | Result | A Verifier appraises Evidence and generates |
| | generation | an Attestation Result. |
+----+------------+-------------------------------------------------+
| RR | Result | A Relying Party relays an Attestation |
| | relayed | Result to a Relying Party. |
+----+------------+-------------------------------------------------+
| RA | Result | The Relying Party appraises Attestation |
| | appraised | Results. |
+----+------------+-------------------------------------------------+
| OP | Operation | The Relying Party performs some operation |
| | performed | requested by the Attester via a resource |
| | | access protocol as depicted in Figure 8, |
| | | e.g., across a session created earlier at |
| | | time(RA). |
+----+------------+-------------------------------------------------+
| RX | Result | An Attestation Result should no longer be |
| | expiry | accepted, according to the Verifier that |
| | | generated it. |
+----+------------+-------------------------------------------------+
Table 1: Relevant Events over Time
Using the table above, a number of hypothetical examples of how a
solution might be built are illustrated below. This list is not
intended to be complete; it is just representative enough to
highlight various timing considerations.
All times are relative to the local clocks, indicated by an "_a"
(Attester), "_v" (Verifier), or "_r" (Relying Party) suffix.
Times with an appended Prime (') indicate a second instance of the
same event.
How and if clocks are synchronized depends upon the model.
In the figures below, curly braces indicate containment. For
example, the notation Evidence{foo} indicates that 'foo' is contained
in the Evidence; thus, it is covered by its signature.
A.1. Example 1: Timestamp-Based Passport Model
Figure 10 illustrates a hypothetical Passport Model solution that
uses timestamps and requires roughly synchronized clocks between the
Attester, Verifier, and Relying Party, which depends on using a
secure clock synchronization mechanism. As a result, the receiver of
a conceptual message containing a timestamp can directly compare it
to its own clock and timestamps.
.----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party |
'----+-----' '-----+----' '-------+-------'
| | |
time(VG_a) | |
| | |
~ ~ ~
| | |
time(EG_a) | |
| | |
+------Evidence{time(EG_a)}------>| |
| | |
| time(RG_v) |
| | |
|<-----Attestation Result---------+ |
| {time(RG_v),time(RX_v)} | |
~ ~
| |
+--Attestation Result{time(RG_v),time(RX_v)}--> time(RA_r)
| |
~ ~
| |
| time(OP_r)
Figure 10: Timestamp-Based Passport Model
The Verifier can check whether the Evidence is fresh when appraising
it at time(RG_v) by checking time(RG_v) - time(EG_a) < Threshold,
where the Verifier's threshold is large enough to account for the
maximum permitted clock skew between the Verifier and the Attester.
If time(VG_a) is included in the Evidence along with the Claim value
generated at that time, and the Verifier decides that it can trust
the time(VG_a) value, the Verifier can also determine whether the
Claim value is recent by checking time(RG_v) - time(VG_a) <
Threshold. The threshold is decided by the Appraisal Policy for
Evidence and, again, needs to take into account the maximum permitted
clock skew between the Verifier and the Attester.
The Attester does not consume the Attestation Result but might cache
it.
The Relying Party can check whether the Attestation Result is fresh
when appraising it at time(RA_r) by checking the time(RA_r) -
time(RG_v) < Threshold, where the Relying Party's threshold is large
enough to account for the maximum permitted clock skew between the
Relying Party and the Verifier. The result might then be used for
some time (e.g., throughout the lifetime of a connection established
at time(RA_r)). However, the Relying Party must be careful not to
allow continued use beyond the period for which it deems the
Attestation Result to remain fresh enough. Thus, it might allow use
(at time(OP_r)) as long as time(OP_r) - time(RG_v) < Threshold.
However, if the Attestation Result contains an expiry time
time(RX_v), then it could explicitly check time(OP_r) < time(RX_v).
A.2. Example 2: Nonce-Based Passport Model
Figure 11 illustrates a hypothetical Passport Model solution that
uses nonces instead of timestamps. Compared to the timestamp-based
example, it requires an extra round trip to retrieve a nonce and
requires that the Verifier and Relying Party track state to remember
the nonce for some period of time.
The advantage is that it does not require that any clocks are
synchronized. As a result, the receiver of a conceptual message
containing a timestamp cannot directly compare it to its own clock or
timestamps. Thus, we use a suffix ("a" for Attester, "v" for
Verifier, and "r" for Relying Party) on the IDs below indicating
which clock generated them since times from different clocks cannot
be compared. Only the delta between two events from the sender can
be used by the receiver.
.----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party |
'----+-----' '-----+----' '-------+-------'
| | |
time(VG_a) | |
| | |
~ ~ ~
| | |
|<--Nonce1---------------------time(NS_v) |
| | |
time(EG_a) | |
| | |
+---Evidence--------------------->| |
| {Nonce1, time(EG_a)-time(VG_a)} | |
| | |
| time(RG_v) |
| | |
|<--Attestation Result------------+ |
| {time(RX_v)-time(RG_v)} | |
~ ~
| |
|<--Nonce2-------------------------------------time(NS_r)
| |
time(RR_a) |
| |
+--[Attestation Result{time(RX_v)-time(RG_v)}, -->|time(RA_r)
| Nonce2, time(RR_a)-time(EG_a)] |
| |
~ ~
| |
| time(OP_r)
Figure 11: Nonce-Based Passport Model
In this example solution, the Verifier can check whether the Evidence
is fresh at time(RG_v) by verifying that time(RG_v)-time(NS_v) <
Threshold.
However, the Verifier cannot simply rely on a Nonce to determine
whether the value of a Claim is recent since the Claim value might
have been generated long before the nonce was sent by the Verifier.
Nevertheless, if the Verifier decides that the Attester can be
trusted to correctly provide the delta time(EG_a)-time(VG_a), then it
can determine recency by checking time(RG_v)-time(NS_v) + time(EG_a)-
time(VG_a) < Threshold.
Similarly if, based on an Attestation Result from a Verifier it
trusts, the Relying Party decides that the Attester can be trusted to
correctly provide time deltas, then it can determine whether the
Attestation Result is fresh by checking time(OP_r)-time(NS_r) +
time(RR_a)-time(EG_a) < Threshold. Although the Nonce2 and
time(RR_a)-time(EG_a) values cannot be inside the Attestation Result,
they might be signed by the Attester such that the Attestation Result
vouches for the Attester's signing capability.
However, the Relying Party must still be careful not to allow
continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying
Party can check time(OP_r)-time(NS_r) < time(RX_v)-time(RG_v).
A.3. Example 3: Passport Model Based on Epoch ID
The example in Figure 12 illustrates a hypothetical Passport Model
solution that uses epoch IDs instead of nonces or timestamps.
The epoch ID distributor broadcasts epoch ID I, which starts a new
epoch E for a protocol participant upon reception at time(IR).
The Attester generates Evidence incorporating epoch ID I and conveys
it to the Verifier.
The Verifier appraises that the received epoch ID I is "fresh"
according to the definition provided in Section 10.3 whereby retries
are required in the case of mismatching epoch IDs; then the Verifier
generates an Attestation Result. The Attestation Result is conveyed
to the Attester.
After the transmission of epoch ID I' a new epoch E' is established
when I' is received by each protocol participant. The Attester
relays the Attestation Result obtained during epoch E (associated
with epoch ID I) to the Relying Party using the epoch ID for the
current epoch I'. If the Relying Party had not yet received I', then
the Attestation Result would be rejected. The Attestation Result is
received in this example.
In Figure 12, the epoch ID for relaying an Attestation Result to the
Relying Party is current while a previous epoch ID was used to
generate Verifier evaluated Evidence. This indicates that at least
one epoch transition has occurred and the Attestation Results may
only be as fresh as the previous epoch. If the Relying Party
remembers the previous epoch ID I during an epoch window as discussed
in Section 10.3, and the message is received during that window, the
Attestation Result is accepted as fresh; otherwise, it is rejected as
stale.
.-------------.
.----------. | Epoch ID | .----------. .---------------.
| Attester | | Distributor | | Verifier | | Relying Party |
'----+-----' '------+------' '-----+----' '-------+-------'
| | | |
time(VG_a) | | |
| | | |
~ | ~ ~
| | | |
time(IR_a) <-----I--o--I------> time(IR_v) ---> time(IR_r)
| | | |
time(EG_a) | | |
| | | |
+---Evidence--------------------->| |
| {I,time(EG_a)-time(VG_a)} | |
| | | |
| | time(RG_v) |
| | | |
|<--Attestation Result------------+ |
| {I,time(RX_v)-time(RG_v)} | |
| | | |
time(IR'_a) <----I'-o--I' ----> time(IR'_v) --> time(IR'_r)
| | |
+---[Attestation Result--------------------> time(RA_r)
| {I,time(RX_v)-time(RG_v)},I'] | |
| | |
~ ~ ~
| | |
| | time(OP_r)
Figure 12: Epoch ID-Based Passport Model
A.4. Example 4: Timestamp-Based Background-Check Model
Figure 13 illustrates a hypothetical Background-Check Model solution
that uses timestamps and requires roughly synchronized clocks between
the Attester, Verifier, and Relying Party. The Attester conveys
Evidence to the Relying Party, which treats it as opaque and simply
forwards it on to the Verifier.
.----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier |
'-------+--' '-------+-------' '----+-----'
| | |
time(VG_a) | |
| | |
~ ~ ~
| | |
time(EG_a) | |
| | |
+----Evidence------->| |
| {time(EG_a)} | |
| time(ER_r) ---Evidence{time(EG_a)}---->|
| | |
| | time(RG_v)
| | |
| time(RA_r) <---Attestation Result------+
| | {time(RX_v)} |
~ ~ ~
| | |
| time(OP_r) |
Figure 13: Timestamp-Based Background-Check Model
The time considerations in this example are equivalent to those
discussed under Example 1.
A.5. Example 5: Nonce-Based Background-Check Model
Figure 14 illustrates a hypothetical Background-Check Model solution
that uses nonces; thus, it does not require that any clocks be
synchronized. In this example solution, a nonce is generated by a
Verifier at the request of a Relying Party when the Relying Party
needs to send one to an Attester.
.----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier |
'----+-----' '-------+-------' '----+-----'
| | |
time(VG_a) | |
| | |
~ ~ ~
| | |
| |<-------Nonce-----------time(NS_v)
| | |
|<---Nonce-----------time(NR_r) |
| | |
time(EG_a) | |
| | |
+----Evidence{Nonce}--->| |
| | |
| time(ER_r) ---Evidence{Nonce}--->|
| | |
| | time(RG_v)
| | |
| time(RA_r) <---Attestation Result--+
| | {time(RX_v)-time(RG_v)} |
~ ~ ~
| | |
| time(OP_r) |
Figure 14: Nonce-Based Background-Check Model
The Verifier can check whether the Evidence is fresh and a Claim
value is recent, which is the same as Example 2.
However, unlike in Example 2, the Relying Party can use the Nonce to
determine whether the Attestation Result is fresh by verifying that
time(OP_r)-time(NR_r) < Threshold.
However, the Relying Party must still be careful not to allow
continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX_v)-time(RG_v), then the Relying
Party can check time(OP_r)-time(ER_r) < time(RX_v)-time(RG_v).
Acknowledgments
The authors would like to thank the following people for their input:
Joerg Borchert, Carsten Bormann, Nancy Cam-Winget, Guy Fedorkow,
Jessica Fitzgerald-McKay, Thomas Fossati, Simon Frost, Andrew Guinn,
Thomas Hardjano, Eliot Lear, Diego Lopez, Peter Loscocco, Laurence
Lundblade, Giri Mandyam, Daniel Migault, Kathleen Moriarty, Paul
Rowe, Hannes Tschofenig, Eric Voit, Monty Wiseman, David Wooten, and
Liang Xia.
Contributors
Thomas Hardjono created initial versions of the terminology section
in collaboration with Ned Smith. Eric Voit provided the conceptual
separation between Attestation Provision Flows and Attestation
Evidence Flows. Monty Wisemen was a key author of a document that
was merged to create this document. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat
Model.
Peter Loscocco contributed critical review feedback as part of the
weekly design team meetings that added precision and depth to several
sections.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Dave Thaler
Microsoft
United States of America
Email: dthaler@microsoft.com
Michael Richardson
Sandelman Software Works
Canada
Email: mcr+ietf@sandelman.ca
Ned Smith
Intel Corporation
United States of America
Email: ned.smith@intel.com