RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler
Expires: 13 August 2021 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
W. Pan
Huawei Technologies
9 February 2021
Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-10
Abstract
In network protocol exchanges it is often the case that one entity
requires believable evidence about the operational state of a remote
peer. Such evidence is typically conveyed as claims about the peer's
software and hardware platform, and is subsequently appraised in
order to assess the peer's trustworthiness. The process of
generating and appraising this kind of evidence is known as remote
attestation. This document describes an architecture for remote
attestation procedures that generate, convey, and appraise evidence
about a peer's operational state.
Note to Readers
Discussion of this document takes place on the RATS Working Group
mailing list (rats@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/rats/
(https://mailarchive.ietf.org/arch/browse/rats/).
Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats-wg/architecture (https://github.com/
ietf-rats-wg/architecture).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 13 August 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4
2.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 4
2.2. Confidential Machine Learning (ML) Model Protection . . . 5
2.3. Confidential Data Protection . . . . . . . . . . . . . . 5
2.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
2.5. Trusted Execution Environment (TEE) Provisioning . . . . 6
2.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 6
2.7. FIDO Biometric Authentication . . . . . . . . . . . . . . 7
3. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
3.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9
3.2. Reference Values . . . . . . . . . . . . . . . . . . . . 9
3.3. Two Types of Environments of an Attester . . . . . . . . 9
3.4. Layered Attestation Environments . . . . . . . . . . . . 11
3.5. Composite Device . . . . . . . . . . . . . . . . . . . . 13
3.6. Implementation Considerations . . . . . . . . . . . . . . 15
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Artifacts . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Topological Patterns . . . . . . . . . . . . . . . . . . . . 18
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 18
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 19
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 20
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 21
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 22
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7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 22
7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 23
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 23
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 23
7.5. Endorser, Reference Value Provider, and Verifier Owner . 25
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 25
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 25
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 26
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 26
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 27
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Explicit Timekeeping using Synchronized Clocks . . . . . 30
10.2. Implicit Timekeeping using Nonces . . . . . . . . . . . 30
10.3. Implicit Timekeeping using Epoch Handles . . . . . . . . 30
10.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 31
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 32
12. Security Considerations . . . . . . . . . . . . . . . . . . . 33
12.1. Attester and Attestation Key Protection . . . . . . . . 33
12.1.1. On-Device Attester and Key Protection . . . . . . . 33
12.1.2. Attestation Key Provisioning Processes . . . . . . . 34
12.2. Integrity Protection . . . . . . . . . . . . . . . . . . 35
12.3. Handle-based Attestation . . . . . . . . . . . . . . . . 36
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 37
15. Notable Contributions . . . . . . . . . . . . . . . . . . . . 37
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 37
16.1. Example 1: Timestamp-based Passport Model Example . . . 38
16.2. Example 2: Nonce-based Passport Model Example . . . . . 40
16.3. Example 3: Handle-based Passport Model Example . . . . . 42
16.4. Example 4: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 43
16.5. Example 5: Nonce-based Background-Check Model Example . 44
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
17.1. Normative References . . . . . . . . . . . . . . . . . . 45
17.2. Informative References . . . . . . . . . . . . . . . . . 45
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
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 to consider that
Attester a trustworthy peer or not. RATS are facilitated by an
additional vital party, the Verifier.
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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 models and the 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 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 models, and
appraisal 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 subtle
difference and being familiar with the difference is crucial for
using this document. Additionally, the concepts of freshness and
trust relationships with respect to RATS are elaborated on to enable
implementers to choose appropriate solutions to compose their Remote
Attestation Procedures.
2. Reference Use Cases
This section covers a number of representative use cases for remote
attestation, independent of specific solutions. The purpose is to
provide motivation for various aspects of the architecture presented
in this draft. Many other use cases exist, and this document does
not intend to have a complete list, only to have a set of use cases
that collectively cover all the functionality required in the
architecture.
Each use case includes a description followed by a summary of the
Attester and Relying Party roles.
2.1. Network Endpoint Assessment
Network operators want a trustworthy report that includes identity
and version information about the hardware and software on the
machines attached to their network, for purposes such as inventory,
audit, anomaly detection, record maintenance and/or trending reports
(logging). 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
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its validity. Remote attestation is desired to prevent vulnerable or
compromised devices from getting access to the network and
potentially harming others.
Typically, solutions start with a specific component (called a "root
of trust") that provides device identity and protected storage for
measurements. The system components perform a series of measurements
that may be signed by the root of trust, considered as Evidence about
the hardware, firmware, BIOS, software, etc. that is present.
Attester: A device desiring access to a network
Relying Party: Network equipment such as a router, switch, or access
point, responsible for admission of the device into the network
2.2. Confidential Machine Learning (ML) Model Protection
A device manufacturer wants to protect its intellectual property.
This is primarily the ML model it developed and runs in the devices
purchased by its customers. The goals for the protection include
preventing attackers, potentially the customer themselves, from
seeing the details of the model.
This typically 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 the Attester already has 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 to compromised devices.
Attester: An entity desiring to retrieve confidential data
Relying Party: An entity that holds confidential data for release to
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authorized entities
2.4. Critical Infrastructure Control
In this use case, potentially harmful physical equipment (e.g., power
grid, traffic control, hazardous chemical processing, etc.) is
connected to a network. The organization managing such
infrastructure needs to ensure that only authorized code and users
can control such processes, and that these processes are protected
from unauthorized manipulation or other threats. When a protocol
operation can affect a component of a critical system, the device
attached to the critical equipment requires some assurances depending
on the security context, including that: the 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 (TEE) Provisioning
A "Trusted Application Manager (TAM)" server is responsible for
managing the applications running in the TEE of a client device. To
do this, the TAM wants to assess the state of a TEE, or of
applications in the TEE, of a client device. The TEE conducts a
remote attestation procedure with the TAM, which can then decide
whether the TEE is already in compliance with the TAM's latest
policy, or if the TAM needs 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 trusted execution environment capable of
running trusted applications that can be updated
Relying Party: A Trusted Application Manager
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.
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A solution to this problem is a watchdog timer implemented in a
protected environment such as a Trusted Platform Module (TPM), as
described in [TCGarch] section 43.3. If the watchdog does not
receive regular, and fresh, Attestation Results as to 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. Remote
attestation defined by RATS is thus a candidate for use in the FIDO
protocol.
Other biometric authentication protocols such as the Chinese IFAA
standard and WeChat Pay as well as Google Pay make use of attestation
in one form or another.
Attester: Every FIDO Authenticator contains an Attester.
Relying Party: Any web site, mobile application back-end, 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.
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************ ************* ************ *****************
* Endorser * * Reference * * Verifier * * Relying Party *
************ * Value * * Owner * * Owner *
| * Provider * ************ *****************
| ************* | |
| | | |
|Endorsements |Reference |Appraisal |Appraisal
| |Values |Policy |Policy for
| | |for |Attestation
.-----------. | |Evidence |Results
| | | |
| | | |
v v v |
.---------------------------. |
.----->| Verifier |------. |
| '---------------------------' | |
| | |
| Attestation| |
| Results | |
| Evidence | |
| | |
| v v
.----------. .---------------.
| Attester | | Relying Party |
'----------' '---------------'
Figure 1: Conceptual Data Flow
An Attester creates Evidence that is conveyed to a Verifier.
The Verifier uses 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, and generates Attestation Results for use by Relying
Parties. The Appraisal Policy for Evidence might be obtained from an
Endorser along with the Endorsements, and/or might be obtained via
some other mechanism such as being configured in the Verifier by the
Verifier Owner.
The Relying Party uses Attestation Results by applying its own
appraisal policy to make application-specific decisions such as
authorization decisions. The Appraisal Policy for Attestation
Results is configured in the Relying Party by the Relying Party
Owner, and/or is programmed into the Relying Party.
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3.1. Appraisal Policies
The Verifier, when appraising Evidence, or the Relying Party, when
appraising Attestation Results, checks the values of some Claims
against constraints specified in its appraisal policy. Such
constraints might involve a comparison for equality against a
Reference Value, or a check for being in a range bounded by Reference
Values, or membership in a set of Reference Values, or a check
against values in other Claims, or any other test.
3.2. Reference Values
Reference Values used in appraisal come from a Reference Value
Provider and are then used by the appraisal policy. They might be
conveyed in any number of ways, including:
* as part of the appraisal policy itself, if the Verifier Owner
either: acquires Reference Values from a Reference Value Provider
or is itself a Reference Value Provider;
* as part of an Endorsement, if the Endorser either acquires
Reference Values from a Reference Value Provider or is itself a
Reference Value Provider; or
* via separate communication.
The actual data format and semantics of any Reference Values are
specific to Claims and implementations. This architecture document
does not define any general purpose format for them or general means
for comparison.
3.3. 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. In some
implementations, the Attesting and Target Environments might be
combined. Other implementations might have multiple Attesting and
Target Environments, such as in the examples described in more detail
in Section 3.4 and Section 3.5. Other examples may exist. Besides,
the examples discussed could be combined into even more complex
implementations.
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.--------------------------------.
| |
| Verifier |
| |
'--------------------------------'
^
|
.-------------------------|----------.
| | |
| .----------------. | |
| | Target | | |
| | Environment | | |
| | | | Evidence |
| '----------------' | |
| | | |
| | | |
| Collect | | |
| Claims | | |
| | | |
| v | |
| .-------------. |
| | Attesting | |
| | Environment | |
| | | |
| '-------------' |
| Attester |
'------------------------------------'
Figure 2: Two Types of Environments
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, taking measurements on code, memory, or other security
related assets of the Target Environment. Attesting Environments
then format the Claims appropriately, and typically use key material
and cryptographic functions, such as signing or cipher algorithms, to
create Evidence. There is no limit to or requirement on the types of
hardware or software environments that can be used to implement an
Attesting Environment, for example: Trusted Execution Environments
(TEEs), embedded Secure Elements (eSEs), Trusted Platform Modules
(TPMs), 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
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itself, it instead 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.
3.4. Layered Attestation Environments
By definition, the Attester role generates Evidence. An Attester may
consist of one or more nested environments (layers). The root layer
of an Attester includes at least one root of trust. In order to
appraise Evidence generated by an Attester, the Verifier needs to
trust the Attester's root of trust. Trust in the Attester's root of
trust can be established either directly (e.g., the Verifier puts the
root of trust's public key into its trust anchor store) or
transitively via an Endorser (e.g., the Verifier puts the Endorser's
public key into its trust anchor store). In layered attestation, a
root of trust is the initial Attesting Environment. Claims can be
collected from or about each 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 a root of trust typically are asserted by Endorsers.
The device illustrated in Figure 3 includes (A) a BIOS stored in
read-only memory, (B) an operating system kernel, and (C) an
application or workload.
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.----------. .----------.
| | | |
| Endorser |------------------->| Verifier |
| | Endorsements | |
'----------' for A, B, and C '----------'
^
.------------------------------------. |
| | |
| .---------------------------. | |
| | Target | | | Layered
| | Environment | | | Evidence
| | C | | | for
| '---------------------------' | | B and C
| Collect | | |
| Claims | | |
| .---------------|-----------. | |
| | Target v | | |
| | Environment .-----------. | | |
| | B | Attesting | | | |
| | |Environment|-----------'
| | | B | | |
| | '-----------' | |
| | ^ | |
| '---------------------|-----' |
| Collect | | Evidence |
| Claims v | for B |
| .-----------. |
| | Attesting | |
| |Environment| |
| | A | |
| '-----------' |
| |
'------------------------------------'
Figure 3: Layered Attester
Attesting Environment A, the read-only BIOS in this example, has to
ensure the integrity of the bootloader (Target Environment B). There
are potentially multiple kernels to boot, and 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, which is that
the successfully measured Target Environment B now becomes (or
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contains) an Attesting Environment for the next layer. This
procedure in Layered Attestation is sometimes called "staging". It
is important that the new Attesting Environment B not be able to
alter any Claims about its own Target Environment B. This can be
ensured having those Claims be either signed by Attesting Environment
A or stored in an untamperable manner by Attesting Environment A.
Continuing with this example, the bootloader's Attesting Environment
B is now in charge of collecting Claims about Target Environment C,
which in this example 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, plus a 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 in Figure 3.
3.5. 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 which 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. But
other slots can communicate with the main slot by the links between
them inside the router. So the main slot collects the Evidence of
other slots, produces the final Evidence of the whole router and
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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. The multi-chassis router provides higher
throughput by interconnecting multiple routers and can be logically
treated as one router for simpler management. A multi-chassis router
provides a management point that connects to the Verifier. Other
routers are only connected to the main router by the network cables,
and therefore they are managed and appraised via this main router's
help. So, in this case, the multi-chassis router is the 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 the 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 the Evidence from the
other Attesters and conveys it to a Verifier. Collection of Evidence
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from sub-entities may itself be a form of Claims collection that
results in Evidence asserted by the lead Attester. The lead Attester
generates the Evidence about the layout of the Composite Device,
while sub-Attesters generate Evidence about their respective modules.
In this situation, the trust model described in Section 7 is also
suitable for this inside Verifier.
3.6. 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. 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 any other Verifier)
in the verification process to 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 the Evidence or not.
4. Terminology
This document uses the following terms.
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/ in [RFC4949].
Consumes: 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.
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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 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 Appraisal Policy
for Evidence in a Verifier.
Produces: Appraisal Policy for Evidence
Endorser: A role performed by an entity (typically a manufacturer)
whose Endorsements help Verifiers appraise the authenticity of
Evidence.
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 artifacts. Compare /claim/ in [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.
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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 informs how a
Verifier evaluates the validity of information about an Attester.
Compare /security policy/ in [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/ in
[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,
although those 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
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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 gets
the Attestation Result from the Verifier. This section refines it 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 patterns.
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 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 passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party.
In this model, an Attester conveys Evidence to a Verifier, which
compares the Evidence against its appraisal policy. The Verifier
then gives back an Attestation Result. If the Attestation Result was
a successful one, 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.
Three ways in which the process may fail include:
* 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 pass the policy.
* The third way is when the Verifier is unreachable or unavailable.
Since the resource access protocol between the Attester and Relying
Party includes an Attestation Result, in this model the details of
that protocol constrain the serialization format of the Attestation
Result. The format of the Evidence on the other hand is only
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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
simply passes it on to a Verifier. The Verifier then 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.
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However, like 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
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use it with other Relying Parties. This is the model that the
Trusted Application Manager plans to support in the TEEP architecture
[I-D.ietf-teep-architecture].
+-------------+
| | 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. These collapsed roles combine the duties of multiple
roles.
In these cases, interaction between these roles do not necessarily
use the Internet Protocol. They can be using 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, GPIO interfaces, local busses, or
hypervisor calls. This type of conveyance is typically found in
Composite Devices. Most importantly, these conveyance methods are
out-of-scope of RATS, but they are presumed to exist in order to
convey conceptual messages appropriately between roles.
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For example, an entity that both connects to a wide-area network and
to a system bus is taking on both the Attester and Verifier roles.
As a system bus-connected entity, a Verifier consumes Evidence from
other devices connected to the system bus that implement Attester
roles. As a wide-area network connected entity, it may implement an
Attester role.
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 might come by the Relying Party trusting the
Verifier (or its public key) directly, or might come by trusting an
entity (e.g., a Certificate Authority) that is in the Verifier's
certificate chain.
The Relying Party might implicitly trust a Verifier, such as in a
Verifier/Relying Party combination where the Verifier and Relying
Party roles are combined. Or, 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.
For example, one explicit way for a Relying Party "A" to establish
such trust in 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.
As another example, the Relying Party can establish trust in the
Verifier by out of band establishment of key material, combined with
a protocol like TLS to communicate. There is an assumption that
between the establishment of the trusted key material and the
creation of the Evidence, that the Verifier has not been compromised.
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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 similarly to how a Relying Party could establish trust in a
Verifier as discussed above.
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 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 address 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 Party(s).
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, 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, in
which case 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. 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.
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In some solutions, a Verifier might be configured to directly trust
an Attester by having the Verifier have 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 to either 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 becomes more
scalable.
That is, it 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 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 an
Attestation system, but is not the subject of standardization within
this architecture.
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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., [I-D.birkholz-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. The root of trust protects both the conveyance channel key
material and the Attesting Environment with equivalent strength
protections.
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 similarly to how a Relying Party might establish trust in a
Verifier.
As discusssed in Section 7.3, authentication or attestation in both
directions might be needed, in which case typically one side's
identity or Evidence 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
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 evaluated by a Verifier to establish
its relevance, compliance, and timeliness. Claims need to be
collected in a manner that is reliable. 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 man-in-the-middle attackers who may observe, change or
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misdirect Evidence as it travels from Attester to 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 signing
capability. For example, if the 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 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, or an appraisal policy might 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 may not wish to let every healthy laptop from the same
manufacturer onto the network, but instead 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 which
may need device-specific information such as a public key for the
device or component or user on the device.
8.3. 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.
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The quality of the Attestation Results depend 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, then 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 often need to include detailed information
about the Attester, for use by Relying Parties, much like physical
passports and drivers licenses include personal information such as
name and date of birth. 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.
9. Claims Encoding Formats
The following diagram illustrates a relationship to which remote
attestation is desired to be added:
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+-------------+ +------------+ Evaluate
| |-------------->| | request
| Attester | Access some | Relying | against
| | resource | Party | security
+-------------+ +------------+ policy
Figure 8: Typical Resource Access
In this diagram, the protocol between Attester and a Relying Party
can be any new or existing protocol (e.g., HTTP(S), COAP(S), ROLIE
[RFC8322], 802.1x, OPC UA [OPCUA], etc.), depending on the use case.
Such protocols typically already have mechanisms for passing security
information for purposes of authentication and authorization. Common
formats include 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 doesn't degrade the security
properties of the system and should use the native extension
mechanisms provided by the underlying protocol. For example, if the
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., [I-D.tschofenig-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 to minimize the attack surface area. So 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
natively in the format that is natural for 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 allowing multiple encoding formats (CWT, JWT, X.509, etc.)
that encode the same information into the Claims format needed by the
Relying Party.
The following diagram illustrates that Evidence and Attestation
Results might each have multiple possible 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.
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Evidence Attestation Results
.--------------. CWT CWT .-------------------.
| Attester-A |------------. .----------->| Relying Party V |
'--------------' v | `-------------------'
.--------------. JWT .------------. JWT .-------------------.
| Attester-B |-------->| Verifier |-------->| Relying Party W |
'--------------' | | `-------------------'
.--------------. X.509 | | X.509 .-------------------.
| Attester-C |-------->| |-------->| 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 may 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
and their values 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.
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.
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10.1. Explicit Timekeeping using Synchronized Clocks
The first approach is to rely on synchronized and trustworthy clocks,
and include a signed timestamp (see [I-D.birkholz-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
creation of Evidence or Attestation Result from the time that a
specific Claim was generated. The clock's trustworthiness typically
requires additional Claims about the signer's time synchronization
mechanism.
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), and might be suitable, for example, in case the Attester
does not have a reliable clock or time synchronization is otherwise
impaired. In this approach, a non-predictable 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 Handles
A third approach relies on having epoch "handles" periodically sent
to both the sender and receiver of Evidence or Attestation Results by
some "Handle Distributor".
Handles 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.
Handles 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.
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Like the nonce approach, this allows associating a "rough" epoch
without requiring a reliable clock or time synchronization in order
to generate or appraise the freshness of Evidence or Attestation
Results. Only the Handle Distributor requires access to a clock so
it can periodically send new epoch handles.
The most recent handle is included in the produced Evidence or
Attestation Results, and the appraising entity can compare the handle
in received Evidence or Attestation Results against the latest handle
it received from the Handle 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 Handle Distributor as the handle, or by
including both the current and previous handles in messages and/or
checks, by requiring retries in case of mismatching handles, or by
buffering incoming messages that might be associated with a handle
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 handles. The depth
of such epoch window is directly proportional to the maximum network
propagation delay between the first to receive the handle and the
last to receive the handle, and it is inversely proportional to the
epoch duration. The appraising entity shall compare the handle
carried in the received Evidence or Attestation Result with the
handles 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 handle 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 Handle Distributor.
10.4. Discussion
Implicit and explicit timekeeping can be combined into hybrid
mechanisms. For example, if clocks exist and are considered
trustworthy 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.
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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 correct
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 Section 16.
For a discussion on the security of handles 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 the Attestation process 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.
Many claims in Attestation Evidence and Attestation Results are
potentially Personally Identifying Information) depending on the end-
to-end use case of the attestation. Attestation that goes up to
include containers and applications may further reveal details about
a specific system or user.
In some cases, an attacker may be able to make inferences about
attestations from the results 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.
Evidence and Attestation Results data structures are expected to
support integrity protection encoding (e.g., COSE, JOSE, X.509) and
optionally might support confidentiality protection (e.g., COSE,
JOSE). Therefore, if confidentiality protection is omitted or
unavailable, the protocols that convey Evidence or Attestation
Results are responsible for detailing what kinds of information are
disclosed, and to whom they are exposed.
Furthermore, because Evidence might contain sensitive 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
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for the Verifier's own Attestation Result, and 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 [CCC-DeepDive] section 6.2 for more discussion.
12. Security Considerations
12.1. Attester and Attestation Key Protection
Implementers need to pay close attention to the protection of the
Attester and the factory processes for provisioning the Attestation
key material. If either of these are compromised, the remote
attestation becomes worthless because an attacker 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, so the protections discussed here range from low to
high security where low security may be only application or process
isolation by the device's operating system and high security involves
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 for and signs them
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. There must also be confidentiality so that the signing
key is not captured and used elsewhere to forge Evidence.
In many cases the user or owner of the device must not be able to
modify or exfiltrate keys from the Attesting Environment of the
Attester. For example the owner or user of a mobile phone or FIDO
authenticator, having full control over the keys, might not be
trusted to use the keys to report Evidence about the environment that
protects the keys. The point of remote attestation is for the
Relying Party to be able to trust the Attester even though they don't
trust the user or owner.
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Some of the measures for a minimally protected system might include
process or application isolation by a high-level operating system,
and perhaps restricting access to root or system privilege. For
extremely simple single-use devices that don't use a protected mode
operating system, like a Bluetooth speaker, the isolation might only
be the plastic housing for the device.
Measures for a moderately protected system could include a special
restricted operating environment like a Trusted Execution Environment
(TEE) might be used. 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 that establishes the signing key material on the
device and the verification key material off the device. Sometimes
this is referred to as "personalization".
One way to provision a key is to first generate it external to the
device and then copy the key onto the device. In this case,
confidentiality of the generator, as well as the path over which the
key is provisioned, is necessary. The manufacturer needs to take
care to protect it with measures consistent with its value. This can
be achieved in a number of ways.
Confidentiality can be achieved entirely with physical provisioning
facility security involving no encryption at all. For low-security
use cases, this might be simply locking doors and limiting personnel
that can enter the facility. For high-security use cases, this might
involve a special area of the facility accessible only to select
security-trained personnel.
Cryptography can also be used to support confidentiality, but keys
that are used to then provision attestation keys must somehow have
been provisioned securely beforehand (a recursive problem).
In many cases both some physical security and some cryptography will
be necessary and useful to establish confidentiality.
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Another way to provision the key material is to generate it on the
device and export the verification key. If public key cryptography
is being used, then only integrity is necessary. Confidentiality is
not necessary.
In all cases, the Attestation Key provisioning process must ensure
that only attestation key material that is generated by a valid
Endorser is established in Attesters and then configured correctly.
For many use cases, this will involve physical security at the
facility, to prevent unauthorized devices from being manufactured
that may be counterfeit or incorrectly configured.
12.2. Integrity Protection
Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation
Results, Endorsements, or appraisal policy must support end-to-end
integrity protection and replay attack prevention, and 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 itself obtained
securely. If an attacker can configure appraisal policies for a
Relying Party or for a Verifier, then integrity of the process is
compromised.
The security protecting conveyed information may be applied at
different layers, whether by a conveyance protocol, or an information
encoding format. This architecture expects attestation messages
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(i.e., Evidence, Attestation Results, Endorsements, Reference Values,
and Policies) are 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.
12.3. Handle-based Attestation
Handles, described in Section 10.3, can be tampered with, dropped,
delayed and reordered by an attacker.
An attacker could be either 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 handles can potentially lock
all the participants in a certain epoch of choice for ever,
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 handles is equivalent to pinning the victim
node to a past epoch. An attacker could drop handles 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 handles 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 where,
for example, a Verifier is 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.
13. IANA Considerations
This document does not require any actions by IANA.
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14. Acknowledgments
Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Fitzgerald-McKay, Diego Lopez, Laurence Lundblade, Paul Rowe, Hannes
Tschofenig, Frank Xia, and David Wooten.
15. Notable Contributions
Thomas Hardjono created older 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 created the content structure of the
first three architecture drafts. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat
Model.
16. Appendix A: Time Considerations
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 Coordinated
Universal Time, or might be defined relative to some other timestamp
or timeticks counter.
+====+============+=================================================+
| 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. |
+----+------------+-------------------------------------------------+
| HR | Handle | A handle is successfully received and |
| | received | processed by an entity. |
+----+------------+-------------------------------------------------+
| EG | Evidence | An Attester creates Evidence from collected |
| | generation | Claims. |
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+----+------------+-------------------------------------------------+
| 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. For example, |
| | | acting upon some message just received |
| | | 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
Using the table above, a number of hypothetical examples of how a
solution might be built are illustrated below. a solution might be
built. This list is not intended to be complete, but 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.
16.1. Example 1: Timestamp-based Passport Model Example
The following example 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.
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.----------. .----------. .---------------.
| 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)
In the figures above and in subsequent sections, curly braces
indicate containment. For example, the notation Evidence{foo}
indicates that 'foo' is contained in the Evidence and is thus covered
by its signature.
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 also 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.
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The Relying Party can check whether the Attestation Result is fresh
when appraising it at time(RA_r) by checking "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)). The Relying Party must be careful, however, to not
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)".
16.2. Example 2: Nonce-based Passport Model Example
The following example 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.
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.----------. .----------. .---------------.
| 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)
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".
The Verifier cannot, however, 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.
However, 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.
The Relying Party must still be careful, however, to not allow
continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a
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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)".
16.3. Example 3: Handle-based Passport Model Example
The example in Figure 10 illustrates a hypothetical Passport Model
solution that uses handles instead of nonces or timestamps.
The Handle Distributor broadcasts handle "H" which starts a new epoch
"E" for a protocol participant upon reception at "time(HR)".
The Attester generates Evidence incorporating handle "H" and conveys
it to the Verifier.
The Verifier appraises that the received handle "H" is "fresh"
according to the definition provided in Section 10.3 whereby retries
are required in the case of mismatching handles, and generates an
Attestation Result. The Attestation Result is conveyed to the
Attester.
After the transmission of handle "H'" a new epoch "E'" is established
when "H'" is received by each protocol participant. The Attester
relays the Attestation Result obtained during epoch "E" (associated
with handle "H") to the Relying Party using the handle for the
current epoch "H'". If the Relying Party had not yet received "H'",
then the Attestation Result would be rejected, but in this example,
it is received.
In the illustrated scenario, the handle for relaying an Attestation
Result to the Relying Party is current, while a previous handle 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 handle H 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, and otherwise it is rejected
as stale.
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.-------------.
.----------. | Handle | .----------. .---------------.
| Attester | | Distributor | | Verifier | | Relying Party |
'----------' '-------------' '----------' '---------------'
time(VG_a) | | |
| | | |
~ ~ ~ ~
| | | |
time(HR_a)<------H--+--H--------time(HR_v)----->time(HR_r)
| | | |
time(EG_a) | | |
|---Evidence--------------------->| |
| {H,time(EG_a)-time(VG_a)} | |
| | | |
| | time(RG_v) |
|<--Attestation Result------------| |
| {H,time(RX_v)-time(RG_v)} | |
| | | |
time(HR'_a)<-----H'-+--H'-------time(HR'_v)---->time(HR'_r)
| | | |
|---[Attestation Result--------------------->time(RA_r)
| {H,time(RX_v)-time(RG_v)},H'] | |
| | | |
~ ~ ~ ~
| | | |
| | | time(OP_r)
Figure 10: Handle-based Passport Model
16.4. Example 4: Timestamp-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check
Model solution that uses timestamps and requires roughly synchronized
clocks between the Attester, Verifier, and Relying Party.
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.----------. .---------------. .----------.
| 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) |
The time considerations in this example are equivalent to those
discussed under Example 1 above.
16.5. Example 5: Nonce-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check
Model solution that uses nonces and thus does not require that any
clocks are 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) |
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The Verifier can check whether the Evidence is fresh, and whether a
claim value is recent, the same as in Example 2 above.
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".
The Relying Party must still be careful, however, to not 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)".
17. References
17.1. Normative References
[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>.
17.2. Informative References
[CCC-DeepDive]
Confidential Computing Consortium, "Confidential Computing
Deep Dive", n.d.,
<https://confidentialcomputing.io/whitepaper-02-latest>.
[CTAP] FIDO Alliance, "Client to Authenticator Protocol", n.d.,
<https://fidoalliance.org/specs/fido-v2.0-id-20180227/
fido-client-to-authenticator-protocol-v2.0-id-
20180227.html>.
[I-D.birkholz-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-04, 13
January 2021, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-04.txt>.
[I-D.birkholz-rats-uccs]
Birkholz, H., O'Donoghue, J., Cam-Winget, N., and C.
Bormann, "A CBOR Tag for Unprotected CWT Claims Sets",
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Work in Progress, Internet-Draft, draft-birkholz-rats-
uccs-02, 2 December 2020, <http://www.ietf.org/internet-
drafts/draft-birkholz-rats-uccs-02.txt>.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-13, 2 November 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-13.txt>.
[I-D.tschofenig-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,
<http://www.ietf.org/internet-drafts/draft-tschofenig-tls-
cwt-02.txt>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security-
model/>.
[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>.
[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]
NISC, "Strength of Function", n.d.,
<https://csrc.nist.gov/glossary/term/
strength_of_function>.
[TCG-DICE] Trusted Computing Group, "DICE Certificate Profiles",
n.d., <https://trustedcomputinggroup.org/wp-
content/uploads/DICE-Certificate-Profiles-
r01_3june2020-1.pdf>.
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[TCGarch] Trusted Computing Group, "Trusted Platform Module Library
- Part 1: Architecture", 8 November 2019,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p59_Part1_Architecture_pub.pdf>.
[WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>.
Contributors
Monty Wiseman
Email: montywiseman32@gmail.com
Liang Xia
Email: frank.xialiang@huawei.com
Laurence Lundblade
Email: lgl@island-resort.com
Eliot Lear
Email: elear@cisco.com
Jessica Fitzgerald-McKay
Sarah C. Helbe
Andrew Guinn
Peter Loscocco
Email: pete.loscocco@gmail.com
Eric Voit
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Thomas Fossati
Email: thomas.fossati@arm.com
Paul Rowe
Carsten Bormann
Email: cabo@tzi.org
Giri Mandyam
Email: mandyam@qti.qualcomm.com
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
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Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com
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