Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-08
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| Authors | Henk Birkholz , Dave Thaler , Michael Richardson , Ned Smith , Wei Pan | ||
| Last updated | 2020-12-08 | ||
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draft-ietf-rats-architecture-08
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler
Expires: 11 June 2021 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
W. Pan
Huawei Technologies
8 December 2020
Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-08
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-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 June 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Simplified BSD License.
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 Retrieval . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . 10
3.5. Composite Device . . . . . . . . . . . . . . . . . . . . 12
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Roles . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Artifacts . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 16
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 17
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 18
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 19
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 20
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 21
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7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 22
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 22
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 22
7.5. Endorser, Reference Value Provider, and Verifier Owner . 24
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 24
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 24
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 25
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 25
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 26
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 30
12. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12.1. Attester and Attestation Key Protection . . . . . . . . 31
12.1.1. On-Device Attester and Key Protection . . . . . . . 31
12.1.2. Attestation Key Provisioning Processes . . . . . . . 32
12.2. Integrity Protection . . . . . . . . . . . . . . . . . . 32
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
15. Notable Contributions . . . . . . . . . . . . . . . . . . . . 34
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 34
16.1. Example 1: Timestamp-based Passport Model Example . . . 35
16.2. Example 2: Nonce-based Passport Model Example . . . . . 37
16.3. Example 3: Handle-based Passport Model Example . . . . . 38
16.4. Example 4: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 40
16.5. Example 5: Nonce-based Background-Check Model Example . 41
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 42
17.1. Normative References . . . . . . . . . . . . . . . . . . 42
17.2. Informative References . . . . . . . . . . . . . . . . . 42
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
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.
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
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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 of information of 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
its validity. Remote attestation is desired to prevent vulnerable or
compromised devices from getting access to the network and
potentially harming others.
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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 running.
Attester: A device desiring access to a network
Relying Party: A network infrastructure device such as a router,
switch, or access point
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 Retrieval
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.
An assessment of system state is made against a set of policies to
evaluate the state of a system using attestations for the system
requesting 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
authorized entities
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2.4. Critical Infrastructure Control
In this use case, potentially dangerous 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 they are protected from malware or
other threats. When a protocol operation can affect some critical
system, the device attached to the critical equipment thus wants some
assurance that the requester has not been compromised. 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.
In the case, the Relying Party is the watchdog timer in the TPM/
secure enclave itself, as described in [TCGarch] section 43.3. The
Attestation Results are returned to the device, and provided to the
enclave.
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If the watchdog does not receive regular, and fresh, Attestation
Results as to the systems' 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 remote server that will securely grant the Attester
permission to continue operating (i.e., not reboot) for a period
of time
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 does biometric authentication.
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, 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
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, and the examples discussed could even be
combined into even more complex implementations.
Claims are collected from Target Environments, as shown in Figure 2.
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
or memory and so on 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 places that an Attesting Environment can exist, but they
typically are in Trusted Execution Environments (TEE), embedded
Secure Elements (eSE), and BIOS firmware. An execution environment
may not, by default, be capable of claims collection for a given
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Target Environment. Execution environments that are designed to be
capable of claims collection are referred to in this document as
Attesting Environments.
.--------------------------------.
| |
| Verifier |
| |
'--------------------------------'
^
|
.-------------------------|----------.
| | |
| .----------------. | |
| | Target | | |
| | Environment | | |
| | | | Evidence |
| '----------------' | |
| | | |
| | | |
| Collect | | |
| Claims | | |
| | | |
| v | |
| .-------------. |
| | Attesting | |
| | Environment | |
| | | |
| '-------------' |
| Attester |
'------------------------------------'
Figure 2: Two Types of Environments
3.4. Layered Attestation Environments
By definition, the Attester role creates Evidence. An Attester may
consist of one or more nested or staged environments, adding
complexity to the architectural structure. The unifying component is
the root of trust and the nested, staged, or chained attestation
Evidence produced. The nested or chained structure includes Claims,
collected by the Attester to aid in the assurance or believability of
the attestation Evidence.
Figure 3 depicts an example of a device that includes (A) a BIOS
stored in read-only memory in this example, (B) an updatable
bootloader, and (C) an operating system kernel.
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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, these Claims 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
contains) an Attesting Environment for the next layer. This
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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.
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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
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.
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.-----------------------------.
| 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 of all other
Attesters and then generates the Evidence of the whole Composite
Attester.
An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. The combination of roles can
be arbitrary. For example, in this Composite Device scenario, the
entity inside the lead Attester can also take on the role of a
Verifier, and the outside 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, whether in the
same conveyance protocol as the Evidence or not, to the outside
Verifier.
In this situation, the trust model described in Section 7 is also
suitable for this inside Verifier.
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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. Consumes: Evidence, Reference Values,
Endorsements, Appraisal Policy for Evidence; Produces: Attestation
Results.
Relying Party Owner: 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: An entity (typically an administrator), that is
authorized to configure Appraisal Policy for Evidence in a
Verifier. Produces: Appraisal Policy for Evidence.
Endorser: An entity (typically a manufacturer) whose Endorsements
help Verifiers appraise the authenticity of Evidence. Produces:
Endorsements.
Reference Value Provider: 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.
Compare /claim/ in [RFC7519].
Endorsement: A secure statement that an Endorser vouches for the
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integrity of an Attester's various capabilities such as Claims
collection and Evidence signing Used 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. Used 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. Used 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]. Used 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]. Used 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. Used By: Verifier; Produced By: Reference
Value Provider.
5. Topological Models
Figure 1 shows a basic model 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. There are multiple other
possible models. This section includes some reference models. This
is not intended to be a restrictive list, and other variations may
exist.
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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.
There are three ways in which the process may fail. First, the
Verifier may refuse to issue the Attestation Result due to some error
in processing, or some missing input to the Verifier. 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,
the result does not pass the policy. The third way is when the
Verifier is unreachable.
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
constrained by the Attester-Verifier remote attestation protocol.
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+-------------+
| | 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.
However, like in the Passport model, an Attestation Result is still
consumed by the Relying Party and so the serialization format of the
Attestation Result is still important. If the Relying Party is a
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constrained node whose purpose is to serve a given type resource
using a standard resource access protocol, it already needs the
parser(s) required by that existing protocol. Hence, the ability to
let the Relying Party obtain an Attestation Result in the same
serialization format allows minimizing the code footprint and attack
surface area of the Relying Party, especially 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 is generally
up to the Relying Party. The same device may need to create Evidence
for different Relying Parties and/or different use cases. For
instance, it would provide Evidence to a network infrastructure
device to gain access to the network, and to a server holding
confidential data to gain access to that data. As such, both models
may simultaneously be in use by the same device.
Figure 7 shows another example of a combination where Relying Party 1
uses the passport model, whereas Relying Party 2 uses an extension of
the background-check model. Specifically, in addition to the basic
functionality shown in Figure 6, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the
Trusted Application Manager plans to support in the TEEP architecture
[I-D.ietf-teep-architecture].
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+-------------+
| | 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 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. The entity, as a system bus Verifier, may choose to fully
isolate its role as a wide-area network Attester.
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
The scope of this document is 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. 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 rules that it controls.
In the background-check model, this Evidence may also be revealed to
Relying Party(s).
In some cases where Evidence contains sensitive information, an
Attester might even require that a Verifier first go through a TLS
authentication or a remote attestation procedure with it before the
Attester will send 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,
mutual authentication or attestation 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.
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.
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.
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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 of the Attester arranges for its Attesting
Environment to be provisioned with key material. The key material is
typically in the form of an asymmetric key pair (e.g., an RSA or
ECDSA private key and a manufacturer-signed IDevID certificate)
secured in the Attester.
The Verifier is provided with an appropriate trust anchor, or
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 an Attestation system,
but is not the subject of standardization within this architecture.
A conveyance protocol that provides authentication and integrity
protection can be used to convey unprotected Evidence, assuming the
following properties exists:
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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 discussed above, and in such a case, mutual
authentication or attestation might even be needed as discussed in
Section 7.3.
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
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.
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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 be a Boolean simply indicating compliance or
non-compliance with a Verifier's appraisal policy, or a rich set of
Claims about the Attester, against which the Relying Party applies
its Appraisal Policy for Attestation Results.
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.
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A 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.
An Attestation Result that indicates compliance can be used by a
Relying Party to make authorization decisions based on the Relying
Party's appraisal policy. The simplest such policy might be to
simply authorize any party supplying a compliant Attestation Result
signed by a trusted Verifier. A more complex policy might also
entail comparing information provided in the result against Reference
Values, or applying more complex logic on such information.
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:
+-------------+ +------------+ 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, etc.), depending on the use case. Such
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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.
To enable remote attestation to be added to existing protocols,
enabling a higher level of assurance against malware for example, it
is important that information needed for appraising the Attester be
usable with existing protocols that have constraints around what
formats they can transport. For example, OPC UA [OPCUA] (probably
the most common protocol in industrial IoT environments) is defined
to carry X.509 certificates and so security information must be
embedded into an X.509 certificate to be passed in the protocol.
Thus, remote attestation related information could be natively
encoded in X.509 certificate extensions, or could be natively encoded
in some other format (e.g., a CWT) which in turn is then encoded in
an X.509 certificate extension.
Especially for constrained nodes, however, 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.
Freshness 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 two common approaches for determining the epoch of an
Evidence or Attestation Result.
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
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can 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.
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.
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. In
another setup where all Roles (Attesters, Verifiers and Relying
Parties) share the same broadcast channel, the nonce-based approach
may be used to anchor all parties to the same (relative) timeline,
without requiring synchronized clocks, by having a central entity
emit nonces at regular intervals and have the "current" nonce
included in the produced Evidence or Attestation Result.
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.
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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 PII (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
for the Verifier's own Attestation Result, and appraising it just as
a Relying Party would appraise an Attestation Result for any other
purpose.
12. Security Considerations
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12.1. Attester and Attestation Key Protection
Implementers need to pay close attention to the isolation and
protection of the Attester and the factory processes for provisioning
the Attestation key material. When either of these are compromised,
the remote attestation becomes worthless because the attacker can
forge Evidence.
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 the Attester is located in an isolated environment
of a device like a process, a dedicated chip a TEE or such that
collects the Claims, formats them and signs them with an Attestation
Key. The Attester 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 is not trusted. 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.
Some of the measures for low level security 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.
At medium level security, 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.
For high level security, specialized hardware will likely be used
providing protection against chip decapping attacks, power supply and
clock glitching, faulting injection and RF and power side channel
attacks.
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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. 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.
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,
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* 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. As such, if appraisal policies for a Relying Party or for
a Verifier can be configured via a network protocol, the ability to
create Evidence about the integrity of the entity providing the
appraisal policy needs to be considered.
The security of conveyed information may be applied at different
layers, whether by a conveyance protocol, or an information encoding
format. This architecture expects attestation messages (i.e.,
Evidence, Attestation Results, Endorsements 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.
13. IANA Considerations
This document does not require any actions by IANA.
14. Acknowledgments
Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade,
Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
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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. |
+----+--------------+---------------------------------------------+
| HD | Handle | A centrally generated identifier for time- |
| | distribution | bound recentness across a domain of devices |
| | | is successfully distributed to Attesters. |
+----+--------------+---------------------------------------------+
| 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 distributed by a Handle |
| | received | Distributor was received. |
+----+--------------+---------------------------------------------+
| EG | Evidence | An Attester creates Evidence from collected |
| | generation | Claims. |
+----+--------------+---------------------------------------------+
| ER | Evidence | A Relying Party relays Evidence to a |
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| | 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)
| |
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 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."time(RG_v) - time(VG_a) < Threshold".
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)".
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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.
.----------. .----------. .---------------.
| 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.
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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
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
Handles are a third option to establish time-keeping next to nonces
or timestamps. Handles are opaque data intended to be available to
all RATS roles that interact with each other, such as the Attester or
Verifier, in specified intervals. To enable this availability,
handles are distributed centrally by the Handle Distributor role over
the network. As any other role, the Handle Distributor role can be
taken on by a dedicated entity or collapsed with other roles, such as
a Verifier. The use of handles can compensate for a lack of clocks
or other sources of time on entities taking on RATS roles. The only
entity that requires access to a source of time is the entity taking
on the role of Handle Distributor.
Handles are different from nonces as they can be used more than once
and can 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, but their reception creates that
information. The reception of a handle is similar to the event that
increments a relative tickcounter. Receipt of a new handle
invalidates a previously received handle.
In this example, Evidence generation based on received handles always
uses the current (most recent) handle. As handles are distributed
over the network, all involved entities receive a fresh handle at
roughly the same time. Due to distribution over the network, there
is some jitter with respect to the time the Handle is received,
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time(HR), for each involved entity. To compensate for this jitter,
there is a small period of overlap (a specified offset) in which both
a current handle and corresponding former handle are valid in
Evidence appraisal: "validity-duration = time(HR'_v) + offset -
time(HR_v)". The offset is typically based on a network's round trip
time. Analogously, the generation of valid Evidence is only
possible, if the age of the handle used is lower than the validity-
duration: "time(HR_v) - time(EG_a) < validity-duration".
From the point of view of a Verifier, the generation of valid
Evidence is only possible, if the age of the handle used in the
Evidence generation is younger than the duration of the distribution
interval - "(time(HR'_v)-time(HR_v)) - (time(HR_a)-time(EG_a)) <
validity-duration".
Due to the validity-duration of handles, multiple different pieces of
Evidence can be generated based on the same handle. The resulting
granularity (time resolution) of Evidence freshness is typically
lower than the resolution of clock-based tickcounters.
The following example illustrates a hypothetical Background-Check
Model solution that uses handles and requires a trustworthy time
source available to the Handle Distributor role.
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.-------------.
.----------. | Handle | .----------. .---------------.
| Attester | | Distributor | | Verifier | | Relying Party |
'----------' '-------------' '----------' '---------------'
time(VG_a) | | |
| | | |
~ ~ ~ ~
| | | |
time(HR_a)<---------+-------------time(HR_v)------>time(HR_r)
| | | |
time(EG_a) | | |
|----Evidence{time(EG_a)}-------->| |
| {Handle1,time(EG_a)-time(VG_a)}| |
| | time(RG_v) |
|<-----Attestation Result---------| |
| {time(RG_v),time(RX_v)} | |
| | |
~ ~ ~
| | |
time(HR_a')<--------'---------------------------->time(HR_r')
| |
time(RR_a) /
|--Attestation Result{time(RX_v)-time(RG_v)}-->time(RA_r)
| {Handle2, time(RR_a)-time(EG_a)} |
~ ~
| |
| time(OP_r)
| |
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
[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-03, 13
July 2020, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-03.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>.
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[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>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library
- Part 1: Architecture", n.d.,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p62_Part1_Architecture_7july2020.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
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Jessica Fitzgerald-McKay
Sarah C. Helbe
Andrew Guinn
Peter Lostcco
Email: pete.loscocco@gmail.com
Eric Voit
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
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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
Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com
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