Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-04
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| Authors | Henk Birkholz , Dave Thaler , Michael Richardson , Ned Smith , Wei Pan | ||
| Last updated | 2020-05-21 | ||
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draft-ietf-rats-architecture-04
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler
Expires: 22 November 2020 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
W. Pan
Huawei Technologies
21 May 2020
Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-04
Abstract
In network protocol exchanges, it is often the case that one entity
(a Relying Party) requires evidence about a remote peer to assess the
peer's trustworthiness, and a way to appraise such evidence. The
evidence is typically a set of claims about its software and hardware
platform. This document describes an architecture for such remote
attestation procedures (RATS).
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 22 November 2020.
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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5
3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5
3.2. Confidential Machine Learning (ML) Model Protection . . . 6
3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7
3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9
4.2. Two Types of Environments of an Attester . . . . . . . . 9
4.3. Layered Attestation Environments . . . . . . . . . . . . 10
4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 15
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 20
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 22
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10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
12. Security Considerations . . . . . . . . . . . . . . . . . . . 26
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27
16.1. Example 1: Timestamp-based Passport Model Example . . . 29
16.2. Example 2: Nonce-based Passport Model Example . . . . . 30
16.3. Example 3: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 31
16.4. Example 4: Nonce-based Background-Check Model Example . 31
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
17.1. Normative References . . . . . . . . . . . . . . . . . . 32
17.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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 documents defines a flexible architecture with corresponding
roles and their interaction 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 enable readers of this document to
map their current and emerging solutions to the architecture provided
and the corresponding terminology defined.
A common terminology that provides a well-understood semantic meaning
to the concepts, roles, and models in this document is vital to
create semantic interoperability between solutions and across
different platforms.
Amongst other things, this document is about trust and
trustworthiness. Trust is a decision being made. Trustworthiness is
a quality that is assessed via evidence created. This is a subtle
difference and being familiar with the difference is crucial for
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using this document. Additionally, the concepts of freshness and
trust relationships with respect to RATS are elaborated on to enable
implementers in order to choose appropriate solutions to compose
their Remote Attestation Procedures.
2. Terminology
This document uses the following terms.
Appraisal Policy for Evidence: A set of rules that direct how a
Verifier evaluates the validity of information about an Attester.
Compare /security policy/ in [RFC4949]
Appraisal Policy for Attestation Result: 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]
Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier
vouches for the validity of the results
Attester: An entity whose attributes must be appraised in order to
determine whether the entity is considered trustworthy, such as
when deciding whether the entity is authorized to perform some
operation
Claim: A piece of asserted information, often in the form of a name/
value pair. (Compare /claim/ in [RFC7519])
Endorsement: A secure statement that some entity (typically a
manufacturer) vouches for the integrity of an Attester's signing
capability
Endorser: An entity that creates Endorsements that can be used to
help to appraise the trustworthiness of Attesters
Evidence: A set of information about an Attester that is to be
appraised by a Verifier
Relying Party: An entity that depends on the validity of information
about another entity, typically for purposes of authorization.
Compare /relying party/ in [RFC4949]
Relying Party Owner: An entity, such as an administrator, that is
authorized to configure Appraisal Policy for Attestation Results
in a Relying Party.
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Verifier: An entity that appraises the validity of Evidence about an
Attester
Verifier Owner: An entity, such as an administrator, that is
authorized to configure Appraisal Policy for Evidence in a
Verifier
3. 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, and a summary of what an
Attester and a Relying Party refer to in the use case.
3.1. Network Endpoint Assessment
Network operators want a trustworthy report of identity and version
of information of the hardware and software on the machines attached
to their network, for purposes such as inventory, auditing, and/or
logging. The network operator may also want a policy by which full
access is only granted to devices that meet some definition of
health, and so wants to get claims about such information and verify
their 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. These components perform a series of measurements, and
express this with Evidence as to the hardware and firmware/software
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
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3.2. Confidential Machine Learning (ML) Model Protection
A device manufacturer wants to protect its intellectual property in
terms of the ML model it developed and that runs in the devices that
its customers purchased, and it wants to prevent attackers,
potentially including the customer themselves, from seeing the
details of the model.
This typically works by having some protected environment in the
device attest to some manufacturer service. 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 to do inferencing
Relying Party: A server or service holding ML models it desires to
protect
3.3. 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.
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 retrieval
by other entities
3.4. 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 adversaries. 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
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Relying Party: A device or application connected to potentially
dangerous physical equipment (hazardous chemical processing,
traffic control, power grid, etc.)
3.5. 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 attests to 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
3.6. Hardware Watchdog
One significant problem is malware that holds a device hostage and
does not allow it to reboot to prevent updates to be applied. This
is a significant problem, because it allows a fleet of devices to be
held hostage for ransom.
A hardware watchdog can be implemented by forcing a reboot unless
remote attestation to a server succeeds within a periodic interval,
and having the reboot do remediation by bringing a device into
compliance, including installation of patches as needed.
Attester: The device that is desired to keep 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
4. Architectural Overview
Figure 1 depicts the data that flows between different roles,
independent of protocol or use case.
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************ ************ ****************
* Endorser * * Verifier * * Relying Party*
************ * Owner * * Owner *
| ************ ****************
| | |
Endorsements| | |
| |Appraisal |
| |Policy |
| |for | Appraisal
| |Evidence | Policy for
| | | Attestation
| | | Result
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 Evidence Appraisal Policy to assess the
trustworthiness of the Attester, and generates Attestation Results
for use by Relying Parties. The Evidence Appraisal Policy might be
obtained from an Endorser along with the Endorsements, or might be
obtained via some other mechanism such as being configured in the
Verifier by an administrator.
The Relying Party uses Attestation Results by applying its own
Appraisal Policy to make application-specific decisions such as
authorization decisions. The Attestation Result Appraisal Policy
might, for example, be configured in the Relying Party by an
administrator.
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4.1. 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.
Such reference values might be specified as part of the Appraisal
Policy itself, or might be obtained from a separate source, such as
an Endorsement, and then used by the Appraisal Policy.
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.
4.2. 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 4.3 and Section 4.4.
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 raw values and the
information to be represented in claims, such as by doing some
measurement of a Target Environment's code, memory, and/or registers.
Attesting Environments then format the claims appropriately, and
typically use key material and cryptographic functions, such as
signing or cipher algorithms, to create Evidence. Places that
Attesting Environments can exist include 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 Target Environment. Attesting
Environments are designed specifically with claims collection in
mind.
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.--------------------------------.
| |
| Verifier |
| |
'--------------------------------'
^
|
.-------------------------|----------.
| | |
| .----------------. | |
| | Target | | |
| | Environment | | |
| | | | Evidence |
| '----------------' | |
| | | |
| | | |
| Collect | | |
| Claims | | |
| | | |
| v | |
| .-------------. |
| | Attesting | |
| | Environment | |
| | | |
| '-------------' |
| Attester |
'------------------------------------'
Figure 2: Two Types of Environments
4.3. Layered Attestation Environments
By definition, the Attester role takes on the duty to create
Evidence. The fact that an Attester role is composed of environments
that can be nested or staged adds complexity to the architectural
layout of how an Attester can be composed and therefore has to
conduct the Claims collection in order to create believable
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, say, making the kernel
become another Attesting Environment for an application as another
Target Environment, resulting 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.
4.4. 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. Among these routers,
there is only one main router 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: Conceptual Data Flow for a 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 verifies them using Endorsements and Appraisal
Policies (obtained the same way as any other Verifier), 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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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 Results from the Verifier. There are multiple other
possible models. This section includes some reference models, but
this is not intended to be a restrictive list, and other variations
may exist.
5.1. Passport Model
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 to a Relying Party, which then compares the Attestation Result
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 resulting 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.
+-------------+
| | Compare Evidence
| Verifier | against Appraisal Policy
| |
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+----------+ +---------+
| |------------->| |Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party | Appraisal
+----------+ +---------+ Policy
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Figure 5: 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.
5.2. Background-Check Model
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
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.
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+-------------+
| | 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
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 a former employer or government agency that
issues a report is a Verifier.
5.3. Combinations
One variation of the background-check model is where the Relying
Party and the Verifier on the same machine, and so 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, and the same device may need to create
Evidence for different Relying Parties and different use cases (e.g.,
a network infrastructure device to gain access to the network, and
then a server holding confidential data to get access to that data).
As such, both models may simultaneously be in use by the same device.
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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].
+-------------+
| | 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. As a result, the entity can participate as
a constituent of the RATS architecture. Additionally, 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
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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.
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 also creates
and consumes the corresponding conceptual messages as defined in this
document.
7. Trust Model
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 the Verifying Relying Party
combination). Or, for a stronger level of security, the Relying
Party might require that the Verifier itself provide information
about itself that the Relying Party can use to assess the
trustworthiness of the Verifier before accepting its Attestation
Results.
The Endorser and Verifier Owner may need to trust the Verifier before
giving the Endorsement and Appraisal Policy to it. Such trust can
also be established directly or indirectly, implicitly or explicitly.
One explicit way to establish such trust may be the Verifier first
acts as an Attester and creates Evidence about itself to be consumed
by the Endorser and/or Verifier Owner as the Relying Parties. If it
is accepted as trustworthy, then they can provide Endorsements and
Appraisal Policies that enable it to act as a 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
solutions with weaker security, a Verifier might be configured to
implicitly trust firmware or even software (e.g., a hypervisor).
That is, it might appraise the trustworthiness of an application
component, or operating system component or service, under the
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assumption that information provided about it by the lower-layer
hypervisor or firmware is true. A stronger level 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. The component that is implicitly trusted is often
referred to as a Root of Trust.
A conveyance protocol that provides authentication and integrity
protection can be used to convey unprotected Evidence, assuming the
following properties exists:
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.
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 rules that it controls. In
the background-check model, this Evidence may also be revealed to
Relying Party(s).
8. Conceptual Messages
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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.
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 known-good
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.
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8.3. Attestation Results
Attestation Results may indicate compliance or non-compliance with a
Verifier's Appraisal Policy. 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 known-
good 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
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In this diagram, the protocol between Attester and a Relying Party
can be any new or existing protocol (e.g., HTTP(S), COAP(S), 802.1x,
OPC UA, 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.
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
It is important to prevent replay attacks where an attacker replays
old Evidence or an old Attestation Result that is no longer correct.
To do so, some mechanism of ensuring that the Evidence and
Attestation Result are fresh, meaning that there is some degree of
assurance that they still reflect the latest state of the Attester,
and that any Attestation Result was generated using the latest
Appraisal Policy for Evidence. There is, however, always a race
condition possible in that the state of the Attester, and the
Appraisal Policy for Evidence, might change immediately after the
Evidence or Attestation Result was generated. The goal is merely to
narrow the time window to something the Verifier (for Evidence) or
Relying Party (for an Attestation Result) is willing to accept.
There are two common approaches to providing some assurance of
freshness. The first approach is that a nonce is generated by a
remote entity (e.g., the Verifier for Evidence, or the Relying Party
for an Attestation Result), and the nonce is then signed and included
along with the claims in the Evidence or Attestation Result, so that
the remote entity knows that the claims were signed after the nonce
was generated.
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A second approach is to rely on synchronized clocks, and include a
signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with
the claims in the Evidence or Attestation Result, so that the remote
entity knows that the claims were signed at that time, as long as it
has some assurance that the timestamp is correct. This typically
requires additional claims about the signer's time synchronization
mechanism in order to provide such assurance.
In either approach, 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 might
have been generated at boot, and then used in claims as long as the
signer can guarantee that they cannot have changed since boot.
A more detailed discussion with examples appears in Section 16.
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. 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.
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.
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12. Security Considerations
Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation
Results, Endorsements, or Appraisal Policy, needs to support end-to-
end integrity protection and replay attack prevention, and often also
needs to support additional security protections. For example,
additional means of authentication, confidentiality, integrity,
replay, denial of service and privacy protection are needed in many
use cases. 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,
Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
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15. Contributors
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.
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+----+------------+-----------------------------------------------+
| ID | Event | Explanation of event |
+====+============+===============================================+
| VG | Value | A value to appear in a claim was created |
| | generation | |
+----+------------+-----------------------------------------------+
| NS | Nonce sent | A random number not predictable to an |
| | | Attester is sent |
+----+------------+-----------------------------------------------+
| NR | Nonce | The nonce is relayed to an Attester by |
| | relayed | enother entity |
+----+------------+-----------------------------------------------+
| EG | Evidence | An Attester collects claims and generates |
| | generation | Evidence |
+----+------------+-----------------------------------------------+
| ER | Evidence | A Relying Party relays Evidence to a Verifier |
| | relayed | |
+----+------------+-----------------------------------------------+
| RG | Result | A Verifier appraises Evidence and generates |
| | generation | an Attestation Result |
+----+------------+-----------------------------------------------+
| RR | Result | A Relying Party relays an Attestation Result |
| | relayed | 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
We now walk through a number of hypothetical examples of how a
solution might be built. This list is not intended to be complete,
but is just representative enough to highlight various timing
considerations.
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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.
.----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party |
'----------' '----------' '---------------'
time(VG) | |
| | |
~ ~ ~
| | |
time(EG) | |
|------Evidence{time(EG)}-------->| |
| time(RG) |
|<-----Attestation Result---------| |
| {time(RG),time(RX)} | |
~ ~
| |
|------Attestation Result{time(RG),time(RX)}-->time(RA)
| |
~ ~
| |
| time(OP)
| |
The Verifier can check whether the Evidence is fresh when appraising
it at time(RG) by checking "time(RG) - time(EG) < 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) 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) value, the Verifier can also determine whether the
claim value is recent by checking "time(RG) - time(VG) < Threshold",
again where the threshold is large enough to account for the maximum
permitted clock skew between the Verifier and the Attester.
The Relying Party can check whether the Attestation Result is fresh
when appraising it at time(RA) by checking "time(RA) - time(RG) <
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)). The Relying Party must be careful, however, to not allow
continued use beyond the period for which it deems the Attestation
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Result to remain fresh enough. Thus, it might allow use (at
time(OP)) as long as "time(OP) - time(RG) < Threshold". However, if
the Attestation Result contains an expiry time time(RX) then it could
explicitly check "time(OP) < time(RX)".
16.2. Example 2: Nonce-based Passport Model Example
The following example illustrates a hypothetical Passport Model
solution that uses nonces and thus does not require that any clocks
are synchronized.
.----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party |
'----------' '----------' '---------------'
time(VG) | |
| | |
~ ~ ~
| | |
|<---Nonce1--------------------time(NS) |
time(EG) | |
|----Evidence-------------------->| |
| {Nonce1, time(EG)-time(VG)} | |
| time(RG) |
|<---Attestation Result-----------| |
| {time(RX)-time(RG)} | |
~ ~
| |
|<---Nonce2------------------------------------time(NS')
time(RR)
|----Attestation Result{time(RX)-time(RG)}---->time(RA)
| Nonce2, time(RR)-time(EG) |
~ ~
| |
| time(OP)
In this example solution, the Verifier can check whether the Evidence
is fresh at time(RG) by verifying that "time(RG) - time(NS) <
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)-time(VG), then it can determine
recency by checking "time(RG)-time(NS) + time(EG)-time(VG) <
Threshold".
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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) - time(NS') +
time(RR)-time(EG) < Threshold". Although the Nonce2 and time(RR)-
time(EG) 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)-time(RG), then the Relying
Party can check "time(OP) - time(NS') < time(RX)-time(RG)".
16.3. Example 3: 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.
.----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------'
time(VG) | |
| | |
~ ~ ~
| | |
time(EG) | |
|----Evidence------->| |
| {time(EG)} time(ER)--Evidence{time(EG)}-->|
| | time(RG)
| time(RA)<-Attestation Result---|
| | {time(RX)} |
~ ~ ~
| | |
| time(OP) |
The time considerations in this example are equivalent to those
discussed under Example 1 above.
16.4. Example 4: 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.
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.----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------'
time(VG) | |
| | |
~ ~ ~
| | |
| |<-----Nonce-------------time(NS)
|<---Nonce-----------time(NR) |
time(EG) | |
|----Evidence{Nonce}--->| |
| time(ER)--Evidence{Nonce}----->|
| | time(RG)
| time(RA)<-Attestation Result---|
| | {time(RX)-time(RG)} |
~ ~ ~
| | |
| time(OP) |
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) - time(NR) < 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)-time(RG), then the Relying
Party can check "time(OP) - time(ER) < time(RX)-time(RG)".
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
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[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>.
[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/>.
[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-02, 9
March 2020, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-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-08, 4 April 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-08.txt>.
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
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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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