draft-ietf-rats-architecture-02.txt   draft-ietf-rats-architecture-03.txt 
RATS Working Group H. Birkholz RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler Intended status: Informational D. Thaler
Expires: 8 September 2020 Microsoft Expires: 22 November 2020 Microsoft
M. Richardson M. Richardson
Sandelman Software Works Sandelman Software Works
N. Smith N. Smith
Intel Intel
W. Pan W. Pan
Huawei Technologies Huawei Technologies
7 March 2020 21 May 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-02 draft-ietf-rats-architecture-03
Abstract Abstract
In network protocol exchanges, it is often the case that one entity In network protocol exchanges, it is often the case that one entity
(a Relying Party) requires evidence about a remote peer to assess the (a Relying Party) requires evidence about a remote peer to assess the
peer's trustworthiness, and a way to appraise such evidence. The peer's trustworthiness, and a way to appraise such evidence. The
evidence is typically a set of claims about its software and hardware evidence is typically a set of claims about its software and hardware
platform. This document describes an architecture for such remote platform. This document describes an architecture for such remote
attestation procedures (RATS). attestation procedures (RATS).
skipping to change at page 2, line 7 skipping to change at page 2, line 10
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 8 September 2020. This Internet-Draft will expire on 22 November 2020.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5
3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5 3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5
3.2. Confidential Machine Learning (ML) Model Protection . . . 5 3.2. Confidential Machine Learning (ML) Model Protection . . . 5
3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6 3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
3.5. Trusted Execution Environment (TEE) Provisioning . . . . 6 3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7
3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7 3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
4.1. Two Types of Environments of an Attester . . . . . . . . 9 4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9
4.2. Layered Attestation Procedures . . . . . . . . . . . . . 9 4.2. Two Types of Environments of an Attester . . . . . . . . 9
4.3. Composite Device . . . . . . . . . . . . . . . . . . . . 12 4.3. Layered Attestation Environments . . . . . . . . . . . . 10
4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 14 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 14
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 14 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 15 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 16 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17
6. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 17 6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18
7. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 18 7. Role Hosting and Composition . . . . . . . . . . . . . . . . 19
7.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 18 8. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 19 9. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 21
7.3. Attestation Results . . . . . . . . . . . . . . . . . . . 19 9.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 20 9.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21
9. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 22 9.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 23
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 11. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24 13. Security Considerations . . . . . . . . . . . . . . . . . . . 26
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
15.1. Normative References . . . . . . . . . . . . . . . . . . 24 15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
15.2. Informative References . . . . . . . . . . . . . . . . . 24 16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 17. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27
17.1. Example 1: Timestamp-based Passport Model Example . . . 29
17.2. Example 2: Nonce-based Passport Model Example . . . . . 30
17.3. Example 3: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 31
17.4. Example 4: Nonce-based Background-Check Model Example . 31
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
18.1. Normative References . . . . . . . . . . . . . . . . . . 32
18.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction 1. Introduction
In Remote Attestation Procedures (RATS), one peer (the "Attester") In Remote Attestation Procedures (RATS), one peer (the "Attester")
produces believable information about itself - Evidence - to enable a produces believable information about itself - Evidence - to enable a
remote peer (the "Relying Party") to decide whether to consider that remote peer (the "Relying Party") to decide whether to consider that
Attester a trustworthy peer or not. RATS are facilitated by an Attester a trustworthy peer or not. RATS are facilitated by an
additional vital party, the Verifier. The Verifier appraises additional vital party, the Verifier.
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 This documents defines a flexible architecture consisting of
roles and their interaction via conceptual messages. Additionally, attestation roles and their interactions via conceptual messages.
this document defines a universal set of terms that can be mapped to Additionally, this document defines a universal set of terms that can
various existing and emerging Remote Attestation Procedures. Common be mapped to various existing and emerging Remote Attestation
role compositions and data flows, such as the "Passport Model" and Procedures. Common topological models and the data flows associated
the "Background-Check Model" are illustrated to enable readers of with them, such as the "Passport Model" and the "Background-Check
this document to map their current and emerging solutions to the Model" are illustrated. The purpose is to enable readers to map
architecture provided and the corresponding terminology defined. A their solution architecture to the canonical attestation architecture
common terminology that provides a well-understood semantic meaning provided here and to define useful terminology for attestation.
to the concepts, roles, and models in this document is vital to Having a common terminology that provides well-understood meanings
create semantic interoperability between solutions and across for common themes such as, roles, device composition, topological
different platforms. 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 Amongst other things, this document is about trust and
trustworthiness. Trust is a decision being made. Trustworthiness is trustworthiness. Trust is a decision being made. Trustworthiness is
a quality that is assessed via evidence created. This is a subtle a quality that is assessed via evidence created. This is a subtle
difference and being familiar with the difference is crucial for difference and being familiar with the difference is crucial for
using this document. Additionally, the concepts of freshness and using this document. Additionally, the concepts of freshness and
trust relationships with respect to RATS are elaborated on to enable trust relationships with respect to RATS are elaborated on to enable
implementers in order to choose appropriate solutions to compose implementers in order to choose appropriate solutions to compose
their Remote Attestation Procedures. their Remote Attestation Procedures.
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Verifier evaluates the validity of information about an Attester. Verifier evaluates the validity of information about an Attester.
Compare /security policy/ in [RFC4949] Compare /security policy/ in [RFC4949]
Appraisal Policy for Attestation Result: A set of rules that direct Appraisal Policy for Attestation Result: A set of rules that direct
how a Relying Party uses the Attestation Results regarding an how a Relying Party uses the Attestation Results regarding an
Attester generated by the Verifiers. Compare /security policy/ in Attester generated by the Verifiers. Compare /security policy/ in
[RFC4949] [RFC4949]
Attestation Result: The output generated by a Verifier, typically Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier including information about an Attester, where the Verifier
vouches for the validity of the results vouches for the validity of the Evidence it has appraised
Attester: An entity whose attributes must be appraised in order to Attester: An entity (typically a device), whose Evidence must be
determine whether the entity is considered trustworthy, such as appraised in order to infer the extent to which the Attester is
when deciding whether the entity is authorized to perform some considered trustworthy, such as when deciding whether it is
operation authorized to perform some operation
Endorsement: A secure statement that some entity (typically a Claim: A piece of asserted information, often in the form of a name/
manufacturer) vouches for the integrity of an Attester's signing value pair. (Compare /claim/ in [RFC7519])
capability
Endorser: An entity that creates Endorsements that can be used to Endorsement: Statements that Endorsers make (typically a
help to appraise the trustworthiness of Attesters manufacturer) that vouches for the design and implementation of
the Attester. Often this includes statements about the integrity
of an Attester's signing capability
Evidence: A set of information about an Attester that is to be Endorser: An entity (typically a manufacturer) whose Endorsements
appraised by a Verifier help Verifiers appraise the authenticity of Evidence
Relying Party: An entity that depends on the validity of information Evidence: A set of information that asserts the trustworthiness
about another entity, typically for purposes of authorization. status of an Attester, that is appraised by a Verifier
Compare /relying party/ in [RFC4949]
Relying Party Owner: An entity, such as an administrator, that is Relying Party: 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]
Relying Party Owner: An entity (typically an administrator), that is
authorized to configure Appraisal Policy for Attestation Results authorized to configure Appraisal Policy for Attestation Results
in a Relying Party. in a Relying Party
Verifier: An entity that appraises the validity of Evidence about an Verifier: An entity (typically a service), that appraises the
Attester validity of Evidence about an Attester and produces Attestation
Results to be used by a Relying Party
Verifier Owner: An entity, such as an administrator, that is Verifier Owner: An entity (typically an administrator), that is
authorized to configure Appraisal Policy for Evidence in a authorized to configure Appraisal Policy for Evidence in a
Verifier Verifier
3. Reference Use Cases 3. Reference Use Cases
This section covers a number of representative use cases for remote This section covers a number of representative use cases for remote
attestation, independent of specific solutions. The purpose is to attestation, independent of specific solutions. The purpose is to
provide motivation for various aspects of the architecture presented provide motivation for various aspects of the architecture presented
in this draft. Many other use cases exist, and this document does 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 not intend to have a complete list, only to have a set of use cases
skipping to change at page 5, line 26 skipping to change at page 5, line 40
their validity. Remote attestation is desired to prevent vulnerable their validity. Remote attestation is desired to prevent vulnerable
or compromised devices from getting access to the network and or compromised devices from getting access to the network and
potentially harming others. potentially harming others.
Typically, solutions start with a specific component (called a "Root Typically, solutions start with a specific component (called a "Root
of Trust") that provides device identity and protected storage for of Trust") that provides device identity and protected storage for
measurements. These components perform a series of measurements, and measurements. These components perform a series of measurements, and
express this with Evidence as to the hardware and firmware/software express this with Evidence as to the hardware and firmware/software
that is running. that is running.
FIXME from Henk: Measurements at early stages of
Layered Attestation are NOT evidence yet.
This text does not cover that yet
Attester: A device desiring access to a network Attester: A device desiring access to a network
Relying Party: A network infrastructure device such as a router, Relying Party: A network infrastructure device such as a router,
switch, or access point switch, or access point
3.2. Confidential Machine Learning (ML) Model Protection 3.2. Confidential Machine Learning (ML) Model Protection
A device manufacturer wants to protect its intellectual property in A device manufacturer wants to protect its intellectual property in
terms of the ML model it developed and that runs in the devices that terms of the ML model it developed and that runs in the devices that
its customers purchased, and it wants to prevent attackers, its customers purchased, and it wants to prevent attackers,
skipping to change at page 8, line 5 skipping to change at page 8, line 5
Relying Party: A remote server that will securely grant the Attester Relying Party: A remote server that will securely grant the Attester
permission to continue operating (i.e., not reboot) for a period permission to continue operating (i.e., not reboot) for a period
of time of time
4. Architectural Overview 4. Architectural Overview
Figure 1 depicts the data that flows between different roles, Figure 1 depicts the data that flows between different roles,
independent of protocol or use case. independent of protocol or use case.
************ ************ ***************** ************ ************ ****************
* Endorser * * Verifier * * Relying Party * * Endorser * * Verifier * * Relying Party*
************ * Owner * * Owner * ************ * Owner * * Owner *
| ************ ***************** | ************ ****************
| | | | | |
Endorsements| | | Endorsements| | |
| |Appraisal | | |Appraisal |
| |Policy for | | |Policy |
| |Evidence | Appraisal | |for | Appraisal
| | | Policy for | |Evidence | Policy for
| | | Attestation | | | Attestation
| | | Result | | | Result
v v | v v |
.-----------------. | .-----------------. |
.----->| Verifier |------. | .----->| Verifier |------. |
| '-----------------' | | | '-----------------' | |
| | | | | |
| Attestation| | | Attestation| |
| Results | | | Results | |
| Evidence | | | Evidence | |
skipping to change at page 9, line 5 skipping to change at page 9, line 5
obtained from an Endorser along with the Endorsements, or might be obtained from an Endorser along with the Endorsements, or might be
obtained via some other mechanism such as being configured in the obtained via some other mechanism such as being configured in the
Verifier by an administrator. Verifier by an administrator.
The Relying Party uses Attestation Results by applying its own The Relying Party uses Attestation Results by applying its own
Appraisal Policy to make application-specific decisions such as Appraisal Policy to make application-specific decisions such as
authorization decisions. The Attestation Result Appraisal Policy authorization decisions. The Attestation Result Appraisal Policy
might, for example, be configured in the Relying Party by an might, for example, be configured in the Relying Party by an
administrator. administrator.
4.1. Two Types of Environments of an Attester 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 An Attester consists of at least one Attesting Environment and at
least one Target Environment. In some implementations, the Attesting least one Target Environment. In some implementations, the Attesting
and Target Environments might be combined. Other implementations and Target Environments might be combined. Other implementations
might have multiple Attesting and Target Environments, such as in the might have multiple Attesting and Target Environments, such as in the
examples described in more detail in Section 4.2 and Section 4.3. examples described in more detail in Section 4.3 and Section 4.4.
Other examples may exist, and the examples discussed could even be Other examples may exist, and the examples discussed could even be
combined into even more complex implementations. combined into even more complex implementations.
Claims are collected from Target Environments. That is, Attesting Claims are collected from Target Environments, as shown in Figure 2.
Environments collect the raw values and the information to be That is, Attesting Environments collect the raw values and the
represented in claims, such as by doing some measurement of a Target information to be represented in claims, such as by doing some
Environment's code, memory, and/or registers. Attesting Environments measurement of a Target Environment's code, memory, and/or registers.
then format the claims appropriately, and typically use key material Attesting Environments then format the claims appropriately, and
and cryptographic functions, such as signing or cipher algorithms, to typically use key material and cryptographic functions, such as
create Evidence. Examples of environments that can be used as signing or cipher algorithms, to create Evidence. Places that
Attesting Environments include Trusted Execution Environments (TEE), Attesting Environments can exist include Trusted Execution
embedded Secure Elements (eSE), or Hardware Security Modules (HSM). 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.
4.2. Layered Attestation Procedures .--------------------------------.
| |
| 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 By definition, the Attester role takes on the duty to create
Evidence. The fact that an Attester role is composed of several Evidence. The fact that an Attester role is composed of environments
types of environments that can be nested or staged adds complexity to that can be nested or staged adds complexity to the architectural
the architectural layout of how an Attester - in itself - is composed layout of how an Attester can be composed and therefore has to
and therefore has to conduct the Claims collection in order to create conduct the Claims collection in order to create believable
believable Attestation Evidence. The following example is intended attestation Evidence.
to illustrate this composition:
A very common example is elaborated on to illustrate Layered Figure 3 depicts an example of a device that includes (A) a BIOS
Attestation. stored in read-only memory in this example, (B) an updatable
bootloader, and (C) an operating system kernel.
.----------. .----------. .----------. .----------.
| | | | | | | |
| Endorser |------------------->| Verifier | | Endorser |------------------->| Verifier |
| | Endorsements | | | | Endorsements | |
'----------' for A, B, and C '----------' '----------' for A, B, and C '----------'
^ ^
.------------------------------------. | .------------------------------------. |
| | | | | |
| .---------------------------. | | | .---------------------------. | |
skipping to change at page 10, line 39 skipping to change at page 11, line 39
| Collect | | Evidence | | Collect | | Evidence |
| claims v | for B | | claims v | for B |
| .-----------. | | .-----------. |
| | Attesting | | | | Attesting | |
| |Environment| | | |Environment| |
| | A | | | | A | |
| '-----------' | | '-----------' |
| | | |
'------------------------------------' '------------------------------------'
Figure 2: Layered Attester Figure 3: Layered Attester
The very first Attesting Environment has to ensure the integrity of
the (U)EFI / BIOS / Firmware that initially boots up a composite
device (e.g., a cell phone).
Henk: we are looking for a better term than UEFI/BIOS/Firmware
These Claims have to be measured securely. At this stage of the
boot-cycle of a composite device, the Claims collected typically
cannot be composed into Evidence.
The very first Attesting Environment in this example can be a
hardware component that is a Static Code Root of Trust. As in any
other scenario, this hardware component is the first Attesting
Environment. It collects a rather concise number of Claims about the
Target Environment. The Target Environment in this example is the
(U)EFI / BIOS / Firmware After the boot sequence started, the Target
Environment conducts the most important and defining feature of
Layered Attestation: The successfully measured environment that is
the (U)EFI / BIOS / Firmware now becomes the Attesting Environment.
Analogously, the Attesting Environment hands off its duty to one of
its Target Environments. This procedure in Layered Attestation is
called Staging.
Now, the duties have been transferred and Layered Attestation takes
place. The initial Attesting Environment relinquishes its duties to
the Target Environment. It is important to note that the new
Attesting Environment cannot alter the content about its own
measurements. If the Attesting Environment would be able to do that,
Layered Attestation would become unfeasible.
In this example the duty of being the Attesting Environment is now Attesting Environment A, the read-only BIOS in this example, has to
taken over by the (U)EFI / BIOS / Firmware that was the Attested ensure the integrity of the bootloader (Target Environment B). There
Environment before. This transfer of duty is the essential part of are potentially multiple kernels to boot, and the decision is up to
Layered Attestation. The (U)EFI / BIOS / Firmware now is the the bootloader. Only a bootloader with intact integrity will make an
Attesting Environment. The next Target Environment is, in this appropriate decision. Therefore, these Claims have to be measured
example, a bootloader. There are potentially multiple kernels to securely. At this stage of the boot-cycle of the device, the Claims
boot, the decision is up to the bootloader. Only a bootloader with collected typically cannot be composed into Evidence.
intact integrity will make an appropriate decision. Therefore,
Claims about the integrity of a bootloader are now collected by the
freshly appointed Attesting Environment that is the (U)EFI / BIOS /
Firmware. Collected Claims have to be stored by the current
Attesting Environment in a similar shielded and secured manner, so
that the next Attesting Environment is not capable of altering the
collection of claims stored.
Continuing with this example, the bootloader is now in charge of After the boot sequence is started, the BIOS conducts the most
collecting Claims about the next execution environment. The next important and defining feature of layered attestation, which is that
execution environment in this example is the kernel to be booted up. the successfully measured Target Environment B now becomes (or
Analogously, the next transfer of duties in this Layered Attestation contains) an Attesting Environment for the next layer. This
example occurs: The duty of being an Attesting Environment is procedure in Layered Attestation is sometimes called "staging". It
transferred to a successfully measured kernel. In this sequence, the is important that the new Attesting Environment B not be able to
kernel is now collecting additional Claims and is storing them in a alter any Claims about its own Target Environment B. This can be
secure and shielded manner. ensured having those Claims be either signed by Attesting Environment
A or stored in an untamperable manner by Attesting Environment A.
[Henk: we might have to define what successful Continuing with this example, the bootloader's Attesting Environment
means in this example and beyond] 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.
The essence of this example is a cascade of staged boot environments. This example could be extended further by, say, making the kernel
Each environment (after the initial one that is a root-of-trust) has become another Attesting Environment for an application as another
the duty of measuring its next environment before it is started. Target Environment, resulting in a third set of Claims in the
Evidence pertaining to that application.
Therefore, creating a layered boot sequence and correspondingly The essence of this example is a cascade of staged environments.
enabling Layered Attestation. 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.
4.3. Composite Device 4.4. Composite Device
A Composite Device is an entity composed of multiple sub-entities A Composite Device is an entity composed of multiple sub-entities
such that its trustworthiness has to be determined by the appraisal such that its trustworthiness has to be determined by the appraisal
of all these sub-entities. of all these sub-entities.
Each sub-entity has at least one Attesting Environment collecting the Each sub-entity has at least one Attesting Environment collecting the
claims from at least one Target Environment, then this sub-entity claims from at least one Target Environment, then this sub-entity
generates Evidence about its trustworthiness. Therefore each sub- generates Evidence about its trustworthiness. Therefore each sub-
entity can be called an Attester. Among all the Attesters, there may entity can be called an Attester. Among all the Attesters, there may
be only some which have the ability to communicate with the Verifier be only some which have the ability to communicate with the Verifier
while others do not. while others do not.
For example, a carrier-grade router is consists of a chassis and For example, a carrier-grade router consists of a chassis and
multiple slots. The trustworthiness of the router depends on all its multiple slots. The trustworthiness of the router depends on all its
slots' trustworthiness. Each slot has an Attesting Environment such slots' trustworthiness. Each slot has an Attesting Environment such
as a TEE collecting the claims of its boot process, after which it as a TEE collecting the claims of its boot process, after which it
generates Evidence from the claims. Among these slots, only a main generates Evidence from the claims. Among these slots, only a main
slot can communicate with the Verifier while other slots cannot. But slot can communicate with the Verifier while other slots cannot. But
other slots can communicate with the main slot by the links between other slots can communicate with the main slot by the links between
them inside the router. So the main slot collects the Evidence of them inside the router. So the main slot collects the Evidence of
other slots, produces the final Evidence of the whole router and other slots, produces the final Evidence of the whole router and
conveys the final Evidence to the Verifier. Therefore the router is conveys the final Evidence to the Verifier. Therefore the router is
a Composite Device, each slot is an Attester, and the main slot is a Composite Device, each slot is an Attester, and the main slot is
the transiting Attester. the lead Attester.
Another example is a multi-chassis router composed of multiple single Another example is a multi-chassis router composed of multiple single
carrier-grade routers. The multi-chassis router provides higher carrier-grade routers. The multi-chassis router provides higher
throughput by interconnecting multiple routers and can be logically throughput by interconnecting multiple routers and can be logically
treated as one router for simpler management. Among these routers, treated as one router for simpler management. Among these routers,
there is only one main router that connects to the Verifier. Other there is only one main router that connects to the Verifier. Other
routers are only connected to the main router by the network cables, routers are only connected to the main router by the network cables,
and therefore they are managed and appraised via this main router's and therefore they are managed and appraised via this main router's
help. So, in this case, the multi-chassis router is the Composite 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 Device, each router is an Attester and the main router is the lead
Attester. Attester.
Figure 3 depicts the conceptual data flow for a Composite Device. Figure 4 depicts the conceptual data flow for a Composite Device.
.-----------------------------. .-----------------------------.
| Verifier | | Verifier |
'-----------------------------' '-----------------------------'
^ ^
| |
| Evidence of | Evidence of
| Composite Device | Composite Device
| |
.----------------------------------|-------------------------------. .----------------------------------|-------------------------------.
skipping to change at page 13, line 30 skipping to change at page 14, line 30
| | | Environment(s) | | |<------------| ... | | | | | Environment(s) | | |<------------| ... | |
| | | | '------------' | Evidence '------------' | | | | | '------------' | Evidence '------------' |
| | '----------------' | of | | | '----------------' | of |
| | | Attesters | | | | Attesters |
| | lead Attester A | (via Internal Links or | | | lead Attester A | (via Internal Links or |
| '--------------------------------------' Network Connections) | | '--------------------------------------' Network Connections) |
| | | |
| Composite Device | | Composite Device |
'------------------------------------------------------------------' '------------------------------------------------------------------'
Figure 3: Conceptual Data Flow for a Composite Device Figure 4: Conceptual Data Flow for a Composite Device
In the Composite Device, each Attester generates its own Evidence by In the Composite Device, each Attester generates its own Evidence by
its Attesting Environment(s) collecting the claims from its Target its Attesting Environment(s) collecting the claims from its Target
Environment(s). The lead Attester collects the Evidence of all other Environment(s). The lead Attester collects the Evidence of all other
Attesters and then generates the Evidence of the whole Composite Attesters and then generates the Evidence of the whole Composite
Attester. 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 sends 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 6 is also
suitable for this inside Verifier.
5. Topological Models 5. Topological Models
There are multiple possible models for communication between an Figure 1 shows a basic model for communication between an Attester, a
Attester, a Verifier, and a Relying Party. This section includes Verifier, and a Relying Party. The Attester conveys its Evidence to
some reference models, but this is not intended to be a restrictive the Verifier for appraisal, and the Relying Party gets the
list, and other variations may exist. 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 5.1. Passport Model
In this model, an Attester sends Evidence to a Verifier, which In this model, an Attester conveys Evidence to a Verifier, which
compares the Evidence against its Appraisal Policy. The Verifier compares the Evidence against its Appraisal Policy. The Verifier
then gives back an Attestation Result. If the Attestation Result was then gives back an Attestation Result. If the Attestation Result was
a successful one, the Attester can then present the Attestation a successful one, the Attester can then present the Attestation
Result to a Relying Party, which then compares the Attestation Result Result to a Relying Party, which then compares the Attestation Result
against its own Appraisal Policy. against its own Appraisal Policy.
There are three ways in which the process may fail. First, the There are three ways in which the process may fail. First, the
Verifier may refuse to issue the Attestation Result due to some error 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 processing, or some missing input to the Verifier. The second way
in which the process may fail is when the resulting Result is in which the process may fail is when the resulting Result is
skipping to change at page 14, line 50 skipping to change at page 15, line 43
^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+----------+ +---------+ +----------+ +---------+
| |------------->| |Compare Attestation | |------------->| |Compare Attestation
| Attester | Attestation | Relying | Result against | Attester | Attestation | Relying | Result against
| | Result | Party | Appraisal | | Result | Party | Appraisal
+----------+ +---------+ Policy +----------+ +---------+ Policy
Figure 4: Passport Model Figure 5: Passport Model
The passport model is so named because of its resemblance to how The passport model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity claim, such as an when it needs to assert a citizenship or identity claim, such as an
airport immigration desk. The passport is considered sufficient airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity claims, and it is because it vouches for the citizenship and identity claims, and it is
issued by a trusted authority. Thus, in this immigration desk issued by a trusted authority. Thus, in this immigration desk
analogy, the passport issuing agency is a Verifier, the passport is analogy, the passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party. an Attestation Result, and the immigration desk is a Relying Party.
5.2. Background-Check Model 5.2. Background-Check Model
In this model, an Attester sends Evidence to a Relying Party, which In this model, an Attester conveys Evidence to a Relying Party, which
simply passes it on to a Verifier. The Verifier then compares the simply passes it on to a Verifier. The Verifier then compares the
Evidence against its Appraisal Policy, and returns an Attestation Evidence against its Appraisal Policy, and returns an Attestation
Result to the Relying Party. The Relying Party then compares the Result to the Relying Party. The Relying Party then compares the
Attestation Result against its own appraisal policy. Attestation Result against its own appraisal policy.
The resource access protocol between the Attester and Relying Party The resource access protocol between the Attester and Relying Party
includes Evidence rather than an Attestation Result, but that includes Evidence rather than an Attestation Result, but that
Evidence is not processed by the Relying Party. Since the Evidence Evidence is not processed by the Relying Party. Since the Evidence
is merely forwarded on to a trusted Verifier, any serialization is merely forwarded on to a trusted Verifier, any serialization
format can be used for Evidence because the Relying Party does not format can be used for Evidence because the Relying Party does not
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^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+------------+ +-------------+ +------------+ +-------------+
| |-------------->| | Compare Attestation | |-------------->| | Compare Attestation
| Attester | Evidence | Relying | Result against | Attester | Evidence | Relying | Result against
| | | Party | Appraisal Policy | | | Party | Appraisal Policy
+------------+ +-------------+ +------------+ +-------------+
Figure 5: Background-Check Model Figure 6: Background-Check Model
The background-check model is so named because of the resemblance of The background-check model is so named because of the resemblance of
how employers and volunteer organizations perform background checks. how employers and volunteer organizations perform background checks.
When a prospective employee provides claims about education or When a prospective employee provides claims about education or
previous experience, the employer will contact the respective previous experience, the employer will contact the respective
institutions or former employers to validate the claim. Volunteer institutions or former employers to validate the claim. Volunteer
organizations often perform police background checks on volunteers in organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and a former employer or government agency that the Relying Party, and a former employer or government agency that
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Party and the Verifier on the same machine, and so there is no need Party and the Verifier on the same machine, and so there is no need
for a protocol between the two. for a protocol between the two.
It is also worth pointing out that the choice of model is generally 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 up to the Relying Party, and the same device may need to create
Evidence for different Relying Parties and different use cases (e.g., Evidence for different Relying Parties and different use cases (e.g.,
a network infrastructure device to gain access to the network, and a network infrastructure device to gain access to the network, and
then a server holding confidential data to get access to that data). 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. As such, both models may simultaneously be in use by the same device.
Figure 6 shows another example of a combination where Relying Party 1 Figure 7 shows another example of a combination where Relying Party 1
uses the passport model, whereas Relying Party 2 uses an extension of uses the passport model, whereas Relying Party 2 uses an extension of
the background-check model. Specifically, in addition to the basic the background-check model. Specifically, in addition to the basic
functionality shown in Figure 5, Relying Party 2 actually provides functionality shown in Figure 6, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the use it with other Relying Parties. This is the model that the
Trusted Application Manager plans to support in the TEEP architecture Trusted Application Manager plans to support in the TEEP architecture
[I-D.ietf-teep-architecture]. [I-D.ietf-teep-architecture].
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal Policy | Verifier | against Appraisal Policy
| | | |
+-------------+ +-------------+
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^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+----------+ +----------+ +----------+ +----------+
| |-------------->| | Compare Attestation | |-------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against | Attester | Attestation | Relying | Result against
| | Result | Party 1 | Appraisal Policy | | Result | Party 1 | Appraisal Policy
+----------+ +----------+ +----------+ +----------+
Figure 6: Example Combination Figure 7: Example Combination
6. Trust Model 6. Roles and Entities
HENK VERSION
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
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. Role Hosting and Composition
NED VERSION
The RATS architecture includes the definition of Roles (e.g.
Attester, Verifier, Relying Party, Endorser) and conceptual messages
(e.g., Evidence, Attestation Results, Endorsements, Appraisal
Policies) that captures canonical attestation behaviors, that are
common to a broad range of attestation-enabled systems. An entity
that combines multiple Roles produces and consumes the associated
Role Messages.
The RATS architecture is not prescriptive about deployment
configuration options of attestation-enabled systems, therefore the
various Roles can be hosted on any participating entity. This
implies, for a given entity, that multiple Roles could be co-resident
so that the duties of multiple roles could be performed
simultaneously. Nevertheless, the semantics of which Role Messages
are inputs and outputs to a Role entity remains constant. As a
result, the entity can participate as a constituent of the RATS
architecture while flexibly accommodating the needs of various
deployment architectures.
Interactions between Roles do not necessarily require use of Internet
protocols. They could, for example, use inter-process communication,
local system buses, shared memory, hypervisors, IP-loopback devices
or any communication path between the various environments that may
exist on the entity that combines multiple Roles.
The movement of Role Messages between locally hosted Roles is
referred to as "local conveyance". Most importantly, the definition
of local conveyance methods is out-of-scope for the RATS
architecture.
The following paragraph elaborates on an exemplary usage scenario:
In a Composite Device scenario, in addition to local entities that
host the lead Attester and other subordinate Attesters, the Composite
Device can host the Verifier role locally to appraise Evidence from
one or more subordinate Attesters. The local Verifier might convey
local Attestation Results to a remote Relying party or the Relying
Party role also could become local where an application-specific
action is taken locally. For example, a secure boot scenario
prevents system software from loading if the firmware fails to
satisfy a local trustworthiness appraisal policy.
In a multi-network scenario, a network node might bridge a wide-area
network, local-area network, and span various system buses. In so
doing, the bridge node might need to host multiple Roles depending on
the type of behavior each connected domain expects. For example, the
node might be an Attester to a wide-area network, a Verifier to the
local-area network, and a Relying Party to components attached to a
local system bus.
8. Trust Model
The scope of this document is scenarios for which a Relying Party The scope of this document is scenarios for which a Relying Party
trusts a Verifier that can appraise the trustworthiness of trusts a Verifier that can appraise the trustworthiness of
information about an Attester. Such trust might come by the Relying information about an Attester. Such trust might come by the Relying
Party trusting the Verifier (or its public key) directly, or might Party trusting the Verifier (or its public key) directly, or might
come by trusting an entity (e.g., a Certificate Authority) that is in come by trusting an entity (e.g., a Certificate Authority) that is in
the Verifier's certificate chain. The Relying Party might implicitly the Verifier's certificate chain. The Relying Party might implicitly
trust a Verifier (such as in the Verifying Relying Party trust a Verifier (such as in the Verifying Relying Party
combination). Or, for a stronger level of security, the Relying combination). Or, for a stronger level of security, the Relying
Party might require that the Verifier itself provide information Party might require that the Verifier itself provide information
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component, or operating system component or service, under the component, or operating system component or service, under the
assumption that information provided about it by the lower-layer assumption that information provided about it by the lower-layer
hypervisor or firmware is true. A stronger level of security comes hypervisor or firmware is true. A stronger level of security comes
when information can be vouched for by hardware or by ROM code, when information can be vouched for by hardware or by ROM code,
especially if such hardware is physically resistant to hardware especially if such hardware is physically resistant to hardware
tampering. The component that is implicitly trusted is often tampering. The component that is implicitly trusted is often
referred to as a Root of Trust. referred to as a Root of Trust.
In some scenarios, Evidence might contain sensitive information such In some scenarios, Evidence might contain sensitive information such
as Personally Identifiable Information. Thus, an Attester must trust as Personally Identifiable Information. Thus, an Attester must trust
entities to which it sends Evidence, to not reveal sensitive data to entities to which it conveys Evidence, to not reveal sensitive data
unauthorized parties. The Verifier might share this information with to unauthorized parties. The Verifier might share this information
other authorized parties, according rules that it controls. In the with other authorized parties, according rules that it controls. In
background-check model, this Evidence may also be revealed to Relying the background-check model, this Evidence may also be revealed to
Party(s). Relying Party(s).
7. Conceptual Messages 9. Conceptual Messages
7.1. Evidence 9.1. Evidence
Today, Evidence tends to be highly device-specific, since the Evidence is a set of claims about the target environment that reveal
information in the Evidence often includes vendor-specific operational status, health, configuration or construction that have
information that is necessary to fully describe the manufacturer and security relevance. Evidence is evaluated by a Verifier to establish
model of the device including its security properties, the health of its relevance, compliance, and timeliness. Claims need to be
the device, and the level of confidence in the correctness of the collected in a manner that is reliable. Evidence needs to be
information. Evidence is typically signed by the device (whether by securely associated with the target environment so that the Verifier
hardware, firmware, or software on the device), and its appraisal in cannot be tricked into accepting claims originating from a different
isolation would require Appraisal Policy to be based on device- environment (that may be more trustworthy). Evidence also must be
specific details (e.g., a device public key). 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.
7.2. Endorsements 9.2. Endorsements
An Endorsement is a secure statement that some entity (e.g., a An Endorsement is a secure statement that some entity (e.g., a
manufacturer) vouches for the integrity of the device's signing manufacturer) vouches for the integrity of the device's signing
capability. For example, if the signing capability is in hardware, capability. For example, if the signing capability is in hardware,
then an Endorsement might be a manufacturer certificate that signs a then an Endorsement might be a manufacturer certificate that signs a
public key whose corresponding private key is only known inside the public key whose corresponding private key is only known inside the
device's hardware. Thus, when Evidence and such an Endorsement are device's hardware. Thus, when Evidence and such an Endorsement are
used together, an appraisal procedure can be conducted based on used together, an appraisal procedure can be conducted based on
Appraisal Policies that may not be specific to the device instance, Appraisal Policies that may not be specific to the device instance,
but merely specific to the manufacturer providing the Endorsement. but merely specific to the manufacturer providing the Endorsement.
skipping to change at page 19, line 35 skipping to change at page 22, line 19
compliant devices are considered authorized for some purpose. For compliant devices are considered authorized for some purpose. For
example, an enterprise using remote attestation for Network Endpoint example, an enterprise using remote attestation for Network Endpoint
Assessment may not wish to let every healthy laptop from the same Assessment may not wish to let every healthy laptop from the same
manufacturer onto the network, but instead only want to let devices manufacturer onto the network, but instead only want to let devices
that it legally owns onto the network. Thus, an Endorsement may be that it legally owns onto the network. Thus, an Endorsement may be
helpful information in authenticating information about a device, but helpful information in authenticating information about a device, but
is not necessarily sufficient to authorize access to resources which is not necessarily sufficient to authorize access to resources which
may need device-specific information such as a public key for the may need device-specific information such as a public key for the
device or component or user on the device. device or component or user on the device.
7.3. Attestation Results 9.3. Attestation Results
Attestation Results may indicate compliance or non-compliance with a Attestation Results may indicate compliance or non-compliance with a
Verifier's Appraisal Policy. A result that indicates non-compliance Verifier's Appraisal Policy. A result that indicates non-compliance
can be used by an Attester (in the passport model) or a Relying Party 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 (in the background-check model) to indicate that the Attester should
not be treated as authorized and may be in need of remediation. In 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 some cases, it may even indicate that the Evidence itself cannot be
authenticated as being correct. authenticated as being correct.
An Attestation Result that indicates compliance can be used by a An Attestation Result that indicates compliance can be used by a
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allows a Relying Party's Appraisal Policy to be simpler, potentially allows a Relying Party's Appraisal Policy to be simpler, potentially
based on standard ways of expressing the information, while still based on standard ways of expressing the information, while still
allowing interoperability with heterogeneous devices. allowing interoperability with heterogeneous devices.
Finally, whereas Evidence is signed by the device (or indirectly by a Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer, if Endorsements are used), Attestation Results are manufacturer, if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity, rather than a larger set of entities, relationship with one entity, rather than a larger set of entities,
for purposes of its Appraisal Policy. for purposes of its Appraisal Policy.
8. Claims Encoding Formats 10. Claims Encoding Formats
The following diagram illustrates a relationship to which remote The following diagram illustrates a relationship to which remote
attestation is desired to be added: attestation is desired to be added:
+-------------+ +------------+ Evaluate +-------------+ +------------+ Evaluate
| |-------------->| | request | |-------------->| | request
| Attester | Access some | Relying | against | Attester | Access some | Relying | against
| | resource | Party | security | | resource | Party | security
+-------------+ +------------+ policy +-------------+ +------------+ policy
Figure 7: Typical Resource Access Figure 8: Typical Resource Access
In this diagram, the protocol between Attester and a Relying Party 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, 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 OPC UA, etc.), depending on the use case. Such protocols typically
already have mechanisms for passing security information for purposes already have mechanisms for passing security information for purposes
of authentication and authorization. Common formats include JWTs of authentication and authorization. Common formats include JWTs
[RFC7519], CWTs [RFC8392], and X.509 certificates. [RFC7519], CWTs [RFC8392], and X.509 certificates.
To enable remote attestation to be added to existing protocols, To enable remote attestation to be added to existing protocols,
enabling a higher level of assurance against malware for example, it enabling a higher level of assurance against malware for example, it
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motivates why the Verifier might also be responsible for accepting motivates why the Verifier might also be responsible for accepting
Evidence that encodes claims in one format, while issuing Attestation Evidence that encodes claims in one format, while issuing Attestation
Results that encode claims in a different format. Results that encode claims in a different format.
Evidence Attestation Results Evidence Attestation Results
.--------------. CWT CWT .-------------------. .--------------. CWT CWT .-------------------.
| Attester-A |------------. .----------->| Relying Party V | | Attester-A |------------. .----------->| Relying Party V |
'--------------' v | `-------------------' '--------------' v | `-------------------'
.--------------. JWT .------------. JWT .-------------------. .--------------. JWT .------------. JWT .-------------------.
| Attester-B |-------->| Verifier |-------->| Relying Party W | | Attester-B |-------->| Verifier |-------->| Relying Party W |
--------------' | | `-------------------' '--------------' | | `-------------------'
.--------------. X.509 | | X.509 .-------------------. .--------------. X.509 | | X.509 .-------------------.
| Attester-C |-------->| |-------->| Relying Party X | | Attester-C |-------->| |-------->| Relying Party X |
'--------------' | | `-------------------' '--------------' | | `-------------------'
.--------------. TPM | | TPM .-------------------. .--------------. TPM | | TPM .-------------------.
| Attester-D |-------->| |-------->| Relying Party Y | | Attester-D |-------->| |-------->| Relying Party Y |
'--------------' '------------' `-------------------' '--------------' '------------' `-------------------'
.--------------. other ^ | other .-------------------. .--------------. other ^ | other .-------------------.
| Attester-E |------------' '----------->| Relying Party Z | | Attester-E |------------' '----------->| Relying Party Z |
'--------------' `-------------------' '--------------' `-------------------'
Figure 8: Multiple Attesters and Relying Parties with Different Figure 9: Multiple Attesters and Relying Parties with Different
Formats Formats
9. Freshness 11. Freshness
It is important to prevent replay attacks where an attacker replays It is important to prevent replay attacks where an attacker replays
old Evidence or an old Attestation Result that is no longer correct. old Evidence or an old Attestation Result that is no longer correct.
To do so, some mechanism of ensuring that the Evidence and To do so, some mechanism of ensuring that the Evidence and
Attestation Result are fresh, meaning that there is some degree of Attestation Result are fresh, meaning that there is some degree of
assurance that they still reflect the latest state of the Attester, assurance that they still reflect the latest state of the Attester,
and that any Attestation Result was generated using the latest and that any Attestation Result was generated using the latest
Appraisal Policy for Evidence. There is, however, always a race Appraisal Policy for Evidence. There is, however, always a race
condition possible in that the state of the Attester, and the condition possible in that the state of the Attester, and the
Appraisal Policy for Evidence, might change immediately after the Appraisal Policy for Evidence, might change immediately after the
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requires additional claims about the signer's time synchronization requires additional claims about the signer's time synchronization
mechanism in order to provide such assurance. mechanism in order to provide such assurance.
In either approach, it is important to note that the actual values in In either approach, it is important to note that the actual values in
claims might have been generated long before the claims are signed. claims might have been generated long before the claims are signed.
If so, it is the signer's responsibility to ensure that the values If so, it is the signer's responsibility to ensure that the values
are still correct when they are signed. For example, values might 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 have been generated at boot, and then used in claims as long as the
signer can guarantee that they cannot have changed since boot. signer can guarantee that they cannot have changed since boot.
10. Privacy Considerations A more detailed discussion with examples appears in Section 17.
12. Privacy Considerations
The conveyance of Evidence and the resulting Attestation Results The conveyance of Evidence and the resulting Attestation Results
reveal a great deal of information about the internal state of a reveal a great deal of information about the internal state of a
device. In many cases, the whole point of the Attestation process is device. In many cases, the whole point of the Attestation process is
to provide reliable information about the type of the device and the to provide reliable information about the type of the device and the
firmware/software that the device is running. This information might firmware/software that the device is running. This information might
be particularly interesting to many attackers. For example, knowing be particularly interesting to many attackers. For example, knowing
that a device is running a weak version of firmware provides a way to that a device is running a weak version of firmware provides a way to
aim attacks better. aim attacks better.
Evidence and Attestation Results data structures are expected to Evidence and Attestation Results data structures are expected to
support integrity protection encoding (e.g., COSE, JOSE, X.509) and support integrity protection encoding (e.g., COSE, JOSE, X.509) and
optionally might support confidentiality protection (e.g., COSE, optionally might support confidentiality protection (e.g., COSE,
JOSE). Therefore, if confidentiality protection is omitted or JOSE). Therefore, if confidentiality protection is omitted or
unavailable, the protocols that convey Evidence or Attestation unavailable, the protocols that convey Evidence or Attestation
Results are responsible for detailing what kinds of information are Results are responsible for detailing what kinds of information are
disclosed, and to whom they are exposed. disclosed, and to whom they are exposed.
11. Security Considerations 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.
13. Security Considerations
Any solution that conveys information used for security purposes, Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation whether such information is in the form of Evidence, Attestation
Results, Endorsements, or Appraisal Policy, needs to support end-to- Results, Endorsements, or Appraisal Policy, needs to support end-to-
end integrity protection and replay attack prevention, and often also end integrity protection and replay attack prevention, and often also
needs to support additional security protections. For example, needs to support additional security protections. For example,
additional means of authentication, confidentiality, integrity, additional means of authentication, confidentiality, integrity,
replay, denial of service and privacy protection are needed in many replay, denial of service and privacy protection are needed in many
use cases. Section 9 discusses ways in which freshness can be used use cases. Section 11 discusses ways in which freshness can be used
in this architecture to protect against replay attacks. in this architecture to protect against replay attacks.
To assess the security provided by a particular Appraisal Policy, it To assess the security provided by a particular Appraisal Policy, it
is important to understand the strength of the Root of Trust, e.g., 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 whether it is mutable software, or firmware that is read-only after
boot, or immutable hardware/ROM. boot, or immutable hardware/ROM.
It is also important that the Appraisal Policy was itself obtained It is also important that the Appraisal Policy was itself obtained
securely. As such, if Appraisal Policies for a Relying Party or for securely. As such, if Appraisal Policies for a Relying Party or for
a Verifier can be configured via a network protocol, the ability to a Verifier can be configured via a network protocol, the ability to
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The security of conveyed information may be applied at different The security of conveyed information may be applied at different
layers, whether by a conveyance protocol, or an information encoding layers, whether by a conveyance protocol, or an information encoding
format. This architecture expects attestation messages (i.e., format. This architecture expects attestation messages (i.e.,
Evidence, Attestation Results, Endorsements and Policies) are end-to- Evidence, Attestation Results, Endorsements and Policies) are end-to-
end protected based on the role interaction context. For example, if end protected based on the role interaction context. For example, if
an Attester produces Evidence that is relayed through some other an Attester produces Evidence that is relayed through some other
entity that doesn't implement the Attester or the intended Verifier entity that doesn't implement the Attester or the intended Verifier
roles, then the relaying entity should not expect to have access to roles, then the relaying entity should not expect to have access to
the Evidence. the Evidence.
12. IANA Considerations 14. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
13. Acknowledgments 15. Acknowledgments
Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade, Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade,
Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten. Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
14. Contributors 16. Contributors
Thomas Hardjono created older versions of the terminology section in Thomas Hardjono created older versions of the terminology section in
collaboration with Ned Smith. Eric Voit provided the conceptual collaboration with Ned Smith. Eric Voit provided the conceptual
separation between Attestation Provision Flows and Attestation separation between Attestation Provision Flows and Attestation
Evidence Flows. Monty Wisemen created the content structure of the Evidence Flows. Monty Wisemen created the content structure of the
first three architecture drafts. Carsten Bormann provided many of first three architecture drafts. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat the motivational building blocks with respect to the Internet Threat
Model. Model.
15. References 17. Appendix A: Time Considerations
15.1. Normative References 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 |
| | 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.
17.1. 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
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)".
17.2. 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".
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)".
17.3. 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.
17.4. 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.
.----------. .---------------. .----------.
| 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)".
18. References
18.1. Normative References
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>. <https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>. May 2018, <https://www.rfc-editor.org/info/rfc8392>.
15.2. Informative References 18.2. Informative References
[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] [I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in "Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-01, 11 Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9
September 2019, <http://www.ietf.org/internet-drafts/ March 2020, <http://www.ietf.org/internet-drafts/draft-
draft-birkholz-rats-tuda-01.txt>. birkholz-rats-tuda-02.txt>.
[I-D.ietf-teep-architecture] [I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler, Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP) "Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft- Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-06, 8 February 2020, ietf-teep-architecture-08, 4 April 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep- <http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-06.txt>. architecture-08.txt>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security-
model/>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
Authors' Addresses Authors' Addresses
Henk Birkholz Henk Birkholz
Fraunhofer SIT Fraunhofer SIT
Rheinstrasse 75 Rheinstrasse 75
64295 Darmstadt 64295 Darmstadt
Germany Germany
Email: henk.birkholz@sit.fraunhofer.de Email: henk.birkholz@sit.fraunhofer.de
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