draft-ietf-rats-architecture-06.txt   draft-ietf-rats-architecture-07.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: 5 March 2021 Microsoft Expires: 19 April 2021 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
1 September 2020 16 October 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-06 draft-ietf-rats-architecture-07
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 10 skipping to change at page 2, line 10
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This Internet-Draft will expire on 5 March 2021. This Internet-Draft will expire on 19 April 2021.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 5
3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5 3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 6
3.2. Confidential Machine Learning (ML) Model Protection . . . 6 3.2. Confidential Machine Learning (ML) Model Protection . . . 6
3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6 3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 7
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 7 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 7
3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7 3.5. Trusted Execution Environment (TEE) Provisioning . . . . 7
3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7 3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 8
3.7. FIDO Biometric Authentication . . . . . . . . . . . . . . 8 3.7. FIDO Biometric Authentication . . . . . . . . . . . . . . 8
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 8 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 9
4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 10 4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 10
4.2. Two Types of Environments of an Attester . . . . . . . . 10 4.2. Reference Values . . . . . . . . . . . . . . . . . . . . 10
4.3. Layered Attestation Environments . . . . . . . . . . . . 11 4.3. Two Types of Environments of an Attester . . . . . . . . 10
4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 13 4.4. Layered Attestation Environments . . . . . . . . . . . . 11
4.5. Composite Device . . . . . . . . . . . . . . . . . . . . 13
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 16 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 16
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 16 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 16
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 17 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 17
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 18 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 18
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 19 6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 19
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 20 7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 21 7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 21
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 21 7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 21
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 21 7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 21
7.5. Endorser and Verifier Owner . . . . . . . . . . . . . . . 22 7.5. Endorser, Reference Value Provider, and Verifier Owner . 23
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 22 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 23
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 22 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 23
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 22 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 24
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 23 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 24
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 24 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 25
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 26 10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 27
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 28 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 29
12. Security Considerations . . . . . . . . . . . . . . . . . . . 28 12. Security Considerations . . . . . . . . . . . . . . . . . . . 29
12.1. Attester and Attestation Key Protection . . . . . . . . 29 12.1. Attester and Attestation Key Protection . . . . . . . . 30
12.1.1. On-Device Attester and Key Protection . . . . . . . 29 12.1.1. On-Device Attester and Key Protection . . . . . . . 30
12.1.2. Attestation Key Provisioning Processes . . . . . . . 30 12.1.2. Attestation Key Provisioning Processes . . . . . . . 31
12.2. Integrity Protection . . . . . . . . . . . . . . . . . . 30 12.2. Integrity Protection . . . . . . . . . . . . . . . . . . 31
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 32 15. Notable Contributions . . . . . . . . . . . . . . . . . . . . 33
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 32 16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 33
16.1. Example 1: Timestamp-based Passport Model Example . . . 33 16.1. Example 1: Timestamp-based Passport Model Example . . . 34
16.2. Example 2: Nonce-based Passport Model Example . . . . . 35 16.2. Example 2: Nonce-based Passport Model Example . . . . . 36
16.3. Example 3: Handle-based Passport Model Example . . . . . 36 16.3. Example 3: Handle-based Passport Model Example . . . . . 37
16.4. Example 4: Timestamp-based Background-Check Model 16.4. Example 4: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 38 Example . . . . . . . . . . . . . . . . . . . . . . . . 39
16.5. Example 5: Nonce-based Background-Check Model Example . 38 16.5. Example 5: Nonce-based Background-Check Model Example . 39
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 39 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
17.1. Normative References . . . . . . . . . . . . . . . . . . 39 17.1. Normative References . . . . . . . . . . . . . . . . . . 40
17.2. Informative References . . . . . . . . . . . . . . . . . 39 17.2. Informative References . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
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. additional vital party, the Verifier.
The Verifier appraises Evidence via Appraisal Policies and creates The Verifier appraises Evidence via appraisal policies and creates
the Attestation Results to support Relying Parties in their decision the Attestation Results to support Relying Parties in their decision
process. This documents defines a flexible architecture consisting process. This documents defines a flexible architecture consisting
of attestation roles and their interactions via conceptual messages. of attestation roles and their interactions via conceptual messages.
Additionally, this document defines a universal set of terms that can Additionally, this document defines a universal set of terms that can
be mapped to various existing and emerging Remote Attestation be mapped to various existing and emerging Remote Attestation
Procedures. Common topological models and the data flows associated Procedures. Common topological models and the data flows associated
with them, such as the "Passport Model" and the "Background-Check with them, such as the "Passport Model" and the "Background-Check
Model" are illustrated. The purpose is to define useful terminology Model" are illustrated. The purpose is to define useful terminology
for attestation and enable readers to map their solution architecture for attestation and enable readers to map their solution architecture
to the canonical attestation architecture provided here. Having a to the canonical attestation architecture provided here. Having a
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vouches for the validity of the results vouches for the validity of the results
Attester: A role performed by an entity (typically a device) whose Attester: A role performed by an entity (typically a device) whose
Evidence must be appraised in order to infer the extent to which Evidence must be appraised in order to infer the extent to which
the Attester is considered trustworthy, such as when deciding the Attester is considered trustworthy, such as when deciding
whether it is authorized to perform some operation whether it is authorized to perform some operation
Claim: A piece of asserted information, often in the form of a name/ Claim: A piece of asserted information, often in the form of a name/
value pair. (Compare /claim/ in [RFC7519]) value pair. (Compare /claim/ in [RFC7519])
Endorsement: A secure statement that some entity (typically a Endorsement: A secure statement that an Endorser vouches for the
manufacturer) vouches for the integrity of an Attester's signing integrity of an Attester's various capabilities such as Claims
capability collection and Evidence signing
Endorser: An entity (typically a manufacturer) whose Endorsements Endorser: An entity (typically a manufacturer) whose Endorsements
help Verifiers appraise the authenticity of Evidence help Verifiers appraise the authenticity of Evidence
Evidence: A set of information about an Attester that is to be Evidence: A set of information about an Attester that is to be
appraised by a Verifier. Evidence may include configuration data, appraised by a Verifier. Evidence may include configuration data,
measurements, telemetry, or inferences. measurements, telemetry, or inferences.
Reference Value Provider: An entity (typically a manufacturer) whose
Reference Values help Verifiers appraise the authenticity of
Evidence.
Reference Values: A set of values against which values of Claims can
be compared as part of applying an Appraisal Policy for Evidence.
Reference Values are sometimes referred to in other documents as
known-good values, golden measurements, or nominal values,
although those terms typically assume comparison for equality,
whereas here Reference Values might be more general and be used in
any sort of comparison.
Relying Party: A role performed by an entity that depends on the Relying Party: A role performed by an entity that depends on the
validity of information about an Attester, for purposes of validity of information about an Attester, for purposes of
reliably applying application specific actions. Compare /relying reliably applying application specific actions. Compare /relying
party/ in [RFC4949] party/ in [RFC4949]
Relying Party Owner: An entity (typically an administrator), that is 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: A role performed by an entity that appraises the validity Verifier: A role performed by an entity that appraises the validity
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and version of information of the hardware and software on the and version of information of the hardware and software on the
machines attached to their network, for purposes such as inventory, machines attached to their network, for purposes such as inventory,
audit, anomaly detection, record maintenance and/or trending reports audit, anomaly detection, record maintenance and/or trending reports
(logging). The network operator may also want a policy by which full (logging). The network operator may also want a policy by which full
access is only granted to devices that meet some definition of access is only granted to devices that meet some definition of
hygiene, and so wants to get claims about such information and verify hygiene, and so wants to get claims about such information and verify
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. The system components perform a series of measurements measurements. The system components perform a series of measurements
that may be signed by the Root of Trust, considered as Evidence about that may be signed by the root of trust, considered as Evidence about
the hardware, firmware, BIOS, software, etc. that is running. the hardware, firmware, BIOS, software, etc. that is running.
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. A device manufacturer wants to protect its intellectual property.
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preventing attackers, potentially the customer themselves, from preventing attackers, potentially the customer themselves, from
seeing the details of the model. seeing the details of the model.
This typically works by having some protected environment in the This typically works by having some protected environment in the
device go through a remote attestation with some manufacturer service device go through a remote attestation with some manufacturer service
that can assess its trustworthiness. If remote attestation succeeds, that can assess its trustworthiness. If remote attestation succeeds,
then the manufacturer service releases either the model, or a key to then the manufacturer service releases either the model, or a key to
decrypt a model the Attester already has in encrypted form, to the decrypt a model the Attester already has in encrypted form, to the
requester. requester.
Attester: A device desiring to run an ML model to do inferencing Attester: A device desiring to run an ML model
Relying Party: A server or service holding ML models it desires to Relying Party: A server or service holding ML models it desires to
protect protect
3.3. Confidential Data Retrieval 3.3. Confidential Data Retrieval
This is a generalization of the ML model use case above, where the 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 data can be any highly confidential data, such as health data about
customers, payroll data about employees, future business plans, etc. customers, payroll data about employees, future business plans, etc.
An assessment of system state is made against a set of policies to An assessment of system state is made against a set of policies to
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Attester: Every FIDO Authenticator contains an Attester. Attester: Every FIDO Authenticator contains an Attester.
Relying Party: Any web site, mobile application back end or service Relying Party: Any web site, mobile application back end or service
that does biometric authentication. that does biometric authentication.
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 * * Reference * * Verifier * * Relying Party *
************ * Owner * * Owner * ************ * Value * * Owner * * Owner *
| ************ **************** | * Provider * ************ *****************
| | | | ************* | |
Endorsements| | | | | | |
| |Appraisal | |Endorsements |Reference |Appraisal |Appraisal
| |Policy | | |Values |Policy |Policy for
| |for | Appraisal | | |for |Attestation
| |Evidence | Policy for .-----------. | |Evidence |Results
| | | Attestation | | | |
| | | Results | | | |
v v | v v v |
.-----------------. | .---------------------------. |
.----->| Verifier |------. | .----->| Verifier |------. |
| '-----------------' | | | '---------------------------' | |
| | | | | |
| Attestation| | | Attestation| |
| Results | | | Results | |
| Evidence | | | Evidence | |
| | | | | |
| v v | v v
.----------. .-----------------. .----------. .---------------.
| Attester | | Relying Party | | Attester | | Relying Party |
'----------' '-----------------' '----------' '---------------'
Figure 1: Conceptual Data Flow Figure 1: Conceptual Data Flow
An Attester creates Evidence that is conveyed to a Verifier. An Attester creates Evidence that is conveyed to a Verifier.
The Verifier uses the Evidence, and any Endorsements from Endorsers, The Verifier uses the Evidence, and any Endorsements from Endorsers,
by applying an Appraisal Policy for Evidence to assess the by applying an Appraisal Policy for Evidence to assess the
trustworthiness of the Attester, and generates Attestation Results trustworthiness of the Attester, and generates Attestation Results
for use by Relying Parties. The Appraisal Policy for Evidence might for use by Relying Parties. The Appraisal Policy for Evidence might
be obtained from an Endorser along with the Endorsements, and/or be obtained from an Endorser along with the Endorsements, and/or
might be obtained via some other mechanism such as being configured might be obtained via some other mechanism such as being configured
in the Verifier by the Verifier Owner. in the Verifier by the Verifier Owner.
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 pppraisal policy to make application-specific decisions such as
authorization decisions. The Appraisal Policy for Attestation authorization decisions. The Appraisal Policy for Attestation
Results is configured in the Relying Party by the Relying Party Results is configured in the Relying Party by the Relying Party
Owner, and/or is programmed into the Relying Party. Owner, and/or is programmed into the Relying Party.
4.1. Appraisal Policies 4.1. Appraisal Policies
The Verifier, when appraising Evidence, or the Relying Party, when The Verifier, when appraising Evidence, or the Relying Party, when
appraising Attestation Results, checks the values of some claims appraising Attestation Results, checks the values of some claims
against constraints specified in its Appraisal Policy. Such against constraints specified in its appraisal policy. Such
constraints might involve a comparison for equality against a constraints might involve a comparison for equality against a
reference value, or a check for being in a range bounded by reference 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 Values, or membership in a set of Reference Values, or a check
against values in other claims, or any other test. against values in other claims, or any other test.
Such reference values might be specified as part of the Appraisal 4.2. Reference Values
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 Reference Values used in appraisal 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 specific to claims and implementations. This architecture document
does not define any general purpose format for them or general means does not define any general purpose format for them or general means
for comparison. for comparison.
4.2. Two Types of Environments of an Attester 4.3. 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 4.3 and Section 4.4. examples described in more detail in Section 4.4 and Section 4.5.
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, as shown in Figure 2. Claims are collected from Target Environments, as shown in Figure 2.
That is, Attesting Environments collect the values and the That is, Attesting Environments collect the values and the
information to be represented in Claims, by reading system registers information to be represented in Claims, by reading system registers
and variables, calling into subsystems, taking measurements on code and variables, calling into subsystems, taking measurements on code
or memory and so on of the Target Environment. Attesting or memory and so on of the Target Environment. Attesting
Environments then format the claims appropriately, and typically use Environments then format the claims appropriately, and typically use
key material and cryptographic functions, such as signing or cipher key material and cryptographic functions, such as signing or cipher
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| .-------------. | | .-------------. |
| | Attesting | | | | Attesting | |
| | Environment | | | | Environment | |
| | | | | | | |
| '-------------' | | '-------------' |
| Attester | | Attester |
'------------------------------------' '------------------------------------'
Figure 2: Two Types of Environments Figure 2: Two Types of Environments
4.3. Layered Attestation Environments 4.4. Layered Attestation Environments
By definition, the Attester role creates Evidence. An Attester may By definition, the Attester role creates Evidence. An Attester may
consist of one or more nested or staged environments, adding consist of one or more nested or staged environments, adding
complexity to the architectural structure. The unifying component is complexity to the architectural structure. The unifying component is
the Root of Trust and the nested, staged, or chained attestation the root of trust and the nested, staged, or chained attestation
Evidence produced. The nested or chained structure includes Claims, Evidence produced. The nested or chained structure includes Claims,
collected by the Attester to aid in the assurance or believability of collected by the Attester to aid in the assurance or believability of
the attestation Evidence. the attestation Evidence.
Figure 3 depicts an example of a device that includes (A) a BIOS Figure 3 depicts an example of a device that includes (A) a BIOS
stored in read-only memory in this example, (B) an updatable stored in read-only memory in this example, (B) an updatable
bootloader, and (C) an operating system kernel. bootloader, and (C) an operating system kernel.
.----------. .----------. .----------. .----------.
| | | | | | | |
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Environment. This would result in a third set of Claims in the Environment. This would result in a third set of Claims in the
Evidence pertaining to that application. Evidence pertaining to that application.
The essence of this example is a cascade of staged environments. The essence of this example is a cascade of staged environments.
Each environment has the responsibility of measuring the next Each environment has the responsibility of measuring the next
environment before the next environment is started. In general, the environment before the next environment is started. In general, the
number of layers may vary by device or implementation, and an number of layers may vary by device or implementation, and an
Attesting Environment might even have multiple Target Environments Attesting Environment might even have multiple Target Environments
that it measures, rather than only one as shown in Figure 3. that it measures, rather than only one as shown in Figure 3.
4.4. Composite Device 4.5. 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
skipping to change at page 15, line 30 skipping to change at page 15, 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 4: Conceptual Data Flow for a Composite Device Figure 4: 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, An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. The combination of roles can Relying Party, etc.) at the same time. The combination of roles can
be arbitrary. For example, in this Composite Device scenario, the be arbitrary. For example, in this Composite Device scenario, the
entity inside the lead Attester can also take on the role of a 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 Verifier, and the outside entity of Verifier can take on the role of
a Relying Party. After collecting the Evidence of other Attesters, a Relying Party. After collecting the Evidence of other Attesters,
this inside Verifier uses Endorsements and Appraisal Policies this inside Verifier uses Endorsements and appraisal policies
(obtained the same way as any other Verifier) in the verification (obtained the same way as any other Verifier) in the verification
process to generate Attestation Results. The inside Verifier then process to generate Attestation Results. The inside Verifier then
conveys the Attestation Results of other Attesters, whether in the conveys the Attestation Results of other Attesters, whether in the
same conveyance protocol as the Evidence or not, to the outside same conveyance protocol as the Evidence or not, to the outside
Verifier. Verifier.
In this situation, the trust model described in Section 7 is also In this situation, the trust model described in Section 7 is also
suitable for this inside Verifier. suitable for this inside Verifier.
5. Topological Models 5. Topological Models
skipping to change at page 16, line 30 skipping to change at page 16, line 30
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.
In this model, an Attester conveys 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 Attestation Result is in which the process may fail is when the Attestation Result is
examined by the Relying Party, and based upon the Appraisal Policy, examined by the Relying Party, and based upon the appraisal policy,
the result does not pass the policy. The third way is when the the result does not pass the policy. The third way is when the
Verifier is unreachable. Verifier is unreachable.
Since the resource access protocol between the Attester and Relying Since the resource access protocol between the Attester and Relying
Party includes an Attestation Result, in this model the details of Party includes an Attestation Result, in this model the details of
that protocol constrain the serialization format of the Attestation that protocol constrain the serialization format of the Attestation
Result. The format of the Evidence on the other hand is only Result. The format of the Evidence on the other hand is only
constrained by the Attester-Verifier remote attestation protocol. constrained by the Attester-Verifier remote attestation protocol.
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal Policy | Verifier | against appraisal policy
| | | |
+-------------+ +-------------+
^ | ^ |
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 5: Passport Model Figure 5: Passport Model
5.2. Background-Check Model 5.2. 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
issues a report is a Verifier. issues a report is a Verifier.
In this model, an Attester conveys 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
need a parser for it. The only requirement is that the Evidence can need a parser for it. The only requirement is that the Evidence can
be _encapsulated in_ the format required by the resource access be _encapsulated in_ the format required by the resource access
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constrained node whose purpose is to serve a given type resource constrained node whose purpose is to serve a given type resource
using a standard resource access protocol, it already needs the using a standard resource access protocol, it already needs the
parser(s) required by that existing protocol. Hence, the ability to parser(s) required by that existing protocol. Hence, the ability to
let the Relying Party obtain an Attestation Result in the same let the Relying Party obtain an Attestation Result in the same
serialization format allows minimizing the code footprint and attack serialization format allows minimizing the code footprint and attack
surface area of the Relying Party, especially if the Relying Party is surface area of the Relying Party, especially if the Relying Party is
a constrained node. a constrained node.
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal | Verifier | against appraisal
| | Policy | | policy
+-------------+ +-------------+
^ | ^ |
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 6: Background-Check Model Figure 6: Background-Check Model
5.3. Combinations 5.3. Combinations
One variation of the background-check model is where the Relying One variation of the background-check model is where the Relying
Party and the Verifier are on the same machine, performing both Party and the Verifier are on the same machine, performing both
functions together. In this case, there is no need for a protocol functions together. In this case, there is no need for a protocol
between the two. between the two.
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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 6, 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
| | | |
+-------------+ +-------------+
^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+-------------+ +-------------+
| | Compare | | Compare
| Relying | Attestation Result | Relying | Attestation Result
| Party 2 | against Appraisal Policy | Party 2 | against appraisal policy
+-------------+ +-------------+
^ | ^ |
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 7: Example Combination Figure 7: Example Combination
6. Roles and Entities 6. Roles and Entities
An entity in the RATS architecture includes at least one of the roles An entity in the RATS architecture includes at least one of the roles
defined in this document. An entity can aggregate more than one role defined in this document. An entity can aggregate more than one role
into itself. These collapsed roles combine the duties of multiple into itself. These collapsed roles combine the duties of multiple
roles. roles.
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information about itself that the Relying Party can use to assess the information about itself that the Relying Party can use to assess the
trustworthiness of the Verifier before accepting its Attestation trustworthiness of the Verifier before accepting its Attestation
Results. Results.
For example, one explicit way for a Relying Party "A" to establish For example, one explicit way for a Relying Party "A" to establish
such trust in a Verifier "B", would be for B to first act as an such trust in a Verifier "B", would be for B to first act as an
Attester where A acts as a combined Verifier/Relying Party. If A Attester where A acts as a combined Verifier/Relying Party. If A
then accepts B as trustworthy, it can choose to accept B as a then accepts B as trustworthy, it can choose to accept B as a
Verifier for other Attesters. Verifier for other Attesters.
As another example, the Relying Party can establish trust in the
Verifier by out of band establishment of key material, combined with
a protocol like TLS to communicate. There is an assumption that
between the establishment of the trusted key material and the
creation of the Evidence, that the Verifier has not been compromised.
Similarly, the Relying Party also needs to trust the Relying Party Similarly, the Relying Party also needs to trust the Relying Party
Owner for providing its Appraisal Policy for Attestation Results, and Owner for providing its Appraisal Policy for Attestation Results, and
in some scenarios the Relying Party might even require that the in some scenarios the Relying Party might even require that the
Relying Party Owner go through a remote attestation procedure with it Relying Party Owner go through a remote attestation procedure with it
before the Relying Party will accept an updated policy. This can be before the Relying Party will accept an updated policy. This can be
done similarly to how a Relying Party could establish trust in a done similarly to how a Relying Party could establish trust in a
Verifier as discussed above. Verifier as discussed above.
7.2. Attester 7.2. Attester
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 conveys Evidence, to not reveal sensitive data entities to which it conveys Evidence, to not reveal sensitive data
to unauthorized parties. The Verifier might share this information to unauthorized parties. The Verifier might share this information
with other authorized parties, according to rules that it controls. with other authorized parties, according to rules that it controls.
In the background-check model, this Evidence may also be revealed to In the background-check model, this Evidence may also be revealed to
Relying Party(s). Relying Party(s).
In some cases where Evidence contains sensitive information, an In some cases where Evidence contains sensitive information, an
Attester might even require that a Verifier first go through a remote Attester might even require that a Verifier first go through a TLS
attestation procedure with it before the Attester will send the authentication or a remote attestation procedure with it before the
sensitive Evidence. This can be done by having the Attester first Attester will send the sensitive Evidence. This can be done by
act as a Verifier/Relying Party, and the Verifier act as its own having the Attester first act as a Verifier/Relying Party, and the
Attester, as discussed above. Verifier act as its own Attester, as discussed above.
7.3. Relying Party Owner 7.3. Relying Party Owner
The Relying Party Owner might also require that the Relying Party The Relying Party Owner might also require that the Relying Party
first act as an Attester, providing Evidence that the Owner can first act as an Attester, providing Evidence that the Owner can
appraise, before the Owner would give the Relying Party an updated appraise, before the Owner would give the Relying Party an updated
policy that might contain sensitive information. In such a case, policy that might contain sensitive information. In such a case,
mutual attestation might be needed, in which case typically one mutual authentication or attestation might be needed, in which case
side's Evidence must be considered safe to share with an untrusted typically one side's Evidence must be considered safe to share with
entity, in order to bootstrap the sequence. an untrusted entity, in order to bootstrap the sequence.
7.4. Verifier 7.4. Verifier
The Verifier trusts (or more specifically, the Verifier's security The Verifier trusts (or more specifically, the Verifier's security
policy is written in a way that configures the Verifier to trust) a 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 manufacturer, or the manufacturer's hardware, so as to be able to
appraise the trustworthiness of that manufacturer's devices. In appraise the trustworthiness of that manufacturer's devices. In a
solutions with weaker security, a Verifier might be configured to typical solution, a Verifier comes to trust an Attester indirectly by
implicitly trust firmware or even software (e.g., a hypervisor). having an Endorser (such as a manufacturer) vouch for the Attester's
ability to securely generate Evidence.
In some solutions, a Verifier might be configured to directly trust
an Attester by having the Verifier have the Attester's key material
(rather than the Endorser's) in its trust anchor store.
Such direct trust must first be established at the time of trust
anchor store configuration either by checking with an Endorser at
that time, or by conducting a security analysis of the specific
device. Having the Attester directly in the trust anchor store
narrows the Verifier's trust to only specific devices rather than all
devices the Endorser might vouch for, such as all devices
manufactured by the same manufacturer in the case that the Endorser
is a manufacturer.
Such narrowing is often important since physical possession of a
device can also be used to conduct a number of attacks, and so a
device in a physically secure environment (such as one's own
premises) may be considered trusted whereas devices owned by others
would not be. This often results in a desire to either have the
owner run their own Endorser that would only Endorse devices one
owns, or to use Attesters directly in the trust anchor store. When
there are many Attesters owned, the use of an Endorser becomes more
scalable.
That is, it might appraise the trustworthiness of an application That is, it might appraise the trustworthiness of an application
component, operating system component, or service under the component, 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 assurance of firmware or software is true. A stronger level of assurance of
security comes when information can be vouched for by hardware or by security comes when information can be vouched for by hardware or by
ROM code, especially if such hardware is physically resistant to ROM code, especially if such hardware is physically resistant to
hardware tampering. The component that is implicitly trusted is hardware tampering. In most cases, components that have to be
often referred to as a Root of Trust. vouched for via Endorsements because no Evidence is generated about
them are referred to as roots of trust.
The manufacturer of the Attester arranges for its Attesting
Environment to be provisioned with key material. The key material is
typically in the form of an asymmetric key pair (e.g., an RSA or
ECDSA private key and a manufacturer-signed IDevID certificate)
secured in the Attester.
The Verifier is provided with an appropriate trust anchor, or
provided with a database of public keys (rather than certificates),
or even carefully secured lists of symmetric keys. The nature of how
the Verifier manages to validate the signatures produced by the
Attester is critical to the secure operation an Attestation system,
but is not the subject of standardization within this architecture.
A conveyance protocol that provides authentication and integrity A conveyance protocol that provides authentication and integrity
protection can be used to convey unprotected Evidence, assuming the protection can be used to convey unprotected Evidence, assuming the
following properties exists: following properties exists:
1. The key material used to authenticate and integrity protect the 1. The key material used to authenticate and integrity protect the
conveyance channel is trusted by the Verifier to speak for the conveyance channel is trusted by the Verifier to speak for the
Attesting Environment(s) that collected claims about the Target Attesting Environment(s) that collected claims about the Target
Environment(s). Environment(s).
2. All unprotected Evidence that is conveyed is supplied exclusively 2. All unprotected Evidence that is conveyed is supplied exclusively
by the Attesting Environment that has the key material that by the Attesting Environment that has the key material that
protects the conveyance channel protects the conveyance channel
3. The Root of Trust protects both the conveyance channel key 3. The root of trust protects both the conveyance channel key
material and the Attesting Environment with equivalent strength material and the Attesting Environment with equivalent strength
protections. protections.
7.5. Endorser and Verifier Owner See Section 12 for discussion on security strength.
In some scenarios, the Endorser and Verifier Owner may need to trust 7.5. Endorser, Reference Value Provider, and Verifier Owner
the Verifier before giving the Endorsement and Appraisal Policy to
it. This can be done similarly to how a Relying Party might In some scenarios, the Endorser, Reference Value Provider, and
establish trust in a Verifier as discussed above, and in such a case, Verifier Owner may need to trust the Verifier before giving the
mutual attestation might even be needed as discussed in Section 7.3. Endorsement, Reference Values, or appraisal policy to it. This can
be done similarly to how a Relying Party might establish trust in a
Verifier as discussed above, and in such a case, mutual
authentication or attestation might even be needed as discussed in
Section 7.3.
8. Conceptual Messages 8. Conceptual Messages
8.1. Evidence 8.1. Evidence
Evidence is a set of claims about the target environment that reveal Evidence is a set of claims about the target environment that reveal
operational status, health, configuration or construction that have operational status, health, configuration or construction that have
security relevance. Evidence is evaluated by a Verifier to establish security relevance. Evidence is evaluated by a Verifier to establish
its relevance, compliance, and timeliness. Claims need to be its relevance, compliance, and timeliness. Claims need to be
collected in a manner that is reliable. Evidence needs to be collected in a manner that is reliable. Evidence needs to be
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8.2. Endorsements 8.2. 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.
For example, an Appraisal Policy might simply check that devices from For example, an appraisal policy might simply check that devices from
a given manufacturer have information matching a set of known-good a given manufacturer have information matching a set of Reference
reference values, or an Appraisal Policy might have a set of more Values, or an appraisal policy might have a set of more complex logic
complex logic on how to appraise the validity of information. on how to appraise the validity of information.
However, while an Appraisal Policy that treats all devices from a However, while an appraisal policy that treats all devices from a
given manufacturer the same may be appropriate for some use cases, it given manufacturer the same may be appropriate for some use cases, it
would be inappropriate to use such an Appraisal Policy as the sole would be inappropriate to use such an appraisal policy as the sole
means of authorization for use cases that wish to constrain _which_ means of authorization for use cases that wish to constrain _which_
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.
8.3. Attestation Results 8.3. Attestation Results
Attestation Results are the input used by the Relying Party to decide Attestation Results are the input used by the Relying Party to decide
the extent to which it will trust a particular Attester, and allow it the extent to which it will trust a particular Attester, and allow it
to access some data or perform some operation. Attestation Results to access some data or perform some operation. Attestation Results
may be a Boolean simply indicating compliance or non-compliance with may be a Boolean simply indicating compliance or non-compliance with
a Verifier's Appraisal Policy, or a rich set of Claims about the a Verifier's appraisal policy, or a rich set of Claims about the
Attester, against which the Relying Party applies its Appraisal Attester, against which the Relying Party applies its Appraisal
Policy for Attestation Results. Policy for Attestation Results.
A result that indicates non-compliance can be used by an Attester (in A result that indicates non-compliance can be used by an Attester (in
the passport model) or a Relying Party (in the background-check the passport model) or a Relying Party (in the background-check
model) to indicate that the Attester should not be treated as model) to indicate that the Attester should not be treated as
authorized and may be in need of remediation. In some cases, it may authorized and may be in need of remediation. In some cases, it may
even indicate that the Evidence itself cannot be authenticated as even indicate that the Evidence itself cannot be authenticated as
being correct. being correct.
An Attestation Result that indicates compliance can be used by a An Attestation Result that indicates compliance can be used by a
Relying Party to make authorization decisions based on the Relying Relying Party to make authorization decisions based on the Relying
Party's Appraisal Policy. The simplest such policy might be to Party's appraisal policy. The simplest such policy might be to
simply authorize any party supplying a compliant Attestation Result simply authorize any party supplying a compliant Attestation Result
signed by a trusted Verifier. A more complex policy might also signed by a trusted Verifier. A more complex policy might also
entail comparing information provided in the result against known- entail comparing information provided in the result against Reference
good reference values, or applying more complex logic on such Values, or applying more complex logic on such information.
information.
Thus, Attestation Results often need to include detailed information Thus, Attestation Results often need to include detailed information
about the Attester, for use by Relying Parties, much like physical about the Attester, for use by Relying Parties, much like physical
passports and drivers licenses include personal information such as passports and drivers licenses include personal information such as
name and date of birth. Unlike Evidence, which is often very device- name and date of birth. Unlike Evidence, which is often very device-
and vendor-specific, Attestation Results can be vendor-neutral if the and vendor-specific, Attestation Results can be vendor-neutral if the
Verifier has a way to generate vendor-agnostic information based on Verifier has a way to generate vendor-agnostic information based on
the appraisal of vendor-specific information in Evidence. This the appraisal of vendor-specific information in Evidence. This
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.
9. Claims Encoding Formats 9. 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
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'--------------' '------------' `-------------------' '--------------' '------------' `-------------------'
.--------------. other ^ | other .-------------------. .--------------. other ^ | other .-------------------.
| Attester-E |------------' '----------->| Relying Party Z | | Attester-E |------------' '----------->| Relying Party Z |
'--------------' `-------------------' '--------------' `-------------------'
Figure 9: Multiple Attesters and Relying Parties with Different Figure 9: Multiple Attesters and Relying Parties with Different
Formats Formats
10. Freshness 10. Freshness
A remote entity (Verifier or Relying Party) may need to learn the A Verifier or Relying Party may need to learn the point in time
point in time (i.e., the "epoch") an Evidence or Attestation Result (i.e., the "epoch") an Evidence or Attestation Result has been
has been produced. This is essential in deciding whether the produced. This is essential in deciding whether the included Claims
included Claims and their values can be considered fresh, meaning and their values can be considered fresh, meaning they still reflect
they still reflect the latest state of the Attester, and that any the latest state of the Attester, and that any Attestation Result was
Attestation Result was generated using the latest Appraisal Policy generated using the latest Appraisal Policy for Evidence.
for Evidence.
Freshness is assessed based on a policy defined by the consuming Freshness is assessed based on the Appraisal Policy for Evidence or
entity, Verifier or Relying Party, that compares the estimated epoch Attestation Results, that compares the estimated epoch against an
against an "expiry" threshold defined locally to that policy. There "expiry" threshold defined locally to that policy. There is,
is, however, always a race condition possible in that the state of however, always a race condition possible in that the state of the
the Attester, and the Appraisal Policy for Evidence, might change Attester, and the appraisal policies might change immediately after
immediately after the Evidence or Attestation Result was generated. the Evidence or Attestation Result was generated. The goal is merely
The goal is merely to narrow their recentness to something the to narrow their recentness to something the Verifier (for Evidence)
Verifier (for Evidence) or Relying Party (for Attestation Result) is or Relying Party (for Attestation Result) is willing to accept.
willing to accept. Freshness is a key component for enabling caching Freshness is a key component for enabling caching and reuse of both
and reuse of both Evidence and Attestation Results, which is Evidence and Attestation Results, which is especially valuable in
especially valuable in cases where their computation uses a cases where their computation uses a substantial part of the resource
substantial part of the resource budget (e.g., energy in constrained budget (e.g., energy in constrained devices).
devices).
There are two common approaches for determining the epoch of an There are two common approaches for determining the epoch of an
Evidence or Attestation Result. Evidence or Attestation Result.
The first approach is to rely on synchronized and trustworthy clocks, The first approach is to rely on synchronized and trustworthy clocks,
and include a signed timestamp (see [I-D.birkholz-rats-tuda]) along and include a signed timestamp (see [I-D.birkholz-rats-tuda]) along
with the Claims in the Evidence or Attestation Result. Timestamps with the Claims in the Evidence or Attestation Result. Timestamps
can be added on a per-Claim basis, to distinguish the time of can be added on a per-Claim basis, to distinguish the time of
creation of Evidence or Attestation Result from the time that a creation of Evidence or Attestation Result from the time that a
specific Claim was generated. The clock's trustworthiness typically specific Claim was generated. The clock's trustworthiness typically
requires additional Claims about the signer's time synchronization requires additional Claims about the signer's time synchronization
mechanism. mechanism.
A second approach places the onus of timekeeping solely on the A second approach places the onus of timekeeping solely on the
appraising entity, i.e., the Verifier (for Evidence), or the Relying Verifier (for Evidence), or the Relying Party (for Attestation
Party (for Attestation Results), and might be suitable, for example, Results), and might be suitable, for example, in case the Attester
in case the Attester does not have a reliable clock or time does not have a reliable clock or time synchronisation is otherwise
synchronisation is otherwise impaired. In this approach, a non- impaired. In this approach, a non-predictable nonce is sent by the
predictable nonce is sent by the appraising entity, and the nonce is appraising entity, and the nonce is then signed and included along
then signed and included along with the Claims in the Evidence or with the Claims in the Evidence or Attestation Result. After
Attestation Result. After checking that the sent and received nonces checking that the sent and received nonces are the same, the
are the same, the appraising entity knows that the Claims were signed appraising entity knows that the Claims were signed after the nonce
after the nonce was generated. This allows associating a "rough" was generated. This allows associating a "rough" epoch to the
epoch to the Evidence or Attestation Result. In this case the epoch Evidence or Attestation Result. In this case the epoch is said to be
is said to be rough because: rough because:
* The epoch applies to the entire claim set instead of a more * The epoch applies to the entire claim set instead of a more
granular association, and granular association, and
* The time between the creation of Claims and the collection of * The time between the creation of Claims and the collection of
Claims is indistinguishable. Claims is indistinguishable.
Implicit and explicit timekeeping can be combined into hybrid Implicit and explicit timekeeping can be combined into hybrid
mechanisms. For example, if clocks exist and are considered mechanisms. For example, if clocks exist and are considered
trustworthy but are not synchronized, a nonce-based exchange may be trustworthy but are not synchronized, a nonce-based exchange may be
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that only attestation key material that is generated by a valid that only attestation key material that is generated by a valid
Endorser is established in Attesters and then configured correctly. Endorser is established in Attesters and then configured correctly.
For many use cases, this will involve physical security at the For many use cases, this will involve physical security at the
facility, to prevent unauthorized devices from being manufactured facility, to prevent unauthorized devices from being manufactured
that may be counterfeit or incorrectly configured. that may be counterfeit or incorrectly configured.
12.2. Integrity Protection 12.2. Integrity Protection
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 must support end-to-end Results, Endorsements, or appraisal policy must support end-to-end
integrity protection and replay attack prevention, and often also integrity protection and replay attack prevention, and often also
needs to support additional security properties, including: needs to support additional security properties, including:
* end-to-end encryption, * end-to-end encryption,
* denial of service protection, * denial of service protection,
* authentication, * authentication,
* auditing, * auditing,
* fine grained access controls, and * fine grained access controls, and
* logging. * logging.
Section 10 discusses ways in which freshness can be used in this Section 10 discusses ways in which freshness can be used in this
architecture to protect against replay attacks. 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
create Evidence about the integrity of the entity providing the create Evidence about the integrity of the entity providing the
Appraisal Policy needs to be considered. appraisal policy needs to be considered.
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.
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13. IANA Considerations 13. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
14. Acknowledgments 14. 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,
Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten. Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
15. Contributors 15. Notable Contributions
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.
16. Appendix A: Time Considerations 16. Appendix A: Time Considerations
skipping to change at page 40, line 48 skipping to change at page 41, line 48
<https://www.rfc-editor.org/info/rfc8322>. <https://www.rfc-editor.org/info/rfc8322>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library [TCGarch] Trusted Computing Group, "Trusted Platform Module Library
- Part 1: Architecture", n.d., - Part 1: Architecture", n.d.,
<https://trustedcomputinggroup.org/wp-content/uploads/ <https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>. TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>.
[WebAuthN] W3C, "Web Authentication: An API for accessing Public Key [WebAuthN] W3C, "Web Authentication: An API for accessing Public Key
Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>. Credentials", n.d., <https://www.w3.org/TR/webauthn-1/>.
Contributors
Monty Wiseman
Email: montywiseman32@gmail.com
Liang Xia
Email: frank.xialiang@huawei.com
Laurence Lundblade
Email: lgl@island-resort.com
Eliot Lear
Email: elear@cisco.com
Jessica Fitzgerald-McKay
Sarah C. Helbe
Andrew Guinn
Peter Lostcco
Email: pete.loscocco@gmail.com
Eric Voit
Thomas Fossati
Email: thomas.fossati@arm.com
Paul Rowe
Carsten Bormann
Email: cabo@tzi.org
Giri Mandyam
Email: mandyam@qti.qualcomm.com
Authors' Addresses 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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