draft-ietf-rats-architecture-04.txt   draft-ietf-rats-architecture-05.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: 22 November 2020 Microsoft Expires: 11 January 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
21 May 2020 10 July 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-04 draft-ietf-rats-architecture-05
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).
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 November 2020. This Internet-Draft will expire on 11 January 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . 6
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . 7
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 8
4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9 4.1. Appraisal Policies . . . . . . . . . . . . . . . . . . . 9
4.2. Two Types of Environments of an Attester . . . . . . . . 9 4.2. Two Types of Environments of an Attester . . . . . . . . 9
4.3. Layered Attestation Environments . . . . . . . . . . . . 10 4.3. Layered Attestation Environments . . . . . . . . . . . . 10
4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12 4.4. Composite Device . . . . . . . . . . . . . . . . . . . . 12
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 15 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 15
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 15
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 16
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 17
6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18 6. Roles and Entities . . . . . . . . . . . . . . . . . . . . . 18
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 20 7.1. Relying Party . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Attester . . . . . . . . . . . . . . . . . . . . . . . . 20
7.3. Relying Party Owner . . . . . . . . . . . . . . . . . . . 20
7.4. Verifier . . . . . . . . . . . . . . . . . . . . . . . . 20
7.5. Endorser and Verifier Owner . . . . . . . . . . . . . . . 21
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 21
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 22 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 23
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24 10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 25
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27
12. Security Considerations . . . . . . . . . . . . . . . . . . . 26 12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 29
16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27 16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 29
16.1. Example 1: Timestamp-based Passport Model Example . . . 29 16.1. Example 1: Timestamp-based Passport Model Example . . . 30
16.2. Example 2: Nonce-based Passport Model Example . . . . . 30 16.2. Example 2: Nonce-based Passport Model Example . . . . . 32
16.3. Example 3: Timestamp-based Background-Check Model 16.3. Example 3: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 31 Example . . . . . . . . . . . . . . . . . . . . . . . . 33
16.4. Example 4: Nonce-based Background-Check Model Example . 31 16.4. Example 4: Nonce-based Background-Check Model Example . 33
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
17.1. Normative References . . . . . . . . . . . . . . . . . . 32 17.1. Normative References . . . . . . . . . . . . . . . . . . 34
17.2. Informative References . . . . . . . . . . . . . . . . . 32 17.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
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. process. This documents defines a flexible architecture consisting
of attestation roles and their interactions via conceptual messages.
This documents defines a flexible architecture with corresponding Additionally, this document defines a universal set of terms that can
roles and their interaction via conceptual messages. Additionally, be mapped to various existing and emerging Remote Attestation
this document defines a universal set of terms that can be mapped to Procedures. Common topological models and the data flows associated
various existing and emerging Remote Attestation Procedures. Common with them, such as the "Passport Model" and the "Background-Check
topological models and the data flows associated with them, such as Model" are illustrated. The purpose is to define useful terminology
the "Passport Model" and the "Background-Check Model" are for attestation and enable readers to map their solution architecture
illustrated. The purpose is to enable readers of this document to to the canonical attestation architecture provided here. Having a
map their current and emerging solutions to the architecture provided common terminology that provides well-understood meanings for common
and the corresponding terminology defined. themes such as roles, device composition, topological models, and
appraisal is vital for semantic interoperability across solutions and
A common terminology that provides a well-understood semantic meaning platforms involving multiple vendors and providers.
to the concepts, roles, and models in this document is vital to
create semantic interoperability between solutions and across
different platforms.
Amongst other things, this document is about trust and Amongst other things, this document is about trust and
trustworthiness. Trust is a decision being made. Trustworthiness is trustworthiness. Trust is a choice one makes about another system.
a quality that is assessed via evidence created. This is a subtle Trustworthiness is a quality about the other system that can be used
in making one's decision to trust it or not. This is subtle
difference and being familiar with the difference is crucial for 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.
2. Terminology 2. Terminology
This document uses the following terms. This document uses the following terms.
Appraisal Policy for Evidence: A set of rules that direct how a Appraisal Policy for Evidence: A set of rules that informs how a
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 Results: 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 results
Attester: An entity whose attributes must be appraised in order to Attester: A role performed by an entity (typically a device) whose
determine whether the entity is considered trustworthy, such as Evidence must be appraised in order to infer the extent to which
when deciding whether the entity is authorized to perform some the Attester is considered trustworthy, such as when deciding
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 some entity (typically a
manufacturer) vouches for the integrity of an Attester's signing manufacturer) vouches for the integrity of an Attester's signing
capability capability
Endorser: An entity that creates Endorsements that can be used to Endorser: An entity (typically a manufacturer) whose Endorsements
help to appraise the trustworthiness of Attesters 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 appraised by a Verifier. Evidence may include configuration data,
measurements, telemetry, or inferences.
Relying Party: An entity that depends on the validity of information Relying Party: A role performed by an entity that depends on the
about another entity, typically for purposes of authorization. validity of information about an Attester, for purposes of
Compare /relying party/ in [RFC4949] reliably applying application specific actions. Compare /relying
party/ in [RFC4949]
Relying Party Owner: An entity, such as 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: An entity that appraises the validity of Evidence about an Verifier: A role performed by an entity that appraises the validity
Attester 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
that collectively cover all the functionality required in the that collectively cover all the functionality required in the
architecture. architecture.
Each use case includes a description, and a summary of what an Each use case includes a description followed by a summary of the
Attester and a Relying Party refer to in the use case. Attester and a Relying Party roles.
3.1. Network Endpoint Assessment 3.1. Network Endpoint Assessment
Network operators want a trustworthy report of identity and version Network operators want a trustworthy report that includes identity
of information of the hardware and software on the machines attached and version of information of the hardware and software on the
to their network, for purposes such as inventory, auditing, and/or machines attached to their network, for purposes such as inventory,
logging. The network operator may also want a policy by which full audit, anomaly detection, record maintenance and/or trending reports
(logging). The network operator may also want a policy by which full
access is only granted to devices that meet some definition of access is only granted to devices that meet some definition of
health, 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. These components perform a series of measurements, and measurements. The system components perform a series of measurements
express this with Evidence as to the hardware and firmware/software that may be signed by the Root of Trust, considered as Evidence about
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 in A device manufacturer wants to protect its intellectual property.
terms of the ML model it developed and that runs in the devices that This is primarily the ML model it developed and runs in the devices
its customers purchased, and it wants to prevent attackers, purchased by its customers. The goals for the protection include
potentially including the customer themselves, from seeing the preventing attackers, potentially the customer themselves, from
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 attest to some manufacturer service. If remote attestation device go through a remote attestation with some manufacturer service
succeeds, then the manufacturer service releases either the model, or that can assess its trustworthiness. If remote attestation succeeds,
a key to decrypt a model the Attester already has in encrypted form, then the manufacturer service releases either the model, or a key to
to the requester. decrypt a model the Attester already has in encrypted form, to the
requester.
Attester: A device desiring to run an ML model to do inferencing Attester: A device desiring to run an ML model to do inferencing
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.
Attestation is desired to prevent leaking data to compromised An assessment of system state is made against a set of policies to
devices. evaluate the state of a system using attestations for the system
requesting data. Attestation is desired to prevent leaking data to
compromised devices.
Attester: An entity desiring to retrieve confidential data Attester: An entity desiring to retrieve confidential data
Relying Party: An entity that holds confidential data for retrieval Relying Party: An entity that holds confidential data for retrieval
by other entities by other entities
3.4. Critical Infrastructure Control 3.4. Critical Infrastructure Control
In this use case, potentially dangerous physical equipment (e.g., In this use case, potentially dangerous physical equipment (e.g.,
power grid, traffic control, hazardous chemical processing, etc.) is power grid, traffic control, hazardous chemical processing, etc.) is
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Relying Party: A device or application connected to potentially Relying Party: A device or application connected to potentially
dangerous physical equipment (hazardous chemical processing, dangerous physical equipment (hazardous chemical processing,
traffic control, power grid, etc.) traffic control, power grid, etc.)
3.5. Trusted Execution Environment (TEE) Provisioning 3.5. Trusted Execution Environment (TEE) Provisioning
A "Trusted Application Manager (TAM)" server is responsible for A "Trusted Application Manager (TAM)" server is responsible for
managing the applications running in the TEE of a client device. To managing the applications running in the TEE of a client device. To
do this, the TAM wants to assess the state of a TEE, or of do this, the TAM wants to assess the state of a TEE, or of
applications in the TEE, of a client device. The TEE attests to the applications in the TEE, of a client device. The TEE conducts a
TAM, which can then decide whether the TEE is already in compliance remote attestation procedure with the TAM, which can then decide
with the TAM's latest policy, or if the TAM needs to uninstall, whether the TEE is already in compliance with the TAM's latest
update, or install approved applications in the TEE to bring it back policy, or if the TAM needs to uninstall, update, or install approved
into compliance with the TAM's policy. applications in the TEE to bring it back into compliance with the
TAM's policy.
Attester: A device with a trusted execution environment capable of Attester: A device with a trusted execution environment capable of
running trusted applications that can be updated running trusted applications that can be updated
Relying Party: A Trusted Application Manager Relying Party: A Trusted Application Manager
3.6. Hardware Watchdog 3.6. Hardware Watchdog
One significant problem is malware that holds a device hostage and One significant problem is malware that holds a device hostage and
does not allow it to reboot to prevent updates to be applied. This does not allow it to reboot to prevent updates from being applied.
is a significant problem, because it allows a fleet of devices to be This is a significant problem, because it allows a fleet of devices
held hostage for ransom. to be held hostage for ransom.
A hardware watchdog can be implemented by forcing a reboot unless In the case, the Relying Party is the watchdog timer in the TPM/
remote attestation to a server succeeds within a periodic interval, secure enclave itself, as described in [TCGarch] section 43.3. The
and having the reboot do remediation by bringing a device into Attestation Results are returned to the device, and provided to the
compliance, including installation of patches as needed. enclave.
If the watchdog does not receive regular, and fresh, Attestation
Results as to the systems' health, then it forces a reboot.
Attester: The device that is desired to keep from being held hostage Attester: The device that is desired to keep from being held hostage
for a long period of time for a long period of time
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
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* Endorser * * Verifier * * Relying Party* * Endorser * * Verifier * * Relying Party*
************ * Owner * * Owner * ************ * Owner * * Owner *
| ************ **************** | ************ ****************
| | | | | |
Endorsements| | | Endorsements| | |
| |Appraisal | | |Appraisal |
| |Policy | | |Policy |
| |for | Appraisal | |for | Appraisal
| |Evidence | Policy for | |Evidence | Policy for
| | | Attestation | | | Attestation
| | | Result | | | Results
v v | v v |
.-----------------. | .-----------------. |
.----->| Verifier |------. | .----->| Verifier |------. |
| '-----------------' | | | '-----------------' | |
| | | | | |
| Attestation| | | Attestation| |
| Results | | | Results | |
| Evidence | | | Evidence | |
| | | | | |
| v v | v v
skipping to change at page 8, line 38 skipping to change at page 8, line 45
| 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 Evidence Appraisal Policy to assess the by applying an Evidence Appraisal Policy 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 Evidence Appraisal Policy might be for use by Relying Parties. The Appraisal Policy for Evidence might
obtained from an Endorser along with the Endorsements, or might be 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 Appraisal Policy for Attestation
might, for example, be configured in the Relying Party by an Results might, for example, be configured in the Relying Party by an
administrator. administrator.
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
skipping to change at page 9, line 44 skipping to change at page 9, line 50
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 raw values and the That is, Attesting Environments collect the raw values and the
information to be represented in claims, such as by doing some information to be represented in claims, such as by doing some
measurement of a Target Environment's code, memory, and/or registers. measurement of a Target Environment's code, memory, and/or registers.
Attesting Environments then format the claims appropriately, and Attesting Environments then format the claims appropriately, and
typically use key material and cryptographic functions, such as typically use key material and cryptographic functions, such as
signing or cipher algorithms, to create Evidence. Places that signing or cipher algorithms, to create Evidence. Places that
Attesting Environments can exist include Trusted Execution Attesting Environments can exist include Trusted Execution
Environments (TEE), embedded Secure Elements (eSE), and BIOS Environments (TEE), embedded Secure Elements (eSE), and BIOS
firmware. An execution environment may not, by default, be capable firmware. An execution environment may not, by default, be capable
of claims collection for a given Target Environment. Attesting of claims collection for a given Target Environment. Execution
Environments are designed specifically with claims collection in environments that are designed to be capable of claims collection are
mind. referred to in this document as Attesting Environments.
.--------------------------------. .--------------------------------.
| | | |
| Verifier | | Verifier |
| | | |
'--------------------------------' '--------------------------------'
^ ^
| |
.-------------------------|----------. .-------------------------|----------.
| | | | | |
skipping to change at page 10, line 37 skipping to change at page 10, line 37
| | Environment | | | | Environment | |
| | | | | | | |
| '-------------' | | '-------------' |
| Attester | | Attester |
'------------------------------------' '------------------------------------'
Figure 2: Two Types of Environments Figure 2: Two Types of Environments
4.3. Layered Attestation Environments 4.3. Layered Attestation Environments
By definition, the Attester role takes on the duty to create By definition, the Attester role creates Evidence. An Attester may
Evidence. The fact that an Attester role is composed of environments consist of one or more nested or staged environments, adding
that can be nested or staged adds complexity to the architectural complexity to the architectural structure. The unifying component is
layout of how an Attester can be composed and therefore has to the Root of Trust and the nested, staged, or chained attestation
conduct the Claims collection in order to create believable Evidence produced. The nested or chained structure includes Claims,
attestation Evidence. collected by the Attester to aid in the assurance or believability of
the attestation Evidence.
Figure 3 depicts an example of a device that includes (A) a BIOS 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.
.----------. .----------. .----------. .----------.
| | | | | | | |
| Endorser |------------------->| Verifier | | Endorser |------------------->| Verifier |
| | Endorsements | | | | Endorsements | |
'----------' for A, B, and C '----------' '----------' for A, B, and C '----------'
skipping to change at page 12, line 17 skipping to change at page 12, line 17
ensured having those Claims be either signed by Attesting Environment ensured having those Claims be either signed by Attesting Environment
A or stored in an untamperable manner by Attesting Environment A. A or stored in an untamperable manner by Attesting Environment A.
Continuing with this example, the bootloader's Attesting Environment Continuing with this example, the bootloader's Attesting Environment
B is now in charge of collecting Claims about Target Environment C, 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 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 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 measured and signed by the BIOS, plus a set of Claims about the
kernel as measured and signed by the bootloader. kernel as measured and signed by the bootloader.
This example could be extended further by, say, making the kernel This example could be extended further by making the kernel become
become another Attesting Environment for an application as another another Attesting Environment for an application as another Target
Target Environment, resulting 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 4.4. Composite Device
skipping to change at page 13, line 21 skipping to change at page 13, line 21
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 lead 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. A multi-chassis router
there is only one main router that connects to the Verifier. Other provides a management point 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 4 depicts the conceptual data flow for a Composite Device. Figure 4 depicts the conceptual data flow for a Composite Device.
.-----------------------------. .-----------------------------.
| Verifier | | Verifier |
skipping to change at page 14, line 44 skipping to change at page 14, line 44
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 verifies them using Endorsements and Appraisal this inside Verifier uses Endorsements and Appraisal Policies
Policies (obtained the same way as any other Verifier), to generate (obtained the same way as any other Verifier) in the verification
Attestation Results. The inside Verifier then conveys the process to generate Attestation Results. The inside Verifier then
Attestation Results of other Attesters, whether in the same conveys the Attestation Results of other Attesters, whether in the
conveyance protocol as the Evidence or not, to the outside Verifier. same conveyance protocol as the Evidence or not, to the outside
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
Figure 1 shows a basic model for communication between an Attester, a Figure 1 shows a basic model for communication between an Attester, a
Verifier, and a Relying Party. The Attester conveys its Evidence to Verifier, and a Relying Party. The Attester conveys its Evidence to
the Verifier for appraisal, and the Relying Party gets the the Verifier for appraisal, and the Relying Party gets the
Attestation Results from the Verifier. There are multiple other Attestation Result from the Verifier. There are multiple other
possible models. This section includes some reference models, but possible models. This section includes some reference models. This
this is not intended to be a restrictive list, and other variations is not intended to be a restrictive list, and other variations may
may exist. exist.
5.1. Passport Model 5.1. Passport Model
The passport model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity claim, such as an
airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity claims, and it is
issued by a trusted authority. Thus, in this immigration desk
analogy, the passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party.
In this model, an Attester conveys Evidence to a Verifier, which 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 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.
skipping to change at page 16, line 4 skipping to change at page 16, line 19
+-------------+ +-------------+
^ | ^ |
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
The passport model is so named because of its resemblance to how Figure 5: Passport Model
nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity claim, such as an
airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity claims, and it is
issued by a trusted authority. Thus, in this immigration desk
analogy, the passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party.
5.2. Background-Check Model 5.2. Background-Check Model
The background-check model is so named because of the resemblance of
how employers and volunteer organizations perform background checks.
When a prospective employee provides claims about education or
previous experience, the employer will contact the respective
institutions or former employers to validate the claim. Volunteer
organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and a former employer or government agency that
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
skipping to change at page 17, line 22 skipping to change at page 17, line 29
| | 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
The background-check model is so named because of the resemblance of
how employers and volunteer organizations perform background checks.
When a prospective employee provides claims about education or
previous experience, the employer will contact the respective
institutions or former employers to validate the claim. Volunteer
organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and a former employer or government agency that
issues a report is a Verifier.
5.3. Combinations 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 on the same machine, and so there is no need Party and the Verifier are on the same machine, performing both
for a protocol between the two. functions together. In this case, there is no need for a protocol
between the two.
It is also worth pointing out that the choice of model is generally 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. The same device may need to create Evidence
Evidence for different Relying Parties and different use cases (e.g., for different Relying Parties and/or different use cases. For
a network infrastructure device to gain access to the network, and instance, it would provide Evidence to a network infrastructure
then a server holding confidential data to get access to that data). device to gain access to the network, and to a server holding
As such, both models may simultaneously be in use by the same device. confidential data to gain access to that data. As such, both models
may simultaneously be in use by the same device.
Figure 7 shows another example of a combination where Relying Party 1 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 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].
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| |-------------->| | 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. As a result, the entity can participate as defined in this document. An entity can aggregate more than one role
a constituent of the RATS architecture. Additionally, an entity can into itself. These collapsed roles combine the duties of multiple
aggregate more than one role into itself. These collapsed roles roles.
combine the duties of multiple roles. In these cases, interaction
between these roles do not necessarily use the Internet Protocol. In these cases, interaction between these roles do not necessarily
They can be using a loopback device or other IP-based communication use the Internet Protocol. They can be using a loopback device or
between separate environments, but they do not have to. Alternative other IP-based communication between separate environments, but they
channels to convey conceptual messages include function calls, do not have to. Alternative channels to convey conceptual messages
sockets, GPIO interfaces, local busses, or hypervisor calls. This include function calls, sockets, GPIO interfaces, local busses, or
type of conveyance is typically found in Composite Devices. Most hypervisor calls. This type of conveyance is typically found in
importantly, these conveyance methods are out-of-scope of RATS, but Composite Devices. Most importantly, these conveyance methods are
they are presumed to exist in order to convey conceptual messages out-of-scope of RATS, but they are presumed to exist in order to
appropriately between roles. convey conceptual messages appropriately between roles.
For example, an entity that both connects to a wide-area network and 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. to a system bus is taking on both the Attester and Verifier roles.
As a system bus entity, a Verifier consumes Evidence from other As a system bus entity, a Verifier consumes Evidence from other
devices connected to the system bus that implement Attester roles. devices connected to the system bus that implement Attester roles.
As a wide-area network connected entity, it may implement an Attester As a wide-area network connected entity, it may implement an Attester
role. The entity, as a system bus Verifier, may choose to fully role. The entity, as a system bus Verifier, may choose to fully
isolate its role as a wide-area network Attester. isolate its role as a wide-area network Attester.
In essence, an entity that combines more than one role also creates In essence, an entity that combines more than one role creates and
and consumes the corresponding conceptual messages as defined in this consumes the corresponding conceptual messages as defined in this
document. document.
7. Trust Model 7. Trust Model
7.1. Relying Party
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.
trust a Verifier (such as in the Verifying Relying Party
combination). Or, for a stronger level of security, the Relying The Relying Party might implicitly trust a Verifier, such as in a
Party might require that the Verifier itself provide information Verifier/Relying Party combination where the Verifier and Relying
about itself that the Relying Party can use to assess the Party roles are combined. Or, for a stronger level of security, the
Relying Party might require that the Verifier first provide
information about itself that the Relying Party can use to assess the
trustworthiness of the Verifier before accepting its Attestation trustworthiness of the Verifier before accepting its Attestation
Results. Results.
The Endorser and Verifier Owner may need to trust the Verifier before For example, one explicit way for a Relying Party "A" to establish
giving the Endorsement and Appraisal Policy to it. Such trust can such trust in a Verifier "B", would be for B to first act as an
also be established directly or indirectly, implicitly or explicitly. Attester where A acts as a combined Verifier/Relying Party. If A
One explicit way to establish such trust may be the Verifier first then accepts B as trustworthy, it can choose to accept B as a
acts as an Attester and creates Evidence about itself to be consumed Verifier for other Attesters.
by the Endorser and/or Verifier Owner as the Relying Parties. If it
is accepted as trustworthy, then they can provide Endorsements and Similarly, the Relying Party also needs to trust the Relying Party
Appraisal Policies that enable it to act as a Verifier. Owner for providing its Appraisal Policy for Attestation Results, and
in some scenarios the Relying Party might even require that the
Relying Party Owner go through a remote attestation procedure with it
before the Relying Party will accept an updated policy. This can be
done similarly to how a Relying Party could establish trust in a
Verifier as discussed above.
7.2. Attester
In some scenarios, Evidence might contain sensitive information such
as Personally Identifiable Information. Thus, an Attester must trust
entities to which it conveys Evidence, to not reveal sensitive data
to unauthorized parties. The Verifier might share this information
with other authorized parties, according to rules that it controls.
In the background-check model, this Evidence may also be revealed to
Relying Party(s).
In some cases where Evidence contains sensitive information, an
Attester might even require that a Verifier first go through a remote
attestation procedure with it before the Attester will send the
sensitive Evidence. This can be done by having the Attester first
act as a Verifier/Relying Party, and the Verifier act as its own
Attester, as discussed above.
7.3. Relying Party Owner
The Relying Party Owner might also require that the Relying Party
first act as an Attester, providing Evidence that the Owner can
appraise, before the Owner would give the Relying Party an updated
policy that might contain sensitive information. In such a case,
mutual attestation might be needed, in which case typically one
side's Evidence must be considered safe to share with an untrusted
entity, in order to bootstrap the sequence.
7.4. Verifier
The Verifier trusts (or more specifically, the Verifier's security 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
solutions with weaker security, a Verifier might be configured to solutions with weaker security, a Verifier might be configured to
implicitly trust firmware or even software (e.g., a hypervisor). implicitly trust firmware or even software (e.g., a hypervisor).
That is, it might appraise the trustworthiness of an application That is, it might appraise the trustworthiness of an application
component, or 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 security comes hypervisor or firmware is true. A stronger level of assurance of
when information can be vouched for by hardware or by ROM code, security comes when information can be vouched for by hardware or by
especially if such hardware is physically resistant to hardware ROM code, especially if such hardware is physically resistant to
tampering. The component that is implicitly trusted is often hardware tampering. The component that is implicitly trusted is
referred to as a Root of Trust. often referred to as a Root of Trust.
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.
In some scenarios, Evidence might contain sensitive information such 7.5. Endorser and Verifier Owner
as Personally Identifiable Information. Thus, an Attester must trust
entities to which it conveys Evidence, to not reveal sensitive data In some scenarios, the Endorser and Verifier Owner may need to trust
to unauthorized parties. The Verifier might share this information the Verifier before giving the Endorsement and Appraisal Policy to
with other authorized parties, according rules that it controls. In it. This can be done similarly to how a Relying Party might
the background-check model, this Evidence may also be revealed to establish trust in a Verifier as discussed above, and in such a case,
Relying Party(s). mutual 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
securely associated with the target environment so that the Verifier securely associated with the target environment so that the Verifier
cannot be tricked into accepting claims originating from a different cannot be tricked into accepting claims originating from a different
environment (that may be more trustworthy). Evidence also must be environment (that may be more trustworthy). Evidence also must be
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+-------------+ +------------+ Evaluate +-------------+ +------------+ Evaluate
| |-------------->| | request | |-------------->| | request
| Attester | Access some | Relying | against | Attester | Access some | Relying | against
| | resource | Party | security | | resource | Party | security
+-------------+ +------------+ policy +-------------+ +------------+ policy
Figure 8: 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), ROLIE
OPC UA, etc.), depending on the use case. Such protocols typically [RFC8322], 802.1x, OPC UA, etc.), depending on the use case. Such
already have mechanisms for passing security information for purposes protocols typically already have mechanisms for passing security
of authentication and authorization. Common formats include JWTs information for purposes of authentication and authorization. Common
[RFC7519], CWTs [RFC8392], and X.509 certificates. formats include JWTs [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
is important that information needed for appraising the Attester be is important that information needed for appraising the Attester be
usable with existing protocols that have constraints around what usable with existing protocols that have constraints around what
formats they can transport. For example, OPC UA [OPCUA] (probably formats they can transport. For example, OPC UA [OPCUA] (probably
the most common protocol in industrial IoT environments) is defined the most common protocol in industrial IoT environments) is defined
to carry X.509 certificates and so security information must be to carry X.509 certificates and so security information must be
embedded into an X.509 certificate to be passed in the protocol. embedded into an X.509 certificate to be passed in the protocol.
Thus, remote attestation related information could be natively Thus, remote attestation related information could be natively
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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
It is important to prevent replay attacks where an attacker replays A remote entity (Verifier or Relying Party) may need to learn the
old Evidence or an old Attestation Result that is no longer correct. point in time (i.e., the "epoch") an Evidence or Attestation Result
To do so, some mechanism of ensuring that the Evidence and has been produced. This is essential in deciding whether the
Attestation Result are fresh, meaning that there is some degree of included Claims and their values can be considered fresh, meaning
assurance that they still reflect the latest state of the Attester, they still reflect the latest state of the Attester, and that any
and that any Attestation Result was generated using the latest Attestation Result was generated using the latest Appraisal Policy
Appraisal Policy for Evidence. There is, however, always a race for Evidence.
condition possible in that the state of the Attester, and the
Appraisal Policy for Evidence, might change immediately after the
Evidence or Attestation Result was generated. The goal is merely to
narrow the time window to something the Verifier (for Evidence) or
Relying Party (for an Attestation Result) is willing to accept.
There are two common approaches to providing some assurance of Freshness is assessed based on a policy defined by the consuming
freshness. The first approach is that a nonce is generated by a entity, Verifier or Relying Party, that compares the estimated epoch
remote entity (e.g., the Verifier for Evidence, or the Relying Party against an "expiry" threshold defined locally to that policy. There
for an Attestation Result), and the nonce is then signed and included is, however, always a race condition possible in that the state of
along with the claims in the Evidence or Attestation Result, so that the Attester, and the Appraisal Policy for Evidence, might change
the remote entity knows that the claims were signed after the nonce immediately after the Evidence or Attestation Result was generated.
was generated. The goal is merely to narrow their recentness to something the
Verifier (for Evidence) or Relying Party (for Attestation Result) is
willing to accept. Freshness is a key component for enabling caching
and reuse of both Evidence and Attestation Results, which is
especially valuable in cases where their computation uses a
substantial part of the resource budget (e.g., energy in constrained
devices).
A second approach is to rely on synchronized clocks, and include a There are two common approaches for determining the epoch of an
signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with Evidence or Attestation Result.
the claims in the Evidence or Attestation Result, so that the remote
entity knows that the claims were signed at that time, as long as it
has some assurance that the timestamp is correct. This typically
requires additional claims about the signer's time synchronization
mechanism in order to provide such assurance.
In either approach, it is important to note that the actual values in The first approach is to rely on synchronized and trustworthy clocks,
claims might have been generated long before the claims are signed. and include a signed timestamp (see [I-D.birkholz-rats-tuda]) along
If so, it is the signer's responsibility to ensure that the values with the Claims in the Evidence or Attestation Result. Timestamps
are still correct when they are signed. For example, values might can be added on a per-Claim basis, to distinguish the time of
have been generated at boot, and then used in claims as long as the creation of Evidence or Attestation Result from the time that a
signer can guarantee that they cannot have changed since boot. specific Claim was generated. The clock's trustworthiness typically
requires additional Claims about the signer's time synchronization
mechanism.
A second approach places the onus of timekeeping solely on the
appraising entity, i.e., the Verifier (for Evidence), or the Relying
Party (for Attestation Results), and might be suitable, for example,
in case the Attester does not have a reliable clock or time
synchronisation is otherwise impaired. In this approach, a non-
predictable nonce is sent by the appraising entity, and the nonce is
then signed and included along with the Claims in the Evidence or
Attestation Result. After checking that the sent and received nonces
are the same, the appraising entity knows that the Claims were signed
after the nonce was generated. This allows associating a "rough"
epoch to the Evidence or Attestation Result. In this case the epoch
is said to be rough because:
* The epoch applies to the entire claim set instead of a more
granular association, and
* The time between the creation of Claims and the collection of
Claims is indistinguishable.
Implicit and explicit timekeeping can be combined into hybrid
mechanisms. For example, if clocks exist and are considered
trustworthy but are not synchronized, a nonce-based exchange may be
used to determine the (relative) time offset between the involved
peers, followed by any number of timestamp based exchanges. In
another setup where all Roles (Attesters, Verifiers and Relying
Parties) share the same broadcast channel, the nonce-based approach
may be used to anchor all parties to the same (relative) timeline,
without requiring synchronized clocks, by having a central entity
emit nonces at regular intervals and have the "current" nonce
included in the produced Evidence or Attestation Result.
It is important to note that the actual values in Claims might have
been generated long before the Claims are signed. If so, it is the
signer's responsibility to ensure that the values are still correct
when they are signed. For example, values generated at boot time
might have been saved to secure storage until network connectivity is
established to the remote Verifier and a nonce is obtained.
A more detailed discussion with examples appears in Section 16. A more detailed discussion with examples appears in Section 16.
11. Privacy Considerations 11. 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 as well as any users the device is associated with. In many
to provide reliable information about the type of the device and the cases, the whole point of the Attestation process is to provide
firmware/software that the device is running. This information might reliable information about the type of the device and the firmware/
be particularly interesting to many attackers. For example, knowing software that the device is running. This information might be
particularly interesting to many attackers. For example, knowing
that a device is running a weak version of firmware provides a way to that a device is running a weak version of firmware provides a way to
aim attacks better. aim attacks better.
Many claims in Attestation Evidence and Attestation Results are
potentially PII (Personally Identifying Information) depending on the
end-to-end use case of the attestation. Attestation that goes up to
include containers and applications may further reveal details about
a specific system or user.
In some cases, an attacker may be able to make inferences about
attestations from the results or timing of the processing. For
example, an attacker might be able to infer the value of specific
claims if it knew that only certain values were accepted by the
Relying Party.
Evidence and Attestation Results data structures are expected to 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.
Furthermore, because Evidence might contain sensitive information, Furthermore, because Evidence might contain sensitive information,
Attesters are responsible for only sending such Evidence to trusted Attesters are responsible for only sending such Evidence to trusted
skipping to change at page 26, line 9 skipping to change at page 27, line 50
of the trustworthiness of a Verifier before sending Evidence to it. 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 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 for the Verifier's own Attestation Result, and appraising it just as
a Relying Party would appraise an Attestation Result for any other a Relying Party would appraise an Attestation Result for any other
purpose. purpose.
12. Security Considerations 12. 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 must support end-to-end
end integrity protection and replay attack prevention, and often also integrity protection and replay attack prevention, and often also
needs to support additional security protections. For example, needs to support additional security properties, including:
additional means of authentication, confidentiality, integrity,
replay, denial of service and privacy protection are needed in many * end-to-end encryption,
use cases. Section 10 discusses ways in which freshness can be used
in this architecture to protect against replay attacks. * denial of service protection,
* authentication,
* auditing,
* fine grained access controls, and
* logging.
Section 10 discusses ways in which freshness can be used in this
architecture to protect against replay attacks.
To assess the security provided by a particular Appraisal Policy, it 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
skipping to change at page 26, line 46 skipping to change at page 28, line 49
the Evidence. the Evidence.
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,
Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten. Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
15. Contributors 15. 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.
16. Appendix A: Time Considerations 16. Appendix A: Time Considerations
The table below defines a number of relevant events, with an ID that 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 is used in subsequent diagrams. The times of said events might be
defined in terms of an absolute clock time such as Coordinated defined in terms of an absolute clock time such as Coordinated
Universal Time, or might be defined relative to some other timestamp Universal Time, or might be defined relative to some other timestamp
or timeticks counter. or timeticks counter.
+----+------------+-----------------------------------------------+ +====+==============+===============================================+
| ID | Event | Explanation of event | | ID | Event | Explanation of event |
+====+============+===============================================+ +====+==============+===============================================+
| VG | Value | A value to appear in a claim was created | | VG | Value | A value to appear in a Claim was |
| | generation | | | | generation | created. |
+----+------------+-----------------------------------------------+ +----+--------------+-----------------------------------------------+
| NS | Nonce sent | A random number not predictable to an | | AA | Attester | An Attesting Environment starts to |
| | | Attester is sent | | | awareness | be aware of a new/changed Claim |
+----+------------+-----------------------------------------------+ | | | value. |
| NR | Nonce | The nonce is relayed to an Attester by | +----+--------------+-----------------------------------------------+
| | relayed | enother entity | | HD | Handle | A centrally generated identifier for |
+----+------------+-----------------------------------------------+ | | distribution | time-bound recentness across a |
| EG | Evidence | An Attester collects claims and generates | | | | domain of devices is successfully |
| | generation | Evidence | | | | distributed to Attesters. |
+----+------------+-----------------------------------------------+ +----+--------------+-----------------------------------------------+
| ER | Evidence | A Relying Party relays Evidence to a Verifier | | NS | Nonce sent | A nonce not predictable to an |
| | relayed | | | | | Attester (recentness & uniqueness) |
+----+------------+-----------------------------------------------+ | | | is sent to an Attester. |
| RG | Result | A Verifier appraises Evidence and generates | +----+--------------+-----------------------------------------------+
| | generation | an Attestation Result | | NR | Nonce | A nonce is relayed to an Attester by |
+----+------------+-----------------------------------------------+ | | relayed | another entity. |
| RR | Result | A Relying Party relays an Attestation Result | +----+--------------+-----------------------------------------------+
| | relayed | to a Relying Party | | EG | Evidence | An Attester creates Evidence from |
+----+------------+-----------------------------------------------+ | | generation | collected Claims. |
| RA | Result | The Relying Party appraises Attestation | +----+--------------+-----------------------------------------------+
| | appraised | Results | | ER | Evidence | A Relying Party relays Evidence to a |
+----+------------+-----------------------------------------------+ | | relayed | Verifier. |
| OP | Operation | The Relying Party performs some operation | +----+--------------+-----------------------------------------------+
| | performed | requested by the Attester. For example, | | RG | Result | A Verifier appraises Evidence and |
| | | acting upon some message just received across | | | generation | generates an Attestation Result. |
| | | a session created earlier at time(RA). | +----+--------------+-----------------------------------------------+
+----+------------+-----------------------------------------------+ | RR | Result | A Relying Party relays an |
| RX | Result | An Attestation Result should no longer be | | | relayed | Attestation Result to a Relying |
| | expiry | accepted, according to the Verifier that | | | | Party. |
| | | generated it | +----+--------------+-----------------------------------------------+
+----+------------+-----------------------------------------------+ | RA | Result | The Relying Party appraises |
| | appraised | Attestation Results. |
+----+--------------+-----------------------------------------------+
| OP | Operation | The Relying Party performs some |
| | performed | operation 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 |
| | expiry | longer be accepted, according to the |
| | | Verifier that generated it. |
+----+--------------+-----------------------------------------------+
Table 1 Table 1
We now walk through a number of hypothetical examples of how a Using the table above, a number of hypothetical examples of how a
solution might be built. This list is not intended to be complete, solution might be built are illustrated below. a solution might be
but is just representative enough to highlight various timing built. This list is not intended to be complete, but is just
considerations. representative enough to highlight various timing considerations.
16.1. Example 1: Timestamp-based Passport Model Example 16.1. Example 1: Timestamp-based Passport Model Example
The following example illustrates a hypothetical Passport Model The following example illustrates a hypothetical Passport Model
solution that uses timestamps and requires roughly synchronized solution that uses timestamps and requires roughly synchronized
clocks between the Attester, Verifier, and Relying Party, which clocks between the Attester, Verifier, and Relying Party, which
depends on using a secure clock synchronization mechanism. depends on using a secure clock synchronization mechanism.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
skipping to change at page 33, line 9 skipping to change at page 35, line 9
[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>.
17.2. Informative References 17.2. Informative References
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>. <https://www.rfc-editor.org/info/rfc4949>.
[RFC8322] Field, J., Banghart, S., and D. Waltermire, "Resource-
Oriented Lightweight Information Exchange (ROLIE)",
RFC 8322, DOI 10.17487/RFC8322, February 2018,
<https://www.rfc-editor.org/info/rfc8322>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification, [OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25 Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/ November 2015, <https://opcfoundation.org/developer-tools/
specifications-unified-architecture/part-2-security- specifications-unified-architecture/part-2-security-
model/>. 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-02, 9 Progress, Internet-Draft, draft-birkholz-rats-tuda-02, 9
March 2020, <http://www.ietf.org/internet-drafts/draft- March 2020, <http://www.ietf.org/internet-drafts/draft-
birkholz-rats-tuda-02.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-08, 4 April 2020, ietf-teep-architecture-11, 2 July 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep- <http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-08.txt>. architecture-11.txt>.
[TCGarch] Trusted Computing Group, "Trusted Platform Module Library
- Part 1: Architecture", 7 July 2020,
<https://trustedcomputinggroup.org/wp-content/uploads/
TCG_TPM2_r1p62_Part1_Architecture_7july2020.pdf>.
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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