draft-ietf-rats-architecture-03.txt   draft-ietf-rats-architecture-04.txt 
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Expires: 22 November 2020 Microsoft Expires: 22 November 2020 Microsoft
M. Richardson M. Richardson
Sandelman Software Works Sandelman Software Works
N. Smith N. Smith
Intel Intel
W. Pan W. Pan
Huawei Technologies Huawei Technologies
21 May 2020 21 May 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-03 draft-ietf-rats-architecture-04
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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extracted from this document must include Simplified BSD License text extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License. provided without warranty as described in the Simplified BSD License.
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 . . . . . . . . . . . . . . . 5
3.2. Confidential Machine Learning (ML) Model Protection . . . 5 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 . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . . . 14 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. Role Hosting and Composition . . . . . . . . . . . . . . . . 19 7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 20
9. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 21 8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 21 8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21
9.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 21 8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22
9.3. Attestation Results . . . . . . . . . . . . . . . . . . . 22 9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 22
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 23 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25
11. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 24 12. Security Considerations . . . . . . . . . . . . . . . . . . . 26
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 25 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
13. Security Considerations . . . . . . . . . . . . . . . . . . . 26 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27
15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 26 16. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27 16.1. Example 1: Timestamp-based Passport Model Example . . . 29
17. Appendix A: Time Considerations . . . . . . . . . . . . . . . 27 16.2. Example 2: Nonce-based Passport Model Example . . . . . 30
17.1. Example 1: Timestamp-based Passport Model Example . . . 29 16.3. Example 3: Timestamp-based Background-Check Model
17.2. Example 2: Nonce-based Passport Model Example . . . . . 30
17.3. Example 3: Timestamp-based Background-Check Model
Example . . . . . . . . . . . . . . . . . . . . . . . . 31 Example . . . . . . . . . . . . . . . . . . . . . . . . 31
17.4. Example 4: Nonce-based Background-Check Model Example . 31 16.4. Example 4: Nonce-based Background-Check Model Example . 31
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
18.1. Normative References . . . . . . . . . . . . . . . . . . 32 17.1. Normative References . . . . . . . . . . . . . . . . . . 32
18.2. Informative References . . . . . . . . . . . . . . . . . 32 17.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction 1. Introduction
In Remote Attestation Procedures (RATS), one peer (the "Attester") In Remote Attestation Procedures (RATS), one peer (the "Attester")
produces believable information about itself - Evidence - to enable a produces believable information about itself - Evidence - to enable a
remote peer (the "Relying Party") to decide whether to consider that remote peer (the "Relying Party") to decide whether to consider that
Attester a trustworthy peer or not. RATS are facilitated by an Attester a trustworthy peer or not. RATS are facilitated by an
additional vital party, the Verifier. additional vital party, the Verifier.
This documents defines a flexible architecture consisting of The Verifier appraises Evidence via Appraisal Policies and creates
attestation roles and their interactions via conceptual messages. the Attestation Results to support Relying Parties in their decision
Additionally, this document defines a universal set of terms that can process.
be mapped to various existing and emerging Remote Attestation
Procedures. Common topological models and the data flows associated This documents defines a flexible architecture with corresponding
with them, such as the "Passport Model" and the "Background-Check roles and their interaction via conceptual messages. Additionally,
Model" are illustrated. The purpose is to enable readers to map this document defines a universal set of terms that can be mapped to
their solution architecture to the canonical attestation architecture various existing and emerging Remote Attestation Procedures. Common
provided here and to define useful terminology for attestation. topological models and the data flows associated with them, such as
Having a common terminology that provides well-understood meanings the "Passport Model" and the "Background-Check Model" are
for common themes such as, roles, device composition, topological illustrated. The purpose is to enable readers of this document to
models and appraisal is vital for semantic interoperability across map their current and emerging solutions to the architecture provided
solutions and platforms involving multiple vendors and providers. and the corresponding terminology defined.
A common terminology that provides a well-understood semantic meaning
to the concepts, roles, and models in this document is vital to
create semantic interoperability between solutions and across
different platforms.
Amongst other things, this document is about trust and Amongst other things, this document is about trust and
trustworthiness. Trust is a decision being made. Trustworthiness is trustworthiness. Trust is a decision being made. Trustworthiness is
a quality that is assessed via evidence created. This is a subtle a quality that is assessed via evidence created. This is a subtle
difference and being familiar with the difference is crucial for difference and being familiar with the difference is crucial for
using this document. Additionally, the concepts of freshness and using this document. Additionally, the concepts of freshness and
trust relationships with respect to RATS are elaborated on to enable trust relationships with respect to RATS are elaborated on to enable
implementers in order to choose appropriate solutions to compose implementers in order to choose appropriate solutions to compose
their Remote Attestation Procedures. their Remote Attestation Procedures.
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Verifier evaluates the validity of information about an Attester. Verifier evaluates the validity of information about an Attester.
Compare /security policy/ in [RFC4949] Compare /security policy/ in [RFC4949]
Appraisal Policy for Attestation Result: A set of rules that direct Appraisal Policy for Attestation Result: A set of rules that direct
how a Relying Party uses the Attestation Results regarding an how a Relying Party uses the Attestation Results regarding an
Attester generated by the Verifiers. Compare /security policy/ in Attester generated by the Verifiers. Compare /security policy/ in
[RFC4949] [RFC4949]
Attestation Result: The output generated by a Verifier, typically Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier including information about an Attester, where the Verifier
vouches for the validity of the Evidence it has appraised vouches for the validity of the results
Attester: An entity (typically a device), whose Evidence must be Attester: An entity whose attributes must be appraised in order to
appraised in order to infer the extent to which the Attester is determine whether the entity is considered trustworthy, such as
considered trustworthy, such as when deciding whether it is when deciding whether the entity is authorized to perform some
authorized to perform some operation 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: Statements that Endorsers make (typically a Endorsement: A secure statement that some entity (typically a
manufacturer) that vouches for the design and implementation of manufacturer) vouches for the integrity of an Attester's signing
the Attester. Often this includes statements about the integrity capability
of an Attester's signing capability
Endorser: An entity (typically a manufacturer) whose Endorsements Endorser: An entity that creates Endorsements that can be used to
help Verifiers appraise the authenticity of Evidence help to appraise the trustworthiness of Attesters
Evidence: A set of information that asserts the trustworthiness Evidence: A set of information about an Attester that is to be
status of an Attester, that is appraised by a Verifier appraised by a Verifier
Relying Party: An entity, that depends on the validity of Relying Party: An entity that depends on the validity of information
information about an Attester, for purposes of reliably applying about another entity, typically for purposes of authorization.
application specific actions. Compare /relying party/ in Compare /relying party/ in [RFC4949]
[RFC4949]
Relying Party Owner: An entity (typically an administrator), that is Relying Party Owner: An entity, such as 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 (typically a service), that appraises the Verifier: An entity that appraises the validity of Evidence about an
validity of Evidence about an Attester and produces Attestation Attester
Results to be used by a Relying Party
Verifier Owner: An entity (typically an administrator), that is Verifier Owner: An entity, such as 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
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'------------------------------------------------------------------' '------------------------------------------------------------------'
Figure 4: Conceptual Data Flow for a Composite Device Figure 4: Conceptual Data Flow for a Composite Device
In the Composite Device, each Attester generates its own Evidence by In the Composite Device, each Attester generates its own Evidence by
its Attesting Environment(s) collecting the claims from its Target its Attesting Environment(s) collecting the claims from its Target
Environment(s). The lead Attester collects the Evidence of all other Environment(s). The lead Attester collects the Evidence of all other
Attesters and then generates the Evidence of the whole Composite Attesters and then generates the Evidence of the whole Composite
Attester. Attester.
An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. The combination of roles can
be arbitrary. For example, in this Composite Device scenario, the
entity inside the lead Attester can also take on the role of a
Verifier, and the outside entity of Verifier can take on the role of
a Relying Party. After collecting the Evidence of other Attesters,
this inside Verifier verifies them using Endorsements and Appraisal
Policies (obtained the same way as any other Verifier), to generate
Attestation Results. The inside Verifier then conveys the
Attestation Results of other Attesters, whether in the same
conveyance protocol as the Evidence or not, to the outside Verifier.
In this situation, the trust model described in Section 7 is also
suitable for this inside Verifier.
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 Results from the Verifier. There are multiple other
possible models. This section includes some reference models, but possible models. This section includes some reference models, but
this is not intended to be a restrictive list, and other variations this is not intended to be a restrictive list, and other variations
may exist. may exist.
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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
HENK VERSION
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. As a result, the entity can participate as
a constituent of the RATS architecture. Additionally, an entity can a constituent of the RATS architecture. Additionally, an entity can
aggregate more than one role into itself. These collapsed roles aggregate more than one role into itself. These collapsed roles
combine the duties of multiple roles. In these cases, interaction combine the duties of multiple roles. In these cases, interaction
between these roles do not necessarily use the Internet Protocol. between these roles do not necessarily use the Internet Protocol.
They can be using a loopback device or other IP-based communication They can be using a loopback device or other IP-based communication
between separate environments, but they do not have to. Alternative between separate environments, but they do not have to. Alternative
channels to convey conceptual messages include function calls, channels to convey conceptual messages include function calls,
sockets, GPIO interfaces, local busses, or hypervisor calls. This sockets, GPIO interfaces, local busses, or hypervisor calls. This
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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 also creates
and consumes the corresponding conceptual messages as defined in this and consumes the corresponding conceptual messages as defined in this
document. document.
7. Role Hosting and Composition 7. Trust Model
NED VERSION
The RATS architecture includes the definition of Roles (e.g.
Attester, Verifier, Relying Party, Endorser) and conceptual messages
(e.g., Evidence, Attestation Results, Endorsements, Appraisal
Policies) that captures canonical attestation behaviors, that are
common to a broad range of attestation-enabled systems. An entity
that combines multiple Roles produces and consumes the associated
Role Messages.
The RATS architecture is not prescriptive about deployment
configuration options of attestation-enabled systems, therefore the
various Roles can be hosted on any participating entity. This
implies, for a given entity, that multiple Roles could be co-resident
so that the duties of multiple roles could be performed
simultaneously. Nevertheless, the semantics of which Role Messages
are inputs and outputs to a Role entity remains constant. As a
result, the entity can participate as a constituent of the RATS
architecture while flexibly accommodating the needs of various
deployment architectures.
Interactions between Roles do not necessarily require use of Internet
protocols. They could, for example, use inter-process communication,
local system buses, shared memory, hypervisors, IP-loopback devices
or any communication path between the various environments that may
exist on the entity that combines multiple Roles.
The movement of Role Messages between locally hosted Roles is
referred to as "local conveyance". Most importantly, the definition
of local conveyance methods is out-of-scope for the RATS
architecture.
The following paragraph elaborates on an exemplary usage scenario:
In a Composite Device scenario, in addition to local entities that
host the lead Attester and other subordinate Attesters, the Composite
Device can host the Verifier role locally to appraise Evidence from
one or more subordinate Attesters. The local Verifier might convey
local Attestation Results to a remote Relying party or the Relying
Party role also could become local where an application-specific
action is taken locally. For example, a secure boot scenario
prevents system software from loading if the firmware fails to
satisfy a local trustworthiness appraisal policy.
In a multi-network scenario, a network node might bridge a wide-area
network, local-area network, and span various system buses. In so
doing, the bridge node might need to host multiple Roles depending on
the type of behavior each connected domain expects. For example, the
node might be an Attester to a wide-area network, a Verifier to the
local-area network, and a Relying Party to components attached to a
local system bus.
8. Trust Model
The scope of this document is scenarios for which a Relying Party The scope of this document is scenarios for which a Relying Party
trusts a Verifier that can appraise the trustworthiness of trusts a Verifier that can appraise the trustworthiness of
information about an Attester. Such trust might come by the Relying information about an Attester. Such trust might come by the Relying
Party trusting the Verifier (or its public key) directly, or might Party trusting the Verifier (or its public key) directly, or might
come by trusting an entity (e.g., a Certificate Authority) that is in come by trusting an entity (e.g., a Certificate Authority) that is in
the Verifier's certificate chain. The Relying Party might implicitly the Verifier's certificate chain. The Relying Party might implicitly
trust a Verifier (such as in the Verifying Relying Party trust a Verifier (such as in the Verifying Relying Party
combination). Or, for a stronger level of security, the Relying combination). Or, for a stronger level of security, the Relying
Party might require that the Verifier itself provide information Party might require that the Verifier itself provide information
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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, or operating system component or service, under the
assumption that information provided about it by the lower-layer assumption that information provided about it by the lower-layer
hypervisor or firmware is true. A stronger level of security comes hypervisor or firmware is true. A stronger level of security comes
when information can be vouched for by hardware or by ROM code, when information can be vouched for by hardware or by ROM code,
especially if such hardware is physically resistant to hardware especially if such hardware is physically resistant to hardware
tampering. The component that is implicitly trusted is often tampering. The component that is implicitly trusted is often
referred to as a Root of Trust. referred to as a Root of Trust.
A conveyance protocol that provides authentication and integrity
protection can be used to convey unprotected Evidence, assuming the
following properties exists:
1. The key material used to authenticate and integrity protect the
conveyance channel is trusted by the Verifier to speak for the
Attesting Environment(s) that collected claims about the Target
Environment(s).
2. All unprotected Evidence that is conveyed is supplied exclusively
by the Attesting Environment that has the key material that
protects the conveyance channel
3. The Root of Trust protects both the conveyance channel key
material and the Attesting Environment with equivalent strength
protections.
In some scenarios, Evidence might contain sensitive information such 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 rules that it controls. In with other authorized parties, according rules that it controls. In
the background-check model, this Evidence may also be revealed to the background-check model, this Evidence may also be revealed to
Relying Party(s). Relying Party(s).
9. Conceptual Messages 8. Conceptual Messages
8.1. Evidence
9.1. 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
protected from man-in-the-middle attackers who may observe, change or protected from man-in-the-middle attackers who may observe, change or
misdirect Evidence as it travels from Attester to Verifier. The misdirect Evidence as it travels from Attester to Verifier. The
timeliness of Evidence can be captured using claims that pinpoint the timeliness of Evidence can be captured using claims that pinpoint the
time or interval when changes in operational status, health, and so time or interval when changes in operational status, health, and so
forth occur. forth occur.
9.2. 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.
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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.
9.3. Attestation Results 8.3. Attestation Results
Attestation Results may indicate compliance or non-compliance with a Attestation Results may indicate compliance or non-compliance with a
Verifier's Appraisal Policy. A result that indicates non-compliance Verifier's Appraisal Policy. A result that indicates non-compliance
can be used by an Attester (in the passport model) or a Relying Party can be used by an Attester (in the passport model) or a Relying Party
(in the background-check model) to indicate that the Attester should (in the background-check model) to indicate that the Attester should
not be treated as authorized and may be in need of remediation. In not be treated as authorized and may be in need of remediation. In
some cases, it may even indicate that the Evidence itself cannot be some cases, it may even indicate that the Evidence itself cannot be
authenticated as being correct. authenticated as being correct.
An Attestation Result that indicates compliance can be used by a An Attestation Result that indicates compliance can be used by a
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allows a Relying Party's Appraisal Policy to be simpler, potentially allows a Relying Party's Appraisal Policy to be simpler, potentially
based on standard ways of expressing the information, while still based on standard ways of expressing the information, while still
allowing interoperability with heterogeneous devices. allowing interoperability with heterogeneous devices.
Finally, whereas Evidence is signed by the device (or indirectly by a Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer, if Endorsements are used), Attestation Results are manufacturer, if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity, rather than a larger set of entities, relationship with one entity, rather than a larger set of entities,
for purposes of its Appraisal Policy. for purposes of its Appraisal Policy.
10. 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
+-------------+ +------------+ policy +-------------+ +------------+ policy
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.--------------. TPM | | TPM .-------------------. .--------------. TPM | | TPM .-------------------.
| Attester-D |-------->| |-------->| Relying Party Y | | Attester-D |-------->| |-------->| Relying Party Y |
'--------------' '------------' `-------------------' '--------------' '------------' `-------------------'
.--------------. other ^ | other .-------------------. .--------------. other ^ | other .-------------------.
| Attester-E |------------' '----------->| Relying Party Z | | Attester-E |------------' '----------->| Relying Party Z |
'--------------' `-------------------' '--------------' `-------------------'
Figure 9: Multiple Attesters and Relying Parties with Different Figure 9: Multiple Attesters and Relying Parties with Different
Formats Formats
11. Freshness 10. Freshness
It is important to prevent replay attacks where an attacker replays It is important to prevent replay attacks where an attacker replays
old Evidence or an old Attestation Result that is no longer correct. old Evidence or an old Attestation Result that is no longer correct.
To do so, some mechanism of ensuring that the Evidence and To do so, some mechanism of ensuring that the Evidence and
Attestation Result are fresh, meaning that there is some degree of Attestation Result are fresh, meaning that there is some degree of
assurance that they still reflect the latest state of the Attester, assurance that they still reflect the latest state of the Attester,
and that any Attestation Result was generated using the latest and that any Attestation Result was generated using the latest
Appraisal Policy for Evidence. There is, however, always a race Appraisal Policy for Evidence. There is, however, always a race
condition possible in that the state of the Attester, and the condition possible in that the state of the Attester, and the
Appraisal Policy for Evidence, might change immediately after the Appraisal Policy for Evidence, might change immediately after the
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requires additional claims about the signer's time synchronization requires additional claims about the signer's time synchronization
mechanism in order to provide such assurance. mechanism in order to provide such assurance.
In either approach, it is important to note that the actual values in In either approach, it is important to note that the actual values in
claims might have been generated long before the claims are signed. claims might have been generated long before the claims are signed.
If so, it is the signer's responsibility to ensure that the values If so, it is the signer's responsibility to ensure that the values
are still correct when they are signed. For example, values might are still correct when they are signed. For example, values might
have been generated at boot, and then used in claims as long as the have been generated at boot, and then used in claims as long as the
signer can guarantee that they cannot have changed since boot. signer can guarantee that they cannot have changed since boot.
A more detailed discussion with examples appears in Section 17. A more detailed discussion with examples appears in Section 16.
12. 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. In many cases, the whole point of the Attestation process is
to provide reliable information about the type of the device and the to provide reliable information about the type of the device and the
firmware/software that the device is running. This information might firmware/software that the device is running. This information might
be particularly interesting to many attackers. For example, knowing be particularly interesting to many attackers. For example, knowing
that a device is running a weak version of firmware provides a way to that a device is running a weak version of firmware provides a way to
aim attacks better. aim attacks better.
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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
Verifiers. Some Attesters might want a stronger level of assurance Verifiers. Some Attesters might want a stronger level of assurance
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.
13. 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, needs to support end-to-
end integrity protection and replay attack prevention, and often also end integrity protection and replay attack prevention, and often also
needs to support additional security protections. For example, needs to support additional security protections. For example,
additional means of authentication, confidentiality, integrity, additional means of authentication, confidentiality, integrity,
replay, denial of service and privacy protection are needed in many replay, denial of service and privacy protection are needed in many
use cases. Section 11 discusses ways in which freshness can be used use cases. Section 10 discusses ways in which freshness can be used
in this architecture to protect against replay attacks. in this architecture to protect against replay attacks.
To assess the security provided by a particular Appraisal Policy, it To assess the security provided by a particular Appraisal Policy, it
is important to understand the strength of the Root of Trust, e.g., is important to understand the strength of the Root of Trust, e.g.,
whether it is mutable software, or firmware that is read-only after whether it is mutable software, or firmware that is read-only after
boot, or immutable hardware/ROM. boot, or immutable hardware/ROM.
It is also important that the Appraisal Policy was itself obtained It is also important that the Appraisal Policy was itself obtained
securely. As such, if Appraisal Policies for a Relying Party or for securely. As such, if Appraisal Policies for a Relying Party or for
a Verifier can be configured via a network protocol, the ability to a Verifier can be configured via a network protocol, the ability to
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The security of conveyed information may be applied at different The security of conveyed information may be applied at different
layers, whether by a conveyance protocol, or an information encoding layers, whether by a conveyance protocol, or an information encoding
format. This architecture expects attestation messages (i.e., format. This architecture expects attestation messages (i.e.,
Evidence, Attestation Results, Endorsements and Policies) are end-to- Evidence, Attestation Results, Endorsements and Policies) are end-to-
end protected based on the role interaction context. For example, if end protected based on the role interaction context. For example, if
an Attester produces Evidence that is relayed through some other an Attester produces Evidence that is relayed through some other
entity that doesn't implement the Attester or the intended Verifier entity that doesn't implement the Attester or the intended Verifier
roles, then the relaying entity should not expect to have access to roles, then the relaying entity should not expect to have access to
the Evidence. the Evidence.
14. IANA Considerations 13. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
15. 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. Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
16. 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.
17. 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 |
+====+============+===============================================+ +====+============+===============================================+
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| | | generated it | | | | generated it |
+----+------------+-----------------------------------------------+ +----+------------+-----------------------------------------------+
Table 1 Table 1
We now walk through a number of hypothetical examples of how a We now walk through a number of hypothetical examples of how a
solution might be built. This list is not intended to be complete, solution might be built. This list is not intended to be complete,
but is just representative enough to highlight various timing but is just representative enough to highlight various timing
considerations. considerations.
17.1. 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 |
'----------' '----------' '---------------' '----------' '----------' '---------------'
time(VG) | | time(VG) | |
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account for the maximum permitted clock skew between the Relying account for the maximum permitted clock skew between the Relying
Party and the Verifier. The result might then be used for some time Party and the Verifier. The result might then be used for some time
(e.g., throughout the lifetime of a connection established at (e.g., throughout the lifetime of a connection established at
time(RA)). The Relying Party must be careful, however, to not allow time(RA)). The Relying Party must be careful, however, to not allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain fresh enough. Thus, it might allow use (at Result to remain fresh enough. Thus, it might allow use (at
time(OP)) as long as "time(OP) - time(RG) < Threshold". However, if time(OP)) as long as "time(OP) - time(RG) < Threshold". However, if
the Attestation Result contains an expiry time time(RX) then it could the Attestation Result contains an expiry time time(RX) then it could
explicitly check "time(OP) < time(RX)". explicitly check "time(OP) < time(RX)".
17.2. Example 2: Nonce-based Passport Model Example 16.2. Example 2: Nonce-based Passport Model Example
The following example illustrates a hypothetical Passport Model The following example illustrates a hypothetical Passport Model
solution that uses nonces and thus does not require that any clocks solution that uses nonces and thus does not require that any clocks
are synchronized. are synchronized.
.----------. .----------. .---------------. .----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party | | Attester | | Verifier | | Relying Party |
'----------' '----------' '---------------' '----------' '----------' '---------------'
time(VG) | | time(VG) | |
| | | | | |
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time(EG) values cannot be inside the Attestation Result, they might time(EG) values cannot be inside the Attestation Result, they might
be signed by the Attester such that the Attestation Result vouches be signed by the Attester such that the Attestation Result vouches
for the Attester's signing capability. for the Attester's signing capability.
The Relying Party must still be careful, however, to not allow The Relying Party must still be careful, however, to not allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX)-time(RG), then the Relying validity lifetime in terms of time(RX)-time(RG), then the Relying
Party can check "time(OP) - time(NS') < time(RX)-time(RG)". Party can check "time(OP) - time(NS') < time(RX)-time(RG)".
17.3. Example 3: Timestamp-based Background-Check Model Example 16.3. Example 3: Timestamp-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check The following example illustrates a hypothetical Background-Check
Model solution that uses timestamps and requires roughly synchronized Model solution that uses timestamps and requires roughly synchronized
clocks between the Attester, Verifier, and Relying Party. clocks between the Attester, Verifier, and Relying Party.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------' '----------' '---------------' '----------'
time(VG) | | time(VG) | |
| | | | | |
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| | time(RG) | | time(RG)
| time(RA)<-Attestation Result---| | time(RA)<-Attestation Result---|
| | {time(RX)} | | | {time(RX)} |
~ ~ ~ ~ ~ ~
| | | | | |
| time(OP) | | time(OP) |
The time considerations in this example are equivalent to those The time considerations in this example are equivalent to those
discussed under Example 1 above. discussed under Example 1 above.
17.4. Example 4: Nonce-based Background-Check Model Example 16.4. Example 4: Nonce-based Background-Check Model Example
The following example illustrates a hypothetical Background-Check The following example illustrates a hypothetical Background-Check
Model solution that uses nonces and thus does not require that any Model solution that uses nonces and thus does not require that any
clocks are synchronized. In this example solution, a nonce is clocks are synchronized. In this example solution, a nonce is
generated by a Verifier at the request of a Relying Party, when the generated by a Verifier at the request of a Relying Party, when the
Relying Party needs to send one to an Attester. Relying Party needs to send one to an Attester.
.----------. .---------------. .----------. .----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier | | Attester | | Relying Party | | Verifier |
'----------' '---------------' '----------' '----------' '---------------' '----------'
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However, unlike in Example 2, the Relying Party can use the Nonce to However, unlike in Example 2, the Relying Party can use the Nonce to
determine whether the Attestation Result is fresh, by verifying that determine whether the Attestation Result is fresh, by verifying that
"time(OP) - time(NR) < Threshold". "time(OP) - time(NR) < Threshold".
The Relying Party must still be careful, however, to not allow The Relying Party must still be careful, however, to not allow
continued use beyond the period for which it deems the Attestation continued use beyond the period for which it deems the Attestation
Result to remain valid. Thus, if the Attestation Result sends a Result to remain valid. Thus, if the Attestation Result sends a
validity lifetime in terms of time(RX)-time(RG), then the Relying validity lifetime in terms of time(RX)-time(RG), then the Relying
Party can check "time(OP) - time(ER) < time(RX)-time(RG)". Party can check "time(OP) - time(ER) < time(RX)-time(RG)".
18. References 17. References
18.1. Normative References 17.1. Normative References
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>. <https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>. May 2018, <https://www.rfc-editor.org/info/rfc8392>.
18.2. 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>.
[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/>.
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