draft-ietf-rats-architecture-01.txt   draft-ietf-rats-architecture-02.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: 7 August 2020 Microsoft Expires: 8 September 2020 Microsoft
M. Richardson M. Richardson
Sandelman Software Works Sandelman Software Works
N. Smith N. Smith
Intel Intel
4 February 2020 W. Pan
Huawei Technologies
7 March 2020
Remote Attestation Procedures Architecture Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-01 draft-ietf-rats-architecture-02
Abstract Abstract
In network protocol exchanges, it is often the case that one entity In network protocol exchanges, it is often the case that one entity
(a Relying Party) requires evidence about a remote peer to assess the (a Relying Party) requires evidence about a remote peer to assess the
peer's trustworthiness, and a way to appraise such evidence. The peer's trustworthiness, and a way to appraise such evidence. The
evidence is typically a set of claims about its software and hardware evidence is typically a set of claims about its software and hardware
platform. This document describes an architecture for such remote platform. This document describes an architecture for such remote
attestation procedures (RATS). attestation procedures (RATS).
skipping to change at page 2, line 4 skipping to change at page 2, line 6
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-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
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 7 August 2020.
This Internet-Draft will expire on 8 September 2020.
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.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4 3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 4 3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5
4.1. Composite Attester . . . . . . . . . . . . . . . . . . . 5 3.2. Confidential Machine Learning (ML) Model Protection . . . 5
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 7 3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 7 3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 8 3.5. Trusted Execution Environment (TEE) Provisioning . . . . 6
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 9 3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7
6. Two Types of Environments of an Attester . . . . . . . . . . 10 4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1. Two Types of Environments of an Attester . . . . . . . . 9
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 11 4.2. Layered Attestation Procedures . . . . . . . . . . . . . 9
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3. Composite Device . . . . . . . . . . . . . . . . . . . . 12
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 12 5. Topological Models . . . . . . . . . . . . . . . . . . . . . 14
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 13 5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 14
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 13 5.2. Background-Check Model . . . . . . . . . . . . . . . . . 15
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 16
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16 6. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 17
12. Security Considerations . . . . . . . . . . . . . . . . . . . 16 7. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 18
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 7.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 18
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 7.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 19
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17 7.3. Attestation Results . . . . . . . . . . . . . . . . . . . 19
16. Informative References . . . . . . . . . . . . . . . . . . . 17 8. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 9. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
15.1. Normative References . . . . . . . . . . . . . . . . . . 24
15.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
<more text to be added here> In Remote Attestation Procedures (RATS), one peer (the "Attester")
Remote Attestation, as used in this document, is a process by which produces believable information about itself - Evidence - to enable a
one entity (the "Attester") provides evidence about its identity and remote peer (the "Relying Party") to decide whether to consider that
state to another remote entity (the "Relying Party"), which then Attester a trustworthy peer or not. RATS are facilitated by an
assesses the Attester's trustworthiness for the Relying Party's own additional vital party, the Verifier. The Verifier appraises
purposes. Evidence via Appraisal Policies and creates the Attestation Results
to support Relying Parties in their decision process.
This documents defines a flexible architecture with corresponding
roles and their interaction via conceptual messages. Additionally,
this document defines a universal set of terms that can be mapped to
various existing and emerging Remote Attestation Procedures. Common
role compositions and data flows, such as the "Passport Model" and
the "Background-Check Model" are illustrated to enable readers of
this document to map their current and emerging solutions to the
architecture provided and the corresponding terminology defined. A
common terminology that provides a well-understood semantic meaning
to the concepts, roles, and models in this document is vital to
create semantic interoperability between solutions and across
different platforms.
Amongst other things, this document is about trust and
trustworthiness. Trust is a decision being made. Trustworthiness is
a quality that is assessed via evidence created. This is a subtle
difference and being familiar with the difference is crucial for
using this document. Additionally, the concepts of freshness and
trust relationships with respect to RATS are elaborated on to enable
implementers in order to choose appropriate solutions to compose
their Remote Attestation Procedures.
2. Terminology 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 direct 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 Appraisal Policy for Attestation Result: A set of rules that direct
direct how a Relying Party uses the evaluation results about 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 evaluation results generated by a Attestation Result: The output generated by a Verifier, typically
Verifier, typically including information about an Attester, where including information about an Attester, where the Verifier
the Verifier vouches for the validity of the results. vouches for the validity of the results
* Attester: An entity whose attributes must be evaluated in order to Attester: An entity whose attributes must be appraised in order to
determine whether the entity is considered trustworthy, such as determine whether the entity is considered trustworthy, such as
when deciding whether the entity is authorized to perform some when deciding whether the entity is authorized to perform some
operation. operation
* 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 that creates Endorsements that can be used to
help evaluate trustworthiness of Attesters. help to appraise the trustworthiness of Attesters
* Evidence: A set of information about an Attester that is to be Evidence: A set of information about an Attester that is to be
evaluated 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 another entity, typically for purposes of about another entity, typically for purposes of authorization.
authorization. Compare /relying party/ in [RFC4949]. Compare /relying party/ in [RFC4949]
* Relying Party Owner: An entity, such as 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 that evaluates the validity of Evidence about Verifier: An entity that appraises the validity of Evidence about an
an Attester. Attester
* Verifier Owner: An entity, such as 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
<unclear if the WG wants this section in the arch doc> This section covers a number of representative use cases for remote
attestation, independent of specific solutions. The purpose is to
provide motivation for various aspects of the architecture presented
in this draft. Many other use cases exist, and this document does
not intend to have a complete list, only to have a set of use cases
that collectively cover all the functionality required in the
architecture.
Each use case includes a description, and a summary of what an
Attester and a Relying Party refer to in the use case.
3.1. Network Endpoint Assessment
Network operators want a trustworthy report of identity and version
of information of the hardware and software on the machines attached
to their network, for purposes such as inventory, auditing, and/or
logging. The network operator may also want a policy by which full
access is only granted to devices that meet some definition of
health, and so wants to get claims about such information and verify
their validity. Remote attestation is desired to prevent vulnerable
or compromised devices from getting access to the network and
potentially harming others.
Typically, solutions start with a specific component (called a "Root
of Trust") that provides device identity and protected storage for
measurements. These components perform a series of measurements, and
express this with Evidence as to the hardware and firmware/software
that is running.
FIXME from Henk: Measurements at early stages of
Layered Attestation are NOT evidence yet.
This text does not cover that yet
Attester: A device desiring access to a network
Relying Party: A network infrastructure device such as a router,
switch, or access point
3.2. Confidential Machine Learning (ML) Model Protection
A device manufacturer wants to protect its intellectual property in
terms of the ML model it developed and that runs in the devices that
its customers purchased, and it wants to prevent attackers,
potentially including the customer themselves, from seeing the
details of the model.
This typically works by having some protected environment in the
device attest to some manufacturer service. If remote attestation
succeeds, then the manufacturer service releases either the model, or
a key to decrypt a model the Attester already has in encrypted form,
to the requester.
Attester: A device desiring to run an ML model to do inferencing
Relying Party: A server or service holding ML models it desires to
protect
3.3. Confidential Data Retrieval
This is a generalization of the ML model use case above, where the
data can be any highly confidential data, such as health data about
customers, payroll data about employees, future business plans, etc.
Attestation is desired to prevent leaking data to compromised
devices.
Attester: An entity desiring to retrieve confidential data
Relying Party: An entity that holds confidential data for retrieval
by other entities
3.4. Critical Infrastructure Control
In this use case, potentially dangerous physical equipment (e.g.,
power grid, traffic control, hazardous chemical processing, etc.) is
connected to a network. The organization managing such
infrastructure needs to ensure that only authorized code and users
can control such processes, and they are protected from malware or
other adversaries. When a protocol operation can affect some
critical system, the device attached to the critical equipment thus
wants some assurance that the requester has not been compromised. As
such, remote attestation can be used to only accept commands from
requesters that are within policy.
Attester: A device or application wishing to control physical
equipment
Relying Party: A device or application connected to potentially
dangerous physical equipment (hazardous chemical processing,
traffic control, power grid, etc.)
3.5. Trusted Execution Environment (TEE) Provisioning
A "Trusted Application Manager (TAM)" server is responsible for
managing the applications running in the TEE of a client device. To
do this, the TAM wants to assess the state of a TEE, or of
applications in the TEE, of a client device. The TEE attests to the
TAM, which can then decide whether the TEE is already in compliance
with the TAM's latest policy, or if the TAM needs to uninstall,
update, or install approved applications in the TEE to bring it back
into compliance with the TAM's policy.
Attester: A device with a trusted execution environment capable of
running trusted applications that can be updated
Relying Party: A Trusted Application Manager
3.6. Hardware Watchdog
One significant problem is malware that holds a device hostage and
does not allow it to reboot to prevent updates to be applied. This
is a significant problem, because it allows a fleet of devices to be
held hostage for ransom.
A hardware watchdog can be implemented by forcing a reboot unless
remote attestation to a server succeeds within a periodic interval,
and having the reboot do remediation by bringing a device into
compliance, including installation of patches as needed.
Attester: The device that is desired to keep from being held hostage
for a long period of time
Relying Party: A remote server that will securely grant the Attester
permission to continue operating (i.e., not reboot) for a period
of time
4. Architectural Overview 4. Architectural Overview
Figure 1 depicts the data that flows between different roles, Figure 1 depicts the data that flows between different roles,
independent of protocol or use case. independent of protocol or use case.
************ ************ ***************** ************ ************ *****************
* Endorser * * Verifier * * Relying Party * * Endorser * * Verifier * * Relying Party *
************ * Owner * * Owner * ************ * Owner * * Owner *
| ************ ***************** | ************ *****************
skipping to change at page 5, line 13 skipping to change at page 9, line 5
obtained from an Endorser along with the Endorsements, or might be obtained from an Endorser along with the Endorsements, or might be
obtained via some other mechanism such as being configured in the obtained via some other mechanism such as being configured in the
Verifier by an administrator. Verifier by an administrator.
The Relying Party uses Attestation Results by applying its own The Relying Party uses Attestation Results by applying its own
Appraisal Policy to make application-specific decisions such as Appraisal Policy to make application-specific decisions such as
authorization decisions. The Attestation Result Appraisal Policy authorization decisions. The Attestation Result Appraisal Policy
might, for example, be configured in the Relying Party by an might, for example, be configured in the Relying Party by an
administrator. administrator.
4.1. Composite Attester 4.1. Two Types of Environments of an Attester
A Composite Attester is an entity composed of multiple sub-entities An Attester consists of at least one Attesting Environment and at
such that its trustworthiness has to be determined by evaluating all least one Target Environment. In some implementations, the Attesting
these sub-entities. Each sub-entity has at least one Attesting and Target Environments might be combined. Other implementations
Environment collecting the claims from at least one Target might have multiple Attesting and Target Environments, such as in the
Environment, then this sub-entity generates Evidence about its examples described in more detail in Section 4.2 and Section 4.3.
trustworthiness. Therefore each sub-entity can be called an Other examples may exist, and the examples discussed could even be
Attester. Among these Attesters, there may be only some, which can combined into even more complex implementations.
be called Lead Attesters, that have the ability to communicate with
the Verifier. Other Attesters don't have this ability, but they are
connected to the Lead Attesters via internal links or network
connections, and they are evaluated via the Lead Attester's help.
For example, a carrier-grade router is a composite device consisting Claims are collected from Target Environments. That is, Attesting
of a chassis and multiple slots. The trustworthiness of the router Environments collect the raw values and the information to be
depends on all its slots' trustworthiness. Each slot has an represented in claims, such as by doing some measurement of a Target
Attesting Environment such as a TPM or TEE collecting the claims of Environment's code, memory, and/or registers. Attesting Environments
its boot process, after which it generates Evidence from the claims. then format the claims appropriately, and typically use key material
Among these slots, only a main slot can communicate with the Verifier and cryptographic functions, such as signing or cipher algorithms, to
while other slots cannot. But other slots can communicate with the create Evidence. Examples of environments that can be used as
main slot by the links between them inside the router. So the main Attesting Environments include Trusted Execution Environments (TEE),
slot collects the Evidence of other slots, produces the final embedded Secure Elements (eSE), or Hardware Security Modules (HSM).
Evidence of the whole router and conveys the final Evidence to the
Verifier. Therefore the router is a Composite Attester, each slot is 4.2. Layered Attestation Procedures
an Attester, and the main slot is the Lead Attester.
By definition, the Attester role takes on the duty to create
Evidence. The fact that an Attester role is composed of several
types of environments that can be nested or staged adds complexity to
the architectural layout of how an Attester - in itself - is composed
and therefore has to conduct the Claims collection in order to create
believable Attestation Evidence. The following example is intended
to illustrate this composition:
A very common example is elaborated on to illustrate Layered
Attestation.
.----------. .----------.
| | | |
| Endorser |------------------->| Verifier |
| | Endorsements | |
'----------' for A, B, and C '----------'
^
.------------------------------------. |
| | |
| .---------------------------. | |
| | Target | | | Layered
| | Environment | | | Evidence
| | C | | | for
| '---------------------------' | | B and C
| Collect | | |
| claims | | |
| .---------------|-----------. | |
| | Target v | | |
| | Environment .-----------. | | |
| | B | Attesting | | | |
| | |Environment|-----------'
| | | B | | |
| | '-----------' | |
| | ^ | |
| '---------------------|-----' |
| Collect | | Evidence |
| claims v | for B |
| .-----------. |
| | Attesting | |
| |Environment| |
| | A | |
| '-----------' |
| |
'------------------------------------'
Figure 2: Layered Attester
The very first Attesting Environment has to ensure the integrity of
the (U)EFI / BIOS / Firmware that initially boots up a composite
device (e.g., a cell phone).
Henk: we are looking for a better term than UEFI/BIOS/Firmware
These Claims have to be measured securely. At this stage of the
boot-cycle of a composite device, the Claims collected typically
cannot be composed into Evidence.
The very first Attesting Environment in this example can be a
hardware component that is a Static Code Root of Trust. As in any
other scenario, this hardware component is the first Attesting
Environment. It collects a rather concise number of Claims about the
Target Environment. The Target Environment in this example is the
(U)EFI / BIOS / Firmware After the boot sequence started, the Target
Environment conducts the most important and defining feature of
Layered Attestation: The successfully measured environment that is
the (U)EFI / BIOS / Firmware now becomes the Attesting Environment.
Analogously, the Attesting Environment hands off its duty to one of
its Target Environments. This procedure in Layered Attestation is
called Staging.
Now, the duties have been transferred and Layered Attestation takes
place. The initial Attesting Environment relinquishes its duties to
the Target Environment. It is important to note that the new
Attesting Environment cannot alter the content about its own
measurements. If the Attesting Environment would be able to do that,
Layered Attestation would become unfeasible.
In this example the duty of being the Attesting Environment is now
taken over by the (U)EFI / BIOS / Firmware that was the Attested
Environment before. This transfer of duty is the essential part of
Layered Attestation. The (U)EFI / BIOS / Firmware now is the
Attesting Environment. The next Target Environment is, in this
example, a bootloader. There are potentially multiple kernels to
boot, the decision is up to the bootloader. Only a bootloader with
intact integrity will make an appropriate decision. Therefore,
Claims about the integrity of a bootloader are now collected by the
freshly appointed Attesting Environment that is the (U)EFI / BIOS /
Firmware. Collected Claims have to be stored by the current
Attesting Environment in a similar shielded and secured manner, so
that the next Attesting Environment is not capable of altering the
collection of claims stored.
Continuing with this example, the bootloader is now in charge of
collecting Claims about the next execution environment. The next
execution environment in this example is the kernel to be booted up.
Analogously, the next transfer of duties in this Layered Attestation
example occurs: The duty of being an Attesting Environment is
transferred to a successfully measured kernel. In this sequence, the
kernel is now collecting additional Claims and is storing them in a
secure and shielded manner.
[Henk: we might have to define what successful
means in this example and beyond]
The essence of this example is a cascade of staged boot environments.
Each environment (after the initial one that is a root-of-trust) has
the duty of measuring its next environment before it is started.
Therefore, creating a layered boot sequence and correspondingly
enabling Layered Attestation.
4.3. Composite Device
A Composite Device is an entity composed of multiple sub-entities
such that its trustworthiness has to be determined by the appraisal
of all these sub-entities.
Each sub-entity has at least one Attesting Environment collecting the
claims from at least one Target Environment, then this sub-entity
generates Evidence about its trustworthiness. Therefore each sub-
entity can be called an Attester. Among all the Attesters, there may
be only some which have the ability to communicate with the Verifier
while others do not.
For example, a carrier-grade router is consists of a chassis and
multiple slots. The trustworthiness of the router depends on all its
slots' trustworthiness. Each slot has an Attesting Environment such
as a TEE collecting the claims of its boot process, after which it
generates Evidence from the claims. Among these slots, only a main
slot can communicate with the Verifier while other slots cannot. But
other slots can communicate with the main slot by the links between
them inside the router. So the main slot collects the Evidence of
other slots, produces the final Evidence of the whole router and
conveys the final Evidence to the Verifier. Therefore the router is
a Composite Device, each slot is an Attester, and the main slot is
the transiting 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 simpler management throughput by interconnecting multiple routers and can be logically
by being logically treated as one router. Among these routers, there treated as one router for simpler management. Among these routers,
is only one main router that connects to the Verifier. Other routers there is only one main router that connects to the Verifier. Other
are only connected to the main router by the network cables, and routers are only connected to the main router by the network cables,
therefore they are managed and verified via this main router. So, in and therefore they are managed and appraised via this main router's
this case, the multi-chassis router is the Composite Attester, each help. So, in this case, the multi-chassis router is the Composite
router is an Attester and the main router is the Lead Attester. Device, each router is an Attester and the main router is the lead
Attester.
Figure 2 depicts the conceptual data flow for a Composite Attester. Figure 3 depicts the conceptual data flow for a Composite Device.
.-----------------------------. .-----------------------------.
| Verifier | | Verifier |
'-----------------------------' '-----------------------------'
^ ^
| |
| Composite | Evidence of
| Evidence | Composite Device
| |
.----------------------------------|-------------------------------. .----------------------------------|-------------------------------.
| .--------------------------------|-----. .------------. | | .--------------------------------|-----. .------------. |
| | .------------. | | | | | | Collect .------------. | | | |
| | | Attesting |<--------| Attester B |-. | | | Claims .--------->| Attesting |<--------| Attester B |-. |
| | |Environment | | '------------. | | | | | |Environment | | '------------. | |
| | .----------------. | |<----------| Attester C |-. | | | .----------------. | |<----------| Attester C |-. |
| | | Target | | | | '------------' | | | | | Target | | | | '------------' | |
| | | Environment(s) | | |<------------| ... | | | | | Environment(s) | | |<------------| ... | |
| | | | '------------' | Evidence '------------' | | | | | '------------' | Evidence '------------' |
| | | | ^ | of | | | '----------------' | of |
| | | |------------/ | Attesters | | | | Attesters |
| | '----------------' Collecting | (via Internal Links or | | | lead Attester A | (via Internal Links or |
| | Claims | Network Connections) | | '--------------------------------------' Network Connections) |
| | | |
| | Lead Attester A | |
| '--------------------------------------' |
| | | |
| Device/Composite Device/Attester/TBD #33 | | Composite Device |
'------------------------------------------------------------------' '------------------------------------------------------------------'
Figure 2: Conceptual Data Flow for a Composite Attester Figure 3: Conceptual Data Flow for a Composite Device
In the Composite Attester, each Attester generates its own Evidence In the Composite Device, each Attester generates its own Evidence by
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.
The Lead Attester's Attesting Environment may or may not include its An entity can take on multiple RATS roles (e.g., Attester, Verifier,
own Verifier. One situation is that the Attesting Environment has no Relying Party, etc.) at the same time. The combination of roles can
internal Verifier. In this situation, the Lead Attesting Environment be arbitrary. For example, in this Composite Device scenario, the
simply combines the various Evidences into the final Evidence that is entity inside the lead Attester can also take on the role of a
sent off to the remote Verifier, which evaluates the Composite Verifier, and the outside entity of Verifier can take on the role of
Attester's, including the Lead Attester's and other Attesters', a Relying Party. After collecting the Evidence of other Attesters,
trustworthiness. this inside Verifier verifies them using Endorsements and Appraisal
Policies (obtained the same way as any other Verifier), to generate
Attestation Results. The inside Verifier then sends the Attestation
Results of other Attesters, whether in the same conveyance protocol
as the Evidence or not, to the outside Verifier.
The other situation is that the Lead Attesting Environment has an In this situation, the trust model described in Section 6 is also
internal Verifier. After collecting the Evidence of other Attesters, suitable for this inside Verifier.
this Attesting Environment verifies them using Endorsements and
Appraisal Policies (obtained the same way as any other Verifier), for
evaluating these Attesters' trustworthiness. Then the Lead Attesting
Environment combines the Attestation Results into the final Evidence
of the whole Composite Attester which is sent off to the remote
Verifier, which might treat the claims obtained from the local
Attestation Results as if they were Evidence.
5. Topological Models 5. Topological Models
There are multiple possible models for communication between an There are multiple possible models for communication between an
Attester, a Verifier, and a Relying Party. This section includes Attester, a Verifier, and a Relying Party. This section includes
some reference models, but this is not intended to be a restrictive some reference models, but this is not intended to be a restrictive
list, and other variations may exist. list, and other variations may exist.
5.1. Passport Model 5.1. Passport Model
In this model, an Attester sends Evidence to a Verifier, which In this model, an Attester sends 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
Verifier may refuse to issue the Attestation Result due to some error
in processing, or some missing input to the Verifier. The second way
in which the process may fail is when the resulting Result is
examined by the Relying Party, and based upon the Appraisal Policy,
the result does not pass the policy. The third way is when the
Verifier is unreachable.
Since the resource access protocol between the Attester and Relying 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 attestation protocol. constrained by the Attester-Verifier remote attestation protocol.
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal Policy | Verifier | against Appraisal Policy
| | | |
+-------------+ +-------------+
^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+-------------+ +-------------+ +----------+ +---------+
| |-------------->| | Compare Attestation | |------------->| |Compare Attestation
| Attester | Attestation | Relying | Result against | Attester | Attestation | Relying | Result against
| | Result | Party | Appraisal Policy | | Result | Party | Appraisal
+-------------+ +-------------+ +----------+ +---------+ Policy
Figure 3: Passport Model Figure 4: Passport Model
The passport model is so named because of its resemblance to how The passport model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity claim, such as an when it needs to assert a citizenship or identity claim, such as an
airport immigration desk. The passport is considered sufficient airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity claims, and it is because it vouches for the citizenship and identity claims, and it is
issued by a trusted authority. Thus, in this immigration desk issued by a trusted authority. Thus, in this immigration desk
analogy, the passport issuing agency is a Verifier, the passport is analogy, the passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party. an Attestation Result, and the immigration desk is a Relying Party.
5.2. Background-Check Model 5.2. Background-Check Model
In this model, an Attester sends Evidence to a Relying Party, which In this model, an Attester sends 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 security policy. Attestation Result against its own appraisal policy.
The resource access protocol between the Attester and Relying Party The resource access protocol between the Attester and Relying Party
includes Evidence rather than an Attestation Result, but that includes Evidence rather than an Attestation Result, but that
Evidence is not processed by the Relying Party. Since the Evidence Evidence is not processed by the Relying Party. Since the Evidence
is merely forwarded on to a trusted Verifier, any serialization is merely forwarded on to a trusted Verifier, any serialization
format can be used for Evidence because the Relying Party does not format can be used for Evidence because the Relying Party does not
need a parser for it. The only requirement is that the Evidence can need a parser for it. The only requirement is that the Evidence can
be _encapsulated in_ the format required by the resource access be _encapsulated in_ the format required by the resource access
protocol between the Attester and Relying Party. protocol between the Attester and Relying Party.
skipping to change at page 9, line 5 skipping to change at page 16, line 5
consumed by the Relying Party and so the serialization format of the consumed by the Relying Party and so the serialization format of the
Attestation Result is still important. If the Relying Party is a Attestation Result is still important. If the Relying Party is a
constrained node whose purpose is to serve a given type resource constrained node whose purpose is to serve a given type resource
using a standard resource access protocol, it already needs the using a standard resource access protocol, it already needs the
parser(s) required by that existing protocol. Hence, the ability to parser(s) required by that existing protocol. Hence, the ability to
let the Relying Party obtain an Attestation Result in the same let the Relying Party obtain an Attestation Result in the same
serialization format allows minimizing the code footprint and attack serialization format allows minimizing the code footprint and attack
surface area of the Relying Party, especially if the Relying Party is surface area of the Relying Party, especially if the Relying Party is
a constrained node. a constrained node.
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal Policy | Verifier | against Appraisal
| | | | Policy
+-------------+ +-------------+
^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+-------------+ +-------------+ +------------+ +-------------+
| |-------------->| | Compare Attestation | |-------------->| | Compare Attestation
| Attester | Evidence | Relying | Result against | Attester | Evidence | Relying | Result against
| | | Party | Appraisal Policy | | | Party | Appraisal Policy
+-------------+ +-------------+ +------------+ +-------------+
Figure 4: Background-Check Model Figure 5: Background-Check Model
The background-check model is so named because of the resemblance of The background-check model is so named because of the resemblance of
how employers and volunteer organizations perform background checks. how employers and volunteer organizations perform background checks.
When a prospective employee provides claims about education or When a prospective employee provides claims about education or
previous experience, the employer will contact the respective previous experience, the employer will contact the respective
institutions or former employers to validate the claim. Volunteer institutions or former employers to validate the claim. Volunteer
organizations often perform police background checks on volunteers in organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and a former employer or government agency that the Relying Party, and a former employer or government agency that
issues a report is a Verifier. issues a report is a Verifier.
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 on the same machine, and so there is no need
for a protocol between the two. for a protocol between the two.
It is also worth pointing out that the choice of model is generally It is also worth pointing out that the choice of model is generally
up to the Relying Party, and the same device may need to attest to up to the Relying Party, and the same device may need to create
different Relying Parties for different use cases (e.g., a network Evidence for different Relying Parties and different use cases (e.g.,
infrastructure device to gain access to the network, and then a a network infrastructure device to gain access to the network, and
server holding confidential data to get access to that data). As then a server holding confidential data to get access to that data).
such, both models may simultaneously be in use by the same device. As such, both models may simultaneously be in use by the same device.
Figure 5 shows another example of a combination where Relying Party 1 Figure 6 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 4, Relying Party 2 actually provides functionality shown in Figure 5, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the use it with other Relying Parties. This is the model that the
Trusted Application Manager plans to support in the TEEP architecture Trusted Application Manager plans to support in the TEEP architecture
[I-D.ietf-teep-architecture]. [I-D.ietf-teep-architecture].
+-------------+ +-------------+
| | Compare Evidence | | Compare Evidence
| Verifier | against Appraisal Policy | Verifier | against Appraisal Policy
| | | |
+-------------+ +-------------+
skipping to change at page 10, line 31 skipping to change at page 17, line 31
^ | ^ |
Evidence| |Attestation Evidence| |Attestation
| | Result | | Result
| v | v
+----------+ +----------+ +----------+ +----------+
| |-------------->| | Compare Attestation | |-------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against | Attester | Attestation | Relying | Result against
| | Result | Party 1 | Appraisal Policy | | Result | Party 1 | Appraisal Policy
+----------+ +----------+ +----------+ +----------+
Figure 5: Example Combination Figure 6: Example Combination
6. Two Types of Environments of an Attester
An Attester consists of at least one Attesting Environment and at
least one Target Environment. In some implementations, the Attesting
and Target Environments might be combined. Other implementations
might have multiple Attesting and Target Environments. One example
is a set of components in a boot sequence (e.g., ROM, firmware, OS,
and application) where a Target Environment is the Attesting
Environment for the next environment in the boot sequence.
Claims are collected from Target Environments. That is, Attesting
Environments collect the raw values and the information to be
represented in claims. Attesting Environments then format them
appropriately, and typically use key material and cryptographic
functions, such as signing or cipher algorithms, to create Evidence.
Examples of environments that can be used as Attesting Environments
include Trusted Execution Environments (TEE), embedded Secure
Elements (eSE), or Hardware Security Modules (HSM).
7. Trust Model 6. 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 evaluate 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
about itself that the Relying Party can use to evaluate the 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.
In solutions following the background-check model, the Attester is The Endorser and Verifier Owner may need to trust the Verifier before
assumed to trust the Verifier (again, whether directly or indirectly giving the Endorsement and Appraisal Policy to it. Such trust can
via a Certificate Authority that it trusts), since the Attester also be established directly or indirectly, implicitly or explicitly.
relies on an Attestation Result it obtains from the Verifier, in One explicit way to establish such trust may be the Verifier first
order to access resources. acts as an Attester and creates Evidence about itself to be consumed
by the Endorser and/or Verifier Owner as the Relying Parties. If it
is accepted as trustworthy, then they can provide Endorsements and
Appraisal Policies that enable it to act as a Verifier.
The Verifier trusts (or more specifically, the Verifier's security 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
evaluate 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 evaluate 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.
8. Conceptual Messages In some scenarios, Evidence might contain sensitive information such
8.1. Evidence as Personally Identifiable Information. Thus, an Attester must trust
entities to which it sends Evidence, to not reveal sensitive data to
unauthorized parties. The Verifier might share this information with
other authorized parties, according rules that it controls. In the
background-check model, this Evidence may also be revealed to Relying
Party(s).
7. Conceptual Messages
7.1. Evidence
Today, Evidence tends to be highly device-specific, since the Today, Evidence tends to be highly device-specific, since the
information in the Evidence often includes vendor-specific information in the Evidence often includes vendor-specific
information that is necessary to fully describe the manufacturer and information that is necessary to fully describe the manufacturer and
model of the device including its security properties, the health of model of the device including its security properties, the health of
the device, and the level of confidence in the correctness of the the device, and the level of confidence in the correctness of the
information. Evidence is typically signed by the device (whether by information. Evidence is typically signed by the device (whether by
hardware, firmware, or software on the device), and evaluating it in hardware, firmware, or software on the device), and its appraisal in
isolation would require Appraisal Policy to be based on device- isolation would require Appraisal Policy to be based on device-
specific details (e.g., a device public key). specific details (e.g., a device public key).
8.2. Endorsements 7.2. 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, evaluating them can be done against Appraisal Policy used together, an appraisal procedure can be conducted based on
that may not be specific to the device instance, but merely specific Appraisal Policies that may not be specific to the device instance,
to the manufacturer providing the Endorsement. For example, an but merely specific to the manufacturer providing the Endorsement.
Appraisal Policy might simply check that devices from a given For example, an Appraisal Policy might simply check that devices from
manufacturer have information matching a set of known-good reference a given manufacturer have information matching a set of known-good
values, or an Appraisal Policy might have a set of more complex logic reference values, or an Appraisal Policy might have a set of more
on how to evaluate the validity of information. complex logic on how to appraise the validity of information.
However, while an Appraisal Policy that treats all devices from a However, while an Appraisal Policy that treats all devices from a
given manufacturer the same may be appropriate for some use cases, it given manufacturer the same may be appropriate for some use cases, it
would be inappropriate to use such an Appraisal Policy as the sole would be inappropriate to use such an Appraisal Policy as the sole
means of authorization for use cases that wish to constrain _which_ means of authorization for use cases that wish to constrain _which_
compliant devices are considered authorized for some purpose. For compliant devices are considered authorized for some purpose. For
example, an enterprise using attestation for Network Endpoint example, an enterprise using remote attestation for Network Endpoint
Assessment may not wish to let every healthy laptop from the same Assessment may not wish to let every healthy laptop from the same
manufacturer onto the network, but instead only want to let devices manufacturer onto the network, but instead only want to let devices
that it legally owns onto the network. Thus, an Endorsement may be that it legally owns onto the network. Thus, an Endorsement may be
helpful information in authenticating information about a device, but helpful information in authenticating information about a device, but
is not necessarily sufficient to authorize access to resources which is not necessarily sufficient to authorize access to resources which
may need device-specific information such as a public key for the may need device-specific information such as a public key for the
device or component or user on the device. device or component or user on the device.
8.3. Attestation Results 7.3. 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
Relying Party to make authorization decisions based on the Relying Relying Party to make authorization decisions based on the Relying
Party's Appraisal Policy. The simplest such policy might be to Party's Appraisal Policy. The simplest such policy might be to
simply authorize any party supplying a compliant Attestation Result simply authorize any party supplying a compliant Attestation Result
signed by a trusted Verifier. A more complex policy might also signed by a trusted Verifier. A more complex policy might also
entail comparing information provided in the result against known- entail comparing information provided in the result against known-
good reference values, or applying more complex logic such good reference values, or applying more complex logic on such
information. information.
Thus, Attestation Results often need to include detailed information Thus, Attestation Results often need to include detailed information
about the Attester, for use by Relying Parties, much like physical about the Attester, for use by Relying Parties, much like physical
passports and drivers licenses include personal information such as passports and drivers licenses include personal information such as
name and date of birth. Unlike Evidence, which is often very device- name and date of birth. Unlike Evidence, which is often very device-
and vendor-specific, Attestation Results can be vendor-neutral if the and vendor-specific, Attestation Results can be vendor-neutral if the
Verifier has a way to generate vendor-agnostic information based on Verifier has a way to generate vendor-agnostic information based on
evaluating vendor-specific information in Evidence. This allows a the appraisal of vendor-specific information in Evidence. This
Relying Party's Appraisal Policy to be simpler, potentially based on allows a Relying Party's Appraisal Policy to be simpler, potentially
standard ways of expressing the information, while still allowing based on standard ways of expressing the information, while still
interoperability with heterogeneous devices. allowing interoperability with heterogeneous devices.
Finally, whereas Evidence is signed by the device (or indirectly by a Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer, if Endorsements are used), Attestation Results are manufacturer, if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity, rather than a larger set of entities, relationship with one entity, rather than a larger set of entities,
for purposes of its Appraisal Policy. for purposes of its Appraisal Policy.
9. Claims Encoding Formats 8. Claims Encoding Formats
The following diagram illustrates a relationship to which attestation The following diagram illustrates a relationship to which remote
is desired to be added: attestation is desired to be added:
+-------------+ +-------------+ +-------------+ +------------+ Evaluate
| |-------------->| | | |-------------->| | request
| Attester | Access some | Relying | Evaluate request | Attester | Access some | Relying | against
| | resource | Party | against security policy | | resource | Party | security
+-------------+ +-------------+ +-------------+ +------------+ policy
Figure 6: Typical Resource Access Figure 7: Typical Resource Access
In this diagram, the protocol between Attester and a Relying Party In this diagram, the protocol between Attester and a Relying Party
can be any new or existing protocol (e.g., HTTP(S), COAP(S), 802.1x, can be any new or existing protocol (e.g., HTTP(S), COAP(S), 802.1x,
OPC UA, etc.), depending on the use case. Such protocols typically OPC UA, etc.), depending on the use case. Such protocols typically
already have mechanisms for passing security information for purposes already have mechanisms for passing security information for purposes
of authentication and authorization. Common formats include JWTs of authentication and authorization. Common formats include JWTs
[RFC7519], CWTs [RFC8392], and X.509 certificates. [RFC7519], CWTs [RFC8392], and X.509 certificates.
To enable attestation to be added to existing protocols, enabling a To enable remote attestation to be added to existing protocols,
higher level of assurance against malware for example, it is enabling a higher level of assurance against malware for example, it
important that information needed for evaluating 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, attestation-related information could be natively encoded in Thus, remote attestation related information could be natively
X.509 certificate extensions, or could be natively encoded in some encoded in X.509 certificate extensions, or could be natively encoded
other format (e.g., a CWT) which in turn is then encoded in an X.509 in some other format (e.g., a CWT) which in turn is then encoded in
certificate extension. an X.509 certificate extension.
Especially for constrained nodes, however, there is a desire to Especially for constrained nodes, however, there is a desire to
minimize the amount of parsing code needed in a Relying Party, in minimize the amount of parsing code needed in a Relying Party, in
order to both minimize footprint and to minimize the attack surface order to both minimize footprint and to minimize the attack surface
area. So while it would be possible to embed a CWT inside a JWT, or area. So while it would be possible to embed a CWT inside a JWT, or
a JWT inside an X.509 extension, etc., there is a desire to encode a JWT inside an X.509 extension, etc., there is a desire to encode
the information natively in the format that is natural for the the information natively in the format that is natural for the
Relying Party. Relying Party.
This motivates having a common "information model" that describes the This motivates having a common "information model" that describes the
set of attestation related information in an encoding-agnostic way, set of remote attestation related information in an encoding-agnostic
and allowing multiple encoding formats (CWT, JWT, X.509, etc.) that way, and allowing multiple encoding formats (CWT, JWT, X.509, etc.)
encode the same information into the claims format needed by the that encode the same information into the claims format needed by the
Relying Party. Relying Party.
The following diagram illustrates that Evidence and Attestation The following diagram illustrates that Evidence and Attestation
Results might each have multiple possible encoding formats, so that Results might each have multiple possible encoding formats, so that
they can be conveyed by various existing protocols. It also they can be conveyed by various existing protocols. It also
motivates why the Verifier might also be responsible for accepting motivates why the Verifier might also be responsible for accepting
Evidence that encodes claims in one format, while issuing Attestation Evidence that encodes claims in one format, while issuing Attestation
Results that encode claims in a different format. Results that encode claims in a different format.
Evidence Attestation Results Evidence Attestation Results
.--------------. CWT CWT .-------------------.
.--------------. CWT CWT .-------------------. | Attester-A |------------. .----------->| Relying Party V |
| Attester-A |------------. .----------->| Relying Party V | '--------------' v | `-------------------'
'--------------' v | `-------------------' .--------------. JWT .------------. JWT .-------------------.
.--------------. JWT .------------. JWT .-------------------. | Attester-B |-------->| Verifier |-------->| Relying Party W |
| Attester-B |-------->| Verifier |-------->| Relying Party W | --------------' | | `-------------------'
'--------------' | | `-------------------' .--------------. X.509 | | X.509 .-------------------.
.--------------. X.509 | | X.509 .-------------------. | Attester-C |-------->| |-------->| Relying Party X |
| Attester-C |-------->| |-------->| Relying Party X | '--------------' | | `-------------------'
'--------------' | | `-------------------' .--------------. TPM | | TPM .-------------------.
.--------------. TPM | | TPM .-------------------. | Attester-D |-------->| |-------->| Relying Party Y |
| Attester-D |-------->| |-------->| Relying Party Y | '--------------' '------------' `-------------------'
'--------------' '------------' `-------------------' .--------------. other ^ | other .-------------------.
.--------------. other ^ | other .-------------------. | Attester-E |------------' '----------->| Relying Party Z |
| Attester-E |------------' '----------->| Relying Party Z | '--------------' `-------------------'
'--------------' `-------------------'
Figure 7: Multiple Attesters and Relying Parties with Different Figure 8: Multiple Attesters and Relying Parties with Different
Formats Formats
10. Freshness 9. 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, may change immediately after the Appraisal Policy for Evidence, might change immediately after the
Evidence or Attestation Result was generated. The goal is merely to Evidence or Attestation Result was generated. The goal is merely to
narrow the time window to something the Verifier (for Evidence) or narrow the time window to something the Verifier (for Evidence) or
Relying Party (for an Attestation Result) is willing to accept. Relying Party (for an Attestation Result) is willing to accept.
There are two common approaches to providing some assurance of There are two common approaches to providing some assurance of
freshness. The first approach is that a nonce is generated by a freshness. The first approach is that a nonce is generated by a
remote entity (e.g., the Verifier for Evidence, or the Relying Party remote entity (e.g., the Verifier for Evidence, or the Relying Party
for an Attestation Result), and the nonce is then signed and included for an Attestation Result), and the nonce is then signed and included
along with the claims in the Evidence or Attestation Result, so that along with the claims in the Evidence or Attestation Result, so that
the remote entity knows that the claims were signed after the nonce the remote entity knows that the claims were signed after the nonce
skipping to change at page 16, line 15 skipping to change at page 22, line 43
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.
11. Privacy Considerations 10. 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 is firmware/software that the device is running. This information might
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.
Protocols that convey Evidence or Attestation Results are responsible Evidence and Attestation Results data structures are expected to
for detailing what kinds of information are disclosed, and to whom support integrity protection encoding (e.g., COSE, JOSE, X.509) and
they are exposed. optionally might support confidentiality protection (e.g., COSE,
JOSE). Therefore, if confidentiality protection is omitted or
unavailable, the protocols that convey Evidence or Attestation
Results are responsible for detailing what kinds of information are
disclosed, and to whom they are exposed.
12. Security Considerations 11. 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, or Endorsements, or Appraisal Policy, needs to support end- Results, Endorsements, or Appraisal Policy, needs to support end-to-
to-end integrity protection and replay attack prevention, and often end integrity protection and replay attack prevention, and often also
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 10 discusses ways in which freshness can be used use cases. Section 9 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 evaluate the security provided by a particular Appraisal Policy, To assess the security provided by a particular Appraisal Policy, it
it is important to understand the strength of the Root of Trust, is important to understand the strength of the Root of Trust, e.g.,
e.g., whether it is mutable software, or firmware that is read-only whether it is mutable software, or firmware that is read-only after
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 Policy in a Relying Party or securely. As such, if Appraisal Policies for a Relying Party or for
Verifier can be configured via a network protocol, the ability to a Verifier can be configured via a network protocol, the ability to
attest to the health of the client providing the Appraisal Policy create Evidence about the integrity of the entity providing the
needs to be considered. Appraisal Policy needs to be considered.
13. IANA Considerations The security of conveyed information may be applied at different
layers, whether by a conveyance protocol, or an information encoding
format. This architecture expects attestation messages (i.e.,
Evidence, Attestation Results, Endorsements and Policies) are end-to-
end protected based on the role interaction context. For example, if
an Attester produces Evidence that is relayed through some other
entity that doesn't implement the Attester or the intended Verifier
roles, then the relaying entity should not expect to have access to
the Evidence.
12. IANA Considerations
This document does not require any actions by IANA. This document does not require any actions by IANA.
14. Acknowledgments 13. Acknowledgments
Special thanks go to David Wooten, Joerg Borchert, Hannes Tschofenig, Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Laurence Lundblade, Diego Lopez, Jessica Fitzgerald-McKay, Frank Xia, Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade,
and Nancy Cam-Winget. Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
15. Contributors 14. 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. Informative References 15. References
15.1. Normative References
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
15.2. Informative References
[I-D.birkholz-rats-tuda] [I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann, Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in "Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-01, 11 Progress, Internet-Draft, draft-birkholz-rats-tuda-01, 11
September 2019, <http://www.ietf.org/internet-drafts/ September 2019, <http://www.ietf.org/internet-drafts/
draft-birkholz-rats-tuda-01.txt>. draft-birkholz-rats-tuda-01.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-05, 12 December 2019, ietf-teep-architecture-06, 8 February 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep- <http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-05.txt>. architecture-06.txt>.
[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/>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token [RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc7519>. <https://www.rfc-editor.org/info/rfc4949>.
[RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
"CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
May 2018, <https://www.rfc-editor.org/info/rfc8392>.
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
skipping to change at line 812 skipping to change at page 25, line 38
Sandelman Software Works Sandelman Software Works
Canada Canada
Email: mcr+ietf@sandelman.ca Email: mcr+ietf@sandelman.ca
Ned Smith Ned Smith
Intel Corporation Intel Corporation
United States of America United States of America
Email: ned.smith@intel.com Email: ned.smith@intel.com
Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com
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