Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-02
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| Authors | Henk Birkholz , Dave Thaler , Michael Richardson , Ned Smith , Wei Pan | ||
| Last updated | 2020-03-07 | ||
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draft-ietf-rats-architecture-02
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler
Expires: 8 September 2020 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
W. Pan
Huawei Technologies
7 March 2020
Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-02
Abstract
In network protocol exchanges, it is often the case that one entity
(a Relying Party) requires evidence about a remote peer to assess the
peer's trustworthiness, and a way to appraise such evidence. The
evidence is typically a set of claims about its software and hardware
platform. This document describes an architecture for such remote
attestation procedures (RATS).
Note to Readers
Discussion of this document takes place on the RATS Working Group
mailing list (rats@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/rats/
(https://mailarchive.ietf.org/arch/browse/rats/).
Source for this draft and an issue tracker can be found at
https://github.com/ietf-rats-wg/architecture (https://github.com/
ietf-rats-wg/architecture).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4
3.1. Network Endpoint Assessment . . . . . . . . . . . . . . . 5
3.2. Confidential Machine Learning (ML) Model Protection . . . 5
3.3. Confidential Data Retrieval . . . . . . . . . . . . . . . 6
3.4. Critical Infrastructure Control . . . . . . . . . . . . . 6
3.5. Trusted Execution Environment (TEE) Provisioning . . . . 6
3.6. Hardware Watchdog . . . . . . . . . . . . . . . . . . . . 7
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 7
4.1. Two Types of Environments of an Attester . . . . . . . . 9
4.2. Layered Attestation Procedures . . . . . . . . . . . . . 9
4.3. Composite Device . . . . . . . . . . . . . . . . . . . . 12
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 14
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 14
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 15
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 16
6. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 18
7.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 19
7.3. Attestation Results . . . . . . . . . . . . . . . . . . . 19
8. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 20
9. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 22
11. Security Considerations . . . . . . . . . . . . . . . . . . . 23
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
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13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 24
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
15.1. Normative References . . . . . . . . . . . . . . . . . . 24
15.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
In Remote Attestation Procedures (RATS), one peer (the "Attester")
produces believable information about itself - Evidence - to enable a
remote peer (the "Relying Party") to decide whether to consider that
Attester a trustworthy peer or not. RATS are facilitated by an
additional vital party, the Verifier. The Verifier appraises
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
This document uses the following terms.
Appraisal Policy for Evidence: A set of rules that direct how a
Verifier evaluates the validity of information about an Attester.
Compare /security policy/ in [RFC4949]
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Appraisal Policy for Attestation Result: A set of rules that direct
how a Relying Party uses the Attestation Results regarding an
Attester generated by the Verifiers. Compare /security policy/ in
[RFC4949]
Attestation Result: The output generated by a Verifier, typically
including information about an Attester, where the Verifier
vouches for the validity of the results
Attester: An entity whose attributes must be appraised in order to
determine whether the entity is considered trustworthy, such as
when deciding whether the entity is authorized to perform some
operation
Endorsement: A secure statement that some entity (typically a
manufacturer) vouches for the integrity of an Attester's signing
capability
Endorser: An entity that creates Endorsements that can be used to
help to appraise the trustworthiness of Attesters
Evidence: A set of information about an Attester that is to be
appraised by a Verifier
Relying Party: An entity that depends on the validity of information
about another entity, typically for purposes of authorization.
Compare /relying party/ in [RFC4949]
Relying Party Owner: An entity, such as an administrator, that is
authorized to configure Appraisal Policy for Attestation Results
in a Relying Party.
Verifier: An entity that appraises the validity of Evidence about an
Attester
Verifier Owner: An entity, such as an administrator, that is
authorized to configure Appraisal Policy for Evidence in a
Verifier
3. Reference Use Cases
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.
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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
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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
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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
Figure 1 depicts the data that flows between different roles,
independent of protocol or use case.
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************ ************ *****************
* Endorser * * Verifier * * Relying Party *
************ * Owner * * Owner *
| ************ *****************
| | |
Endorsements| | |
| |Appraisal |
| |Policy for |
| |Evidence | Appraisal
| | | Policy for
| | | Attestation
| | | Result
v v |
.-----------------. |
.----->| Verifier |------. |
| '-----------------' | |
| | |
| Attestation| |
| Results | |
| Evidence | |
| | |
| v v
.----------. .-----------------.
| Attester | | Relying Party |
'----------' '-----------------'
Figure 1: Conceptual Data Flow
An Attester creates Evidence that is conveyed to a Verifier.
The Verifier uses the Evidence, and any Endorsements from Endorsers,
by applying an Evidence Appraisal Policy to assess the
trustworthiness of the Attester, and generates Attestation Results
for use by Relying Parties. The Evidence Appraisal Policy might be
obtained from an Endorser along with the Endorsements, or might be
obtained via some other mechanism such as being configured in the
Verifier by an administrator.
The Relying Party uses Attestation Results by applying its own
Appraisal Policy to make application-specific decisions such as
authorization decisions. The Attestation Result Appraisal Policy
might, for example, be configured in the Relying Party by an
administrator.
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4.1. 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, such as in the
examples described in more detail in Section 4.2 and Section 4.3.
Other examples may exist, and the examples discussed could even be
combined into even more complex implementations.
Claims are collected from Target Environments. That is, Attesting
Environments collect the raw values and the information to be
represented in claims, such as by doing some measurement of a Target
Environment's code, memory, and/or registers. Attesting Environments
then format the claims 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).
4.2. Layered Attestation Procedures
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.
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.----------. .----------.
| | | |
| 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
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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.
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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
carrier-grade routers. The multi-chassis router provides higher
throughput by interconnecting multiple routers and can be logically
treated as one router for simpler management. Among these routers,
there is only one main router that connects to the Verifier. Other
routers are only connected to the main router by the network cables,
and therefore they are managed and appraised via this main router's
help. So, in this case, the multi-chassis router is the Composite
Device, each router is an Attester and the main router is the lead
Attester.
Figure 3 depicts the conceptual data flow for a Composite Device.
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.-----------------------------.
| Verifier |
'-----------------------------'
^
|
| Evidence of
| Composite Device
|
.----------------------------------|-------------------------------.
| .--------------------------------|-----. .------------. |
| | Collect .------------. | | | |
| | Claims .--------->| Attesting |<--------| Attester B |-. |
| | | |Environment | | '------------. | |
| | .----------------. | |<----------| Attester C |-. |
| | | Target | | | | '------------' | |
| | | Environment(s) | | |<------------| ... | |
| | | | '------------' | Evidence '------------' |
| | '----------------' | of |
| | | Attesters |
| | lead Attester A | (via Internal Links or |
| '--------------------------------------' Network Connections) |
| |
| Composite Device |
'------------------------------------------------------------------'
Figure 3: Conceptual Data Flow for a Composite Device
In the Composite Device, each Attester generates its own Evidence by
its Attesting Environment(s) collecting the claims from its Target
Environment(s). The lead Attester collects the Evidence of all other
Attesters and then generates the Evidence of the whole Composite
Attester.
An entity can take on multiple RATS roles (e.g., Attester, Verifier,
Relying Party, etc.) at the same time. The combination of roles can
be arbitrary. For example, in this Composite Device scenario, the
entity inside the lead Attester can also take on the role of a
Verifier, and the outside entity of Verifier can take on the role of
a Relying Party. After collecting the Evidence of other Attesters,
this inside Verifier verifies them using Endorsements and Appraisal
Policies (obtained the same way as any other Verifier), to generate
Attestation Results. The inside Verifier then sends the Attestation
Results of other Attesters, whether in the same conveyance protocol
as the Evidence or not, to the outside Verifier.
In this situation, the trust model described in Section 6 is also
suitable for this inside Verifier.
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5. Topological Models
There are multiple possible models for communication between an
Attester, a Verifier, and a Relying Party. This section includes
some reference models, but this is not intended to be a restrictive
list, and other variations may exist.
5.1. Passport Model
In this model, an Attester sends Evidence to a Verifier, which
compares the Evidence against its Appraisal Policy. The Verifier
then gives back an Attestation Result. If the Attestation Result was
a successful one, the Attester can then present the Attestation
Result to a Relying Party, which then compares the Attestation Result
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
Party includes an Attestation Result, in this model the details of
that protocol constrain the serialization format of the Attestation
Result. The format of the Evidence on the other hand is only
constrained by the Attester-Verifier remote attestation protocol.
+-------------+
| | Compare Evidence
| Verifier | against Appraisal Policy
| |
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+----------+ +---------+
| |------------->| |Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party | Appraisal
+----------+ +---------+ Policy
Figure 4: Passport Model
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The passport model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority
is specific to the country involved. The citizen retains control of
the resulting passport document and presents it to other entities
when it needs to assert a citizenship or identity claim, such as an
airport immigration desk. The passport is considered sufficient
because it vouches for the citizenship and identity claims, and it is
issued by a trusted authority. Thus, in this immigration desk
analogy, the passport issuing agency is a Verifier, the passport is
an Attestation Result, and the immigration desk is a Relying Party.
5.2. Background-Check Model
In this model, an Attester sends Evidence to a Relying Party, which
simply passes it on to a Verifier. The Verifier then compares the
Evidence against its Appraisal Policy, and returns an Attestation
Result to the Relying Party. The Relying Party then compares the
Attestation Result against its own appraisal policy.
The resource access protocol between the Attester and Relying Party
includes Evidence rather than an Attestation Result, but that
Evidence is not processed by the Relying Party. Since the Evidence
is merely forwarded on to a trusted Verifier, any serialization
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
be _encapsulated in_ the format required by the resource access
protocol between the Attester and Relying Party.
However, like in the Passport model, an Attestation Result is still
consumed by the Relying Party and so the serialization format of the
Attestation Result is still important. If the Relying Party is a
constrained node whose purpose is to serve a given type resource
using a standard resource access protocol, it already needs the
parser(s) required by that existing protocol. Hence, the ability to
let the Relying Party obtain an Attestation Result in the same
serialization format allows minimizing the code footprint and attack
surface area of the Relying Party, especially if the Relying Party is
a constrained node.
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+-------------+
| | Compare Evidence
| Verifier | against Appraisal
| | Policy
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+------------+ +-------------+
| |-------------->| | Compare Attestation
| Attester | Evidence | Relying | Result against
| | | Party | Appraisal Policy
+------------+ +-------------+
Figure 5: Background-Check Model
The background-check model is so named because of the resemblance of
how employers and volunteer organizations perform background checks.
When a prospective employee provides claims about education or
previous experience, the employer will contact the respective
institutions or former employers to validate the claim. Volunteer
organizations often perform police background checks on volunteers in
order to determine the volunteer's trustworthiness. Thus, in this
analogy, a prospective volunteer is an Attester, the organization is
the Relying Party, and a former employer or government agency that
issues a report is a Verifier.
5.3. Combinations
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
for a protocol between the two.
It is also worth pointing out that the choice of model is generally
up to the Relying Party, and the same device may need to create
Evidence for different Relying Parties and different use cases (e.g.,
a network infrastructure device to gain access to the network, and
then a server holding confidential data to get access to that data).
As such, both models may simultaneously be in use by the same device.
Figure 6 shows another example of a combination where Relying Party 1
uses the passport model, whereas Relying Party 2 uses an extension of
the background-check model. Specifically, in addition to the basic
functionality shown in Figure 5, Relying Party 2 actually provides
the Attestation Result back to the Attester, allowing the Attester to
use it with other Relying Parties. This is the model that the
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Trusted Application Manager plans to support in the TEEP architecture
[I-D.ietf-teep-architecture].
+-------------+
| | Compare Evidence
| Verifier | against Appraisal Policy
| |
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+-------------+
| | Compare
| Relying | Attestation Result
| Party 2 | against Appraisal Policy
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+----------+ +----------+
| |-------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party 1 | Appraisal Policy
+----------+ +----------+
Figure 6: Example Combination
6. Trust Model
The scope of this document is scenarios for which a Relying Party
trusts a Verifier that can appraise the trustworthiness of
information about an Attester. Such trust might come by the Relying
Party trusting the Verifier (or its public key) directly, or might
come by trusting an entity (e.g., a Certificate Authority) that is in
the Verifier's certificate chain. The Relying Party might implicitly
trust a Verifier (such as in the Verifying Relying Party
combination). Or, for a stronger level of security, the Relying
Party might require that the Verifier itself provide information
about itself that the Relying Party can use to assess the
trustworthiness of the Verifier before accepting its Attestation
Results.
The Endorser and Verifier Owner may need to trust the Verifier before
giving the Endorsement and Appraisal Policy to it. Such trust can
also be established directly or indirectly, implicitly or explicitly.
One explicit way to establish such trust may be the Verifier first
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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
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
appraise the trustworthiness of that manufacturer's devices. In
solutions with weaker security, a Verifier might be configured to
implicitly trust firmware or even software (e.g., a hypervisor).
That is, it might appraise the trustworthiness of an application
component, or operating system component or service, under the
assumption that information provided about it by the lower-layer
hypervisor or firmware is true. A stronger level of security comes
when information can be vouched for by hardware or by ROM code,
especially if such hardware is physically resistant to hardware
tampering. The component that is implicitly trusted is often
referred to as a Root of Trust.
In some scenarios, Evidence might contain sensitive information such
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
information in the Evidence often includes vendor-specific
information that is necessary to fully describe the manufacturer and
model of the device including its security properties, the health of
the device, and the level of confidence in the correctness of the
information. Evidence is typically signed by the device (whether by
hardware, firmware, or software on the device), and its appraisal in
isolation would require Appraisal Policy to be based on device-
specific details (e.g., a device public key).
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7.2. Endorsements
An Endorsement is a secure statement that some entity (e.g., a
manufacturer) vouches for the integrity of the device's signing
capability. For example, if the signing capability is in hardware,
then an Endorsement might be a manufacturer certificate that signs a
public key whose corresponding private key is only known inside the
device's hardware. Thus, when Evidence and such an Endorsement are
used together, an appraisal procedure can be conducted based on
Appraisal Policies that may not be specific to the device instance,
but merely specific to the manufacturer providing the Endorsement.
For example, an Appraisal Policy might simply check that devices from
a given manufacturer have information matching a set of known-good
reference values, or an Appraisal Policy might have a set of more
complex logic on how to appraise the validity of information.
However, while an Appraisal Policy that treats all devices from a
given manufacturer the same may be appropriate for some use cases, it
would be inappropriate to use such an Appraisal Policy as the sole
means of authorization for use cases that wish to constrain _which_
compliant devices are considered authorized for some purpose. For
example, an enterprise using remote attestation for Network Endpoint
Assessment may not wish to let every healthy laptop from the same
manufacturer onto the network, but instead only want to let devices
that it legally owns onto the network. Thus, an Endorsement may be
helpful information in authenticating information about a device, but
is not necessarily sufficient to authorize access to resources which
may need device-specific information such as a public key for the
device or component or user on the device.
7.3. Attestation Results
Attestation Results may indicate compliance or non-compliance with a
Verifier's Appraisal Policy. A result that indicates non-compliance
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
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
authenticated as being correct.
An Attestation Result that indicates compliance can be used by a
Relying Party to make authorization decisions based on the Relying
Party's Appraisal Policy. The simplest such policy might be to
simply authorize any party supplying a compliant Attestation Result
signed by a trusted Verifier. A more complex policy might also
entail comparing information provided in the result against known-
good reference values, or applying more complex logic on such
information.
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Thus, Attestation Results often need to include detailed information
about the Attester, for use by Relying Parties, much like physical
passports and drivers licenses include personal information such as
name and date of birth. Unlike Evidence, which is often very device-
and vendor-specific, Attestation Results can be vendor-neutral if the
Verifier has a way to generate vendor-agnostic information based on
the appraisal of vendor-specific information in Evidence. This
allows a Relying Party's Appraisal Policy to be simpler, potentially
based on standard ways of expressing the information, while still
allowing interoperability with heterogeneous devices.
Finally, whereas Evidence is signed by the device (or indirectly by a
manufacturer, if Endorsements are used), Attestation Results are
signed by a Verifier, allowing a Relying Party to only need a trust
relationship with one entity, rather than a larger set of entities,
for purposes of its Appraisal Policy.
8. Claims Encoding Formats
The following diagram illustrates a relationship to which remote
attestation is desired to be added:
+-------------+ +------------+ Evaluate
| |-------------->| | request
| Attester | Access some | Relying | against
| | resource | Party | security
+-------------+ +------------+ policy
Figure 7: Typical Resource Access
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,
OPC UA, etc.), depending on the use case. Such protocols typically
already have mechanisms for passing security information for purposes
of authentication and authorization. Common formats include JWTs
[RFC7519], CWTs [RFC8392], and X.509 certificates.
To enable remote attestation to be added to existing protocols,
enabling a higher level of assurance against malware for example, it
is important that information needed for appraising the Attester be
usable with existing protocols that have constraints around what
formats they can transport. For example, OPC UA [OPCUA] (probably
the most common protocol in industrial IoT environments) is defined
to carry X.509 certificates and so security information must be
embedded into an X.509 certificate to be passed in the protocol.
Thus, remote attestation related information could be natively
encoded in X.509 certificate extensions, or could be natively encoded
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in some other format (e.g., a CWT) which in turn is then encoded in
an X.509 certificate extension.
Especially for constrained nodes, however, there is a desire to
minimize the amount of parsing code needed in a Relying Party, in
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
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
Relying Party.
This motivates having a common "information model" that describes the
set of remote attestation related information in an encoding-agnostic
way, and allowing multiple encoding formats (CWT, JWT, X.509, etc.)
that encode the same information into the claims format needed by the
Relying Party.
The following diagram illustrates that Evidence and Attestation
Results might each have multiple possible encoding formats, so that
they can be conveyed by various existing protocols. It also
motivates why the Verifier might also be responsible for accepting
Evidence that encodes claims in one format, while issuing Attestation
Results that encode claims in a different format.
Evidence Attestation Results
.--------------. CWT CWT .-------------------.
| Attester-A |------------. .----------->| Relying Party V |
'--------------' v | `-------------------'
.--------------. JWT .------------. JWT .-------------------.
| Attester-B |-------->| Verifier |-------->| Relying Party W |
--------------' | | `-------------------'
.--------------. X.509 | | X.509 .-------------------.
| Attester-C |-------->| |-------->| Relying Party X |
'--------------' | | `-------------------'
.--------------. TPM | | TPM .-------------------.
| Attester-D |-------->| |-------->| Relying Party Y |
'--------------' '------------' `-------------------'
.--------------. other ^ | other .-------------------.
| Attester-E |------------' '----------->| Relying Party Z |
'--------------' `-------------------'
Figure 8: Multiple Attesters and Relying Parties with Different
Formats
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9. Freshness
It is important to prevent replay attacks where an attacker replays
old Evidence or an old Attestation Result that is no longer correct.
To do so, some mechanism of ensuring that the Evidence and
Attestation Result are fresh, meaning that there is some degree of
assurance that they still reflect the latest state of the Attester,
and that any Attestation Result was generated using the latest
Appraisal Policy for Evidence. There is, however, always a race
condition possible in that the state of the Attester, and the
Appraisal Policy for Evidence, might change immediately after the
Evidence or Attestation Result was generated. The goal is merely to
narrow the time window to something the Verifier (for Evidence) or
Relying Party (for an Attestation Result) is willing to accept.
There are two common approaches to providing some assurance of
freshness. The first approach is that a nonce is generated by a
remote entity (e.g., the Verifier for Evidence, or the Relying Party
for an Attestation Result), and the nonce is then signed and included
along with the claims in the Evidence or Attestation Result, so that
the remote entity knows that the claims were signed after the nonce
was generated.
A second approach is to rely on synchronized clocks, and include a
signed timestamp (e.g., using [I-D.birkholz-rats-tuda]) along with
the claims in the Evidence or Attestation Result, so that the remote
entity knows that the claims were signed at that time, as long as it
has some assurance that the timestamp is correct. This typically
requires additional claims about the signer's time synchronization
mechanism in order to provide such assurance.
In either approach, it is important to note that the actual values in
claims might have been generated long before the claims are signed.
If so, it is the signer's responsibility to ensure that the values
are still correct when they are signed. For example, values might
have been generated at boot, and then used in claims as long as the
signer can guarantee that they cannot have changed since boot.
10. Privacy Considerations
The conveyance of Evidence and the resulting Attestation Results
reveal a great deal of information about the internal state of a
device. In many cases, the whole point of the Attestation process is
to provide reliable information about the type of the device and the
firmware/software that the device is running. This information might
be particularly interesting to many attackers. For example, knowing
that a device is running a weak version of firmware provides a way to
aim attacks better.
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Evidence and Attestation Results data structures are expected to
support integrity protection encoding (e.g., COSE, JOSE, X.509) and
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.
11. Security Considerations
Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation
Results, Endorsements, or Appraisal Policy, needs to support end-to-
end integrity protection and replay attack prevention, and often also
needs to support additional security protections. For example,
additional means of authentication, confidentiality, integrity,
replay, denial of service and privacy protection are needed in many
use cases. Section 9 discusses ways in which freshness can be used
in this architecture to protect against replay attacks.
To assess the security provided by a particular Appraisal Policy, it
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
boot, or immutable hardware/ROM.
It is also important that the Appraisal Policy was itself obtained
securely. As such, if Appraisal Policies for a Relying Party or for
a Verifier can be configured via a network protocol, the ability to
create Evidence about the integrity of the entity providing the
Appraisal Policy needs to be considered.
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.
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13. Acknowledgments
Special thanks go to Joerg Borchert, Nancy Cam-Winget, Jessica
Fitzgerald-McKay, Thomas Fossati, Diego Lopez, Laurence Lundblade,
Wei Pan, Paul Rowe, Hannes Tschofenig, Frank Xia, and David Wooten.
14. Contributors
Thomas Hardjono created older versions of the terminology section in
collaboration with Ned Smith. Eric Voit provided the conceptual
separation between Attestation Provision Flows and Attestation
Evidence Flows. Monty Wisemen created the content structure of the
first three architecture drafts. Carsten Bormann provided many of
the motivational building blocks with respect to the Internet Threat
Model.
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]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", Work in
Progress, Internet-Draft, draft-birkholz-rats-tuda-01, 11
September 2019, <http://www.ietf.org/internet-drafts/
draft-birkholz-rats-tuda-01.txt>.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-06, 8 February 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-06.txt>.
[OPCUA] OPC Foundation, "OPC Unified Architecture Specification,
Part 2: Security Model, Release 1.03", OPC 10000-2 , 25
November 2015, <https://opcfoundation.org/developer-tools/
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specifications-unified-architecture/part-2-security-
model/>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Dave Thaler
Microsoft
United States of America
Email: dthaler@microsoft.com
Michael Richardson
Sandelman Software Works
Canada
Email: mcr+ietf@sandelman.ca
Ned Smith
Intel Corporation
United States of America
Email: ned.smith@intel.com
Wei Pan
Huawei Technologies
Email: william.panwei@huawei.com
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