Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-01
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| Authors | Henk Birkholz , Dave Thaler , Michael Richardson , Ned Smith | ||
| Last updated | 2020-02-04 | ||
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draft-ietf-rats-architecture-01
RATS Working Group H. Birkholz
Internet-Draft Fraunhofer SIT
Intended status: Informational D. Thaler
Expires: 7 August 2020 Microsoft
M. Richardson
Sandelman Software Works
N. Smith
Intel
4 February 2020
Remote Attestation Procedures Architecture
draft-ietf-rats-architecture-01
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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 7 August 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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Reference Use Cases . . . . . . . . . . . . . . . . . . . . . 4
4. Architectural Overview . . . . . . . . . . . . . . . . . . . 4
4.1. Composite Attester . . . . . . . . . . . . . . . . . . . 5
5. Topological Models . . . . . . . . . . . . . . . . . . . . . 7
5.1. Passport Model . . . . . . . . . . . . . . . . . . . . . 7
5.2. Background-Check Model . . . . . . . . . . . . . . . . . 8
5.3. Combinations . . . . . . . . . . . . . . . . . . . . . . 9
6. Two Types of Environments of an Attester . . . . . . . . . . 10
7. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Conceptual Messages . . . . . . . . . . . . . . . . . . . . . 11
8.1. Evidence . . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. Endorsements . . . . . . . . . . . . . . . . . . . . . . 12
8.3. Attestation Results . . . . . . . . . . . . . . . . . . . 13
9. Claims Encoding Formats . . . . . . . . . . . . . . . . . . . 13
10. Freshness . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16
12. Security Considerations . . . . . . . . . . . . . . . . . . . 16
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
16. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction
<more text to be added here>
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Remote Attestation, as used in this document, is a process by which
one entity (the "Attester") provides evidence about its identity and
state to another remote entity (the "Relying Party"), which then
assesses the Attester's trustworthiness for the Relying Party's own
purposes.
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].
* Appraisal Policy for Attestation Result: A set of rules that
direct how a Relying Party uses the evaluation results about an
Attester generated by the Verifiers. Compare /security policy/ in
[RFC4949].
* Attestation Result: The evaluation results 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 evaluated 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 evaluate trustworthiness of Attesters.
* Evidence: A set of information about an Attester that is to be
evaluated 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 evaluates the validity of Evidence about
an Attester.
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* Verifier Owner: An entity, such as an administrator, that is
authorized to configure Appraisal Policy for Evidence in a
Verifier.
3. Reference Use Cases
<unclear if the WG wants this section in the arch doc>
4. Architectural Overview
Figure 1 depicts the data that flows between different roles,
independent of protocol or use case.
************ ************ *****************
* 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
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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.
4.1. Composite Attester
A Composite Attester is an entity composed of multiple sub-entities
such that its trustworthiness has to be determined by evaluating 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 these Attesters, there may be only some, which can
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
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 TPM or 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 Attester, each slot is
an Attester, and the main slot is the Lead 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 simpler management
by being logically treated as one router. 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 verified via this main router. So, in
this case, the multi-chassis router is the Composite Attester, each
router is an Attester and the main router is the Lead Attester.
Figure 2 depicts the conceptual data flow for a Composite Attester.
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.-----------------------------.
| Verifier |
'-----------------------------'
^
|
| Composite
| Evidence
|
.----------------------------------|-------------------------------.
| .--------------------------------|-----. .------------. |
| | .------------. | | | |
| | | Attesting |<--------| Attester B |-. |
| | |Environment | | '------------. | |
| | .----------------. | |<----------| Attester C |-. |
| | | Target | | | | '------------' | |
| | | Environment(s) | | |<------------| ... | |
| | | | '------------' | Evidence '------------' |
| | | | ^ | of |
| | | |------------/ | Attesters |
| | '----------------' Collecting | (via Internal Links or |
| | Claims | Network Connections) |
| | | |
| | Lead Attester A | |
| '--------------------------------------' |
| |
| Device/Composite Device/Attester/TBD #33 |
'------------------------------------------------------------------'
Figure 2: Conceptual Data Flow for a Composite Attester
In the Composite Attester, 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.
The Lead Attester's Attesting Environment may or may not include its
own Verifier. One situation is that the Attesting Environment has no
internal Verifier. In this situation, the Lead Attesting Environment
simply combines the various Evidences into the final Evidence that is
sent off to the remote Verifier, which evaluates the Composite
Attester's, including the Lead Attester's and other Attesters',
trustworthiness.
The other situation is that the Lead Attesting Environment has an
internal Verifier. After collecting the Evidence of other Attesters,
this Attesting Environment verifies them using Endorsements and
Appraisal Policies (obtained the same way as any other Verifier), for
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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
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.
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 attestation protocol.
+-------------+
| | Compare Evidence
| Verifier | against Appraisal Policy
| |
+-------------+
^ |
Evidence| |Attestation
| | Result
| v
+-------------+ +-------------+
| |-------------->| | Compare Attestation
| Attester | Attestation | Relying | Result against
| | Result | Party | Appraisal Policy
+-------------+ +-------------+
Figure 3: Passport Model
The passport model is so named because of its resemblance to how
nations issue passports to their citizens. The nature of the
Evidence that an individual needs to provide to its local authority
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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 security 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 4: 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 attest to
different Relying Parties for 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 5 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 4, 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 5: 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).
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7. Trust Model
The scope of this document is scenarios for which a Relying Party
trusts a Verifier that can evaluate 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 evaluate the
trustworthiness of the Verifier before accepting its Attestation
Results.
In solutions following the background-check model, the Attester is
assumed to trust the Verifier (again, whether directly or indirectly
via a Certificate Authority that it trusts), since the Attester
relies on an Attestation Result it obtains from the Verifier, in
order to access resources.
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
evaluate 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 evaluate 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.
8. Conceptual Messages
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8.1. 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 evaluating it in
isolation would require Appraisal Policy to be based on device-
specific details (e.g., a device public key).
8.2. 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, evaluating them can be done against Appraisal Policy
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 evaluate 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 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.
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8.3. 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 such
information.
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
evaluating 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.
9. Claims Encoding Formats
The following diagram illustrates a relationship to which attestation
is desired to be added:
+-------------+ +-------------+
| |-------------->| |
| Attester | Access some | Relying | Evaluate request
| | resource | Party | against security policy
+-------------+ +-------------+
Figure 6: Typical Resource Access
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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 attestation to be added to existing protocols, enabling a
higher level of assurance against malware for example, it is
important that information needed for evaluating 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, attestation-related information could be natively encoded in
X.509 certificate extensions, or could be natively encoded 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 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.
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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 7: Multiple Attesters and Relying Parties with Different
Formats
10. 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, may 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
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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.
11. 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 is
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.
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
Any solution that conveys information used for security purposes,
whether such information is in the form of Evidence, Attestation
Results, or 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 10 discusses ways in which freshness can be used
in this architecture to protect against replay attacks.
To evaluate 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 Policy in a Relying Party or
Verifier can be configured via a network protocol, the ability to
attest to the health of the client providing the Appraisal Policy
needs to be considered.
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13. IANA Considerations
This document does not require any actions by IANA.
14. Acknowledgments
Special thanks go to David Wooten, Joerg Borchert, Hannes Tschofenig,
Laurence Lundblade, Diego Lopez, Jessica Fitzgerald-McKay, Frank Xia,
and Nancy Cam-Winget.
15. 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.
16. 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-05, 12 December 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-teep-
architecture-05.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/
specifications-unified-architecture/part-2-security-
model/>.
[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>.
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[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
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
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