Attestation Results for Secure Interactions
draft-ietf-rats-ar4si-00
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| Authors | Eric Voit , Henk Birkholz , Thomas Hardjono , Thomas Fossati , Vincent Scarlata | ||
| Last updated | 2021-12-02 | ||
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draft-ietf-rats-ar4si-00
RATS Working Group E. Voit
Internet-Draft Cisco
Intended status: Standards Track H. Birkholz
Expires: 5 June 2022 Fraunhofer SIT
T. Hardjono
MIT
T. Fossati
Arm Limited
V. Scarlata
Intel
2 December 2021
Attestation Results for Secure Interactions
draft-ietf-rats-ar4si-00
Abstract
This document defines reusable Attestation Result information
elements. When these elements are offered to Relying Parties as
Evidence, different aspects of Attester trustworthiness can be
evaluated. Additionally, where the Relying Party is interfacing with
a heterogenous mix of Attesting Environment and Verifier types,
consistent policies can be applied to subsequent information exchange
between each Attester and the Relying Party.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 5 June 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. AR Augmented Evidence and Actions . . . . . . . . . . . . . . 5
2.1. Attestation Results for Secure Interactions . . . . . . . 5
2.2. Non-repudiable Identity . . . . . . . . . . . . . . . . . 6
2.2.1. Attester and Attesting Environment . . . . . . . . . 7
2.2.2. Verifier . . . . . . . . . . . . . . . . . . . . . . 9
2.2.3. Communicating Identity . . . . . . . . . . . . . . . 10
2.3. Trustworthiness Claims . . . . . . . . . . . . . . . . . 10
2.3.1. Specific Claims . . . . . . . . . . . . . . . . . . . 10
2.3.2. Trustworthiness Vector . . . . . . . . . . . . . . . 13
2.3.3. Trustworthiness Vector for a type of Attesting
Environment . . . . . . . . . . . . . . . . . . . . . 14
2.4. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 14
3. Secure Interactions Model . . . . . . . . . . . . . . . . . . 15
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Normative References . . . . . . . . . . . . . . . . . . 18
7.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Supportable Trustworthiness Claims . . . . . . . . . 20
A.1. Supportable Trustworthiness Claims for HSM-based CC . . . 20
A.2. Supportable Trustworthiness Claims for process-based
CC . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
A.3. Supportable Trustworthiness Claims for VM-based CC . . . 23
Appendix B. Some issues being worked . . . . . . . . . . . . . . 24
Appendix C. Contributors . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
The first paragraph of the May 2021 US Presidential Executive Order
on Improving the Nation's Cybersecurity [US-Executive-Order] ends
with the statement "the trust we place in our digital infrastructure
should be proportional to how trustworthy and transparent that
infrastructure is." Later this order explores aspects of
trustworthiness such as an auditable trust relationship, which it
defines as an "agreed-upon relationship between two or more system
elements that is governed by criteria for secure interaction,
behavior, and outcomes."
The Remote ATtestation procedureS (RATS) architecture
[I-D.ietf-rats-architecture] provides a useful context for
programmatically establishing and maintaining such auditable trust
relationships. Specifically, the architecture defines conceptual
messages conveyed between architectural subsystems to support
trustworthiness appraisal. The RATS conceptual message used to
convey evidence of trustworthiness is the Attestation Results. The
Attestation Results includes Verifier generated appraisals of an
Attester including such information as the identity of the Attester,
the security mechanisms employed on this Attester, and the Attester's
current state of trustworthiness.
Generated Attestation Results are ultimately conveyed to one or more
Relying Parties. Reception of an Attestation Result enables a
Relying Party to determine what action to take with regards to an
Attester. Frequently, this action will be to choose whether to allow
the Attester to securely interact with the Relying Party over some
connection between the two.
When determining whether to allow secure interactions with an
Attester, a Relying Party is challenged with a number of difficult
problems which it must be able to handle successfully. These
problems include:
* What types of Attestation Results (AR) might a Relying Party be
willing to trust from a specific type of Verifier?
* What supplemental information must the Verifier need to include
within Attestation Results to convince a Relying Party to allow
interactions, or to apply policies to any connections, based on
these Attestation Results?
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* What are the operating/environmental realities of the Attesting
Environment where a Relying Party should only be able to associate
a certain confidence regarding Attestation Results out of the
Verifier? (In other words, different types of Trusted Execution
Environments (TEE) need not be treated as equivalent.)
* How to make direct comparisons where there is a heterogeneous mix
of Attesting Environments and Verifier types.
To address these problems, it is important that specific Attestation
Result information elements are framed independently of Attesting
Environment specific constraints. If they are not, a Relying Party
would be forced to adapt to the syntax and semantics of many vendor
specific environments. This is not a reasonable ask as there can be
many types of Attesters interacting with or connecting to a Relying
Party.
The business need therefore is for common Attestation Result
information element definitions. With these definitions, consistent
interaction or connectivity decisions can be made by a Relying Party
where there is a heterogenous mix of Attesting Environment types and
Verifier types.
This document defines information elements for Attestation Results in
a way which normalizes the trustworthiness assertions that can be
made from a diverse set of Attesters.
1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
The following terms are imported from [I-D.ietf-rats-architecture]:
Appraisal Policy for Attestation Results, Attester, Attesting
Environment, Claims, Evidence, Relying Party, Target Environment and
Verifier.
[I-D.ietf-rats-architecture] also describes topological patterns that
illustrate the need for interoperable conceptual messages. The two
patterns called "background-check model" and "passport model" are
imported from the RATS architecture and used in this document as a
reference to the architectural concepts: Background-Check Model and
Passport Model.
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Newly defined terms for this document:
AR-augmented Evidence: a bundle of Evidence which includes at least
the following:
1. Verifier signed Attestation Results. These Attestation
Results must include Identity Evidence for the Attester, a
Trustworthiness Vector describing a Verifier's most recent
appraisal of an Attester, and some Verifier Proof-of-Freshness
(PoF).
2. A Relying Party PoF which is bound to the Attestation Results
of (1) by the Attester's Attesting Environment signature.
3. Sufficient information to determine the elapsed interval
between the Verifier PoF and Relying Party PoF.
Identity Evidence: Evidence which unambiguously identifies an
identity. Identity Evidence could take different forms, such as a
certificate, or a signature which can be appraised to have only
been generated by a specific private/public key pair.
Trustworthiness Claim: a specific quanta of trustworthiness which
can be assigned by a Verifier based on its appraisal policy.
Trustworthiness Vector: a set of zero to many Trustworthiness Claims
assigned during a single appraisal procedure by a Verifier using
Evidence generated by an Attester. The vector is included within
Attestation Results.
2. AR Augmented Evidence and Actions
An Attester creates AR Augmented Evidence by appending Attestation
Results with supplemental Evidence. When a Relying Party receives AR
Augmented Evidence, it will receive them as part of a protocol from
an Attesting endpoint which expects some result from this
communication. Upon receipt, the Relying Party will apply an
Appraisal Policy for Attestation Results. This policy will consider
both the Attestation Results as well as additional information about
the Attester within the AR Agumented Evidence the when determining
what action to take.
2.1. Attestation Results for Secure Interactions
When the action is a communication establishment attempt with an
Attester, there is only a limited set of actions which a Relying
Party might take. These actions include:
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* Allow or deny information exchange with the Attester (i.e.,
connectivity). When there is a deny, reasons should be returned
to the Attester.
* Connect the Attester to a specific context within a Relying Party.
* Apply policies on the connection to or from the Attester (e.g.,
rate limits).
There are three categories of information which must be conveyed to
the Relying Party (which also is integrated with a Verifier) before
it determines which of these actions to take.
1. Non-repudiable Identity Evidence - Evidence which undoubtably
identifies one or more entities involved with a connection.
2. Trustworthiness Claims - Specifics a Verifier asserts with
regards to its trustworthiness findings about an Attester.
3. Claim Freshness - Establishes the time of last update (or
refresh) of Trustworthiness Claims.
The following sections detail requirements for these three
categories.
2.2. Non-repudiable Identity
Identity Evidence must be conveyed during the establishment of any
trust-based relationship. Specific use cases will define the minimum
types of identities required by a particular Relying Party as it
evaluates AR-Augmented Evidence. At a bare minimum, a Relying Party
MUST start with the ability to verify the identity of a Verifier it
chooses to trust. Attester identities may then be acquired through
signed communications with the Verifier identity and/or the pre-
provisioning Attester public keys in the Attester.
During the Remote Attestation process, the Verifier's identity will
be established with a Relying Party via a Verifier signature across
recent Attestation Results. This Verifier identity could only have
come from a key pair maintained by a trusted developer or operator of
the Verifier.
Additionally, each set of AR Augmented Evidence must be provably and
non-reputably bound to the identity of the original Attesting
Environment which was evaluated by the Verifier. This will be
accomplished via two items. First the Verifier signed Attestation
Results MUST include sufficient Identity Evidence to ensure that this
Attesting Environment signature refers to the same Attesting
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Environment appraised by the Verifier. Second, an Attesting
Environment signature which includes the Verifier signature of the
Attestation Results MUST also be included.
In a subset of use cases, these two pieces of Identity Evidence may
be sufficient for a Relying Party to successfully meet the criteria
for its Appraisal Policy for Attestation Results. If the use case is
a connection request, a Relying Party may simply then establish a
transport session with an Attester after successfully appraising
verified by a Verifier. However an Appraisal Policy for Attestation
Results will often be more nuanced, and the Relying Party may need
additional information. Some Identity Evidence related policy
questions which the Relying Party may consider include:
* Does the Relying Party only trust this Verifier to make
Trustworthiness Claims on behalf a specific type of hardware
rooted Attesting Environment? Might a mix of Verifiers be
necessary to cover all mandatory Trustworthiness Claims?
* Does the Relying Party only accept connections from a verified-
authentic software build from a specific software developer?
* Does the Relying Party only accept connections from specific
preconfigured list of Attesters?
For any of these more nuanced appraisals, additional Identity
Evidence or other policy related information must be conveyed or pre-
provisioned during the formation of a trust context between the
Relying Party, the Attester, the Attester's Attesting Environment,
and the Verifier.
2.2.1. Attester and Attesting Environment
Per [I-D.ietf-rats-architecture] Figure 2, an Attester and a
corresponding Attesting Environment might not share common code or
even hardware boundaries. Consequently, an Attester implementation
needs to ensure that any Evidence which originates from outside the
Attesting Environment MUST have been collected and delivered securely
before any Attesting Environment signing may occur. After the
Verifier performs its appraisal, it will include sufficient
information in Attestation Results to enable a Relying Party to have
confidence that the Attester's trustworthiness is represented via
Trustworthiness Claims signed by the appropriate Attesting
Environment.
This document recognizes three general categories of Attesters.
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1. HSM-based: A Hardware Security Module (HSM) based cryptoprocessor
which continually hashes security measurements in a way which
prevents an Attester from lying about measurements which have
been extended into the Attesting Environment (e.g., TPM2.0.)
2. Process-based: An individual process which has its runtime memory
encrypted by an Attesting Environment in a way that no other
processes can read and decrypt that memory (e.g., [SGX] or
[I-D.tschofenig-rats-psa-token].)
3. VM-based: An entire Guest VM (or a set of containers within a
host) have been encrypted as a walled-garden unit by an Attesting
Environment. The result is that the host operating system cannot
read and decrypt what is executing within that VM (e.g., SEV or
TDX.)
Each of these categories of Attesters abover will be capable of
generating Evidence which is protected using private keys /
certificates which are not accessible outside of the corresponding
Attesting Environment. The owner of these secrets is the owner of
the identity which is bound within the Attesting Environment.
Effectively this means that for any Attester identity, there will
exist a chain of trust ultimately bound to a hardware-based root of
trust in the Attesting Environment. It is upon this root of trust
that unique, non-repudiable Attester identities may be founded.
There are several types of Attester identities defined in this
document. This list is extensible:
* chip-vendor: the vendor of the hardware chip used for the
Attesting Environment (e.g., a primary Endorsement Key from a TPM)
* chip-hardware: specific hardware with specific firmware from an
'ae-vendor'
* target-environment: a unique instance of a software build running
in an Attester (e.g., MRENCLAVE [SGX], an Instance ID
[I-D.tschofenig-rats-psa-token], or a hash which represents a set
of software loaded since boot (e.g., TPM based integrity
verification.))
* target-developer: the organizational unit responsible for a
particular 'target-environment' (e.g., MRSIGNER [SGX])
* ae-instance: a unique deployed instance of an Attesting
Environment running on 'chip-hardware' (e.g., an LDevID
[IEEE802.1AR])
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* (need to map SEV into above.)
Based on the category of the Attesting Environment, different types
of identities might be exposed by an Attester.
+========================+===============+===========+===========+
| Attester Identity type | Process-based | VM-based | HSM-based |
+========================+===============+===========+===========+
| chip-vendor | Mandatory | Mandatory | Mandatory |
+------------------------+---------------+-----------+-----------+
| chip-hardware | Mandatory | Mandatory | Mandatory |
+------------------------+---------------+-----------+-----------+
| target-environment | Mandatory | Mandatory | Optional |
+------------------------+---------------+-----------+-----------+
| target-developer | Mandatory | Optional | Optional |
+------------------------+---------------+-----------+-----------+
| ae-instance | Optional | Optional | Optional |
+------------------------+---------------+-----------+-----------+
Table 1
It is expected that drafts subsequent to this specification will
provide the definitions and value domains for specific identities,
each of which falling within the Attester identity types listed
above. In some cases the actual unique identities might encoded as
complex structures. An example complex structure might be a 'target-
environment' encoded as a Software Bill of Materials (SBOM).
With the identity definitions and value domains, a Relying Party will
have sufficient information to ensure that the Attester identities
and Trustworthiness Claims asserted are actually capable of being
supported by the underlying type of Attesting Environment.
Consequently, the Relying Party SHOULD require Identity Evidence
which indicates of the type of Attesting Environment when it
considers its Appraisal Policy for Attestation Results.
For more see Appendix A.
2.2.2. Verifier
For the Verifier identity, it is critical for a Relying Party to
review the certificate and chain of trust for that Verifier.
Additionally, the Relying Party must have confidence that the
Trustworthiness Claims being relied upon from the Verifier considered
the chain of trust for the Attesting Environment .
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2.2.3. Communicating Identity
Any of the above identities used by the Appraisal Policy for
Attestation Results needed to be pre-established by the Relying Party
before, or provided during, the exchange of Attestation Results.
When provided during this exchange, the identity may be communicated
either implicitly or explicitly.
An example of explicit communication would be to include the
following Identity Evidence directly within the Attestation Results:
a unique identifier for an Attesting Environment, the name of a key
which can be provably associated with that unique identifier, and the
set of Attestation Results which are signed using that key. As these
Attestation Results are signed by the Verifier, it is the Verifier
which is explicitly asserting the credentials it believes are
trustworthy.
An example of implicit communication would be to include Identity
Evidence in the form of a signature which has been placed over the
Attestation Results asserted by a Verifier. It would be then up to
the Relying Party's Appraisal Policy for Attestation Results to
extract this signature and confirm that it only could have been
generated by an Attesting Environment having access to a specific
private key. This implicit identity communication is only viable if
the Attesting Environment's public key is already known by the
Relying Party.
One final step in communicating identity is proving the freshness of
the Attestation Results to the degree needed by the Relying Party. A
typical way to accomplish this is to include an element of freshness
be embedded within a signed portion of the Attestation Results. This
element of freshness reduces the identity spoofing risks from a
replay attack. For more on this, see Section 2.4.
2.3. Trustworthiness Claims
2.3.1. Specific Claims
Trust is not absolute. Trust is a belief in some aspect of an
Attester, and that particular aspect is something upon which a
Relying Party depends. Consequently, a Verifier must be able to
assert different aspects of Attester trustworthiness.
Specific Claims of Verifier appraised trustworthiness have been
defined in this section. These are known as Trustworthiness Claims.
These Trustworthiness Claims may be either affirming (positive) or
detracting (negative). It is these Trustworthiness Claims which are
asserted within the Attestation Results produced by a Verifier. It
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is up to the Verifier to publish the types of evaluations it performs
when determining how Trustworthiness Claims are derived for a type of
Attester. This is one of the ways a Verifier can establish its
trustworthiness to a Relying Party. But it is out of the scope of
this document for the Verifier to provide proof or specific logic on
how an particular Trustworthiness Claim which has been asserted was
derived.
Following are the set of Trustworthiness Claims defined within this
document:
+========================+=============================+============+
| Trustworthiness Claim | Definition | +/- |
+========================+=============================+============+
| ae-instance-recognized | A Verifier has verified an | affirming |
| | Attesting Environment's | |
| | unique identity based on | |
| | some hardware based | |
| | private key signing | |
+------------------------+-----------------------------+------------+
| ae-instance-unknown | A Verifier has attempted | detracting |
| | and failed to verify an | |
| | Attesting Environment's | |
| | unique hardware protected | |
| | identity | |
+------------------------+-----------------------------+------------+
| config-insecure | A Verifier has appraised | detracting |
| | an Attester's | |
| | configuration, and has | |
| | found security issues | |
| | which should be addressed | |
+------------------------+-----------------------------+------------+
| config-secure | A Verifier has appraised | affirming |
| | an Attester's | |
| | configuration, and has | |
| | found no security issues | |
+------------------------+-----------------------------+------------+
| executables-fail | A Verifier has appraised | detracting |
| | that an Attester has | |
| | installed into runtime | |
| | memory executables, | |
| | scripts, or files other | |
| | than approved ones | |
+------------------------+-----------------------------+------------+
| executables-verified | A Verifier has appraised | affirming |
| | that an Attester has | |
| | installed into runtime | |
| | memory only a genuine set | |
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| | of approved executables, | |
| | scripts, and files during | |
| | and after boot | |
+------------------------+-----------------------------+------------+
| file-system-anomaly | A Verifier has found a | detracting |
| | passively stored file on | |
| | an Attester which should | |
| | not be present | |
+------------------------+-----------------------------+------------+
| hw-authentic | A Verifier has appraised | affirming |
| | an Attester as having | |
| | authentic hardware and | |
| | firmware | |
+------------------------+-----------------------------+------------+
| hw-verification-fail | A Verifier has appraised | detracting |
| | that an Attester has | |
| | failed its hardware or | |
| | firmware verification | |
+------------------------+-----------------------------+------------+
| runtime-confidential | A Verifier has appraised | affirming |
| | that an Attester's | |
| | executing target | |
| | environment is opaque to | |
| | the operating system, any | |
| | virtual machine manager, | |
| | and any applications | |
| | outside the target | |
| | environment. This is a | |
| | more secure superset of | |
| | 'target-isolation'. See | |
| | O.RUNTIME_CONFIDENTIALITY | |
| | from [GP-TEE-PP] | |
+------------------------+-----------------------------+------------+
| secure-storage | A Verifier has appraised | affirming |
| | that an Attester has a | |
| | Trusted Execution | |
| | Environment which encrypts | |
| | persistent storage using | |
| | keys unavailable outside | |
| | protected hardware. | |
| | Protections must meet the | |
| | capabilities of [OMTP-ATE] | |
| | Section 5, but need not be | |
| | hardware tamper resistant. | |
+------------------------+-----------------------------+------------+
| source-data-integrity | A Verifier has appraised | affirming |
| | that the Attester is | |
| | operating upon data inputs | |
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| | from an external Attester | |
| | having a Trustworthiness | |
| | Vector with no less than | |
| | the current Vector. | |
+------------------------+-----------------------------+------------+
| target-isolation | A Verifier has appraised | affirming |
| | that an Attester has both | |
| | execution and storage | |
| | space which is | |
| | inaccessible from any | |
| | other parallel application | |
| | or Guest VM running on the | |
| | Attester's physical | |
| | device. Note that a host | |
| | operator may still have | |
| | target environment | |
| | visibility however. See | |
| | O.TA_ISOLATION from | |
| | [GP-TEE-PP] | |
+------------------------+-----------------------------+------------+
Table 2
Each type of Attesting Environment MUST be able to support one or
more of the set of affirming Trustworthiness Claims listed above.
Additional Trustworthiness Claims may be defined in subsequent
documents, but the goal is to minimize these Trustworthiness Claims
to just Verifier appraisals which are directly actionable by the
Relying Party.
2.3.2. Trustworthiness Vector
Multiple Trustworthiness Claims may be asserted about an Attesting
Environment at single point in time. The set of Trustworthiness
Claims inserted into an instance of Attestation Results by a Verifier
is known as a Trustworthiness Vector. The order of Claims in the
vector is NOT meaningful. A Trustworthiness Vector with no
Trustworthiness Claims (i.e., a null Trustworthiness Vector) is a
valid construct. In this case, the Verifier is making no affirming
or detracting Trustworthiness Claims but is confirming that a
appraisal has been made.
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2.3.3. Trustworthiness Vector for a type of Attesting Environment
Some Trustworthiness Claims are implicit based on the underlying type
of Attesting Environment. For example, a validated MRSIGNER identity
can be present where the underlying [SGX] hardware is 'hw-authentic'.
Where such implicit Trustworthiness Claims exist, they do not have to
be explicitly included in the Trustworthiness Vector. However these
implicit Trustworthiness Claims SHOULD be considered as being present
by the Relying Party. Another way of saying this is if a
Trustworthiness Claim is automatically supported as a result of
coming from a specific type of TEE, that claim need not be
redundantly articulated. Such implicit Trustworthiness Claims can be
seen in the tables within Appendix A.2 and Appendix A.3.
Additionally, there are some Trustworthiness Claims which cannot be
adequately supported by an Attesting Environment. For example, it
would be difficult for an Attester that includes only a TPM (and no
other TEE) from ever having a Verifier appraise support for 'runtime-
confidential'. As such, a Relying Party would be acting properly if
it rejects any non-supportable Trustworthiness Claims asserted from a
Verifier.
As a result, the need for the ability to carry a specific
Trustworthiness Claim will vary by the type of Attesting Environment.
Example mappings can be seen in Appendix A.
2.4. Freshness
A Relying Party will care about the recentness of the Attestation
Results, and the specific Trustworthiness Claims which are embedded.
All freshness mechanisms of [I-D.ietf-rats-architecture], Section 10
are supportable by this specification.
Additionally, a Relying Party may track when a Verifier expires its
confidence for the Trustworthiness Claims or the Trustworthiness
Vector as a whole. Mechanisms for such expiry are not defined within
this document.
There is a subset of secure interactions where the freshness of
Trustworthiness Claims may need to be revisited asynchronously. This
subset is when trustworthiness depends on the continuous availability
of a transport session between the Attester and Relying Party. With
such connectivity dependent Attestation Results, if there is a reboot
which resets transport connectivity, all established Trustworthiness
Claims should be cleared. Subsequent connection re-establishment
will allow fresh new Trustworthiness Claims to be delivered.
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3. Secure Interactions Model
The establishment and maintenance of a connection between an Attester
and a Relying Party will follow the Passport Model from Section 5.1
of [I-D.ietf-rats-architecture]. Figure 1 describes this flow of
information using the time definitions described in
[I-D.ietf-rats-architecture]. Corresponding messages are passed
within an authentication framework, such the EAP protocol [RFC5247]
over TLS [RFC8446].
.----------------.
| Attester |
| .-------------.|
| | Attesting || .----------. .---------------.
| | Environment || | Verifier | | Relying Party |
| '-------------'| | A | | / Verifier B |
'----------------' '----------' '---------------'
time(VG) | |
|<------Verifer PoF--------time(NS) |
| | |
time(EG)(1)------Evidence------------>| |
| time(RG) |
|<------Attestation Results-(2) |
~ ~ ~
time(VG')? | |
~ ~ ~
|<------Relying Party PoF-----------------(3)time(NS')
| | |
time(EG')(4)------AR-augmented Evidence----------------->|
| | time(RG',RA')(5)
(6)
~
time(RX')
Figure 1: Secure Interactions Model
Figure 1 assumes that some form of time interval tracking is possible
between the Verifer PoF and Relying Party PoF. However, there is a
simplified case that does not require a Relying Party's PoF. In that
second variant, the Relying Party trusts that the Attester cannot be
meaningfully changed from the outside during that interval. Based on
that assumption, the Relying Party PoF can be safely omitted. In
essence, the AR-augmented Evidence is replaced by the stand-alone
Attestation Results.
In the first variant illustrated in Figure 1, a Verifier B is often
implemented as a code module within the Relying Party. In these
cases, the role Relying Party and the role Verifier are collapsed in
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one entity. As a result, the entity can appraise both the
Attestation Result parts as well as the Evidence parts of AR-
augmented Evidence to determine whether an Attester qualifies for
connection to the Relying Party's resources. Appraisal policies
define the conditions and prerequisites for when an Attester
qualifies for connection. In essence, an Attester has to be able to
provide all of the mandatory affirming Trustworthiness Claims needed
by a Relying Party's Appraisal Policy for Attestation Results, and
none of the disqualifying detracting Trustworthiness Claims.
More details on each interaction step are as follows. The numbers
used in this sequence match to the numbered steps in Figure 1:
1. An Attester sends Evidence which is provably fresh to Verifier A
at time(EG). Freshness from the perspective of Verifier A MAY be
established with Verifier PoF such as a nonce.
2. Verifier A appraises (1), then sends the following items back to
that Attester within Attestation Results:
1. the verified identity of the Attesting Environment,
2. the Verifier A appraised Trustworthiness Vector of an
Attester,
3. a freshness proof associated with the Attestation Results,
4. a Verifier signature across (2.1) though (2.3).
3. At time(EG') a Relying Party PoF (such as a nonce) known to the
Relying Party is sent to the Attester.
4. The Attester generates and sends AR-augmented Evidence to the
Relying Party/Verifier B. This AR-augmented Evidence includes:
1. The Attestation Results from (2)
2. Attestation Environment signing of a hash of the Attestation
Results plus the proof-of-freshness from (3). This allows
the delta of time between (2.3) and (3) to be definitively
calculated by the Relying Party.
5. On receipt of (4), the Relying Party applies its Appraisal Policy
for Attestation Results. At minimum, this appraisal policy
process must include the following:
1. Verify that (4.2) includes the nonce from (3).
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2. Use a local certificate to validate the signature (4.1).
3. Verify that the hash from (4.2) matches (4.1)
4. Use the identity of (2.1) to validate the signature of (4.2).
5. Failure of any steps (5.1) through (5.4) means the link does
not meet minimum validation criteria, therefore appraise the
link as having a null Verifier B Trustworthiness Vector.
Jump to step (6.1).
6. When there is large or uncertain time gap between time(EG)
and time(EG'), the link should be assigned a null Verifier B
Trustworthiness Vector. Jump to step (6.1).
7. Assemble the Verifier B Trustworthiness Vector
1. Copy Verifier A Trustworthiness Vector to Verifier B
Trustworthiness Vector
2. Add implicit Trustworthiness Claims inherent to the type
of TEE.
3. Prune any unbelievable Trustworthiness Claims
4. Prune any Trustworthiness Claims the Relying Party
doesn't accept from this Verifier.
6. The Relying Party takes action based on Verifier B's appraised
Trustworthiness Vector:
1. Prune any Trustworthiness Claims not used in the Appraisal
Policy for Attestion Results.
2. Allow the information exchange from the Attester into a
Relying Party context where the Verifier B appraised
Trustworthiness Vector includes all the mandatory affirming
Trustworthiness Claims, and none of the disqualifying
detracting Trustworthiness Claims.
3. Disallow any information exchange into a Relying Party
context for which that Verifier B appraised Trustworthiness
Vector is not qualified.
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As link layer protocols re-authenticate, steps (1) to (2) and steps
(3) to (6) will independently refresh. This allows the
Trustworthiness of Attester to be continuously re-appraised. There
are only specific triggers which will refresh Evidence generation
(1), Attestation Result generation (2), and in consequence AR-
augmented Evidence generation (4):
* life-cycle events, e.g. a change to an Authentication Secret of
the Attester or an update of a software component
* uptime-cycle events, e.g. a hard reset of a composite device or a
re-initialization of a TEE.
* authentication-cycle events, e.g. a link-layer interface resets or
new TLS session is spawned.
Additionally, it is common that each device on either side of a
connection will requires fresh remote attestation of its
corresponding peer. This process is known as mutual-attestation. To
support mutual-attestation, the process listed above may be run
independently on each side of the connection.
4. Privacy Considerations
Privacy Considerations Text
5. Security Considerations
Security Considerations Text
6. IANA Considerations
See Body.
7. References
7.1. Normative References
[GP-TEE-PP]
"Global Platform TEE Protection Profile v1.3", September
2020, <https://globalplatform.org/specs-library/tee-
protection-profile-v1-3/>.
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[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
13, 8 November 2021, <https://www.ietf.org/archive/id/
draft-ietf-rats-architecture-13.txt>.
[OMTP-ATE] "Open Mobile Terminal Platform - Advanced Trusted
Environment", May 2009, <https://www.gsma.com/newsroom/wp-
content/uploads/2012/03/
omtpadvancedtrustedenvironmentomtptr1v11.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[I-D.tschofenig-rats-psa-token]
Tschofenig, H., Frost, S., Brossard, M., Shaw, A., and T.
Fossati, "Arm's Platform Security Architecture (PSA)
Attestation Token", Work in Progress, Internet-Draft,
draft-tschofenig-rats-psa-token-08, 24 March 2021,
<https://www.ietf.org/archive/id/draft-tschofenig-rats-
psa-token-08.txt>.
[IEEE802.1AR]
"802.1AR: Secure Device Identity", 2 August 2018,
<https://ieeexplore.ieee.org/document/8423794>.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<https://www.rfc-editor.org/info/rfc5247>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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[SGX] "Supporting Third Party Attestation for Intel SGX with
Intel Data Center Attestation Primitives", 2017, <https://
software.intel.com/content/dam/develop/external/us/en/
documents/intel-sgx-support-for-third-party-attestation-
801017.pdf>.
[TPM-ID] "TPM Keys for Platform Identity for TPM 1.2", August 2015,
<https://www.trustedcomputinggroup.org/wp-content/uploads/
TPM_Keys_for_Platform_Identity_v1_0_r3_Final.pdf>.
[US-Executive-Order]
"Executive Order on Improving the Nation's Cybersecurity",
12 May 2021, <https://www.whitehouse.gov/briefing-room/
presidential-actions/2021/05/12/executive-order-on-
improving-the-nations-cybersecurity/>.
Appendix A. Supportable Trustworthiness Claims
The following is a table which shows what Claims are supportable by
different Attesting Environment types. Note that claims MAY BE
implicit to an Attesting Environment type, and therefore do not have
to be included in the Trustworthiness Vector to be considered as set
by the Relying Party.
A.1. Supportable Trustworthiness Claims for HSM-based CC
Following are Trustworthiness Claims which MAY be set for a HSM-based
Confidential Computing Attester. (Such as a TPM.)
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+========================+=======================================+
| Trustworthiness Claim | TPM |
+========================+=======================================+
| ae-instance-recognized | Optional |
+------------------------+---------------------------------------+
| ae-instance-unknown | Optional |
+------------------------+---------------------------------------+
| config-insecure | Optional |
+------------------------+---------------------------------------+
| config-secure | Verifier evaluation of Attester |
| | reveals no configuration lines which |
| | expose the Attester to known security |
| | vulnerabilities. |
+------------------------+---------------------------------------+
| executables-refuted | If PCR checks fail for the static |
| | operating system, and for any tracked |
| | files subsequently loaded |
+------------------------+---------------------------------------+
| executables-verified | If PCRs check for the static |
| | operating system, and for any tracked |
| | files subsequently loaded |
+------------------------+---------------------------------------+
| file-system-anomaly | Verifier evaluation of Attester |
| | reveals an unexpected file. |
+------------------------+---------------------------------------+
| hw-authentic | If PCR check ok from BIOS checks, |
| | through Master Boot Record |
| | configuration |
+------------------------+---------------------------------------+
| hw-verification-fail | If PCR don't check ok |
+------------------------+---------------------------------------+
| runtime-confidential | TPMs do not provide a sufficient |
| | technology base for this claim. |
+------------------------+---------------------------------------+
| secure-storage | Minimal secure storage space exists |
| | and is writeable by external |
| | applications. This space would |
| | typically just be used to store keys. |
+------------------------+---------------------------------------+
| source-data-integrity | Optional |
+------------------------+---------------------------------------+
| target-isolation | This can be set only if no other |
| | applications are running on the |
| | Attester |
+------------------------+---------------------------------------+
Table 3
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Setting the Trustworthiness Claims may follow the following logic at
the Verifier A within (2) of Figure 1:
Start: Evidence received starts the generation of a new
Trustworthiness Vector. (e.g., TPM Quote Received, log received,
or appraisal timer expired)
Step 0: set Trustworthiness Vector = Null
Step 1: Is there sufficient fresh signed evidence to appraise?
(yes) - No Action
(no) - Goto Step 6
Step 2: Appraise Hardware Integrity PCRs
(if hw-verification-fail) - push onto vector, go to Step 6
else (if hw-authentic) - push onto vector
(if not evaluated, or insufficient data to conclude: take no action)
Step 3: Appraise Attesting Environment identity
(if hw-instance-recognized) - push onto vector
else (if hw-instance-unknown) - push onto vector
(if not evaluated, or insufficient data to conclude: take no action)
Step 4: Appraise executable loaded and filesystem integrity
(if executables-verified) - push onto vector
else (if executables-refuted) - push onto vector, go to Step 6
(if file-system-anomaly) - push onto vector, go to Step 6
(if not evaluated, or insufficient data to conclude: take no action)
Step 5: Appraise all remaining Trustworthiness Claims and set as
appropriate.
Step 6: Assemble Attestation Results, and push to Attester
End
A.2. Supportable Trustworthiness Claims for process-based CC
Following are Trustworthiness Claims which MAY be set for a process-
based Confidential Computing based Attester. (Such as a SGX Enclaves
and TrustZone.)
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+========================+==============================+
| Trustworthiness Claim | Process-based |
+========================+==============================+
| ae-instance-recognized | Optional |
+------------------------+------------------------------+
| ae-instance-unknown | Optional |
+------------------------+------------------------------+
| config-insecure | Optional |
+------------------------+------------------------------+
| config-secure | Optional |
+------------------------+------------------------------+
| executables-refuted | Optional |
+------------------------+------------------------------+
| executables-verified | Optional |
+------------------------+------------------------------+
| file-system-anomaly | n/a |
+------------------------+------------------------------+
| hw-authentic | Implicit in signature |
+------------------------+------------------------------+
| hw-verification-fail | Implicit if signature not ok |
+------------------------+------------------------------+
| runtime-confidential | Implicit in signature |
+------------------------+------------------------------+
| target-isolation | Implicit in signature |
+------------------------+------------------------------+
| secure-storage | Implicit in signature |
+------------------------+------------------------------+
| source-data-integrity | Optional |
+------------------------+------------------------------+
Table 4
A.3. Supportable Trustworthiness Claims for VM-based CC
Following are Trustworthiness Claims which MAY be set for a VM-based
Confidential Computing based Attester. (Such as SEV, TDX, ACCA, SEV-
SNP.)
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+========================+=======================+
| Trustworthiness Claim | Process-based |
+========================+=======================+
| ae-instance-recognized | Optional |
+------------------------+-----------------------+
| ae-instance-unknown | Optional |
+------------------------+-----------------------+
| config-insecure | Optional |
+------------------------+-----------------------+
| config-secure | Optional |
+------------------------+-----------------------+
| executables-refuted | Optional |
+------------------------+-----------------------+
| executables-verified | Optional |
+------------------------+-----------------------+
| file-system-anomaly | Optional |
+------------------------+-----------------------+
| hw-authentic | Chip dependent |
+------------------------+-----------------------+
| hw-verification-fail | Chip dependent |
+------------------------+-----------------------+
| runtime-confidential | Implicit |
+------------------------+-----------------------+
| target-isolation | Implicit in signature |
+------------------------+-----------------------+
| secure-storage | Chip dependent |
+------------------------+-----------------------+
| source-data-integrity | Optional |
+------------------------+-----------------------+
Table 5
Appendix B. Some issues being worked
It is possible for a cluster/hierarchy of Verifiers to have aggregate
AR which are perhaps signed/endorsed by a lead Verifier. What should
be the Proof-of-Freshness or Verifier associated with any of the
aggregate set of Trustworthiness Claims?
There will need to be a subsequent document which documents how these
objects which will be translated into a protocol on a wire (e.g. EAP
on TLS). Some breakpoint between what is in this draft, and what is
in specific drafts for wire encoding will need to be determined.
Questions like architecting the cluster/hierarchy of Verifiers fall
into this breakdown.
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For Trustworthiness Claims such as 'exectables-verified', there could
be value in identifying a specific Appraisal Policy for Attestation
Results applied. One way this could be done would be a URI which
identifies this policy. As the URI also could encode the version of
the software, it might also act as a mechanism to signal the Relying
Party to refresh/re-evaluate its view of Verifier A.
Expand the variant of Figure 1 which requires no Relying Party PoF
into its own picture.
Rather than duplicating claim concepts for affirming vs detracting,
perhaps we could collapse them and have affirming vs detracting be
part of the value. Not collapsing complicates the test matrix.
Normalization of the identity claims between different types of TEE.
E.g., does MRSIGNER plus extra loaded software = the sum of TrustZone
Signer IDs for loaded components?
Appendix C. Contributors
Guy Fedorkow
Email: gfedorkow@juniper.net
Dave Thaler
Email: dthaler@microsoft.com
Authors' Addresses
Eric Voit
Cisco Systems
Email: evoit@cisco.com
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
Thomas Hardjono
MIT
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Email: hardjono@mit.edu
Thomas Fossati
Arm Limited
Email: Thomas.Fossati@arm.com
Vincent Scarlata
Intel
Email: vincent.r.scarlata@intel.com
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