Internet DRAFT - draft-birkholz-rats-reference-interaction-model
draft-birkholz-rats-reference-interaction-model
RATS Working Group H. Birkholz
Internet-Draft M. Eckel
Intended status: Informational Fraunhofer SIT
Expires: January 9, 2021 C. Newton
L. Chen
University of Surrey
July 08, 2020
Reference Interaction Models for Remote Attestation Procedures
draft-birkholz-rats-reference-interaction-model-03
Abstract
This document describes interaction models for remote attestation
procedures (RATS). Three conveying mechanisms - Challenge/Response,
Uni-Directional, and Streaming Remote Attestation - are illustrated
and defined. Analogously, a general overview about the information
elements typically used by corresponding conveyance protocols are
highlighted. Privacy preserving conveyance of Evidence via Direct
Anonymous Attestation is elaborated on for each interaction model,
individually.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 9, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Disambiguation . . . . . . . . . . . . . . . . . . . . . . . 4
4. Scope and Intent . . . . . . . . . . . . . . . . . . . . . . 4
5. Direct Anonymous Attestation . . . . . . . . . . . . . . . . 5
5.1. Endorsers . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2. Endorsers for Direct Anonymous Attestation . . . . . . . 6
6. Normative Prerequisites . . . . . . . . . . . . . . . . . . . 6
7. Generic Information Elements . . . . . . . . . . . . . . . . 7
8. Interaction Models . . . . . . . . . . . . . . . . . . . . . 9
8.1. Challenge/Response Remote Attestation . . . . . . . . . . 10
8.2. Uni-Directional Remote Attestation . . . . . . . . . . . 11
8.3. Streaming Remote Attestation . . . . . . . . . . . . . . 13
9. Additional Application-Specific Requirements . . . . . . . . 15
9.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 15
9.2. Mutual Authentication . . . . . . . . . . . . . . . . . . 15
9.3. Hardware-Enforcement/Support . . . . . . . . . . . . . . 15
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 15
10.1. Implementer . . . . . . . . . . . . . . . . . . . . . . 16
10.2. Implementation Name . . . . . . . . . . . . . . . . . . 16
10.3. Implementation URL . . . . . . . . . . . . . . . . . . . 16
10.4. Maturity . . . . . . . . . . . . . . . . . . . . . . . . 16
10.5. Coverage and Version Compatibility . . . . . . . . . . . 16
10.6. License . . . . . . . . . . . . . . . . . . . . . . . . 16
10.7. Implementation Dependencies . . . . . . . . . . . . . . 16
10.8. Contact . . . . . . . . . . . . . . . . . . . . . . . . 17
11. Security and Privacy Considerations . . . . . . . . . . . . . 17
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 17
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
14.1. Normative References . . . . . . . . . . . . . . . . . . 19
14.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. CDDL Specification for a simple CoAP
Challenge/Response Interaction . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Remote ATtestation procedureS (RATS, [I-D.ietf-rats-architecture])
are workflows composed of roles and interactions, in which Verifiers
create Attestation Results about the trustworthiness of an Attester's
system component characteristics. The Verifier's assessment in the
form of Attestation Results is created based on Attestation Policies
and Evidence - trustable and tamper-evident Claims Sets about an
Attester's system component characteristics - created by an Attester.
The roles _Attester_ and _Verifier_, as well as the Conceptual
Messages _Evidence_ and _Attestation Results_ are terms defined by
the RATS Architecture [I-D.ietf-rats-architecture]. This documents
captures interaction models that can be used in specific RATS-related
solution documents. The primary focus of this document is the
conveyance of attestation Evidence. Specific goals of this document
are to:
o prevent inconsistencies in descriptions of these interaction
models in other documents (due to text cloning over time),
o enable to highlight an exact delta/divergence between the core set
of characteristics captured here in this document and variants of
these interaction models used in other specifications or
solutions, and to
o illustrate the application of Direct Anonymous Attestation (DAA)
for each of the interaction models described.
In summary, this document enables the specification and design of
trustworthy and privacy preserving conveyance methods for attestation
Evidence from an Attester to a Verifier. While the conveyance of
other Conceptual Messages is out-of-scope the methods described can
also be applied to the conveyance of Endorsements or Attestation
Results.
2. Terminology
This document uses the terms, roles, and concepts defined in
[I-D.ietf-rats-architecture]:
Attester, Verifier, Relying Party, Conceptual Message, Evidence,
Endorsement, Attestation Result, Appraisal Policy, Attesting
Environment, Target Environment
A PKIX Certificate is an X.509v3 format certificate as specified by
[RFC5280].
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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.
3. Disambiguation
The term "Remote Attestation" is a common expression and often
associated or connoted with certain properties. The term "Remote" in
this context does not necessarily refer to a remote entity in the
scope of network topologies or the Internet. It rather refers to a
decoupled system or entities that exchange the payload of the
Conceptual Message type called Evidence [I-D.ietf-rats-architecture].
This conveyance can also be "local", if the Verifier is part of the
same entity as the Attester, e.g., separate system components of a
Composite Device (a single RATS Entity). Examples of these types of
co-located environments include: a Trusted Execution Environment
(TEE), Baseboard Management Controllers (BMCs), as well as other
physical or logical protected/isolated/shielded Computing
Environments (e.g. embedded Secure Elements (eSE) or Trusted Platform
Modules (TPM)).
4. Scope and Intent
This document focuses on generic interaction models between Attesters
and Verifiers in order to convey Evidence. Complementary procedures,
functions, or services that are required for a complete semantic
binding of the concepts defined in [I-D.ietf-rats-architecture] are
out-of-scope of this document. Examples include: identity
establishment, key distribution and enrollment, time synchronization,
as well as certificate revocation.
Furthermore, any processes and duties that go beyond carrying out
remote attestation procedures are out-of-scope. For instance, using
the results of a remote attestation that are created by the Verifier,
e.g., how to triggering remediation actions or recovery processes, as
well as such remediation actions and recovery processes themselves,
are also out-of-scope.
The interaction models illustrated in this document are intended to
provide a stable basis and reference for other solutions documents
inside or outside the IETF. Solution documents of any kind can
reference the interaction models in order to avoid text clones and to
avoid the danger of subtle discrepancies. Analogously, deviations
from the generic model descriptions in this document can be
illustrated in solutions documents to highlight distinct
contributions.
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5. Direct Anonymous Attestation
DAA [DAA] is a signature scheme used in RATS that allows preservation
of the privacy of users that are associated with an Attester (e.g.
its owner). Essentially, DAA can be seen as a group signature scheme
with the feature that given a DAA signature no-one can find out who
the signer is, i.e., the anonymity is not revocable. To be able to
sign anonymously an Attester has to obtain a credential from a DAA
Issuer. The DAA Issuer uses a private/public key pair to generate a
credential for an Attester and makes the public key (in the form of a
public key certificate) available to the verifier to enable them to
validate the DAA signature obtained as part of the Evidence.
In order to support these DAA signatures, the DAA Issuer MUST
associate a single key pair with each group of Attesters and use the
same key pair when creating the credentials for all of the Attesters
in this group. The DAA Issuer's public key certificate for the group
replaces the Attester Identity documents in the verification of the
Evidence (instead of unique Attester Identity documents). This is in
contrast to intuition that there has to be a unique Attester Identity
per device.
This document extends the duties of the Endorser role as defined by
the RATS architecture with respect to the provision of these Attester
Identity documents to Attesters. The existing duties of the Endorser
role and the duties of a DAA Issuer are quite similar as illustrated
in the following subsections.
5.1. Endorsers
Via its Attesting Environments, an Attester can only create Evidence
about its Target Environments. After being appraised to be
trustworthy, a Target Environment may become a new Attesting
Environment in charge of creating Evidence for further Target
Environments. [I-D.ietf-rats-architecture] explains this as Layered
Attestation. Layered Attestation has to start with an initial
Attesting Environment (i.e., there cannot be turtles all the way down
[turtles]). At this rock bottom of Layered Attestation, the
Attesting Environments are called Roots of Trust (RoT). An Attester
cannot create Evidence about its own RoTs by design. As a
consequence, a Verifier requires trustable statements about this
subset of Attesting Environments from a different source than the
Attester itself. The corresponding trustable statements are called
Endorsements and originate from external, trustable entities that
take on the role of an Endorser (e.g., supply chain entities).
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5.2. Endorsers for Direct Anonymous Attestation
In order to enable DAA to be used, an Endorser role takes on the
duties of a DAA Issuer in addition to its already defined duties.
DAA Issuers offer zero-knowledge proofs based on public key
certificates used for a group of Attesters [DAA]. Effectively, these
certificates share the semantics of Endorsements. The differences
are:
o The associated private keys are used by the DAA Issuer to provide
an Attester with a credential that it can use to convince the
Verifier that its Evidence is valid. To keep their anonymity the
Attester randomises this credential each time that it is used.
o The Verifier can use the DAA Issuer's public key certificate,
together with the randomised credential from the Attester, to
confirm that the Evidence comes from a valid Attester.
o A credential is conveyed from an Endorser to an Attester together
with the transfer of the public key certificates from Endorser to
Verifier.
The zero-knowledge proofs required cannot be created by an Attester
alone - like the Endorsements of RoTs - and have to be created by a
trustable third entity - like an Endorser. Due to that vast semantic
overlap (XXX-mcr:explain), an Endorser in this document can convey
trustable third party statements both to a Verifier and an Attester.
6. Normative Prerequisites
In order to ensure an appropriate conveyance of Evidence, the
following set of prerequisites MUST be in place to support the
implementation of interaction models:
Attester Identity: The provenance of Evidence with respect to a
distinguishable Attesting Environment MUST be correct and
unambiguous.
An Attester Identity MAY be a unique identity, it MAY be included
in a zero-knowledge proof (ZKP), or it MAY be part of a group
signature, or it MAY be a randomised DAA credential.
Attestation Evidence Authenticity: Attestation Evidence MUST be
correct and authentic.
In order to provide proofs of authenticity, Attestation Evidence
SHOULD be cryptographically associated with an identity document
(e.g. an PKIX certificate or trusted key material, or a randomised
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DAA credential), or SHOULD include a correct and unambiguous and
stable reference to an accessible identity document.
Authentication Secret: An Authentication Secret MUST be available
exclusively to an Attester's Attesting Environment.
The Attester MUST protect Claims with that Authentication Secret,
thereby proving the authenticity of the Claims included in
Evidence. The Authentication Secret MUST be established before
RATS can take place.
Evidence Freshness: Evidence MUST include an indicator about its
Freshness that can be understood by a Verifier. Analogously,
interaction models MUST support the conveyance of proofs of
freshness in a way that is useful to Verifiers and their appraisal
procedures.
Evidence Protection: Evidence MUST be a set of well-formatted and
well-protected Claims that an Attester can create and convey to a
Verifier in a tamper-evident manner.
7. Generic Information Elements
This section defines the information elements that are vital to all
kinds interaction models. Varying from solution to solution, generic
information elements can be either included in the scope of protocol
messages or can be included in their payload. Ultimately, the
following information elements are required by any kind of scalable
remote attestation procedure using one or more of the interaction
models provided.
Attester Identity ('attesterIdentity'): _mandatory_
A statement about a distinguishable Attester made by an Endorser
without accompanying evidence about its validity - used as proof
of identity.
In DAA the Attester's identity is not revealed to the verifier.
The Attester is issued with a credential by the Endorser that is
randomised and then used to anonymously confirm the validity of
their evidence. The evidence is verified using the Endorser's
public key.
Authentication Secret IDs ('authSecID'): _mandatory_
A statement representing an identifier list that MUST be
associated with corresponding Authentication Secrets used to
protect Evidence. In DAA, Authentication Secret IDs are
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represented by the Endorser (DAA issuer)'s public key that MUST be
used to create DAA credentials for the corresponding
Authentication Secrets used to protect Evidence.
Each Authentication Secret is uniquely associated with a
distinguishable Attesting Environment. Consequently, an
Authentication Secret ID also identifies an Attesting Environment.
In DAA an Authentication Secret ID does not identify a unique
Attesting Environment but associated with a group of Attesting
Environments. This is because an Attesting Environment should not
be distinguishable and the DAA credential which represents the
Attesting Environment is randomised each time it used.
Handle ('handle'): _mandatory_
A statement that is intended to uniquely distinguish received
Evidence and/or determine the Freshness of Evidence.
A Verifier can also use a Handle as an indicator for authenticity
or attestation provenance, as only Attesters and Verifiers that
are intended to exchange Evidence should have knowledge of the
corresponding Handles. Examples include Nonces or signed
timestamps.
Claims ('claims'): _mandatory_
Claims are assertions that represent characteristics of an
Attester's Target Environment.
Claims are part Conceptual Message and are, for example, used to
appraise the integrity of Attesters via a Verifiers. The other
information elements in this section can be expressed as Claims in
any type of Conceptional Messages.
Reference Claims ('refClaims') _mandatory_
Reference Claims are a specific subset of Appraisal Policies as
defined in [I-D.ietf-rats-architecture].
Reference Claims are used to appraise the Claims received from an
Attester via appraisal by direct comparison. For example,
Reference Claims MAY be Reference Integrity Measurements (RIM) or
assertions that are implicitly trusted because they are signed by
a trusted authority (see Endorsements in
[I-D.ietf-rats-architecture]). Reference Claims typically
represent (trusted) Claim sets about an Attester's intended
platform operational state.
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Claim Selection ('claimSelection'): _optional_
A statement that represents a (sub-)set of Claims that can be
created by an Attester.
Claim Selections can act as filters that can specify the exact set
of Claims to be included in Evidence. An Attester MAY decide
whether or not to provide all Claims as requested via a Claim
Selection.
Evidence ('signedAttestationEvidence'): _mandatory_
A set of Claims that consists of a list of Authentication Secret
IDs that each identifies an Authentication Secret in a single
Attesting Environment, the Attester Identity, Claims, and a
Handle. Attestation Evidence MUST cryptographically bind all of
these information elements. The Evidence MUST be protected via
the Authentication Secret. The Authentication Secret MUST be
trusted by the Verifier as authoritative.
Attestation Result ('attestationResult'): _mandatory_
An Attestation Result is produced by the Verifier as the output of
the appraisal of Evidence. Attestation Results include condensed
assertions about integrity or other characteristics of the
corresponding Attester.
8. Interaction Models
The following subsections introduce and illustrate the interaction
models:
1. Challenge/Response Remote Attestation
2. Uni-Directional Remote Attestation
3. Streaming Remote Attestation
Each section starts with a sequence diagram illustrating the
interactions between Attester and Verifier. The other roles RATS
roles - mainly Relying Parties and Endorsers - are not relevant for
this interaction model. While the interaction models presented focus
on the conveyance of Evidence, future work could apply this to the
conveyance of other Conceptual Messages, namely Attestation Results,
Endorsements, or Appraisal Policies.
All interaction model have a strong focus on the use of a handle to
incorporate a proof of freshness. The ways these handles are
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processed is the most prominent difference between the three
interaction models.
8.1. Challenge/Response Remote Attestation
.----------. .----------.
| Attester | | Verifier |
'----------' '----------'
| |
| |
valueGeneration(targetEnvironment) |
| => claims |
| |
| <------requestEvidence(handle, authSecIDs, claimSelection) |
| |
claimsCollection(claimSelection) |
| => collectedClaims |
| |
evidenceGeneration(handle, authSecIDs, collectedClaims) |
| => evidence |
| |
| returnEvidence-------------------------------------------> |
| returnEventLog-------------------------------------------> |
| |
| evidenceAppraisal(evidence, eventLog, refClaims)
| attestationResult <= |
| |
This Challenge/Response Remote Attestation procedure is initiated by
the Verifier, by sending a remote attestation request to the
Attester. A request includes a Handle, a list of Authentication
Secret IDs, and a Claim Selection.
In the Challenge/Response model, the handle is composed of qualifying
data in the form of a cryptographically strongly randomly generated,
and therefore unpredictable, nonce. The Verifier-generated nonce is
intended to guarantee Evidence freshness.
The list of Authentication Secret IDs selects the attestation keys
with which the Attester is requested to sign the Attestation
Evidence. Each selected key is uniquely associated with an Attesting
Environment of the Attester. As a result, a single Authentication
Secret ID identifies a single Attesting Environment.
Analogously, a particular set of Evidence originating from a
particular Attesting Environments in a composite device can be
requested via multiple Authentication Secret IDs. Methods to acquire
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Authentication Secret IDs or mappings between Attesting Environments
to Authentication Secret IDs are out-of-scope of this document.
The Claim Selection narrows down the set of Claims collected and used
to create Evidence to those that the Verifier requires. If the Claim
Selection is omitted, then by default all Claims that are known and
available on the Attester MUST be used to create corresponding
Evidence. For example when performing a boot integrity evaluation, a
Verifier may only be requesting a particular subset of claims about
the Attester, such as Evidence about BIOS and firmware the Attester
booted up, and not include information about all currently running
software.
While it is crucial that Claims, the Handle, as well as the Attester
Identity information MUST be cryptographically bound to the signature
of Evidence, they may be presented in an encrypted form.
Cryptographic blinding MAY be used at this point. For further
reference see section Section 11.
As soon as the Verifier receives signed Evidence, it validates the
signature, the Attester Identity, as well as the Nonce, and appraises
the Claims. Appraisal procedures are application-specific and can be
conducted via comparison of the Claims with corresponding Reference
Claims, such as Reference Integrity Measurements. The final output
of the Verifier are Attestation Results. Attestation Results
constitute new Claims Sets about an Attester's properties and
characteristics that enables Relying Parties, for example, to assess
an Attester's trustworthiness.
8.2. Uni-Directional Remote Attestation
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.----------. .----------.
| Attester | | Verifier |
'----------' '----------'
| |
valueGeneration(targetEnvironment) |
| => claims |
| |
| .--------------------. |
| <----------handle | | |
| | Handle Distributor | |
| | | handle----------> |
| '--------------------' |
| |
evidenceGeneration(handle, authSecIDs, collectedClaims) |
| => evidence |
| |
| pushEventLog---------------------------------------------> |
| pushEvidence---------------------------------------------> |
| |
| appraiseEvidence(evidence, eventLog, refClaims)
| evidenceAppraisal(evidence, refClaims)
| attestationResult <= |
~ ~
| |
valueGeneration(targetEnvironment) |
| => claimsDelta |
| |
evidenceGeneration(handle, authSecIDs, collectedClaims) |
| => evidence |
| |
| pushEventLogDelta----------------------------------------> |
| pushEvidence---------------------------------------------> |
| |
| appraiseEvidence(evidence, eventLogDelta, refClaims)
| evidenceAppraisal(evidence, refClaims)
| attestationResult <= |
| |
Uni-Directional Remote Attestation procedures can be initiated both
by the Attester and by the Verifier. Initiation by the Attester can
result in unsolicited pushes of Evidence to the Verifier. Initiation
by the Verifier always results in solicited pushes to the Verifier.
The Uni-Directional model uses the same information elements as the
Challenge/Response model. In the sequence diagram above, the
Attester initiates the conveyance of Evidence (comparable with a
RESTful POST operation or the emission of a beacon). While a request
of evidence from the Verifier would result in a sequence diagram more
similar to the Challenge/Response model (comparable with a RESTful
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GET operation), the specific manner how handles are created and used
always remains as the distinguishing quality of this model. In the
Uni-Directional model, handles are composed of trustable signed
timestamps as shown in [I-D.birkholz-rats-tuda], potentially
including other qualifying data. The handles are created by an
external 3rd entity - the Handle Distributor - that includes a
trustworthy source of time and takes on the role of a Time Stamping
Authority (TSA, as initially defined in [RFC3161]). Timstamps
created from local clocks (absolute clocks using a global timescale,
as well as relative clocks, such as tick-counters) of Attesters and
Verifiers MUST be cryptographically bound to fresh Handles received
from the Handle Distributor. This binding provides a proof of
synchronization that MUST be included in every evidence created.
Correspondingly, evidence created for conveyance via this model
provides a proof that it was fresh at a certain point in time.
Effectively, this allows for series of evidence to be pushed to
multiple Verifiers, simultaniously. Methods to detect excessive time
drift that would mandate a fresh Handle to be received by the Handle
Distributor, as well as timing of handle distribution are out-of-
scope of this document.
8.3. Streaming Remote Attestation
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.----------. .----------.
| Attester | | Verifier |
'----------' '----------'
| |
valueGeneration(targetEnvironment) |
| => claims |
| |
| <----subscribeEvidence(handle, authSecIDs, claimSelection) |
| subscriptionResult --------------------------------------> |
| |
evidenceGeneration(handle, authSecIDs, collectedClaims) |
| => evidence |
| |
| pushEventLog---------------------------------------------> |
| pushEvidence---------------------------------------------> |
| |
| evidenceAppraisal(evidence, eventLog, refClaims)
| attestationResult <= |
~ ~
| |
valueGeneration(targetEnvironment) |
| => claimsDelta |
| |
evidenceGeneration(handle, authSecIDs, collectedClaims) |
| => evidence |
| |
| pushEventLogDelta----------------------------------------> |
| pushEvidence---------------------------------------------> |
| |
| evidenceAppraisal(evidence, eventLogDelta, refClaims)
| attestationResult <= |
| |
Streaming Remote Attestation procedures require the setup of
subscription state. Setting up subscription state between a Verifier
and an Attester is conducted via a subscribe operation. This
subscribe operation is used to convey the handles required for
Evidence generation. Effectively, this allows for series of evidence
to be pushed to a Verifier similar to the Uni-Directional model.
While a Handle Distributor is not required in this model, it is also
limited to bi-lateral subscription relationships, in which each
Verifier has to create and provide its individual handle. Handles
provided by a specific subscribing Verifier MUST be used in Evidence
generation for that specific Verifier. The Streaming model uses the
same information elements as the Challenge/Response and the Uni-
Directional model. Methods to detect excessive time drift that would
mandate a refreshed Handle to be conveyed via another subscribe
operation are out-of-scope of this document.
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9. Additional Application-Specific Requirements
Depending on the use cases covered, there can be additional
requirements. An exemplary subset is illustrated in this section.
9.1. Confidentiality
Confidentiality of exchanged attestation information may be
desirable. This requirement usually is present when communication
takes place over insecure channels, such as the public Internet. In
such cases, TLS may be uses as a suitable communication protocol that
preserves confidentiality. In private networks, such as carrier
management networks, it must be evaluated whether or not the
transport medium is considered confidential.
9.2. Mutual Authentication
In particular use cases mutual authentication may be desirable in
such a way that a Verifier also needs to prove its identity to the
Attester, instead of only the Attester proving its identity to the
Verifier.
9.3. Hardware-Enforcement/Support
Depending on given usage scenarios, hardware support for secure
storage of cryptographic keys, crypto accelerators, as well as
protected or isolated execution environments can be mandatory
requirements. Well-known technologies in support of these
requirements are roots of trusts, such as Hardware Security Modules
(HSM), Physically Unclonable Functions (PUFs), Shielded Secrets, or
Trusted Executions Environments (TEEs).
10. Implementation Status
Note to RFC Editor: Please remove this section as well as references
to [BCP205] before AUTH48.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [BCP205].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
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features. Readers are advised to note that other implementations may
exist.
According to [BCP205], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
10.1. Implementer
The open-source implementation was initiated and is maintained by the
Fraunhofer Institute for Secure Information Technology - SIT.
10.2. Implementation Name
The open-source implementation is named "CHAllenge-Response based
Remote Attestation" or in short: CHARRA.
10.3. Implementation URL
The open-source implementation project resource can be located via:
https://github.com/Fraunhofer-SIT/charra
10.4. Maturity
The code's level of maturity is considered to be "prototype".
10.5. Coverage and Version Compatibility
The current version (commit '847bcde') is aligned with the exemplary
specification of the CoAP FETCH bodies defined in section Appendix A
of this document.
10.6. License
The CHARRA project and all corresponding code and data maintained on
github are provided under the BSD 3-Clause "New" or "Revised"
license.
10.7. Implementation Dependencies
The implementation requires the use of the official Trusted Computing
Group (TCG) open-source Trusted Software Stack (TSS) for the Trusted
Platform Module (TPM) 2.0. The corresponding code and data is also
maintained on github and the project resources can be located via:
https://github.com/tpm2-software/tpm2-tss/
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The implementation uses the Constrained Application Protocol
[RFC7252] (http://coap.technology/) and the Concise Binary Object
Representation [RFC7049] (https://cbor.io/).
10.8. Contact
Michael Eckel (michael.eckel@sit.fraunhofer.de)
11. Security and Privacy Considerations
In a remote attestation procedure the Verifier or the Attester MAY
want to cryptographically blind several attributes. For instance,
information can be part of the signature after applying a one-way
function (e. g. a hash function).
There is also a possibility to scramble the Nonce or Attester
Identity with other information that is known to both the Verifier
and Attester. A prominent example is the IP address of the Attester
that usually is known by the Attester itself as well as the Verifier.
This extra information can be used to scramble the Nonce in order to
counter certain types of relay attacks.
12. Acknowledgments
Olaf Bergmann, Michael Richardson, and Ned Smith
13. Change Log
o Initial draft -00
o Changes from version 00 to version 01:
* Added details to the flow diagram
* Integrated comments from Ned Smith (Intel)
* Reorganized sections and
* Updated interaction model
* Replaced "claims" with "assertions"
* Added proof-of-concept CDDL for CBOR via CoAP based on a TPM
2.0 quote operation
o Changes from version 01 to version 02:
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* Revised the relabeling of "claims" with "assertion" in
alignment with the RATS Architecture I-D.
* Added Implementation Status section
* Updated interaction model
* Text revisions based on changes in [I-D.ietf-rats-architecture]
and comments provided on rats@ietf.org.
o Changes from version 02 to version 00 RATS related document
* update of the challenge/response diagram
* minor rephrasing of Prerequisites section
* rephrasing to information elements and interaction model
section
o Changes from version 00 to version 01
* added Attestation Authenticity, updated Identity and Secret
* relabeled Secret ID to Authentication Secret ID + rephrasing
* relabeled Claim Selection to Assertion Selection + rephrasing
* relabeled Evidence to (Signed) Attestation Evidence
* Added Attestation Result and Reference Assertions
* update of the challenge/response diagram and expositional text
* added CDDL spec for CoAP FETCH operation proof-of-concept
o Changes from version 01 to version 02
* prepared the inclusion of additional reference models
* update to Introduction and Scope section
* major update to (Normative) Prerequisites
* relabeled Attestation Authenticity to Att. Evidence
Authenticity
* relabeled Assertion term back to Claim terms
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* added BCP205 Implementation Status section related to
Appendix CDDL
o Changes from version 02 to version 03
* major refactoring to now accommodate three interaction models
* updated existing and added two new diagrams for models
* major refactoring of existing and adding of new diagram
description
* incorporated content about Direct Anonymous Attestation
* integrated comments from Michael Richardson
* updated roster
14. References
14.1. Normative References
[BCP205] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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>.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
2001, <https://www.rfc-editor.org/info/rfc3161>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
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[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[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>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
14.2. Informative References
[DAA] Brickell, E., Camenisch, J., and L. Chen, "Direct
Anonymous Attestation", ACM Proceedings of the 11rd ACM
conference on Computer and Communications Security ,
page 132-145, 2004.
[I-D.birkholz-rats-tuda]
Fuchs, A., Birkholz, H., McDonald, I., and C. Bormann,
"Time-Based Uni-Directional Attestation", draft-birkholz-
rats-tuda-02 (work in progress), March 2020.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-04 (work in progress), May
2020.
[turtles] Wikipedia, "Turrles all the way down", July 2020,
<https://en.wikipedia.org/wiki/Turtles_all_the_way_down>.
Appendix A. CDDL Specification for a simple CoAP Challenge/Response
Interaction
The following CDDL specification is an exemplary proof-of-concept to
illustrate a potential implementation of the Challenge/Response
Interaction Model. The transfer protocol used is CoAP using the
FETCH operation. The actual resource operated on can be empty. Both
the Challenge Message and the Response Message are exchanged via the
FETCH operation and corresponding FETCH Request and FETCH Response
body.
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In this example, evidence is created via the root-of-trust for
reporting primitive operation "quote" that is provided by a TPM 2.0.
RAIM-Bodies = CoAP-FETCH-Body / CoAP-FETCH-Response-Body
CoAP-FETCH-Body = [ hello: bool, ; if true, the AK-Cert is conveyed
nonce: bytes,
pcr-selection: [ + [ tcg-hash-alg-id: uint .size 2, ; TPM2_ALG_ID
[ + pcr: uint .size 1 ],
]
],
]
CoAP-FETCH-Response-Body = [ attestation-evidence: TPMS_ATTEST-quote,
tpm-native-signature: bytes,
? ak-cert: bytes, ; attestation key certificate
]
TPMS_ATTEST-quote = [ qualifiediSigner: uint .size 2, ;TPM2B_NAME
TPMS_CLOCK_INFO,
firmwareVersion: uint .size 8
quote-responses: [ * [ pcr: uint .size 1,
+ [ pcr-value: bytes,
? hash-alg-id: uint .size 2,
],
],
? pcr-digest: bytes,
],
]
TPMS_CLOCK_INFO = [ clock: uint .size 8,
resetCounter: uint .size 4,
restartCounter: uint .size 4,
save: bool,
]
Authors' Addresses
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
Germany
Email: henk.birkholz@sit.fraunhofer.de
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Michael Eckel
Fraunhofer SIT
Rheinstrasse 75
Darmstadt 64295
Germany
Email: michael.eckel@sit.fraunhofer.de
Christopher Newton
University of Surrey
Email: cn0016@surrey.ac.uk
Liqun Chen
University of Surrey
Email: liqun.chen@surrey.ac.uk
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