title: Reference Interaction Models for Remote Attestation Procedures abbrev: REIM docname: draft-ietf-rats-reference-interaction-models-latest wg: RATS Working Group stand_alone: true ipr: trust200902 area: Security kw: Internet-Draft cat: info pi: toc: yes sortrefs: yes symrefs: yes
author:
- ins: H. Birkholz name: Henk Birkholz org: Fraunhofer SIT abbrev: Fraunhofer SIT email: [email protected] street: Rheinstrasse 75 code: '64295' city: Darmstadt country: Germany
- ins: M. Eckel name: Michael Eckel org: Fraunhofer SIT abbrev: Fraunhofer SIT email: [email protected] street: Rheinstrasse 75 code: '64295' city: Darmstadt country: Germany
- ins: W. Pan name: Wei Pan org: Huawei Technologies email: [email protected]
- ins: E. Voit name: Eric Voit org: Cisco Systems abbrev: Cisco email: [email protected]
normative: RFC2119: RFC3161: TSA RFC5280: X509 RFC7049: CBOR RFC7252: COAP RFC8174: BCP205: RFC7942 RFC8610: CDDL RFC9334: RATS I-D.birkholz-rats-epoch-markers: epoch-markers
informative: I-D.ietf-rats-tpm-based-network-device-attest: RIV I-D.birkholz-rats-tuda: TUDA DAA: title: Direct Anonymous Attestation author: - ins: E. Brickell name: Ernie Brickell - ins: J. Camenisch name: Jan Camenisch - ins: L. Chen name: Liqun Chen seriesinfo: ACM: > Proceedings of the 11th ACM conference on Computer and Communications Security page: 132-145 date: 2004 turtles: title: "Turtles All the Way Down: Foundation, Edifice, and Ruin in Faulkner and McCarthy" author: - ins: R. Rudnicki name: Robert Rudnicki seriesinfo: The Faulkner Journal: 25.2 DOI: 10.1353/fau.2010.0002 date: 2010 TNC: title: TCG Trusted Network Communications TNC Architecture for Interoperability author: - ins: TCG name: Trusted Computing Group seriesinfo: Specification: Version 2.0 Revision 13 date: 2017 MQTT: title: Message Queuing Telemetry Transport (MQTT) Version 5.0 Committee Specification 02 author: - ins: OASIS name: Organization for the Advancement of Structured Information Standards seriesinfo: Specification: Version 5.0 date: 2018 DesignPatterns: title: Design Patterns - Elements of Reusable Object-Oriented Software author: - ins: E. Gamma name: Erich Gamma - ins: R. Helm name: Richard Helm - ins: R. Johnson name: Ralph Johnson - ins: J. Vlissides name: John Vlissides seriesinfo: Publisher: Addison-Wesley date: 1994 ISIS: title: Exploiting Virtual Synchrony in Distributed Systems author: - ins: K. Birman name: Ken Paul Birman - ins: T. Joseph name: Thomas A. Joseph seriesinfo: DOI: 10.1145/41457.37515 date: 1987
--- 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.
--- middle
Remote ATtestation procedureS (RATS, {{-RATS}}) 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 -- generated by an Attester. The roles Attester and Verifier, as well as the Conceptual Messages Evidence and Attestation Results are concepts defined by the RATS Architecture {{-RATS}}. This document defines interaction models that can be used in specific RATS-related solution documents. The primary focus of this document is the conveyance of attestation Evidence. The reference models defined can also be applied to the conveyance of other Conceptual Messages in RATS. This document aims to:
-
prevent inconsistencies in descriptions of interaction models in other documents, which may occur due to text cloning and evolution over time, and to
-
highlight the exact delta/divergence between the core characteristics captured in this document and variants of these interaction models used in other specifications or solutions.
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, for example, Endorsements or Attestation Results.
This document uses the following terms defined in {{Section 4 of -RATS}}: Attester, Verifier, Relying Party, Conceptual Message, Evidence, Endorsement, Attestation Result, Appraisal Policy, Attesting Environment, Target Environment.
A PKIX Certificate is an X.509v3 certificate as specified by {{RFC5280}}.
{::boilerplate bcp14}
"Remote Attestation" is a common expression often associated or connoted with certain properties. In the context of this document, the term "Remote" does not necessarily refer to a remote entity in the scope of network topologies or the Internet. It rather refers to decoupled systems or entities that exchange the Conceptual Message type called Evidence {{-RATS}}. This conveyance can also be "Local", if the Verifier role is part of the same entity as the Attester role, e.g., separate system components of the same Composite Device (a single RATS entity), or the Verifier and Relying Party roles are hosted by the same entity, for example in a cryptographic key broker system. Even if an entity takes on two or more different roles, the functions they provide typically reside in isolated environments that are components of the same entity. Examples of such isolated 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)). Readers of this document should be familiar with the concept of Layered Attestation as described in {{Section 3.1 of {{-RATS}} and the definition of Attestation as described in {{-RIV}}.
This document outlines the interaction models between Attesters and Verifiers to convey Evidence. Procedures, functions, and services needed for a complete semantic binding of the concepts defined in {{-RATS}} are not covered in this document. Examples of such procedures include: identity establishment, key distribution and enrollment, time synchronization, and certificate revocation.
Furthermore, any processes and duties beyond conducting remote attestation procedures are out of scope. For example, utilizing the results of a remote attestation procedure generated by the Verifier, such as triggering remediation actions or recovery processes, as well as the remediation actions and recovery processes themselves, are also out of scope.
The interaction models described in this document are meant to serve as a solid foundation and reference for other solution documents within or outside the IETF. Solution documents of any kind can refer to these interaction models to prevent duplicating text and to avoid the risk of subtle discrepancies. Similarly, deviations from the generic model described in this document can be illustrated in solution documents to highlight distinct contributions.
In order to ensure appropriate conveyance of Evidence, the following requirements MUST be fulfilled:
Integrity: : Information provided by an Attester MUST NOT have been altered since it was created. This may be achieved through a digital signature over Attestation Evidencewhich may be symmetric, like an HMAC, or asymmetric, like ECDSA.
Authentication: : The information provided by the Attester MUST be authentic. To do this, the Attester should authenticate itself to the Verifier. This can be done through implicit authentication using a digital signature over the Attestation Evidence, which does not require additional protocol steps, or by using a confidential channel with encryption.
An Attester only generates Evidence about its Target Environments through its Attesting Environments. Once a Target Environment is appraised as trustworthy, it may become a new Attesting Environment responsible for generating Evidence for other Target Environments. This is known as Layered Attestation, as explained in {{Section 3.2 of -RATS}}.
Since there can't be an infinite chain of Attesting Environments {{turtles}}, Layered Attestation has to start with an initial Attesting Environment. The Attesting Environments at the "rock bottom" of layered attestation are referred to as Roots of Trust (RoT). By design, an Attester cannot produce Evidence about its RoTs. Therefore, a Verifier needs trustworthy statements about this subset of Attesting Environments from a source other than the Attester itself. These trustworthy statements are called Endorsements and come from external, trusted entities that act as Endorsers (for example, supply chain entities).
In order to ensure Evidence is appropriately conveyed through the interaction models described in this document, the following prerequisites MUST be in place to support their implementation:
Authentication Secret:
: An Authentication Secret MUST be exclusively available to an Attesting Environment of the Attester.
: The Attester MUST protect Claims with this Authentication Secret to prove the authenticity of the Claims included in Evidence. The Authentication Secret MUST be established before RATS take place.
Attester Identity:
: A statement made by an Endorser about an Attester that affirms the Attester's distinguishability. (Note that distinguishability does not imply uniqueness.)
: The provenance of Evidence for a distinguishable Attesting Environment MUST be correct and unambiguous.
: An Attester Identity MAY be an Authentication Secret which is available exclusively to one of the Attesting Environments of the Attester. It MAY be a unique identity, it MAY be included in a zero-knowledge proof (ZKP), it MAY be part of a group signature, or it MAY be a randomized DAA credential {{DAA}}.
Attestation Evidence Authenticity:
: Attestation Evidence MUST be authentic.
: In order to provide a proof of authenticity, Attestation Evidence SHOULD be cryptographically associated with an identity document (e.g., a PKIX certificate or trusted key material, or a randomized DAA credential {{DAA}}), or SHOULD include a correct, unambiguous and stable reference to an accessible identity document.
Evidence Freshness:
: Evidence MUST include an indicator about its freshness that can be understood by a Verifier (See also {{Section 10 of -RATS}}). Similarly, 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.
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 (instantiating Conceptual Messages) or can be included in additional protocol parameters or 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.
Attestation Key IDs ('authSecIDs'):
: optional
: A statement representing an identifier list that MUST be associated with corresponding Attestation Keys (authentication secrets) used to protect Claims in Evidence produced by Attesting Environments of an Attester.
: While a verifier does not necessarily have knowledge about an Attesting Environment's Attestation Key (ID), each distinguishable Attesting Environment has access to a protected capability that includes an Attestation Key (Authentication Secret). Consequently, an Attestation Key ID can also identify an Attesting Environment.
Handle ('handle'):
: mandatory
: A statement provided to the Attester from the outside to be included in Evidence (or other RATS Conceptual Messages) to determine recentness, freshness, or to protect against replay attacks.
: Handle is an umbrella term for existing data types that accomplish one or more of (a) determining recentness, (b) determining freshness, or (c) provide replay protection. Examples include: Nonces that are used to protect from replay attacks or Epoch Markers that identify distinct periods (Epoch) of freshness {{-epoch-markers}}. Handles can also be used as an indicator for authenticity or attestation Evidence provenance, as only a select number of RATS Roles (e.g., an Attester and a Verifier in a challenge-response interaction) are intended to have knowledge of a current Handle.
Claims ('claims'):
: mandatory
: Claims are assertions that represent characteristics of an Attester's Target Environment.
: Claims are part of a Conceptual Message and are, for example, used to appraise the integrity of Attesters via Verifiers. The other information elements in this section can be expressed as Claims in any type of Conceptional Messages.
Event Logs ('eventLogs'):
: optional
: Event Logs accompany Claims by providing event trails of security-critical events in a system. The primary purpose of Event Logs is to support Claim reproducibility by providing information on how Claims originated.
Reference Values ('refValues')
: mandatory
: Reference Values as defined in {{-RATS}}. This specific type of Claims is used to appraise Claims incorporated in Evidence. For example, Reference Values MAY be Reference Integrity Measurements (RIM) or assertions that are implicitly trusted because they are signed by a trusted authority (see Endorsements in {{-RATS}}). Reference Values typically represent (trusted) Claim sets about an Attester's intended platform operational state.
Claim Selection ('claimSelection'):
: optional
: A (sub-)set of Claims which can be created by an Attester.
: Claim Selections act as optional filters to specify the exact set of Claims to be included in Evidence. For example, a Verifier could send a Claim Selection, among other elements, to an Attester. An Attester MAY decide whether or not to provide all requested Claims from a Claim Selection to the Verifier. If there is no way to convey a Claim Selection in a remote attestation protocol, a default Claim Selection (e.g., "all") MUST be defined be the Attester and SHOULD be known to the Verifier.
Collected Claims ('collectedClaims'):
: mandatory
: Collected Claims represent a (sub-)set of Claims created by an Attester.
: Collected Claims are gathered based on the Claims selected in the Claim Selection. If a Verifier does not provide a Claim Selection, then all available Claims on the Attester are part of the Collected Claims.
Evidence ('evidence'):
: 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. Evidence MUST be protected via an 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 that are processible by Relying Parties.
The following subsections introduce and illustrate the interaction models:
- Challenge/Response Remote Attestation
- Uni-Directional Remote Attestation
- Streaming Remote Attestation
Each section starts with a sequence diagram illustrating the interactions between Attester and Verifier. While the presented interaction models focus on the conveyance of Evidence, the intention of this document is in support of future work that applies the presented models to the conveyance of other Conceptual Messages, namely Attestation Results, Endorsements, Reference Values, or Appraisal Policies.
All interaction models have a strong focus on the use of a handle to incorporate a type of proof of freshness and to prevent replay attacks. The way these handles are processed is the most prominent difference between the three interaction models.
.----------. .----------.
| Attester | | Verifier |
'----+-----' '-----+----'
| |
=================[Evidence Generation and Conveyance]===================
| |
generateClaims(attestingEnvironment) |
| => claims, eventLogs |
| |
|<--- requestAttestation(handle, attKeyIDs, claimSelection) |
| |
collectClaims(claims, claimSelection) |
| => collectedClaims |
| |
generateEvidence(handle, attKeyIDs, collectedClaims) |
| => evidence |
| |
| evidence, eventLogs -------------------------------------->|
| |
==========================[Evidence Appraisal]==========================
| |
| appraiseEvidence(evidence, eventLogs, refValues)
| attestationResult <= |
| |
The Attester boots up and thereby produces claims about its boot state and its operational state. Event Logs accompany the produced claims by providing an event trail of security-critical events in a system. Claims are produced by all attesting Environments of an Attester system.
The 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 practically infeasible to guess nonce, such as a cryptographically strong random number. The Verifier-generated nonce is intended to guarantee Evidence freshness and to prevent replay attacks.
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. Correspondingly, a particular set of Evidence originating from a particular Attesting Environment in a composite device can be requested via multiple Authentication Secret IDs. Methods to acquire Authentication Secret IDs or mappings between Attesting Environments to Authentication Secret IDs are out-of-scope of this document.
The Attester collects Claims based on the Claim Selection. With the Claim Selection the Verifier defines the set of Claims it requires. Correspondingly, collected Claims can be a subset of the produced Claims. This could be all available Claims, depending on the Claim Selection. 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/UEFI and firmware that the Attester booted up, and not include information about all currently running software.
With the Handle, the Authentication Secret IDs, and the collected Claims, the Attester produces signed Evidence. That is, it digitally signs the Handle and the collected Claims with a cryptographic secret identified by the Authentication Secret ID. This is done once per Attesting Environment which is identified by the particular Authentication Secret ID. The Attester communicates the signed Evidence as well as all accompanying Event Logs back to the Verifier.
While it is crucial that Claims, the Handle, and the Attester Identity information (i.e., the Authentication Secret) MUST be cryptographically bound to the signature of Evidence, they MAY be presented obfuscated, encrypted, or cryptographically blinded. For further reference see section {{security-and-privacy-considerations}}.
As soon as the Verifier receives the Evidence and the Event Logs, it appraises the Evidence. For this purpose, it validates the signature, the Attester Identity, and the Handle, and then appraises the Claims. Appraisal procedures are application-specific and can be conducted via comparison of the Claims with corresponding Reference Values, such as Reference Integrity Measurements. The final output of the Verifier are Attestation Results. Attestation Results constitute new Claim Sets about the properties and characteristics of an Attester, which enables Relying Parties, for example, to assess an Attester's trustworthiness.
According to the RATS Architecture, two reference models for Challenge/Response Attestation have been proposed. This section highlights the information flows between the Attester, Verifier, and Relying Party undergoing Remote Attestation Procedure, using these models.
The passport model is so named because of its resemblance to how nations issue passports to their citizens. In this model, the attestation sequence is a two-step procedure. In the first step, an Attester conveys Evidence to a Verifier, which compares the Evidence against its appraisal policy. The Verifier then gives back an Attestation Result to the Attester, which simply caches it. In the second step, the Attester presents the Attestation Result (and possibly additional Claims/Evidence) to a Relying Party, which then compares this information against its own appraisal policy to establish the trustworthiness of the Attester.
.----------. .----------. .---------------.
| Attester | | Verifier | | Relying Party |
'----+-----' '-----+----' '-------+-------'
| | |
=================[Evidence Generation and Conveyance]===================
| | |
generateClaims(attestingEnvironment) | |
| => claims, eventLogs | |
| | |
|<--------------------- requestAttestation(handle, |
| attKeyIDs, claimSelection) |
| | |
collectClaims(claims, claimSelection) | |
| => collectedClaims | |
| | |
generateEvidence(handle, | |
attKeyIDs, collectedClaims) | |
| => evidence | |
| | |
| {evidence, eventLogs} -------------->| |
| | |
==========================[Evidence Appraisal]==========================
| | |
| appraiseEvidence(evidence, |
| eventLogs, refValues) |
| attestationResult <= | |
| | |
|<------------------ attestationResult | |
| | |
| {evidence, attestationResult} ------------------------>|
| | |
====================[Attestation Result Generation]=====================
| | |
| | appraiseResult(policy,
| | attestationResult)
| | |
The background-check model is so named because of the resemblance of how employers and volunteer organizations perform background checks. In this model, the attestation sequence is initiated by a Relying Party. The Attester conveys Evidence to the Relying Party, which does not process its payload, but relays the message and optionally checks its signature against a policed trust anchor store. Upon receiving the Evidence, the Relying Party initiates a session with the Verifier. Once the session is established, it forwards the received Evidence to the Verifier. The Verifier appraises the received Evidence according to its appraisal policy for Evidence and returns a corresponding Attestation Result to the Relying Party. The Relying Party then checks the Attestation Result against its own appraisal policy to conclude attestation.
.----------. .---------------. .----------.
| Attester | | Relying Party | | Verifier |
'----+-----' '-------+-------' '-----+----'
| | |
=================[Evidence Generation and Conveyance]===================
| | |
|<--------------------- requestAttestation(handle, |
| attKeyIDs, claimSelection) |
| | |
generateClaims(attestingEnvironment) | |
| => {claims, eventLogs} | |
| | |
collectClaims(claims, | |
claimSelection) | |
| => collectedClaims | |
| | |
generateEvidence(handle, | |
attKeyIDs, collectedClaims) | |
| => evidence | |
| | |
| {evidence, eventLogs} ----------->| |
| | |
==========================[Evidence Appraisal]==========================
| | |
| | {handle, evidence, |
| | eventLogs} --------->|
| | |
| | appraiseEvidence(evidence,
| | eventLogs, refValues)
| | attestationResult <= |
| | |
| |<---------- {evidence, |
| | attestationResult} |
| | |
====================[Attestation Result Generation]=====================
| | |
| appraiseResult(policy, |
| attestationResult) |
| | |
.----------. .--------------------. .----------.
| Attester | | Handle Distributor | | Verifier |
'----+-----' '---------+----------' '-----+----'
| | |
==========================[Handle Generation]===========================
| generateHandle() |
| | => handle |
| | |
|<---------------------------- {handle} | {handle} --------->|
| | |
| x |
| |
=================[Evidence Generation and Conveyance]===================
| |
generateClaims(attestingEnvironment) |
| => claims, eventLogs |
| |
collectClaims(claims, claimSelection) |
| => collectedClaims |
| |
generateEvidence(handle, attKeyIDs, collectedClaims) |
| => evidence |
| |
| {evidence, eventLogs} ------------------------------------>|
| |
==========================[Evidence Appraisal]==========================
| |
| appraiseEvidence(evidence,
| eventLogs,
| refValues)
| attestationResult <= |
~ ~
| |
.--------[loop]------------------------------------------------------.
| | | |
| =============[Delta Evidence Generation and Conveyance]============= |
| | | |
| generateClaims(attestingEnvironment) | |
| | => claimsDelta, eventLogsDelta | |
| | | |
| collectClaims(claimsDelta, claimSelection) | |
| | => collectedClaimsDelta | |
| | | |
| generateEvidence(handle, attKeyIDs, collectedClaimsDelta) | |
| | => evidence | |
| | | |
| | {evidence, eventLogsDelta} ------------------------------->| |
| | | |
| =====================[Delta Evidence Appraisal]===================== |
| | | |
| | appraiseEvidence(evidence, |
| | eventLogsDelta, |
| | refValues) |
| | 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 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 cryptographically signed trusted timestamps as shown in {{-TUDA}}, potentially including other qualifying data. The Handles are created by an external 3rd entity -- the Handle Distributor -- which includes a trustworthy source of time, and takes on the role of a Time Stamping Authority (TSA, as initially defined in {{-TSA}}). Timestamps 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 all produced Evidence. Correspondingly, conveyed Evidence in this model provides a proof that it was fresh at a certain point in time.
While periodically pushing Evidence to the Verifier, the Attester only needs to generate and convey evidence generated from Claim values that have changed and new Event Log entries since the previous conveyance. These updates reflecting the differences are called "delta" in the sequence diagram above.
Effectively, the Uni-Directional model allows for a series of Evidence to be pushed to multiple Verifiers simultaneously. 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.
Streaming Remote Attestation serves as the foundational concept for both the observer pattern ({{ISIS}}) and the publish-subscribe pattern ({{DesignPatterns}}). It entails establishing subscription states to enable continuous remote attestation. The observer pattern directly connects observers to subjects without a broker, while the publish-subscribe pattern involves a central broker for message distribution. In the following Subsections, streaming remote attestation without a broker (observer pattern) as well as with a broker (publish-subscribe pattern) are illustrated.
.----------. .----------.
| Attester | | Verifier |
'----+-----' '-----+----'
| |
==========================[Handle Generation]===========================
| |
| generateHandle()
| handle<= |
| |
|<------------ subscribe(handle, attKeyIDs, claimSelection) |
| {handle} ------------------------------------------------->|
| |
=================[Evidence Generation and Conveyance]===================
| |
generateClaims(attestingEnvironment) |
| => claims, eventLogs |
| |
collectClaims(claims, claimSelection) |
| => collectedClaims |
| |
generateEvidence(handle, attKeyIDs, collectedClaims) |
| => evidence |
| |
==========================[Evidence Appraisal]==========================
| |
| {handle, evidence, eventLogs} ---------------------------->|
| |
| appraiseEvidence(evidence,
| eventLogs,
| refValues)
| attestationResult <= |
~ ~
| |
.--------[loop]------------------------------------------------------.
| | | |
| =============[Delta Evidence Generation and Conveyance]============= |
| | | |
| generateClaims(attestingEnvironment) | |
| | => claimsDelta, eventLogsDelta | |
| | | |
| collectClaims(claimsDelta, claimSelection) | |
| | => collectedClaimsDelta | |
| | | |
| generateEvidence(handle, attKeyIDs, collectedClaimsDelta) | |
| | => evidence | |
| | | |
| =====================[Delta Evidence Appraisal]===================== |
| | | |
| | {evidence, eventLogsDelta} ------------------------------->| |
| | | |
| | appraiseEvidence(evidence, |
| | eventLogsDelta, |
| | refValues) |
| | attestationResult <= | |
| | | |
'--------------------------------------------------------------------'
| |
The observer pattern is employed in scenarios where message delivery does not involve a central broker. Instead, an observer directly subscribes to observed resources via a dedicated mechanism. Consequently, these dedicated mechanisms contain information about the observer and are responsible for maintaining subscription state. Setting up subscription state between a Verifier and an Attester is conducted via a subscribe operation. The subscribe operation is used to convey Handles required for Evidence generation. Effectively, this allows for a series of Evidence to be pushed to a Verifier, similar to the Uni-Directional model. While a Handle Distributor is not mandatory in this model, the model is also limited to bi-lateral subscription relationships, in which each Verifier has to create and provide Handles individually. Handles provided by a specific subscribing Verifier MUST be used in Evidence generation for that specific Verifier. The streaming model without a broker uses the same information elements as the Challenge/Response and the Uni-Directional model. Methods to detect excessive time drift that would render Handles stale and mandate a fresh Handles to be conveyed via another subscribe operation are out-of-scope of this document.
The publish-subscribe messaging pattern is widely used for communication in different areas. Unlike the Streaming Remote Attestation without a Broker interaction model, Attesters do not (need to) be aware of corresponding Verifiers. In scenarios with large numbers of Attesters and Verifiers, the publish-subscribe pattern may reduce interdependencies and improve scalability.
With publish-subscribe, clients typically connect to (or register with) a publish-subscribe server (PubSub server or Broker). Clients may publish data in the form of a message under a certain topic. Subscribers to that topic get notified whenever a message arrives under a topic, and the appropriate message is forwarded to them. Depending on the particular publish-subscribe model and implementation, clients can be either publishers or subscribers or both.
In the following sections, the interaction models Challenge/Response Remote Attestation over Publish-Subscribe and Uni-Directional Remote Attestation over Publish-Subscribe are described. There are different phases that both models go through:
- Handle Generation
- Evidence Generation and Conveyance
- Evidence Appraisal
- Attestation Result Generation
The models only differ in the handle generation phase. From a remote attestations procedure's point of view Evidence Generation, Conveyance, and Appraisal, as well as Attestation Result Generation are identical in both models.
.----------. .---------------. .----------.
| Attester | | PubSub Server | | Verifier |
'----+-----' '-------+-------' '-----+----'
| | |
==========================[Handle Generation]===========================
| | |
sub(topic=AttReq) ---------------->| |
| |<------------ pub(topic=AttReq,
| | handle)
|<------------------- notify(topic=AttReq, handle) |
| | |
~ ~ ~
The Challenge/Response Remote Attestation over Publish-Subscribe interaction model uses the same information elements as the Challenge/Response Remote Attestation interaction model. Handles are provided by a Verifier on a per-request basis. In the sequence diagram above, an Attester subscribes to the "AttReq" (= Attestation Request) topic on the PubSub server. The Verifier publishes a Handle to the "AttReq" topic, which the PubSub server forwards to the Attester by notifying it.
.----------. .-------------. .---------------. .----------.
| Attester | | Handle | | PubSub Server | | Verifier |
'----+-----' | Distributor | '-------+-------' '-----+----'
| '------+------' | |
| | | |
==========================[Handle Generation]===========================
| | | |
| | | |
sub(topic=Handle) ------------------->| |
| | | |
| | |<--------- sub(topic=Handle)
| | | |
| | | |
| generateHandle() | |
| | => handle | |
| | | |
| pub(topic=Handle, | |
| | handle) --------->| |
| x | |
| | |
|<--------------------- notify(topic=Handle, handle) |
| | |
| notify(topic=Handle, handle) ------->|
| | |
~ ~ ~
The Uni-Directional Remote Attestation over Publish-Subscribe model uses the same information elements as the Uni-Directional Remote Attestation model. Accordingly, Handles are created by a 3rd party, the Handle Distributor. In the sequence diagram above, both an Attester and a Verifier subscribe to the topic "Handle" on the PubSub server. When the Handle Distributor generates and publishes a Handle to the "Handle" topic on the PubSub server, the PubSub server notifies the subscribers, Attester and Verifier, and forwards ("notify") the Handle to them during Handle Generation.
~ ~ ~
| | |
.----+-----. .-------+-------. .-----+----.
| Attester | | PubSub Server | | Verifier |
'----+-----' '-------+-------' '-----+----'
| | |
| |<---------- sub(topic=AttEv)
| | |
.--------[loop]------------------------------------------------------.
| | | | |
| ===============[Evidence Generation and Conveyance]================= |
| | | | |
| generateClaims(attestingEnvironment) | | |
| | => claims, eventLogs | | |
| | | | |
| collectClaims(claims, claimSelection) | | |
| | => collectedClaims | | |
| | | | |
| generateEvidence(handle, attKeyIDs, | | |
| | collectedClaims) | | |
| | => evidence | | |
| | | | |
| pub(topic=AttEv, | | |
| | evidence, eventLogs) ------------>| | |
| | | | |
| | notify(topic=AttEv, | |
| | | evidence, | |
| | | eventLogs) ---------->| |
| | | | |
| ========================[Evidence Appraisal]======================== |
| | | | |
| | | appraiseEvidence( |
| | | evidence, |
| | | eventLogs, |
| | | refValues) |
| | | attestationResult <= | |
| | | | |
| ==================[Attestation Result Generation]=================== |
| | | | |
| | |<--------- pub(topic=AttRes, |
| | | attestationResult) |
| | | | |
'--------------------------------------------------------------------'
| | |
~ ~ ~
Exactly as in the Challenge/Response and Uni-Directional interaction models, there is an Evidence Generation-Appraisal loop, in which the Attester generates Evidence and the Verifier appraises it. In the Publish-Subscribe model above, the Attester publishes Evidence to the topic "AttEv" (= Attestation Evidence) on the PubSub server, to which a Verifier subscribed before. The PubSub server notifies Verifiers, accordingly, by forwarding the attestation Evidence. Although the above diagram depicts only full attestation Evidence and Event Logs, later attestations may use "deltas' for Evidence and Event Logs. Verifiers appraise the Evidence and publish the Attestation Result to topic "AttRes" (= Attestation Result) on the PubSub server.
~ ~ ~ ~
| | | |
.----+-----. .--+------------. .-------+-------. .-----+----.
| Attester | | Relying Party | | PubSub Server | | Verifier |
'----+-----' '--+------------' '-------+-------' '-----+----'
| | | |
====================[Attestation Result Generation]=====================
| | | |
| sub(topic=AttRes) | |
| handle) ----------------->| |
| | | |
.--------[loop]------------------------------------------------------.
| | | | | |
| | | |<--------- pub(topic=AttRes, |
| | | | attestationResult) |
| | | | | |
| | |<----------------- notify(topic=AttRes | |
| | | | attestationResult) | |
| | | | | |
'--------------------------------------------------------------------'
| | | |
~ ~ ~ ~
Attestation Result Generation is the same for both publish-subscribe models,Challenge/Response Remote Attestation over Publish-Subscribe and Uni-Directional Remote Attestation over Publish-Subscribe.
Relying Parties subscribe to topic AttRes (= Attestation Result) on the PubSub server.
The PubSub server forwards Attestation Results to the Relying Parties as soon as they are published to topic AttRes.
Many publish-subscribe models provide hierarchical organization of topics.
This way, subscribers can subscribe to either all attestations (topic AttRes), or, for example, to topic AttRes/DbServers/Germany to receive only attestations from database servers in Germany.
Further, it may be required to distinguish between uni-directional and challenge-response attestation evidence.
Depending on the use cases covered, there can be additional requirements. An exemplary subset is illustrated in this section.
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 used as a suitable communication protocol which provides confidentiality protection. In private networks, such as carrier management networks, it must be evaluated whether or not the transport medium is considered confidential.
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.
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).
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 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".
The open-source implementation was initiated and is maintained by the Fraunhofer Institute for Secure Information Technology SIT.
The open-source implementation is named "CHAllenge-Response based Remote Attestation" or in short: CHARRA.
The open-source implementation project resource can be located via: https://github.com/fraunhofer-sit/charra
The code's level of maturity is considered to be "prototype".
The current version ('6194b3b') implements a challenge/response interaction model and is aligned with the exemplary specification of the CoAP FETCH bodies defined in Section {{coap-fetch-bodies}} of this document.
The CHARRA project and all corresponding code and data maintained on GitHub are provided under the BSD 3-Clause "New" or "Revised" license.
The implementation requires the use of the Trusted Computing Group (TCG) Trusted Software Stack (TSS), and an HSM interoperable with the Trusted Platform Module Library specifications, e.g., a Trusted Platform Module (TPM) 2.0 or equivalent implementation. The corresponding project resources (code and data) for Linux-based operating systems are maintained on GitHub at https://github.com/tpm2-software/tpm2-tss/.
The implementation uses the Constrained Application Protocol {{-COAP}} (http://coap.technology/) and the Concise Binary Object Representation {{-CBOR}} (https://cbor.io/).
Michael Eckel ([email protected])
{: #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.
Olaf Bergmann, Michael Richardson, and Ned Smith
--- back
{: #coap-fetch-bodies}
The following CDDL specification is an exemplary proof-of-concept to illustrate a potential implementation of the Challenge/Response Interaction Model. The communication protocol used is CoAP. Both the request message and the response message are exchanged via the FETCH operation and corresponding FETCH request and FETCH response body.
In this example, Evidence is created via the root-of-trust for reporting primitive operation "quote" that is provided by a TPM 2.0.
charra-bodies = charra-attestation-request / charra-attestation-response
charra-attestation-request = [
hello: bool, ; if true, the TPM 2.0 AK Cert shall be conveyed
key-id: bytes, ; the key ID to use for signing
nonce: bytes, ; a (random) nonce, providing freshness and/or recentness
pcr-selections: [ * pcr-selection ]
]
pcr-selection = [
tcg-hash-alg-id: uint .size 2, ; TPM2_ALG_ID
pcrs: [
pcr: uint .size 2
]
]
charra-attestation-response = [
attestation-data: bytes, ; TPMS_ATTEST.quoted
tpm2-signature: bytes,
? ak-cert: bytes, ; TPM2 attestation key certificate (AK Cert)
]