]> Arm

paul.howard@arm.com

Linaro

Thomas.Fossati@linaro.org

Security Remote ATtestation ProcedureS RATS attestation endorsement reference value In the Remote Attestation Procedures (RATS) architecture, Verifiers require Endorsements and Reference Values to assess the trustworthiness of Attesters. This document specifies the Concise Selector for Endorsements and Reference Values (CoSERV), a structured query format designed to facilitate the discovery and retrieval of these artifacts from various providers. CoSERV defines a query language using CDDL that can be serialized in CBOR format, enabling interoperability across diverse systems. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Remote ATtestation ProcedureS Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Remote Attestation Procedures (RATS) enable Relying Parties to evaluate the trustworthiness of remote Attesters by appraising Evidence. This appraisal necessitates access to Endorsements and Reference Values, which are often distributed across multiple providers, including hardware manufacturers, firmware developers, and software vendors. The lack of standardized methods for querying and retrieving these artifacts poses challenges in achieving seamless interoperability. The Concise Selector for Endorsements and Reference Values (CoSERV) addresses this challenge by defining a query language that allows Verifiers to specify the environment characteristics of the desired artifacts. This facilitates the efficient discovery and retrieval of relevant Endorsements and Reference Values from providers. The CoSERV query language is intended to form the input data type for tools and services that provide access to Endorsements and Reference Values. This document does not define the complete APIs or interaction models for such tools and services. Nor does this document constrain the format of the output data that such tools and services might produce. The scope of this document is limited to the definition of the query language only. The environment characteristics of Endorsements and Reference Values are derived from the equivalent concepts in CoRIM . CoSERV therefore borrows heavily from CoRIM, and shares some data types for its fields. And, like CoRIM, the CoSERV schema is defined using CDDL . A CoSERV query can be serialized in CBOR format.

Terminology and Requirements Language 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 when, and only when, they appear in all capitals, as shown here. This document uses terms and concepts defined by the RATS architecture. For a complete glossary, see . This document uses terms and concepts defined by the CoRIM specification. For a complete glossary, see . This document uses the terms "actual state" and "reference state" as defined in . The terminology from CBOR , CDDL and COSE applies; in particular, CBOR diagnostic notation is defined in and . Terms and concepts are always referenced as proper nouns, i.e., with Capital Letters.

CoSERV Query Language The CoSERV query language enables Verifiers to specify the desired characteristics of Endorsements and Reference Values based on the environment in which they are applicable. This section presents the CBOR data model for CoSERV queries. CDDL is used to express rules and constraints of the data model for CBOR. These rules must be strictly followed when creating or validating CoSERV data objects.

Common Data Types CoSERV inherits the following types from the CoRIM data model class-map, $class-id-type-choice, $instance-id-type-choice and $group-id-type-choice. The collated CDDL is in .

Query Structure The top-level structure of a CoSERV query is given by the following CDDL: artifact-type &(profile: 1) => profile-type &(environment-selector: 2) => environment-selector-map } artifact-type = &(endorsed-values: 0) / &(trust-anchors: 1) / &(reference-values: 2) profile-type = oid-type / ~uri ]]> The meanings of these fields are detailed in the following subsections.

Artifact Type The artifact-type field is the foremost discriminator of the query. It is a top-level category selector. Its three permissible values are trust-anchors (codepoint 1), endorsed-values (codepoint 0) and reference-values (codepoint 2). These correspond to the following three categories of endorsement artifact that can be identified in the RATS architecture:

• Trust Anchor (trust-anchors): A trust anchor is as defined in . An example of a trust anchor would be the public part of the asymmetric signing key that is used by the Attester to sign Evidence, such that the Verifier is able to verify the cryptographic signature.
• Endorsed Value (endorsed-values): An endorsed value is as defined in .
• Reference Value (reference-values): A reference value is as defined in . A reference value specifies an individual aspect of the Attester's desired state. Reference values are sometimes informally called "golden values". An example of a reference value would be the expected hash or checksum of a binary firmware or software image running in the Attester's environment. Evidence from the Attester would then include claims about the Attester's actual state, which the Verifier can then compare with the reference values at Evidence appraisal time.

It is expected that implementations might choose to store these different categories of artifacts in different top-level stores or database tables. Where this is the case, the artifact-type field serves to narrow the query down to the correct store or table. Even where this is not the case, the discriminator is useful as a filter for the consumer, resulting in an efficiency gain by avoiding the transfer of unwanted data items.

Profile In common with EAT and CoRIM, CoSERV supports the notion of profiles. As with EAT and CoRIM, profiles are a way to extend or specialize the structure of a generic CoSERV query in order to cater for a specific use case or environment. In a CoSERV query, the profile can be identified by either a Uniform Resource Identifier (URI) or an Object Identifier (OID). This convention is identical to how EAT profiles are identified using the eat_profile claim as described in .

Environment Selector The environment selector forms the main body of the query, and its CDDL is given below: [ * class-map ] ) selector //= ( &(instance: 1) => [ * $instance-id-type-choice ] ) selector //= ( &(group: 2) => [ * $group-id-type-choice ] ) ]]> The environment defines the scope (or scopes) in which the endorsement artifacts are applicable. Given that the consumer of these artifacts is likely to be a Verifier in the RATS model, the typical interpretation of the environment would be that of an Attester that either has produced evidence, or is expected to produce evidence, that the Verifier needs to appraise. The Verifier consequently needs to query the Endorser or Reference Value Provider for artifacts that are applicable in that environment. There are three mutually-exclusive methods for defining the environment within a CoSERV query. Exactly one of these three methods must be used for the query to be valid. All three methods correspond to environments that are also defined within CoRIM.

• Class: A class is an environment that is expected to be common to a group of similarly-constructed Attesters, who might therefore share the same set of endorsed characteristics. An example of this might be a fleet of computing devices of the same model and manufacturer.
• Instance: An instance is an environment that is unique to an individual and identifiable Attester, such as a single computing device or component.
• Group: A group is a collection of common Attester instances that are collected together based on some defined semantics. For example, Attesters may be put into groups for the purpose of anonymity.

Although these three environment definitions are mutually-exclusive in a CoSERV query, all three support multiple entries. This is to gain efficiency by allowing the consumer (such as a Verifier) to query for multiple artifacts in a single transaction. For example, where artifacts are being indexed by instance, it would be possible to specify an arbitrary number of instances in a single query, and therefore obtain the artifacts for all of them in a single transaction. Likewise for classes and groups. However, it would not be possible for a single query to specify more than one kind of environment. For example, it would not be possible to query for both class-level and instance-level artifacts in a single CoSERV transaction.

Encoding Requirements Implementations may wish to use serialized CoSERV queries as canonical identifiers for artifact collections. For example, a Reference Value Provider service may wish the cache the results of a CoSERV query to gain efficiency when responding to a future identical query. For these use cases to be effective, it is essential that any given CoSERV query is always serialized to the same fixed sequence of CBOR bytes. Therefore, CoSERV queries MUST always use CBOR deterministic encoding as specified in . Further, CoSERV queries MUST use CBOR definite-length encoding.

Examples This section provides some illustrative examples of valid CoSERV query objects. The following example shows a query for Reference Values scoped by a single class. The artifact-type is set to 2 (reference-values), indicating a query for Reference Values. The profile is given the example value of tag:example.com,2025:cc-platform#1.0.0. Finally, the environment-selector uses the key 0 to select for class, and the value contains a single entry with illustrative settings for the identifier, vendor and model. The next example is similar, but adds a second entry to the set of classes in the environment-map, showing how multiple classes can be queried at the same time. The following example shows a query for Reference Values scoped by instance. Again, the artifact-type is set to 2, and profile is given a demonstration value. The environment-selector now uses the key 1 to select for instances, and the value contains two entries with example instance identifiers.

Implementation Status RFC Editor: please remove this section prior to publication. 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 . 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 , "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".

Veraison Responsible Organisation: Veraison (open source project within the Confidential Computing Consortium). Location: https://github.com/veraison Description: Veraison provides components that can be used to build a Verifier, and also exemplifies adjacent RATS roles such as the Relying Party. There is an active effort to extend Veraison so that it can act in the capacity of an Endorser or Reference Value Provider, showing how CoSERV can be used as a query language for such services. This includes library code to assist with the creation, parsing and manipulation of CoSERV queries. Level of Maturity: This is a proof-of-concept prototype implementation. License: Apache-2.0. Coverage: This implementation covers all aspects of the CoSERV query language. Contact: Thomas Fossati, Thomas.Fossati@linaro.org

Security Considerations The CoSERV data type serves an auxiliary function in the RATS architecture. It does not directly convey Evidence, Endorsements, Reference Values, Policies or Attestation Results. CoSERV exists only to facilitate the interactions between the Verifier and the Endorser or Reference Value Provider roles. Consequently, there are fewer security considerations for CoSERV, particularly when compared with data objects such as EAT or CoRIM. Certain security characteristics are desirable for interactions between the Verifier and the Endorser or Reference Value Provider. However, these characteristics would be the province of the specific implementations of these roles, and of the transport protocols in between them. They would not be the province of the CoSERV data object itself. Examples of such desirable characteristics might be:

• The Endorser or Reference Value Provider is available to the Verifier when needed.
• The Verifier is authorised to query data from the Endorser or Reference Value Provider.
• Queries cannot be intercepted or undetectably modified by an entity that is interposed between the Verifier and the Endorser or Reference Value Provider.

Forming Native Database Queries from CoSERV Implementations should take care when transforming CoSERV queries into native query types that are compatible with their underlying storage technology (such as SQL queries). There is a risk of injection attacks arising from poorly-formed or maliciously-formed CoSERV queries. Implementations must ensure that suitable sanitization procedures are in place when performing such translations.

Privacy Considerations A CoSERV query can potentially contain privacy-sensitive information. Specifically, the environment-selector field of the query may reference identifiable Attester instances in some cases. This concern naturally also extends to the data objects that might be returned to the consumer in response to the query, although the specifications of such data objects are beyond the scope of this document. Implementations should ensure that appropriate attention is paid to this. Suitable mitigations include the following:

• The use of authenticated secure channels between the producers and the consumers of CoSERV queries and returned artifacts.
• Collating Attester instances into anonymity groups, and referencing the groups rather than the individual instances.

IANA Considerations RFC Editor: replace "RFCthis" with the RFC number assigned to this document.

Media Types Registrations IANA is requested to add the following media types to the "Media Types" registry . CoSERV Media Types

Name	Template	Reference
coserv+cbor	application/coserv+cbor	RFCthis

application/coserv+cbor

Type name:

application

Subtype name:

coserv+cbor

Required parameters:

n/a

Optional parameters:

n/a

Encoding considerations:

binary (CBOR)

Security considerations:

of RFCthis

Interoperability considerations:

n/a

Published specification:

RFCthis

Applications that use this media type:

Verifiers, Endorsers, Reference Value Providers

Fragment identifier considerations:

The syntax and semantics of fragment identifiers are as specified for "application/cbor". (No fragment identification syntax is currently defined for "application/cbor".)

Person & email address to contact for further information:

RATS WG mailing list (rats@ietf.org)

Intended usage:

COMMON

Restrictions on usage:

none

Author/Change controller:

IETF

Provisional registration:

no

References Normative References This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. In network protocol exchanges, it is often useful for one end of a communication to know whether the other end is in an intended operating state. This document provides an architectural overview of the entities involved that make such tests possible through the process of generating, conveying, and evaluating evidentiary Claims. It provides a model that is neutral toward processor architectures, the content of Claims, and protocols. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. IANA Informative References A trust anchor represents an authoritative entity via a public key and associated data. The public key is used to verify digital signatures, and the associated data is used to constrain the types of information for which the trust anchor is authoritative. A relying party uses trust anchors to determine if a digitally signed object is valid by verifying a digital signature using the trust anchor's public key, and by enforcing the constraints expressed in the associated data for the trust anchor. This document describes some of the problems associated with the lack of a standard trust anchor management mechanism and defines requirements for data formats and push-based protocols designed to address these problems. This document is not an Internet Standards Track specification; it is published for informational purposes. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. 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. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice. Armidale Consulting Fraunhofer SIT Linaro In the IETF Remote Attestation Procedures (RATS) architecture, a Verifier accepts Evidence and, using Appraisal Policy typically with additional input from Endorsements and Reference Values, generates Attestation Results in formats that are useful for Relying Parties. This document illustrates the purpose and role of Endorsements and discusses some considerations in the choice of message format for Endorsements in the scope of the RATS architecture. This document does not aim to define a conceptual message format for Endorsements and Reference Values. Instead, it extends RFC9334 to provide further details on Reference Values and Endorsements, as these topics were outside the scope of the RATS charter when RFC9334 was developed. Fraunhofer SIT Linaro arm Intel Huawei Technologies Remote Attestation Procedures (RATS) enable Relying Parties to assess the trustworthiness of a remote Attester and therefore to decide whether or not to engage in secure interactions with it. Evidence about trustworthiness can be rather complex and it is deemed unrealistic that every Relying Party is capable of the appraisal of Evidence. Therefore that burden is typically offloaded to a Verifier. In order to conduct Evidence appraisal, a Verifier requires not only fresh Evidence from an Attester, but also trusted Endorsements and Reference Values from Endorsers and Reference Value Providers, such as manufacturers, distributors, or device owners. This document specifies the information elements for representing Endorsements and Reference Values in CBOR format. Security Theory LLC Mediatek USA Qualcomm Technologies Inc. Red Hound Software, Inc. An Entity Attestation Token (EAT) provides an attested claims set that describes state and characteristics of an entity, a device like a smartphone, IoT device, network equipment or such. This claims set is used by a relying party, server or service to determine the type and degree of trust placed in the entity. An EAT is either a CBOR Web Token (CWT) or JSON Web Token (JWT) with attestation-oriented claims.

Collated CoSERV CDDL artifact-type &(profile: 1) => profile-type &(environment-selector: 2) => environment-selector-map } artifact-type = &(endorsed-values: 0) / &(trust-anchors: 1) / &(reference-values: 2) profile-type = oid-type / ~uri environment-selector-map = { selector } selector //= ( &(class: 0) => [ * class-map ] ) selector //= ( &(instance: 1) => [ * $instance-id-type-choice ] ) selector //= ( &(group: 2) => [ * $group-id-type-choice ] ) class-map = non-empty<{ ? &(class-id: 0) => $class-id-type-choice ? &(vendor: 1) => tstr ? &(model: 2) => tstr ? &(layer: 3) => uint ? &(index: 4) => uint }> $class-id-type-choice /= tagged-oid-type $class-id-type-choice /= tagged-uuid-type $class-id-type-choice /= tagged-bytes $instance-id-type-choice /= tagged-ueid-type $instance-id-type-choice /= tagged-uuid-type $instance-id-type-choice /= $crypto-key-type-choice $instance-id-type-choice /= tagged-bytes $group-id-type-choice /= tagged-uuid-type $group-id-type-choice /= tagged-bytes tagged-bytes = #6.560(bytes) oid-type = bytes tagged-oid-type = #6.111(oid-type) ueid-type = bytes .size (7..33) tagged-ueid-type = #6.550(ueid-type) uuid-type = bytes .size 16 tagged-uuid-type = #6.37(uuid-type) $crypto-key-type-choice /= tagged-pkix-base64-key-type $crypto-key-type-choice /= tagged-pkix-base64-cert-type $crypto-key-type-choice /= tagged-pkix-base64-cert-path-type $crypto-key-type-choice /= tagged-cose-key-type $crypto-key-type-choice /= tagged-thumbprint-type $crypto-key-type-choice /= tagged-cert-thumbprint-type $crypto-key-type-choice /= tagged-cert-path-thumbprint-type $crypto-key-type-choice /= tagged-pkix-asn1der-cert-type $crypto-key-type-choice /= tagged-bytes tagged-pkix-base64-key-type = #6.554(tstr) tagged-pkix-base64-cert-type = #6.555(tstr) tagged-pkix-base64-cert-path-type = #6.556(tstr) tagged-thumbprint-type = #6.557(digest) tagged-cose-key-type = #6.558(COSE_KeySet / COSE_Key) tagged-cert-thumbprint-type = #6.559(digest) tagged-cert-path-thumbprint-type = #6.561(digest) tagged-pkix-asn1der-cert-type = #6.562(bstr) digest = [ alg: (int / text), val: bytes ] digests-type = [ + digest ] non-empty = (M) .and ({ + any => any }) COSE_KeySet = [ + COSE_Key ] COSE_Key = { 1 => tstr / int ? 2 => bstr ? 3 => tstr / int ? 4 => [+ (tstr / int) ] ? 5 => bstr * cose-label => cose-value } cose-label = int / tstr cose-value = any ]]>

Acknowledgments TODO acknowledge.