]> Arm

Simon.Frost@arm.com

Security Remote ATtestation ProcedureS EAT collection container The default top-level definitions for an EAT assume a hierarchy involving a leading signer within the Attester. Some token use cases do not match that model. This specification defines an extension to EAT allowing the top-level of the token to consist of a collection of otherwise defined tokens. About This Document 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 .

Introduction An Attestation Token conforming to EAT has a default top level definition for a token to be constructed principally as a claim set within a CBOR Web Token (CWT) with the associated COSE envelope providing at least integrity and authentication. An equivalent JSON encoding for a JWT in a JWS envelope is supported as an alternative at the top-level definition. The top level token can be augmented with related claims in a Detached EAT Bundle. For the use case of transmitting a claim set through a secure channel, the top-level definition can be extended to use an Unprotected CWT Claim Set (UCCS) . This document outlines an additional top-level extension for which neither of the above top level definitions match exactly: the attestation token consists of a collection of objects, each with their own integrity and some internally defined relationship through which the integrity of the whole collection can be determined. i.e. there is no top-level signer for the set. The objects may all share the same logical hierarchy in a device or have a hierarchy which is internally defined within the object set.

Terminology and Requirements Language Readers are also expected to be familiar with the terminology from and . In this document, the structure of data is specified in CDDL . 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.

Design Considerations / Use Cases Take a device with an attestation system consisting of a platform claim set and a workload claim set, each controlled by different components and with an underlying hardware Root of Trust. The two claim sets are delivered together to make up the overall attestation token. Depending upon the implementation and deployment use case, the signing system can either be entirely centric to the platform RoT or can have separate signers for the two claim sets. In either case, a cryptographic binding is established between the two parts of the token. A specific manifestation of such a device is one incorporating the Arm Confidential Compute Architecture (CCA) attestation token . In trying to prepare the attestation token using EAT, there were no issues constructing the claim sets or incorporating them into individual CWTs where appropriate. However, in trying to design an 'envelope structure' to convey the two parts as a single report it was found that maintaining EAT compatibility would require very different shaped compound tokens for different models, for example one based on a submod arrangement and another based on a Detached EAT Bundle, though with different 'leading' objects. This would create extra code and explanation in areas where keeping things simple is desirable. There was an alternative approach considered, which stays close to existing thinking on EAT, which would be to create the wrapper from the UCCS EAT extension containing only submods for the respective components. This however stretches the current use case for UCCS beyond its existing description. The RATS WG approach of separating UCCS from the core EAT specification to be an extension also encourages proposing this further extension. To support the CCA use case, it is also relevant to consider current attestation technologies which are based on certificate chains (e.g. SPDM, DICE, several key attestation systems). Here also are multiple objects with their own integrity and an internally defined relationship. If attempting to move such a technology to the EAT world, the same challenges apply. An additional use case beyond the production of tokens from an Attester occurs when using EAT to convey Attestation Results from a Verifier. Attestation results may be separated into different sections depending upon what aspects of Appraisal Policy are applied by the Verifier. For example, the set of validated evidence claims may form one section, while claims reflecting semantic conclusions drawn by an Appraisal Policy could form another section. Given the role of different authorities in concluding result sections, each could have a different signer rather than all results being under a single signature from the Verifier. In this case, a collection can be used to collate all result sections into a single response message. Using a collection simplifies operations if individual sections from the collated result sections need to be later dispersed to different Relying Parties. A further Attestation Result use case can be seen in the "Below Zero Trust" system described in where the AR-augmented Evidence credential has compound form.

Token Collection The proposed extension for the top-level definition is to add a 'Token Collection' type. The contents of the type are a map of CWTs (JWTs). The Detached EAT Bundle top-level entry for EAT is included for completeness, and the UCCS extension can also be embraced, though the use cases for these have not been explored. The identification of collection members and the intra collection integrity mechanism is considered usage specific. A verifier will be expected to extract each of the members of the collection and check their validity both individually and as a set. In addition to entries which have their own integrity, it is also supported to include an unsigned Claims Set, provided that the integrity for that Claims Set is provided within another entry that uses one of the signed forms. A map was chosen rather than an unbounded array to give the opportunity to add identifying map tags to each entry. The interpretation of the tags will be usage specific, but may correspond to registered identities of specific token types. To assist a verifier correlate the expected contents a profile entry can be added as the 'profile-label' identity in the map. See for a CDDL description of the proposed extension. While most of the use cases for collections are for scenarios where there will be at least two entries in a collection, the CDDL allows for >= 1 entries in a collection to allow for the scenario where only one entry is currently available even though the normal set is larger.

Binder Definition This specification includes a proposal for a Collection Binder claim (see ). This claim would be included within any collection entry as a definiton of the integrity mechanism that binds that collection entry to another collection entry. A verifier can use the information within this claim to drive inter collection entry integrity checking. This claim would not be mandatory within a collection entry as a verifier may implement the integrity checking based upon information in the profile alone.

EAT collection binder ; By definition, the binder-function is applied to a concatenation ; of the ordered list of source claims. ; If the array is empty, the function is applied to the whole ; contents of the token. binder-source-label = Claim-Label destination-collection-entry = collection-entry-label destination-claim = Claim-Label Claim-Label = JC<"text", int> collection-entry-label = JC JC = J .feature "json" / C .feature "cbor" ]]>

The attributes within the binder claim are:

• binder-function: the identity of the binding cryptographic function, usually a hash function, applied to the values identified by the binder-source-label array.
• binder-source-label: an array defining the set of claims providing the binding information within the collection entry. It is assumed that the values corresponding to this (ordered) list will be concatenated and have the binder-function applied to produce a binder value. If the array is empty, the entire source collection entry is used as input to the binder-function. This latter condition would normally be applied to a collection entry consisting of a Claim Set.
• destination-collection-entry: this defines the collection entry that will hold (receive) the binding for this (source) collection entry.
• destination-claim: this defines the claim label within destination-collection-entry which will store the binder value.

A verifier can check the binding between two collection entries by computing the binder value for one entry and comparing the result stored within the value of the destination claim (in the destination collection entry).

Security Considerations A verifier for an attestation token must apply a verification process for the full set of entries contained within the Token Collection. This process will be custom to the relevant profile for the Token Collection and take into account any individual verification per entry and/or verification for the objects considered collectively, including the intra token integrity scheme. As there is no overall signature for the Collection, protection against malicious modification must be contained within the entries. It is expected that there exists a cryptographic binding between entries, this can for example be one to many or one to one in a (chain) series. The implementation of creating these bindings may involve passing data across ABIs. This provides an attack vector on the integrity of the collection which must be considered within any threat model. With respect to binder claims, these require integrity protection. This protection can either be provided by the signature on the token entry which contains the binder or, in the case where the entry does not have a signature, by including the binder claim with any other claims when preparing input into the cryptographic binding function. Depending upon the use case and associated threat model, the freshness of entries may need extra consideration.

IANA Considerations In the registry , IANA is requested to allocate the tag in from the FCFS space, with the present document as the specification reference. EAT Collection

Tag	Data Item	Semantics
TBD399	map	EAT Collection RFCthis

References Normative References Security Theory LLC Qualcomm Technologies Inc. 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 how much it wishes to trust the entity. An EAT is either a CBOR Web Token (CWT) or JSON Web Token (JWT) with attestation-oriented claims. IANA 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. The Concise Data Definition Language (CDDL), standardized in RFC 8610, provides "control operators" as its main language extension point. The present document defines a number of control operators that were not yet ready at the time RFC 8610 was completed: , , and for the construction of constants; / for including ABNF (RFC 5234 and RFC 7405) in CDDL specifications; and for indicating the use of a non-basic feature in an instance. 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. Informative References Fraunhofer SIT Qualcomm Technologies Inc. Cisco Systems Universität Bremen TZI CBOR Web Token (CWT, RFC 8392) Claims Sets sometimes do not need the protection afforded by wrapping them into COSE, as is required for a true CWT. This specification defines a CBOR tag for such unprotected CWT Claims Sets (UCCS) and discusses conditions for its proper use. Fraunhofer SIT Microsoft Sandelman Software Works Intel Corporation Huawei Technologies 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. An attempt is made to provide for a model that is neutral toward processor architectures, the content of claims, and protocols. Cisco Systems Fraunhofer SIT MIT Arm Limited Intel This document defines reusable Attestation Result information elements. When these elements are offered to Relying Parties as Evidence, different aspects of Attester trustworthiness can be evaluated. Additionally, where the Relying Party is interfacing with a heterogeneous mix of Attesting Environment and Verifier types, consistent policies can be applied to subsequent information exchange between each Attester and the Relying Party. CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. 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. JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. Arm Ltd

CDDL general-uri / general-oid ) all-collection-types = ( cwt-collection-entries // jwt-collection-entries // claim-set-collection-entries // detatched-eat-bundle-collection-entries ) cwt-collection-entries = ( collection-entry-label => CWT-Messages ) jwt-collection-entries = ( collection-entry-label => JWT-Messages ) claim-set-collection-entries = ( collection-entry-label => JC ) detatched-eat-bundle-collection-entries = ( collection-entry-label => BUNDLE-Messages ) collection-entry-label = JC ; The Collection Binder is a formal declaration of the inter entry ; binding mechanism. It would be included within the body of one or ; more of the collection entries. Tagged-Collection-Binder = #6.TBD99(Collection-Binder) Collection-Binder = [ binder-function, [*binder-source-label], destination-collection-entry, destination-claim ] ; binder function is normally the name/id of a hash algorithm binder-function = JC ; by definition, the binder-function is applied to a concatenation ; of the ordered list of source claims ; If the array is empty, the function is applied to the whole ; contents of the token binder-source-label = Claim-Label destination-collection-entry = collection-entry-label destination-claim = Claim-Label ]]>

Acknowledgments Thomas Fossati proposed the inclusion of the Binder definiton and collaborated on the CDDL. Yogesh Deshpande provided insightful comments and review for this proposal.

Contributors Arm Limited

thomas.fossati@arm.com