]> Transmute

United States orie@transmute.industries

Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@ietf.contact

Microsoft

UK antdl@microsoft.com

Microsoft

UK fournet@microsoft.com

Security COSE Internet-Draft COSE (CBOR Object Signing and Encryption) Receipts prove properties of a verifiable data structure to a verifier. Verifiable data structures and associated proof types enable security properties, such as minimal disclosure, transparency and non-equivocation. Transparency helps maintain trust over time, and has been applied to certificates, end to end encrypted messaging systems, and supply chain security. This specification enables concise transparency oriented systems, by building on CBOR (Concise Binary Object Representation) and COSE. The extensibility of the approach is demonstrated by providing CBOR encodings for RFC9162. Discussion Venues Discussion of this document takes place on the CBOR Object Signing and Encryption Working Group mailing list (cose@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction COSE Receipts are signed proofs that include metadata about certain states of a verifiable data structure (VDS) that are true when the COSE Receipt was issued. COSE Receipts can include proves that a document is in a database (proof of inclusion), that a database is append only (proof of consistency), that a smaller set of statements are contained in a large set of statements (proof of disclosure, a special case of proof of inclusion), or proof that certain data is not yet present in a database (proofs of non inclusion). Different VDS can produce different verifiable data structure proofs (VDP). The combination of representations of various VDS and VDP can significantly increase burden for implementers and create interoperability challenges for transparency services. This document describes how to convey VDS and associated VDP types in unified COSE envelopes.

Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

New COSE Header Parameters This document defines three new COSE header parameters, which are introduced up-front in this Section and elaborated on later in this document.

TBD_0 (requested assignment 394):

A COSE header parameter named receipts with a value type of array where the array contains one ore more COSE Receipts as specified in this document.

TBD_1 (requested assignment 395):

A COSE header parameter named vds (Verifiable Data Structure), which conveys the algorithm identifier for a verifiable data structure. Correspondingly, this document introduces a new verifiable data structure registry that registers the integers used to identify verifiable data structures.

TBD_2 (requested assignment 396):

A COSE header parameter named vdp (short for "verifiable data structure proofs"), which conveys a map containing verifiable data structure proofs organized by proof type. Correspondingly, this document introduces a new verifiable data structure proof registry that registers the integers used to identify verifiable data structure proof types.

The other codepoints in this document are assigned from the registries established in this draft, they are therefore not marked TBD.

Terminology

CDDL:

Concise Data Definition Language (CDDL) is defined in .

EDN:

CBOR Extended Diagnostic Notation (EDN) is defined in , where it is referred to as "diagnostic notation", and is revised in .

Verifiable Data Structure (VDS):

A data structure which supports one or more Verifiable Data Structure Proof Types. This property describes an algorithm used to maintain a verifiable data structure, for example a binary merkle tree algorithm.

Verifiable Data Structure Proofs (VDP):

A data structure used to convey proof types for proving different properties, such as authentication, inclusion, consistency, and freshness. Parameters can include multiple proofs of a given type, or multiple types of proof (inclusion and consistency).

Proof Type:

A verifiable process, that proves properties of a Verifiable Data Structure. For example, a VDS, such as a binary merkle tree, can support multiple proofs of type "inclusion" where each proof confirms that a given entry is included in a merkle root.

Proof Value:

An encoding of a Proof Type in CBOR .

Entry:

An entry in a verifiable data structure for which proofs can be derived.

Receipt:

A COSE object, as defined in , containing the header parameters necessary to convey VDP for an associated VDS.

Verifiable Data Structures in CBOR This section describes representations of verifiable data structure proofs in . For example, construction of a merkle tree leaf, or an inclusion proof from a leaf to a merkle root, might have several different representations, depending on the verifiable data structure used. Differences in representations are necessary to support efficient verification, unique security or privacy properties, and for compatibility with specific implementations. This document defines two extension points for enabling verifiable data structures with COSE and provides concrete examples for the structures and proofs defined in . The design of these structures is influenced by the conventions established for COSE Keys.

Structures Similar to COSE Key Types, different verifiable data structures support different algorithms. As EC2 keys (1: 2) support both digital signature and key agreement algorithms, RFC9162_SHA256 (TBD_1 : 1) supports both inclusion and consistency proofs. This document establishes a registry of verifiable data structure algorithms, with the following initial contents: COSE Verifiable Data Structures

Name	Value	Description	Reference
Reserved	0	Reserved	Reserved
RFC9162_SHA256	1	SHA256 Binary Merkle Tree

Proofs Similar to COSE Key Type Parameters, as EC2 keys (1: 2) keys require and give meaning to specific parameters, such as -1 (crv), -2 (x), -3 (y), -4 (d), RFC9162_SHA256 (TBD_1 : 1) supports both (-1) inclusion and (-2) consistency proofs. This document establishes a registry of verifiable data structure algorithms, with the following initial contents: COSE Verifiable Data Structure Proofs

Verifiable Data Structure	Name	Label	CBOR Type	Description	Reference
1	inclusion proofs	-1	array (of bstr)	Proof of inclusion
1	consistency proofs	-2	array (of bstr)	Proof of append only property

Proof types are specific to their associated "verifiable data structure", for example, different Merkle trees might support different representations of "inclusion proof" or "consistency proof". Implementers should not expect interoperability across "verifiable data structures", but they should expect conceptually similar properties across the different registered proof types. For example, 2 different merkle tree based verifiable data structures might both support proofs of inclusion. Security analysis SHOULD be conducted prior to migrating to new structures to ensure the new security and privacy assumptions are acceptable for the use case.

Usage This document registered a new COSE Header Parameter receipts (394) to enable this Receipts to be conveyed in the protected and unprotected headers of COSE Objects. When the receipts header parameter is present, the associated verifiable data structure and verifiable data structure proofs MUST match entries present in the registries established in this specification. The following informative CDDL is provided:

CDDL for a COSE Sign1 with attached receipts cose-value } Unprotected_Header = { &(receipts: 394) => [+ Receipt] * cose-label => cose-value } COSE_Sign1 = [ protected : bstr .cbor Protected_Header, unprotected : Unprotected_Header, payload : bstr / nil, signature : bstr ] ]]>

The following informative EDN is provided:

A COSE Signature with multiple receipts >, / unprotected / { / receipts / 394 : { <>, / unprotected / { / proofs / 396 : { / inclusion / -1 : [ <<[ / size / 9, / leaf / 8, / inclusion path / h'7558a95f...e02e35d6' ]>> ], }, }, / payload / null, / signature / h'02d227ed...ccd3774f' ])>>, <>, / unprotected / { / proofs / 396 : { / inclusion / -1 : [ <<[ / size / 6, / leaf / 5, / inclusion path / h'9352f974...4ffa7ce0', h'54806f32...f007ea06' ]>> ], }, }, / payload / null, / signature / h'36581f38...a5581960' ])>> }, }, / payload / h'0167c57c...deeed6d4', / signature / h'2544f2ed...5840893b' ]) ]]>

Registration Requirements Each specification MUST define how to encode the verifiable data structure identifier and its proof types in CBOR. Each specification MUST define how to produce and consume the supported proof types. See as an example. Where a specification supports a choice of hash algorithm, an IANA registration must be made for each individually supported algorithm. For example, to provide for both SHA256 and SHA3_256 with , both "RFC9162_SHA256" and "RFC9162_SHA3_256" require entries in the relevant IANA registries.

RFC9162_SHA256 This section defines how the data structures described in are mapped to the terminology defined in this document, using and .

Verifiable Data Structure The integer identifier for this Verifiable Data Structure is 1. The string identifier for this Verifiable Data Structure is "RFC9162_SHA256". See . See , 2.1.1. Definition of the Merkle Tree, for a complete description of this verifiable data structure.

Inclusion Proof See , 2.1.3.1. Generating an Inclusion Proof, for a complete description of this verifiable data structure proof type. The CBOR representation of an inclusion proof for RFC9162_SHA256 is:

CBOR Encoded RFC9162 Inclusion Proof

The term leaf-index is used for alignment with the use established in Note that defines that verification MUST fail if leaf-index is >= tree-size, and inclusion proofs are defined only for leaf nodes. The identifying index of a leaf node is relative to all nodes in the tree size for which the proof was obtained.

Receipt of Inclusion In a signed inclusion proof, the payload is the merkle tree root which corresponds to the log at size tree-size. Specifications are encouraged to make payloads detached when possible, forcing validation-time comparison. Profiles of proof signatures are encouraged to make additional protected header parameters mandatory, to ensure that claims are processed with their intended semantics. One way to include this information in the COSE structure is use of the typ (type) Header Parameter, see and the similar guidance provided in . The protected header for an RFC9162_SHA256 inclusion proof signature is:

Protected Header for a Receipt of Inclusion int &(vds: 395) => int * cose-label => cose-value } ]]>

• alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int.
• vds (label: 395): REQUIRED. Verifiable data structure algorithm identifier. Value type: int.

The unprotected header for an RFC9162_SHA256 inclusion proof signature is:

A Verifiable Data Structure Proofs in an Unprotected Header inclusion-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value } ]]>

• vdp (label: 396): REQUIRED. Verifiable data structure proofs. Value type: Map.
• inclusion-proof (label: -1): REQUIRED. Inclusion proofs. Value type: Array of bstr.

The payload of an RFC9162_SHA256 inclusion proof signature is the Merkle tree hash as defined in . The payload SHOULD be detached. Detaching the payload forces verifiers to recompute the root from the inclusion proof, this protects against implementation errors where the signature is verified but the merkle root does not match the inclusion proof. The EDN for a Receipt containing an inclusion proof for RFC9162_SHA256 is:

Receipt of Inclusion >, / unprotected / { / proofs / 396 : { / inclusion / -1 : [ <<[ / size / 20, / leaf / 17, / inclusion path / h'fc9f050f...221c92cb', h'bd0136ad...6b28cf21', h'd68af9d6...93b1632b' ]>> ], }, }, / payload / null, / signature / h'de24f0cc...9a5ade89' ]) ]]>

The VDS in the protected header is necessary to understand the inclusion proof structure in the unprotected header. The inclusion proof and signature are verified in order. First the verifiers applies the inclusion proof to a possible entry (set member) bytes. If this process fails, the inclusion proof may have been tampered with. If this process succeeds, the result is a merkle root, which in the attached as the COSE Sign1 payload. Second the verifier checks the signature of the COSE Sign1. If the resulting signature verifies, the Receipt has proved inclusion of the entry in the verifiable data structure. If the resulting signature does not verify, the signature may have been tampered with. It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid inclusion proof.

Consistency Proof See , 2.1.4.1. Generating a Consistency Proof, for a complete description of this verifiable data structure proof type. The cbor representation of a consistency proof for RFC9162_SHA256 is:

CBOR Encoded RFC9162 Consistency Proof

Receipt of Consistency In a signed consistency proof, the newer merkle tree root (proven to be consistent with an older merkle tree root) is an attached payload and corresponds to the log at size tree-size-2. The protected header for an RFC9162_SHA256 consistency proof signature is:

Protected Header for a Receipt of Consistency int &(vds: 395) => int * cose-label => cose-value } ]]>

• alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int.
• vds (label: TBD_1): REQUIRED. Verifiable data structure algorithm identifier. Value type: int.

The unprotected header for an RFC9162_SHA256 consistency proof signature is: consistency-proofs } unprotected-header-map = { &(vdp: 396) => verifiable-proofs * cose-label => cose-value } ]]>

• vdp (label: 396): REQUIRED. Verifiable data structure proofs. Value type: Map.
• consistency-proof (label: -2): REQUIRED. Consistency proofs. Value type: Array of bstr.

The payload of an RFC9162_SHA256 consistency proof signature is: The newer Merkle tree hash as defined in . The payload SHOULD be detached. Detaching the payload forces verifiers to recompute the root from the consistency proof, this protects against implementation errors where the signature is verified but the merkle root does not match the proof. The EDN for a Receipt containing a consistency proof for RFC9162_SHA256 is:

Example consistency receipt >, / unprotected / { / proofs / 396 : { / consistency / -2 : [ <<[ / old / 20, / new / 104, / consistency path / h'e5b3e764...c4a813bc', h'87e8a084...4f529f69', h'f712f76d...92a0ff36', h'd68af9d6...93b1632b', h'249efab6...b7614ccd', h'85dd6293...38914dc1' ]>> ], }, }, / payload / null, / signature / h'94469f73...52de67a1' ]) ]]>

The VDS in the protected header is necessary to understand the consistency proof structure in the unprotected header. The signature and consistency proof are verified in order. First the verifier checks the signature on the COSE Sign1. If the verification fails, the consistency proof is not checked. Second the consistency proof is checked by applying a previous inclusion proof, to the consistency proof. If the verification fails, the append only property of the verifiable data structure is not assured. This approach is specific to RFC9162_SHA256, different verifiable data structures may not support consistency proofs. It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid consistency proof.

Privacy Considerations See the privacy considerations section of:

Log Length Some structures and proofs leak the size of the log at the time of inclusion. In the case that a log only stores certain kinds of information, this can reveal details that could impact reputation. For example, if a transparency log only stored breach notices, a receipt for a breach notice would reveal the number of previous breaches at the time the notice was made transparent.

Header Parameters Additional header parameters can reveal information about the transparency service or its log entries. A privacy analysis MUST be performed for all mandatory fields in profiles based on this specification.

Security Considerations See the security considerations section of:

Choice of Signature Algorithms A security analysis MUST be performed to ensure that the digital signature algorithm alg has the appropriate strength to secure receipts. It is recommended to select signature algorithms that share cryptographic components with the verifiable data structure used, for example: Both RFC9162_SHA256 and ES256 depend on the sha-256 hash function.

Validity Period In some cases, receipts MAY include strict validity periods, for example, activation not too far in the future, or expiration, not too far in the past. See the iat, nbf, and exp claims in , for one way to accomplish this. The details of expressing validity periods are out of scope for this document.

Status Updates In some cases, receipts should be "revocable" or "suspendible", after being issued, regardless of their validity period. The details of expressing statuses are out of scope for this document.

IANA Considerations

COSE Header Parameter IANA is requested to add the COSE header parameters defined in , as listed in , to the "COSE Header Parameters" registry in the 'Integer values from 256 to 65535' range ('Specification Required' Registration Procedure). All new entries use https://www.iana.org/assignments/cose/cose.xhtml#header-parameters as the value for the "Value Registry" column. Newly registered COSE Header Parameters

Name	Label	Value Type	Description	Reference
receipts	TBD_0 (requested assignment: 394)	array	Priority ordered sequence of CBOR encoded Receipts	RFCthis,
vds	TBD_1 (requested assignment: 395)	int	Algorithm identifier for verifiable data structure, used to produce verifiable data structure proofs	RFCthis,
vdp	TBD_2 (requested assignment: 396)	map	Location for verifiable data structure proofs in COSE Header Parameters	RFCthis,

Verifiable Data Structure Registries IANA established the COSE Verifiable Data Structures and COSE Verifiable Data Structure Parameters Registries under a Specification Required policy as described in .

Expert Review Expert reviewers should take into consideration the following points:

• Experts are advised to assign the next available positive integer for verifiable data structures.
• Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in deployments.
• Specifications are required for all point assignments. Early Allocation is permissible, see Section 2 of .
• It is not permissible to assign points in COSE Verifiable Data Structures, for which no corresponding COSE Verifiable Data Structure Proofs entry exists, and vice versa.

COSE Verifiable Data Structures Registration Template:

• Name: The name of the verifiable data structure
• Value: The identifier for the verifiable data structure
• Description: A brief description of the verifiable data structure
• Reference: Where the verifiable data structure is defined

Initial contents: Provided in

COSE Verifiable Data Structure Proofs Registration Template:

• Verifiable Data Structure: The identifier for the verifiable data structure
• Name: The name of the proof type
• Label: The integer of the proof type
• CBOR Type: The cbor data type of the proof
• Description: The description of the proof type
• Reference: Where the proof type is defined

Initial contents: Provided in

Acknowledgements We would like to thank Maik Riechert, Jon Geater, Michael B. Jones, Mike Prorock, Ilari Liusvaara, Amaury Chamayou, for their contributions (some of which substantial) to this draft and to the initial set of implementations.

References Normative References 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. 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 a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. This document describes how to include CBOR Web Token (CWT) claims in the header parameters of any CBOR Object Signing and Encryption (COSE) structure. This functionality helps to facilitate applications that wish to make use of CWT claims in encrypted COSE structures and/or COSE structures featuring detached signatures, while having some of those claims be available before decryption and/or without inspecting the detached payload. Another use case is using CWT claims with payloads that are not CWT Claims Sets, including payloads that are not CBOR at all. This specification adds the equivalent of the JSON Object Signing and Encryption (JOSE) "typ" (type) header parameter to CBOR Object Signing and Encryption (COSE). This enables the benefits of explicit typing (as defined in RFC 8725, "JSON Web Token Best Current Practices") to be brought to COSE objects. The syntax of the COSE type header parameter value is the same as the existing COSE content type header parameter. 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 This memo describes the process for early allocation of code points by IANA from registries for which "Specification Required", "RFC Required", "IETF Review", or "Standards Action" policies apply. This process can be used to alleviate the problem where code point allocation is needed to facilitate desired or required implementation and deployment experience prior to publication of an RFC, which would normally trigger code point allocation. The procedures in this document are intended to apply only to IETF Stream documents. 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. 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 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. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. 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. 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. Universität Bremen TZI This document formalizes and consolidates the definition of the Extended Diagnostic Notation (EDN) of the Concise Binary Object Representation (CBOR), addressing implementer experience. Replacing EDN's previous informal descriptions, it updates RFC 8949, obsoleting its Section 8, and RFC 8610, obsoleting its Appendix G. It also specifies and uses registry-based extension points, using one to support text representations of epoch-based dates/times and of IP addresses and prefixes. // (This cref will be removed by the RFC editor:) The present // revision (–16) addresses the first half of the WGLC comments, // except for the issues around the specific way how to best achieve // pluggable ABNF grammars for application-extensions. It is // intended for use as a reference document for the mid-WGLC CBOR WG // interim meeting on 2025-01-08.

Implementation Status Note to RFC Editor: Please remove this section as well as references to 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 . 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".

Transmute Prototype An open-source implementation was initiated and is maintained by the Transmute Industries Inc. - Transmute. An application demonstrating the concepts is available at COSE SCITT Receipts Implementation URL: https://github.com/transmute-industries/cose Maturity: The code's level of maturity is considered to be "prototype". Coverage and Version Compatibility: The current version ('main') implements the verifiable data structure algorithm, inclusion proof and consistency proof concepts of this draft. License: The project and all corresponding code and data maintained on GitHub are provided under the Apache License, version 2. Contact: Orie Steele (orie@transmute.industries)