COSE (CBOR Object Signing and Encryption) Receipts
draft-ietf-cose-merkle-tree-proofs-18

COSE O. Steele
Internet-Draft Tradeverifyd
Intended status: Standards Track H. Birkholz
Expires: 5 June 2026 Fraunhofer SIT
 A. Delignat-Lavaud
 C. Fournet
 Microsoft
 2 December 2025
 COSE (CBOR Object Signing and Encryption) Receipts
 draft-ietf-cose-merkle-tree-proofs-18
Abstract
 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 Merkle inclusion and
 consistency proofs.
Discussion Venues
 This note is to be removed before publishing as an RFC.
 Discussion of this document takes place on the CBOR Object Signing
 and Encryption Working Group mailing list (cose@ietf.org), which is
 archived at https://mailarchive.ietf.org/arch/browse/cose/.
 Source for this draft and an issue tracker can be found at
 https://github.com/cose-wg/draft-ietf-cose-merkle-tree-proofs.
Status of This Memo
 This Internet-Draft is submitted in full conformance with the
 provisions of BCP 78 and BCP 79.
 Internet-Drafts are working documents of the Internet Engineering
 Task Force (IETF). Note that other groups may also distribute
 working documents as Internet-Drafts. The list of current Internet-
 Drafts is at https://datatracker.ietf.org/drafts/current/.
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 Internet-Drafts are draft documents valid for a maximum of six months
 and may be updated, replaced, or obsoleted by other documents at any
 time. It is inappropriate to use Internet-Drafts as reference
 material or to cite them other than as "work in progress."
 This Internet-Draft will expire on 5 June 2026.
Copyright Notice
 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents (https://trustee.ietf.org/
 license-info) in effect on the date of publication of this document.
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document. Code Components
 extracted from this document must include Revised BSD License text as
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 provided without warranty as described in the Revised BSD License.
Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
 2. New COSE Header Parameters . . . . . . . . . . . . . . . . . 3
 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
 4. Verifiable Data Structures in CBOR . . . . . . . . . . . . . 5
 4.1. Structures . . . . . . . . . . . . . . . . . . . . . . . 5
 4.2. Proofs . . . . . . . . . . . . . . . . . . . . . . . . . 5
 4.3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 6
 4.4. Profiles . . . . . . . . . . . . . . . . . . . . . . . . 9
 4.4.1. Registration Requirements . . . . . . . . . . . . . . 10
 5. RFC9162_SHA256 . . . . . . . . . . . . . . . . . . . . . . . 10
 5.1. Verifiable Data Structure . . . . . . . . . . . . . . . . 10
 5.2. Inclusion Proof . . . . . . . . . . . . . . . . . . . . . 10
 5.2.1. Receipt of Inclusion . . . . . . . . . . . . . . . . 11
 5.3. Consistency Proof . . . . . . . . . . . . . . . . . . . . 13
 5.3.1. Receipt of Consistency . . . . . . . . . . . . . . . 13
 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 15
 6.1. Log Length . . . . . . . . . . . . . . . . . . . . . . . 16
 6.2. Header Parameters . . . . . . . . . . . . . . . . . . . . 16
 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
 7.1. Choice of Signature Algorithms . . . . . . . . . . . . . 16
 7.2. Validity Period . . . . . . . . . . . . . . . . . . . . . 16
 7.3. Status Updates . . . . . . . . . . . . . . . . . . . . . 16
 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
 8.1. COSE Header Parameter . . . . . . . . . . . . . . . . . . 17
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 8.2. Verifiable Data Structure Registries . . . . . . . . . . 18
 8.2.1. Expert Review . . . . . . . . . . . . . . . . . . . . 18
 8.2.2. COSE Verifiable Data Structure Algorithms . . . . . . 18
 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
 10.1. Normative References . . . . . . . . . . . . . . . . . . 20
 10.2. Informative References . . . . . . . . . . . . . . . . . 21
 Appendix A. Implementation Status . . . . . . . . . . . . . . . 22
 A.1. Transmute Prototype . . . . . . . . . . . . . . . . . . . 22
 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 22
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. 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 proofs 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 the 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.
1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
2. 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
 or more COSE Receipts as specified in this document.
 TBD_1 (requested assignment 395): A COSE header parameter named vds
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 (Verifiable Data Structure), which conveys the algorithm
 identifier for a verifiable data structure. Correspondingly, this
 document introduces a new registry (Section 8.2.2) defining 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 registry
 (Table 2) defining the integers used to identify verifiable data
 structure proof types.
3. Terminology
 CDDL: Concise Data Definition Language (CDDL) is defined in
 [RFC8610].
 EDN: CBOR Extended Diagnostic Notation (EDN) is defined in
 [RFC8949], where it is referred to as "diagnostic notation", and
 is revised in [I-D.draft-ietf-cbor-edn-literals].
 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 property that can be obtained by verifying a given
 proof over one or more entries in a Verifiable Data Structure.
 For example, a VDS, such as a binary Merkle tree, can support
 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 [RFC8949].
 Entry: An entry in a verifiable data structure for which proofs can
 be derived.
 Receipt: A COSE object, as defined in [RFC9052], containing the
 header parameters necessary to convey VDP for an associated VDS.
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4. Verifiable Data Structures in CBOR
 This section describes representations of verifiable data structure
 proofs in [RFC8949]. 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 Section 2.1.3 of [RFC9162] and
 Section 2.1.4 of [RFC9162]. The design of these structures is
 influenced by the conventions established for COSE Keys.
4.1. Structures
 Similar to COSE Key Types (https://www.iana.org/assignments/cose/
 cose.xhtml#key-type), different verifiable data structures support
 different algorithms.
 This document establishes a registry of verifiable data structure
 algorithms, see Section 8.2.2 for details.
4.2. Proofs
 Similar to COSE Key Type Parameters
 (https://www.iana.org/assignments/cose/cose.xhtml#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 (requested assignment 395) : 1) supports both
 (-1) inclusion and (-2) consistency proofs.
 This document establishes a registry of verifiable data structure
 algorithm proofs, see Table 2 for details.
 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". Security analysis MUST be conducted
 prior to migrating to new structures to ensure the new security and
 privacy assumptions are acceptable for the use case.
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4.3. Usage
 This document registers a new COSE Header Parameter receipts (TBD_0
 (requested assignment 394)) to enable Receipts to be conveyed in the
 protected and unprotected headers of COSE Objects.
 When the receipts header parameter is present, the verifier MUST
 confirm that the associated verifiable data structure and verifiable
 data structure proofs match entries present in the registries
 established in this specification, including values added in
 subsequent registrations..
 Receipts MUST be tagged as COSE_Sign1.
 The following [RFC8610] definition is provided:
 Signature_With_Receipt = #6.18(COSE_Sign1)
 cose.label = int / text
 cose.values = any
 Protected_Header = {
 * cose.label => cose.values
 }
 Unprotected_Header = {
 &(receipts: 394) => [+ bstr .cbor Receipt]
 * cose.label => cose.values
 }
 COSE_Sign1 = [
 protected : bstr .cbor Protected_Header,
 unprotected : Unprotected_Header,
 payload : bstr / nil,
 signature : bstr
 ]
 Receipt = Receipt_For_Inclusion / Receipt_For_Consistency
 ; Note the the proof formats shown here are for RFC9162_SHA256.
 ; Other verifiable data structures may have different proof formats.
 Receipt_For_Inclusion = #6.18(Signed_Inclusion_Proof)
 Signed_Inclusion_Proof = [
 protected : bstr .cbor RFC9162_SHA256_Inclusion_Protected_Header
 unprotected : RFC9162_SHA256_Inclusion_Unprotected_Header
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 payload : bstr / nil
 signature : bstr
 ]
 RFC9162_SHA256_Inclusion_Protected_Header = {
 &(alg: 1) => int
 &(vds: 395) => int
 * cose.label => cose.values
 }
 RFC9162_SHA256_Inclusion_Unprotected_Header = {
 &(vdp: 396) => RFC9162_SHA256_Verifiable_Inclusion_Proofs
 * cose.label => cose.values
 }
 RFC9162_SHA256_Verifiable_Inclusion_Proofs = {
 &(inclusion-proof: -1) => RFC9162_SHA256_Inclusion_Proofs
 }
 RFC9162_SHA256_Inclusion_Proofs = [ + RFC9162_SHA256_Inclusion_Proof ]
 RFC9162_SHA256_Inclusion_Proof = bstr .cbor [
 tree_size: uint,
 leaf_index: uint,
 inclusion_path: [ + bstr ]
 ]
 Receipt_For_Consistency = #6.18(Signed_Consistency_Proof)
 Signed_Consistency_Proof = [
 protected : bstr .cbor RFC9162_SHA256_Consistency_Protected_Header,
 unprotected : RFC9162_SHA256_Consistency_Unprotected_Header,
 payload : bstr / nil, ; Newer Merkle tree root
 signature : bstr
 ]
 RFC9162_SHA256_Consistency_Protected_Header = {
 &(alg: 1) => int,
 &(vds: 395) => int,
 * cose.label => cose.values
 }
 RFC9162_SHA256_Consistency_Unprotected_Header = {
 &(vdp: 396) => RFC9162_SHA256_Verifiable_Consistency_Proofs
 * cose.label => cose.values
 }
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 RFC9162_SHA256_Verifiable_Consistency_Proofs = {
 &(consistency-proof: -2) => RFC9162_SHA256_Consistency_Proofs
 }
 RFC9162_SHA256_Consistency_Proofs = [ + RFC9162_SHA256_Consistency_Proof ]
 RFC9162_SHA256_Consistency_Proof = bstr .cbor [
 tree_size_1: uint,
 tree_size_2: uint,
 consistency_path: [ + bstr ]
 ]
 Figure 1: CDDL for a COSE Sign1 with attached receipts
 The following informative EDN is provided:
 / cose-sign1 / 18([
 / protected / <<{
 / kid / 4 : h'bc297b51...e4edf0de',
 / algorithm / 1 : -7, # ES256
 }>>,
 / unprotected / {
 / receipts / 394 : {
 <</ cose-sign1 / 18([
 / protected / <<{
 / kid / 4 : h'abcdef12...34567890',
 / algorithm / 1 : -7, # ES256
 / vds / 395 : 1, # RFC9162 SHA-256
 }>>,
 / unprotected / {
 / proofs / 396 : {
 / inclusion / -1 : [
 <<[
 / size / 9, / leaf / 8,
 / inclusion path /
 h'7558a95f...e02e35d6'
 ]>>
 ],
 },
 },
 / payload / null,
 / signature / h'02d227ed...ccd3774f'
 ])>>,
 <</ cose-sign1 / 18([
 / protected / <<{
 / kid / 4 : h'abcdef12...34567890',
 / algorithm / 1 : -7, # ES256
 / vds / 395 : 1, # RFC9162 SHA-256
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 }>>,
 / 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'
 ])
 Figure 2: An example COSE Signature with multiple receipts
 The specific structure of COSE Receipts is dependent on the structure
 of the COSE_Sign1 payload and the verifiable data structure proofs
 contained in the COSE_Sign1 unprotected header. The CDDL definition
 for verifiable data structure proofs is specific to each verifiable
 data structure. This document describes proofs for RFC9162_SHA256 in
 the following sections.
4.4. Profiles
 New verifiable data structures can require the definition of a
 profile. The payload in such definitions SHOULD be detached.
 Detached payloads force verifiers to recompute the root from the
 proof and protect against implementation errors where the signature
 is verified but the payload is incompatible with the proof. Profiles
 of proof signatures that define additional protected header
 parameters are encouraged to make their presence 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 [RFC9596] and the similar guidance
 provided in [RFC9597].
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4.4.1. Registration Requirements
 Each verifiable data structure specification applying for inclusion
 in this registry 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 Section 5 as an example.
 Where a specification supports a choice of hash algorithm, a separate
 IANA registration must be made for each supported algorithm. For
 example, to provide support for SHA256 and SHA3_256 with Merkle
 Consistency and Inclusion Proofs defined respectively in
 Section 2.1.3 of [RFC9162] and Section 2.1.4 of [RFC9162], both
 "RFC9162_SHA256" and "RFC9162_SHA3_256" require entries in the
 relevant IANA registries. This document only defines
 "RFC9162_SHA256".
5. RFC9162_SHA256
 This section defines how the data structure described in Section 2.1
 of [RFC9162] is mapped to the terminology defined in this document,
 using [RFC8949] and [RFC9053].
5.1. Verifiable Data Structure
 The integer identifier for this Verifiable Data Structure is 1. The
 string identifier for this Verifiable Data Structure is
 "RFC9162_SHA256", a Merkle Tree where SHA256 is used as the hash
 algorithm. See Table 2. See Section 2.1.1 of [RFC9162] (Definition
 of the Merkle Tree), for a complete description of this verifiable
 data structure.
5.2. Inclusion Proof
 See Section 2.1.3.1 of [RFC9162] (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:
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 inclusion-proof = bstr .cbor [
 ; tree size at current Merkle root
 tree-size: uint
 ; index of leaf in tree
 leaf-index: uint
 ; path from leaf to current Merkle root
 inclusion-path: [ + bstr ]
 ]
 Figure 3: CBOR Encoded RFC9162 Inclusion Proof
 The term leaf-index is used for alignment with the use established in
 Section 2.1.3.2 of [RFC9162].
 Note that [RFC9162] defines inclusion proofs only for leaf nodes, and
 that:
 If leaf_index is greater than or equal to tree_size, then fail the
 proof verification.
 The identifying index of a leaf node is relative to all nodes in the
 tree size for which the proof was obtained.
5.2.1. Receipt of Inclusion
 In a signed inclusion proof, the payload is the Merkle tree root that
 corresponds to the log at size tree-size. The protected header for
 an RFC9162_SHA256 inclusion proof signature is:
 protected-header-map = {
 &(alg: 1) => int
 &(vds: 395) => int
 * cose-label => cose-value
 }
 Figure 4: Protected Header for a Receipt of Inclusion
 * alg (label: 1): REQUIRED. Signature algorithm identifier. Value
 type: int.
 * vds (label: TBD_1 (requested assignment 395)): REQUIRED.
 Verifiable data structure algorithm identifier. Value type: int.
 The unprotected header for an RFC9162_SHA256 inclusion proof
 signature is:
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 inclusion-proofs = [ + inclusion-proof ]
 verifiable-proofs = {
 &(inclusion-proof: -1) => inclusion-proofs
 }
 unprotected-header-map = {
 &(vdp: 396) => verifiable-proofs
 * cose-label => cose-value
 }
 Figure 5: A Verifiable Data Structure Proofs in an Unprotected Header
 * vdp (label: TBD_2 (requested assignment 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 [RFC9162].
 An EDN example for a Receipt containing an inclusion proof for
 RFC9162_SHA256 with a detached payload (see Section 4.4) is:
 / cose-sign1 / 18([
 / protected / <<{
 / algorithm / 1 : -7, # ES256
 / vds / 395 : 1, # RFC9162 SHA-256
 }>>,
 / 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'
 ])
 Figure 6: Receipt of Inclusion
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 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
 verifier 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.
5.3. Consistency Proof
 See Section 2.1.4.1 of [RFC9162] (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:
 consistency-proof = bstr .cbor [
 ; older Merkle root tree size
 tree-size-1: uint
 ; newer Merkle root tree size
 tree-size-2: uint
 ; path from older Merkle root to newer Merkle root.
 consistency-path: [ + bstr ]
 ]
 Figure 7: CBOR Encoded RFC9162 Consistency Proof
5.3.1. 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:
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 protected-header-map = {
 &(alg: 1) => int
 &(vds: 395) => int
 * cose-label => cose-value
 }
 Figure 8: Protected Header for a Receipt of Consistency
 * alg (label: 1): REQUIRED. Signature algorithm identifier. Value
 type: int.
 * vds (label: TBD_1 (requested assignment 395)): REQUIRED.
 Verifiable data structure algorithm identifier. Value type: int.
 The unprotected header for an RFC9162_SHA256 consistency proof
 signature is:
 consistency-proofs = [ + consistency-proof ]
 verifiable-proofs = {
 &(consistency-proof: -2) => consistency-proofs
 }
 unprotected-header-map = {
 &(vdp: 396) => verifiable-proofs
 * cose-label => cose-value
 }
 * vdp (label: TBD_2 (requested assignment 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 [RFC9162].
 An example EDN for a Receipt containing a consistency proof for
 RFC9162_SHA256 with a detached payload (see Section 4.4) is:
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 / cose-sign1 / 18([
 / protected / <<{
 / algorithm / 1 : -7, # ES256
 / vds / 395 : 1, # RFC9162 SHA-256
 }>>,
 / 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'
 ])
 Figure 9: Example consistency receipt
 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.
6. Privacy Considerations
 The privacy considerations section of [RFC9162] and [RFC9053] apply
 to this document.
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6.1. 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.
6.2. Header Parameters
 Additional header parameters can reveal information about the
 transparency service or its log entries. The receipt producer MUST
 perform a privacy analysis for all mandatory fields in profiles based
 on this specification.
7. Security Considerations
 See the security considerations section of:
 * [RFC9162]
 * [RFC9053]
7.1. Choice of Signature Algorithms
 A security analysis ought to 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.
7.2. 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 [RFC8392], for
 one way to accomplish this. The details of expressing validity
 periods are out of scope for this document.
7.3. 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.
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8. IANA Considerations
8.1. COSE Header Parameter
 IANA is requested to add the COSE header parameters defined in
 Section 2, as listed in Table 1, to the "COSE Header Parameters"
 registry [IANA.cose_header-parameters] in the 'Integer values from
 256 to 65535' range ('Specification Required' Registration
 Procedure). The Value Registry for "vds" is the COSE Verifiable Data
 Structure registry. The map labels in the "vdp" are assigned from
 the COSE Verifiable Data Structure Proofs registry.
 +========+=============+=====+============+=============+=========+
 |Name | Label |Value| Value | Description |Reference|
 | | |Type | Registry | | |
 +========+=============+=====+============+=============+=========+
 |receipts| TBD_0 |array| | Priority |RFCthis, |
 | | (requested | | | ordered |Section 2|
 | | assignment: | | | sequence of | |
 | | 394) | | | CBOR | |
 | | | | | encoded | |
 | | | | | Receipts | |
 +--------+-------------+-----+------------+-------------+---------+
 |vds | TBD_1 |int | COSE | Algorithm |RFCthis, |
 | | (requested | | Verifiable | identifier |Section 2|
 | | assignment: | | Data | for | |
 | | 395) | | Structure | verifiable | |
 | | | | | data | |
 | | | | | structures, | |
 | | | | | used to | |
 | | | | | produce | |
 | | | | | verifiable | |
 | | | | | data | |
 | | | | | structure | |
 | | | | | proofs | |
 +--------+-------------+-----+------------+-------------+---------+
 |vdp | TBD_2 |map | map key in | Location |RFCthis, |
 | | (requested | | COSE | for |Section 2|
 | | assignment: | | Verifiable | verifiable | |
 | | 396) | | Data | data | |
 | | | | Structure | structure | |
 | | | | Proofs | proofs in | |
 | | | | | COSE Header | |
 | | | | | Parameters | |
 +--------+-------------+-----+------------+-------------+---------+
 Table 1: Newly registered COSE Header Parameters
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8.2. Verifiable Data Structure Registries
 IANA established the COSE Verifiable Data Structure Algorithms and
 COSE Verifiable Data Structure Proofs registries under a
 Specification Required policy as described in Section 4.6 of
 [RFC8126].
8.2.1. 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 [RFC7120].
 * It is not permissible to assign points in COSE Verifiable Data
 Structure Algorithms, for which no corresponding COSE Verifiable
 Data Structure Proofs entry exists, and vice versa.
 * The Change Controller for related registrations of structures and
 proofs should be the same.
8.2.2. COSE Verifiable Data Structure Algorithms
 Registration Template:
 * Name: This is a descriptive name for the verifiable data structure
 that enables easier reference to the item.
 * Value: This is the value used to identify the verifiable data
 structure.
 * Description: This field contains a brief description of the
 verifiable data structure.
 * Reference: This contains a pointer to the public specification for
 the verifiable data structure.
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 * Change Controller: For Standards Track RFCs, list the "IETF". For
 others, give the name of the responsible party. Other details
 (e.g., postal address, email address, home page URI) may also be
 included.
 Initial contents:
 +================+=======+===========================+==============+
 | Name | Value | Description | Reference |
 +================+=======+===========================+==============+
 | Reserved | 0 | Reserved | Reserved |
 +----------------+-------+---------------------------+--------------+
 | RFC9162_SHA256 | 1 | SHA256 Binary | Section 2.1 |
 | | | Merkle Tree | of [RFC9162] |
 +----------------+-------+---------------------------+--------------+
 Table 2: COSE Verifiable Data Structure Algorithms
 Registration Template:
 * Verifiable Data Structure: This value used identifies the related
 verifiable data structure.
 * Name: This is a descriptive name for the proof type that enables
 easier reference to the item.
 * Label: This is the value used to identify the verifiable data
 structure proof type.
 * CBOR Type: This contains the CBOR type for the value portion of
 the label.
 * Description: This field contains a brief description of the proof
 type.
 * Reference: This contains a pointer to the public specification for
 the proof type.
 * Change Controller: For Standards Track RFCs, list the "IETF". For
 others, give the name of the responsible party. Other details
 (e.g., postal address, email address, home page URI) may also be
 included.
 Initial contents:
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 +============+=============+=====+=======+=============+===========+
 | Verifiable | Name |Label| CBOR | Description | Reference |
 | Data | | | Type | | |
 | Structure | | | | | |
 +============+=============+=====+=======+=============+===========+
 | 1 | inclusion |-1 | array | Proof of | RFCthis, |
 | | proofs | | (of | inclusion | Section |
 | | | | bstr) | | 5.2 |
 +------------+-------------+-----+-------+-------------+-----------+
 | 1 | consistency |-2 | array | Proof of | RFCthis, |
 | | proofs | | (of | append only | Section |
 | | | | bstr) | property | 5.3 |
 +------------+-------------+-----+-------+-------------+-----------+
 Table 3: COSE Verifiable Data Structure Proofs
9. 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.
10. References
10.1. Normative References
 [IANA.cose_header-parameters]
 IANA, "COSE Header Parameters",
 <https://www.iana.org/assignments/cose>.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/rfc/rfc2119>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
 [RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
 Definition Language (CDDL): A Notational Convention to
 Express Concise Binary Object Representation (CBOR) and
 JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
 June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.
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 [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
 Representation (CBOR)", STD 94, RFC 8949,
 DOI 10.17487/RFC8949, December 2020,
 <https://www.rfc-editor.org/rfc/rfc8949>.
 [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
 Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
 August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
 [RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
 Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
 December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.
 [RFC9596] Jones, M.B. and O. Steele, "CBOR Object Signing and
 Encryption (COSE) "typ" (type) Header Parameter",
 RFC 9596, DOI 10.17487/RFC9596, June 2024,
 <https://www.rfc-editor.org/rfc/rfc9596>.
 [RFC9597] Looker, T. and M.B. Jones, "CBOR Web Token (CWT) Claims in
 COSE Headers", RFC 9597, DOI 10.17487/RFC9597, June 2024,
 <https://www.rfc-editor.org/rfc/rfc9597>.
10.2. Informative References
 [BCP205] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
 Code: The Implementation Status Section", BCP 205,
 RFC 7942, DOI 10.17487/RFC7942, July 2016,
 <https://www.rfc-editor.org/rfc/rfc7942>.
 [I-D.draft-ietf-cbor-edn-literals]
 Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
 Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
 literals-19, 16 October 2025,
 <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
 edn-literals-19>.
 [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
 Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
 2014, <https://www.rfc-editor.org/rfc/rfc7120>.
 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
 Writing an IANA Considerations Section in RFCs", BCP 26,
 RFC 8126, DOI 10.17487/RFC8126, June 2017,
 <https://www.rfc-editor.org/rfc/rfc8126>.
 [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
 "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
 May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.
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 [RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
 Structures and Process", STD 96, RFC 9052,
 DOI 10.17487/RFC9052, August 2022,
 <https://www.rfc-editor.org/rfc/rfc9052>.
Appendix A. Implementation Status
 Note to RFC Editor: Please remove this section as well as references
 to [BCP205] before AUTH48.
 This section records the status of known implementations of the
 protocol defined by this specification at the time of posting of this
 Internet-Draft, and is based on a proposal described in [BCP205].
 The description of implementations in this section is intended to
 assist the IETF in its decision processes in progressing drafts to
 RFCs. Please note that the listing of any individual implementation
 here does not imply endorsement by the IETF. Furthermore, no effort
 has been spent to verify the information presented here that was
 supplied by IETF contributors. This is not intended as, and must not
 be construed to be, a catalog of available implementations or their
 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".
A.1. 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 (https://github.com/
 transmute-industries/cose?tab=readme-ov-file#transparent-statement)
 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)
Contributors
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 Amaury Chamayou
 Microsoft
 United Kingdom
 Email: amaury.chamayou@microsoft.com
 Steve Lasker
 Email: stevenlasker@hotmail.com
 Robert Martin
 MITRE Corporation
 United States
 Email: ramartin@mitre.org
 Monty Wiseman
 United States of America
 Email: mwiseman32@acm.org
 Roy Williams
 United States of America
 Email: roywill@msn.com
Authors' Addresses
 Orie Steele
 Tradeverifyd
 United States
 Email: orie@or13.io
 Henk Birkholz
 Fraunhofer SIT
 Rheinstrasse 75
 64295 Darmstadt
 Germany
 Email: henk.birkholz@ietf.contact
 Antoine Delignat-Lavaud
 Microsoft
 United Kingdom
 Email: antdl@microsoft.com
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 Cedric Fournet
 Microsoft
 United Kingdom
 Email: fournet@microsoft.com
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