COSE O. Steele Internet-Draft Transmute Intended status: Standards Track H. Birkholz Expires: 20 April 2025 Fraunhofer SIT A. Delignat-Lavaud C. Fournet Microsoft 17 October 2024 COSE Receipts draft-ietf-cose-merkle-tree-proofs-07 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 RFC9162. 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/. Steele, et al. Expires 20 April 2025 [Page 1] Internet-Draft COSE Receipts October 2024 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 20 April 2025. Copyright Notice Copyright (c) 2024 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 described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. CBOR Tags . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Verifiable Data Structures in CBOR . . . . . . . . . . . . . 5 4.1. Structures . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Parameters . . . . . . . . . . . . . . . . . . . . . . . 6 4.3. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.3.1. Registration Requirements . . . . . . . . . . . . . . 10 5. RFC9162_SHA256 . . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Verifiable Data Structure . . . . . . . . . . . . . . . . 11 5.2. Inclusion Proof . . . . . . . . . . . . . . . . . . . . . 11 5.2.1. Receipt of Inclusion . . . . . . . . . . . . . . . . 11 5.3. Consistency Proof . . . . . . . . . . . . . . . . . . . . 13 5.3.1. Receipt of Consistency . . . . . . . . . . . . . . . 14 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16 6.1. Log Length . . . . . . . . . . . . . . . . . . . . . . . 17 6.2. Header Parameters . . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7.1. Choice of Signature Algorithms . . . . . . . . . . . . . 17 7.2. Validity Period . . . . . . . . . . . . . . . . . . . . . 17 7.3. Status Updates . . . . . . . . . . . . . . . . . . . . . 18 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 9.1. Additions to Existing Registries . . . . . . . . . . . . 18 Steele, et al. Expires 20 April 2025 [Page 2] Internet-Draft COSE Receipts October 2024 9.1.1. New Entries to the COSE Header Parameters Registry . 18 9.1.2. COSE Verifiable Data Structures . . . . . . . . . . . 19 9.1.3. COSE Verifiable Data Structure Parameters . . . . . . 20 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 10.1. Normative References . . . . . . . . . . . . . . . . . . 21 10.2. Informative References . . . . . . . . . . . . . . . . . 22 Appendix A. Implementation Status . . . . . . . . . . . . . . . 23 A.1. Transmute Prototype . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 1. Introduction COSE Receipts are signed proofs that include metadata about 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. 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. CBOR Tags Editorial Note (To be removed by RFC Editor). This section will be removed before the document is completed, its purpose is to track the TBD code points references throughout the draft. 395 is TBD_1: A requested cose header parameter representing the verifiable data structure used. 396 is TBD_2: A requested cose header parameter representing the verifiable data structure parameters map (proofs map). Steele, et al. Expires 20 April 2025 [Page 3] Internet-Draft COSE Receipts October 2024 The other codepoints are assigned from the registries established in this draft, they are therefore not marked TBD. 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 Proof Types. This property is conceptually similar to "alg" (1), it described an algorithm used to maintain the verifiable data structure, for example a binary merkle tree algorithm. Verifiable Data Structure Parameters (VDP): Parameters to a verifiable data structure that are used to prove 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). This property is conceptually similar to COSE Header Parameter "epk" (-1) or CBOR Web Token (CWT) claim "cnf" (8), it is applied to a verifiable data structure, to confirm a property. For example an encrypted message might be decrypted using epk and a private key, a digital signature for authentication might be verified using cnf and the (CWT) claim "nonce" and "audience", and an inclusion proof for a binary merkle tree might be verified with VDP and some entry that is being tested or inclusion in the tree. 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 [RFC9052], containing the header parameters necessary to convey VDP for an associated VDS. Steele, et al. Expires 20 April 2025 [Page 4] Internet-Draft COSE Receipts October 2024 4. Verifiable Data Structures in CBOR This section describes representations of verifiable data structure proofs in CBOR. 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 [RFC9162]. The design of these structures is influenced by the conventions established for COSE Keys. During testing and development the experimental range SHOULD be used, unless early assignment for a provisional entry has been completed. 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. 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: +================+=======+===========================+===========+ | Name | Value | Description | Reference | +================+=======+===========================+===========+ | N/A | 0 | N/A | N/A | +----------------+-------+---------------------------+-----------+ | RFC9162_SHA256 | 1 | SHA256 Binary Merkle Tree | [RFC9162] | +----------------+-------+---------------------------+-----------+ | EXPERIMENTAL | 11 | Unknown | RFC XXXX | +----------------+-------+---------------------------+-----------+ | EXPERIMENTAL | 22 | Unknown | RFC XXXX | +----------------+-------+---------------------------+-----------+ | EXPERIMENTAL | 33 | Unknown | RFC XXXX | +----------------+-------+---------------------------+-----------+ Table 1: COSE Verifiable Data Structures When designing new verifiable data structures, please request the next available positive integer as your requested assignment, for example: Steele, et al. Expires 20 April 2025 [Page 5] Internet-Draft COSE Receipts October 2024 +================+================+===============+===============+ | Name | Value | Description | Reference | +================+================+===============+===============+ | N/A | 0 | N/A | N/A | +----------------+----------------+---------------+---------------+ | RFC9162_SHA256 | 1 | SHA256 Binary | [RFC9162] | | | | Merkle Tree | | +----------------+----------------+---------------+---------------+ | Your name | TBD (requested | tbd | Your | | | assignment 2) | | specification | +----------------+----------------+---------------+---------------+ Table 2: How to register new structures 4.2. Parameters 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 : 1) supports both (-1) inclusion and (-2) consistency proofs. This document establishes a registry of verifiable data structure algorithms, with the following initial contents: Steele, et al. Expires 20 April 2025 [Page 6] Internet-Draft COSE Receipts October 2024 +============+=============+=====+=======+=============+===========+ | Verifiable | Name |Label| CBOR | Description | Reference | | Data | | | Type | | | | Structure | | | | | | +============+=============+=====+=======+=============+===========+ | 1 | inclusion |-1 | array | Proof of | Section | | | proofs | | (of | inclusion | 5.2 | | | | | bstr) | | | +------------+-------------+-----+-------+-------------+-----------+ | 1 | consistency |-2 | array | Proof of | Section | | | proofs | | (of | append only | 5.3 | | | | | bstr) | property | | +------------+-------------+-----+-------+-------------+-----------+ | 11 | unknown |-1 | array | Unknown | RFC XXXX | | | | | (of | | | | | | | bstr) | | | +------------+-------------+-----+-------+-------------+-----------+ | 22 | unknown |-1 | array | Unknown | RFC XXXX | | | | | (of | | | | | | | bstr) | | | +------------+-------------+-----+-------+-------------+-----------+ | 33 | unknown |-1 | array | Unknown | RFC XXXX | | | | | (of | | | | | | | bstr) | | | +------------+-------------+-----+-------+-------------+-----------+ Table 3: COSE Verifiable Data Structure Parameters 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. When designing new verifiable data structure parameters (or proof types), please start with -1, and count down for each proof type supported by your verifiable data structure: Steele, et al. Expires 20 April 2025 [Page 7] Internet-Draft COSE Receipts October 2024 +==========+===========+=====+=====+===========+==================+ |Verifiable|Name |Label|CBOR |Description|Reference | |Data | | |Type | | | |Structure | | | | | | +==========+===========+=====+=====+===========+==================+ |1 |inclusion |-1 |array|Proof of |Section 5.2 | | |proofs | |(of |inclusion | | | | | |bstr)| | | +----------+-----------+-----+-----+-----------+------------------+ |1 |consistency|-2 |array|Proof of |Section 5.3 | | |proofs | |(of |append only| | | | | |bstr)|property | | +----------+-----------+-----+-----+-----------+------------------+ |TBD |new proof |-1 |tbd |tbd |Your_Specification| |(requested|type | | | | | |assignment| | | | | | |2) | | | | | | +----------+-----------+-----+-----+-----------+------------------+ |TBD |new proof |-2 |tbd |tbd |Your_Specification| |(requested|type | | | | | |assignment| | | | | | |2) | | | | | | +----------+-----------+-----+-----+-----------+------------------+ |TBD |new proof |-3 |tbd |tbd |Your_Specification| |(requested|type | | | | | |assignment| | | | | | |2) | | | | | | +----------+-----------+-----+-----+-----------+------------------+ Table 4: How to register new parameters 4.3. 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 parameter is present, the associated verifiable data structure and verifiable data structure proofs MUST match entries present in the registries established in RFC XXXX. The following informative CDDL is provided: Steele, et al. Expires 20 April 2025 [Page 8] Internet-Draft COSE Receipts October 2024 Receipt = #6.18(COSE_Sign1) Protected_Header = { * cose-label => 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 ] Figure 1: CDDL for a COSE Sign1 with attached receipts The following informative EDN is provided: / cose-sign1 / 18([ / protected / <<{ / key / 4 : "vCl7UcS0ZZY99VpRthDc-0iUjLdfLtnmFqLJ2-Tt8N4", / algorithm / 1 : -7, # ES256 }>>, / unprotected / { / receipts / 394 : { <>, / unprotected / { / proofs / 396 : { / inclusion / -1 : [ <<[ / size / 9, / leaf / 8, / inclusion path / h'7558a95f...e02e35d6' ]>> ], }, }, / payload / null, / signature / h'02d227ed...ccd3774f' Steele, et al. Expires 20 April 2025 [Page 9] Internet-Draft COSE Receipts October 2024 ])>>, <>, / 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: A COSE Signature with multiple receipts 4.3.1. Registration Requirements Each specification MUST define how to encode the verifiable data structure and its parameters (also called 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, an IANA registration must be made for each individually supported algorithm. For example, to provide for both SHA256 and SHA3_256 with [RFC9162], both "RFC9162_SHA256" and "RFC9162_SHA3_256" require entries in the relevant IANA registries. 5. RFC9162_SHA256 This section defines how the data structures described in [RFC9162] are mapped to the terminology defined in this document, using CBOR and COSE. Steele, et al. Expires 20 April 2025 [Page 10] Internet-Draft COSE Receipts October 2024 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". See Table 1. See [RFC9162], 2.1.1. Definition of the Merkle Tree, for a complete description of this verifiable data structure. 5.2. Inclusion Proof See [RFC9162], 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: 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 [RFC9162] Note that [RFC9162] 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. 5.2.1. 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 [I-D.ietf-cose-typ-header-parameter] and the similar guidance Steele, et al. Expires 20 April 2025 [Page 11] Internet-Draft COSE Receipts October 2024 provided in [I-D.ietf-cose-cwt-claims-in-headers]. 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: 395): REQUIRED. verifiable data structure algorithm identifier. Value type: int. The unprotected header for an RFC9162_SHA256 inclusion proof signature is: 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: 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]. 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: Steele, et al. Expires 20 April 2025 [Page 12] Internet-Draft COSE Receipts October 2024 / 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 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. 5.3. Consistency Proof See [RFC9162], 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: Steele, et al. Expires 20 April 2025 [Page 13] Internet-Draft COSE Receipts October 2024 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: 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): REQUIRED. Verifiable data structure algorithm identifier. Value type: int. The unprotected header for an RFC9162_SHA256 consistency proof signature is: Steele, et al. Expires 20 April 2025 [Page 14] Internet-Draft COSE Receipts October 2024 consistency-proofs = [ + consistency-proof ] verifiable-proofs = { &(consistency-proof: -2) => 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 [RFC9162]. 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: Steele, et al. Expires 20 April 2025 [Page 15] Internet-Draft COSE Receipts October 2024 / 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 See the privacy considerations section of: * [RFC9162] Steele, et al. Expires 20 April 2025 [Page 16] Internet-Draft COSE Receipts October 2024 * [RFC9053] 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. A privacy analysis MUST be performed 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 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. 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. Steele, et al. Expires 20 April 2025 [Page 17] Internet-Draft COSE Receipts October 2024 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. 8. Acknowledgements We would like to thank Maik Riechert, Jon Geater, Mike Jones, Mike Prorock, Ilari Liusvaara, for their contributions (some of which substantial) to this draft and to the initial set of implementations. 9. IANA Considerations 9.1. Additions to Existing Registries 9.1.1. New Entries to the COSE Header Parameters Registry This document requests IANA to add new values to the 'COSE Header Parameters' registries in the 'Integer values between 1 and 255' range with 'Specification Required' Registration Procedure. 9.1.1.1. COSE Header Parameters 9.1.1.1.1. Receipts * Name: receipts * Label: TBD_0 (requested assignment 394) * Value type: array (of bstr) * Value registry: https://www.iana.org/assignments/cose/ cose.xhtml#header-parameters * Description: Priority ordered list of CBOR encoded Receipts. * Reference: RFC XXXX 9.1.1.1.2. Verifiable Data Structure * Name: vds * Label: TBD_1 (requested assignment 395) * Value type: int Steele, et al. Expires 20 April 2025 [Page 18] Internet-Draft COSE Receipts October 2024 * Value registry: https://www.iana.org/assignments/cose/ cose.xhtml#header-parameters * Description: Algorithm name for verifiable data structure, used to produce verifiable data structure proofs. * Reference: RFC XXXX 9.1.1.1.3. Verifiable Data Structure Proofs * Name: vdp (requested assignment 396) * Label: TBD_2 * Value type: map * Value registry: https://www.iana.org/assignments/cose/ cose.xhtml#header-parameters * Description: Location for verifiable data structure proofs in COSE Header Parameters. * Reference: RFC XXXX 9.1.2. COSE Verifiable Data Structures IANA will be asked to establish a registry of verifiable data structure identifiers, named "COSE Verifiable Data Structures" to be administered under a Specification Required policy [RFC8126]. 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 Table 1 9.1.2.1. Expert Review This IANA registries is established under a Specification Required policy. Steele, et al. Expires 20 April 2025 [Page 19] Internet-Draft COSE Receipts October 2024 This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude. Expert reviewers should take into consideration the following points: * 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]. Provisional assignments to expired drafts MUST be removed from the registry. * Points assigned in this registry MUST have references that match the COSE Verifiable Data Structure Parameters registry. It is not permissible to assign points in this registry, for which no Verifiable Data Structure Parameters entries exist. 9.1.3. COSE Verifiable Data Structure Parameters IANA will be asked to establish a registry of verifiable data structure parameters, named "COSE Verifiable Data Structure Parameters" to be administered under a Specification Required policy [RFC8126]. 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 Table 3 Steele, et al. Expires 20 April 2025 [Page 20] Internet-Draft COSE Receipts October 2024 9.1.3.1. Expert Review This IANA registries is established under a Specification Required policy. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude. Expert reviewers should take into consideration the following points: * 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]. Provisional assignments to expired drafts MUST be removed from the registry. * Points assigned in this registry MUST have references that match the COSE Verifiable Data Structures registry. It is not permissible to assign points in this registry, for which no Verifiable Data Structure entry exists. 10. References 10.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, . Steele, et al. Expires 20 April 2025 [Page 21] Internet-Draft COSE Receipts October 2024 [RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, December 2021, . 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, . [I-D.draft-ietf-cbor-edn-literals] Bormann, C., "CBOR Extended Diagnostic Notation (EDN)", Work in Progress, Internet-Draft, draft-ietf-cbor-edn- literals-12, 1 September 2024, . [I-D.ietf-cose-cwt-claims-in-headers] Looker, T. and M. B. Jones, "CBOR Web Token (CWT) Claims in COSE Headers", Work in Progress, Internet-Draft, draft- ietf-cose-cwt-claims-in-headers-10, 29 November 2023, . [I-D.ietf-cose-typ-header-parameter] Jones, M. B. and O. Steele, "COSE "typ" (type) Header Parameter", Work in Progress, Internet-Draft, draft-ietf- cose-typ-header-parameter-05, 3 April 2024, . [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2014, . [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, . [RFC8392] Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, May 2018, . Steele, et al. Expires 20 April 2025 [Page 22] Internet-Draft COSE Receipts October 2024 [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, . [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, . [RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, . 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) Steele, et al. Expires 20 April 2025 [Page 23] Internet-Draft COSE Receipts October 2024 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) Authors' Addresses Orie Steele Transmute United States Email: orie@transmute.industries Henk Birkholz Fraunhofer SIT Rheinstrasse 75 64295 Darmstadt Germany Email: henk.birkholz@ietf.contact Antoine Delignat-Lavaud Microsoft United Kingdom Email: antdl@microsoft.com Cedric Fournet Microsoft United Kingdom Email: fournet@microsoft.com Steele, et al. Expires 20 April 2025 [Page 24]