Concise Encoding of Signed Merkle Tree Proofs
draftietfcosemerkletreeproofs01
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Authors  Orie Steele , Henk Birkholz , Antoine DelignatLavaud , Cedric Fournet  
Last updated  20231020 (Latest revision 20230817)  
Replaces  draftsteelecosemerkletreeproofs  
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draftietfcosemerkletreeproofs01
COSE O. Steele InternetDraft Transmute Intended status: Standards Track H. Birkholz Expires: 22 April 2024 Fraunhofer SIT A. DelignatLavaud C. Fournet Microsoft 20 October 2023 Concise Encoding of Signed Merkle Tree Proofs draftietfcosemerkletreeproofs01 Abstract This specification describes verifiable data structures and associated proof types for use with 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. Source for this draft and an issue tracker can be found at https://github.com/cosewg/draftietfcosemerkletreeproofs. Status of This Memo This InternetDraft is submitted in full conformance with the provisions of BCP 78 and BCP 79. InternetDrafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as InternetDrafts. The list of current Internet Drafts is at https://datatracker.ietf.org/drafts/current/. InternetDrafts 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 InternetDrafts as reference material or to cite them other than as "work in progress." This InternetDraft will expire on 22 April 2024. Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. Steele, et al. Expires 22 April 2024 [Page 1] InternetDraft CoMETRE October 2023 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ licenseinfo) 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Verifiable Data Structures in CBOR . . . . . . . . . . . . . 4 3.1. Algorithms Registry . . . . . . . . . . . . . . . . . . . 4 3.1.1. Registration Requirements . . . . . . . . . . . . . . 4 4. Proof Types in CBOR . . . . . . . . . . . . . . . . . . . . . 5 4.1. Proof Types Registry . . . . . . . . . . . . . . . . . . 5 4.2. Inclusion Proof . . . . . . . . . . . . . . . . . . . . . 5 4.3. Consistency Proof . . . . . . . . . . . . . . . . . . . . 6 5. RFC9162_SHA256 as a Verifiable Data Structure . . . . . . . . 6 5.1. Algorithm Definition . . . . . . . . . . . . . . . . . . 6 5.2. Inclusion Proof Definition . . . . . . . . . . . . . . . 6 5.2.1. Inclusion Proof Signature . . . . . . . . . . . . . . 7 5.3. Consistency Proof Definition . . . . . . . . . . . . . . 8 5.3.1. Consistency Proof Signature . . . . . . . . . . . . . 9 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8.1. Additions to Existing Registries . . . . . . . . . . . . 11 8.1.1. New Entries to the COSE Header Parameters Registry . 11 8.1.2. Verifiable Data Structures . . . . . . . . . . . . . 11 8.1.3. Verifiable Data Structure Proof Types . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 13 Appendix A. Implementation Status . . . . . . . . . . . . . . . 14 A.1. Implementer . . . . . . . . . . . . . . . . . . . . . . . 14 A.2. Implementation Name . . . . . . . . . . . . . . . . . . . 14 A.3. Implementation URL . . . . . . . . . . . . . . . . . . . 14 A.4. Maturity . . . . . . . . . . . . . . . . . . . . . . . . 14 A.5. Coverage and Version Compatibility . . . . . . . . . . . 15 A.6. License . . . . . . . . . . . . . . . . . . . . . . . . . 15 A.7. Implementation Dependencies . . . . . . . . . . . . . . . 15 A.8. Contact . . . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 Steele, et al. Expires 22 April 2024 [Page 2] InternetDraft CoMETRE October 2023 1. Introduction Merkle trees are one of many verifiable data structures that enable tamper evident secure information storage, through their ability to protect the integrity of batches of documents or collections of statements. Merkle trees can be constructed from simple operations such as concatenation and digest via a cryptographic hash function, however, more advanced constructions enable proofs of different properties of the underlying verifiable data structure. Verifiable data structure proofs can be used to prove 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). Differences in the representation of verifiable data structures, and verifiable data structure proof types, can increase the burden for implementers, and create interoperability challenges for transparency services. This document describes how to convey verifiable data structures, and associated proof types in 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. Terminology Verifiable Data Structure: A data structure which supports one or more Proof Types. Proof Type: A verifiable process, that proves properties of one or more Verifiable Data Structures. Proof Value: An encoding of a Proof Type in CBOR. Proof Signature: A COSE Sign1 encoding of a specific Proof Type for a specific Verifiable Data Structure. Steele, et al. Expires 22 April 2024 [Page 3] InternetDraft CoMETRE October 2023 3. Verifiable Data Structures in CBOR This section describes representations of verifiable data structure proofs structures in CBOR. Different verifiable data structures support the same proof types, but the representations of the proofs varies greatly. 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. Some differences in representations are necessary to support efficient verification of different kinds of proofs and for compatibility with specific implementations. Some proof types benefit from standard envelope formats for signing and encryption, whilst others require no further cryptographic intervention at all. In order to improve interoperability we define two extension points for enabling verifiable data structures with COSE, and we provide concrete examples for the structures and proofs defined in [RFC9162]. 3.1. Algorithms Registry This document establishes a registry of verifiable data structure algorithms, with the following initial contents: +============+================+===========+  Identifier  Algorithm  Reference  +============+================+===========+  0  N/A   ++++  1  RFC9162_SHA256  [RFC9162]  ++++ Table 1: Verifiable Data Structure Alogrithms 3.1.1. Registration Requirements Each specification MUST define how to encode the algorithm and proof types in CBOR. Each specification MUST define how to produce and consume the supported proof types. Steele, et al. Expires 22 April 2024 [Page 4] InternetDraft CoMETRE October 2023 See Section 5 as an example. 4. Proof Types in CBOR 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 accross "verifiable data structures", but they should expect conceptually similar properties across registered proof types. For example, 2 different merkle tree based verifiable data structures might both support proofs of inclusion. Protocols requiring proof of inclusion ought to be able to preserve their functionality, while switching from one verifiable data structure to another, so long as both structures support the same proof types. 4.1. Proof Types Registry This document establishes a registry of verifiable data structure proof types tags, with the following initial contents: +============+=============+===============+=============+  Identifier  Proof Type  Proof Value  Reference  +============+=============+===============+=============+  0  N/A  N/A  N/A  +++++  1  inclusion  array of bstr  Section 4.2  +++++  2  consistency  array of bstr  Section 4.3  +++++ Table 2: Verifiable Data Structure Proof Types 4.2. Inclusion Proof Inclusion proofs provide a mechanism for a verifier to validate set membership. The integer identifier for this Proof Type is 1. The string identifier for this Proof Type is "inclusion". The value of this Proof Type is array of bstr. Section 5.2 provides a concrete example. Steele, et al. Expires 22 April 2024 [Page 5] InternetDraft CoMETRE October 2023 4.3. Consistency Proof Consistency proofs provide a mechanism for a verifier to validate the consistency of a verifiable data structure. The integer identifier for this Proof Type is 2. The string identifier for this Proof Type is "consistency". The value of this Proof Type is array of bstr. Section 5.3 provides a concrete example. 5. RFC9162_SHA256 as a Verifiable Data Structure This section defines how the data structures described in [RFC9162] are mapped to the terminology defined in this document, using cbor and cose. RFC9162_SHA256 requires the following: * 11111 (verifiabledatastructure): 1, the integer representing the RFC9162_SHA256 verifiable data structure algorithm. * 22222 (verifiabledataproofs): a map supporting the following proof types: * 1 (inclusionproof): an array of bstr representing RFC9162_SHA256 inclusion proofs * 2 (consistencyproof): an array of bstr representing RFC9162_SHA256 consistency proofs 5.1. Algorithm Definition The integer identifier for this Verifiable Data Structure is 1. The string identifier for this Verifiable Data Structure is "RFC9162_SHA256". See Section 3.1. See [RFC9162], 2.1.1. Definition of the Merkle Tree, for a complete description of this verifiable data structure. 5.2. Inclusion Proof Definition 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: Steele, et al. Expires 22 April 2024 [Page 6] InternetDraft CoMETRE October 2023 inclusionproof = [ treesize: int leafindex: int inclusionpath: [+ bstr] ] 5.2.1. Inclusion Proof Signature In a signed inclusion proof, the previous merkle tree root, maps to treesize1, and is a detached payload. Other specifications refer to signed inclusion proofs as "receipts", profiles of proof signatures are encouraged to make additional protected header parameters mandatory. TODO: reference to scitt receipts. The protected header for an RFC9162_SHA256 inclusion proof signature is: * alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int / tstr. * verifiabledatastructure (label: 11111): REQUIRED. verifiable data structure algorithm identifier. Value type: int / tstr. The unprotected header for an RFC9162_SHA256 inclusion proof signature is: inclusionproofs = [ + bstr ] verifiableproofs = { &(inclusionproof: 1) => inclusionproofs } unprotectedheadermap = { &(verifiabledataproof: 22222) => verifiableproofs * coselabel => cosevalue } * inclusionproof (label: 1): REQUIRED. proof type identifier. Value type: [ + bstr ]. The payload of an RFC9162_SHA256 inclusion proof signature is the previous Merkle tree hash as defined in [RFC9162]. The payload MUST be detached. Steele, et al. Expires 22 April 2024 [Page 7] InternetDraft CoMETRE October 2023 Detaching the payload forces verifiers to recompute the root from the inclusion proof signature, this protects against implementation errors where the signature is verified but the root does not match the inclusion proof. 18( / COSE Single Signer Data Object / [ h'a3012604...392b6601', / Protected header / { / Unprotected header / 22222: { / Proofs / 1: [ / Inclusion proofs (1) / h'83040282...1f487bb1', / Inclusion proof 1 / ] }, }, h'', / Detached payload / h'1c0f970e...bf4bae7f' / Signature / ] ) { / Protected header / 1: 7, / Cryptographic algorithm to use / 4: h'68747470...6d706c65', / Key identifier / 11111: 1 / Verifiable data structure / } [ / Inclusion proof 1 / 4, / Tree size / 2, / Leaf index / [ / Inclusion hashes (2) / h'a39655d4...d29a968a' / Intermediate hash 1 / h'57187dff...1f487bb1' / Intermediate hash 2 / ] ] 5.3. Consistency Proof Definition 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: consistencyproof = [ treesize1: int ; size of the tree, when the previous root was produced. treesize2: int ; size of the tree, when the latest root was produced. consistencypath: [+ bstr] ; consistency path, from previous root to latest root. ] Steele, et al. Expires 22 April 2024 [Page 8] InternetDraft CoMETRE October 2023 Editors note: treesize1, could be ommited, if an inclusionproof is always present, since the inclusion proof contains, treesize1. 5.3.1. Consistency Proof Signature In a signed consistency proof, the latest merkle tree root, maps to treesize2, and is an attached payload. The protected header for an RFC9162_SHA256 consistency proof signature is: * alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int / tstr. * verifiabledatastructure (label: TBD_1): REQUIRED. verifiable data structure algorithm identifier. Value type: int / tstr. * kid (label: 4): OPTIONAL. Key identifier. Value type: bstr * crit (label: 2): OPTIONAL. Criticality marker. Value type: [ + label ] The unprotected header for an RFC9162_SHA256 consistency proof signature is: consistencyproofs = [ + bstr ] verifiableproofs = { &(consistencyproof: 2) => consistencyproofs } unprotectedheadermap = { &(verifiabledataproof: 22222) => verifiableproofs * coselabel => cosevalue } * consistencyproof (label: 2): REQUIRED. proof type identifier. Value type: [ + bstr ]. The payload of an RFC9162_SHA256 consistency proof signature is: The latest Merkle tree hash as defined in [RFC9162]. The payload MUST be attached. Steele, et al. Expires 22 April 2024 [Page 9] InternetDraft CoMETRE October 2023 18( / COSE Single Signer Data Object / [ h'a3012604...392b6601', / Protected header / { / Unprotected header / 22222: { / Proofs / 2: [ / Consistency proofs (1) / h'83040682...2e73a8ab', / Consistency proof 1 / ] }, }, h'430b6fd7...f74c7fc4', / Payload / h'8bcb1b79...78829bca' / Signature / ] ) { / Protected header / 1: 7, / Cryptographic algorithm to use / 4: h'68747470...6d706c65', / Key identifier / 11111: 1 / Verifiable data structure / } [ / Consistency proof 1 / 4, / Tree size 1 / 6, / Tree size 2 / [ / Consistency hashes (2) / h'0bdaaed3...32568964' / Intermediate hash 1 / h'75f177fd...2e73a8ab' / Intermediate hash 2 / ] ] 6. Privacy Considerations See the privacy considerations section of: * [RFC9162] * [RFC9053] 7. Security Considerations See the security considerations section of: * [RFC9162] * [RFC9053] 8. IANA Considerations Steele, et al. Expires 22 April 2024 [Page 10] InternetDraft CoMETRE October 2023 8.1. Additions to Existing Registries 8.1.1. New Entries to the COSE Header Parameters Registry This document requests IANA to add new values to the 'COSE Algorithms' and to the 'COSE Header Algorithm Parameters' registries in the 'Standards Action With Expert Review category. 8.1.1.1. COSE Header Algorithm Parameters * Name: verifiabledatastructure * Label: 11111 * Value type: int / tstr * Value registry: https://www.iana.org/assignments/cose/ cose.xhtml#headerparameters * Description: Algorithm name for verifiable data structure, used to produce verifiable data proofs. * Name: verifiabledataproof * Label: 222222 * Value type: int / tstr * Value registry: https://www.iana.org/assignments/cose/ cose.xhtml#headerparameters * Description: Location for verifiable data proofs in COSE Header Parameters. 8.1.2. Verifiable Data Structures IANA will be asked to establish a registry of tree algorithm identifiers, named "Verifiable Data Structures" to be administered under a Specification Required policy [RFC8126]. Template: * Identifier: The twobyte identifier for the algorithm * Algorithm: The name of the data structure * Reference: Where the data structure is defined Steele, et al. Expires 22 April 2024 [Page 11] InternetDraft CoMETRE October 2023 Initial contents: Provided in Table 1 8.1.3. Verifiable Data Structure Proof Types IANA will be asked to establish a registry of tree algorithm identifiers, named "Verifiable Data Structures Proof Types" to be administered under a Specification Required policy [RFC8126]. Template: * Identifier: The twobyte identifier for the algorithm * Algorithm: The name of the proof type algorithm * Reference: Where the algorithm is defined Initial contents: Provided in Table 2 9. References 9.1. Normative 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.rfceditor.org/rfc/rfc7942>. [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.rfceditor.org/rfc/rfc2119>. [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHAbased HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, <https://www.rfceditor.org/rfc/rfc6234>. [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, <https://www.rfceditor.org/rfc/rfc6962>. [RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August 2013, <https://www.rfceditor.org/rfc/rfc6979>. Steele, et al. Expires 22 April 2024 [Page 12] InternetDraft CoMETRE October 2023 [RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013, <https://www.rfceditor.org/rfc/rfc7049>. [RFC8032] Josefsson, S. and I. Liusvaara, "EdwardsCurve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, <https://www.rfceditor.org/rfc/rfc8032>. [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.rfceditor.org/rfc/rfc8126>. [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.rfceditor.org/rfc/rfc8174>. [RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", STD 94, RFC 8949, DOI 10.17487/RFC8949, December 2020, <https://www.rfceditor.org/rfc/rfc8949>. [RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, August 2022, <https://www.rfceditor.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.rfceditor.org/rfc/rfc9162>. 9.2. Informative References [ID.ietfcosecountersign] Schaad, J., "CBOR Object Signing and Encryption (COSE): Countersignatures", Work in Progress, InternetDraft, draftietfcosecountersign10, 20 September 2022, <https://datatracker.ietf.org/doc/html/draftietfcose countersign10>. [ID.ietfscittarchitecture] Birkholz, H., DelignatLavaud, A., Fournet, C., Deshpande, Y., and S. Lasker, "An Architecture for Trustworthy and Transparent Digital Supply Chains", Work in Progress, InternetDraft, draftietfscittarchitecture03, 16 October 2023, <https://datatracker.ietf.org/doc/html/ draftietfscittarchitecture03>. Steele, et al. Expires 22 April 2024 [Page 13] InternetDraft CoMETRE October 2023 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 InternetDraft, 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. Implementer An opensource implementation was initiated and is maintained by the Transmute Industries Inc.  Transmute. A.2. Implementation Name An application demonstrating the concepts is available at https://scitt.xyz (https://scitt.xyz). A.3. Implementation URL An opensource implementation is available at: * https://github.com/transmuteindustries/cose A.4. Maturity The code's level of maturity is considered to be "prototype". Steele, et al. Expires 22 April 2024 [Page 14] InternetDraft CoMETRE October 2023 A.5. Coverage and Version Compatibility The current version ('main') implements the tree algorithm, inclusion proof and consistency proof concepts of this draft. A.6. License The project and all corresponding code and data maintained on GitHub are provided under the Apache License, version 2. A.7. Implementation Dependencies The implementation builds on concepts described in SCITT [ID.ietfscittarchitecture] (https://scitt.io/). The implementation uses the Concise Binary Object Representation [RFC7049] (https://cbor.io/). The implementation uses the CBOR Object Signing and Encryption [RFC9053], maintained at:  https://github.com/erdtman/cosejs The implementation uses an implementation of [RFC9162], maintained at: * https://github.com/transmuteindustries/rfc9162/tree/main/src/ CoMETRE A.8. 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@sit.fraunhofer.de Steele, et al. Expires 22 April 2024 [Page 15] InternetDraft CoMETRE October 2023 Antoine DelignatLavaud Microsoft United Kingdom Email: antdl@microsoft.com Cedric Fournet Microsoft United Kingdom Email: fournet@microsoft.com Steele, et al. Expires 22 April 2024 [Page 16]