Internet-Draft | CoMETRE | October 2023 |
Steele, et al. | Expires 22 April 2024 | [Page] |
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.¶
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/cose-wg/draft-ietf-cose-merkle-tree-proofs.¶
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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.¶
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.¶
A data structure which supports one or more Proof Types.¶
A verifiable process, that proves properties of one or more Verifiable Data Structures.¶
An encoding of a Proof Type in CBOR.¶
A COSE Sign1 encoding of a specific Proof Type for a specific Verifiable Data Structure.¶
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].¶
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] |
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.¶
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 |
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.¶
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.¶
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 (verifiable-data-structure): 1, the integer representing the RFC9162_SHA256 verifiable data structure algorithm.¶
-22222 (verifiable-data-proofs): a map supporting the following proof types:¶
1 (inclusion-proof): an array of bstr representing RFC9162_SHA256 inclusion proofs¶
2 (consistency-proof): an array of bstr representing RFC9162_SHA256 consistency proofs¶
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.¶
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 = [ tree-size: int leaf-index: int inclusion-path: [+ bstr] ]¶
In a signed inclusion proof, the previous merkle tree root, maps to tree-size-1, 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.¶
verifiable-data-structure (label: -11111): REQUIRED. verifiable data structure algorithm identifier. Value type: int / tstr.¶
The unprotected header for an RFC9162_SHA256 inclusion proof signature is:¶
inclusion-proofs = [ + bstr ] verifiable-proofs = { &(inclusion-proof: 1) => inclusion-proofs } unprotected-header-map = { &(verifiable-data-proof: -22222) => verifiable-proofs * cose-label => cose-value }¶
inclusion-proof (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.¶
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 / ] ]¶
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:¶
consistency-proof = [ tree-size-1: int ; size of the tree, when the previous root was produced. tree-size-2: int ; size of the tree, when the latest root was produced. consistency-path: [+ bstr] ; consistency path, from previous root to latest root. ]¶
Editors note: tree-size-1, could be ommited, if an inclusion-proof is always present, since the inclusion proof contains, tree-size-1.¶
In a signed consistency proof, the latest merkle tree root, maps to tree-size-2, 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.¶
verifiable-data-structure (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:¶
consistency-proofs = [ + bstr ] verifiable-proofs = { &(consistency-proof: 2) => consistency-proofs } unprotected-header-map = { &(verifiable-data-proof: -22222) => verifiable-proofs * cose-label => cose-value }¶
consistency-proof (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.¶
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 / ] ]¶
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.¶
Name: verifiable-data-structure¶
Label: -11111¶
Value type: int / tstr¶
Value registry: https://www.iana.org/assignments/cose/cose.xhtml#header-parameters¶
Description: Algorithm name for verifiable data structure, used to produce verifiable data proofs.¶
Name: verifiable-data-proof¶
Label: -222222¶
Value type: int / tstr¶
Value registry: https://www.iana.org/assignments/cose/cose.xhtml#header-parameters¶
Description: Location for verifiable data proofs in COSE Header Parameters.¶
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:¶
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:¶
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".¶
An open-source implementation was initiated and is maintained by the Transmute Industries Inc. - Transmute.¶
An application demonstrating the concepts is available at https://scitt.xyz.¶
An open-source implementation is available at:¶
https://github.com/transmute-industries/cose¶
The current version ('main') implements the tree algorithm, inclusion proof and consistency proof concepts of this draft.¶
The project and all corresponding code and data maintained on GitHub are provided under the Apache License, version 2.¶
The implementation builds on concepts described in SCITT [I-D.ietf-scitt-architecture] (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/cose-js¶
The implementation uses an implementation of [RFC9162], maintained at:¶
https://github.com/transmute-industries/rfc9162/tree/main/src/CoMETRE¶