Concise Encoding of Signed Merkle Tree Proofs

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Table of Contents

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.¶

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.¶

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].¶

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.¶

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 (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¶

inclusion-proof = [
    tree-size: int
    leaf-index: int
    inclusion-path: [+ bstr]
]

inclusion-proofs = [ + bstr ]

verifiable-proofs = {
  &(inclusion-proof: 1) => inclusion-proofs
}

unprotected-header-map = {
  &(verifiable-data-proof: -22222) => verifiable-proofs
  * cose-label => cose-value
}

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                   /
  ]
]

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.
]

consistency-proofs = [ + bstr ]

verifiable-proofs = {
  &(consistency-proof: 2) => consistency-proofs
}

unprotected-header-map = {
  &(verifiable-data-proof: -22222) => verifiable-proofs
  * cose-label => cose-value
}

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

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".¶

Authors' Addresses