COSE Receipts

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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. For conciseness, we refer to a COSE object securing a verifiable data structure and its associated proof types, as a COSE Receipt.¶

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.¶

-111 is TBD_1:

A requested cose header parameter representing the verifiable data structure used.¶

-222 is TBD_2:

A requested cose header parameter representing the verifiable data structure parameters map (proofs map).¶

The other codepoints are assigned from the registries established in this draft, they are therefore not marked TBD.¶

3. Terminology

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 messsage 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.¶

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 containing the header parameters necessary to convey a proof types for verifiable data structures.¶

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. 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]. The design of these structures is influenced by the conventions established for COSE Keys.¶

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.¶

inclusion-proof = bstr .cbor [

    ; tree size at current merkle root
    tree-size: int

    ; index of leaf in tree
    leaf-index: int

    ; path from leaf to current merkle root
    inclusion-path: [ + bstr ]
]

protected-header-map = {
  &(alg: 1) => int
  &(vds: -111) => int
  * cose-label => cose-value
}

inclusion-proofs = [ + inclusion-proof ]

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

unprotected-header-map = {
  &(vdp: -222) => verifiable-proofs
  * cose-label => cose-value
}

18(                                 / COSE Sign 1                   /
    [
      h'a4012604...6d706c65',       / Protected                     /
      {                             / Unprotected                   /
        -222: {                     / Proofs                        /
          -1: [                     / Inclusion proofs (1)          /
            h'83080783...32568964', / Inclusion proof 1             /
          ]
        },
      },
      nil,                          / Detached payload              /
      h'2e34df43...8d74d55e'        / Signature                     /
    ]
)

{                                   / Protected                     /
  1: -7,                            / Algorithm                     /
  4: h'4930714e...7163316b',        / Key identifier                /
  -111: 1,                          / Verifiable Data Structure     /
}

[                                   / Inclusion proof 1             /
  8,                                / Tree size                     /
  7,                                / Leaf index                    /
  [                                 / Inclusion hashes (3)          /
     h'2a8d7dfc...15d10b22'         / Intermediate hash 1           /
     h'75f177fd...2e73a8ab'         / Intermediate hash 2           /
     h'0bdaaed3...32568964'         / Intermediate hash 3           /
  ]
]

consistency-proof =  bstr .cbor [

    ; previous merkle root tree size
    tree-size-1: int

    ; latest merkle root tree size
    tree-size-2: int

    ; path from previous merkle root to latest merkle root.
    consistency-path: [ + bstr ]

]

protected-header-map = {
  &(alg: 1) => int
  &(vds: -111) => int
  * cose-label => cose-value
}

consistency-proofs = [ + consistency-proof ]

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

unprotected-header-map = {
  &(vdp: -222) => verifiable-proofs
  * cose-label => cose-value
}

18(                                 / COSE Sign 1                   /
    [
      h'a3012604...392b6601',       / Protected                     /
      {                             / Unprotected                   /
        -222: {                     / Proofs                        /
          -2: [                     / Consistency proofs (1)        /
            h'83040682...2e73a8ab', / Consistency proof 1           /
          ]
        },
      },
      h'430b6fd7...f74c7fc4',       / Payload (Attached)            /
      h'd97befea...f30631cb'        / Signature                     /
    ]
)

{                                   / Protected                     /
  1: -7,                            / Algorithm                     /
  4: h'68747470...6d706c65',        / Key identifier                /
  -111: 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. 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

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