Internet-Draft | CoMTRE CFF Ledger | January 2024 |

Birkholz, et al. | Expires 15 July 2024 | [Page] |

- Workgroup:
- TBD
- Internet-Draft:
- draft-birkholz-cose-cometre-ccf-profile-01
- Published:
- Intended Status:
- Standards Track
- Expires:

# A Transaction Ledger Verifiable Structure for COSE Merkle Tree Proofs

## Abstract

This document defines a new verifiable data structure type for COSE Signed Merkle Tree Proofs specifically designed for transaction ledgers produced by Trusted Execution Environments (TEEs) to provide stronger tamper-evidence guarantees.¶

## 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/.¶

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This Internet-Draft will expire on 15 July 2024.¶

## 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.¶

## 1. Introduction

The Concise Encoding of Signed Merkle Tree Proofs (CoMeTre) [I-D.ietf-cose-merkle-tree-proofs] defines a common framework for defining different types of proofs about verifiable data structures (also abbreviated as "logs" in this document). For instance, inclusion proofs guarantee to a verifier that a given serializable element is recorded at a given state of the log, while consistency proofs are used to establish that an inclusion proof is still consistent with the new state of the log at a later time.¶

In this document, we define a new type of log, associated with the Confidential Consortium Framework (CCF) ledger. This log carries indexed transaction information in a binary Merkle Tree, where new transactions are appended to the right, so that the binary decomposition of the index of a transaction can be interpreted as the position in the tree if 0 represents the left branch and 1 the right branch. Compared to [RFC9162], the leaves of CCF trees carry additional opaque information that is used to verify that elements are only written by the Trusted Execution Environment, which addresses the persistence of committed transactions that happen between new signatures of the Merkle Tree root.¶

### 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. Format of CCF Ledger

This documents extends the verifiable data structure registry of [I-D.ietf-cose-merkle-tree-proofs] with the following value:¶

Name | Value | Description | Reference |
---|---|---|---|

CCF_LEDGER_SHA256 | TBD_1 (requested assignment 2) | Historical transaction ledgers, such as the CCF ledger | This document |

This document defines inclusion proofs and consistency proof formats for CCF ledgers. Verifiers MUST reject all other proof types.¶

### 2.1. Merkle Tree Shape

A CCF ledger is a binary Merkle Tree constructed from a hash function H, which is defined from the log type. For instance, the hash function for `CCF_LEDGER_SHA256`

is `SHA256`

, whose `HASH_SIZE`

is 32 bytes.
The Merkle tree encodes an ordered list of `n`

transactions T_n = {T[0], T[1], ..., T[n-1]}. We define the Merkle Tree Hash (MTH) function, which takes as input a list of serialized transactions (as byte strings), and outputs a single HASH_SIZE byte string called the Merkle root hash, by induction on the list:¶

The hash of an empty list is the hash of an empty string:¶

MTH({}) = HASH().¶

The hash of a list with one entry (also known as a leaf hash) is:¶

MTH({d[0]}) = HASH(d[0]).¶

For n > 1, let k be the largest power of two smaller than n (i.e., k < n <= 2k). The Merkle Tree Hash of an n-element list D_n is then defined recursively as:¶

MTH(D_n) = HASH(MTH(D[0:k]) || MTH(D[k:n])),¶

where:¶

### 2.2. Transaction Components

Each leaf transaction in a CCF ledger carries the following components:¶

CCF-leaf = [ internal-hash: bstr ; a string of HASH_SIZE bytes; internal-data: bstr; a string of at most 1024 bytes; and data_hash: bstr ; the serialization of the element stored at this leaf. ]¶

The `internal_hash`

and `internal_data`

byte strings are internal to the CCF implementation. Similarly, the auxiliary tree entries are internal to CCF. They are opaque to receipt Verifiers, but they commit the TS to the whole tree contents and may be used for additional, CCF-specific auditing.¶

## 3. CCF Inclusion Proofs

CCF inclusion proofs consist of a list of digests tagged with a single left-or-right bit.¶

CCF-inclusion-proof: [ leaf: CCF-leaf ; path: [+ ccf-proof-element] ; ] ccf-proof-element = [ left: bool hash: bstr ]¶

Unlike some other tree algorithms, the index of the element in the tree is not explicit in the inclusion proof, but the list of left-or-right bits can be treated as the binary decomposition of the index, from the least significant (leaf) to the most significant (root).¶

### 3.1. Inclusion Proof Verification Algorithm

CCF uses the following algorithm to validate an inclusion receipt:¶

compute_root(proof): let h = proof.leaf.internal-hash || HASH(proof.leaf.internal-data) || proof.leaf.data-hash for [left, hash] in proof.path: h := HASH(hash + h) if left HASH(h + hash) else return h verify_inclusion_receipt(inclusion_receipt): let proofs = inclusion_receipt.unprotected_headers[-222] or fail let payload = nil assert(inclusion_receipt.payload == nil) for proof in proofs let root = compute_root(proof) if payload = nil then payload := root else assert(root == payload) # Use the Merkle Root as the detached payload return verif_cose(inclusion_receipt, payload)¶

## 4. Privacy Considerations

Privacy Considerations¶

## 5. Security Considerations

Security Considerations¶

## 7. References

### 7.1. Normative References

- [RFC2119]
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://doi.org/10.17487/RFC2119>.
- [RFC8174]
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://doi.org/10.17487/RFC8174>.
- [RFC9162]
- Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, , <https://doi.org/10.17487/RFC9162>.

### 7.2. Informative References

- [I-D.ietf-cose-merkle-tree-proofs]
- Steele, O., Birkholz, H., Delignat-Lavaud, A., and C. Fournet, "Concise Encoding of Signed Merkle Tree Proofs", Work in Progress, Internet-Draft, draft-ietf-cose-merkle-tree-proofs-03, , <https://datatracker.ietf.org/doc/html/draft-ietf-cose-merkle-tree-proofs-03>.

## Appendix A. Attic

Not ready to throw these texts into the trash bin yet.¶