TBD H. Birkholz Internet-Draft Fraunhofer SIT Intended status: Standards Track A. Delignat-Lavaud Expires: 15 July 2024 C. Fournet Microsoft Research 12 January 2024 A Transaction Ledger Verifiable Structure for COSE Merkle Tree Proofs draft-birkholz-cose-cometre-ccf-profile-01 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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Expires 15 July 2024 [Page 1] Internet-Draft CoMTRE CFF Ledger January 2024 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3 2. Format of CCF Ledger . . . . . . . . . . . . . . . . . . . . 3 2.1. Merkle Tree Shape . . . . . . . . . . . . . . . . . . . . 3 2.2. Transaction Components . . . . . . . . . . . . . . . . . 4 3. CCF Inclusion Proofs . . . . . . . . . . . . . . . . . . . . 4 3.1. Inclusion Proof Verification Algorithm . . . . . . . . . 5 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6.1. Additions to Existing Registries . . . . . . . . . . . . 5 6.1.1. Tree Algorithms . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 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. Birkholz, et al. Expires 15 July 2024 [Page 2] Internet-Draft CoMTRE CFF Ledger January 2024 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 | Historical | This | | | (requested | transaction | document | | | assignment 2) | ledgers, such as | | | | | the CCF ledger | | +-------------------+---------------+------------------+-----------+ Table 1: Verifiable Data Structure Algorithms 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]). Birkholz, et al. Expires 15 July 2024 [Page 3] Internet-Draft CoMTRE CFF Ledger January 2024 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: * || denotes concatenation * : denotes concatenation of lists * D[k1:k2] = D'_(k2-k1) denotes the list {d'[0] = d[k1], d'[1] = d[k1+1], ..., d'[k2-k1-1] = d[k2-1]} of length (k2 - k1). 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 ] Birkholz, et al. Expires 15 July 2024 [Page 4] Internet-Draft CoMTRE CFF Ledger January 2024 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 6. IANA Considerations 6.1. Additions to Existing Registries 6.1.1. Tree Algorithms This document requests IANA to add the following new value to the 'COSE Verifiable Data Structures' registry: Birkholz, et al. Expires 15 July 2024 [Page 5] Internet-Draft CoMTRE CFF Ledger January 2024 * Name: CCF_LEDGER_SHA256 * Value: TBD_1 (requested assignment 2) * Description: Historical transaction ledgers, such as the CCF ledger * Reference: This document 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, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, December 2021, . 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, 11 December 2023, . Appendix A. Attic Not ready to throw these texts into the trash bin yet. Authors' Addresses Henk Birkholz Fraunhofer SIT Rheinstrasse 75 64295 Darmstadt Germany Email: henk.birkholz@sit.fraunhofer.de Birkholz, et al. Expires 15 July 2024 [Page 6] Internet-Draft CoMTRE CFF Ledger January 2024 Antoine Delignat-Lavaud Microsoft Research 21 Station Road Cambridge CB1 2FB United Kingdom Email: antdl@microsoft.com Cedric Fournet Microsoft Research 21 Station Road Cambridge CB1 2FB United Kingdom Email: fournet@microsoft.com Birkholz, et al. Expires 15 July 2024 [Page 7]