Transmute

orie@transmute.industries

Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@sit.fraunhofer.de

Microsoft

UK Maik.Riechert@microsoft.com

Microsoft

UK antdl@microsoft.com

Microsoft

UK fournet@microsoft.com

Security TBD Internet-Draft This specification describes three CBOR data structures for primary use in COSE envelopes. A format for Merkle Tree Root Signatures with metadata, a format for Inclusions Paths, and a format for disclosure of a single hadh tree leaf payload (Merkle Tree Proofs).

Introduction Merkle proofs are verifiable data structures that support secure data storage, through their ability to protect the integrity of batches of documents or collections of statements. Merkle proofs can be used to prove a document is in a database (proof of existence), or that a smaller set of statements are contained in a large set of statements (proof of disclosure). A merkle proof is a path from a leaf to a root in a merkle tree. Merkle trees are constructed from simple operations such as concatenation and digest via a cryptographic hash function. The simple design and valuable cryptographic properties of merkle trees have been leveraged in many network and database applications. Differences in the representation of a merkle tree, merkle leaf and merkle inclusion proof can increase the burden for implementers, and create interoperability challenges. This document describes the three data structures necessary to use merkle proofs with COSE envelopes.

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 when, and only when, they appear in all capitals, as shown here.

Terminology

Leaf Bytes:

A merkle tree leaf is labelled with the cryptographic hash of a sequence of bytes. These bytes may be structured as a combination of Payload and Extra Data.

Merkle Tree:

A Merkle tree is a tree where every leaf is labelled with the cryptographic hash of a sequence of bytes and every node that is not a leaf is labeled with the cryptographic hash of the labels of its child nodes.

Merkle Tree Root:

A Merkle tree root is the root node of a tree which represents the cryptographic hash that commits to all leaves in the tree.

Merkle Tree Algorithm:

A Merkle tree algorithm specifies how nodes in the tree must be hashed to compute the root node.

Payload and Extra Data:

A payload is data bound to in a Merkle tree leaf. The Merkle tree algorithm determines how a payload together with extra data is bound to a leaf. The simplest case is that the payload is the leaf itself without extra data.

Inclusion Path:

An inclusion path confirms that a value is a leaf of a Merkle tree known only by its root hash (and tree size, possibly).

Signed Merkle Tree Proof:

A signed Merkle tree proof is the combination of signed Merkle tree root hash, inclusion path, extra data, and payload.

CBOR Merkle Structures This section describes representations of merkle tree structures in CBOR. Some of the structures such as the construction of a merkle tree leaf, or an inclusion proof from a leaf to a merkle root, might have several different representations. Some differences in representations are necessary to support efficient verification of proofs and compatibility with deployed tree algorithms used in specific implementations.

Signed Merkle Tree Root A Merkle tree root is signed with COSE_Sign1, creating a Signed Merkle Tree Root: SMTR = THIS.COSE.profile .and COSE_Sign1_Tagged Protected header parameters:

• alg (label: 1): REQUIRED. Signature algorithm. Value type: int / tstr.
• tree alg (label: TBD): REQUIRED. Merkle tree algorithm. Value type: int / tstr.
• tree size (label: TBD): OPTIONAL. Merkle tree size as the number of leaves. Value type: uint.

A COSE profile of this specification may add further header parameters, for example to identify the signer. Payload: Merkle tree root hash bytes according to tree alg (i.e., header params tell you what the alg id is here) Note: The payload is just a byte string representing the Merkle tree root hash (and not some wrapper structure) so that it can be detached (see defintion of payload in https://www.rfc-editor.org/rfc/rfc9052#section-4.1) and easily re-computed from an inclusion path and leaf bytes. This allows to design other structures that force re-computation and prevent faulty implementations (forgetting to match a computed root with one embedded in a signature). One example of a Signed Merkle Tree Proof is a "transparent signed statement" or "claim" as defined in .

Inclusion Paths defines a merkle audit path for a leaf in a merkle tree as the shortest list of additional nodes in the merkle tree required to compute the merkle root for that tree. changed the term from "merkle audit path" to "merkle inclusion proof". We prefer to use the term "inclusion path" to avoid confusion with Signed Merkle Tree Proof. If the tree size and leaf index is known, then a compact inclusion path variant can be used: IndexAwareInclusionPath = #6.1234([ leaf_index: int hashes: [+ bstr] ]) Otherwise, the direction for each path step must be included: FIXME bit vector: 0 right, 1 left, so no bit labels IndexUnawareInclusionPath = #6.1235([ hashes: [+ bstr] left: uint ; bit vector ]) For some tree algorithms, the direction is derived from the hashes themselves and both the index and direction can be left out in the path: ; TODO: find a better name for this UndirectionalInclusionPath = #6.1236([+ bstr]) InclusionPath = IndexAwareInclusionPath / IndexUnawareInclusionPath / UndirectionalInclusionPath Note: Including the tree size and leaf index may not be appropriate in certain privacy-focused applications as an attacker may be able to derive private information from them. TODO: Should leaf index be part of inclusion path (IndexAwareInclusionPath) or outside? TODO: Define root computation algorithm for each inclusion path type TODO: Do we need both inclusion path types? what properties does each type have? TODO: Should the inclusion path be opaque (bstr) and fixed by the tree algorithm? It seems this is orthogonal and the choice of inclusion path type should be application-specific.

Signed Merkle Tree Proof A signed Merkle tree proof is a CBOR array containing a signed tree root, an inclusion path, extra data for the tree algorithm, and the payload. SignedMerkleTreeProof = [ signed_tree_root: bstr .cbor SMTR ; payload of COSE_Sign1_Tagged is detached inclusion_path: bstr .cbor InclusionPath extra_data: bstr / nil payload: bstr ] extra_data is an additional input to the tree algorithm and is used together with the payload to compute the leaf hash. A use case for this field is to implement blinding. TODO: maybe rename extra_data

Signed Merkle Tree Multiproof TODO: define a multi-leaf variant of a signed Merkle tree proof like in:

• https://github.com/transmute-industries/merkle-proof
• https://transmute-industries.github.io/merkle-disclosure-proof-2021/

TODO: consider using sparse multiproofs, see https://medium.com/@jgm.orinoco/understanding-sparse-merkle-multiproofs-9b9f049e8f08 and https://arxiv.org/pdf/2002.07648.pdf

Merkle Tree Algorithms This document establishes a registry of Merkle tree algorithms with the following initial contents: [FIXME] exploration table, what should go into -00? Merke Tree Alogrithms

Name	Label	Description
Reserved	0
RFC9162_SHA256	1	RFC9162 with SHA-256

Each tree algorithm defines how to compute the root node from a sequence of leaves each represented by payload and extra data. Extra data is algorithm-specific and should be considered opaque.

RFC9162_SHA256 The RFC9162_SHA256 tree algorithm uses the Merkle tree definition from with SHA-256 hash algorithm. For n > 1 inputs, let k be the largest power of two smaller than n. where d(0) is the payload. This algorithm takes no extra data.

Privacy Considerations TBD

Security Considerations TBD

IANA Considerations

Additions to Existing Registries

New Entries to the COSE Header Parameters Registry IANA will be requested to register the new COSE Header parameters defined below in the "COSE Header Parameters" registry at some point.

New SCITT-Related Registries IANA will be asked to add a new registry "TBD" to the list that appears at https://www.iana.org/assignments/. The rest of this section defines the subregistries that are to be created within the new "TBD" registry.

Tree Algorithms IANA will be asked to establish a registry of tree algorithm identifiers, named "Tree Algorithms", with the following registration procedures: TBD The "Tree Algorithms" registry initially consists of: Initial content of Tree Algorithms registry

Identifier	Tree Algorithm	Reference
TBD	TBD tree algorithm	This document

The designated expert(s) should ensure that the proposed algorithm has a public specification and is suitable for use as [TBD].

References Normative References The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. This document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. Federal Information Processing Standard, FIPS This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. This document defines a deterministic digital signature generation procedure. Such signatures are compatible with standard Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures and can be processed with unmodified verifiers, which need not be aware of the procedure described therein. Deterministic signatures retain the cryptographic security features associated with digital signatures but can be more easily implemented in various environments, since they do not need access to a source of high-quality randomness. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References August Cellars Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. CBOR Object Signing and Encryption (COSE) defines a set of security services for CBOR. This document defines a countersignature algorithm along with the needed header parameters and CBOR tags for COSE. This document updates RFC 9052. Fraunhofer SIT Microsoft Research Microsoft Research ARM Traceability of physical and digital artifacts in supply chains is a long-standing, but increasingly serious security concern. The rise in popularity of verifiable data structures as a mechanism to make actors more accountable for breaching their compliance promises has found some successful applications to specific use cases (such as the supply chain for digital certificates), but lacks a generic and scalable architecture that can address a wider range of use cases. This memo defines a generic and scalable architecture to enable transparency across any supply chain with minimum adoption barriers for producers (who can register their Signed Statements on any Transparency Service, with the guarantee that all consumers will be able to verify them) and enough flexibility to allow different implementations of Transparency Services with various auditing and compliance requirements.