Transmute

United States orie@transmute.industries

Fraunhofer SIT

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

Microsoft

UK antdl@microsoft.com

Microsoft

UK fournet@microsoft.com

Security TBD Internet-Draft This specification describes verifiable data structures and associated proof types for use with COSE. The extensibility of the approach is demonstrated by providing CBOR encodings for RFC9162.

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.

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

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.

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 .

Algorithms Registry This document establishes a registry of verifiable data structure algorithms, with the following initial contents: Verifiable Data Structure Alogrithms

Identifier	Algorithm	Reference
0	N/A
1	RFC9162_SHA256

Registration Requirements Each specification MUST define how to encode the algorithm and proof types in CBOR. Each specification MUST define how to produce and consume the supported proof types. See as an example.

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.

Proof Types Registry This document establishes a registry of verifiable data structure proof types tags, with the following initial contents: Verifiable Data Structure Proof Types

Identifier	Proof Type	Reference
0	N/A
TBD_2	inclusion
TBD_3	consistency

Editors note: The registry requirements needs to address the case of multiple proofs of a given type.

Inclusion Proof Inclusion proofs provide a mechanism for a verifier to validate set membership. The integer identifier for this Proof Type is TBD_2. The string identifier for this Proof Type is "inclusion". provides a concrete example.

Consistency Proof Consistency proofs provide a mechanism for a verifier to validate the consistency of a verifiable data structure. The integer identifier for this Proof Type is TBD_3. The string identifier for this Proof Type is "consistency". provides a concrete example.

RFC9162_SHA256 as a Verifiable Data Structure This section defines how the data structures described in are mapped to the terminology defined in this document, using cbor and cose. RFC9162_SHA256 requires the following:

• TBD_1 (verifiable-data-structure): 1, the integer representing the RFC9162_SHA256 verifiable data structure algorithm.
• TBD_2 (inclusion-proof): a bstr representing the RFC9162_SHA256 inclusion proof
• TBD_3 (consistency-proof): a bstr representing the RFC9162_SHA256 consistency proof

Algorithm Definition The integer identifier for this Verifiable Data Structure is 1. The string identifier for this Verifiable Data Structure is "RFC9162_SHA256". See . See , 2.1.1. Definition of the Merkle Tree, for a complete description of this verifiable data structure.

Inclusion Proof Definition See , 2.1.3.1. Generating an Inclusion Proof, for a complete description of this verifiable data structure proof type. The cbor representation of an inclusion proof for RFC9162_SHA256 is: inclusion-proof = #TBD_2([ tree-size: int leaf-index: int inclusion-path: [+ bstr] ])

Inclusion Proof Signature In a signed inclusion proof, the previous merkle tree root, maps to tree-size-1, and is a detached payload. Other specifications refer to signed inclusion proofs as "receipts", profiles of proof signatures are encouraged to make additional protected header parameters mandatory. TODO: reference to scitt receipts. The protected header for an RFC9162_SHA256 inclusion proof signature is:

• alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int / tstr.
• verifiable-data-structure (label: TBD_1): REQUIRED. verifiable data structure algorithm identifier. Value type: int / tstr.
• crit (label: 2): OPTIONAL. Criticality marker. Value type: [ +label ]

Editors note: Recommend removing crit and mandating kid. See issue 21. The unprotected header for an RFC9162_SHA256 inclusion proof signature is:

• inclusion-proof (label: TBD_2): REQUIRED. proof type identifier. Value type: bstr.

The payload of an RFC9162_SHA256 inclusion proof signature is: A previous Merkle tree hash as defined in . The payload MUST be detached. Detaching the payload forces verifiers to recompute the root from the inclusion proof signature, this protects against implementation errors where the signature is verified but to root does not match the inclusion proof. The following example needs to be converted to proper CDDL:

Consistency Proof Definition See , 2.1.4.1. Generating a Consistency Proof, for a complete description of this verifiable data structure proof type. The cbor representation of a consistency proof for RFC9162_SHA256 is: consistency-proof = #TBD_3([ 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. ]) Editors note: tree-size-1, could be ommited, if an inclusion-proof is always present, since the inclusion proof contains, tree-size-1.

Consistency Proof Signature In a signed consistency proof, the latest merkle tree root, maps to tree-size-2, and is an attached payload. The protected header for an RFC9162_SHA256 consistency proof signature is:

• alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int / tstr.
• verifiable-data-structure (label: TBD_1): REQUIRED. verifiable data structure algorithm identifier. Value type: int / tstr.
• crit (label: 2): OPTIONAL. Criticality marker. Value type: [ +label ]

Editors note: Recommend removing crit and mandating kid. See issue 21. The unprotected header for an RFC9162_SHA256 consistency proof signature is:

• consistency-proof (label: TBD_2): REQUIRED. proof type identifier. Value type: bstr.

The payload of an RFC9162_SHA256 consistency proof signature is: The latest Merkle tree hash as defined in . The payload MUST be attached. The following example needs to be converted to proper CDDL:

Privacy Considerations See the privacy considerations section of:

Although the word transparency implies to some degree read access, it is important to note that transparency logs might include sensitive information. Depending on the verifiable data structure used, a service provider might be able to count unique entries. In the case that an entry is produced from a cose sign 1 envelope, adding information to the unprotected header can be used to produce a unique entry. However, this could impact privacy, and some transparency service operators might prefer only integrity protected content be made transparent.

Leaf Blinding In cases where a single merkle root and multiple inclusion paths are used to prove inclusion for multiple payloads. There is a risk that an attacker may be able to learn the content of undisclosed payloads, by brute forcing the values adjacent to the disclosed payloads through application of the cryptographic hash function and comparison to the the disclosed inclusion paths. This kind of attack can be mitigated by including a cryptographic nonce in the construction of the leaf, however this nonce must then disclosed along side an inclusion proof which increases the size of multiple payload signed inclusion proofs. Tree algorithm designers are encouraged to comment on this property of their leaf construction algorithm.

Blinding Example Implementers wishing to leverage multiple inclusion proofs to support selective disclosure, can prepend each payload with extra data before computing the inclusion proof, where extra data is a cryptographic nonce.

Security Considerations See the security considerations section of:

Hash Function Agility The choice of cryptographic hash function is the primary primitive impacting the security of authenticating payload inclusion in a merkle root. Tree algorithm designers should review the latest guidance on selecting a suitable cryptographic hash function.

IANA Considerations

Additions to Existing Registries

New Entries to the COSE Header Parameters Registry This document requests IANA to add new values to the 'COSE Algorithms' and to the 'COSE Header Algorithm Parameters' registries in the 'Standards Action With Expert Review category.

COSE Header Algorithm Parameters

• Name: verifiable-data-structure
• Label: TBD_1 (requested assignment 12)
• Value type: int / tstr
• Value registry: See
• Description: Tag indicating the Verifiable Data Structure, see .

Editors note: Authors are discussing how to avoid flooding the cose header parameters registry with new proof types.

• Name: inclusion-proof
• Label: TBD_2 (requested assignment 13)
• Value type: bstr
• Value registry: See
• Description: Tag indicating the "inclusion" Proof Type, see .
• Name: consistency-proof
• Label: TBD_2 (requested assignment 14)
• Value type: bstr
• Value registry: See
• Description: Tag indicating the "consistency" Proof Type, see .

Verifiable Data Structures IANA will be asked to establish a registry of tree algorithm identifiers, named "Verifiable Data Structures" to be administered under a Specification Required policy . Template:

• Identifier: The two-byte identifier for the algorithm
• Algorithm: The name of the data structure
• Reference: Where the data structure is defined

Initial contents: Provided in

Verifiable Data Structure Proof Types IANA will be asked to establish a registry of tree algorithm identifiers, named "Verifiable Data Structures Proof Types" to be administered under a Specification Required policy . Template:

• Identifier: The two-byte identifier for the algorithm
• Algorithm: The name of the proof type algorithm
• Reference: Where the algorithm is defined

Initial contents: Provided in

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. 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. Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines a set of algorithms that can be used with the CBOR Object Signing and Encryption (COSE) protocol (RFC 9052). This document, along with RFC 9052, obsoletes RFC 8152. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. 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. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice. 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.

Implementation Status Note to RFC Editor: Please remove this section as well as references to 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 . 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 , "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".

Implementer An open-source implementation was initiated and is maintained by the Transmute Industries Inc. - Transmute.

Implementation Name An application demonstrating the concepts is available at https://scitt.xyz.

Implementation URL An open-source implementation is available at:

• https://github.com/transmute-industries/cose

Maturity The code's level of maturity is considered to be "prototype".

Coverage and Version Compatibility The current version ('main') implements the tree algorithm, inclusion proof and consistency proof concepts of this draft.

License The project and all corresponding code and data maintained on GitHub are provided under the Apache License, version 2.

Implementation Dependencies The implementation builds on concepts described in SCITT (https://scitt.io/). The implementation uses the Concise Binary Object Representation (https://cbor.io/). The implementation uses the CBOR Object Signing and Encryption , maintained at: - https://github.com/erdtman/cose-js The implementation uses an implementation of , maintained at:

• https://github.com/transmute-industries/rfc9162/tree/main/src/CoMETRE

Contact Orie Steele (orie@transmute.industries)