COSE Receipts

COSE Receipts 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 COSE Internet-Draft COSE (CBOR Object Signing and Encryption) Receipts prove properties of a verifiable data structure to a verifier. Verifiable data structures and associated proof types enable security properties, such as minimal disclosure, transparency and non-equivocation. Transparency helps maintain trust over time, and has been applied to certificates, end to end encrypted messaging systems, and supply chain security. This specification enables concise transparency oriented systems, by building on CBOR (Concise Binary Object Representation) and COSE. The extensibility of the approach is demonstrated by providing CBOR encodings for RFC9162. Discussion Venues Discussion of this document takes place on the CBOR Object Signing and Encryption Working Group mailing list (cose@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

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.

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.

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.

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.

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

Structures Similar to COSE Key Types, different verifiable data structures support different algorithms. As EC2 keys (1: 2) support both digital signature and key agreement algorithms, RFC9162_SHA256 (TBD_1 : 1) supports both inclusion and consistency proofs. This document establishes a registry of verifiable data structure algorithms, with the following initial contents:

Name: The name of the verifiable data structure
Value: The identifier for the verifiable data structure
Description: The identifier for the verifiable data structure
Reference: Where the verifiable data structure is defined

COSE Verifiable Data Structures

Name	Value	Description	Reference
N/A	0	N/A	N/A
RFC9162_SHA256	1	SHA256 Binary Merkle Tree

When desigining new verifiable data structures, please request the next available positive integer as your requested assignment, for example: How to register new structures

Name	Value	Description	Reference
N/A	0	N/A	N/A
RFC9162_SHA256	1	SHA256 Binary Merkle Tree
Your name	TBD (requested assignment 2)	tbd	Your specification

Parameters Similar to COSE Key Type Parameters, as EC2 keys (1: 2) keys require and give meaning to specific parameters, such as -1 (crv), -2 (x), -3 (y), -4 (d), RFC9162_SHA256 (TBD_1 : 1) supports both (-1) inclusion and (-2) consistency proofs. This document establishes a registry of verifiable data structure algorithms, with the following initial contents: COSE Verifiable Data Structure Parameters

Verifiable Data Structure	Name	Label	CBOR Type	Description	Reference
1	inclusion proofs	-1	array (of bstr)	Proof of inclusion
1	consistency proofs	-2	array (of bstr)	Proof of append only property

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 the different 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. Security analysis SHOULD be conducted prior to migrating to new structures to ensure the new security and privacy assumptions are acceptable for the use case. When designing new verifiable data structure parameters (or proof types), please start with -1, and count down for each proof type supported by your verifiable data structure: How to register new parameters

Verifiable Data Structure	Name	Label	CBOR Type	Description	Reference
1	inclusion proofs	-1	array (of bstr)	Proof of inclusion
1	consistency proofs	-2	array (of bstr)	Proof of append only property
TBD (requested assignment 2)	new proof type	-1	tbd	tbd	Your_Specification
TBD (requested assignment 2)	new proof type	-2	tbd	tbd	Your_Specification
TBD (requested assignment 2)	new proof type	-3	tbd	tbd	Your_Specification

Registration Requirements Each specification MUST define how to encode the verifiable data structure and its parameters (also called proof types) in CBOR. Each specification MUST define how to produce and consume the supported proof types. See as an example.

RFC9162_SHA256 This section defines how the data structures described in are mapped to the terminology defined in this document, using CBOR and COSE.

Verifiable Data Structure 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 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:

CBOR Encoded RFC9162 Inclusion Proof

Receipt of Inclusion In a signed inclusion proof, the previous merkle tree root, maps to tree-size-1, and is a detached payload. Profiles of proof signatures are encouraged to make additional protected header parameters mandatory, to ensure that claims are processed with their intended semantics. One way to include this information in the COSE structure is use of the typ (type) Header Parameter, see and the similar guidance provided in . The protected header for an RFC9162_SHA256 inclusion proof signature is:

Protected Header for a Receipt of Inclusion int &(vds: -111) => int * cose-label => cose-value } ]]>

alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int.
vds (label: -111): REQUIRED. verifiable data structure algorithm identifier. Value type: int.

The unprotected header for an RFC9162_SHA256 inclusion proof signature is:

A Verifiable Data Structure Proofs in an Unprotected Header inclusion-proofs } unprotected-header-map = { &(vdp: -222) => verifiable-proofs * cose-label => cose-value } ]]>

vdp (label: -222): REQUIRED. Verifiable data structure proofs. Value type: Map.
inclusion-proof (label: -1): REQUIRED. Inclusion proofs. Value type: Array of bstr.

The payload of an RFC9162_SHA256 inclusion proof signature is the 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 the merkle root does not match the inclusion proof. The CBOR Extended Diagnostic Notation (EDN) for a Receipt containing an inclusion proof for RFC9162_SHA256 is:

Example inclusion receipt The CBOR Extended Diagnostic Notation (EDN) for the Protected Header in the example above is:

Example inclusion receipt decoded protected header The vds in the protected header is necessary to understand the vdp in the unprotected header. The CBOR Extended Diagnostic Notation (EDN) for the inclusion proof in the Unprotected Header is:

Example inclusion receipt decoded inclusion proof The vds in the protected header is necessary to understand the inclusion proof structure in the unprotected header. The inclusion proof and signature are verified in order. First the verifiers applies the inclusion proof to a possible entry (set member) bytes. If this process fails, the inclusion proof may have been tampered with. If this process succeeds, the result is a merkle root, which in the attached as the COSE Sign1 payload. Second the verifier checks the signature of the COSE Sign1. If the resulting signature verifies, the Receipt has proved inclusion of the entry in the verifiable data structure. If the resulting signature does not verify, the signature may have been tampered with. It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid inclusion proof.

Consistency Proof 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:

CBOR Encoded RFC9162 Consistency Proof Editors note: tree-size-1, could be omitted, if an inclusion-proof is always present, since the inclusion proof contains, tree-size-1.

Receipt of Consistency 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:

Protected Header for a Receipt of Consistency int &(vds: -111) => int * cose-label => cose-value } ]]>

alg (label: 1): REQUIRED. Signature algorithm identifier. Value type: int.
vds (label: TBD_1): REQUIRED. Verifiable data structure algorithm identifier. Value type: int.

The unprotected header for an RFC9162_SHA256 consistency proof signature is: consistency-proofs } unprotected-header-map = { &(vdp: -222) => verifiable-proofs * cose-label => cose-value } ]]>

vdp (label: -222): REQUIRED. Verifiable data structure proofs. Value type: Map.
consistency-proof (label: -2): REQUIRED. Consistency proofs. Value type: Array of 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 CBOR Extended Diagnostic Notation (EDN) for a Receipt containing a consistency proof for RFC9162_SHA256 is:

Example consistency receipt The vds in the protected header is necessary to understand the vdp in the unprotected header. The CBOR Extended Diagnostic Notation (EDN) for the Protected Header in the example above is:

Example consistency receipt decoded protected header The CBOR Extended Diagnostic Notation (EDN) for the consistency proof in the Unprotected Header is:

Example consistency receipt decoded consistency proof The vds in the protected header is necessary to understand the consistency proof structure in the unprotected header. The signature and consistency proof are verified in order. First the verifier checks the signature on the COSE Sign1. If the verification fails, the consistency proof is not checked. Second the consistency proof is checked by applying a previous inclusion proof, to the consistency proof. If the verification fails, the append only property of the verifiable data structure is not assured. This approach is specific to RFC9162_SHA256, different verifiable data structures may not support consistency proofs. It is recommended that implementations return a single boolean result for Receipt verification operations, to reduce the chance of accepting a valid signature over an invalid consistency proof.

Privacy Considerations See the privacy considerations section of:

Log Length Some structures and proofs leak the size of the log at the time of inclusion. In the case that a log only stores certain kinds of information, this can reveal details that could impact reputation. For example, if a transparency log only stored breach notices, a receipt for a breach notice would reveal the number of previous breaches at the time the notice was made transparent.

Header Parameters Additional header parameters can reveal information about the transparency service or its log entries. A privacy analysis MUST be performed for all mandatory fields in profiles based on this specification.

Security Considerations See the security considerations section of:

Choice of Signature Algorithms A security analysis MUST be performed to ensure that the digital signature algorithm alg has the appropriate strength to secure receipts. It is recommended to select signature algorithms that share cryptographic components with the verifiable data structure used, for example: Both RFC9162_SHA256 and ES256 depend on the sha-256 hash function.

Validity Period In some cases, receipts MAY include strict validity periods, for example, activation not too far in the future, or expiration, not too far in the past. See the iat, nbf, and exp claims in , for one way to accomplish this. The details of expressing validity periods are out of scope for this document.

Status Updates In some cases, receipts should be "revocable" or "suspendible", after being issued, regardless of their validity period. The details of expressing statuses are out of scope for this document.

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.

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: vds
Label: TBD_1
Value type: int
Value registry: https://www.iana.org/assignments/cose/cose.xhtml#header-parameters
Description: Algorithm name for verifiable data structure, used to produce verifiable data structure proofs.
Name: vdp
Label: TBD_2
Value type: int
Value registry: https://www.iana.org/assignments/cose/cose.xhtml#header-parameters
Description: Location for verifiable data structure proofs in COSE Header Parameters.

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

Name: The name of the verifiable data structure
Value: The identifier for the verifiable data structure
Description: A brief description of the verifiable data structure
Reference: Where the verifiable data structure is defined

Initial contents: Provided in

Expert Review This IANA registries is established under a Specification Required policy. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude. Expert reviewers should take into consideration the following points:

Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in deployments.
Specifications are required for all point assignments. Early assignment before a specification is available is considered to be permissible, however, such registrations MUST be marked provisional by prefixing the entry with "PROVISIONAL: ". Provisional assignments to expired drafts MUST be removed from the registry.
Points assigned in this registry MUST have references that match the COSE Verifiable Data Structure Parameters registry. It is not permissible to assign points in this registry, for which no Verifiable Data Structure Parameters entries exist.

COSE Verifiable Data Structure Parameters IANA will be asked to establish a registry of verifiable data structure parameters, named "COSE Verifiable Data Structure Parameters" to be administered under a Specification Required policy . Template:

Verifiable Data Structure: The identifier for the verifiable data structure
Name: The name of the proof type
Label: The integer of the proof type
CBOR Type: The cbor data type of the proof
Description: The description of the proof type
Reference: Where the proof type is defined

Initial contents: Provided in

Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in deployments.
Specifications are required for all point assignments. Early assignment before a specification is available is considered to be permissible, however, such registrations MUST be marked provisional by prefixing the entry with "PROVISIONAL: ". Provisional assignments to expired drafts MUST be removed from the registry.
Points assigned in this registry MUST have references that match the COSE Verifiable Data Structures registry. It is not permissible to assign points in this registry, for which no Verifiable Data Structure entry exists.

References Normative References Concise Binary Object Representation (CBOR) 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. CBOR Object Signing and Encryption (COSE): Initial Algorithms 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. Certificate Transparency Version 2.0 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. Key words for use in RFCs to Indicate Requirement Levels 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. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words 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 Guidelines for Writing an IANA Considerations Section in RFCs 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. Improving Awareness of Running Code: The Implementation Status Section 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. CBOR Web Token (CWT) CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. CBOR Web Token (CWT) Claims in COSE Headers Mattr Self-Issued Consulting This document describes how to include CBOR Web Token (CWT) claims in the header parameters of any COSE structure. This functionality helps to facilitate applications that wish to make use of CBOR Web Token (CWT) claims in encrypted COSE structures and/or COSE structures featuring detached signatures, while having some of those claims be available before decryption and/or without inspecting the detached payload. Another use case is using CWT claims with payloads that are not CWT Claims Sets, including payloads that are not CBOR at all. COSE "typ" (type) Header Parameter Self-Issued Consulting Transmute This specification adds the equivalent of the JSON Object Signing and Encryption (JOSE) typ (type) header parameter to CBOR Object Signing and Encryption (COSE) so that the benefits of explicit typing, as defined in the JSON Web Token Best Current Practices BCP, can be brought to COSE objects. The syntax of the COSE type header parameter value is the same as the existing COSE content type header parameter.

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 COSE SCITT Receipts

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 verifiable data structure 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 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)