]> Transmute

orie@transmute.industries

DataTrails

steve.lasker@datatrails.ai

Security Internet-Draft This document defines new COSE header parameters for signaling that a payload is the output of a hash function. This mechanism enables faster validation as access to the original payload is not required for signature validation. Additionally, hints of the detached payload's content format and availability are defined. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the CBOR Object Signing and Encryption Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction COSE defined detached payloads in Section 2 of , using nil as the payload. In order to verify a signature over a detached payload, the verifier must have access to the payload content. Storing a hash of the content allows for small signature envelopes, that are easy to transport and verify independently. Additional hints in the protected header ensure cryptographic agility for the hashing & signing algorithms, and discoverability for the original content which could be prohibitively large to move over a network.

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.

Header Parameters To represent a hash of a detached payload, the following headers are defined:

TBD 0:

will be assigned by , represents the content type of the code envelope, including the protected header and payload

TBD 1:

the hash algorithm used to generate the hash of the payload

TBD 2:

the content type of the payload the hash represents

TBD 3:

an identifier enabling a verifier to retrieve the full payload preimage.

Signed Hash Envelopes Example int, ; Type of the envelope ? &(typ: TBD_0) => int / tstr ; Hash algorithm used to produce the payload from content ; -16 for SHA-256, ; See https://www.iana.org/assignments/cose/cose.xhtml &(payload_hash_alg: TBD_1) => int ; Content type of the preimage of the payload ; 50 for application/json, ; See https://datatracker.ietf.org/doc/html/rfc7252#section-12.3 &(payload_preimage_content_type: TBD_2) => int ; Identifier for an artifact repository ; For example: ; pkg:container...image@sha256:244f...9c?repo..._url=dev.example ? &(artifact_repository: TBD) => tstr ; Type of Verifiable Data Structure, e.g. RFC9162_SHA256 ; ? &(verifiable-data-structure: -111) => int, ; additional parameters allows. } Verifiable_Data_Proofs = { ? &(inclusion-proofs: -1) => [ bstr .cbor inclusion-proof ] ? &(consistency-proofs: -2) => [ bstr .cbor consistency-proof ] ; ... other proofs are allowed here. } Hash_Envelope_Unprotected_Header = { ; ? &(verifiable-data-proofs: 222) => Verifiable_Data_Proofs ; ... other unprotected header values are still allowed here. } Hash_Envelope_as_COSE_Sign1 = [ protected : bstr .cbor Hash_Envelope_Protected_Header, unprotected : Hash_Envelope_Unprotected_Header, payload: bstr / nil, signature : bstr ] Hash_Envelope = #6.18(Hash_Envelope_as_COSE_Sign1) ]]>

Protected Header TBD 0 (typ), TBD 1 (payload hash alg) and TBD 2 (content type of the preimage of the payload) MUST be present in the protected header and MUST NOT be present in the unprotected header. For example: TBD 0 will be assigned by , it represents the content type of the code envelope, which includes the protected header and payload. TBD 1 will be assigned by this draft. TBD 2 will be assigned by this draft. TBD 3 will be assigned by this draft.

Attached Payload The payload MAY be attached.

Detached Payload The payload MAY be detached.

Encrypted Hashes Should we define this?

Security Considerations TODO Security

Choice of Hash Function It is RECOMMENDED to align the strength of the chosen hash function to the strength of the chosen signature algorithm. For example, when signing with ECDSA using P-256 and SHA-256, use SHA-256 to hash the payload.

IANA Considerations

COSE Header Algorithm Parameters

• Name: payload hash algorithm
• Label: TBD_1
• Value type: int
• Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
• Description: Hash algorithm used to produce the payload.

Named Information Hash Algorithm Registry

• Name: SHAKE256
• Label: TBD_2
• Value type: int
• Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
• Description: SHAKE256 a described in https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf
• Name: ASCON128
• Label: TBD_3
• Value type: int
• Value registry: https://www.iana.org/assignments/named-information/named-information.xhtml
• Description: ASCON128 a described in https://csrc.nist.gov/CSRC/media/Projects/lightweight-cryptography/documents/round-2/spec-doc-rnd2/ascon-spec-round2.pdf

Normative References 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 the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. 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). This enables the benefits of explicit typing, as defined in the JSON Web Token Best Current Practices BCP, to 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. 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.

Acknowledgments The following individuals provided input into the final form of the document: Carsten Bormann, Henk Birkholz, Antoine Delignat-Lavaud, Cedric Fournet.