]> Transmute

orie@transmute.industries

DataTrails

steve.lasker@datatrails.ai

Fraunhofer SIT

Rheinstrasse 75 Darmstadt 64295 Germany henk.birkholz@ietf.contact

Security Internet-Draft This document defines new COSE header parameters for signaling a payload as an 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 providing references to optional discovery mechanisms that can help to find original payload content. 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.

Attached Payload COSE_sign1 envelope with an attached payload, providing for signature validation.

Detached Payload COSE_sign1 envelope with a detached payload (nil), which is compact but the payload must be distributed out of band to validate the signature

Hashed Payload A hashed payload functions equivalently to an attached payload, with the benefits of being compact in size and providing the ability to validate the signature.

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

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: 16) => 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 ; (content to be hashed) 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 ; Location the content of the hashed payload is stored ; For example: ; storage.example/244f...9c19 ? &(payload_location: TBD_3) => tstr * int => any } Hash_Envelope_Unprotected_Header = { * int => any } 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 16 (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. TBD_3 (payload_location) MAY be added to the protected header and MUST NOT be presented in the unprotected header. For example:

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

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.

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. 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.