Internet-Draft COSE Hash Envelope July 2024
Steele, et al. Expires 9 January 2025 [Page]
Network Working Group
Intended Status:
Standards Track
O. Steele
S. Lasker
H. Birkholz
Fraunhofer SIT

COSE Hash Envelope


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

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Table of Contents

1. Introduction

COSE defined detached payloads in Section 2 of [RFC9052], 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.

1.1. Attached Payload

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

18(                                 / COSE Sign 1                   /
      h'a4013822...3a616263',       / Protected                     /
      {}                            / Unprotected                   /
      h'317cedc7...c494e772',       / Payload                       /
      h'15280897...93ef39e5'        / Signature                     /

1.2. 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

18(                                 / COSE Sign 1                   /
      h'a4013822...3a616263',       / Protected                     /
      {}                            / Unprotected                   /
      nil,                          / Detached Payload              /
      h'15280897...93ef39e5'        / Signature                     /

1.3. 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.

18(                                 / COSE Sign 1                   /
      h'a4013822...3a616263',       / Protected                     /
      {}                            / Unprotected                   /
      h'935b5a91...e18a588a',       / Payload                       /
      h'15280897...93ef39e5'        / Signature                     /

2. Header Parameters

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


the hash algorithm used to generate the hash of the payload


the content type of the payload the hash represents


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

2.1. Signed Hash Envelopes Example

Hash_Envelope_Protected_Header = {
    ; Cryptographic algorithm to use
    ? &(alg: 1) => int,

    ; Type of the envelope
    ? &(typ: 16) => int / tstr

    ; Hash algorithm used to produce the payload from content
    ; -16 for SHA-256,
    ; See
    &(payload_hash_alg: TBD_1) => int

    ; Content type of the preimage
    ; (content to be hashed) of the payload
    ; 50 for application/json,
    ; See
    &(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)

2.2. 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:

  / alg : ES384 / 1: -35,
  / kid / 4: h'75726e3a...32636573',
  / typ / 16: application/hashed+cose
  / payload_hash_alg sha-256 / TBD_1: 1
  / payload_preimage_content_type / TBD_2: application/jwk+json
  / payload_location / TBD_3 : storage.example/244f...9c19

3. Encrypted Hashes

Should we define this?

4. Security Considerations

TODO Security

4.1. 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.

5. IANA Considerations

5.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

5.2. COSE Header Algorithm Parameters

  • Name: payload hash algorithm

  • Label: TBD_1

  • Value type: int

  • Value registry:

  • Description: Hash algorithm used to produce the payload.

5.3. Named Information Hash Algorithm Registry

  • Name: SHAKE256

  • Label: TBD_2

  • Value type: int

  • Value registry:

  • Description: SHAKE256 a described in

  • Name: ASCON128

  • Label: TBD_3

  • Value type: int

  • Value registry:

  • Description: ASCON128 a described in

6. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <>.


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

Authors' Addresses

Orie Steele
Steve Lasker
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt