COSE Hash Envelope

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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.¶

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

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

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.¶

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 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)

{
  / 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¶

5. IANA Considerations

6. Normative References

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[RFC9052]

Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, August 2022, <https://www.rfc-editor.org/rfc/rfc9052>.

Acknowledgments

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

Authors' Addresses