Hi Ilari,
> 6) The encoding of the encapsulated key produced by HPKE seems to be
> under-specified.
>
> HPKE gives octet string as encapsulted key. This apparently is placed
> into the ephremeral public key field in unprotected header. However,
> RFC8152 specifies this field to be cose_key, and it is not at all
> clear how to encode the octet string as cose_key. Especially what to
> fill as the kty field, which is mandatory in cose_key.
>
> Searching for existing RFC8152 construct to abuse, there is the
> "Symmetric" kty. Then the encapsulated key would look like:
>
> -1: {
> /* kty => Symmetric */
> 1:4,
> /* The raw encapsulated ciphertext. */
>
> -1:h'04ca591f4b1139c1c325be3265a6ce4dcc79a5895e9ef12a0726406bc72282697c8d12f18230208ebaa769f903917d59284526fd65a27ab5898913af10ed334398'
> }
You raise a couple of good questions in your remark:
1. Is the unprotected header the right place to put the ephemeral public key?
2. What kty field should be used in the ephemeral public key structure?
3. What is the encoding of the public key exported by HPKE?
Add 1) We used the unprotected header because this is also currently done for
similar uses in the COSE RFC. See, for example, Appendix C.3.1.
Add 2) Regarding the kty field the IANA registry contains a number of "COSE Key
Types" in https://www.iana.org/assignments/cose/cose.xhtml. For use with
elliptic curve keys EC2 (2) appears appropriate.
Add 3) HPKE does not define a wire format. The COSE-HPKE draft defines a wire
format. Hence, there is no conflict in terms of encoding.
It appears to make sense to re-use the already existing encoding provided in
the COSE spec.
Ciao
Hannes
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