COSE_Recpient header protection

[laurencelundblade]

In the two-layer option, there is a COSE_Recipient which has a protected header that must be protected.

One option is to use an Enc_structure of type "Enc_Recipient". Another option is to use Context Info Structure like ECDH alg ID -29 does.

This is probably wrapped up with how Context Info Structure is optional or not.

[dajiaji]

I only implemented "Enc_Recipient" and I don't see the need to add support for the latter for now.

From the perspective of simplicity and interoperability of the COSE-HPKE spec, I think it would be better to make it clear that the use of KDF_Context_Info is not recommended for COSE-HPKE. At least, I think it should be optional.