[core] empty KID values

[Michael Richardson <[e-mail]>]

RFC9254-to-be/yang-cbor says:
   Data nodes implemented using a CBOR array, map, byte string, or text
   string can be instantiated but empty. In this case, they are encoded with
   a length of zero.

When encoding/dealing with the COSE Sign0 in
draft-ietf-anima-constrained-voucher, we have some puzzling about what to do
with:

        kid: null
or:     kid: ""
or:     kid: h''

so, two remarks.  First, the kid: field is in the Sign0 structure, not
actually in the YANG-CBOR, so arguably the above comment does *NOT* apply!

My puzzling is about kid.  Should I treat a null/empty kid as if there were
no kid field at all, and then use some other heuristic to find the right
verification key, or should I treat it as a entry null, which must match
a null/""/h'' entry in a database for the key.
Normally, it might be a hash of a public key, so seeing h'xx..xx' would be
reasonable.

I'm curious what COSE people say.
KID is annoyingly use case specific :-(

--
Michael Richardson <[e-mail]>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide