[Cbor] CDDL spec should specify how CBOR items match types

[Jeffrey Yasskin <[e-mail]>]

Apologies if I'm just reading
https://tools.ietf.org/html/draft-greevenbosch-appsawg-cbor-cddl-11
badly, but I don't see a specification of which CBOR items match which
types.

Sections like https://tools.ietf.org/html/draft-greevenbosch-appsawg-cbor-cddl-11#section-3.4
(Arrays) specify how to write an array type, but I don't see a
normative statement that '82 01 02' is an instance of [*uint] or that
'01' and '42 01 02' aren't.

I also don't see a definition of '#' in order to ground the meaning of
the prelude. "How this is used should be evident from the prelude
(Appendix E)." from 2.2.3. Representation Types isn't really
sufficient to specify it.

I think this lack of specification is at the root of Kevin Braun's set
of bugs in https://mailarchive.ietf.org/arch/msg/cbor/Z1XcZSTLbTDnqA9HP8UXjeUOspE.

I'd be inclined to fix this by specifying each CDDL type as a
recursive parser, which accepts or rejects either CBOR items or byte
strings. I think the choice of item vs bytes comes down to where we
want to put the serialization constraints (like
"indefinite-containers") described in
https://mailarchive.ietf.org/arch/msg/cbor/Pv191TJRBiG1QOJbJL8iVHjUDuE,
but it could also have some implications for how parsers return data
from unfinished byte streams.

Jeffrey