[Cbor] Serialization choices vs. CDDL (Re: CDDL optionality and choices and optionality in CBOR and CDDL)

[Carsten Bormann <[e-mail]>]

On 13. Apr 2024, at 02:49, Rohan Mahy <[e-mail]> wrote:
> 
> I interpret the CDDL spec to mean that the encoding of the first item in the array is either F6 (null);  or 58 20 followed by 32 octets. Did I interpret the CDDL spec correctly? 

CDDL operates on the data model level, not on the representation format level.

The .size control operator on an integer is a convenient way to limit the range of an integer based on what the maximum value for the minimum encoding size for that integer is, not the actual size in the representation format.
The latter is usually set by the encoder according to the rules for preferred serialization (Section 4.1 of RFC 8949 [1]), but it is allowed to choose other serialization variants (unless it actually intends to provide deterministic encoding, Section 4.2 of RFC 8949 [2] and draft-ietf-cbor-cde [3]).

I understand that you are looking at providing a fixed size header for a data format.
You may want to look at the fixed-size representations we have in the tagged array specification [4], where most of the tags defined give an interpretation to byte strings (here: as an array of numbers).

Grüße, Carsten

[1]: https://www.rfc-editor.org/rfc/rfc8949.html#name-preferred-serialization
[2]: https://www.rfc-editor.org/rfc/rfc8949.html#name-deterministically-encoded-c
[3]: https://www.ietf.org/archive/id/draft-ietf-cbor-cde-02.html
[4]: https://www.rfc-editor.org/rfc/rfc8746.html