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

[Rohan Mahy <[e-mail]>]

Hi Carsten,

Asking for more clarification inline please.

On Fri, Apr 12, 2024, 22:55 Carsten Bormann <[e-mail]> wrote:

> 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.
>

sure. but if a tool is using a CDDL model to validate a CBOR instance
document, does the CDDL model below mean "an array of two items, the first
of which is either a null or a bstr, the second of which is a tstr"?

foo = [
  item1 = null / bstr,
  item2 = tstr
]

If so, it should validate an instance document whose first array item is a
null, yes?

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.
>

It sounds like you are saying that I can't apply the .size operator to a
byte string? (I was not asking about setting the size of an integer). I
want a 32 octet byte string, not a 4 octet uint32.

Thanks,
-rohan


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