[Cbor] CDDL optionality and choices and optionality in CBOR and CDDL

[Rohan Mahy <[e-mail]>]

Hi,

1) I read RFC 8610 and came up with the following CDDL document. The
cddl-rs tool says that the CDDL is confirmant:

Content = [
>   replaces: null / MessageId,
>   expires: uint .size 4,
> ]
>
> MessageId = bstr .size 32
>

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?

If I try to validate a CBOR document with
[ h'9936b46e7609af57e7f3d8528b6eabc7f52dbf6c01c94f7cff46333cd0b82a0c', 0 ]
this validates, but [ null, 0 ] fails with "error validating type choice at
cbor location /0: expected 32, got Null".

Is this a bug with the cddl-rs validation tool or did I misunderstand how
CDDL is interpreted?

2) In TLS Presentation Language, you can require different elements
depending on the value of a field that appeared previously. It appears that
I could do this in CDDL as well, but the spec is silent about that
application. For example, presumably this describes an array of 3 items,
the first is an array of food items, the second is either the simple value
true or false, the third value is an array of drink items if the second
value is true OR a boolean if the second item is false.

Order = [
  food = [ * food_items ],
  ( includes_alcohol: true, drinks = [ * drink_items ] ) / (
includes_alcohol: false, to_go: bool )
]
bool = true / false

Is this possible in CDDL?

Many Thanks,
-rohan