Re: [Cbor] draft-ietf-cbor-cddl-control-00 should add CDDL notation for CBOR Sequences

[Henk Birkholz <[e-mail]>]

Hi John,

picking the first of the documents you cited, 
I-D.ietf-6tisch-minimal-security states:

>    For encoding compactness, the Link_Layer_Key object is not enclosed
>    in a top-level CBOR object.  Rather, it is transported as a sequence
>    of CBOR elements [I-D.ietf-cbor-sequence], some being optional.

So the solution here is to concatenate four separate CBOR data items 
(key_id, key_usage, key_value, key_addinfo) in order to safe the bytes 
that would wrap them in an array and would result in a single CBOR data 
item.

While I cannot assess, if the bytes saved here warrant the use of a 
cborseq, this seems like a solid spec to me and I wonder why this is not 
a solution therefore needs to be fixed.

Circling back to my initial reply, I am still under the impression that 
you are asking for a CDDL notation that can express CBOR Sequences as a 
top-level item in a CDDL spec, right? (RFC8764 is hinting at that.)

Viele Grüße,

Henk



On 05.11.20 16:10, John Mattsson wrote:
> Hi Henk,
> 
> The CDDL
> 
> my-embedded-cbor-seq = bytes .cborseq my-array
> 
> mathches a CBOR sequence wrapped in a byte string. I.e. "bytes .cborseq [ 1, 2 ]" would match the CBOR encoding 0x420102
> 
> All the document I cited use unadorned CBOR Sequences (i.e. not wrapped in a byte string). As far as I know there is no way to write CDDL that matches the unadorned CBOR Sequence 0x0102 ( 1, 2 ). I think this needs to be fixed.
> 
> John
> 
> -----Original Message-----
> From: Henk Birkholz <[e-mail]>
> Date: Thursday, 5 November 2020 at 16:02
> To: John Mattsson <[e-mail]>, "[e-mail]" <[e-mail]>
> Subject: Re: [Cbor] draft-ietf-cbor-cddl-control-00 should add CDDL notation for CBOR Sequences
> 
> Hi John,
> 
> I might be totally missing something obvious here - it's just that I am
> having a hard time visualizing your problem at the moment.
> 
> Maybe asking the obvious can help (me getting a grasp of the issue). Why
> does the following recommendation from RFC8764 not help you?
> 
>>     my-embedded-cbor-seq = bytes .cborseq my-array
>>     my-array = [* my-element]
>>     my-element = my-foo / my-bar
> 
> Viele Grüße,
> 
> Henk
> 
> 
> On 05.11.20 15:54, John Mattsson wrote:
>> Hi Henk,
>>
>> To quote RFC 8764
>>
>> 1) "CBOR Sequences are already supported as contents of byte strings using the ".cborseq" control operator"
>> 2) "CDDL does not provide for unadorned CBOR Sequences as a top-level subject of a specification"
>>    
>> All the document I cited do 2) not 1) .cborseq as currently specified is not a solution.
>>
>> Cheers,
>> John
>>
>> -----Original Message-----
>> From: Henk Birkholz <[e-mail]>
>> Date: Thursday, 5 November 2020 at 15:29
>> To: John Mattsson <[e-mail]>, "[e-mail]" <[e-mail]>
>> Subject: Re: [Cbor] draft-ietf-cbor-cddl-control-00 should add CDDL notation for CBOR Sequences
>>
>> Hi John,
>>
>> as the control for cborseq is introduced in:
>>
>>> https://tools.ietf.org/html/rfc8610#section-3.8.4
>>
>> and RFC8742 states that:
>>
>>>      Currently, CDDL does not provide for unadorned CBOR Sequences as a
>>>      top-level subject of a specification.  For now, the suggestion is to
>>>      use an array for the top-level rule, as is used for the ".cborseq"
>>>      control operator, and add English text that explains that the
>>>      specification is really about a CBOR Sequence with the elements of
>>>      the array
>>
>> it seems to me that you are asking for a specific CDDL notation that can
>> represent a cborseq as a top-level subject. Why is using an array here
>> not good enough in your case?
>>
>> Viele Grüße,
>>
>> Henk
>>
>> On 05.11.20 13:49, John Mattsson wrote:
>>> Hi,
>>>
>>> I the most important missing piece in RFC 8610 is the lack of CDDL for CBOR Sequences (RFC 8742) and I think draft-ietf-cbor-cddl-control would be a good place to add CDDL for that.
>>>
>>> CBOR sequences has already been standardized in RFC 8742. CBOR sequences are used quite heavily in IETF documents such as RFC 8769, draft-ietf-6tisch-minimal-security, draft-ietf-lake-edhoc, draft-palombini-core-oscore-edhoc, draft-mattsson-cose-cbor-cert-compress, etc.
>>>
>>> I don't understand all the complexities of CDDL formalism, but this seems like a quite easy thing to solve. I do not care exactly which notation is used, but could we please just agree on something and put in a draft.
>>>
>>> The notation could be something like:
>>>
>>> - reuse the CDDL notation for group ( ... )
>>> - reuse the Diagnostic Notation for CBOR sequences << ... >>
>>> - Some modification of the CDDL array notation ] ... [
>>> - Something looking like symbol swearing #$%@#$% ... #$%@#$%
>>> ...
>>>
>>> Cheers,
>>> John
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> CBOR mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/cbor
>>>
>>
>