Re: [COSE] Assigning CBOR tags to key structures

[Renzo Navas <[e-mail]>]

Hello Cose ML!
(Sorry for the late reply, I didn't see the replies the same day; then
I was on holidays)

Jim:
thanks for the suggestion. Indeed I think I will go for this option to
keep the solution non-homebrew. I will  borrow from the CoAP
Content-Formats (application/cose-key : 101) and put this in the
protected headers.
I think this solution will be 3  bytes (1B content-type label + 2B to
represent "101") instead of 1 byte of the hypothetical CBOR Tag for
COSE-Key (19 or 20 looked nice).

Carsten:
I am using CoAP (..for now) but I want the least possible COSE
metadata on the CoAP header; to make the COSE message as agnostic as
possible of the transport (maybe we will transport this message
over-foo). In my particular case, I simply use the CoAP Content-format
60 to indicate that this is an "application/cbor" payload.
Then I use the CBOR-Tag 16 (COSE_Encrypt0), because indeed that is
what this CBOR message is. So even if I use CoAP metadata for COSE I
should indicate this message as "application/cose;
cose-type="cose-encrypt0" (16).

 My 'problem' arose when, once decrypted; the plaintext part of the
COSE_Encrypt0 contains , in some cases, a COSE_Key object; and I
simply wanted to indicate that with a 1-byte tag . Jim's suggestions
of content-type in protected header solves my problem indeed.


So maybe the question I want to ask to the ML is:

why you discarded defining the COSE_Key/COSE_KeySet CBOR tag? ( I see
the pretty tags 19 and 20 unassigned)
Carsten, I really agree with your opinion from 2015 :

>I'd propose this rule:
>If there is a media type, there should be a CBOR tag for those
>environments that don't benefit from media types.
In the end, it's a matter of 2 bytes, 1 byte tag, vs 3 byte protected
header content-type.
The COSE messages are around 52 Bytes, so It's a matter of 4% longer
Messages/Energy.

I found it strange not to find a CBOR tag for a mayor COSE type.

These are my 2 cents,

Thank you for your quick reply Jim and Carsten!

Renzo


On Fri, Apr 7, 2017 at 6:14 PM, Carsten Bormann <[e-mail]> wrote:
> Hi Renzo,
>
> where would you put such a tag?
>
> Starting the encoded CBOR with a tag would amount to a magic number unless the media type already indicates that this is CBOR-encoded.  But then it could indicate that this is a application/cose-key in the first place (Content-Format 101):
>
> https://www.iana.org/assignments/core-parameters/core-parameters.xhtml#content-formats
>
> Grüße, Carsten
>