Re: [Cbor] [COSE] CBOR magic number, file format and tags

Carsten Bormann <cabo@tzi.org> Sat, 23 January 2021 20:21 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sat, 23 Jan 2021 21:21:34 +0100
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To: Michael Richardson <mcr+ietf@sandelman.ca>
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Subject: Re: [Cbor] [COSE] CBOR magic number, file format and tags
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On 23. Jan 2021, at 21:01, Michael Richardson <mcr+ietf@sandelman.ca> wrote:
> 
> 
> Carsten Bormann <cabo@tzi.org> wrote:
>> Right.  Something like this would probably make sense as an alternative
>> to the primary approach proposed, which is based on CBOR sequences.  If
>> you want to keep the magic-numbered file a single data item, this is
>> the way to go.
> 
> I am personally agnostic about CBOR Sequence vs something else.
> 
> I believe that:
>  a) the magic number needs to be at least 4 bytes
>  b) we shouldn't mandate that it be transmitted on the wire.
> 
> Being able to split off the magic part easily make sense to me.
> That's why I proposed either:
>  1) CBOR Sequence

(Which is the only approach to magic-numbering CBOR sequences, and works for CBOR data items as long as you don’t mind them turning into CBOR sequences.)

>  2) Two-element array.

Which is mostly a weird way to express a tag, without the need to register it.
Why would this be better than a tag?
(Unless you believe registering them is too onerous or creates too much noise in the registry.)
So I’d prefer using tags, which otherwise have exactly the same properties.
The 1+2+1+4 proposal looks pretty good to me.

Grüße, Carsten