Re: [Cbor] Why 0x42_4F_52 (was Re: 🔔 WG adoption call on draft-richardson-cbor-file-magic)

Michael Richardson <mcr+ietf@sandelman.ca> Thu, 18 February 2021 17:14 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: Laurence Lundblade <lgl@island-resort.com>
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Subject: Re: [Cbor] =?utf-8?b?V2h5IDB4NDJfNEZfNTIgKHdhcyBSZTog8J+UlCBXRyBh?= =?utf-8?q?doption_call_on_draft-richardson-cbor-file-magic=29?=
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Laurence Lundblade <lgl@island-resort.com> wrote:
    > Can you say why part 3, 0x42_4F_52, is added? The only reason I can
    > think of is that 55799 is not unique enough.

Because the two tags have to actually tag something.
0x42_4F_52 is "BOR", and encoded as a byte string, puts "CBOR" into the file.
This is the CBOR Sequence version.

    > If we need it, can it be at the start so that the part 2 tag can be
    > part 3 so you can just hand the end part to protocol decoders for COSE,
    > CWT, CoSWID and so on? With the 0x42_4F_52 in there, you can’t do that.

In the discussion running up to this document, the question as to whether it
should be a tag preceeding, or a CBOR Sequence occurred.  I concluded on a
CBOR Sequence, but last week during the discussion, it seems that both have
been asked for, and so I guess the document will do that once adopted.

55799 will need to be replaced in the CBOR Sequence version with a different
code which is equally unique and not valid unicode.

--
Michael Richardson <mcr+IETF@sandelman.ca>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide