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

[Michael Richardson <[e-mail]>]

Carsten Bormann <[e-mail]> wrote:
    > For CBOR data items, I prefer registering a tag and prefixing with that
    > head (*) over prefixing with an array head and a string.  See, we have
    > a magic number registry built right into CBOR...

BTW, in case you didn't read far enough, that was variation two:

## Use a CBOR Tag on the CBOR Integer

A two or three byte CBOR Tag could be used in proposal two, applied to the CBOR Integer.

Or, a two byte CBOR Tag could be used in proposal one, applied to the CBOR Integer,
and not applied to the array.
This would make the first four bytes of a CBOR encoded item recognizeably CBOR,
with the next four bytes being the specific CBOR content.

....

I didn't know if using the Tag registry was a welcome or unwelcome hack.

My take right now is that you:
1) prefer a CBOR Sequence
2) prefer a per-protocol CBOR Tag around something.
   * c"bor" or C"BOR" would seem to be the right strings here!
3) probably prefix it all with Tag number 55799.

Tag<55799>Tag<Protocol>C"BOR"
3-bytes   5-bytes      4-bytes   = 12 bytes, with the relevant magic number
                                   being the first 8-bytes


So in section 6, in the case where the CBOR Tag registry is used, then there are two options I
thought of:

1. allow requesters to select their own four (32-bit) or eight (64-bit) tags, from the First Come First Served Registry, using the existing instructions.

2. amend the IANA instructions for {{RFC8949}} and carve out a 30-bit chunk of the four byte registry,
or a 32-bit chunk of the eight byte registry.

While in many cases CBOR encodings strive to be as compact as possible, for the purposes of a magic number registry for objects stored on disk, the use of between eight and twelve bytes is acceptable.

--
Michael Richardson <[e-mail]>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide