Re: [Cbor] Chunks with tags inside indefinite-length string (major type 2 and 3)

I *think*
https://cbor-wg.github.io/CBORbis/draft-ietf-cbor-7049bis.html#indefinite-length-byte-strings-and-text-strings
is
pretty clear that the contained things have to have the same major type as
the indefinite-length string, and tags have a different major type. e.g. "a
series of zero or more byte or text strings" doesn't include the
possibility of tags, and "If any item between the indefinite-length string
indicator (0b010_11111 or 0b011_11111) and the “break” stop code is not a
definite-length string item of the same major type, the string is not
well-formed." says any other content is not well-formed.

The pseudocode includes

if (it != mt)           // need finite-length chunk
          fail();               //    of same type

which I think also rejects tags inside indefinite strings. What am I
missing?

Jeffrey

On Thu, Dec 5, 2019 at 7:27 AM Faye Amacker <faye.github@gmail.com> wrote:

> While implementing support for tags, I ran into a scenario that might be
> errata or could use some clarification in 7049bis.
>
> Section 3.2.3 states:
>
> > Indefinite-length strings are represented by a byte containing the major
> type and additional information value of 31, followed by a series of zero
> or more byte or text strings (“chunks”) that have definite lengths,
> followed by the “break” stop code (Section 3.2.1). The data item
> represented by the indefinite-length string is the concatenation of the
> chunks (i.e., the empty byte or text string, respectively, if no chunk is
> present).
>
> And the pseudocode in Appendix C allows tags for chunks inside
> indefinite-length strings.
>
> Chunks are simply fragments to be concatenated, so tags applied to chunks
> doesn't seem intuitive.  Chunks are not intended for independent access
> like array elements.  Some tags (like #2 and #3) transform byte string into
> bignum which makes sense for arrays, not chunks.
>
> If tags must be applied to each chunk, there should be some text
> mentioning that because library authors might think it should be rejected
> or applied to the concatenated string rather than chunk.
>
> However, if the tag for a chunk applies to the entire concatenated string,
> then what happens when there are multiple chunks with different tags? Which
> tag wins?
>
> A simpler way forward is to treat tags on chunks as malformed.  If this is
> the way to go, then the pseudocode in Appendix C needs to be updated and an
> example could be added to Appendix G.
>
> GitHub issue for RFC 7049bis:
> https://github.com/cbor-wg/CBORbis/issues/148
> GitHub issue for my CBOR library:
> https://github.com/fxamacker/cbor/issues/44
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