Session Date/Time: 15 Dec 2021 15:00

CBOR

Summary

This interim meeting focused on two main technical issues: resolving an issue with CDDL _dot_abnf and _dot_abnf_b controls identified during AUTH48, and discussing the path forward for the "file magic" draft, particularly regarding the handling of CBOR and non-CBOR data sequences. A decision was made to adjust the CDDL text to clarify byte string usage. For the file magic draft, a new tag (55801) was proposed and discussed to explicitly allow non-CBOR data to follow a CBOR header, and a second working group last call was deemed necessary. The regular bi-weekly meeting schedule will be paused for the end-of-year holidays, resuming in the new year.

Key Discussion Points

CDDL Control `_dot_abnf` and `_dot_abnf_b` Issue (AUTH48)

Problem Identification: During AUTH48, it was noted that while CDDL _dot_abnf and _dot_abnf_b are defined to accept both text and byte strings as their target (left-hand side), the right-hand side (the ABNF content) was only described to accept text strings. However, existing examples already utilized byte strings on the right-hand side.
Rationale for Byte Strings: Using byte strings on the right-hand side is often more practical for incorporating line breaks and allows for easier copy-pasting into documents, as byte strings do not have the same JSON compatibility constraints as text strings.
Proposed Resolution: The text should be updated to explicitly permit byte strings on the right-hand side of _dot_abnf.

File Magic Update

Background: The current draft introduces tags 55799 (part of RFC 8949) and 55800. 55800 provides a CBOR sequence with a short (typically 12-byte) CBOR data item as a prefix to identify the sequence. The issue arose because examples in the draft incorrectly showed 55800 being used to prefix non-CBOR data, while the text for 55800 is clear that it should prefix CBOR data.
Proposed Solution: Introduce a new tag, 55801. This tag will have the same meaning as 55800 (a CBOR header followed by data) but without the guarantee that the subsequent data is itself CBOR. This resolves the discrepancy and allows for labeling files where a CBOR header is followed by arbitrary data (which may or may not be CBOR).
- 55800 indicates a CBOR header followed by a CBOR sequence.
- 55801 indicates a CBOR header followed by data that is not necessarily a CBOR sequence.
Discussion on Implications:
- The coexistence of 55800 and 55801 was acknowledged as slightly "weird" but deemed a practical solution.
- A participant raised a point about content format tags changing semantics when enclosed within 55800 or 55801. Specifically, the byte string directly tagged by a content format number within 55800 or 55801 does not represent the content itself but is a fixed 3-byte sequence ('bor'). The actual content representation follows the 55800/55801 structure.
- Concern was expressed that this introduces a new aspect to the general processing model for tags, where processing might need to occur "from the outside-in" rather than strictly from the inside out, similar to how CBOR-packed operates. This suggests that 55800/55801 tags "disarm" the processing of nested content format tags.
Next Steps for File Magic: The updated draft (dash-07), including 55801 and revised examples, requires thorough review.

Decisions and Action Items

CDDL Control Issue:
- Decision: The text for _dot_abnf in CDDL will be changed in AUTH48 to explicitly allow byte strings on its right-hand side.
File Magic Draft:
- Decision: A new tag 55801 will be introduced in the draft to cover cases where a CBOR header prefixes non-CBOR data.
- Action Item: Carsten (draft editor) will revise the draft (dash-07) to include 55801 and update examples accordingly.
- Action Item: Michael to propose text on the implications for the general tag processing model, specifically regarding how 55800/55801 affect nested content format tags.
- Decision: A second working group last call for the file magic draft will be initiated after the proposed changes are incorporated and reviewed, given the non-trivial nature of the changes.

Next Steps

Complete the text changes for the CDDL _dot_abnf control in AUTH48.
Review the updated file magic draft (dash-07) with the new 55801 tag and revised examples.
Draft and review text on the tag processing model implications for the file magic document.
Initiate a second working group last call for the file magic draft.
The next regular CBOR WG meeting is scheduled for January 12th, 2024. The December 29th meeting is cancelled.