JVET-AM0119 AHG9: On start and end flags of digitally signed content [M. M. Hannuksela, J. Boyce (Nokia)]

Chaired by S. Deshpande on 27 June 2025 during 16:20-:16:50

It is asserted that the specification text related to dsci_signed_content_start_flag and dscv_signed_content_end_flag has the following problems:

1. The following issues are caused by the fact that dscv_signed_content_flag is present only when dscv_verification_substream_id is equal to 0:
   1. Signed content cannot end with an AU that has substream ID greater than 0. For example, if substreams correspond to temporal sublayers, signed content cannot end with an AU that has sublayer identifier greater than 0.
   2. It is unclear how to derive the variable signingStatus, which uses dscv_signed_content_end_flag when dscv_signed_content_end_flag is not present in the DSCV SEI message.
2. The integrity of the original content across a splice point is not protected or detectable by the DSC SEI messages:
   1. An attack can remove the signed content that follows the verification period with dscv_signed_content_end_flag equal to 1.
   2. An attack can reorder pieces of signed content starting with dsci_signed_content_start_flag equal to 1 and ending with dscv_signed_content_end_flag equal to 1.

The proposal introduces the following terms:

• A verification unit is a sequence of verification periods within which splicing would cause a failure of digital signature verification, while splicing can be done at the boundaries of a verification unit.
• A verification chain is a sequence of verification units from which no verification unit can be removed or reordered without causing a failure of digital signature verification. However, a verification chain may be intervened at a verification unit boundary by verification unit(s) of other verification chain(s).

The proposal extends the start and end signalling of verification periods so that verification units and verification chains are identified as follows:

1. dsci_signed_content_start_flag is replaced by the 2-bit dsci_start_idc, with the following values:

0: the beginning of a verification period that does not start a verification unit or a verification chain

1: the beginning of a verification period and the beginning of a verification unit that does not start of a verification chain

2: the beginning of a verification period, a verification unit, and a verification chain

1. dscv_signed_content_end_flag is replaced by the 2-bit dscv_end_idc, which is present without any gating condition and has the following values:

0: the end of a verification substream of a verification period that does not end a verification unit or a verification chain

1: the end of a verification substream of a verification period that ends of a verification unit and does not end a verification chain

2: the end of a verification substream of a verification period that ends of a verification unit and a verification chain

It was commented that this is more of an extension of functionality rather than a bug-fix.

It was commented that there are some open questions.

1. Can it be specified how much content can be added in between chains (number of chains, length of video)
2. How is the interaction of different dsc_ids that are used in a same time in one chain vs. using dsc_id to distinguish chains?
3. How is the removal of entire chain handled?

No support was expressed by anyone for the aspect of changing flags to idcs (mainly due to the lateness of adding functionality at this stage)

The “Limited Alternative” in section 4 was discussed as a possible solution for asserted bug-fix for the point #1 above.

JVET-AM0208 item 7 is related/ similar.

Decision: Adopt the Limited Alterative proposed in section 4 of JVET-AM0119.

JVET-AM0119 AHG9: On start and end flags of digitally signed