(CENC) Item auxiliary information box valid values

[bradh]

I am having some trouble understanding the concept for item encryption auxiliary info.

In ISO/IEC 23001-7:2023 Section 8.4.3, the aux_info_type and aux_info_type_parameter semantics just point back to ISO/IEC 14496-12.

However there is some discussion in 8.4.1 that suggests that aux_info_type is intended to be a meaningful value, not just a random integer or 0. For a protected item, it probably should be a known scheme_type value from https://mp4ra.org/registered-types/schemes - or something defined for one of those schemes. Is the intent that aux_info_type is a 4CC?

Or is it just private to and dependent on the scheme_type in the schm box? If so, where is that defined for say cenc? The example I'm looking at has aux_info_type of 0x00 0x00 0x00, 0x00, and aux_info_type_parameter of 0x00, 0x00, 0x00, 0x01,
and I'm lost as to how to interpret that (essential) item property for the auxi item.

Any suggestions?

[denoualf]

Thank you for the comment.

In section 8.4.1 of ISO/IEC 23001-7, there is this statement that may help (seems to confirm that aux_info_type for a protected item should correspond to a protection_scheme, defined as a 4CC):

If no ItemAuxiliaryInformationBox is present for an auxiliary information item, then the implied value of aux_info_type is either (a) in the case of protected content, the scheme_type included in the ProtectionSchemeInfoBox of the referring item or otherwise (b) the item_type of the referring item. The default value of the aux_info_type_parameter is 0.

A possible clarification for the semantics in section 8.4.3 could be to add the indication from section 8.4.1:
aux_info_type: same semantics as sample auxiliary information in ISO/IEC 14496-12. 
 In case of protected content, should? / shall? take the value of the scheme_type included in the ProtectionSchemeInfoBox of the referring item.

We should also probably clarify in ISO/IEC 14496-12 (8.7.8.1 Definition) what does the following mean?

The default value of the aux_info_type_parameter is 0

especially in light of (8.7.8.3 Semantics)

aux_info_type is an integer that identifies the type of the sample auxiliary information