addition of key rotation to DASH-DRM reference app

[jpiesing]

This issue is created for discussion of the proposed addition of key rotation to the DASH-DRM reference app.

I noted that there is a long discussion of how key rotation can be done in ISOBMFF/CMAF in this CTA wave issue.

https://github.com/cta-wave/dash-hls/issues/20

It relates to DASH/HLS compatibility but seems to discuss all the options.

[jpiesing]

Discussions elsewhere pointed out the description of 3 different modes of key rotation in sections 9.2, 9.3 and 9.4 of the DASH-IF IOP guidelines. https://dashif.org/docs/IOP-Guidelines/DASH-IF-IOP-Part6-v5.0.0.pdf. 9.2 being changing the KID at period boundaries. 9.3 being including a PSSH box in the media segements 9.4 being a key hierarchy like the PlayReady root/leaf arrangement where the root authenticates the leaves and changing a root does not trigger an EME license request.

We may discuss which of these (or indeed something else) is used or is going to be used. See also the discussion about the possibility of some compatibility in key rotation between DASH and HLS in https://github.com/cta-wave/dash-hls/issues/54#issuecomment-2401526874 .

EDIT: DVB-DASH 8.2 gives an informative description of key rotation using 'pssh' boxes in media segments and requirements are found in 8.5.0. This reads like it's the same as 9.3 in DASH-IF IOP but DASH-IF IOP 9.3 gives more details, particularly the statement that "The media segments have one PSSH box per DRM allowing to access content and a KID value in a sgpd box with grouping_type ’seig’." DVB-DASH does not reference the 'sgpd' box or 'seig'.