[libdvdcss-devel] CSS authentication changes content returned from drive on some systems only?

[libdvdcss at jmbreuer.net]

I've run into an issue while taking care of DVD resume code in XBMC.

A bit of background: XBMC computes a hash over the content of all .IFO 
files to use as a Disk Unique ID, to make it possible to identify the 
same disk when it is inserted again later and have per-disk persistent 
player state / resume bookmarks.

XBMC uses libdvdnav/libdvdread/libdvdcss for DVD access.


I've observed the following: On both of my systems (gentoo on x86_64 and 
OpenELEC on x86_64 [different HW]), performing CSS authentication (e.g. 
"starting playback") will cause a different hash value to be computed 
than before - see https://github.com/xbmc/xbmc/issues/2936

So it seems that performing CSS (my guess: the drive authentication 
part) causes different data to be returned from the drive - FOR ME.

The hashes returned "before" and "after" authentication each are stable, 
i.e. I always get the same two hashes for a given disc, one before auth, 
the other after.


While discussing the issue with upstream, it turns out that another 
developer on Windows cannot reproduce the issue - he always gets the 
same content/hashes, although the logs show CSS authentication being 
performed against his drive as well.
See https://github.com/xbmc/xbmc/pull/2951


Can anyone shed some light on why our systems would behave differently 
as they do?


Thanks a lot!


So long,
    Joe