I don't normally buy or watch DVDs, since the movie studios and distributors are not treating the linux community fairly. However, I was recently given a movie to watch, which turned out to not be as straight-forward as I would've liked it to be.
Long story short, in spite of following the usual routine of installing libdvdcss2 and other relevant packages, the movie would not play. Starting a movie player from the command line, I could see what error messages where associated with the failure to play, and the key was "Error cracking css key".
After having tried a few different things, I ended up installing regionset. When running regionset (sudo regionset) it did not echo a value for the current region which my dvd player was set to - apparently the device had not been locked to a region yet. Setting the region of my dvd player to the region of the dvd caused playback to work for all movie player clients.
Whether playback did not work because the
1. region was not set at all
2. region was not set to the same region as the DVD
remains to be found out.
Typically, you can only change the region of your playback device a limited number of times (3-5), so you will want to change it as few times as possible.