When trying to play an Encrypted DVD and no Decryption plugin installed Totem gives the following message:
Could not read DVD. This may be because the DVD is encrypted and a
DVD decryption library is not installed.

However the Help contains no further information. Here is an example of what could be added based on what Ubuntu currently has on this topic.

How do I enable restricted codecs to play DVDs?

DVD support cannot be provided by default in Ubuntu due to legal and technical
restrictions. Most commercial DVDs are encrypted and so require the use of
decryption software in order to play them.

Use Fluendo to legally play DVDs

You can buy a commercial DVD decoder that can handle copy protection from
Fluendo. It works with Linux and should be legal to use in all countries.

Use alternative decryption software

In some countries, the use of the below unlicensed decryption software is not
permitted by law. Verify that you are within your rights to use it.

Install libdvdnav4, libdvdread4, gstreamer0.10-plugins-bad, and
gstreamer0.10-plugins-ugly.