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libdvdcss is not to be confused with DeCSS. Whereas DeCSS uses a cracked DVD player key to perform authentication, libdvdcss uses a generated list of possible player keys. If none of them works (for instance, when the DVD drive enforces region coding), a brute force algorithm is tried in which the region code of a DVD is ignored. Unlike DeCSS, libdvdcss has never been legally challenged.
Many GNU/Linux distributions do not contain libdvdcss (for example, Debian, Fedora, SUSE Linux, and Ubuntu) due to fears of running afoul of DMCA-style laws, but they often provide the tools to let the user install it themselves. For example, it used to be available in Ubuntu through Medibuntu, which is no longer available.
Libdvdcss alone is only a library and cannot play DVDs. DVD player applications, such as VLC media player, use this library to decode DVDs. Libdvdcss is optional in many open-source DVD players, but without it, only non-encrypted discs will play.