What's your distro?
You need to have libd0_rijndael.so installed in your lib directory. Or next to the other stuff? I don't know. Maybe there's some info in the package from the download page.
Get that from
http://www.xonotic.org/download/ "Optional encryption download via HTTP"
If you want to do it another way, you have several choices, listed here from preferred to least preferred:
- Get a compiled version from your distro's repositories if that exists, or
- Make a package yourself, if you have the skill, or
- Compile it and load it from your user directories using LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/path/to/directory/containing/lib", or
- Compile and install it, using sh autogen.sh, followed by ./configure (see ./configure --help), followed by make, followed by make install (or make DESTDIR=/path/to/whereever install)