You can encrypt your system and it is even possible to do this for a server and then enter the encryption key during boot via ssh. Tutorials how to this this can be found e.g. here:

https://www.thomas-krenn.com/de/wiki/Voll-verschl%C3%BCsseltes-System_via_SSH_freischalten

I don't like to edit files in /usr/share and actually you don't have to. Leave out the corresponding step, but instead copy the crypt_unlock.sh provided here to /etc/initramfs-tools/hooks/ and run update-initramfs -u afterwards. This way is also safer for updates to the initramfs-related packages.

When logging in during boot to dropbear. Instead of running the passfifo command, run unlock, enter the password and the server will continue to boot.

This is based on the scripts published at

• http://www.emmolution.org/?p=307
• https://stinkyparkia.wordpress.com/2014/10/14/remote-unlocking-luks-encrypted-lvm-using-dropbear-ssh-in-ubuntu-server-14-04-1-with-static-ipst/
• https://bildung.xarif.de/xwiki/bin/Informatik/verschl%C3%BCsselte+Systemfestplatte+per+SSH+freischalten

If you ever get stuck using this or a similar script: Log in, look at ps and run the kill commands by hand. From ubuntu 12.04 to 14.04 there were some subtle changes.