Is it possible to boot from an encrypted external hard drive?

If I have an external hard drive with a bootable cloned backup (via SuperDuper) - if I encrypt it with the Finder will I still be able to boot from it if necessary? Are there any solutions to enable my backup to be both encrypted and also bootable? I currently use FileVault 2 to encrypt my main SSD, and if I try booting from that (i.e. just booting up my computer) it asks for my password. If I try to boot from a bootable external disk (which happens to be encrypted) will it just ask for the disk password on boot [that is, after I select that as the boot disk], similarly to how I would be asked to enter a password to unlock the SSD encrypted with FileVault 2?