Automating backups to an encrypted drive

[lanas]

Folks,

  I'd like to automate periodic backups to an encrypted drive.  The
drive is in fact a removable USB drive.  It's fairly easy, if needed,
to write a Perl script that would use expect and feed the password
needed to mount the encrypted drive but, is there any way to prevent
using a plain text password in any kind of script whatsoever ?  I'm
using dm-crypt, but I could change to another encryption mechanism.

Cheers,

Al