Disk encryption on a headless box?

[Andre Goree]

I was looking at encrypting my data on a headless server I have, and found a
good tutorial using ZFS+geli.  Easy enough to setup, but one thing I just
realized is that I wouldn't be able to enter the passphrase on the server
since I keep it headless.

I wonder if there is some type of encryption I can use that does not require
entering a passphrase.  For example, perhaps I can set it up in a way that
it'll automatically read from certain files on a thumb drive?