ssh, passphrases and stdin

Hi list,

I would like to get your opinion on this: I am developing an application
that backs up data using rsync+ssh+public/private key login. The
application should start as a service upon system boot.

Since I don't want to have an unencrypted key on the system I need to
enable a passphrase. Well, I learned that ssh does not read the password
from stdin and therefore gives me some headache on how to solve this.

I found a possible fix could be ssh-keychain, however the passphrase needs
to be entered upon every system boot. Does anyone know if ssh-keychain
accepts the passphrase from stdin?

I know, having the passphrase, even in an encrypted from, "laying around"
on the system is far from optimal as well. However, I need to get syncing
done upon system boot.

If anyone has a better idea on this matter I'd sure like to hear it!

Dave