Backing up with Duplicity over SSH

I'm attempting to set up Duplicity as a backup tool, running from cron over ssh, to backup my home directory to another server.

I of course already have password-less SSH set up to connect to that server, and use ssh-agent to store the passphrase for my SSH key, as described in another entry. However, I could not get password-less SSH to work from a cron job. If anyone has tips on how to do that, I'd love to hear it.

So I created a second SSH public/private key pair with no passphrase, (in \~/.ssh/backup and \~/.ssh/backup.pub) and figured out how to have Duplicity call ssh, scp, and sftp with the correct parameters to specify the new key pair. (There's about a zillion different ways of specifying that, and only ONE that works across all three programs.)

The passphrase mentioned here is the one used to encrypt the duplicity backups.

PASSPHRASE='YourPassphraseGoesHere' duplicity \
    --no-print-statistics \
    --ssh-options "-oIdentityFile=/home/schof/.ssh/backup" \
    /home/schof \
    scp://johnmarkschofield@example.com/duplicity