Comment 4 for bug 758077

Had the same problem. Solution: Ensure that in your /etc/crontab you set the environment variable $HOME to the home directory of the user that executes the cronjob. Because otherwise:

1) Duplicity won't be able to find it's archive directory and as a consequence duplicity will fetch the encrypted metadata from remote and needs your private key to decrypt the metadata. (Alternatively use duplicity's "--archive-dir" option.)

2) Additionally, without a correct $HOME environment variable, duplicity will use the wrong ~/.gnupg directory and therefore cannot find your public/private keys. (I don't know any workaround here, just set $HOME correctly. :))