Partially encrypted backup?

[Hendrik Boom <hendrik@topoi.pooq.com>]

I'm currently using rdiff-backup onto removable USB drives for backup.  I 
don not encrypt them now because I'm terrified of losing the encryption 
key and hence losing access to my backups.

I'm planning to trade backup drives with an acquaintance for off-site 
backups.  I trust her, but I don't trust not every random person who 
lives in her house or visits.

Is there any way of doing the backup partially encrypted so that files 
are encrypted only if not world-readable?

-- hendrik