Cryptshot
Open and mount a LUKS volume before performing a backup, defaulting to rsnapshot.
This script first checks to see if a volume with the given UUID exists. If the volume is found, it is treated as a LUKS volume and decrypted with the given key file, after which it is mounted. The script then runs the specified backup program. After the backup is complete, the volume is unmounted and the LUKS mapping is removed. Optionally, the mount point can be deleted to complete the clean-up.
Since the first step taken is to check if the given volume exists, it is appropriate for situations where the external backup volume is not always available to the machine (such as a USB backup drive and a laptop).
If using rsnapshot, the interval should be passed with the -i argument. Cryptshot can then replace rsnapshot in your crontab.
# rsnapshot daily
cryptshot.sh -i daily
See source for configuration.