zsync is a file transfer program. It allows you to download a file from a remote server, where you have a copy of an older version of the file on your computer already. zsync downloads only the new parts of the file. It uses the same algorithm as rsync.
zsync -i old-image.iso http://kanotix.acritox.com/files/trialshots/new-image.iso.zsync
How to create a bootable USB stick from a Kanotix-ISO-Image
How to install Kanotix into an encrypted LVM-volume: