This page is very preliminary. On the list of things to do is to improve it.
Cryptar is an implementation of the cryptar algorithm, which in turn is based on the rsync algorithm. By propitious use of a split database, it stores backup data on a remote server in encrypted form, maintaining sufficient signature information in the local database that it can compute efficient differences between the current versions of files and their previously backed up versions.
In doing so it avoids the need to trust the remote database beyond the hope that data stored there will be saved and returned. It is able to guarantee, however, that data returned is legitimate and unmodified. No unencrypted data leaves the client for the server.
To follow: link to code, link to paper from NordU 2004.