[Duplicity-talk] Encrypted Diffs

[Michael Orlitzky]

So, incremental encrypted backups are supposed to be hard. I've searched 
through all of the documentation I can find (short of digging into the 
source), and haven't been able to locate a good description of how 
Duplicity solves the problem. If possible, can someone explain the 
sequence of events that takes place when one creates an incremental 
encrypted backup?

In particular, the question I'm trying to answer is, "how do we 
calculate the difference between two encrypted blobs?". Do GPG/tar 
provide random access within an archive? Or does Duplicity just diff 
whatever comes out of the stream? Etc.

Since we can't send the encryption key to the server, does the entire 
backup history need to be downloaded during this process?