<i>A while back I was getting depressed at the usability of tarsnap</i><p>Please send me an email with details about the usability issues you encountered.<p><i>[...] distribution of chunk sizes [...] I have no idea what Tarsnap does about this; I am guessing the blocks it hashes for deduplication purposes aren't the same as the ones it sends to the server, and I presume it doesn't stream stuff to the server as it scans the filesystem. And there are probably several other things it does that I wouldn't even think of.</i><p>The blocks Tarsnap uses for deduplication are the same as the blocks Tarsnap uploads; but they are encrypted (to avoid leaking file data), identified by HMAC (to avoid leaking information via hash), and the blocking is key dependent, so an attacker can't say "you have chunks of lengths X, Y, and Z, so you probably have this particular block of data".  Also, the chunk size (64 kB on average, pre-compression) is small enough that there would be too much noise to pull off such an attack.