One of the more interesting design requirements here is that the system should work even when the distributed content is _not_ encrypted. If I join this backup

One of the more interesting design requirements here is that the system should work even when the distributed content is _not_ encrypted. If I join this backup effort, and a meteor takes out all of SF (and thus the Internet Archive), and their off-site backups fail, and the aftermath is preventing me from accessing the internet, then that 1TB of data that I had downloaded is all still usable to me. It's not some giant encrypted blob, it's a bunch of zip files (or similar) containing images, sound, video, web pages, and other types of files that I can, in principle, already use. (Maybe I can't do anything much with that Apple II disk image, but I can at least identify it.) Git annex and BitTorrent would both work with that requirement, and most of the commercial, proprietary solutions cannot; their clients wouldn't use them if that data wasn't private.