[Noisebridge-discuss] Is there a good way to do incrementally updated torrents?

[Meredith L. Patterson]

On Tue, Dec 07, 2010 at 01:46:51PM -0600, Sai wrote:

> I think one solution would be to issue new torrents that have "the
> same pointers" as old torrents for the files they have in common, such
> that people hosting the older versions participate fully in sharing
> the subset of the new version that was in the last even if they don't
> switch to the new one. But I don't know whether it's possible to do
> this with the current bittorrent protocol; I suspect not.

Well, one thing to consider here is that the infohash of a torrent (i.e., how you identify it as a torrent containing the files you're interested in) is the SHA1 of the SHA1 hashes of all the pieces in the torrent along with some metadata. The dictionary from which the infohash is generated also contains the number of pieces in the file. You could maybe look into the current state of the art on chosen-prefix attacks -- they were in the news back in '08 with respect to MD5, I have no idea what if anything has been done with SHA1. But even then, the number of pieces at the front changes, so I'm not sure if chosen prefix actually gains you anything.

http://wiki.theory.org/BitTorrentSpecification#Metainfo_File_Structure is the relevant part of the BitTorrent spec.

Note that there are a couple of extension protocols out there, so if you wanted to try to come up with a way to extend BitTorrent to support, oh, I dunno, some kind of way for a tracker or DHT node to advertise that it has a torrent which subsumes (or should replace) some other torrent, you could do so. Torrent versioning, essentially.

It's certainly worth investigating.

Cheers,
--mlp