Sunday, March 15, 2009

What does it take to download a torrent?

The simple answer to this question is "All you need is the .torrent file. Stupid n00bs." This is true. The .torrent file contains all the metadata required to A) download the data and B) verify the data. Without this, you can't really do anything. So, do you actually need the .torrent file to begin a download?

No! All you need is the infohash for the .torrent file! The infohash is the SHA1 hash of the metadata in a .torrent file. As such, it can be used as a unique identifier for a particular .torrent file. With this infohash, you can query the BitTorrent DHT for a list of peers downloading that torrent. Then, with the help of the Metadata Exchange extension, you can connect to these peers and request that they send you the metadata from the .torrent file and you're away and downloading. Great!

"But what if some malicious peer sends you corrupt metadata, then you'd never be able to download the torrent properly!", I hear you asking. Well, in a rather beautiful twist, this is next to impossible. As I said earlier, the infohash is generated by putting the metadata in the .torrent file through a SHA1 hash. So all you have to do is hash the metadata once you have received it and then compare the result of that to the SHA1 hash you used to start the download. If they match, then you can be fairly confident that the metadata has not been corrupted/altered in any way.

As of 17:00GMT, March 15th MonoTorrent has completed its first download using only a 20 byte hash to begin the download. This is possible because of some tireless work by Olivier Dufour, who also implemented Peer Exchange, a good few parts of DHT, WebSeeding and SuperSeeding. The code for this still hasn't quite hit SVN, a bit of refactoring remains to be done. It should be in SVN within a week. I'm looking forward to his next patch of awesomenesss now.

34 comments:

Anonymous said...

Does this mean that now you can download torrents by having just a URL? For instance something like bttp://3874A39239827F2937....

Alan said...

Exactly. The actual format is as follows:

magnet:?xt=urn:btih:INFOHASH&dn=NAME&tr=TRACKERURL

btif = bittorrent infohash
dn = display name (optional)
tr = tracker (optional)

If no tracker is specified, DHT is used to find peers. You can read the spec for more details if you want.

Strob said...

But how and where do I enter that info?

I'm trying to download a toorent but I have only the hash and file name

Anny said...

You may try to download the torrent at this site - http://file.sh

Alan said...

@strob:

There's a constructor overload for TorrentManager which takes an infohash and a path/name instead of an actual Torrent (created from a .torrent file). This is what you need to use.

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