Hash calculation in torrent clients

问题:

I was wondering if someone knows what a "hash" in a BitTorrent client is referring to, it is clearly not the hashcode of the file, but something different.

I think thats more a magnet link to a file, but how is this connected to the file itself?

I just want to understand the construct behind the scene.
File <--> Hash <---> hash in torrent client

回答1:

The hash in a torrent client or the hashyou find in a magnet-URI is the SHA1-hash of the raw bencoded info-dictionary-part of a torrent-file.

To understand how that works you need to know two things:

• How a torrent-file is built.

• How bencodeing is done.

Both of these are explained in the offical bittorrent specification that you can find here: http://bittorrent.org/beps/bep_0003.html

However I recommed that you instead read the inoffical specification that you can find here: https://wiki.theory.org/BitTorrentSpecification
as it is much easier to understand.