Obtaining metadata from magnetlink infohash

Question

I am learning about bittorrent protocols and have a question I'm not too sure about.

According to BEP009,

magnet URI format

The magnet URI format is:

v1: magnet:?xt=urn:btih:info-hash&dn=name&tr=tracker-url

v2: magnet:?xt=urn:btmh:tagged-info-hash&dn=name&tr=tracker-url

info-hash Is the info-hash hex encoded, for a total of 40 characters. For compatability with existing links in the wild, clients should also support the 32 character base32 encoded info-hash.

tagged-info-hash Is the multihash formatted, hex encoded full infohash for torrents in the new metadata format. 'btmh' and 'btih' exact topics may exist in the same magnet if they describe the same hybrid torrent.

example magnet link: magnet:?xt=urn:btih:407AEA6F3D7DC846879449B24CA3F57DB280DE5C&dn=ubuntu-educationpack_14+04_all&tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337%2Fannounce&tr=udp%3A%2F%2Fexplodie.org%3A6969

Correct me if i'm wrong, but urn:btih:407AEA6F3D7DC846879449B24CA3F57DB280DE5C is the info-hash from the magnet link, and i will need to decode it to be able to obtain a bencoded metadata such as listed in BEP015. Things such as: downloaded, left, uploaded, event, etc.

My question is, how do I decode this in python?

Answer 1

The info-hash in Magnet Link is the same as the info-hash required for a UDP Tracker (20-bytes SHA-1 hash of bencoded "info" dictionary of a torrent).

Additionally, a UDP Tracker doesn't use bencoded data at all, just bytes! Bencoded format is used by HTTP/HTTPs trackers though.

Answer 2

You can search some open source code like libtorrent. It's written by C++, so you need to read the bdecode and bencode part. That part is not complex, and then you can write python codes by yourself.

Answer 3

Correct me if i'm wrong, but urn:btih:407AEA6F3D7DC846879449B24CA3F57DB280DE5C is the info-hash from the magnet link, and i will need to decode it to be able to obtain a bencoded metadata such as listed in BEP015. Things such as: downloaded, left, uploaded, event, etc.

Infohash is a unique SHA1 hash that identifies a torrent. Therefore it cannot be further decoded to obtain any further information, it's just an identifier. Furthermore, if you think about it, the link would constantly need to change if it contained this information.

You must use this infohash in the announce request to a tracker. The purpose of the announce request is to let the tracker know that you are downloading the particular hash, how far along you are and to provide you with peers the tracker knows about.

In your example there are two UDP trackers:

tr=udp%3A%2F%2Ftracker.opentrackr.org%3A1337%2Fannounce&tr=udp%3A%2F%2Fexplodie.org%3A6969

After URL decoding these, they become:

tr=udp://tracker.opentrackr.org:1337/announce&tr=udp://explodie.org:6969

So, these are the trackers you must send your announce request to by implementing https://libtorrent.org/udp_tracker_protocol.html

Note that does not give you any information about the torrent file, for that you need to implement BEP-9.