bittorrent-codex-docs/10 Notes/B-encoding using Python.md

See also: Using Python on macOS.

We will use a very simple Python script to validate encodings. You can find the script here: https://gist.github.com/marcinczenko/a87baf506bb0fd4bdff752cda4c685a1

Our input will be the following data1M.bin.torrent.json file:

{
  "info": {
    "length": 1048576,
    "name": "data1M.bin",
    "piece length": 262144,
    "pieces": [
      "111421FEBA308CD51E9ACF88417193A9EA60F0F84646",
      "11143D4A8279853DA2DA355A574740217D446506E8EB",
      "11141AD686B48B9560B15B8843FD00E7EC1B59624B09",
      "11145015E7DA0C40350624C6B5A1FED1DB39720B726C"
    ]
  }
}

To b-encode we run:

python bencoder.py --info-only data1M.bin.torrent.json
Using values from file: data1M.bin.torrent.json
Info only: True
{
  "length": 1048576,
  "name": "data1M.bin",
  "piece length": 262144,
  "pieces": [
    "21FEBA308CD51E9ACF88417193A9EA60F0F84646",
    "3D4A8279853DA2DA355A574740217D446506E8EB",
    "1AD686B48B9560B15B8843FD00E7EC1B59624B09",
    "5015E7DA0C40350624C6B5A1FED1DB39720B726C"
  ]
}
infohash f335440998515770adf47e8a4626889e59d91dde

The --info-only option instructs the script to only b-encode the info dictionary and do not include other attributes like announce.

We get as an output the data1M.bin.torrent file with the following content:

d4:infod6:lengthi1048576e4:name10:data1M.bin12:piece lengthi262144e6:pieces80:!<21><>0<EFBFBD><30><1E>ψAq<41><71><EFBFBD>`<EFBFBD><EFBFBD>FF=J<EFBFBD>y<EFBFBD>=<EFBFBD><EFBFBD>5ZWG@!}De<06><>ֆ<><D686><EFBFBD>`<EFBFBD>[<EFBFBD>C<EFBFBD><EFBFBD><EFBFBD>YbK	P<15><>@5$Ƶ<><C6B5><EFBFBD><EFBFBD>9rrlee

It is not very readable, because pieces has been converted to hex strings to a flattened byte array - in the end this is supposed to be machine readable code.