I don't know a lot about torrents, at least not enough to understand how certain websites can offer both a normal download link and a torrent link to download a file uploaded by a user.
Is generating a torrent link something common and simple to achieve. Would I need a server installation?
I've made an ugly C# implementation from a Java source, and to make sure some of my encoded variables were correct, I used NBEncode from Lars Warholm.
// There are 'args' because I'm using it from command-line. (arg0 is an option not used here)
// Source file
args[1] = Directory.GetCurrentDirectory() + args[1];
// Name to give to the torrent file
args[2] = Directory.GetCurrentDirectory() + args[2];
var inFileStream = new FileStream(args[1], FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
var filename = args[2];
//BEncoding with NBEencode
var transform = new BObjectTransform();
MemoryStream s = new MemoryStream();
OSS.NBEncode.Entities.BDictionary bod = new OSS.NBEncode.Entities.BDictionary();
OSS.NBEncode.Entities.BDictionary meta = new OSS.NBEncode.Entities.BDictionary();
// Preparing the first part of the file by creating BEncoded objects
string announceURL = "https://www.mysite.com/announce";
int pieceLength = 512 * 1024;
bod.Value.Add(new BByteString("name"), new OSS.NBEncode.Entities.BByteString(filename));
bod.Value.Add(new BByteString("length"), new OSS.NBEncode.Entities.BInteger(inFileStream.Length));
bod.Value.Add(new BByteString("piece length"), new OSS.NBEncode.Entities.BInteger(pieceLength));
bod.Value.Add(new BByteString("pieces"), new BByteString(""));
meta.Value.Add(new BByteString("announce"), new BByteString(announceURL));
meta.Value.Add(new BByteString("info"), bod);
byte[] pieces = hashPieces(args[1], pieceLength);
transform.EncodeObject(meta, s);
s.Close();
// Notice that we finish with a dictionary entry of "pieces" with an empty string.
byte[] trs = s.ToArray();
s.Close();
inFileStream.Close();
// I don't know how to write array of bytes using NBEncode library, so let's continue manually
// All data has been written a MemoryStreamp, except the byte array with the hash info about each parts of the file
Stream st = new FileStream(filename + ".torrent", FileMode.Create);
BinaryWriter bw = new BinaryWriter(st);
// Let's write these Bencoded variables to the torrent file:
// The -4 is there to skip the current end of the file created by NBEncode
for (int i = 0; i < trs.Length - 4; i++)
{
bw.BaseStream.WriteByte(trs[i]);
}
// We'll add the length of the pieces SHA1 hashes:
var bt = stringToBytes(pieces.Length.ToString() + ":");
// Then we'll close the Bencoded dictionary with 'ee'
var bu = stringToBytes("ee");
// Let's append this to the end of the file.
foreach (byte b in bt)
{
bw.BaseStream.WriteByte(b);
}
foreach (byte b in pieces)
{
bw.BaseStream.WriteByte(b);
}
foreach (byte b in bu)
{
bw.BaseStream.WriteByte(b);
}
bw.Close();
st.Close();
// That's it.
}
Functions used:
private static byte[] stringToBytes(String str)
{
System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
Byte[] bytes = encoding.GetBytes(str);
return bytes;
}
private static byte[] hashPieces(string file, int pieceLength)
{
SHA1 sha1 = new SHA1CryptoServiceProvider();
StreamReader inn = new StreamReader(file);
MemoryStream pieces = new MemoryStream();
byte[] bytes = new byte[pieceLength];
byte[] digest = new byte[20];
int pieceByteCount = 0, readCount = inn.BaseStream.Read(bytes, 0, pieceLength);
while (readCount != 0)
{
pieceByteCount += readCount;
digest = sha1.ComputeHash(bytes, 0, readCount);
if (pieceByteCount == pieceLength)
{
pieceByteCount = 0;
foreach (byte b in digest)
{
pieces.WriteByte(b);
}
}
readCount = inn.BaseStream.Read(bytes, 0, pieceLength - pieceByteCount);
}
inn.Close();
if (pieceByteCount > 0)
foreach (byte b in digest)
{
pieces.WriteByte(b);
}
return pieces.ToArray();
}
It depends on how you're trying to create it. If you run a website, and want to generate torrent files from uploaded files, then you'll obviously need server-side code.
Generating a torrent file involves: adding the files you want to the torrent, and adding tracker info. Some popular trackers are:
To create the .torrent file, you'll have to read the about the format of the file. A piece of Java that generates .torrent files is given at https://stackoverflow.com/a/2033298/384155