Why is hashing the info dict turning out wrong?

问题

I have been trying for ages to get this hashing thing for BitTorrent to work in Java but it always becomes wrong.

I have narrowed it down to a few lines of code where I'm 99% sure the problem is:

Bencode bencode = new Bencode(Charset.forName("UTF-8"));
byte[] fileBytes = new byte[33237];
Map<String, Object> dict = bencode.decode(fileBytes, Type.DICTIONARY);
Map infoMap = (Map) object.get("info");
ByteArrayOutputStream baos = new ByteArrayOutputStream();
BencodeOutputStream bos = new BencodeOutputStream(baos);
bos.writeDictionary(infoMap);
byte[] hash = DigestUtils.sha1(baos.toByteArray());

I have hardcoded the size of the array just to make sure the issue is not caused by a bunch of zeroes hanging around.

I have tried with both UTF-8 and US-ASCII.

I have tried using two different libraries for the bencoding so it's probably not there where the problem's at.

Edit: From the spec it seems that the info dict should be urlencoded as the info_hash. So I tried writing out the dictionary into a ByteArrayOutputStream and then do the sha1 hashing on the byte[] that ByteArrayOutPutStream is holding.

Will the DigestUtils.sha1method provide a URL encoder? Can't find any information on that.

回答1:

The problem, as Encombe pointed out, was with the encoding. In the Bencode specification it talks about byte strings and this seems to point to it just being a stream of data without any encoding.

Both of the libraries I looked at converted all byte strings to some encoding so I wrote a Bencode library that only did the conversion when specifically asked to.

The code above is basically correct but here is the client code I am using now:

public void readManifest() throws IOException, Exception {
    byte[] fileBytes = FileUtils.readFileToByteArray(file);
    ByteArrayInputStream bis = new ByteArrayInputStream(fileBytes);
    BDecoder decoder = new BDecoder(bis, "UTF-8");
    BDict dict = decoder.decodeDict();
    Map<String, Object> valueMap = dict.getValue();
    infoMap = (Map<String, Object>) valueMap.get("info");
}

public String hash() throws Exception {
    if (hash == null) {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        BEncoder encoder = new BEncoder(baos, "UTF-8");
        encoder.encodeDict(infoMap);
        hash = DigestUtils.sha1Hex(baos.toByteArray());
    }
    return hash;
}

来源：https://stackoverflow.com/questions/45043257/why-is-hashing-the-info-dict-turning-out-wrong

标签

java

bittorrent

bencoding