Calculating the info-hash of a torrent file

Question

I'm using C++ to parse the info hash of a torrent file, and I am having trouble getting a "correct" hash value in comparison to this site:

http://i-tools.org/torrent

I have constructed a very simple toy example just to make sure I have the basics right.

I opened a .torrent file in sublime and stripped off everything except for the info dictionary, so I have a file that looks like this:

d6:lengthi729067520e4:name31:ubuntu-12.04.1-desktop-i386.iso12:piece lengthi524288e6:pieces27820:¡´E¶ˆØËš3í   ..............(more unreadable stuff.....)..........

I read this file in and parse it with this code:

#include <string>
#include <sstream>
#include <iomanip>
#include <fstream>
#include <iostream>

#include <openssl/sha.h>


void printHexRep(const unsigned char * test_sha) {

    std::cout << "CALLED HEX REP...PREPPING TO PRINT!\n";
    std::ostringstream os;
    os.fill('0');
    os << std::hex;
    for (const unsigned char * ptr = test_sha; ptr < test_sha + 20; ptr++) {

        os << std::setw(2) << (unsigned int) *ptr;
    }
    std::cout << os.str() << std::endl << std::endl;
}


int main() {

    using namespace std;

    ifstream myFile ("INFO_HASH__ubuntu-12.04.1-desktop-i386.torrent", ifstream::binary);

    //Get file length
    myFile.seekg(0, myFile.end);
    int fileLength = myFile.tellg();
    myFile.seekg(0, myFile.beg);

    char buffer[fileLength];

    myFile.read(buffer, fileLength);
    cout << "File length == " << fileLength << endl;
    cout << buffer << endl << endl;

    unsigned char datSha[20];
    SHA1((unsigned char *) buffer, fileLength, datSha);
    printHexRep(datSha);

    myFile.close();

    return 0;
}

Compile it like so:

g++ -o hashes info_hasher.cpp -lssl -lcrypto

And I am met with this output:

4d0ca7e1599fbb658d886bddf3436e6543f58a8b

When I am expecting this output:

14FFE5DD23188FD5CB53A1D47F1289DB70ABF31E

Does anybody know what I might be doing wrong here? Could the problem lie with the un-readability of the end of the file? Do I need to parse this as hex first or something?

Answer 1

Make sure you don't have a newline at the end of the file, you may also want to make sure it ends with an 'e'.

The info-hash of a torrent file is the SHA-1 hash of the info-section (in bencoded form) from the .torrent file. Essentially you need to decode the file (it's bencoded) and remember the byte offsets where the content of the value associated with the "info" key begins and end. That's the range of bytes you need to hash.

For example, if this is the torrent file:

d4:infod6:pieces20:....................4:name4:test12:piece lengthi1024ee8:announce27:http://tracker.com/announcee

You wan to just hash this section:

d6:pieces20:....................4:name4:test12:piece lengthi1024ee

For more information on bencoding, see BEP3.

Answer 2

SHA1 calculation is just as simple as what you've written, more or less. The error is probably in the data you're feeding it, if you get the wrong answer from the library function.

I can't speak to the torrent file prep work you've done, but I do see a few problems. If you'll revisit the SHA1 docs, notice the SHA1 function never requires its own digest length as a parameter. Next, you'll want to be quite certain the technique you're using to read the file's contents is faithfully sucking up the exact bytes, no translation.

A less critical style suggestion: make use of the third parameter to SHA1. General rule, static storage in the library is best avoided. Always prefer to supply your own buffer. Also, where you have a hard-coded 20 in your print function, that's a marvelous place for that digest length constant you've been flirting with.