Question

I am using C# with RHash in order to calculate the btih hashes of of file.
Currently I’m using 3 tools in order to generate the btih hash:

1. rhash-1.2.9-src\bindings\mono with librhash-1.2.9-win dll
2. rhash-1.2.9-win32 command line tool
3. uTorrent

The problem is that every tool generates different btih signatures for the same file (the photo was taken by me, it’s royal free):

1:  2FF7858CC0A0B216C3676A807D619FA30101E45F
2:  E6F07BB3C3B3B67531C84E3452980698AC1B0DAA  A:\IMG_0400.JPG
3:  D0B96839A14A8C45BB81AD157805AE73425998E5

For the C# hash generation I use Hasher.GetHashForFile(f.Name, HashType.BTIH); and rhash --bith in the cmd tool.

What am I doing wrong? Is there another way to calculate the btih?

Answer 1

The difference between the first two is that, according to the RHash source code, BTIH hashes requires additional data to be correctly computed.

The init_btih_data function in calc_sums.c is documented with:

Initialize BTIH hash function. Unlike other algorithms BTIH
requires more data for correct computation.

In test_hashes.c, the BTIH examples are actually treated differently depending on whether USE_BTIH_WITH_TEST_FILENAME has been defined.

That init_btih_data function (which only seems to get called when running the command line application) in turn calls the rhash_trasmit function a number of times depending on various parameters. At a minimum, it will call it twice, which probably explains the difference between the first two. It can, however, call it a number of other times, which I think explains the difference we see with uTorrent.

The difficulty is that while the unmanaged DLL exposes the rhash_trasmit function, the .NET bindings do not, which means that it is not possible to supply the additional data that is expected.