Saturday, February 26, 2011


This is a project I've been wanting to do for a while, and I finally got around to putting the code together yesterday.

It's based on a project called JSteg, the idea of which is to simply alter JPEG data after the lossy compression phase has been completed.

Microsoft has a fairly good explanation of how JPEG works and this can be simplified into three main phases:

  1. Downsampling
  2. Transforming and quantising
  3. Lossless compression
The first phase represents the image in terms of brightness and chrominance, brightness being measured on a one dimensional scale, and chrominance on a two dimensional scale with one axis being red-green, and the other being blue-yellow. Experiments have shown that the brightness channel is what effects our perception of images the most - the JSteg page has an example of how far the chrominance channels can be downsampled  before noticable changes start to appear.

Transforming and quantising
The data is then subjected to a discrete cosine transform which effectively replaces the pixel values with frequency values. These are then quantised (smoothing off the rough edges if you like) and then rearranged in order to place all the resultant 0 values next to each other. You can imagine what the next step is now.

Lossless encoding
The adjacent zeroes are perfect fodder for run length encoding, followed by huffman coding. The resultant space savings come not only from direct reductions in the space used by chrominance values, but also a simplification of the data such that traditional compression can be used.

Hiding a 160 bit hash
This is actually the easiest part. All the complicated work is done in the lossy stage, so all that needs to be done to hide our hash is to alter the file before it is losslessly compressed. The basic operation is as follows:
  • Decompression
  • Altering the coefficients
  • Compression
The way we hide the data is overwriting the LSB of the coefficients, which results in a minor change spread across that part of the image. The advantage of saving a fixed length of data is that no extra metadata needs to be stored, so that any file that hasn't had anything encoded will still produce a hash, it just won't have any meaning, so it gives the encoding some amount of deniability.

You can download the project from here if you'd like to try it out for yourself.

Monday, February 14, 2011

Ściągnij z iply

Kolega mi polecił programa "IPLA" abym obejrzeć seriałów polskich. Oczywiście, to mi się spodobało, ale nie lubię objerzeć online - wolę ściągnąć a potem obejrzeć, abym nie wyczerpać internet.

Więc, uradziłem napisać swojego program, przez co mogłbym tak zrobić. Na szczęście było prosto, bo IPLA używa XML, a mogłem tworzeć jakiegoś XSLT który mi pomógł. Wreszice mi się udało, ale trudno użyć UTF-8 z stdin'a z windowsem - nie mogłem po prostu użyć fgets() lub scanf() a raczej potrezebowałem ReadConsoleW(), SetConsoleCP() i SetConsoleOutputCP().

Program ściągnij z iply można pobrać, więc spróbuj, i mów mi co masz na myśli!

Thursday, February 3, 2011

ASFDump: I'm sick of it

The plugin is dead in the water. XPCOM and NSPR lack documentation and working samples, and I've already spent enough time getting this project up and running. The command line app works, but as for the plugin, somebody else will have to make that.

The project can be found here, feel free to let me know what you think of it

Wednesday, February 2, 2011

ASFRecorder part 3: Making the plugin

This, I told myself, is the easy part - it's all smooth sailing from here!

No such luck unfortunately, although I found comfort in the knowledge that making a plugin for firefox meant I could actually believe that the specification was being somewhat followed. This part was much easier in comparison to the ASF parsing, so I can't really complain too much.

The way binaries are included in addons in firefox is fairly straightforward, and there are good tutorials (my plugin was based more or less on this one with a few adjustments to the makefile) so after the issues were sorted out I had my test plugin ready to go.

With the test plugin sorted, the next step was to add functionality to detect the loading of .asf files. There are a number of different plugins that download .flv files, and a quick look through the (copyrighted) source of these turned up extensive use of observer notifications, and a check against the (GPL licensed and therefore stealable) Tamper Data source confirmed this for me. A look over some of the firefox articles showed me how to extract the URLs, which leaves me set for the next steps:

  • Add observers to the test plugin
  • Cache .asf (and .wmv) file requests
  • Maaaaaaaaaaybe look at response headers to check for MIME types I like
  • Load the ASFRecorder and post processing code into the binary part of the module
  • Maybe add some callback options to give an idea of progress
  • Package up and start distributing
It looks fairly easy from here, maybe the callback stuff will be a pain, but I'm hoping I can get it all sorted by the weekend

Tuesday, February 1, 2011

ASFRecorder part 2: fixing up streaming output

The project is coming along quite well, and I mean quite well, and I'm at a stage where I'm almost ready to build a plugin framework to host a beta. Here's a quick run through what I've had to do so far to get the output to a point where more than just VLC will play

  • Remove all references to the extra streams that are filtered out, including removing a few headers, rebuilding the header section in the process
  • Add padding lengths  to each data packet (because packet sizes are fixed, the server actually breaks its own standard by not setting the padding lengths, so although this is a trivial exercise, it was more work than I wanted to do)
  • Add packet length values to a sparse minority of data packets (something like one in 100,000, but it breaks avidemux) by manually summing the payload lengths, and then setting the padding lengths accordingly
  • Remove the null data packets (once again, these are fixed length, so easy to detect and skip, but this not only breaks the standard, I strip about 10% off the size of the file once these are removed)
Was it worth it? Turns out trying to appease avidemux wasn't the best goal, as its wmv reader module looks quite seriously broken (maybe it doesn't read the keyframes but the output has tons of artifacts), but now the output works on that, vlc, windows media player, and totem movie player.

It should all be smooth sailing from here though - just putting the post processing routines into a single module, getting ASFRecorder up to scratch, possibly tying the two modules together (or maybe that can be the second release), then doing all the GPL documentation and putting the demo plugin together.

I just hope I've gotten the hard part out of the way now ;)