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Comment by ique | Hacker News Reader

As someone who has created many puzzle-hunts and participated in some, I really hope that the article is abbreviated because if they went from FFT to Wikipedia article in that few steps they are a truly amazing team (and I would feel bad).

Then again, it is DEFCON CTF they're trying to get into, so it does require some amazing problem-solving skills.

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I elided the hour or so spent trying to figure out what Power PAQ was. Google has no results for PAQ804; as the writeup says, a teammate eventually Binged it. I also elided the debugging needed to get the FFT to work (but you need to know what the decompressor is before deciding that it doesn't work!). I had commented out the line that applies a Hamming window to the FFT because I didn't understand it, but it didn't work right until I added the window. My theory is that it worked OK until the "background" (i.e., the song) got louder.

PAQ is a series of compression algorithm tests led by Matt Mahoney and a ton of compression enthusiasts who keep tweaking PAQ algorithm (be it for better memory use/speed or most importantly: compression ratio on a corpus).

Checkout their fabulous journey at

http://mattmahoney.net/dc/

PAQ804 says that it is fourth modification to the initial PAQ8 archiver.

Well there weren't really many more steps to take. Between FFT and wikipedia article was only PAQ compression, which didn't require prior knowledge. And they had to ignore a few red herrings, but the spectrogram helped there. I wonder how the 30ms rate was ascertained, though - pure guess?

The 30ms rate could be ascertained by zooming in on the spectrogram and looking at the minimum width of a mark or space in the encoded data.

That's exactly what I did -- the spectrogram is from sndfile-spectrogram, not baudline. baudline helpfully draws a line parallel to the frequency axis and shows you what point on the time axis you're at, so measuring times is easy.

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