Monday, March 11, 2013

RuCTF 2013 Quals - Stegano 300 writeup

Strange song
Who is the author of this song?
We are given a file. By file's header we identified that it's midi song.
Also in the header we have found interesting letters sequence: pwriyuajqscet hkbfxmlvdogn.
The sequence consists of 26 chars and contains of letters of the english alphabet without the letter "z", and space char. Letters of this sequence are unique, and that let us suppose that it's some kind of alphabet.
After listening this strange midi song, and trying to recoginze it with music recognition programms, we decided to open it with midi editor.
I've used Audacity to import midi file. Here is how opened midi file looks like:

After looking at the sequence of notes, I've noticed that long and short notes are alternating, and follow one after another, never overlap other. It gave me the idea that this is a message consisting of the notes. So I've wrote out all the notes from song. Totally 88 notes in 3 octaves.
a#3 f4 c5 d#5 | e4 f4 d#5 a4 | f4 f#4 a#3 d4 | g4 g3 f5 e5 | f4 g3 c#4 f4 | e3 c5 a#3 g#3 | g3 f5 e5 f4 | f3 g3 e4 f#4 | f4 d#5 e4 f#4 | d#4 f#3 f4 e3 | d#4 d#5 e3 c5 | d#4 f4 g#3 d#5 | a3 f4 g4 f5 | d#5 f3 f4 e5 | d#4 e4 e4 g3 | f5 e5 f4 e4 | f#4 d#4 b4 f4 | e4 d#5 f4 d5 | d#5 f4 c#4 e4 | f#3 a#3 f5 e5 | d#4 f4 e4 f#4 | g3 f5 e5 c#4

In 3 octaves of piano keyboard the song notes are laid between e3 and f5. It's exactly 26 notes.
e3 f3 f#3 g3 g#3 a3 a#3 b3 c4 c#4 d4 d#4 e4 f4 f#4 g4 g#4 a4 a#4 b4 c5 c#5 d5 d#5 e5 f5
So by replacing piano notes with letters from found sequence, I got a message:
a lot of hacking is playing with other people you know getting them to do strange things 
It's Steve Wozniak's quote. So he is an author of this strange song.

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