No cON Name CTF Quals 2014 - STEGOsaurus (300pts) writeup

The challenge description was: Can you hear it? Sounds like a flag... Note: The flag only contains lowercase characters.

The hint here was a giveaway of what to pay attention for, while playing the mp3 we notice some low frequency clicks. It doesn't sound like noise because of the repetitive pattern. Let's open the mp3 in Audacity and see what is going on by looking at the waveform:

We notice the first part of the mp3 is quite loud and noisy compared to the rest of it, there is this low frequency pattern 'clicking' in the background. Let's isolate it by removing higher frequencies from the mp3:

After zooming on the waveform we can definitely notice a pattern here, it looks a lot like morse code. Let's see:

-. -.-. -. ...-- -.-. -... ..-. -.. -.-. -.-. ---.. -.. --... .- ..--- ..... --... -.. ---.. ----- -.... ..--- ..... -.... ----- . ---.. ---.. -.. -.. --... -.. --... ..-. -.. -.... ..... -.. -.-. -.... ....- --... .-

After running this in a morse decoder we get the following key NCN3CBFDCC8D7A257D8062560E88DD7D7FD65DC647A and since the hint of the flag was to put it all in lowercase characters..

We got our flag:

ncn3cbfdcc8d7a257d8062560e88dd7d7fd65dc647a