CTF Mugardos 2015 Writeup – Stego200

Statement:

AM 200 points

Now seriously, when it comes to listen to music, nothing better than the radio, or not?

Maybe we are too nostalgic.

Format of the answer:
– Uppercase and lowercase as they appear in the flag
– No spaces

Solution:

In this challenge they gave us an ogg file, nothing strange to the naked eye. Lets start openning it with Audacity:

estego1

We can see that the first two tracks are the original song but there is a third track, lets extract it and open it with Sonic Visualiser, app that has helped me a lot in the past with the typical text hidden in audio thanks to its layers:

stego3

As Im used to I add the spectrogram layer just in case there was some text and because I like it more, in this case we didn’t find any text but we were able to locate some audio in an specific part.

After thinking about several ways and doing several tests with software oriented to weatherfax that we already saw in other challenges I decided to try my luck with SSTV. For that we will play the audio with paplay while we set QSSTVE to listen, notice that you need to redirect the audio output to the input so QSSTV can receibe it, in my case using pulseaudio I did this using pavucontrol, in the input sections and the monitor mode interfaces.

Here we can see that QSSTV has recognized the signal as SSTV in Scottie 1 mode and begins to show the result:

stego4

When the audio finish to play we get the following image:

stego5

Calculate the SHA256 of the string “IWannaKnowPulpoKey” and we get the flag we need.

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