• Category: crypto
• Points: 80
• Description:

Watching videos is fun! Hint: Stegano skills required. Service: https://procrastination.ctf.internetwache.org

Write-up

Given was a website including a song.webm file. Running mediainfo got us the hint, that there is a second audio trace inside the file. We used ffmpeg to receive the audio file in wav format:

ffmpeg -i song.webm -map 0:2 out.wav

After listening to it, we concluded that it must be some dial-up noise. So we ran some DTMF analysis.

multimon-ng -t wav -a DTMF out.wav

This gave us some numbers separated by zeros:

111 127 173 104 122 60 116 63 123 137 127 61 124 110 137 120 110 60 116 63 123

We knew that it must be something like IW{..}. So we looked in the ASCII Table and saw that this must be some OCT representation.

So the flag was IW{DR0N3S_W1TH_PH0N3S}.