Problem

Watching videos is fun!

Attachment: https://ctf.internetwache.org/files/crypto80.zip

Solved by 74 teams

Solution

We unpack and get a webm file. When opened, it plays one of the big hits from the 80s for 36 seconds. Let’s run mediainfo on it, we see that it contains one video track and two audio tracks, let’s extract the second audio track.

avconv -i song.webm -map 0:2 audio.wav

When playing it, we can hear phone dial noises, also known as DTMF. So we run DTMF recognition software.

multimon -t wav -a DTMF audio.wav

And we get the DTMF tones as output:

DTMF: 0
DTMF: 1
DTMF: 1
DTMF: 1
DTMF: 0
DTMF: 1
DTMF: 2
DTMF: 7
DTMF: 0
...snip...
DTMF: 0
DTMF: 1
DTMF: 2
DTMF: 3

We can observe that it seems to be 2-3 digits separated by 0. Let’s group them and remove the separating zeros:

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

Oh no, it’s those pesky octal numbers again! Let’s do what we did last time (code reuse FTW!).

import string

f = open('dtmf.txt')
line = f.readline().replace('\n', '')
print ''.join([chr(string.atoi(x, base=8)) for x in line.split(' ')])

(For some reason multimon didn’t get the last 3 digits, 125 = base8(}), but it wasn’t hard to figure out)

Flag: IW{DR0N3S_W1TH_PH0N3S}