PoliCTF 2015 - It’s Hungry (Forensics 100)

A troll challenge that required you to transcribe a melody on a hidden area of the website.

Challenge Description

Points

100

Description

Old McDonald had a farm. Old McDonald liked chiptune. He also needed to remind
its daughter to take care about a zombie animal. But he wanted to do it
discreetly, so he wrote this song. Can you find the message? (all lowercase, no
spaces) N.B. flag is not in format flag{.+}

Solution

This was the Hidden Challenge in the Scoreboard. If you clicked on the numbers in the music control at the top left of the screen, it’ll show.

The description is obviously referring to the chiptune music that’s being played in the background. So let’s download it and examine it.

$ wget http://polictf.it/tunes/oldmcdonald.flac
$ file oldmcdonald.flac
oldmcdonald.flac: FLAC audio bitstream data, 16 bit, stereo, 44.1 kHz, 6323646 samples

Great, it’s a FLAC file and there doesn’t seem to be anything immediately striking on a first listen. Maybe there’s something in the metadata tags.

$ metaflac --list oldmcdonald.flac
METADATA block #0
  type: 0 (STREAMINFO)
  is last: false
  length: 34
  minimum blocksize: 4096 samples
  maximum blocksize: 4096 samples
  minimum framesize: 3729 bytes
  maximum framesize: 14093 bytes
  sample_rate: 44100 Hz
  channels: 2
  bits-per-sample: 16
  total samples: 6323646
  MD5 signature: 74cf7c267ccb4fbec7c6a38297808d8b
METADATA block #1
  type: 4 (VORBIS_COMMENT)
  is last: true
  length: 168
  vendor string: reference libFLAC 1.3.0 20130526
  comments: 7
    comment[0]: GENRE=Chiptune
    comment[1]: TRACKNUMBER=0
    comment[2]: ALBUM=PoliCTF15
    comment[3]: Flag=Yeah, dream on!
    comment[4]: TITLE=oldmcdonald
    comment[5]: DATE=2015
    comment[6]: ARTIST=rbino

Haha, there’s nothing in here and we got trolled. So maybe let’s take a look in Audacity. Very often, CTFs will hide images visible when viewing the audio in a spectrogram. And there are three images!

We should keep listening…

Translates to: YOUAREOVERCOMPLICATINGJUSTLISTEN

Getting trolled

Okay, looks like all signs are telling us to LISTEN. So that’s what we did. For hours.

Then, I thought, what if I transcribed the notes in the melody? There could be a message in there right? So I fired up PulseBoy and spent about half an hour matching notes by ear.

I finally managed to get a piece that matched. It had the following notes (number is the octave):

    F4 E4 E4 D4 D4 A4 D4 E4 A4 D4 B3 E E4 F4

So our message was: feeddadeadbeef.

We submitted that and got our 100 points :)

Flag: feeddadeadbeef