---
title: neverlanCTF 2020 - [Forensic] Listen to this (125pts)
author: Maltemo
tags: CTF, neverlanCTF, Forensic, mp3
---
neverlanCTF 2020 - [Forensic] Listen to this (125pts)
===
Written by [Maltemo](https://twitter.com/Maltemo), member of team [SinHack](https://sinhack.blog/).

[TOC]

## Statement of the challenge

### Description

You hear that?

* Your flag will be in the normal flag{flagGoesHere] syntax

- ps This guy might be important

- ZestyFE

### File

HiddenAudio.mp3

## Analyzing the file

After listening to the audio of the mp3, I can recognize two things :
* The sweet voice of [Jack Rhysider](https://twitter.com/JackRhysider) coming from on of his podcast (DarknetDiaries) episode.
* Morse sound in the background

I assumed that the flag was hidden in the morse. But in order to decode it, I had to isolate that sound.

In order to manipulate the sound, I used audacity.

First step consists in importing the music file in audacity.

If you want to isolate a partcular sound, you have to fond the the exact same mp3, without what you're searching to isolate.

Example : To isolate a voice in a music, you take the original music and the karaoke/instrumental version.

By inverting the sound waves of the instrumental version and merging the two audio files, you will be able to remove the background music and only get the voice.

Here, we are searching for the original podcast in order to isolate the morse sound.

By searching the first setences of this audio file on google, I found the [initial podcast and its retranscription](https://darknetdiaries.com/transcript/22.html) :


Thanks to the soundcloud iframe, I was able to download the original mp3 podcast.

I found a good tutorial about isolating vocals with audacity on this website :
https://www.howtogeek.com/61250/how-to-isolate-and-save-vocals-from-music-tracks-using-audacity/

1. You have to __PERFECTLY__ synchronize the two soundtracks.
2. Invert the the original soundtrack (not the one with morse) with `Effects > Invert`.
3. Select the two audio tracks and merge them with `Tracks > Mix & Render`

With this result, you should obtain the morse sound with more or less parasite in function of how precise you were during step 1.

In order to have a more visual view of the morse code, I used `sonic-visualiser` with the spectrogram view (`Layer > Add Spectrogram > All chanels mixed`).

_The three first letters were : F L A_

Then, I used the morse code and was able to decode the message.


And I got this : FLAGDITSANDDAHSFORLIFE

## TL;DR

After analizing a mp3 file, we can hear some morse sound in the background. After isolating and amplifying the sound, we are able to manualy decode it.

## Flag

The flag is **flag{DITSANDDAHSFORLIFE}**
___
<a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-nd/4.0/88x31.png" /></a><br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License</a>.