Thứ Bảy, 24 tháng 10, 2020

[Learning] Audio Forensics with Audacity


Hello, welcome to my personal blog. 😊

Today, I want to show you another useful tool for doing forensics, especially in audio forensics. It's about Audacity. Let's begin!

+ Explanation:

- Audio Forensics :

The field of forensic science relating to the acquisition, analysis, and evaluation of sound recordings that may ultimately be presented as admissible evidence in a court of law or some other official venue.The fundamentals of audio, and the processing techniques for forensic audio clarification.

- Audio Steganograpy 

The art of covertly embedding secret messages into digital audio. 

- Audio waveform:

Audio waveforms represent the shape and form of sound waves passing through a medium. Wolfram|Alpha enables audio waves of different forms, frequencies and durations to be visualized and heard. Different waves can be combined to create more complex sounds.

+ Audacity tool

Audacity is the premiere open-source audio file and waveform-viewing tool, and CTF challenge authors love to encode text into audio waveforms, which you can see using the spectogram view (although a specialized tool called Sonic Visualiser is better for this task in particular). Audacity can also enable you to slow down, reverse, and do other manipulations that might reveal a hidden message if you suspect there is one (if you can hear garbled audio, interference, or static).

+ Installing:

On Windows: here

On Ubuntu

      sudo add-apt-repository ppa:ubuntuhandbook1/audacitys                    sudo apt install audacity

 I decide to install this software on my Windows 10. Overview

+ Usage

The key to all of the methods that we will discuss is that we are going to exploit the Human Auditory System. Due to it anatomy, the human ear can pick up the vibrations of a membrane between the frequency range of 20 Hz and 20 kHz. The actual range for a person may vary based on various factors such as age, gender, and health.

Follow the tip above, the first feature we must discovery is : Resample

We should resample the provided file if it's size out of reach (20Hz-20kHz)

Amplify: Increase of reduce the volume of the file


An instresting and very useful fearture of Audacity

The Spectrogram View of an audio track provides a visual indication of how the energy in different frequency bands changes over time. The Spectrogram can show sudden onset of a sound, so it can often be easier to see clicks and other glitches or to line up beats in this view rather than in one of the waveform views.

Note: If you want to
 display the spectrum that contained the characters, change frequency between 17k and 18k.

Change Speed

As its name, this feature could change the speed of player slower or faster.


Check this link to listen Reverse Effect Sound
Reverse reverses the selected audio, so that the end of the audio will be heard first and the beginning last.


Vocal Reduction and Isolation
attempts to remove or isolate center-panned audio from a stereo track. Vocals are sometimes (but not always) recorded in this way. The simplest and quickest removal method subtracts one channel from the other, but the result will be (dual) mono

 Split Stereo Track

This feature could split the provided the main channel into 2 channels left and right. In some cases, we should work in each channel to find a key or to reduce noise.

High-Pass Filter: 

Passes frequencies above its cutoff frequency and attenuates frequencies below its cutoff frequency. This effect can therefore be used to reduce low frequency noise.

Use Audacity to hide a secret message

+ Paint, Photoshop or any software which can edit image
+ Coagula (image editor, making sound from those images
+ Audacity (of course)

Let's do this funny stuff !

Creat a .bmp file with Paint which contains your secret message

Render image to sound with Coagula

Save it

Try to open it with Audacity, Seem like other .wav "normal" file

View it via Spectrogram


Audacity in CTF:

Root-me .Challenge at : here

After dowload file,We received ch3.wav. Import it into Audacity

We almost can't define what kind of this sound, seems like it run too fast. Slow it down

This time is better. But the sound just make non-sense. After trying many effects, I make it work by using reverse feature, and can obviously listen this sound file.


Happy Learning

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