audiocat

Audiocat is a command-line utility for Linux that reads and writes encrypted data across peer-to-peer or broadcast audio connections, using minimodem and gpg.

It is a powerful tool that can be combined with any audio infrastructure (like PSTN, cellular network, internet, radio, walkie-talkies) to provide a secure communication channel through an audio tunnel.

The audio interfaces behave like data-diodes, each allowing unidirectional data transmission only, thus preventing data-leaks and malware-injection.

This enables an "enhanced"-end-to-end encryption (E-E2EE) which notably increases security and privacy, especially when the end devices are completely offline (air-gapped-system), thus providing an effective barrier against "legal or illegal" client-side-scanning!

See also https://www.codeproject.com/Articles/5295970/Audio-Chat-for-Quite-Good-Privacy-AC4QGP

Installation

  pip install audiocat-clark

or with git:

  git clone https://github.com/ClarkFieseln/audiocat.git

  cd audiocat_clark

  chmod +x audiocat

  chmod +x *.sh

during first execution you will be asked to install dependencies: minimodem, gpg, bc

but you can also install them yourself with:

  sudo apt install minimodem
  sudo apt install gpg
  sudo apt install bc

The package gnome-terminal is assumed to be already installed, install otherwise or change cfg/terminal.

How to use

Chat/Messenger

  audiocat -c

if installed with git:

  python3 audiocat.py -c
  # or
  ./audiocat -c

enter and confirm password

On the other device a chat or a remote shell can be started.

Remote Shell

  audiocat -s

if installed with git:

  python3 audiocat.py -s
  # or
  ./audiocat -s

then enter and confirm password

On the other device a chat shall be started to command the remote shell.

Note that this is technically a "reverse shell" which gives access to your system!

File Transfer

  audiocat -f

if installed with git:

  python3 audiocat.py -f
  # or
  ./audiocat -f

enter and confirm password

On the other device a file transfer shall be started.

Probe

To check connectivity and adjust volumes if required.

  audiocat -p

if installed with git:

  python3 audiocat.py -p
  # or
  ./audiocat -p

In addition, a separate terminal will be opened to read unencrypted probe messages being sent by the other side.

Configuration

Adapt the configuration as required editing the files in the cfg folder. The 'Location' can be found with:

  audiocat -d

if installed with git:

  python3 audiocat.py -d
  # or
  ./audiocat -d

The most important settings are:

• baud
• keep_alive_time_sec
• retransmission_timeout_sec
• split_tx_lines
• volume_microphone
• volume_speaker_left
• volume_speaker_right

For more information check the documentation.

Features

on top of the audio modem provided by minimodem and encryption provided by GPG, audiocat offers a reliable transport layer and many other features:

• modes: chat, remote-shell, file transfer (future: sniffer)

• full-duplex communication

• retransmit messages automatically after communication errors

• split big messages into smaller data chunks in order to increase the probability of reception, thus reducing retransmissions

• [keepalive] messages

• redundant transmission of "data-messages"

• composition of piped commands hidden to the user

• tmp folder located in a configurable path beneath $HOME, independent of the current path.

• probe, to check volume on receiver and adjust manually if needed

  (very high and very low volumes may produce signal distortions)

• "braodcast" transmissions also possible, e.g. when ACKs are deactivated

  use-case: walkie-talkie, Radio station, ...

• several configuration options: preamble, trailer, delays, cipher algorithm, confidence, log to file, verbose, etc.

Possible Abuses

please don't do the following if you are not allowed (it might be illegal!):

• exfiltrate data over the air or cable to a nearby or remote computer

• remote control over the air or cable from a nearby or remote computer

• exfiltrate data from a computer evading classical auditing

  (be aware that if you do this on your employer's computer you might be infringing the law!)

• use the tool as a "side-channel" for covert communication e.g. to spread or inject malware,

  even worse when combined with steganography (e.g. low volumes, data hidden in noise)

Typical Configuration

A: audiocat in offline PC (Alice)

D: audiocat in offline PC (Bob)

B, C: smartphone with call session (mobile, messenger app, etc.)

diodes: audio connections (sink/speaker -> source/microphone)

Demo - Communication in Linux over Linphone

A: audiocat in offline PC (Alice)

D: audiocat in offline PC (Bob)

B, C: smartphone with Linphone call session

Demo - Communication in Termux over qTox

A: audiocat in offline smartphone with Termux (Alice)

D: audiocat in offline smartphone with Termux (Bob)

B, C: PCs with qTox call session

Demo - Communication in Linux over Walkie Talkies

Split Configuration

A: audiocat in offline PC (Alice)

D: audiocat in offline PC (Bob)

B1, B2, C1, C2: waklie-talkie

Limitations

The data transfer is usually done at low rates, typical of audio systems. Therefore, this tool is not adequate to transmit big files which may take a long time to complete.

PyPi Project

https://pypi.org/project/audiocat-clark

GitHub Project

https://github.com/ClarkFieseln/audiocat

Documentation

https://github.com/ClarkFieseln/audiocat/blob/main/doc/documentation.md

Screenshots

https://github.com/ClarkFieseln/audiocat/tree/main/screenshots

Videos

https://www.youtube.com/playlist?list=PLX24fhcibpHUx7ej_Tp4neobJUqOkqliN

License

(c) 2025 Clark Fieseln

This repository is licensed under the MIT license. See LICENSE for details.