We covered the fourth part of hardware hacking series where we used HackTheBox Signals for demonstration purposes. We had an audio file with .wav format which we discovered it contained SSTV encoded signals. We used special software named RX-SSTV decoder along with a virtual cable software (VB-Audio) to connect the audio file as input to the SSTV decoder and we were able to extract the underlying image file that contained the challenge flag.
Challenge Description
Some amateur radio hackers captured a strange signal from space. A first analysis indicates similarities with signals transmitted by the ISS. Can you decode the signal and get the information?
Initial Observation
Upon downloading the file, you’ll notice it’s a standard .wav waveform audio file.
Naturally, your first instinct might be to open it with a media player like VLC. But when you try that, you’ll notice:
The file plays, but produces no sound.
This immediately raises a flag: the audio file likely contains encoded data rather than actual human-audible audio. This could be data in the form of:
- Encoded text
- Transmitted imagery
- Signal-based communication (e.g., radio frequencies)
Investigating the Signal Type
Going back to the challenge description, I hit a wall initially, until I started researching terms like “radio signal ISS”.
One of the top results was an article about ISS SSTV (Slow-Scan Television), a method used by the International Space Station to transmit images via audio signals.
From that, we learn:
SSTV (Slow-Scan TV) is a method of transmitting still images over audio frequencies, commonly used in amateur radio and space communication.
So it’s now clear:
The audio file contains SSTV-encoded image data.
Tools You’ll Need
To decode the SSTV transmission, we need a tool that can read and convert these audio signals back into images.
Recommended tool: RX-SSTV
A Windows-based application that can decode SSTV signals in real time.
Tools You’ll Need
To decode the SSTV transmission, we need a tool that can read and convert these audio signals back into images.
Recommended tool: RX-SSTV
A Windows-based application that can decode SSTV signals in real time.
The Final Step: Decoding the Image
As VLC plays the audio:
- RX-SSTV will receive the signal via the virtual cable.
- It will decode the SSTV transmission into a visual image.
- The image will appear on RX-SSTV’s interface , containing the hidden flag.
