✅ What is SoX?
SoX (Sound eXchange) is a powerful command-line tool used to:
- Convert audio formats
- Analyze waveforms
- Extract channels
- Modify audio speed, tone, and frequency
- Visualize or transform audio data
In CTF (Capture The Flag) forensics challenges, SoX is one of the most important tools for solving audio-based problems, especially when flags are encoded through:
- Morse code
- Spectrograms
- Frequency modulation
- Hidden channels
- Speed changes
- Noise patterns
🛠️ Basic SoX Commands
1. Play an audio file
sox file.wav -d
2. Convert to WAV format
sox input.mp3 output.wav
Many CTF tasks require WAV format for analysis.
3. Extract the left or right audio channel
sox file.wav left.wav remix 1 sox file.wav right.wav remix 2
4. Change tempo or speed
sox file.wav output.wav tempo 1.2 sox file.wav output.wav speed 0.5
5. Generate a spectrogram
(VERY common in CTF)
sox file.wav -n spectrogram -o spec.png
Often the flag appears visually in the spectrogram.
6. Trim or cut audio
sox file.wav output.wav trim 0 5
Extract only the first 5 seconds.
7. Remove silence
sox file.wav output.wav silence 1 0.1 1%
8. Show audio file info
sox --i file.wav
Shows:
- Sample rate
- Bit depth
- Channels
- Duration
Useful for detecting anomalies.
🎯 How SoX Is Used in CTF Challenges
1. Spectrogram-based flags
One of the most popular CTF audio tricks:
- Creators encode text into audio frequency images.
- Use SoX to extract the spectrogram:
sox file.wav -n spectrogram -o output.png
➡️ View the resulting image and find the flag.
2. Channel-separated messages
Flags or messages may be hidden in:
- Left channel only
- Right channel only
- One channel reversed
Example:
sox challenge.wav left.wav remix 1
3. Reversed audio
Many challenges reverse the audio signal.
Use:
sox file.wav output.wav reverse
4. Speed manipulation
Flags may appear too fast or too slow.
Example:
sox file.wav slow.wav speed 0.5
or speed it up:
sox file.wav fast.wav speed 2
5. Morse code decoding
If the audio sounds like beeps, convert it to spectrogram or increase clarity:
sox file.wav morse.wav highpass 1000
Then decode with tools or manually.
6. Frequency-coded data
Sometimes:
- Each frequency = a number
- The amplitude encodes bits
- Hidden tonal patterns represent binary
Spectrograms help reveal:
- Binary patterns
- QR codes
- Text
- Shapes
🔍 Common CTF Patterns Using SoX
| Pattern | Description | How SoX Helps |
|---|---|---|
| Spectrogram flags | Flag appears in audio spectrogram | Generate spectrogram using sox … spectrogram |
| Hidden channels | Left/right channels differ | Extract channels using remix |
| Reversed audio | Message backwards | Apply reverse |
| Speed manipulation | Audio too fast/slow | Use speed or tempo |
| Morse code | Beeps encode dots and dashes | Filter or normalize audio |
| Frequency encoding | Bits stored in tones | Analyze via spectrogram |
| Noise layers | Noise masking hidden data | Apply equalizers, filtering |
📌 Tips for Using SoX in CTFs
- Always generate a spectrogram first — many flags are hidden visually.
- Try reversing the audio and adjusting speed.
- Check left/right channels individually.
- Use filters to isolate important frequencies.
- Convert everything to WAV for consistent analysis.
- Use SoX in combination with:
- Audacity
- Sonic Visualiser
- Python audio libraries
🧩 Recommended Workflow in CTF
- Convert to WAV:
sox input.* output.wav - Generate spectrogram:
sox output.wav -n spectrogram -o spec.png - Extract channels:
sox output.wav left.wav remix 1 - Try reversing or adjusting speed.
- Analyze spectrogram for visual clues.
- Look for binary, Morse, QR codes, or text patterns.
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