What Is Audacity?
Audacity is a free, open-source digital audio editor widely used in CTF (Capture The Flag) competitions for audio forensics, signal analysis, and data extraction.
It is especially powerful for challenges in:
- Forensics
- OSINT
- Miscellaneous (MISC)
- Steganography
- Reverse engineering
Audacity provides a visual waveform editor, spectrogram tools, filters, and a wide range of sound processing features. Many CTF flags are hidden in audio files using simple or creative techniques that Audacity can expose.
Basic Usage for CTF
1. Open an Audio File
Audacity supports:
- WAV
- MP3
- OGG
- FLAC
- AAC
- RAW formats (with correct settings)
Simply drag the file into the window or use File → Import → Audio.
2. Switch to Spectrogram View
Spectrogram view reveals frequencies, which often hide visual patterns.
Track → Spectrogram → Spectrogram Settings…
Or toggle:
Click ▼ next to track name → Spectrogram
3. Adjust Spectrogram Settings
Important for improving visibility:
- Range: 0–8000 Hz (or higher for encoded data)
- Window size: 2048 or 4096
- Color Range: 60–90 dB
These settings often reveal hidden text, QR codes, or shapes in the spectrogram.
4. Analyze Waveform Directly
Switch back to waveform if needed:
Click ▼ → Waveform
Zooming in can reveal:
- Morse code
- Binary pulses
- Amplitude-modulated patterns
- Reversed audio
5. Apply Filters & Effects
Useful filters:
- Normalize (makes quiet signals visible)
- Amplify (boosts hidden sounds)
- High Pass / Low Pass Filter (remove noise to reveal tones)
- Noise Reduction
- Reverse (frequent trick)
- Change Speed / Tempo (decode slowed/speeded signals)
How Audacity Is Used in CTF Challenges
1. Hidden Spectrogram Images
Most common.
CTF creators embed:
- text
- flags
- barcodes
- coordinates
- shapes
- memes
…into frequency spectrograms using tools like Sonic Visualizer or directly coding the waveform.
Solution:
- Switch to Spectrogram
- Adjust color + range
- Zoom out until image appears
2. Morse Code Hidden in Audio
Waveforms often contain:
- short = dot
- long = dash
You may need to:
- amplify
- reduce noise
- slow down
Decode manually or with a Morse tool.
3. Binary Encoded in Beeps or Clicks
Two-tone encoding or amplitude-based binary:
- High tone = 1
- Low tone = 0
- Loud = 1
- Soft = 0
Convert manually after identifying patterns.
4. Reversed Audio
The challenge may sound normal but contains reversed speech.
Fix:
Effect → Reverse
5. Speed / Pitch Manipulation
Common trick to obscure speech.
Try:
- Change Tempo
- Change Speed
- Change Pitch
Example:
Effect → Change Speed → 50%
6. DTMF Tones (telephone keypad tones)
Each button has two frequencies.
Use spectrogram to identify the pair:
- Frequencies → keypad digits → decode sequence → flag
7. SSTV (Slow-Scan Television) encoded images
Audio contains SSTV signals.
Steps:
- Export audio
- Decode with
qsstv,rx-sstv, or online tools
Often reveals:
- QR codes
- Flags written as images
8. Hidden audio in channels
Stereo files may hide:
- left-only data
- right-only data
- phase cancellation signals
Solution:
Split Stereo Track Mute one channel at a time
Or:
Tracks → Mix → Mix and Render
9. LSB Steganography in WAV
Sometimes bits are hidden in the least significant bits.
Audacity itself can’t decode LSB, but you can visualize strange noise patterns and export the file for stego tools (e.g., stegolsb, wavsteg).
Common Patterns Used in CTF Problems
| Pattern | Example | How Audacity Helps |
|---|---|---|
| Spectrogram image | Flags, QR codes | Switch to spectrogram view |
| Morse code | Tone beeps | Decode from waveform |
| Binary tones | High/low beep patterns | Extract bits from amplitude or pitch |
| Reversed speech | Backwards message | Effect → Reverse |
| Hidden audio channels | Left/right difference | Split stereo |
| SSTV signals | Image encoded in audio | Decode via SSTV |
| DTMF dialing | Phone tones | Identify frequency pairs |
| Speed manipulation | Fast/slow audio | Change Speed / Tempo |
| Inverted audio | Phase-flipped | Try invert or mix tracks |
Recommended Workflow for CTF Audio
- Open file in Audacity
- Check waveform visually
- Switch to spectrogram
- Adjust frequency range
- Look for hidden shapes
- Listen carefully
- Try common fixes:
- Reverse
- Slow down
- Noise reduce
- Normalize/amplify
- Inspect stereo channels separately
- Export for external tools (if needed)
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