Audio Steganography

Audio Steganography Analysis

A skill for extracting hidden data from audio files using various steganographic techniques.

Quick Triage

Start here for any audio steganography investigation:

# Confirm codec/container details
file audio_file

# Inspect audio metadata and format
ffmpeg -v info -i audio_file -f null -

# Generate spectrogram for visual inspection
sox audio_file -n spectrogram -o spectrogram.png

Key indicators to look for:

• Noise-like content that doesn't match the expected audio type
• Tonal structure or regular patterns in spectrograms
• Unusual codec choices or metadata anomalies
• Files that are larger than expected for their duration

Spectrogram Steganography

When to use

• Audio contains noise or unusual tonal patterns
• Spectrogram shows visible text or patterns
• File is flagged as suspicious but sounds normal

Tools and workflow

Sonic Visualiser (primary tool)

• Download: https://www.sonicvisualiser.org/
• Open audio file and enable spectrogram view
• Look for text, QR codes, or patterns in the frequency-time plot
• Adjust frequency range and window size for clarity

Audacity (alternative)

• Download: https://www.audacityteam.org/
• View → Spectrogram
• Use filters to enhance visibility
• Export spectrogram image for analysis

Command-line spectrogram

# Generate spectrogram with sox
sox input.wav -n spectrogram -o spectrogram.png

# With specific parameters for better visibility
sox input.wav -n spectrogram -o spectrogram.png \
  --size 2048 --overlap 0.9 --window 0.9

Analysis tips

• Spectrogram stego often hides data in frequency bands that are inaudible or perceived as noise
• Look for sharp edges, text-like patterns, or QR codes in the spectrogram
• Try different window sizes and frequency ranges
• Some implementations use specific frequency bands (e.g., 20-20000 Hz visible range)

FSK / Modem Decoding

When to use

• Spectrogram shows alternating single tones
• Audio sounds like modem handshakes or beeps
• Regular frequency shifts visible in spectrogram

Workflow with minimodem

# First, visualize to estimate baud rate and frequencies
sox noise.wav -n spectrogram -o spec.png

# Try common baud rates until printable text appears
minimodem -f noise.wav 45
minimodem -f noise.wav 300
minimodem -f noise.wav 1200
minimodem -f noise.wav 2400
minimodem -f noise.wav 4800
minimodem -f noise.wav 9600

Common minimodem options

# Invert signal if output is garbled
minimodem -f noise.wav 1200 --rx-invert

# Specify sample rate if needed
minimodem -f noise.wav 1200 --samplerate 44100

# Output to file
minimodem -f noise.wav 1200 > decoded.txt

# With verbose output for debugging
minimodem -f noise.wav 1200 --rx-verbose

Baud rate selection guide

• 45 baud: Very slow, old modems
• 300 baud: Common for simple FSK
• 1200 baud: Standard for many CTF challenges
• 2400-9600 baud: Faster modems, more complex audio

WAV LSB Extraction

When to use

• Uncompressed PCM WAV files
• Audio sounds normal but file size seems large
• Other techniques haven't revealed anything

Understanding LSB steganography

• Each audio sample is an integer value
• Modifying low bits changes waveform minimally (inaudible)
• Can hide 1+ bits per sample
• May be interleaved across channels
• May use stride/permutation patterns

WavSteg extraction

# Extract 1 bit per sample
python3 WavSteg.py -r -b 1 -s sound.wav -o out.bin

# Extract 2 bits per sample
python3 WavSteg.py -r -b 2 -s sound.wav -o out.bin

# Extract 3 bits per sample
python3 WavSteg.py -r -b 3 -s sound.wav -o out.bin

# Extract 4 bits per sample
python3 WavSteg.py -r -b 4 -s sound.wav -o out.bin

DeepSound tool

• Download: http://jpinsoft.net/deepsound/download.aspx
• GUI-based LSB extraction
• Supports various bit depths and patterns
• Can detect hidden data automatically

Other audio steganography families

• Phase coding: Modifies phase relationships between samples
• Echo hiding: Embeds data in echo patterns
• Spread-spectrum: Spreads data across frequency spectrum
• Codec side-channels: Format-dependent embedding (MP3, AAC, etc.)

DTMF / Dial Tone Decoding

When to use

• Audio resembles telephone keypad tones
• Regular dual-frequency beeps
• Sounds like someone dialing a phone

Online decoders

• https://unframework.github.io/dtmf-detect/
• http://dialabc.com/sound/detect/index.html

DTMF frequency reference

Row 1: 697 Hz, 770 Hz, 852 Hz, 941 Hz
Row 2: 1209 Hz, 1336 Hz, 1477 Hz, 1633 Hz
Row 3: 1770 Hz, 1975 Hz, 2184 Hz, 2400 Hz

DTMF character mapping

1: 697+1209    2: 697+1336    3: 697+1477    A: 697+1633
4: 770+1209    5: 770+1336    6: 770+1477    B: 770+1633
7: 852+1209    8: 852+1336    9: 852+1477    C: 852+1633
*: 941+1209    0: 941+1336    #: 941+1477    D: 941+1633

Investigation Workflow

Step 1: Initial triage

file audio_file
ffmpeg -v info -i audio_file -f null -

Step 2: Visual inspection

sox audio_file -n spectrogram -o spectrogram.png

Step 3: Technique selection

• Visible patterns in spectrogram → Spectrogram stego
• Alternating tones → FSK/modem decoding
• WAV file, normal audio → Try LSB extraction
• Keypad-like tones → DTMF decoding

Step 4: Extract and verify

• Run appropriate extraction tool
• Check output for readable content
• Try multiple bit depths/parameters if needed
• Verify extracted data makes sense

Common Tools Checklist

• ffmpeg - Audio inspection and conversion
• sox - Spectrogram generation and audio processing
• minimodem - FSK/modem decoding
• WavSteg.py - LSB extraction
• Sonic Visualiser - Spectrogram analysis
• Audacity - Audio editing and spectrogram view
• DeepSound - GUI LSB tool
• Online DTMF decoders - Dial tone analysis

Tips and Tricks

1. Always start with triage - Understanding the file format guides your approach
2. Generate spectrograms early - Visual patterns reveal the technique used
3. Try multiple parameters - LSB bit depth, baud rates, and window sizes vary
4. Check file size - Unusually large files may indicate hidden data
5. Listen carefully - Some stego is audible as noise or distortion
6. Save intermediate outputs - Spectrograms and decoded data for analysis
7. Combine techniques - Some files use multiple steganography methods

Example Scenarios

Scenario 1: CTF audio challenge

1. file challenge.wav → "RIFF WAV PCM 16-bit"
2. sox challenge.wav -n spectrogram -o spec.png → Shows text pattern
3. Use Sonic Visualiser to read hidden message from spectrogram

Scenario 2: Suspicious audio file

1. ffmpeg -v info -i suspicious.mp3 -f null - → Shows unusual metadata
2. sox suspicious.wav -n spectrogram -o spec.png → Shows alternating tones
3. minimodem -f suspicious.wav 1200 → Decodes hidden text

Scenario 3: Normal-sounding WAV with hidden data

1. file normal.wav → "RIFF WAV PCM 16-bit"
2. python3 WavSteg.py -r -b 1 -s normal.wav -o out.bin
3. file out.bin → "ASCII text" or "PNG image data"

References

• Sonic Visualiser: https://www.sonicvisualiser.org/
• Audacity: https://www.audacityteam.org/
• WavSteg: https://github.com/ragibson/Steganography#WavSteg
• DeepSound: http://jpinsoft.net/deepsound/download.aspx
• DTMF Decoder: https://unframework.github.io/dtmf-detect/
• Flagvent 2025 examples: https://0xdf.gitlab.io/flagvent2025/medium