Audio Steganography
===================
Hide secret messages or binary data within audio signals so that casual
listeners hear only the original sound, while decoders can extract the
hidden payload.
Two methods
-----------
.. list-table::
:header-rows: 1
:widths: 12 18 18 27 25
* - Method
- Capacity
- Audibility
- Robustness
- Requirement
* - **LSB**
- ~1 bit/sample (~16 KB / 3 s @ 44.1 kHz)
- Inaudible (≈ −90 dB)
- Fragile (destroyed by lossy compression, resampling)
- Any sample rate
* - **Frequency-band**
- ~2.6 kbit/s (~121 bytes / 3 s @ 44.1 kHz)
- Above most listeners' hearing
- Moderate (survives mild noise)
- sample_rate ≥ 40 kHz
**LSB** flips the least-significant bit of a 16-bit PCM representation —
distortion ≈ −90 dB. Best for lossless pipelines (WAV, FLAC).
**Frequency-band** encodes data as brief BFSK tone bursts at 18.5 kHz
(bit 0) or 19.5 kHz (bit 1). Choose this when light interference is
expected.
Message structure
-----------------
Both methods prepend a **32-bit little-endian header**: bits 0–30 hold
the byte count, bit 31 indicates payload type (0 = text, 1 = binary).
This lets the decoder recover messages without prior knowledge of length.
Hiding text
-----------
.. code-block:: python
import pyminidsp as md
host = md.sine_wave(44100, amplitude=0.8, freq=440.0, sample_rate=44100.0)
stego, n = md.steg_encode(host, "secret message",
sample_rate=44100.0, method=md.STEG_LSB)
print(f"Encoded {n} bytes")
**Listen** — compare the host signal and the stego outputs:
*Original host (440 Hz sine):*
.. raw:: html
*LSB-encoded (sounds identical):*
.. raw:: html
*Frequency-band encoded (faint high-frequency tones):*
.. raw:: html
Recovering text
---------------
.. code-block:: python
recovered = md.steg_decode(stego, sample_rate=44100.0, method=md.STEG_LSB)
print(recovered) # "secret message"
Binary data
-----------
.. code-block:: python
data = b"\x00\x01\x02\xff\xfe\xfd"
stego, n = md.steg_encode_bytes(host, data, sample_rate=44100.0)
recovered = md.steg_decode_bytes(stego, sample_rate=44100.0)
assert recovered == data
Automatic detection
-------------------
.. code-block:: python
method, payload_type = md.steg_detect(stego, sample_rate=44100.0)
if method is not None:
print(f"Method: {'LSB' if method == md.STEG_LSB else 'Freq-band'}")
print(f"Type: {'text' if payload_type == md.STEG_TYPE_TEXT else 'binary'}")
Capacity check
--------------
.. code-block:: python
cap = md.steg_capacity(44100, sample_rate=44100.0, method=md.STEG_LSB)
print(f"Can hide up to {cap} bytes")
md.shutdown()