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

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

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):

LSB-encoded (sounds identical):

Frequency-band encoded (faint high-frequency tones):

Recovering text

recovered = md.steg_decode(stego, sample_rate=44100.0, method=md.STEG_LSB)
print(recovered)  # "secret message"

Binary data

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

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

cap = md.steg_capacity(44100, sample_rate=44100.0, method=md.STEG_LSB)
print(f"Can hide up to {cap} bytes")

md.shutdown()