Audio steganography: hide secret messages inside audio signals. More...

#include "minidsp.h"
#include "minidsp_internal.h"

Macros
#define	HEADER_BITS 32
	Bits needed for the message-length header (32-bit unsigned).
#define	PAYLOAD_TYPE_BIT (1u << 31)
	Bit 31 of the header: payload type flag (0 = text, 1 = binary).
#define	LENGTH_MASK 0x7FFFFFFFu
	Mask to extract the message length from the raw header (bits 0–30).
#define	FREQ_LO 18500.0
	Carrier for bit 0 (Hz).
#define	FREQ_HI 19500.0
	Carrier for bit 1 (Hz).
#define	CHIP_MS 3.0
	Duration of one bit chip (ms).
#define	TONE_AMP 0.02
	Additive tone amplitude.
#define	SPECTEXT_FREQ_LO 18000.0
	Bottom of visual band (Hz).
#define	SPECTEXT_FREQ_HI 23500.0
	Top of visual band (Hz, below Nyquist).
#define	SPECTEXT_COL_MS 30.0
	Column duration (ms).
#define	SPECTEXT_COLS_PER_CHAR 8
	Bitmap columns per character.
#define	SPECTEXT_TARGET_SR 48000.0
	Output sample rate (Hz).
#define	SPECTEXT_AMP 0.02
	Spectrogram art amplitude.
#define	SPECTEXT_NORM_PEAK 0.9
	MD_spectrogram_text peak.
#define	SPECTEXT_PAD_SEC 0.25
	Silence before spectrogram text (s).
#define	SPECTEXT_SEC_PER_CHAR (SPECTEXT_COL_MS / 1000.0 * SPECTEXT_COLS_PER_CHAR)
	Seconds per character in the spectrogram visual.

Functions
static double	clamp (double x)
	Clamp a double to [-1, 1].
static int	double_to_pcm16 (double sample)
	Convert a double sample in [-1, 1] to a signed 16-bit PCM value.
static double	pcm16_to_double (int pcm)
	Convert a signed 16-bit PCM value back to double in [-1, 1].
static unsigned	lsb_capacity (unsigned signal_len)
static unsigned	lsb_encode (const double host, double output, unsigned signal_len, const unsigned char *data, unsigned data_len, unsigned flags)
static unsigned	lsb_read_header (const double *signal)
	Read the raw 32-bit LSB header from the first HEADER_BITS samples.
static unsigned	lsb_decode (const double stego, unsigned signal_len, unsigned char data_out, unsigned max_len)
static unsigned	chip_samples (double sample_rate)
static unsigned	freq_capacity_cs (unsigned signal_len, unsigned cs)
static unsigned	freq_capacity (unsigned signal_len, double sample_rate)
static void	_bfsk_setup (double sample_rate)
void	md_steg_teardown (void)
	Tear down the BFSK sine carrier cache.
static void	encode_one_bit (double output, unsigned signal_len, unsigned chip_idx, unsigned cs, const double carrier)
	Encode one bit by adding a precomputed BFSK tone burst at the given chip.
static unsigned	freq_encode (const double host, double output, unsigned signal_len, double sample_rate, const unsigned char *data, unsigned data_len, unsigned flags)
static unsigned	decode_one_bit (const double stego, unsigned signal_len, unsigned chip_idx, unsigned cs, const double sin_lo, const double sin_hi, double corr_out)
	Decode one bit by correlating a chip against precomputed BFSK carriers.
static unsigned	freq_decode (const double stego, unsigned signal_len, double sample_rate, unsigned char data_out, unsigned max_len)
static unsigned	spectext_vis_capacity (double duration_sec)
	Maximum number of visually displayable characters for a given duration.
static unsigned	spectext_output_len (unsigned signal_len, double sample_rate)
	Compute the output signal length at 48 kHz for a given input.
static unsigned	spectext_capacity (unsigned signal_len, double sample_rate)
static unsigned	spectext_encode (const double host, double output, unsigned signal_len, double sample_rate, const unsigned char *data, unsigned data_len, unsigned flags)
static double	spectext_ultrasonic_rms (const double *signal, unsigned signal_len, double sample_rate)
	Probe for ultrasonic energy in the 18-24 kHz band.
unsigned	MD_steg_capacity (unsigned signal_len, double sample_rate, int method)
	Compute the maximum message length (in bytes) that can be hidden.
static unsigned	encode_common (const double host, double output, unsigned signal_len, double sample_rate, const unsigned char *data, unsigned data_len, int method, unsigned flags)
	Shared encode logic for both text and binary public API functions.
unsigned	MD_steg_encode_bytes (const double host, double output, unsigned signal_len, double sample_rate, const unsigned char *data, unsigned data_len, int method)
	Encode arbitrary binary data into a host audio signal.
unsigned	MD_steg_decode_bytes (const double stego, unsigned signal_len, double sample_rate, unsigned char data_out, unsigned max_len, int method)
	Decode binary data from a stego audio signal.
unsigned	MD_steg_encode (const double host, double output, unsigned signal_len, double sample_rate, const char *message, int method)
	Encode a secret message into a host audio signal.
unsigned	MD_steg_decode (const double stego, unsigned signal_len, double sample_rate, char message_out, unsigned max_msg_len, int method)
	Decode a secret message from a stego audio signal.
int	MD_steg_detect (const double signal, unsigned signal_len, double sample_rate, int payload_type_out)
	Detect which steganography method (if any) was used to encode a signal.

Variables
static double *	_bfsk_sin_lo = NULL
static double *	_bfsk_sin_hi = NULL
static unsigned	_bfsk_cs = 0

Detailed Description

Audio steganography: hide secret messages inside audio signals.

Three methods are provided:

LSB (Least Significant Bit) — encodes message bits in the lowest bit of a 16-bit PCM representation of each sample. High capacity, negligible audible distortion, but fragile to any signal processing.
Frequency-band modulation — encodes bits as the presence or absence of a near-ultrasonic tone (default 18.5 kHz / 19.5 kHz BFSK) in short time chips. Lower capacity but survives mild resampling and compression. Requires sample_rate >= 40000 Hz so the carrier frequencies remain below Nyquist.
Spectrogram text (spectext) — hybrid: LSB for machine-readable data + spectrogram text art in the 18-24 kHz ultrasonic band for visual verification. Auto-upsamples to 48 kHz. The visual channel uses a fixed 30 ms column width (240 ms per character).

Both methods prepend a 32-bit little-endian header before the payload. Bits 0–30 hold the message byte count; bit 31 is a payload type flag (0 = text, 1 = binary). This enables blind decoding without knowing the original message length, and allows MD_steg_detect() to identify the payload type.

Definition in file minidsp_steg.c.

Macro Definition Documentation

◆ CHIP_MS

#define CHIP_MS 3.0

Duration of one bit chip (ms).

Definition at line 192 of file minidsp_steg.c.

◆ FREQ_HI

#define FREQ_HI 19500.0

Carrier for bit 1 (Hz).

Definition at line 191 of file minidsp_steg.c.

◆ FREQ_LO

#define FREQ_LO 18500.0

Carrier for bit 0 (Hz).

Definition at line 190 of file minidsp_steg.c.

◆ HEADER_BITS

#define HEADER_BITS 32

Bits needed for the message-length header (32-bit unsigned).

Definition at line 37 of file minidsp_steg.c.

◆ LENGTH_MASK

#define LENGTH_MASK 0x7FFFFFFFu

Mask to extract the message length from the raw header (bits 0–30).

Definition at line 43 of file minidsp_steg.c.

◆ PAYLOAD_TYPE_BIT

#define PAYLOAD_TYPE_BIT (1u << 31)

Bit 31 of the header: payload type flag (0 = text, 1 = binary).

Definition at line 40 of file minidsp_steg.c.

◆ SPECTEXT_AMP

#define SPECTEXT_AMP 0.02

Spectrogram art amplitude.

Definition at line 394 of file minidsp_steg.c.

◆ SPECTEXT_COL_MS

#define SPECTEXT_COL_MS 30.0

Column duration (ms).

Definition at line 391 of file minidsp_steg.c.

◆ SPECTEXT_COLS_PER_CHAR

#define SPECTEXT_COLS_PER_CHAR 8

Bitmap columns per character.

Definition at line 392 of file minidsp_steg.c.

◆ SPECTEXT_FREQ_HI

#define SPECTEXT_FREQ_HI 23500.0

Top of visual band (Hz, below Nyquist).

Definition at line 390 of file minidsp_steg.c.

◆ SPECTEXT_FREQ_LO

#define SPECTEXT_FREQ_LO 18000.0

Bottom of visual band (Hz).

Definition at line 389 of file minidsp_steg.c.

◆ SPECTEXT_NORM_PEAK

#define SPECTEXT_NORM_PEAK 0.9

MD_spectrogram_text peak.

Definition at line 395 of file minidsp_steg.c.

◆ SPECTEXT_PAD_SEC

#define SPECTEXT_PAD_SEC 0.25

Silence before spectrogram text (s).

Definition at line 396 of file minidsp_steg.c.

◆ SPECTEXT_SEC_PER_CHAR

#define SPECTEXT_SEC_PER_CHAR (SPECTEXT_COL_MS / 1000.0 * SPECTEXT_COLS_PER_CHAR)

Seconds per character in the spectrogram visual.

Definition at line 399 of file minidsp_steg.c.

◆ SPECTEXT_TARGET_SR

#define SPECTEXT_TARGET_SR 48000.0

Output sample rate (Hz).

Definition at line 393 of file minidsp_steg.c.

◆ TONE_AMP

#define TONE_AMP 0.02

Additive tone amplitude.

Definition at line 193 of file minidsp_steg.c.

Function Documentation

◆ _bfsk_setup()

void _bfsk_setup ( double sample_rate )

static

Definition at line 227 of file minidsp_steg.c.

◆ chip_samples()

unsigned chip_samples ( double sample_rate )

static

Definition at line 195 of file minidsp_steg.c.

◆ clamp()

double clamp ( double x )

static

Clamp a double to [-1, 1].

Definition at line 57 of file minidsp_steg.c.

◆ decode_one_bit()

unsigned decode_one_bit	(	const double *	stego,
		unsigned	signal_len,
		unsigned	chip_idx,
		unsigned	cs,
		const double *	sin_lo,
		const double *	sin_hi,
		double *	corr_out )

static

Decode one bit by correlating a chip against precomputed BFSK carriers.

Optionally returns the winning correlation magnitude via corr_out.

Definition at line 307 of file minidsp_steg.c.

◆ double_to_pcm16()

int double_to_pcm16 ( double sample )

static

Convert a double sample in [-1, 1] to a signed 16-bit PCM value.

Uses rounding (not truncation) so the conversion survives a double → float → double WAV roundtrip.

Definition at line 67 of file minidsp_steg.c.

◆ encode_common()

unsigned encode_common	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		double	sample_rate,
		const unsigned char *	data,
		unsigned	data_len,
		int	method,
		unsigned	flags )

static

Shared encode logic for both text and binary public API functions.

Definition at line 588 of file minidsp_steg.c.

◆ encode_one_bit()

void encode_one_bit	(	double *	output,
		unsigned	signal_len,
		unsigned	chip_idx,
		unsigned	cs,
		const double *	carrier )

static

Encode one bit by adding a precomputed BFSK tone burst at the given chip.

Definition at line 257 of file minidsp_steg.c.

◆ freq_capacity()

unsigned freq_capacity	(	unsigned	signal_len,
		double	sample_rate )

static

Definition at line 209 of file minidsp_steg.c.

◆ freq_capacity_cs()

unsigned freq_capacity_cs	(	unsigned	signal_len,
		unsigned	cs )

static

Definition at line 200 of file minidsp_steg.c.

◆ freq_decode()

unsigned freq_decode	(	const double *	stego,
		unsigned	signal_len,
		double	sample_rate,
		unsigned char *	data_out,
		unsigned	max_len )

static

Definition at line 326 of file minidsp_steg.c.

◆ freq_encode()

unsigned freq_encode	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		double	sample_rate,
		const unsigned char *	data,
		unsigned	data_len,
		unsigned	flags )

static

Definition at line 266 of file minidsp_steg.c.

◆ lsb_capacity()

unsigned lsb_capacity ( unsigned signal_len )

static

Definition at line 85 of file minidsp_steg.c.

◆ lsb_decode()

unsigned lsb_decode	(	const double *	stego,
		unsigned	signal_len,
		unsigned char *	data_out,
		unsigned	max_len )

static

Definition at line 144 of file minidsp_steg.c.

◆ lsb_encode()

unsigned lsb_encode	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		const unsigned char *	data,
		unsigned	data_len,
		unsigned	flags )

static

Definition at line 92 of file minidsp_steg.c.

◆ lsb_read_header()

unsigned lsb_read_header ( const double * signal )

static

Read the raw 32-bit LSB header from the first HEADER_BITS samples.

Definition at line 133 of file minidsp_steg.c.

◆ MD_steg_capacity()

unsigned MD_steg_capacity	(	unsigned	signal_len,
		double	sample_rate,
		int	method )

Compute the maximum message length (in bytes) that can be hidden.

Parameters

signal_len	Length of the host signal in samples.
sample_rate	Sample rate in Hz.
method	MD_STEG_LSB, MD_STEG_FREQ_BAND, or MD_STEG_SPECTEXT.

Returns: Maximum message bytes that fit (excluding null terminator).

Definition at line 571 of file minidsp_steg.c.

◆ MD_steg_decode()

unsigned MD_steg_decode	(	const double *	stego,
		unsigned	signal_len,
		double	sample_rate,
		char *	message_out,
		unsigned	max_msg_len,
		int	method )

Decode a secret message from a stego audio signal.

Reads the 32-bit length header, then extracts message bytes using the same method that was used for encoding.

Parameters

stego	The stego signal containing the hidden message.
signal_len	Length of the stego signal in samples.
sample_rate	Sample rate in Hz.
message_out	Output buffer for the decoded message (caller-allocated).
max_msg_len	Size of `message_out` buffer (including null terminator).
method	MD_STEG_LSB, MD_STEG_FREQ_BAND, or MD_STEG_SPECTEXT.

Returns: Number of message bytes decoded (0 if none found).

char recovered[256];
unsigned n = MD_steg_decode(stego, 44100, 44100.0,
                            recovered, 256, MD_STEG_LSB);
printf("Hidden message: %s\n", recovered);

Definition at line 656 of file minidsp_steg.c.

◆ MD_steg_decode_bytes()

unsigned MD_steg_decode_bytes	(	const double *	stego,
		unsigned	signal_len,
		double	sample_rate,
		unsigned char *	data_out,
		unsigned	max_len,
		int	method )

Decode binary data from a stego audio signal.

Works like MD_steg_decode() but writes raw bytes without null termination, suitable for recovering binary payloads.

Parameters

stego	The stego signal containing the hidden data.
signal_len	Length of the stego signal in samples.
sample_rate	Sample rate in Hz.
data_out	Output buffer for the decoded bytes (caller-allocated).
max_len	Maximum number of bytes to write to `data_out`.
method	MD_STEG_LSB, MD_STEG_FREQ_BAND, or MD_STEG_SPECTEXT.

Returns: Number of data bytes decoded (0 if none found).

unsigned char recovered[1024];
unsigned n = MD_steg_decode_bytes(stego, 44100, 44100.0,
                                  recovered, 1024, MD_STEG_LSB);
// recovered[0..n-1] contains the original binary data

Definition at line 625 of file minidsp_steg.c.

◆ MD_steg_detect()

int MD_steg_detect	(	const double *	signal,
		unsigned	signal_len,
		double	sample_rate,
		int *	payload_type_out )

Detect which steganography method (if any) was used to encode a signal.

Probes the signal for a valid steganographic header using both LSB and frequency-band (BFSK) methods. If a valid header is found, returns the method identifier and optionally the payload type (text or binary).

BFSK detection is only attempted when sample_rate >= 40000 Hz. If both methods appear valid, BFSK is preferred (lower false-positive rate).

Parameters

signal	The signal to inspect.
signal_len	Length of the signal in samples.
sample_rate	Sample rate in Hz.
payload_type_out	If non-null, receives MD_STEG_TYPE_TEXT or MD_STEG_TYPE_BINARY when a method is detected.

Returns: MD_STEG_LSB, MD_STEG_FREQ_BAND, MD_STEG_SPECTEXT, or -1 if no steganographic content is detected.

int payload_type;
int method = MD_steg_detect(signal, signal_len, 44100.0, &payload_type);
if (method >= 0) {
    printf("Detected %s payload (%s)\n",
           payload_type == MD_STEG_TYPE_BINARY ? "binary" : "text",
           method == MD_STEG_LSB ? "LSB" : "BFSK");
}

Definition at line 670 of file minidsp_steg.c.

◆ MD_steg_encode()

unsigned MD_steg_encode	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		double	sample_rate,
		const char *	message,
		int	method )

Encode a secret message into a host audio signal.

The host signal is copied to output, then the message is embedded using the selected method. A 32-bit length header is prepended so that MD_steg_decode() can recover the message without knowing its length in advance.

MD_STEG_LSB — flips the least-significant bit of a 16-bit PCM representation of each sample. Distortion is at most ±1/32768 (≈ −90 dB). Works at any sample rate.
MD_STEG_FREQ_BAND — adds a low-amplitude BFSK tone in the near-ultrasonic band (18.5 / 19.5 kHz). Requires sample_rate >= 40000 Hz so the carriers remain below Nyquist.
MD_STEG_SPECTEXT — hybrid: LSB data + spectrogram text art in the 18–23.5 kHz band. Auto-upsamples to 48 kHz; output buffer must be sized for the 48 kHz signal length when input rate < 48 kHz.

Parameters

host	Input host signal (not modified).
output	Output stego signal (caller-allocated; for MD_STEG_SPECTEXT with sample_rate < 48 kHz, must hold MD_resample_output_len(signal_len, sample_rate, 48000) samples).
signal_len	Length of host and output in samples.
sample_rate	Sample rate in Hz.
message	Null-terminated message string to hide.
method	MD_STEG_LSB, MD_STEG_FREQ_BAND, or MD_STEG_SPECTEXT.

Returns: Number of message bytes encoded (0 on failure).

double host[44100], stego[44100];
MD_sine_wave(host, 44100, 0.8, 440.0, 44100.0);
unsigned n = MD_steg_encode(host, stego, 44100, 44100.0,
                            "secret", MD_STEG_LSB);

Definition at line 646 of file minidsp_steg.c.

◆ MD_steg_encode_bytes()

unsigned MD_steg_encode_bytes	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		double	sample_rate,
		const unsigned char *	data,
		unsigned	data_len,
		int	method )

Encode arbitrary binary data into a host audio signal.

Works like MD_steg_encode() but accepts a raw byte buffer instead of a null-terminated string, so it can embed data containing 0x00 bytes (e.g. images, compressed archives, cryptographic keys).

Parameters

host	Input host signal (not modified).
output	Output stego signal (caller-allocated, same length).
signal_len	Length of host and output in samples.
sample_rate	Sample rate in Hz.
data	Pointer to the binary data to hide.
data_len	Length of `data` in bytes.
method	MD_STEG_LSB, MD_STEG_FREQ_BAND, or MD_STEG_SPECTEXT.

Returns: Number of data bytes encoded (0 on failure).

double host[44100], stego[44100];
MD_sine_wave(host, 44100, 0.8, 440.0, 44100.0);
unsigned char png_data[332];
// ... read PNG file into png_data ...
unsigned n = MD_steg_encode_bytes(host, stego, 44100, 44100.0,
                                  png_data, 332, MD_STEG_LSB);

Definition at line 616 of file minidsp_steg.c.

◆ md_steg_teardown()

void md_steg_teardown ( void )

Tear down the BFSK sine carrier cache.

Called only from MD_shutdown().

Definition at line 247 of file minidsp_steg.c.

◆ pcm16_to_double()

double pcm16_to_double ( int pcm )

static

Convert a signed 16-bit PCM value back to double in [-1, 1].

The result is snapped to float precision so that a WAV write/read cycle (which stores IEEE float) preserves the exact value.

Definition at line 80 of file minidsp_steg.c.

◆ spectext_capacity()

unsigned spectext_capacity	(	unsigned	signal_len,
		double	sample_rate )

static

Definition at line 419 of file minidsp_steg.c.

◆ spectext_encode()

unsigned spectext_encode	(	const double *	host,
		double *	output,
		unsigned	signal_len,
		double	sample_rate,
		const unsigned char *	data,
		unsigned	data_len,
		unsigned	flags )

static

Definition at line 428 of file minidsp_steg.c.

◆ spectext_output_len()

unsigned spectext_output_len	(	unsigned	signal_len,
		double	sample_rate )

static

Compute the output signal length at 48 kHz for a given input.

Definition at line 412 of file minidsp_steg.c.

◆ spectext_ultrasonic_rms()

double spectext_ultrasonic_rms	(	const double *	signal,
		unsigned	signal_len,
		double	sample_rate )

static

Probe for ultrasonic energy in the 18-24 kHz band.

Computes RMS of the signal bandpass-filtered to that range using a simple DFT bin summation approach.

Definition at line 515 of file minidsp_steg.c.

◆ spectext_vis_capacity()

unsigned spectext_vis_capacity ( double duration_sec )

static

Maximum number of visually displayable characters for a given duration.

Subtracts the leading pad from the available time.

Definition at line 404 of file minidsp_steg.c.

Variable Documentation

◆ _bfsk_cs

unsigned _bfsk_cs = 0

static

Definition at line 225 of file minidsp_steg.c.

◆ _bfsk_sin_hi

double* _bfsk_sin_hi = NULL

static

Definition at line 224 of file minidsp_steg.c.

◆ _bfsk_sin_lo

double* _bfsk_sin_lo = NULL

static

Definition at line 223 of file minidsp_steg.c.

Macros

Functions

Variables

Detailed Description

Macro Definition Documentation

◆ CHIP_MS

◆ FREQ_HI

◆ FREQ_LO

◆ HEADER_BITS

◆ LENGTH_MASK

◆ PAYLOAD_TYPE_BIT

◆ SPECTEXT_AMP

◆ SPECTEXT_COL_MS

◆ SPECTEXT_COLS_PER_CHAR

◆ SPECTEXT_FREQ_HI

◆ SPECTEXT_FREQ_LO

◆ SPECTEXT_NORM_PEAK

◆ SPECTEXT_PAD_SEC

◆ SPECTEXT_SEC_PER_CHAR

◆ SPECTEXT_TARGET_SR

◆ TONE_AMP

Function Documentation

◆ _bfsk_setup()

◆ chip_samples()

◆ clamp()

◆ decode_one_bit()

◆ double_to_pcm16()

◆ encode_common()

◆ encode_one_bit()

◆ freq_capacity()

◆ freq_capacity_cs()

◆ freq_decode()

◆ freq_encode()

◆ lsb_capacity()

◆ lsb_decode()

◆ lsb_encode()

◆ lsb_read_header()

◆ MD_steg_capacity()

◆ MD_steg_decode()

◆ MD_steg_decode_bytes()

◆ MD_steg_detect()

◆ MD_steg_encode()

◆ MD_steg_encode_bytes()

◆ md_steg_teardown()

◆ pcm16_to_double()

◆ spectext_capacity()

◆ spectext_encode()

◆ spectext_output_len()

◆ spectext_ultrasonic_rms()

◆ spectext_vis_capacity()

Variable Documentation

◆ _bfsk_cs

◆ _bfsk_sin_hi

◆ _bfsk_sin_lo