This is a steganography tool enabling you to conceal any file within a PNG file. In order to compile the program you need libpng to be installed on the system. It is one of my personal projects and I would love to receive expert advice.
Running
Hide message in PNG file:
steg -h <file_in> <png_in> <png_out>
Read message from PNG file:
steg -r <png_in> <file_out>
The arguments <file_in> and <file_out> can be filenames of any (binary) files.
Code
I'm following Linus Torvalds's coding style.
In steg.c I have:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "png_io.h"
/* hide one byte in png */
void hide_byte(unsigned char byte, long i)
{
short bit;
for (bit = 0; bit < CHAR_BIT; ++bit) {
long abs_bit = i * CHAR_BIT + bit;
int y = abs_bit / channels / width;
int x = abs_bit % (width * channels);
png_byte *value = &row_pointers[y][x];
if (byte & (1 << (CHAR_BIT - bit - 1)))
*value = (*value & ~1) + 1; /* 1 */
else
*value = *value & ~1; /* 0 */
}
}
/* hide file contents in png */
void hide_file(char *filename, char *src_png_name, char *out_png_name)
{
FILE *fp = fopen(filename, "rb");
if (!fp) {
fprintf(stderr, "Error: failed to open file \"%s\"\n", filename);
exit(EXIT_FAILURE);
}
fseek(fp, 0, SEEK_END);
long fsize = ftell(fp);
fseek(fp, 0, SEEK_SET);
unsigned char buffer[4 + fsize];
buffer[0] = fsize >> 24;
buffer[1] = fsize >> 16;
buffer[2] = fsize >> 8;
buffer[3] = fsize;
fread(&buffer[4], fsize, 1, fp);
printf("sizeof(buffer) = %zu\n", sizeof(buffer));
read_png(src_png_name);
printf("width: %d\nheight: %d\nchannels: %d\n", width, height, channels);
if ((CHAR_BIT * sizeof(buffer)) > (width * height * channels)) {
fprintf(stderr, "Error: binary file doesn't fit into png file\n");
exit(EXIT_FAILURE);
}
long i;
for (i = 0; i < sizeof(buffer); ++i) {
hide_byte(buffer[i], i);
}
write_png(out_png_name);
fclose(fp);
}
/* read one byte from png */
void read_byte(unsigned char *byte, long i)
{
short bit;
for (bit = 0; bit < CHAR_BIT; ++bit) {
long abs_bit = i * CHAR_BIT + bit;
int y = abs_bit / channels / width;
int x = abs_bit % (width * channels);
png_byte *value = &row_pointers[y][x];
if (*value & 1)
*byte += 1 << (7 - bit);
}
}
/* read file contents from png */
void read_file(char *filename, char *png_name)
{
FILE *fp = fopen(filename, "wb");
if (!fp) {
fprintf(stderr, "Error: failed to open file \"%s\"", filename);
exit(EXIT_FAILURE);
}
read_png(png_name);
printf("width: %d\nheight: %d\nchannels: %d\n", width, height, channels);
long fsize = 0;
short ib;
for (ib = 0; ib < 32; ++ib) {
int y = ib / (width * channels);
int x = ib % (width * channels);
png_byte *value = &row_pointers[y][x];
if (*value % 2 == 1)
fsize += 1 << (31 - ib);
}
unsigned char buffer[4 + fsize];
memset(buffer, 0, sizeof(buffer));
printf("sizeof(buffer) = %zu\n", sizeof(buffer));
long i;
for (i = 0; i < sizeof(buffer); ++i) {
read_byte(&buffer[i], i);
}
fwrite(&buffer[4], fsize, 1, fp);
fclose(fp);
}
/* handle command line arguments */
int main(int argc, char *argv[])
{
if (argc == 5 && !strcmp(argv[1], "-h")) {
hide_file(argv[2], argv[3], argv[4]);
} else if (argc == 4 && !strcmp(argv[1], "-r")) {
read_file(argv[3], argv[2]);
} else {
printf("Hide message: %s -h <file_in> <png_in> <png_out>\n"
"Read message: %s -r <png_in> <file_out>\n",
argv[0], argv[0]);
}
return EXIT_SUCCESS;
}
In png_io.c I have:
#include <png.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include "png_io.h"
#define PNG_BYTES_TO_CHECK 4
/* global variables */
png_infop info_ptr;
png_bytepp row_pointers;
png_uint_32 width, height;
png_byte channels;
/* read png file */
void read_png(char *filename)
{
FILE *fp = fopen(filename, "rb");
if (!fp) {
fprintf(stderr, "Error: failed to open file '%s'\n", filename);
exit(EXIT_FAILURE);
}
/* read signature bytes */
unsigned char sig[PNG_BYTES_TO_CHECK];
if (fread(sig, 1, PNG_BYTES_TO_CHECK, fp) != PNG_BYTES_TO_CHECK) {
fprintf(stderr, "Error: failed to read signature bytes"
"from '%s'\n", filename);
exit(EXIT_FAILURE);
}
/* compare first bytes of signature */
if (png_sig_cmp(sig, 0, PNG_BYTES_TO_CHECK)) {
fprintf(stderr, "Error: '%s' is not a PNG file\n", filename);
exit(EXIT_FAILURE);
}
/* initialize png_struct `png_ptr` */
png_structp png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
NULL, NULL, NULL);
if (!png_ptr) {
fclose(fp);
fprintf(stderr, "Error: memory allocation failed\n");
exit(EXIT_FAILURE);
}
/* allocate memory for image information */
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr) {
fclose(fp);
png_destroy_read_struct(&png_ptr, NULL, NULL);
exit(EXIT_FAILURE);
}
/* set error handling */
if (setjmp(png_jmpbuf(png_ptr))) {
fclose(fp);
png_destroy_read_struct(&png_ptr, NULL, NULL);
exit(EXIT_FAILURE);
}
/* set up input control */
png_init_io(png_ptr, fp);
/* because we read some of the signature */
png_set_sig_bytes(png_ptr, PNG_BYTES_TO_CHECK);
/* read entire image into info structure */
png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
/* optain information */
row_pointers = png_get_rows(png_ptr, info_ptr);
width = png_get_image_width(png_ptr, info_ptr);
height = png_get_image_height(png_ptr, info_ptr);
channels = png_get_channels(png_ptr, info_ptr);
/* free allocated memory */
png_destroy_read_struct(&png_ptr, NULL, NULL);
fclose(fp);
}
/* write png file */
void write_png(char *filename)
{
FILE *fp = fopen(filename, "wb");
if (!fp) {
fprintf(stderr, "Error: failed to open file '%s'\n", filename);
exit(EXIT_FAILURE);
}
/* initialize png_struct `png_ptr` */
png_structp png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
NULL, NULL, NULL);
if (!png_ptr) {
fclose(fp);
fprintf(stderr, "Error: memory allocation failed\n");
exit(EXIT_FAILURE);
}
/* set up output control */
png_init_io(png_ptr, fp);
/* save new pixel values */
png_set_rows(png_ptr, info_ptr, row_pointers);
/* all image data in info structure */
png_write_png(png_ptr, info_ptr, PNG_TRANSFORM_IDENTITY, NULL);
/* free allocated memory */
png_destroy_write_struct(&png_ptr, &info_ptr);
fclose(fp);
}
In png_io.h I have:
#ifndef PNG_IO_H
#define PNG_IO_H
#include <png.h>
/* global variables */
extern png_bytepp row_pointers;
extern png_byte channels;
extern png_uint_32 width, height;
/* read png file */
void read_png(char *filename);
/* write png file */
void write_png(char *filename);
#endif /* PNG_IO_H */
