Question

I am playing around with steganography. I am trying to pull a text file from a image. I am able to read the file, get the bits, but I have an issue with the extraction of these bits.

int getbits( pixel p) {
    return p & 0x03;   
}

char extract ( pixel* image ) {
    static int postion;
    postion = 0;

    postion = *image;

    postion++;

    char curChar;
    curChar = '\0';
    for(int i = 0; i<4; ++i) {
        curChar = curChar << 2;
        curChar = curChar | getbits(postion);
    }
    return curChar;
}

Pixel is an unsigned char. I have loop that calls extract() and fputc(3) the return value. I feel like I am getting garbage from these bits. Which causes me to have massive (1.5 gig) txt files in return.

void decode( PgmType* pgm, char output[80] )
{
FILE*outstream;
int i, length;

outstream = fopen(output, "w");

if(!outstream)
{
    fatal("Could not open");
}
for(i=0; i < 16; ++i)
{
    length = length << 2;
    length = length | getbits(*pgm->image);
}
if ((length* 4) < (pgm->width * pgm->height))
{
    fatal("File Too Big");
}
for (i = 0 ;i<length; ++i)
{
    fputc(extract(pgm->image), outstream);

}
fclose(outstream);

}

Answer 1

You are only actually reading the first pixel in the image – [Edit] because while you are trying to use a static var to keep count, as Oli points out you’re immediately overwriting it.

Instead use position to track your count; but hold the data in another var:

Instead extract() should look something like:

char extract ( pixel* image )
{
   static int postion = 0;

   pixel data = image[position];

   postion++;

   // use 'data' for processing
}