Bit extraction and Steganography

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);

}

Show the loop that calls extract - since you should demonstrate that you are correctly looping through the image. — borrible, Jul 03 '11 at 21:04

DaveR · Accepted Answer · 2011-07-03T21:10:28.303

2

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
}

edited Jul 03 '11 at 21:10

answered Jul 03 '11 at 20:59

DaveR

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This is a red herring; the OP is immediately overwriting `position` with `*image`... – Oliver Charlesworth Jul 03 '11 at 21:07
@Joe: I assume that you are calling extract more times than there are pixels in your image array. Can you see on which iteration you get the error? – DaveR Jul 03 '11 at 21:49
@Dave it is not the first one it is creating a file and just over runs the array – Joe Tyman Jul 04 '11 at 00:33
@Dave I also had to place the position++ in the loop. – Joe Tyman Jul 05 '11 at 13:30

score 2 · Answer 2 · edited May 23 '17 at 12:19

Dave Rigby's excellent diagnosis is correct, but passing position as a parameter (and not incrementing it here) would lead to easier to understand and more flexible routines:

char extract ( pixel* image, int position ) {
    char curChar = '\0';
    for(int i = 0; i<4; ++i) {
        curChar = curChar << 2;
        curChar = curChar | getbits(postion);
    }
    return curChar;
}

char *build_string(pixel *image) {
    int i;
    char *ret = malloc(SECRET_SIZE);
    for (i=0; i<SECRET_SIZE; i++) {
        ret[i]=extract(image, i);
    }
    ret[i] = '\0';
    return ret;
}

Then, when you realize that changing all the pixels in a line makes it pretty obvious, and you'd rather use pixels located at Fibonacci values, the change is easy to make:

char *build_string_via_fib(pixel *image) {
    int i;
    char *ret = malloc(SECRET_SIZE);

    for (i=0; i<SECRET_SIZE; i++) {
        ret[i]=extract(image, fib(i));
    }
    ret[i]='\0';
    return ret;
}

You could stuff the Fibonacci computation into your extract() routine too, but decomposing functions into the smallest, most useful, pieces, gives you excellent legibility, excellent testability, and best chances for future code re-use.

Bit extraction and Steganography

2 Answers2