The other day, for kicks, I wrote a small program to embed a hidden image within another image, and another to extract the hidden image. This is called steganography:
…the art and science of writing hidden messages in such a way that no-one apart from the sender and intended recipient even realizes there is a hidden message, a form of security through obscurity. By contrast, cryptography obscures the meaning of a message, but it does not conceal the fact that there is a message. [Wikipedia]
The programs use the GraphicsMagick variant of the ImageMagick library. (I had GM ready to go already because I used it for other projects.) If you want to play with these images yourself, the JPEG conversions on Flickr will not work; download this PNG instead.
Update: The code is now on github.
// Steganographic decoder, requires GraphicsMagick
// Copyright 2009 Christopher League
// This is free software; you may distribute & modify it under GNU GPLv3
#include <Magick++.h>
#include <iostream>
#include <cassert>
using std::cout;
using std::string;
using namespace Magick;
Quantum revealDisguisedColor(Quantum q)
{
// just keep lowest 2 bits, then amplify by 2^6
return (q & 3) << 6;
}
int main(int argc, char** argv)
{
InitializeMagick(*argv);
if(argc != 3) {
cout << "Usage: " << argv[0] << " input.png output.png\n";
exit(1);
}
const string samplePath = argv[1];
const string outputPath = argv[2];
cout << "Loading " << samplePath << '\n';
Image im (samplePath);
Geometry g = im.size();
cout << "Dimensions are " << (string)g << '\n';
cout << "Depth is " << im.depth() << '\n';
assert(im.type() == TrueColorType);
for(unsigned x = 0; x < g.width(); x++) {
for(unsigned y = 0; y < g.height(); y++) {
Color c = im.pixelColor(x, y);
c.redQuantum(revealDisguisedColor(c.redQuantum()));
c.greenQuantum(revealDisguisedColor(c.greenQuantum()));
c.blueQuantum(revealDisguisedColor(c.blueQuantum()));
im.pixelColor(x, y, c);
}
}
cout << "Writing " << outputPath << '\n';
im.write(outputPath);
return 0;
}
// Steganographic encoder, requires GraphicsMagick
// Copyright 2009 Christopher League
// This is free software; you may distribute & modify it under GNU GPLv3
#include <Magick++.h>
#include <iostream>
#include <cassert>
using std::cout;
using std::string;
using namespace Magick;
Quantum mergeQuanta(Quantum pub, Quantum priv)
{
// reduce private color to just 2 bits
priv >>= 6;
// zero out lowest 2 bits of public color
pub &= 0xFC;
// merge
return pub | priv;
}
Color mergeColors(Color pub, Color priv)
{
pub.redQuantum(mergeQuanta(pub.redQuantum(), priv.redQuantum()));
pub.greenQuantum(mergeQuanta(pub.greenQuantum(), priv.greenQuantum()));
pub.blueQuantum(mergeQuanta(pub.blueQuantum(), priv.blueQuantum()));
return pub;
}
int main(int argc, char** argv)
{
InitializeMagick(*argv);
if(argc != 4) {
cout << "Usage: " << argv[0] << " public.png private.png output.png\n";
exit(1);
}
const string publicPath = argv[1];
const string privatePath = argv[2];
const string outputPath = argv[3];
cout << "Loading " << publicPath << '\n';
Image pub (publicPath);
Geometry g = pub.size();
assert(pub.depth() <= 8);
assert(pub.type() == TrueColorType);
cout << "Loading " << privatePath << '\n';
Image priv (privatePath);
assert(priv.size() == g);
assert(priv.depth() <= 8);
assert(priv.type() == TrueColorType);
for(unsigned x = 0; x < g.width(); x++) {
for(unsigned y = 0; y < g.height(); y++) {
pub.pixelColor(x, y, mergeColors(pub.pixelColor(x, y),
priv.pixelColor(x, y)));
}
}
cout << "Writing " << outputPath << '\n';
pub.write(outputPath);
return 0;
}