Steganography

Original images cropped to same size

Image	Image to Hide

Image with Hidden Value hiding in lower 2 bits

Image with Hidden image	Hidden Image Extracted

Algorithm

The algorithm hides image2 inside image1. First it crops the images we want to combine so that they are the same size. Then it loops over all the pixels of image1 and keeps only the first 6 bits of every color, by dividing the pixel's color value with 2²=4, keeping the integer part of the result and multiplying it with 4, as:

newcolor1=floor(oldcolor1/4)*4

Each pixel's color of the combined image is calculated by adding the first 6 bits of each pixel's color of image1 to the first 6 places of the color's byte and the first 2 bits of each pixel's color of image2 to the last 2 places of the color's byte as:

combinedColor=newcolor1+newcolor2

Finally, to extract an image hidden in the last 2 bits of color of the combined image, the algorithm loops over all pixels of the combined image, extracts the last two bits of color and shifts them to the first two places of the byte representing the extracted image's color as:

Code

 //Steganography
function crop(image,width,height){
    //crop an image to the specified width and height
    var newImage= new SimpleImage(width,height);
    for (var pix of image.values()){
        if (pix.getX()<=width-1 && pix.getY()<=height-1){
            newImage.setPixel(pix.getX(),pix.getY(),pix);
        }
    }
    return newImage;
}

function pixchange(pixval,bitNo){
    //keep the first bitNo bits of the color value pixval, remove the remaining 8-bitNo bits
    pixval=Math.floor(pixval/Math.pow(2,8-bitNo))*Math.pow(2,8-bitNo);
    return pixval;
}


function chop2hide(image, bitNo){
    // keep the first bitNo bits of the colors of the image and return the image
    var imageNew= new SimpleImage(image.getWidth(),image.getHeight());
    for (var pix of image.values()){
        var pixNew=imageNew.getPixel(pix.getX(),pix.getY());
        pixNew.setRed(pixchange(pix.getRed(),bitNo));
        pixNew.setBlue(pixchange(pix.getBlue(),bitNo));
        pixNew.setGreen(pixchange(pix.getGreen(),bitNo));
    }
    return imageNew;
}

function shift(image,bitNo){
    // keep the first bitNo bits of the colors of the image, remove the rest and shift the first bitNo bits to the end of the byte 
    var imageNew=new SimpleImage(image.getWidth(), image.getHeight());
    for (var pix of image.values()){
        var pixNew=imageNew.getPixel(pix.getX(),pix.getY());
        pixNew.setRed(Math.floor(pix.getRed()/Math.pow(2,8-bitNo)));
        pixNew.setGreen(Math.floor(pix.getGreen()/Math.pow(2,8-bitNo)));
        pixNew.setBlue(Math.floor(pix.getBlue()/Math.pow(2,8-bitNo)));
    }
    return imageNew;
}

function newpv(p,q){
    // add numbers p and q and return an error message if their value exceeds 255
    if (p+q>255) print('color RGB value cannot be greater than 255!!');
    return p+q;
}

function combine(image1,image2){
    //combine images image1 and image2 by adding their color coordinates
    var imageCombine=new SimpleImage(image1.getWidth(),image1.getHeight());
    for (var pix1 of image1.values()){
        var pix2=image2.getPixel(pix1.getX(),pix1.getY());
        var pixComb=imageCombine.getPixel(pix1.getX(),pix1.getY());
        pixComb.setRed(newpv(pix1.getRed(),pix2.getRed()));
        pixComb.setGreen(newpv(pix1.getGreen(),pix2.getGreen()));
        pixComb.setBlue(newpv(pix1.getBlue(),pix2.getBlue()));
    }
    return imageCombine;
}

function pixRestore(pixval,bitNo){
    //extract the last bitNo bits of a value and restore them to the first bitNo places of the byte 
    pixRest=pixval-Math.floor(pixval/Math.pow(2,bitNo))*Math.pow(2,bitNo);
    pixRest=pixRest*Math.pow(2,8-bitNo);
    return pixRest;
}

function extract(image,bitNo){
    //extract the image hidden in the last bitNo bits of the input image
    imgExtr=new SimpleImage(image.getWidth(),image.getHeight());
    for (var pix of image.values()){
        var pixExtr=imgExtr.getPixel(pix.getX(),pix.getY());
        var red=pix.getRed();var green=pix.getGreen();var blue=pix.getBlue();
        pixExtr.setRed(pixRestore(red,bitNo));
        pixExtr.setGreen(pixRestore(green,bitNo));
        pixExtr.setBlue(pixRestore(blue,bitNo));
    }
    return imgExtr;
}

//Load the images
var image1= new SimpleImage("kazantzakis.jpg");
var image2=new SimpleImage("quote.jpg");
//--------------------------------------------------------------------------------
//Crop the images so that they are the same size
var widthCrop=Math.min(image1.getWidth(),image2.getWidth());
var heightCrop=Math.min(image1.getHeight(), image2.getHeight());
var image1=crop(image1,widthCrop,heightCrop);
var image2=crop(image2, widthCrop, heightCrop);

print(image1);
print(image2);

print("image 1 width:",image1.getWidth(), "height:", image1.getHeight());
print("image 2 width:",image2.getWidth(), "height:", image2.getHeight());

//--------------------------------------------------------------------------------
//Hide image2 inside image1
noBits=2;//the number of bits in which we want to hide the second image
var start=chop2hide(image1,8-noBits);//remove the last 2bits of image1
var middle=shift(image2,noBits);//remove the last 6 bits of image 2
var end=combine(start,middle);//combine the two images
print(end);
//Check the results
print("Original Image 1:",image1.getPixel(100,100));
print("Original Image 2:",image2.getPixel(100,100));
print("Image 1 tweaked:",start.getPixel(100,100));
print("Image 2 tweaked:", middle.getPixel(100,100));
print("Final Image:",end.getPixel(100,100));

//-------------------------------------------------------
//Extract the hidden image
imgExtract=extract(end,2);
print(imgExtract);
print("Extracted Image:",imgExtract.getPixel(100,100));//check the result
//-------------------------------------------------------
var test=new SimpleImage("test (2).png");
print(test);
print(extract(test,2));

Hidden picture in Drew's picture

The hidden message reads:

"Program testing can be used to show the presence of bugs, but never to show their absence! E.Dijkstra"

The hidden image is shown below.