Steamgifts Steganography Extract.Py

from PIL import Image, ImageFilter

def get_num_bits_secret():
    return 1

# im is the image you get when you combine the host image and the hidden image. (In this case, this was the image posted in the steamgifts thread).
im = Image.open( 'combined.png' )
num_bits_secret = get_num_bits_secret()

def extract(im):

    width, height = im.size
    # iterate through the pixels
    for i in range(width):
        for j in range(height):
            pixel = list( im.getpixel((i,j)) )

            # for each color component
            for c in range(3):
                val = pixel[c]

                # "%" is the modulus operator. x % y gives you the remainder of dividing x by y.
                # "<<" and ">>" are bit shifting operators. They make a number lose binary digits at the right side, or add zeros at the right side.
                # e.g.: x >> 2, makes x lose two bits from the end (it's like dividing by 2^2 and taking only the quotient)
                # e.g.: x << 2, adds two 0 bits to the right of x (it's like multiplying by 2^2)

                # get the last num_bits_secret bits in that number
                val = (val % (1 << num_bits_secret))
                # move the number back to the left by placing zeros at the end.
                val = (val << (8 - num_bits_secret))

                pixel[c] = val

            # place the modified pixel back in the image.
            im.putpixel((i,j), tuple(pixel))

    return im

ex = extract(im)
ex.save('extracted.png')
ex.show()