A method including identifying at least two subsets of pixels within a block of an image; forming a plurality of pixel groups from the at least two subsets of pixels, each pixel group having at least one pixel from a first of the at least two subsets and at least one pixel from a second of the at least two subsets; producing a plurality of difference values, each pixel group providing one of said difference values, each difference value being based on differences between pixel values of pixels within one of the pixel groups; and modifying pixel values of pixels in less than all of the at least two subsets, thereby embedding a bit value into the block.