Works on the assumption that the given image has 255 levels of grayness.

Assumes that all brighness levels are equally likely.

Watermark insertion uses a secret key to seed a pseudo-random process that choses pairs of pixels.

For each pair of pixels, the brighness level of one of them is subtracted by a small constant, say k, while this is added to the brighness level of the other pixel. This is done a number of times (typically around 10000).

Watermark extraction requires the same secret key used in insertion.