In a method and arrangement for embedding and detecting a watermark in an information signal, the embedded watermark (W.sub.i) is selected (13) from a plurality of watermarks (W.sub.1 . . . W.sub.N) in dependence upon a property P of the signal. An example of such a property is the distribution of luminance values of the current video image as calculated by an analysis circuit (12). The corresponding watermark detector performs the same operation: the watermark being looked for depends on the same signal property. It is achieved with the invention that the embedded watermark changes from time to time as a function of the information signal content, so that it cannot easily be hacked.