1 : Method in a client device for obtaining a control word, the client device having a unique binary identification, the method comprising: receiving an input transformed control word; receiving a set of compounds of seeds for each bit of the unique binary identification; and mapping the input transformed control word from an input transform domain to an output transform domain to thereby obtain an output transformed control word by successively applying a transformation function to the input transformed control word using each compound of seeds from the set successively as an input to the successive transformation functions, wherein each of the successive transformation functions is one of a regular transformation function, a first special transformation function and a second special transformation function, wherein one of the compounds of seeds from the set is input to one of the first special transformation function and the second special transformation function, and wherein the other compounds of seeds from the set are input to the regular transformation function.