1. Method in a client device for obtaining a control word associated with a value of a particular bit of a unique binary identification of the client device, the method comprising: receiving an input transformed control word; receiving a set of compounds of seeds, wherein each compound of seeds is associated with a respective 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 applying a sequence of transformation functions to the input transformed control word using each compound of seeds from the set successively as an input to a respective transformation function of the sequence, wherein each of the transformation functions is one of a regular transformation function, a first special transformation function and a second special transformation function, wherein the compound of seeds associated with the particular bit is, according to the value of the particular bit, input to either the first special transformation function or the second special transformation function, and wherein the other compounds of seeds from the set are input to the regular transformation function.