The invention provides for a solution enabling obtaining a control word in the client. The client device has a unique binary identification. An input transformed control word is mapped 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. Each of the successive transformation functions is one of a regular transformation function, a first special transformation function and a second special transformation function. The obtained control word can be used to decrypt one of two copies of a part of content data. The copy that can be decrypted contains a watermark representing either a binary “0” or a binary “1” and represents a bit of the unique binary identification of the client device.