A method of manufacturing a series of integrated circuits having related functionality, the method including the steps of: (a) determining an identifier; (b) permanently storing the identifier on one of the integrated circuits; (c) repeating steps (a) and (b) for each integrated circuit in the series; and wherein the identifiers for the series are determined in such a way that knowing the identifier of one of the integrated circuits does not improve the ability of an attacker to determine the identifier of any of the other integrated circuits.