1. A method of fingerprinting a common function in broadcast systems comprising the steps of (a) determining for each customer a unique function, (b) arranging the functions to be identical on a subset X of inputs, and different (with high probability) on other inputs, and (c) broadcasting only inputs in X in order to enable all customers to compute common outputs, whereby to enable pirate cards to be traced to particular customers by analyzing their computations on arguments which are not necessarily in X.