Control word protection

Abstract

A method for securely obtaining a control word in a chip set of a receiver, said control word for descrambling scrambled content received by the receiver, the method comprising, at the chip set: receiving a secured version of a virtual control word from a conditional access/digital rights management client communicably connected to the chip set; obtaining the virtual control word from the secured version of the virtual control word; and using a first cryptographic function to produce a given output from an input that comprises the virtual control word and either a plurality of signature verification keys or one or more values derived from a plurality of signature verification keys, each signature verification key being associated with a conditional access/digital rights management system, the given output comprising at least one control word, wherein the first cryptographic function has the property that it is infeasible to determine a key pair including a signature key and a signature verification key and an input for the first cryptographic function comprising the determined signature verification key or one or more values derived, at least in part, from the determined signature verification key, such that the first cryptographic function produces the given output from the determined input.