Improvements on Sun ´s Conditional Access System in Pay-TV Broadcasting Systems

A conditional access system (CAS) proposed by Sun has a critical security weakness in its inability to preserve backward secrecy; a former subscriber can still access programs despite his or her change in status. This weakness in Sun ´s CAS originates because 1) no change is made to a group key after a new member arrives, and 2) updates of group keys are done in an insecure manner. We show how simple protocol changes can fix these weaknesses and thus render Sun ´s CAS capable of preserving backward secrecy.