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The conditional access system (CAS) is an essential part of digital pay-TV systems for controlling access to the program services. Conventionally, due to the restrictions of bandwidth and computational capability, a CAS only supports period subscription services that are charged on a monthly basis. In this paper, based on the concept of hierarchical key assignment, we propose three key distribution schemes for the access control of pay-TV systems. With these schemes, a CAS can support more charging strategies for service providers, such as adopting a smaller charging unit and allowing a subscription of any subset of channels with little communication and computational overhead. In addition, the piracy management problem can also be dealt with easily.