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Conditional access system (CAS) is the essential basis of a digital pay-TV system, which is used to prevent unauthorized access to audio/video data and ensure the benefit of the service providers. In the CAS, key distribution is an essential part, whether the distribution scheme succeeding or not can directly affect the security of the CAS. In this paper, based on four-level key hierarchy, a new key distribution scheme based on grouping access control for conditional access system in digital pay-TV system is presented, in which we adopt the new scheme to generate and distribute authorization key and group key more efficiently. Our key distribution scheme can greatly reduce computation overhead and the complexity while providing higher security. Moreover, the proposed scheme is scalable in processing joining and leaving of subscribers, which makes service provider easy to manage the subscribers dynamically. In the end, we analyze the security and performance of the proposed scheme and compare it with other schemes. It shows that our proposed scheme is efficient and suitable for a digital pay-TV system which can provide both PPC and PPV services and is feasible for practical application.