In distribution of data encrypted to multiple users, there is a problem that the system will become low efficient if a single key center has to generate keys for a large number of users. Besides, it is a risk that some users could deliberately disclose their keys for some benefits. We in this paper give a key leakage discovering scheme where users are partitioned into groups and groups are hierarchically organized. In our scheme, users in upper-level groups can delegate keys for users in

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... vel groups, which alleviates the key generation burden of the trusted third party. As an interesting feature, our scheme provide a key leakage discovering measure that if some users in groups leaked their decryption keys then at least one of them can be found out. This enables the data owners to accuse the illegal users when they infringed the copyright. At last, we analyze the performance of our system.