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This paper presents the design and implementation of a pay-broadcasting system which can supply the contents to support a number of clients simultaneously on the Internet. The authors have surveyed user-authentication methods on IP multicasting are compared under the constraint that the system should be constructed without any specific hardware for user identification, and it also must be installed without any changes on the network infrastructure of the Internet. To support scalable number of users, the contents of the system should be supplied to the users as the same data-stream. To keep the security of the contents, the encrypted single data-stream should be decoded by different keys individually distributed to the authorized users. Broadcasting encryption and tracing traitors schemes proposed by Naor et al. are considered to be suitable to realize such system. As a prototype of the system, a music broadcasting system named "MusicCast/AS" was implemented with user-authentication on IP multicasting. Its server and clients are implemented as software running on common Linux systems and the system do not need any changes in the existing campus network as a testbed. The practicality of the methodology was verified by the experimentation to broadcast encrypted music data to the campus network. The scalablity of the users and security of the keys are also discussed on the implementation.