Practical pay-TV scheme using traitor tracing scheme for multiple channels

Abstract

A Pay-TV scheme broadcasts a set of services or streams instead of one. That is, a pay-TV scheme is a specific application of a broadcast encryption scheme in which the secret to be broadcast is associated with a number of services. For example, a pay-TV broadcaster offers various channels such as a sports channel, a movie channel, and so on. A traitor tracing scheme is a variant of a broadcast encryption scheme, so can be applied to construct a pay-TV scheme. However, because most known traitor tracing schemes deal with the broadcast of a single stream, a direct extension to multiple streams is too inefficient, i.e., direct extension to an m-stream case would involve an m-fold increase in the user's secret keys. In other words, if the number of streams to be sent increases, the number of secret keys each user must store in a secure memory also linearly increases. Therefore, we require a specific traitor tracing scheme which can be efficiently applied to a pay-TV scheme. We propose a new traitor tracing scheme for multiple channels and its application to a pay-TV scheme. The number of secret keys each user must store in a secure memory is just one regardless of the number of channels and it cannot be changed. Our scheme has a revocation property, i.e., we can revoke some users without redistributing a new secret key to other un-revoked users. Our scheme also provides a so called holding property - we can revoke some users and un-revoke them without redistribution of new keys after some period. This is very useful in a pay-TV scheme since a user may wish to un-subscribe from a channel for some periods and resubscribe again later. Moreover, our pay-TV scheme is based on a public key traitor tracing scheme. Therefore, any content provider can send encrypted data to users with the public key the system manager provides. Our scheme can also be extended to provide asymmetric property and be secure against the adaptive chosen ciphertext attack.