As the development of digital broadcasting or digital medium such as Blu-ray Disc in recent years, the contents protection technology is more important. The broadcast encryption, one of the contents protection technologies can make the privileged users be able to decrypt a encrypted message and non-privileged users not be able to decrypt the message. We measure the efficience of a broadcast encryption by three values, that are the overhead of the encrypted messages, the length of users' private keys and the time to decryption. The number of users are variable as the changes of the users' contract. The effect is desirable to be independent from the changes.
We research on the efficient broadcast encryption whose supposed number of the users are huge.