An identity-based group-oriented threshold encryption scheme

In many broadcast systems, it is useful to send messages to some (not all) members of the whole group. And the ciphertext can only be decrypted with the participation of a certain number of receivers. In this paper, we construct an efficient identity-based group-oriented threshold encryption scheme. The sender can encrypt a single message for a dynamic k-member group which can be a subset of the n-member group. And at least t receivers are needed to decrypt the ciphertext, where 1≼t≼k≼n. Meanwhile, we provide the formal security proofs of this scheme in random oracle model.