Efficient Chosen Ciphertext Secure Threshold Public-Key Encryption with Forward Security

Threshold Public Key Encryption allows a set of servers to decrypt a cipher text if a given threshold of authorized servers cooperate. Forward security allows one to mitigate the damage caused by exposure of secret keys. Forward-secure public encryptions are used to the threshold setting, this model guarantees that even if an adversary have broken into more than t distinct servers(for some i), messages encrypted during all time periods prior to i remain secret. In this paper, we present the first probably secure (non-interactive) forward-secure threshold public-key encryption scheme against chosen-cipher text attacks in the random oracle model. The encryption and update operations are very efficient when compared with the scheme presented by Libert et al. A formal definition, as well as a detailed analysis of the security performance of this scheme, is presented. The security of this scheme is based on the Computational Bilinear Diffie Hellman assumption, which leads to a unique approach to prove security in the random oracle model. Furthermore, in our model, the servers update their keys individually and asynchronously, without any communication between them.