This study deals with the dynamic property of threshold cryptosystem. A dynamic threshold cryptosystem allows the sender to choose the authorized decryption group and the threshold value for each message dynamically. We first introduce an identity based dynamic threshold cryptosystem, and then use the CanettiHalevi-Katz(CHK) transformation to transform it into a fully secure system in the traditional public key setting.Finally, the elegant dual system encryption technique is applied to constructing a fully secure dynamic threshold cryptosystem with adaptive security.
In current cloud computing system, large amounts of sensitive data are shared to other cloud users.To keep these data confidentiality, data owners should encrypt their data before outsourcing. We choose proxy reencryption(PRE) as the cloud data encryption technique. In a PRE system, a semi-trusted proxy can transform a ciphertext under one public key into a ciphertext of the same message under another public key, but the proxy cannot gain any information about the message. In this paper, we propose a certificateless PRE(CL-PRE) scheme without pairings. The security of the proposed scheme can be proved to be equivalent to the computational DiffieHellman(CDH) problem in the random oracle model. Compared with other existing CL-PRE schemes, our scheme requires less computation cost and is significantly more efficient. The new scheme does not need the public key certificates to guarantee validity of public keys and solves the key escrow problem in identity-based public key cryptography.
