Updatable ElGamal Encryption Scheme with Forward and Backward Security for Cloud Storage

Abstract

Updatable encryption plays an important role in cloud storage scenario due to providing update functionality for ciphertext data and resisting key compromise attacks. However, most of updatable encryption schemes adopted a type of partitioning strategy: leakage sets or firewalls technique, which prohibited the adversary from querying the key at the challenge-equal epochs and hardly captured forward and backward security. In this paper, we enhance the existing security model by considering a corruption oracle that allows any secret key query and overcomes the dependence on leakage sets, and then present the first updatable public-key encryption scheme with no-directional key updates and uni-directional ciphertext updates on the basis of the classical ElGamal encryption scheme. The proposed scheme can minimize the information leakage of update tokens by using the indistinguishable obfuscation technique of the punctured program. Moreover, under the framework of the enhanced security model, the proposed scheme is proven to be IND-CPA. Finally, compared with the existing updatable encryption schemes, the proposed scheme eliminates leakage sets and firewalls to capture forward and backward security.