Forward and backward secure updatable ElGamal encryption scheme for cloud storage

Abstract

Updatable encryption can support key/ciphertext update functionality, resist key compromise attacks, and be applicable to cloud storage. However, as a type of partitioning strategy, leakage sets or firewalls technique has been adopted by the existing updatable encryption schemes to build a relaxed security model, which forbids the adversary from querying the secret key at the challenge-equal epochs and hardly captures forward and backward security. In this paper, an enhanced security model can be built by considering a corruption oracle that allows any secret key query without requiring leakage sets. Then we utilize the indistinguishability obfuscation technique of the punctured program to construct an updatable ElGamal encryption scheme with no-directional key update and uni-directional ciphertext update. Furthermore, under the framework of the enhanced security model, the proposed scheme is proven to be IND-CPA. Finally, by comparison with the existing updatable encryption schemes, the proposed scheme eliminates leakage sets and firewalls, and can achieve both forward and backward security.