Quorum controlled homomorphic re-encryption for privacy preserving computations in the cloud

Cloud computing provides services to store users’ data, so that they can access it anytime, anywhere at their convenience. In order to preserve data privacy, the data is encrypted before being uploaded to the cloud platform. But it is challenging to process and analyse the encrypted data as well as manage its access control. In Information Sciences 2017, Ding et al. have proposed a novel solution to address this challenge, which consists of a homomorphic re-encryption scheme (HRES) and a toolkit of several basic operations over ciphertexts. Ding’s work has good potentials for various cloud-based applications in which privacy preserving computations are required and the processed results are shared among multiple users. However, the HRES scheme relies on some relatively strong assumptions, making it less practical for real-world applications. For example, the system parameters have to be generated by a trusted third party (TTP), the two proxies who execute the decryption and re-encryption processes have to follow the protocol, and they can neither be out-of-service nor collude. In this paper, we relax these assumptions by extending the HRES scheme into a quorum controlled homomorphic re-encryption scheme (QHRES). Both the decryption and re-encryption processes are carried out by multiple proxies in a distributed fashion, and all the desirable security properties are guaranteed if there exists a quorum of honest proxies. Moreover, no TTP is required and every proxy’s behaviour can be publicly verified. Our extension inherits the homomorphic property as in HRES and it also supports the basic operations over ciphertexts. Therefore, it contributes to a more practical privacy preserving data processing system for real-world applications in the cloud.