Gordon et al. systematically studied the Universally Composable (UC) security of Multi-client Verifiable Computation (MVC), in which a set of computationally-weak clients delegate the computation of a general function to an untrusted server based on their private inputs, and proposed a UC-secure scheme ensuring that the protocol remains secure even when arbitrarily composed with other UC-secure instances. However, this scheme imposed a significant computational overhead on clients due to the utilization of fully homomorphic encryption, and the plaintext size scaled linearly with function input size. In this work, we present MVOC, a more efficient UC-secure MVC protocol, that significantly reduces the amortized overhead for clients in both semi-honest and malicious settings, by delegating a larger portion of the computation to the server. We enable clients to verify the garbled circuit before entering the online phase, ensuring security against malicious clients without incurring heavy overhead of compiling a semi-honest protocol into a malicious one. We present the detailed proof and analyze the theoretical complexity of MVOC. Furthermore, we implement our protocol and evaluate the performance, and the results demonstrate that the computation and communication overheads during the input phase can be decreased by at least 95.55% and 87.17%, respectively.

Gordon et al. (TCC 2015) systematically studied the security of Multi-client Verifiable Computation (MVC), in which a set of computationally-weak clients outsource the computation of a general function f over their private inputs to an untrusted server. They introduced the universally composable (UC) security of MVC and proposed a scheme achieving UC-security, where the protocol remains secure after arbitrarily composed with other UC-secure instances. However, the clients in their scheme have to undertake the heavy computation overhead caused by fully homomorphic encryption (FHE) and further, the plaintext size is linear to the function input size. In this work, we propose a more efficient UC-secure multi-client privacy-preserving verifiable computation protocol, called MVOC, that sharply reduces amortized overheads for clients, in both semi-honest and malicious settings. In particular, our protocol achieves stronger outsourcability by outsourcing more computation to the server, so that it may be more friendly to those lightweight clients. More specifically, we revisit the definition of garbling scheme, and propose a novel garbled circuit protocol whose circuit randomness is non-interactively provided by multiple parties. We also realize the idea of hybrid homomorphic encryption, which makes the FHE plaintext size independent of the input size. We present the detailed proof and analyze the theoretical complexity of MVOC. We further implement our protocol and evaluate the performance, and the results show that, after adopting our new techniques, the computation and communication overheads during input phase can be decreased by 55.15%–68.05% and 62.55%–75% respectively.

Big data analytics has been regarded as a promising technology to yield better insights into future development by government and industry. Data collection and aggregation are necessary steps to enable data analysis. However, data may be dispersed across multiple places and in different formats. Even worse, data can be encrypted under different encryption mechanisms when different data owners try to secure their data. This makes data aggregation very difficult, if not impossible, especially when the encryption keys cannot be shared for various reasons. In this paper, we take the first step in addressing this problem. More specifically, we propose a new notion of cross-domain encryption switching that bridges two well-studied encryption mechanisms, namely traditional public key encryption and identity-based encryption. As of independent interest, our notion supports keyword search over encrypted data. We provide a provably-secure instantiation satisfying the notion, and further present the efficiency analysis to show the scalability. Our proposed scheme is applicable in multi-domain cloud storage system.

Secure cloud storage, an emerging cloud service, guarantees the confidentiality of outsourced data while providing flexible data access control for cloud users whose data are out of their physical control. Ciphertext-Policy Attribute-Based Encryption (CP-ABE) is one of the promising secure mechanisms to support fine-grained access control on encrypted data in cloud settings. However, due to its inherent "all-or-nothing" decryption control characteristic, there is a risk for the misuse of access credentials. In this paper, we consider the two main types of access credential misuse, namely: semi-trusted authority's illegal access credential (re-)distribution, and cloud user's illegal access credential leakage. To mitigate these two types of access credential misuse, we propose the first accountable authority revokable CP-ABE based cloud storage system with white-box traceability and auditing, referred to as CryptCloud+. We also prove the security of our system and present the experimental results to demonstrate the utility of our system.