Leakage-Resilient Chosen-Ciphertext Secure Public-Key Encryption from Hash Proof System and One-Time Lossy Filter ∗

We present a new generic construction of a public-key encryption (PKE) scheme secure against leakage-resilient chosen-ciphertext attacks (LR-CCA), from any Hash Proof System (HPS) and any one-time lossy filter (OT-LF). Efficient constructions of HPSs and OT-LFs from the DDH and DCR assumptions suggest that our construction is a practical approach to LR-CCA security. Most of practical PKEs with LR-CCA security, like variants of Cramer-Shoup scheme, rooted from Hash Proof Systems, but with leakage rates at most 1/4 − o(1) (defined as the ratio of leakage amount to secret-key size). The instantiations of our construction from the DDH and DCR assumptions result in LR-CCA secure PKEs with leakage rate of 1/2 − o(1). On the other hand, our construction also creates a new approach for constructing IND-CCA secure (leakage-free) PKE schemes, which may be of independent interest

New Smooth Projective Hashing For Oblivious Transfer

Oblivious transfer is an important tool against malicious cloud server providers. Halevi-Kalai OT, which is based on smooth projective hash(SPH), is a famous and the most efficient framework for $1$-out-of-$2$ oblivious transfer (\mbox{OT}^{2}_{1}) against malicious adversaries in plain model. A natural question however, which so far has not been answered, is whether its security level can be improved, i.e., whether it can be made fully-simulatable. In this paper, we press a new SPH variant, which enables a positive answer to above question. In more details, it even makes fully-simulatable \mbox{OT}^{n}_{t} ($n,t\in \mathbb{N}$ and $n>t$) possible. We instantiate this new SPH variant under not only the decisional Diffie-Hellman assumption, the decisional $N$-th residuosity assumption and the decisional quadratic residuosity assumption as currently existing SPH constructions, but also the learning with errors (LWE) problem. Before this paper, there is a folklore that it is technically difficult to instantiate SPH under the lattice assumption (e.g., LWE). Considering quantum adversaries in the future, lattice-based SPH makes important sense

Efficient Compilers for After-the-Fact Leakage: from CPA to CCA-2 secure PKE to AKE

The goal of leakage-resilient cryptography is to construct cryptographic algorithms that are secure even if the adversary obtains side-channel information from the real world implementation of these algorithms. Most of the prior works on leakage-resilient cryptography consider leakage models where the adversary has access to the leakage oracle before the challenge-ciphertext is generated (before-the-fact leakage). In this model, there are generic compilers that transform any leakage-resilient CPA-secure public key encryption (PKE) scheme to its CCA-2 variant using Naor-Yung type of transformations. In this work, we give an efficient generic compiler for transforming a leakage-resilient CPA-secure PKE to leakage-resilient CCA-2 secure PKE in presence of after-the-fact split-state (bounded) memory leakage model, where the adversary has access to the leakage oracle even after the challenge phase. The salient feature of our transformation is that the leakage rate (defined as the ratio of the amount of leakage to the size of secret key) of the transformed after-the-fact CCA-2 secure PKE is same as the leakage rate of the underlying after-the-fact CPA-secure PKE, which is $1-o(1)$. We then present another generic compiler for transforming an after-the-fact leakage-resilient CCA-2 secure PKE to a leakage-resilient authenticated key exchange (AKE) protocol in the bounded after-the-fact leakage-resilient eCK (BAFL-eCK) model proposed by Alawatugoda et al. (ASIACCS\u2714). To the best of our knowledge, this gives the first compiler that transform any leakage-resilient CCA-2 secure PKE to an AKE protocol in the leakage variant of the eCK model