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 (beforethe-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 leakageresilient 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 (BAFLeCK) model proposed by Alawatugoda et al. (ASIACCS’14). 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.