Efficient leakage-resilient identity-based encryption with CCA security

Due to the proliferation of side-channel attacks, lots of efforts have been made to construct cryptographic systems that are still secure even if part of the secret information is leaked to the adversary. Recently, many identity-based encryption (IBE) schemes have been proposed in this context, almost all of which, however, are only proved CPA secure. As far as we know, the IBE scheme presented by Alwen et al. is the unique CCA secure and the most practical one in the standard model. Unfortunately, this scheme suffers from an undesirable shortcoming that the leakage parameter λ and the message length m are subject to λ+m ≤ logp-ω(logκ), where κ is the security parameter and p is the prime order of the underlying group. To overcome this drawback, we designed a new IBE scheme based on Gentry's IBE in this paper, which is λ-leakage resilient CCA2 secure in the standard model where λ ≤ logp-ω(logκ). In contrast, the leakage parameter λ in our proposal is independent of the size of the message space. Moreover, our scheme is quite practical and almost as efficient as the original scheme. To the best of our knowledge, it is the first practical leakage-resilient fully CCA2 secure IBE scheme in the standard model, tolerating up to (logp-ω(logκ))-bit leakage of the private key, the leakage parameter of which is independent of the message length.