Cryptography Resilient to Continual Memory Leakage

In recent years, there has been a major effort to design cryptographic schemes that remain secure even if part of the secret key is leaked. This is due to a recent proliferation of side channel attacks which, through various physical means, can recover part of the secret key. We explore the possibility of achieving security even with continual leakage, i.e., even if some information is leaked each time the key is used. We show how to securely update a secret key while information is leaked: We construct schemes that remain secure even if an attacker, at each time period, can probe the entire memory (containing a secret key) and “leak” up to a (1− o(1)) fraction of the secret key. The attacker may also probe the memory during the updates, and leak O(log k) bits, where k is the security parameter (relying on subexponential hardness allows k bits of leakage during each update process). All of the above is achieved without restricting the model as is done in previous works (e.g. by assuming that “only computation leaks information” [Micali-Reyzin, TCC04]). Specifically, under the decisional linear assumption on bilinear groups (which allows for a leakage rate of (1/2−o(1))) or the symmetric external Diffie-Hellman assumption (which allows for a leakage rate of (1− o(1))), we achieve the above for public key encryption, identity-based encryption, and signature schemes. Prior to this work, it was not known how to construct public-key encryption schemes even in the more restricted model of [MR]. The main contributions of this work are (1) showing how to securely update a secret key while information is leaked (in the more general model) and (2) giving a public key encryption (and IBE) schemes that are resilient to continual leakage. ∗Weizmann Institute of Science and Microsoft Research, [email protected]. †Microsoft Research, [email protected]. ‡University of Maryland, [email protected]. §IBM Research, [email protected].