Efficient Public-Key Cryptography in the Presence of Key Leakage

We study the design of cryptographic primitives resistant to a large class of side-channel attacks, called “memory attacks”, where an attacker can repeatedly and adaptively learn information about the secret key, subject

only

to the constraint that the

overall amount

of such information is bounded by some parameter ℓ. Although the study of such primitives was initiated only recently by Akavia et al. [2], subsequent work already produced many such “leakage-resilient” primitives [48,4,42], including signature, encryption, identification (ID) and authenticated key agreement (AKA) schemes. Unfortunately, every existing scheme, — for any of the four fundamental primitives above, — fails to satisfy at least one of the following desirable properties:

Efficiency.

While the construction may be generic, it should have some

efficient

instantiations, based on standard cryptographic assumptions, and without relying on random oracles.

Strong Security

. The construction should satisfy the strongest possible definition of security (even in the presence of leakage). For example, encryption schemes should be secure against chosen

ciphertext

attack (CCA), while signatures should be

existentially

unforgeable.

Leakage Flexibility.

It should be possible to set the scheme parameters so that the leakage bound ℓ can come arbitrarily close to the secret-key size.

In this work we design the first signature, encryption, ID and AKA schemes which overcome these limitations, and satisfy all the properties above. Moreover, all our constructions are generic, in several cases elegantly simplifying and generalizing the prior constructions (which did not have any efficient instantiations). We also introduce several tools of independent interest, such as the abstraction (and constructions) of

true-simulation extractable

NIZK arguments, and a new

deniable

DH-based AKA protocol based on any CCA-secure encryption.