A Time-Success Ratio Analysis of wPRF-based Leakage-Resilient Stream Ciphers

Maciej Skorski

Abstract: Weak pseudorandom functions (wPRFs) found an important application as main building blocks for leakage-resilient ciphers (EUROCRYPT'09). Several security bounds, based on different techniques, were given to these stream ciphers. The security loss in these reduction-based proofs is always polynomial, but has not been studied in detail. The aim of this paper is twofold. First, we present a clear comparison of quantitatively different security bounds in the literature. Second, we revisit the current proof techniques and answer the natural question of how far we are from meaningful and provable security guarantees, when instantiating weak PRFs with standard primitives (block ciphers or hash functions). In particular, we demonstrate a flaw in the recent (TCC'14) analysis of the EUROCRYPT'09 stream cipher, which means that we still don't know if it offers provable security when instantiated with a standard block cipher. Our approach is a \emph{time-to-success Ratio} analysis, a universal measure introduced by Luby, which allow us to compare different security bounds.