  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.
