A weak pseudorandom function (wPRF) is a cryptographic primitive similar to - but weaker than - a pseudorandom function: for wPRFs one only requires that the output is pseudorandom when queried on random inputs. We show that unlike "normal" PRFs, wPRFs are seed-incompressible, in the sense that the output of a wPRF is pseudorandom even if a bounded amount of information about the key is leaked.rnAs an application of this result we construct a simple mode of operation which - when instantiated with any wPRF - gives a leakage-resilient stream-cipher. The implementation of such a cipher is secure against every side-channel attack, as long as the amount of information leaked per round is bounded, but overall can be arbitrary large. The construction is simpler than the previous one (Dziembowski-Pietrzak FOCS'08) as it only uses a single primitive (a wPRF) in a straight forward manner.
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