[Resource Topic] 2016/1049: Randomized stopping times and provably secure pseudorandom permutation generators

Welcome to the resource topic for 2016/1049

Title:
Randomized stopping times and provably secure pseudorandom permutation generators

Authors: Michal Kulis, Pawel Lorek, Filip Zagorski

Abstract:

Conventionally, key-scheduling algorithm (KSA) of a cryptographic scheme runs for predefined number of steps. We suggest a different approach by utilization of randomized stopping rules to generate permutations which are indistinguishable from uniform ones. We explain that if the stopping time of such a shuffle is a Strong Stationary Time and bits of the secret key are not reused then these algorithms are immune against timing attacks. We also revisit the well known paper of Mironov~\cite{Mironov2002} which analyses a card shuffle which models KSA of RC4. Mironov states that expected time till reaching uniform distribution is 2n H_n - n while we prove that n H_n+ n steps are enough (by finding a new strong stationary time for the shuffle). Nevertheless, both cases require O(n \log^2 n) bits of randomness while one can replace the shuffle used in RC4 (and in Spritz) with a better shuffle which is optimal and needs only O(n \log n) bits.

ePrint: https://eprint.iacr.org/2016/1049

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