איתי דינור

The power of adaptivity in algorithms has been intensively studied in diverse areas of theoretical computer science. In this paper, we obtain a number of sharp lower bound results which show that adaptivity provides a significant extra power in cryptanalytic time-space tradeoffs with (possibly unlimited) preprocessing time. Most notably, we consider the discrete logarithm (DLOG) problem in a generic group of N elements. The classical baby-step giant-step' algorithm for the problem has time complexity T=O(√N), uses O(√N) bits of space (up to logarithmic factors in N) and achieves constant success probability. We examine a generalized setting where an algorithm obtains an advice string of S bits and is allowed to make T arbitrary non-adaptive queries that depend on the advice string (but not on the challenge group element for which the DLOG needs to be computed). We show that in this setting, the T=O(√N) online time complexity of the baby-step giant-step algorithm cannot be improved, unless the advice string is more than ω(√N) bits long. This lies in stark contrast with the classical adaptive Pollard's rho algorithm for DLOG, which can exploit preprocessing to obtain the tradeoff curve ST2=O(N). We obtain similar sharp lower bounds for the problem of breaking the Even-Mansour cryptosystem in symmetric-key cryptography and for several other problems. To obtain our results, we present a new model that allows analyzing non-adaptive preprocessing algorithms for a wide array of search and decision problems in a unified way. Since previous proof techniques inherently cannot distinguish between adaptive and non-adaptive algorithms for the problems in our model, they cannot be used to obtain our results. Consequently, we rely on information-theoretic tools for handling distributions and functions over the space SN of permutations of N elements. Specifically, we use a variant of Shearer's lemma for this setting, due to Barthe, Cordero-Erausquin, Ledoux, and Maurey (2011), and a variant of the concentration inequality of Gavinsky, Lovett, Saks and Srinivasan (2015) for read-k families of functions, that we derive from it. This seems to be the first time a variant of Shearer's lemma for permutations is used in an algorithmic context, and it is expected to be useful in other lower bound arguments.