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Worst-case to average-case reductions for module lattices

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Abstract

Most lattice-based cryptographic schemes are built upon the assumed hardness of the Short Integer Solution (SIS) and Learning With Errors (LWE) problems. Their efficiencies can be drastically improved by switching the hardness assumptions to the more compact Ring-SIS and Ring-LWE problems. However, this change of hardness assumptions comes along with a possible security weakening: SIS and LWE are known to be at least as hard as standard (worst-case) problems on euclidean lattices, whereas Ring-SIS and Ring-LWE are only known to be as hard as their restrictions to special classes of ideal lattices, corresponding to ideals of some polynomial rings. In this work, we define the Module-SIS and Module-LWE problems, which bridge SIS with Ring-SIS, and LWE with Ring-LWE, respectively. We prove that these average-case problems are at least as hard as standard lattice problems restricted to module lattices (which themselves bridge arbitrary and ideal lattices). As these new problems enlarge the toolbox of the lattice-based cryptographer, they could prove useful for designing new schemes. Importantly, the worst-case to average-case reductions for the module problems are (qualitatively) sharp, in the sense that there exist converse reductions. This property is not known to hold in the context of Ring-SIS/Ring-LWE: Ideal lattice problems could reveal easy without impacting the hardness of Ring-SIS/Ring-LWE.

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Notes

  1. With the exception of the recent result of Micciancio and Peikert [26] on the hardness of SIS and LWE with small parameters.

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Acknowledgments

We thank Guillaume Hanrot, Oded Regev and Ron Steinfeld for helpful discussions. We also thank the anonymous reviewers: their comments helped to very significantly improve the paper. Significant parts of the writing of this article were undergone while the authors were visiting Macquarie and Monash Universities, whose hospitalities are gratefully acknowledged. This work has been supported in part by the Australian Research Council under Discovery Grant DP0987734 and by the European Research Council under by ERC Starting Grant ERC-2013-StG-335086-LATTAC.

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Correspondence to Damien Stehlé.

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Communicated by C. Padro.

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Langlois, A., Stehlé, D. Worst-case to average-case reductions for module lattices. Des. Codes Cryptogr. 75, 565–599 (2015). https://doi.org/10.1007/s10623-014-9938-4

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