Abstract
Constructing one-way functions from average-case hardness is a long-standing open problem. A positive result would exclude Pessiland (Impagliazzo ’95) and establish a highly desirable win-win situation: either (symmetric) cryptography exists unconditionally, or all \(\mathsf {NP}\) problems can be solved efficiently on the average. Motivated by the lack of progress on this seemingly very hard question, we initiate the investigation of weaker yet meaningful candidate win-win results of the following type: either there are fine-grained one-way functions (FGOWF), or nontrivial speedups can be obtained for all \(\mathsf {NP} \) problems on the average. FGOWFs only require a fixed polynomial gap (as opposed to superpolynomial) between the running time of the function and the running time of an inverter. We obtain three main results:
Construction. We show that if there is an \(\mathsf {NP} \) language having a very strong form of average-case hardness, which we call block finding hardness, then FGOWF exist. We provide heuristic support for this very strong average-case hardness notion by showing that it holds for a random language. Then, we study whether weaker (and more natural) forms of average-case hardness could already suffice to obtain FGOWF, and obtain two negative results:
Separation I. We provide a strong oracle separation for the implication (\(\exists \) exponentially average-case hard language \(\implies \) \(\exists \) FGOWF).
Separation II. We provide a second strong negative result for an even weaker candidate win-win result. Namely, we rule out a black-box proof for the implication (\(\exists \) exponentially average-case hard language whose hardness amplifies optimally through parallel repetitions \(\implies \) \(\exists \) FGOWF). This separation forms the core technical contribution of our work.
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- 1.
Though we heard that lately, some cryptographers have been found dreaming of an even higher heaven, the mysterious land of Obfustopia.
- 2.
Ralph Merkle, 1974 project proposal for CS 244 at U.C. Berkeley, http://www.merkle.com/1974/.
- 3.
Of course, this heuristic is simplified: most real languages can have more than a single witness, and the choice of having \(|w| = |x|\) is a somewhat arbitrary way of tuning the hardness to make it exactly \(2^n\). Still, we believe that there is value in using a simple model to heuristically analyze the plausibility of an assumption – even though, as any heuristic model, it must fail on artificial counter examples.
- 4.
More precisely, it requires \(2^{n-1}\) calls to \(\mathsf {Chk}\) on the average to find a witness of language membership if x is indeed in the language. In turn, it requires \(2^{n}\) calls to confirm that there is indeed no witness if x is not in the language.
- 5.
More formally, since we consider an oracle sampled from a distribution over oracles, as for the Random Oracle Model, this captures average-case hard language distributions. I.e., the hardness of a language is averaged over the choice of the instance and the sampling of the oracle.
- 6.
We thank an anonymous reviewer for pointing out this construction.
- 7.
Determining an appropriate bound on much higher is crucial to avoid that deciding \(\mathcal {L}^{\mathsf {O}}\) becomes too easy. We return to this issue shortly.
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Acknowledgements
We thank Félix Richart for help with the experimental verification of some probability claims, and the anonymous Eurocrypt reviewers for their careful proofreading of the paper. C. Brzuska supported by the academy of Finland. G. Couteau supported by the ANR SCENE.
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Brzuska, C., Couteau, G. (2022). On Building Fine-Grained One-Way Functions from Strong Average-Case Hardness. In: Dunkelman, O., Dziembowski, S. (eds) Advances in Cryptology – EUROCRYPT 2022. EUROCRYPT 2022. Lecture Notes in Computer Science, vol 13276. Springer, Cham. https://doi.org/10.1007/978-3-031-07085-3_20
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