International Association for Cryptologic Research

International Association
for Cryptologic Research


Karel Král


On Average-Case Hardness in TFNP from One-Way Functions 📺
The complexity class TFNP consists of all NP search problems that are total in the sense that a solution is guaranteed to exist for all instances. Over the years, this class has proved to illuminate surprising connections among several diverse subfields of mathematics like combinatorics, computational topology, and algorithmic game theory. More recently, we are starting to better understand its interplay with cryptography. We know that certain cryptographic primitives (e.g. one-way permutations, collision-resistant hash functions, or indistinguishability obfuscation) imply average-case hardness in TFNP and its important subclasses. However, its relationship with the most basic cryptographic primitive -- \ie one-way functions (OWFs) -- still remains unresolved. Under an additional complexity theoretic assumption, OWFs imply hardness in TFNP (Hubá?ek, Naor, and Yogev, ITCS 2017). It is also known that average-case hardness in most structured subclasses of TFNP does not imply any form of cryptographic hardness in a black-box way (Rosen, Segev, and Shahaf, TCC 2017) and, thus, one-way functions might be sufficient. Specifically, no negative result which would rule out basing average-case hardness in TFNP \emph{solely} on OWFs is currently known. In this work, we further explore the interplay between TFNP and OWFs and give the first negative results. As our main result, we show that there cannot exist constructions of average-case (and, in fact, even worst-case) hard TFNP problem from OWFs with a certain type of simple black-box security reductions. The class of reductions we rule out is, however, rich enough to capture many of the currently known cryptographic hardness results for TFNP. Our results are established using the framework of black-box separations (Impagliazzo and Rudich, STOC 1989) and involve a novel application of the reconstruction paradigm (Gennaro and Trevisan, FOCS 2000).
Stronger Lower Bounds for Online ORAM
Oblivious RAM (ORAM), introduced in the context of software protection by Goldreich and Ostrovsky [JACM’96], aims at obfuscating the memory access pattern induced by a RAM computation. Ideally, the memory access pattern of an ORAM should be independent of the data being processed. Since the work of Goldreich and Ostrovsky, it was believed that there is an inherent $$ \varOmega (\log n) $$ bandwidth overhead in any ORAM working with memory of size n. Larsen and Nielsen [CRYPTO’18] were the first to give a general $$ \varOmega (\log n) $$ lower bound for any online ORAM, i.e., an ORAM that must process its inputs in an online manner.In this work, we revisit the lower bound of Larsen and Nielsen, which was proved under the assumption that the adversarial server knows exactly which server accesses correspond to which input operation. We give an $$\varOmega (\log n) $$ lower bound for the bandwidth overhead of any online ORAM even when the adversary has no access to this information. For many known constructions of ORAM this information is provided implicitly as each input operation induces an access sequence of roughly the same length. Thus, they are subject to the lower bound of Larsen and Nielsen. Our results rule out a broader class of constructions and specifically, they imply that obfuscating the boundaries between the input operations does not help in building a more efficient ORAM.As our main technical contribution and to handle the lack of structure, we study the properties of access graphs induced naturally by the memory access pattern of an ORAM computation. We identify a particular graph property that can be efficiently tested and that all access graphs of ORAM computation must satisfy with high probability. This property is reminiscent of the Larsen-Nielsen property but it is substantially less structured; that is, it is more generic.