## CryptoDB

### Alon Rosen

#### Affiliation: IDC Herzliya

#### Publications

**Year**

**Venue**

**Title**

2020

EUROCRYPT

Fine-Grained Cryptography: A New Frontier?
★
Abstract

Fine-grained cryptography is concerned with adversaries that are only moderately more powerful than the honest parties. We will survey recent results in this relatively underdeveloped area of study and examine whether the time is ripe for further advances in it.

2020

PKC

Limits on the Efficiency of (Ring) LWE Based Non-interactive Key Exchange
📺
Abstract

$$mathsf {LWE}$$ based key-exchange protocols lie at the heart of post-quantum public-key cryptography. However, all existing protocols either lack the non-interactive nature of Diffie-Hellman key-exchange or polynomial $$mathsf {LWE}$$ -modulus, resulting in unwanted efficiency overhead. We study the possibility of designing non-interactive $$mathsf {LWE}$$ -based protocols with polynomial $$mathsf {LWE}$$ -modulus. To this end, We identify and formalize simple non-interactive and polynomial $$mathsf {LWE}$$ -modulus variants of existing protocols, where Alice and Bob simultaneously exchange one or more (ring) $$mathsf {LWE}$$ samples with polynomial $$mathsf {LWE}$$ -modulus and then run individual key reconciliation functions to obtain the shared key. We point out central barriers and show that such non-interactive key-exchange protocols are impossible if: (1) the reconciliation functions first compute the inner product of the received $$mathsf {LWE}$$ sample with their private $$mathsf {LWE}$$ secret. This impossibility is information theoretic. (2) One of the reconciliation functions does not depend on the error of the transmitted $$mathsf {LWE}$$ sample. This impossibility assumes hardness of $$mathsf {LWE}$$ . We give further evidence that progress in either direction, of giving an $$mathsf {LWE}$$ -based $$mathrm {NIKE}$$ protocol or proving impossibility of one will lead to progress on some other well-studied questions in cryptography. Overall, our results show possibilities and challenges in designing simple (ring) $$mathsf {LWE}$$ -based non-interactive key exchange protocols.

2020

TCC

Public-Coin Zero-Knowledge Arguments with (almost) Minimal Time and Space Overheads
📺
Abstract

Zero-knowledge protocols enable the truth of a mathematical statement to be certified by a verifier without revealing any other information. Such protocols are a cornerstone of modern cryptography and recently are becoming more and more practical. However, a major bottleneck in deployment is the efficiency of the prover and, in particular, the space-efficiency of the protocol.
For every $\mathsf{NP}$ relation that can be verified in time $T$ and space $S$, we construct a public-coin zero-knowledge argument in which the prover runs in time $T \cdot \mathrm{polylog}(T)$ and space $S \cdot \mathrm{polylog}(T)$. Our proofs have length $\mathrm{polylog}(T)$ and the verifier runs in time $T \cdot \mathrm{polylog}(T)$ (and space $\mathrm{polylog}(T)$). Our scheme is in the random oracle model and relies on the hardness of discrete log in prime-order groups.
Our main technical contribution is a new space efficient \emph{polynomial commitment scheme} for multi-linear polynomials. Recall that in such a scheme, a sender commits to a given multi-linear polynomial $P:\mathbb{F}^n \to \mathbb{F}$ so that later on it can prove to a receiver statements of the form ``$P(x)=y$''. In our scheme, which builds on commitments schemes of Bootle et al. (Eurocrypt 2016) and B{\"u}nz et al. (S\&P 2018), we assume that the sender is given multi-pass streaming access to the evaluations of $P$ on the Boolean hypercube and we show how to implement both the sender and receiver in roughly time $2^n$ and space $n$ and with communication complexity roughly $n$.

2020

ASIACRYPT

Cryptography from One-Way Communication: On Completeness of Finite Channels
📺
Abstract

Garg et al. (Crypto 2015) initiated the study of cryptographic protocols over noisy channels in the non-interactive setting, namely when only one party speaks. A major question left open by this work is the completeness of {\em finite} channels, whose input and output alphabets do not grow with the desired level of security. In this work, we address this question by obtaining the following results:
Completeness of Bit-ROT with Inverse Polynomial Error: We show that bit-ROT (i.e., Randomized Oblivious Transfer channel, where each of the two messages is a single bit) can be used to realize general randomized functionalities with inverse polynomial error. Towards this, we provide a construction of string-ROT from bit-ROT with inverse polynomial error.
No Finite Channel is Complete with Negligible Error: To complement the above, we show that {\it no} finite channel can be used to realize string-ROT with negligible error, implying that the inverse polynomial error in the completeness of bit-ROT is inherent. This holds even with semi-honest parties and for computational security, and is contrasted with the (negligible-error) completeness of string-ROT shown by Garg et al.
Characterization of Finite Channels Enabling Zero-Knowledge Proofs: An important instance of secure computation is zero-knowledge proofs.
Noisy channels can potentially be used to realize truly non-interactive zero-knowledge proofs, without trusted common randomness, and with non-transferability and deniability features that cannot be realized in the plain model. Garg et al. obtain such zero-knowledge proofs from the binary erasure channel (BEC) and the binary symmetric channel (BSC). We complete the picture by showing that in fact any non-trivial channel suffices.

2020

ASIACRYPT

Non-Interactive Composition of Sigma-Protocols via Share-then-Hash
📺
Abstract

Proofs of partial knowledge demonstrate the possession of certain subsets of witnesses for a given collection of statements x_1,\dots,x_n.
Cramer, Damg{\aa}rd, and Schoenmakers (CDS), built proofs of partial knowledge, given "atomic" protocols for individual statements x_i, by having the prover randomly secret share the verifier's challenge and using the shares as challenges for the atomic protocols. This simple and highly-influential transformation has been used in numerous applications, ranging from anonymous credentials to ring signatures.
We consider what happens if, instead of using the shares directly as challenges, the prover first hashes them. We show that this elementary enhancement can result in significant benefits:
- the proof contains a {\em single} atomic transcript per statement x_i,
- it suffices that the atomic protocols are k-special sound for k \geq 2,
- when compiled using the Fiat-Shamir heuristic, the protocol retains its soundness in the {\em non-programmable} random oracle model.
None of the above features is satisfied by the CDS transformation.

2018

EUROCRYPT

2018

CRYPTO

Proofs of Work From Worst-Case Assumptions
📺
Abstract

We give Proofs of Work (PoWs) whose hardness is based on well-studied worst-case assumptions from fine-grained complexity theory. This extends the work of (Ball et al., STOC ’17), that presents PoWs that are based on the Orthogonal Vectors, 3SUM, and All-Pairs Shortest Path problems. These, however, were presented as a ‘proof of concept’ of provably secure PoWs and did not fully meet the requirements of a conventional PoW: namely, it was not shown that multiple proofs could not be generated faster than generating each individually. We use the considerable algebraic structure of these PoWs to prove that this non-amortizability of multiple proofs does in fact hold and further show that the PoWs’ structure can be exploited in ways previous heuristic PoWs could not.This creates full PoWs that are provably hard from worst-case assumptions (previously, PoWs were either only based on heuristic assumptions or on much stronger cryptographic assumptions (Bitansky et al., ITCS ’16)) while still retaining significant structure to enable extra properties of our PoWs. Namely, we show that the PoWs of (Ball et al., STOC ’17) can be modified to have much faster verification time, can be proved in zero knowledge, and more.Finally, as our PoWs are based on evaluating low-degree polynomials originating from average-case fine-grained complexity, we prove an average-case direct sum theorem for the problem of evaluating these polynomials, which may be of independent interest. For our context, this implies the required non-amortizability of our PoWs.

2010

EPRINT

Sequential Rationality in Cryptographic Protocols
Abstract

Much of the literature on rational cryptography focuses on analyzing the strategic properties of cryptographic protocols. However, due to the presence of computationally-bounded players and the asymptotic nature of cryptographic security, a definition of sequential rationality for this setting has thus far eluded researchers.
We propose a new framework for overcoming these obstacles, and provide the first definitions of computational solution concepts that guarantee sequential rationality. We argue that natural computational variants of subgame perfection are too strong for cryptographic protocols. As an alternative, we introduce a weakening called threat free Nash equilibrium that is more permissive but still eliminates the undesirable ``empty threats'' of non-sequential solution concepts.
To demonstrate the applicability of our framework, we revisit the problem of implementing a mediator for correlated equilibria (Dodis Halevi-Rabin, Crypto'00), and propose a variant of their protocol that is sequentially rational for a non-trivial class of correlated equilibria. Our treatment provides a better understanding of the conditions under which mediators in a correlated equilibrium can be replaced by a stable protocol.

2008

EPRINT

Fairness with an Honest Minority and a Rational Majority
Abstract

We provide a simple protocol for secret reconstruction in any threshold secret sharing scheme, and prove that it is fair when executed with many rational parties together with a small minority of honest parties. That is, all parties will learn the secret with high probability when the honest parties follow the protocol and the rational parties act in their own self-interest (as captured by the notion of a Bayesian subgame perfect equilibrium). The protocol only
requires a standard (synchronous) broadcast channel, and tolerates fail-stop deviations (i.e. early stopping, but not incorrectly computed messages).
Previous protocols for this problem in the cryptographic or economic models have either required an honest majority, used strong communication channels that enable simultaneous exchange of information, or settled for approximate notions of security/equilibria.

2008

EPRINT

Chosen-Ciphertext Security via Correlated Products
Abstract

We initiate the study of one-wayness under {\em correlated products}. We are interested in identifying necessary and sufficient conditions for a function $f$ and a distribution on inputs $(x_1, \ldots, x_k)$, so that the function $(f(x_1), \ldots, f(x_k))$ is one-way. The main motivation of this study is the construction of public-key encryption schemes that are secure against chosen-ciphertext attacks (CCA). We show that any collection of injective trapdoor functions that is secure under very natural correlated products can be used to construct a CCA-secure public-key encryption scheme. The construction is simple, black-box, and admits a direct proof of security.
We provide evidence that security under correlated products is achievable by demonstrating that any collection of lossy trapdoor functions, a powerful primitive introduced by Peikert and Waters (STOC '08), yields a collection of injective trapdoor functions that is secure under the above mentioned natural correlated products. Although we eventually base security under correlated products on lossy trapdoor functions, we argue that the former notion is potentially weaker as a general assumption. Specifically, there is no fully-black-box construction of lossy trapdoor functions from trapdoor functions that are secure under correlated products.

2008

EPRINT

Efficient Lossy Trapdoor Functions based on the Composite Residuosity Assumption
Abstract

Lossy trapdoor functions (Peikert and Waters, STOC '08) are an intriguing and powerful cryptographic primitive. Their main applications are simple and black-box constructions of chosen-ciphertext secure encryption, as well as collision-resistant hash functions and oblivious transfer. An appealing property of lossy trapdoor functions is the ability to realize them from a variety of number-theoretic assumptions, such as the hardness of the decisional Diffie-Hellman problem, and the worst-case hardness of lattice problems.
In this short note we propose a new construction of lossy trapdoor functions based on the Damg{\aa}rd-Jurik encryption scheme (whose security relies on Paillier's decisional composite residuosity assumption). Our approach also yields a direct construction of all-but-one trapdoor functions, an important ingredient of the Peikert-Waters encryption scheme. The functions we propose enjoy short public descriptions, which in turn yield more efficient encryption schemes.

2006

EPRINT

Lattices that Admit Logarithmic Worst-Case to Average-Case Connection Factors
Abstract

We demonstrate an \emph{average-case} problem which is as hard as
finding $\gamma(n)$-approximate shortest vectors in certain
$n$-dimensional lattices in the \emph{worst case}, where $\gamma(n)
= O(\sqrt{\log n})$. The previously best known factor for any class
of lattices was $\gamma(n) = \tilde{O}(n)$.
To obtain our results, we focus on families of lattices having
special algebraic structure. Specifically, we consider lattices
that correspond to \emph{ideals} in the ring of integers of an
algebraic number field. The worst-case assumption we rely on is
that in some $\ell_p$ length, it is hard to find approximate
shortest vectors in these lattices, under an appropriate form of
preprocessing of the number field. Our results build upon prior
works by Micciancio (FOCS 2002), Peikert and Rosen (TCC 2006), and
Lyubashevsky and Micciancio (ICALP 2006).
For the connection factors $\gamma(n)$ we achieve, the corresponding
\emph{decisional} promise problems on ideal lattices are \emph{not}
known to be NP-hard; in fact, they are in P. However, the
\emph{search} approximation problems still appear to be very hard.
Indeed, ideal lattices are well-studied objects in computational
number theory, and the best known algorithms for them seem to
perform \emph{no better} than the best known algorithms for general
lattices.
To obtain the best possible connection factor, we instantiate our
constructions with infinite families of number fields having
constant \emph{root discriminant}. Such families are known to exist
and are computable, though no efficient construction is yet known.
Our work motivates the search for such constructions. Even
constructions of number fields having root discriminant up to
$O(n^{2/3-\epsilon})$ would yield connection factors better than the
current best of~$\tilde{O}(n)$.

2001

EPRINT

Black-Box Concurrent Zero-Knowledge Requires $\tilde\Omega(\log n)$ Rounds
Abstract

We show that any concurrent zero-knowledge protocol for a non-trivial
language (i.e., for a language outside $\BPP$), whose security is
proven via black-box simulation, must use at least
$\tilde\Omega(\log n)$ rounds of interaction. This result achieves a
substantial improvement over previous lower bounds, and is the first
bound to rule out the possibility of constant-round concurrent
zero-knowledge when proven via black-box simulation. Furthermore, the
bound is polynomially related to the number of rounds in the best
known concurrent zero-knowledge protocol for languages in $\NP$.

2001

EPRINT

Pseudo-Random Functions and Factoring
Abstract

Factoring integers is the most established problem on which
cryptographic primitives are based. This work presents an efficient
construction of {\em pseudorandom functions} whose security is based
on the intractability of factoring. In particular, we are able to
construct efficient length-preserving pseudorandom functions where
each evaluation requires only a {\em constant} number of modular
multiplications per output bit. This is substantially more efficient
than any previous construction of pseudorandom functions based on
factoring, and matches (up to a constant factor) the efficiency of
the best known factoring-based {\em pseudorandom bit generators}.

#### Program Committees

- Crypto 2020
- TCC 2020
- TCC 2019
- TCC 2019
- Eurocrypt 2018
- TCC 2018
- Eurocrypt 2015
- Asiacrypt 2014
- TCC 2013
- PKC 2012
- TCC 2010
- PKC 2010
- Crypto 2008
- Eurocrypt 2007
- TCC 2005

#### Coauthors

- Masayuki Abe (1)
- Shweta Agrawal (2)
- Shashank Agrawal (1)
- Giulia Alberini (1)
- Miguel Ambrona (1)
- Prabhanjan Ananth (1)
- Benny Applebaum (2)
- Marshall Ball (1)
- Abhishek Banerjee (2)
- Nir Bitansky (2)
- Alexander R. Block (1)
- Andrey Bogdanov (2)
- Andrej Bogdanov (4)
- Hai Brenner (2)
- Ran Canetti (3)
- Henry Cohn (1)
- Yan Zong Ding (2)
- David Freeman (1)
- Lubos Gaspar (1)
- Oded Goldreich (1)
- Shafi Goldwasser (1)
- Vipul Goyal (2)
- Ronen Gradwohl (1)
- Siyao Guo (6)
- Iftach Haitner (1)
- Danny Harnik (4)
- Brett Hemenway (3)
- Justin Holmgren (1)
- Pavel Hubáček (4)
- Yuval Ishai (1)
- Yael Tauman Kalai (1)
- Pritish Kamath (1)
- Joe Kilian (2)
- Eike Kiltz (1)
- Ilan Komargodski (1)
- Eyal Kushilevitz (1)
- Gaëtan Leurent (2)
- Noam Livne (1)
- Vadim Lyubashevsky (1)
- Tal Malkin (1)
- Daniel Masny (2)
- Daniele Micciancio (1)
- Tal Moran (2)
- Moni Naor (4)
- Varun Narayanan (1)
- Jesper Buus Nielsen (1)
- Miyako Ohkubo (1)
- Igor Carboni Oliveira (1)
- Shien Jin Ong (2)
- Rafail Ostrovsky (3)
- Omer Paneth (2)
- David C. Parkes (2)
- Rafael Pass (1)
- Chris Peikert (6)
- Erez Petrank (1)
- Krzysztof Pietrzak (1)
- Vinod Prabhakaran (1)
- Manoj Prabhakaran (2)
- Omer Reingold (3)
- Silas Richelson (4)
- Ron D. Rothblum (1)
- Manuel Sabin (1)
- Gil Segev (6)
- Ido Shahaf (1)
- Ronen Shaltiel (3)
- Abhi Shelat (1)
- Pratik Soni (1)
- Katerina Sotiraki (1)
- François-Xavier Standaert (1)
- Salil P. Vadhan (2)
- Margarita Vald (4)
- Prashant Nalini Vasudevan (1)
- Eylon Yogev (1)