## CryptoDB

### Tal Malkin

#### Publications

**Year**

**Venue**

**Title**

2023

JOFC

Topology-Hiding Communication from Minimal Assumptions
Abstract

Topology-hiding broadcast ( THB ) enables parties communicating over an incomplete network to broadcast messages while hiding the topology from within a given class of graphs. THB is a central tool underlying general topology-hiding secure computation ( THC ) (Moran et al. TCC’15). Although broadcast is a privacy-free task, it was recently shown that THB for certain graph classes necessitates computational assumptions, even in the semi-honest setting, and even given a single corrupted party. In this work, we investigate the minimal assumptions required for topology-hiding communication: both Broadcast or Anonymous Broadcast (where the broadcaster’s identity is hidden). We develop new techniques that yield a variety of necessary and sufficient conditions for the feasibility of THB / THAB in different cryptographic settings: information theoretic, given existence of key agreement, and given existence of oblivious transfer. Our results show that feasibility can depend on various properties of the graph class, such as connectivity , and highlight the role of different properties of topology when kept hidden, including direction , distance , and/or distance-of-neighbors to the broadcaster. An interesting corollary of our results is a dichotomy for THC with a public number of at least three parties, secure against one corruption: information-theoretic feasibility if all graphs are 2-connected; necessity and sufficiency of key agreement otherwise.

2022

EUROCRYPT

Unclonable Polymers and Their Cryptographic Applications
📺
Abstract

We propose a mechanism for generating and manipulating protein polymers to obtain a new type of *consumable storage* that exhibits intriguing cryptographic "self-destruct" properties, assuming the hardness of certain polymer-sequencing problems.
To demonstrate the cryptographic potential of this technology, we first develop a formalism that captures (in a minimalistic way) the functionality and security properties provided by the technology. Next, using this technology, we construct and prove security of two cryptographic applications that are currently obtainable only via trusted hardware that implements logical circuitry (either classical or quantum). The first application is a password-controlled *secure vault* where the stored data is irrecoverably erased once a threshold of unsuccessful access attempts is reached. The second is (a somewhat relaxed version of) *one time programs*, namely a device that allows evaluating a secret function only a limited number of times before self-destructing, where each evaluation is made on a fresh user-chosen input.
Finally, while our constructions, modeling, and analysis are designed to capture the proposed polymer-based technology, they are sufficiently general to be of potential independent interest.

2022

TCC

Poly Onions: Achieving Anonymity in the Presence of Churn
Abstract

Onion routing is a popular approach towards anonymous communication. Practical implementations are widely used (for example, Tor has millions of users daily), but are vulnerable to various traffic correlation attacks, and the theoretical foundations, despite recent progress, still lag behind.
In particular, all works that model onion routing protocols and prove their security only address a single run, where each party sends and receives a single message of fixed length, once. Moreover, they all assume a static network setting, where the parties are stable throughout the lifetime of the protocol. In contrast, real networks have a high rate of churn (nodes joining and exiting the network), real users want to send multiple messages, and realistic adversaries may observe multiple runs of the protocol.
We initiate a formal treatment of onion routing in a setting with multiple runs over a dynamic network with churn. We provide definitions of both security and anonymity in this setting, and constructions that satisfy them. In particular, we define a new cryptographic primitive called \emph{Poly Onions} and show that it can be used to realize our definitions.

2020

CRYPTO

Non-Malleability against Polynomial Tampering
📺
Abstract

We present the first explicit construction of a non-malleable code that can handle tampering functions that are bounded-degree polynomials.
Prior to our work, this was only known for degree-1 polynomials (affine tampering functions), due to Chattopadhyay and Li (STOC 2017). As a direct corollary, we obtain an explicit non-malleable code that is secure against tampering by bounded-size arithmetic circuits.
We show applications of our non-malleable code in constructing non-malleable secret sharing schemes that are robust against bounded-degree polynomial tampering. In fact our result is stronger: we can handle adversaries that can adaptively choose the polynomial tampering function based on initial leakage of a bounded number of shares.
Our results are derived from explicit constructions of seedless non-malleable extractors that can handle bounded-degree polynomial tampering functions. Prior to our work, no such result was known even for degree-2 (quadratic) polynomials.

2020

TCC

Topology-Hiding Communication from Minimal Assumptions.
📺
Abstract

Topology-hiding broadcast (THB) enables parties communicating over an incomplete network to broadcast messages while hiding the topology from within a given class of graphs. THB is a central tool underlying general topology-hiding secure computation (THC) (Moran et al. TCC’15). Although broadcast is a privacy-free task, it was recently shown that THB for certain graph classes necessitates computational assumptions, even in the semi-honest setting, and even given a single corrupted party.
In this work we investigate the minimal assumptions required for topology-hiding communication—both Broadcast or Anonymous Broadcast (where the broadcaster’s identity is hidden). We develop new techniques that yield a variety of necessary and sufficient conditions for the feasibility of THB/THAB in different cryptographic settings: information theoretic, given existence of key agreement, and given existence of oblivious transfer. Our results show that feasibility can depend on various properties of the graph class, such as connectivity, and highlight the role of different properties of topology when kept hidden, including direction, distance, and/or distance-of-neighbors to the broadcaster. An interesting corollary of our results is a dichotomy for THC with a public number of at least three parties, secure against one corruption: information-theoretic feasibility if all graphs are 2-connected; necessity and sufficiency of key agreement otherwise.

2019

EUROCRYPT

Non-Malleable Codes Against Bounded Polynomial Time Tampering
📺
Abstract

We construct efficient non-malleable codes (NMC) that are (computationally) secure against tampering by functions computable in any fixed polynomial time. Our construction is in the plain (no-CRS) model and requires the assumptions that (1) $$\mathbf {E}$$E is hard for $$\mathbf {NP}$$NP circuits of some exponential $$2^{\beta n}$$2βn ($$\beta >0$$β>0) size (widely used in the derandomization literature), (2) sub-exponential trapdoor permutations exist, and (3) $$\mathbf {P}$$P-certificates with sub-exponential soundness exist.While it is impossible to construct NMC secure against arbitrary polynomial-time tampering (Dziembowski, Pietrzak, Wichs, ICS ’10), the existence of NMC secure against $$O(n^c)$$O(nc)-time tampering functions (for any fixedc), was shown (Cheraghchi and Guruswami, ITCS ’14) via a probabilistic construction. An explicit construction was given (Faust, Mukherjee, Venturi, Wichs, Eurocrypt ’14) assuming an untamperable CRS with length longer than the runtime of the tampering function. In this work, we show that under computational assumptions, we can bypass these limitations. Specifically, under the assumptions listed above, we obtain non-malleable codes in the plain model against $$O(n^c)$$O(nc)-time tampering functions (for any fixed c), with codeword length independent of the tampering time bound.Our new construction of NMC draws a connection with non-interactive non-malleable commitments. In fact, we show that in the NMC setting, it suffices to have a much weaker notion called quasi non-malleable commitments—these are non-interactive, non-malleable commitments in the plain model, in which the adversary runs in $$O(n^c)$$O(nc)-time, whereas the honest parties may run in longer (polynomial) time. We then construct a 4-tag quasi non-malleable commitment from any sub-exponential OWF and the assumption that $$\mathbf {E}$$E is hard for some exponential size $$\mathbf {NP}$$NP-circuits, and use tag amplification techniques to support an exponential number of tags.

2019

TCC

Is Information-Theoretic Topology-Hiding Computation Possible?
Abstract

Topology-hiding computation (THC) is a form of multi-party computation over an incomplete communication graph that maintains the privacy of the underlying graph topology. Existing THC protocols consider an adversary that may corrupt an arbitrary number of parties, and rely on cryptographic assumptions such as DDH.In this paper we address the question of whether information-theoretic THC can be achieved by taking advantage of an honest majority. In contrast to the standard MPC setting, this problem has remained open in the topology-hiding realm, even for simple “privacy-free” functions like broadcast, and even when considering only semi-honest corruptions.We uncover a rich landscape of both positive and negative answers to the above question, showing that what types of graphs are used and how they are selected is an important factor in determining the feasibility of hiding topology information-theoretically. In particular, our results include the following.
We show that topology-hiding broadcast (THB) on a line with four nodes, secure against a single semi-honest corruption, implies key agreement. This result extends to broader classes of graphs, e.g., THB on a cycle with two semi-honest corruptions.On the other hand, we provide the first feasibility result for information-theoretic THC: for the class of cycle graphs, with a single semi-honest corruption.
Given the strong impossibilities, we put forth a weaker definition of distributional-THC, where the graph is selected from some distribution (as opposed to worst-case).
We present a formal separation between the definitions, by showing a distribution for which information theoretic distributional-THC is possible, but even topology-hiding broadcast is not possible information-theoretically with the standard definition.We demonstrate the power of our new definition via a new connection to adaptively secure low-locality MPC, where distributional-THC enables parties to “reuse” a secret low-degree communication graph even in the face of adaptive corruptions.

2019

ASIACRYPT

Public-Key Function-Private Hidden Vector Encryption (and More)
Abstract

We construct public-key function-private predicate encryption for the “small superset functionality,” recently introduced by Beullens and Wee (PKC 2019). This functionality captures several important classes of predicates:Point functions. For point function predicates, our construction is equivalent to public-key function-private anonymous identity-based encryption.Conjunctions. If the predicate computes a conjunction, our construction is a public-key function-private hidden vector encryption scheme. This addresses an open problem posed by Boneh, Raghunathan, and Segev (ASIACRYPT 2013).d-CNFs and read-once conjunctions of d-disjunctions for constant-size d.
Our construction extends the group-based obfuscation schemes of Bishop et al. (CRYPTO 2018), Beullens and Wee (PKC 2019), and Bartusek et al. (EUROCRYPT 2019) to the setting of public-key function-private predicate encryption. We achieve an average-case notion of function privacy, which guarantees that a decryption key
$$\mathsf {sk} _f$$
reveals nothing about f as long as f is drawn from a distribution with sufficient entropy. We formalize this security notion as a generalization of the (enhanced) real-or-random function privacy definition of Boneh, Raghunathan, and Segev (CRYPTO 2013). Our construction relies on bilinear groups, and we prove security in the generic bilinear group model.

2018

EUROCRYPT

2018

CRYPTO

A Simple Obfuscation Scheme for Pattern-Matching with Wildcards
📺
Abstract

We give a simple and efficient method for obfuscating pattern matching with wildcards. In other words, we construct a way to check an input against a secret pattern, which is described in terms of prescribed values interspersed with unconstrained “wildcard” slots. As long as the support of the pattern is sufficiently sparse and the pattern itself is chosen from an appropriate distribution, we prove that a polynomial-time adversary cannot find a matching input, except with negligible probability. We rely upon the generic group heuristic (in a regular group, with no multilinearity). Previous work [9, 10, 32] provided less efficient constructions based on multilinear maps or LWE.

2018

CRYPTO

Hardness of Non-interactive Differential Privacy from One-Way Functions
📺
Abstract

A central challenge in differential privacy is to design computationally efficient non-interactive algorithms that can answer large numbers of statistical queries on a sensitive dataset. That is, we would like to design a differentially private algorithm that takes a dataset $$D \in X^n$$D∈Xn consisting of some small number of elements n from some large data universe X, and efficiently outputs a summary that allows a user to efficiently obtain an answer to any query in some large family Q.Ignoring computational constraints, this problem can be solved even when X and Q are exponentially large and n is just a small polynomial; however, all algorithms with remotely similar guarantees run in exponential time. There have been several results showing that, under the strong assumption of indistinguishability obfuscation, no efficient differentially private algorithm exists when X and Q can be exponentially large. However, there are no strong separations between information-theoretic and computationally efficient differentially private algorithms under any standard complexity assumption.In this work we show that, if one-way functions exist, there is no general purpose differentially private algorithm that works when X and Q are exponentially large, and n is an arbitrary polynomial. In fact, we show that this result holds even if X is just subexponentially large (assuming only polynomially-hard one-way functions). This result solves an open problem posed by Vadhan in his recent survey [52].

2013

ASIACRYPT

2013

EUROCRYPT

2008

TCC

2004

JOFC

2000

CRYPTO

#### Program Committees

- Crypto 2021 (Program chair)
- Crypto 2021
- Eurocrypt 2020
- TCC 2018
- Eurocrypt 2017
- TCC 2016 (Program chair)
- Crypto 2012
- TCC 2012
- Crypto 2008
- Crypto 2006
- TCC 2006
- Crypto 2005
- TCC 2005
- Crypto 2004
- PKC 2003

#### Coauthors

- Ghada Almashaqbeh (1)
- Megumi Ando (1)
- Marshall Ball (8)
- James Bartusek (1)
- Amos Beimel (6)
- Allison Bishop (1)
- Elette Boyle (4)
- Ran Canetti (4)
- Brent Carmer (1)
- Melissa Chase (1)
- Anupam Chattopadhyay (1)
- Seung Geol Choi (7)
- Miranda Christ (1)
- Ran Cohen (3)
- Giovanni Di Crescenzo (1)
- Dana Dachman-Soled (11)
- Ivan Damgård (2)
- Yevgeniy Dodis (1)
- Stefan Dziembowski (2)
- Ariel Elbaz (2)
- Yaniv Erlich (1)
- Rosario Gennaro (2)
- Jonathan Gershoni (1)
- Yael Gertner (1)
- S. Dov Gordon (1)
- Siyao Guo (1)
- Alexander Healy (1)
- Loïs Huguenin-Dumittan (1)
- Yuval Ishai (4)
- Abhishek Jain (1)
- Zhengzhong Jin (1)
- Ari Juels (1)
- Aggelos Kiayias (2)
- Lisa Kohl (2)
- Lucas Kowalczyk (3)
- Hugo Krawczyk (1)
- Mukul Kulkarni (3)
- Tancrède Lepoint (1)
- Jyun-Jie Liao (1)
- Huijia Lin (1)
- Yehuda Lindell (1)
- Anna Lysyanskaya (3)
- Fermi Ma (1)
- Mohammad Mahmoody (2)
- Alex J. Malozemoff (1)
- Pierre Meyer (2)
- Silvio Micali (2)
- Daniele Micciancio (1)
- Sara K. Miner (1)
- Tal Moran (4)
- Ryan Moriarty (1)
- Steven Myers (1)
- Kobbi Nissim (3)
- Satoshi Obana (1)
- Igor Carboni Oliveira (1)
- Rafail Ostrovsky (1)
- Valerio Pastro (1)
- Itsik Pe'er (1)
- Tal Rabin (1)
- Mariana Raykova (3)
- Leonid Reyzin (1)
- Anna Roitburd-Berman (1)
- Alon Rosen (1)
- Mike Rosulek (1)
- Kevin Shi (1)
- François-Xavier Standaert (1)
- Isamu Teranishi (3)
- Eran Tromer (1)
- Jonathan Ullman (2)
- Yevgeniy Vahlis (1)
- Muthuramakrishnan Venkitasubramaniam (1)
- Hoeteck Wee (5)
- Enav Weinreb (2)
- Daniel Wichs (1)
- Nikolai Yakovenko (1)
- Moti Yung (8)
- Mark Zhandry (1)