International Association
for Cryptologic Research

Is it possible to prove the deletion of a computer program after having executed it? While this task is clearly impossible using classical information alone, the laws of quantum mechanics may admit a solution to this problem. In this work, we propose a new approach to answer this question, using quantum information. In the interactive settings, we present the first fully-secure solution for blind delegation with certified deletion, assuming post-quantum hardness of the learning with errors (LWE) problem. In the non-interactive settings, we propose a construction of obfuscation with certified deletion, assuming post-quantum iO and one-way functions. Our main technical contribution is a new deletion theorem for subspace coset states [Vidick and Zhang, EUROCRYPT'21, Coladangelo et al., CRYPTO'21], which enables a generic compiler that adds the certified deletion guarantee to a variety of cryptographic primitives. In addition to our main result, this allows us to obtain a host of new primitives, such as functional encryption with certified deletion and secure software leasing for an interesting class of programs. In fact, we are able for the first time to achieve a stronger notion of secure software leasing, where even a dishonest evaluator cannot evaluate the program after returning it.

Secure multi-party computation (MPC) allows a set of $n$ parties to jointly compute a function over their private inputs. The seminal works of Ben-Or, Canetti and Goldreich [STOC'93] and Ben-Or, Kelmer and Rabin [PODC'94] settled the feasibility of MPC over asynchronous networks. Despite the significant line of work devoted to improving the communication complexity, current protocols with information-theoretic security and optimal resilience $t<n/3$ communicate $\Omega(n^4C)$ field elements for a circuit with $C$ multiplication gates. In contrast, synchronous MPC protocols with $\Omega(nC)$ communication have long been known. In this work we make progress towards closing this gap. We provide a novel MPC protocol that makes black-box use of an asynchronous complete secret-sharing (ACSS), where the cost per multiplication reduces to the cost of distributing a constant number of sharings in the ACSS, improving a linear factor over the state of the art by Choudhury and Patra [IEEE Trans. Inf. Theory '17]. Instantiating the ACSS with the protocol by Choudhury and Patra [J. Crypto '23] we achieve an MPC protocol with $\mathcal{O}(n^3C)$ communication. Moreover, with a recent concurrent work achieving ACSS with linear cost per sharing, we achieve an MPC with $\mathcal{O}(nC)$ communication.

Non-malleable codes are fundamental objects at the intersection of cryptography and coding theory. These codes provide security guarantees even in settings where error correction and detection are impossible, and have found applications to several other cryptographic tasks. One of the strongest and most well-studied adversarial tampering models is 2-split-state tampering. Here, a codeword is split into two parts which are stored in physically distant servers, and the adversary can then independently tamper with each part using arbitrary functions. This model can be naturally extended to the secret sharing setting with several parties by having the adversary independently tamper with each share. Previous works on non-malleable coding and secret sharing in the split-state tampering model only considered the encoding of classical messages. Furthermore, until recent work by Aggarwal, Boddu, and Jain (IEEE Trans. Inf. Theory 2024 & arXiv 2022), adversaries with quantum capabilities and shared entanglement had not been considered, and it is a priori not clear whether previous schemes remain secure in this model. In this work, we introduce the notions of split-state non-malleable codes and secret sharing schemes for quantum messages secure against quantum adversaries with shared entanglement. Then, we present explicit constructions of such schemes that achieve low-error non-malleability. More precisely, for some constant c>0, we construct efficiently encodable and decodable split-state non-malleable codes and secret sharing schemes for quantum messages preserving entanglement with external systems and achieving security against quantum adversaries having shared entanglement with codeword length n, any message length at most n^c, and error \eps=2^{-{n^{c}}}. In the easier setting of average-case non-malleability, we achieve efficient non-malleable coding with rate close to 1/11.

Unclonable cryptography utilizes the principles of quantum mechanics to addresses cryptographic tasks that are impossible classically. We introduce a novel unclonable primitive in the context of secret sharing, called unclonable secret sharing (USS). In a USS scheme, there are shareholders, each holding a share of a classical secret represented as a quantum state. They can recover the secret once all parties (or at least parties) come together with their shares. Importantly, it should be infeasible to copy their own shares and send the copies to two non-communicating parties, enabling both of them to recover the secret. Our work initiates a formal investigation into the realm of unclonable secret sharing, shedding light on its implications, constructions, and inherent limitations. ** Connections: We explore the connections between USS and other quantum cryptographic primitives such as unclonable encryption and position verification, showing the difficulties to achieve USS in different scenarios. **Limited Entanglement: In the case where the adversarial shareholders do not share any entanglement or limited entanglement, we demonstrate information-theoretic constructions for USS. **Large Entanglement: If we allow the adversarial shareholders to have unbounded entanglement resources (and unbounded computation), we prove that unclonable secret sharing is impossible. On the other hand, in the quantum random oracle model where the adversary can only make a bounded polynomial number of queries, we show a construction secure even with unbounded entanglement. Furthermore, even when these adversaries possess only a polynomial amount of entanglement resources, we establish that any unclonable secret sharing scheme with a reconstruction function implementable using Cliffords and logarithmically many T-gates is also unattainable.

In this work, we study constant round multiparty computation (MPC) for Boolean circuits against a fully malicious adversary who may control up to $n-1$ out of $n$ parties. Without relying on fully homomorphic encryption (FHE), the best-known results in this setting are achieved by Wang et al. (CCS 2017) and Hazay et al. (ASIACRYPT 2017) based on garbled circuits, which require a quadratic communication in the number of parties $O(|C|\cdot n^2)$. In contrast, for non-constant round MPC, the recent result by Rachuri and Scholl (CRYPTO 2022) has achieved linear communication $O(|C|\cdot n)$. In this work, we present the first concretely efficient constant round MPC protocol in this setting with linear communication in the number of parties $O(|C|\cdot n)$. Our construction can be based on any public-key encryption scheme that is linearly homomorphic for public keys. Our work gives a concrete instantiation from a variant of the El-Gamal Encryption Scheme assuming the DDH assumption. The analysis shows that when the computational security parameter $\lambda=128$ and statistical security parameter $\kappa=80$, our protocol achieves a smaller communication than Wang et al. (CCS 2017) when there are $16$ parties for AES circuit, and $8$ parties for general Boolean circuits (where we assume that the numbers of AND gates and XOR gates are the same). When comparing with the recent work by Beck et al. (CCS 2023) that achieves constant communication complexity $O(|C|)$ in the strong honest majority setting ($t<(1/2-\epsilon)n$ where $\epsilon$ is a constant), our protocol is better as long as $n<3500$ (when $t=n/4$ for their work).

Non-malleable cryptography, proposed by Dolev, Dwork, and Naor (SICOMP '00), has numerous applications in protocol composition. In the context of proofs, it guarantees that an adversary who receives a proof cannot maul it into another valid proof. However, non-malleable cryptography (particularly in the non-interactive setting) suffers from an important limitation: An attacker can always copy the proof and resubmit it to another verifier (or even multiple verifiers). In this work, we prevent even the possibility of copying the proof as it is, by relying on quantum information. We call the resulting primitive unclonable proofs, making progress on a question posed by Aaronson. We also consider the related notion of unclonable commitments. We introduce formal definitions of these primitives that model security in various settings of interest. We also provide a near tight characterization of the conditions under which these primitives are possible, including a rough equivalence between unclonable proofs and public-key quantum money.

Can an adversary hack into our computer and steal sensitive data such as cryptographic keys? This question is almost as old as the Internet and significant effort has been spent on designing mechanisms to prevent and detect hacking attacks. Once quantum computers arrive, will the situation remain the same or can we hope to live in a better world? We first consider ubiquitous side-channel attacks, which aim to leak side information on secret system components, studied in the leakage-resilient cryptography literature. Classical leakage-resilient cryptography must necessarily impose restrictions on the type of leakage one aims to protect against. As a notable example, the most well-studied leakage model is that of bounded leakage, where it is assumed that an adversary learns at most l bits of leakage on secret components, for some leakage bound l. Although this leakage bound is necessary, many real-world side-channel attacks cannot be captured by bounded leakage. In this work, we design cryptographic schemes that provide guarantees against arbitrary side-channel attacks: • Using techniques from unclonable quantum cryptography, we design several basic leakage- resilient primitives, such as public- and private-key encryption, (weak) pseudorandom func- tions, digital signatures and quantum money schemes which remain secure under (polyno- mially) unbounded classical leakage. In particular, this leakage can be much longer than the (quantum) secret being leaked upon. In our view, leakage is the result of observations of quantities such as power consumption and hence is most naturally viewed as classi- cal information. Notably, the leakage-resilience of our schemes holds even in the stronger “LOCC leakage” model where the adversary can obtain adaptive leakage for (polynomially) unbounded number of rounds. • What if the adversary simply breaks into our system to steal our secret keys, rather than mounting only a side-channel attack? What if the adversary can even tamper with the data arbitrarily, for example to cover its tracks? We initiate the study of intrusion- detection in the quantum setting, where one would like to detect if security has been compromised even in the face of an arbitrary intruder attack which can leak and tamper with classical as well as quantum data. We design cryptographic schemes supporting intrusion detection for a host of primitives such as public- and private-key encryption, digital signature, functional encryption, program obfuscation and software protection. Our schemes are based on techniques from cryptography with secure key leasing and certified deletion

Quantum no-cloning theorem gives rise to the intriguing possibility of quantum copy protection where we encode a program or functionality in a quantum state such that a user in possession of k copies cannot create k + 1 copies, for any k. Introduced by Aaronson (CCC’09) over a decade ago, copy protection has proven to be notoriously hard to achieve. Previous work has been able to achieve copy-protection for various functionalities only in restricted models: (i) in the bounded collusion setting where k → k + 1 security is achieved for a-priori fixed collusion bound k (in the plain model with the same computational assumptions as ours, by Liu, Liu, Qian, Zhandry [TCC’22]), or, (ii) only k → 2k security is achieved (relative to a structured quantum oracle, by Aaronson [CCC’09]). In this work, we give the first unbounded collusion-resistant (i.e. multiple-copy secure) copy- protection schemes, answering the long-standing open question of constructing such schemes, raised by multiple previous works starting with Aaronson (CCC’09). More specifically, we obtain the following results. - We construct (i) public-keyencryption,(ii) public-keyfunctionalencryption,(iii) signature and (iv) pseudorandom function schemes whose keys are copy-protected against unbounded collusions in the plain model (i.e. without any idealized oracles), assuming (post-quantum) subexponentially secure iO and LWE. - We show that any unlearnable functionality can be copy-protected against unbounded collusions, relative to a classical oracle. - As a corollary of our results, we rule out the existence of hyperefficient quantum shadow tomography and hence answer an open question by Aaronson (STOC’18). We obtain our results through a novel technique which uses identity-based encryption to construct multiple copy secure copy-protection schemes from 1-copy → 2-copy secure schemes. We believe our technique is of independent interest. Along the way, we also obtain the following results. - We define and prove the security of new collusion-resistant monogamy-of-entanglement games for coset states. - We construct a classical puncturable functional encryption scheme whose master secret key can be punctured at all functions f such that f(m0) ̸= f(m1). This might also be of independent interest.

In this work we present a novel actively secure dishonest majority MPC protocol, \textsc{SuperPack}, whose efficiency improves as the number of \emph{honest} parties increases. Concretely, let $0<\epsilon<1/2$ and consider an adversary that corrupts $t<n(1-\epsilon)$ out of $n$ parties. \textsc{SuperPack} requires $6/\epsilon$ field elements of online communication per multiplication gate across all parties, assuming circuit-dependent preprocessing, and $10/\epsilon$ assuming circuit-independent preprocessing. In contrast, most of previous works such as SPDZ (Damg\aa rd \emph{et al}, ESORICS 2013) and its derivatives perform the same regardless of whether there is only one honest party, or a constant (non-majority) fraction of honest parties. The only exception is due to Goyal \emph{et al} (CRYPTO 2022), which achieves $58/\epsilon + 96/\epsilon^2$ field elements assuming circuit-independent preprocessing. Our work improves this result substantially by a factor of at least $25$ in the circuit-independent preprocessing model. Practically, we also compare our work with the best concretely efficient online protocol Turbospeedz (Ben-Efraim \emph{et al}, ACNS 2019), which achieves $2(1-\epsilon)n$ field elements per multiplication gate among all parties. Our online protocol improves over Turbospeedz as $n$ grows, and as $\epsilon$ approaches $1/2$. For example, if there are $90\%$ corruptions ($\epsilon=0.1$), with $n=50$ our online protocol is $1.5\times$ better than Turbospeedz and with $n=100$ this factor is $3\times$, but for $70\%$ corruptions ($\epsilon=0.3$) with $n=50$ our online protocol is $3.5\times$ better, and for $n=100$ this factor is $7\times$. Our circuit-dependent preprocessing can be instantiated from OLE/VOLE. The amount of OLE/VOLE correlations required in our work is a factor of $\approx \epsilon n/2$ smaller than these required by Le Mans (Rachuri and Scholl, CRYPTO 2022) leveraged to instantiate the proprocesing of Turbospeedz. Our dishonest majority protocol relies on packed secret-sharing and leverages ideas from the honest majority \textsc{TurboPack} (Escudero \emph{et al}, CCS 2022) protocol to achieve concrete efficiency for any circuit topology, not only SIMD. We implement both \textsc{SuperPack} and Turbospeedz and verify with experimental results that our approach indeed leads to more competitive runtimes in distributed environments with a moderately large number of parties.

Recently, significant progress has been made toward quantumly secure multi-party computation (MPC) in the stand-alone setting. In sharp contrast, the picture of concurrently secure MPC (or even 2PC), for both classical and quantum functionalities, still remains unclear. Quantum information behaves in a fundamentally different way, making the job of adversary harder and easier at the same time. Thus, it is unclear if the positive or negative results from the classical setting still apply. This work initiates a systematic study of concurrent secure computation in the quantum setting. We obtain a mix of positive and negative results. We first show that assuming the existence of post-quantum one-way functions (PQ-OWFs), concurrently secure 2PC (and thus MPC) for quantum functionalities is impossible. Next, we focus on the bounded-concurrent setting, where we obtain simulation-sound zero-knowledge arguments for both NP and QMA, assuming PQ-OWFs. This is obtained by a new design of simulation-sound gadget, relying on the recent post-quantum non-malleable commitments by Liang, Pandey, and Yamakawa [arXiv:2207.05861], and the quantum rewinding strategy recently developed by Ananth, Chung, and La Placa [CRYPTO'21] for bounded-concurrent post-quantum ZK. Moreover, we show that our technique is general enough---It also leads to quantum-secure bounded-concurrent coin-flipping protocols, and eventually general-purpose 2PC and MPC, for both classical and quantum functionalities. All these constructions can be based on the quantum hardness of Learning with Errors.

Consider the standard setting of two-party computation where the sender has a secret function $f$ and the receiver has a secret input $x$ and the output $f(x)$ is delivered to the receiver at the end of the protocol. Let us consider the unidirectional message model where only one party speaks in each round. In this setting, Katz and Ostrovsky (Crypto 2004) showed that at least four rounds of interaction between the parties are needed in the plain model (i.e., no trusted setup) if the simulator uses the adversary in a black-box way (a.k.a. black-box simulation). Suppose the sender and the receiver would like to run multiple sequential iterations of the secure computation protocol on possibly different inputs. For each of these iterations, do the parties need to start the protocol from scratch and exchange four messages? In this work, we explore the possibility of \textit{amortizing} the round complexity or in other words, \textit{reusing} a certain number of rounds of the secure computation protocol in the plain model. We obtain the following results. \begin{itemize} \item Under standard cryptographic assumptions, we construct a four-round two-party computation protocol where (i) the first three rounds of the protocol could be reused an unbounded number of times if the receiver input remains the same and only the sender input changes, and (ii) the first two rounds of the protocol could be reused an unbounded number of times if the receiver input needs to change as well. In other words, the sender sends a single additional message if only its input changes, and in the other case, we need one message each from the receiver and the sender. The number of additional messages needed in each of the above two modes is optimal and, additionally, our protocol allows arbitrary interleaving of these two modes. \item We also extend these results to the multiparty setting (in the simultaneous message exchange model) and give round-optimal protocols such that (i) the first two rounds could be reused an unbounded number of times if the inputs of the parties need to change and (ii) the first three rounds could be reused an unbounded number of times if the inputs remain the same but the functionality to be computed changes. As in the two-party setting, we allow arbitrary interleaving of the above two modes of operation. \end{itemize}

We revisit the well-studied problem of preventing steganographic communication in multi-party communications. While this is known to be a provably impossible task, we propose a new model that allows circumventing this impossibility. In our model, the parties first publish a single message during an honest \emph{non-interactive} pre-processing phase and then later interact in an execution phase. We show that in this model, it is indeed possible to prevent any steganographic communication in zero-knowledge protocols. Our solutions rely on standard cryptographic assumptions.

A secret sharing scheme enables one party to distribute shares of a secret to n parties and ensures that an adversary in control of t out of n parties will learn no information about the secret. However, traditional secret sharing schemes are often insufficient, especially for applications in which the set of parties who hold the secret shares might change over time. To achieve security in this setting, dynamic proactive secret sharing (DPSS) is used. DPSS schemes proactively update the secret shares held by the parties and allow changes to the set of parties holding the secrets. We propose FaB-DPSS (FAst Batched DPSS) -- a new and highly optimized batched DPSS scheme. While previous work on batched DPSS focuses on a single client submitting a batch of secrets and does not allow storing and releasing secrets independently, we allow multiple different clients to dynamically share and release secrets. FaB-DPSS is the most efficient robust DPSS scheme that supports the highest possible adversarial threshold of 1/2. We prove FaB-DPSS secure and implement it. All operations complete in seconds, and we outperform a prior state-of-the-art DPSS scheme by over 6 times. Additionally, we propose new applications of DPSS in the context of blockchains. Specifically, we propose a protocol that uses blockchains and FaB-DPSS to provide conditional secret storage. The protocol allows parties to store secrets along with a release condition, and once a (possibly different) party satisfies this release condition, the secret is privately released to that party. This functionality is similar to extractable witness encryption. While there are numerous compelling applications (e.g., time-lock encryption, one-time programs, and fair multi-party computation) which rely on extractable witness encryption, there are no known efficient constructions (or even constructions based on any well-studied assumptions) of extractable witness encryption. However, by utilizing blockchains and FaB-DPSS, we can easily build all those applications. We provide an implementation of our conditional secret storage protocol as well as several applications building on top of it.

A {\em $t$-private} circuit for a function $f$ is a randomized Boolean circuit $C$ that maps a randomized encoding of an input $x$ to an encoding of the output $f(x)$, such that probing $t$ wires anywhere in $C$ reveals nothing about $x$. Private circuits can be used to protect embedded devices against side-channel attacks. Motivated by the high cost of generating fresh randomness in such devices, several works have studied the question of minimizing the randomness complexity of private circuits. The best known upper bound, due to Coron et al. (Eurocrypt 2020), is $O(t^2\cdot\log s)$ random bits, where $s$ is the circuit size of $f$. We improve this to $O(t\cdot \log s)$, including the randomness used by the input encoder, and extend this bound to the stateful variant of private circuits. Our constructions are semi-explicit in the sense that there is an efficient randomized algorithm that generates the private circuit $C$ from a circuit for $f$ with negligible failure probability.

Byzantine agreement is a fundamental primitive in cryptography and distributed computing, and minimizing its round complexity is of paramount importance. It is long known that any randomized $r$-round protocol must fail with probability at least $(c\cdot r)^{-r}$, for some constant $c$, when the number of corruptions is linear in the number of parties, $t = \theta(n)$. On the other hand, current protocols fail with probability at least $2^{-r}$. Whether we can match the lower bound agreement probability remains unknown. In this work, we resolve this long-standing open question. We present a protocol that matches the lower bound up to constant factors. Our results hold under a (strongly rushing) adaptive adversary that can corrupt up to $t = (1-\epsilon)n/2$ parties, and our protocols use a public-key infrastructure and a trusted setup for unique threshold signatures. This is the first protocol that decreases the failure probability (overall) by a \emph{super-constant} factor per round.

We revisit the question of minimizing the randomness complexity of protocols for secure multiparty computation (MPC) in the setting of perfect information-theoretic security. Kushilevitz and Mansour (SIAM J. Discret. Math., 1997) studied the case of n-party semi-honest MPC for the XOR function with security threshold t<n, showing that O(t^2 * log(n/t)) random bits are sufficient and \Omega(t) random bits are necessary. Their positive result was obtained via a non-explicit protocol, whose existence was proved using the probabilistic method. We essentially close the question by proving an \Omega(t^2) lower bound on the randomness complexity of XOR, matching the previous upper bound up to a logarithmic factor (or constant factor when t=\Omega(n)). We also obtain an explicit protocol that uses O(t^2 * \log^2n) random bits, matching our lower bound up to a polylogarithmic factor. We extend these results from XOR to general symmetric Boolean functions and to addition over a finite Abelian group, showing how to amortize the randomness complexity over multiple additions. Finally, combining our techniques with recent randomness-efficient constructions of private circuits, we obtain an explicit protocol for evaluating a general circuit C using only O(t^2 * \log |C|) random bits, by employing additional ``helper parties'' who do not contribute any inputs. This upper bound too matches our lower bound up to a logarithmic factor.

In the last few years, the efficiency of secure multi-party computation (MPC) in the dishonest majority setting has increased by several orders of magnitudes starting with the SPDZ protocol family which offers a speedy information-theoretic online phase in the prepossessing model. However, state-of-the-art n-party MPC protocols in the dishonest majority setting incur online communication complexity per multiplication gate which is linear in the number of parties, i.e. O(n), per gate across all parties. In this work, we construct the first MPC protocols in the preprocessing model for dishonest majority with sublinear communication complexity per gate in the number of parties n. To achieve our results, we extend the use of packed secret sharing to the dishonest majority setting. For a constant fraction of corrupted parties (i.e. if 99 percent of the parties are corrupt), we can achieve a communication complexity of O(1) field elements per multiplication gate across all parties. At the crux of our techniques lies a new technique called sharing transformation. The sharing transformation technique allows us to transform shares under one type of linear secret sharing scheme into another, and even perform arbitrary linear maps on the secrets of (packed) secret sharing schemes with optimal communication complexity. This technique can be of independent interest since transferring shares from one type of scheme into another (e.g., for degree reduction) is ubiquitous in MPC. Furthermore, we introduce what we call sparsely packed Shamir sharing which allows us to address the issue of network routing efficiently, and packed Beaver triples which is an extension of the widely used technique of Beaver triples for packed secret sharing (for dishonest majority).

We propose to use blockchains to achieve MPC which does not require the participating parties to be online simultaneously or interact with each other. Parties who contribute inputs but do not wish to receive outputs can go offline after submitting a single message. In addition to our main result, we study combined communication- and state-complexity in MPC, as it has implications for the communication complexity of our main construction. Finally, we provide a variation of our main protocol which additionally provides guaranteed output delivery.

We introduce a natural generalization of two-source non-malleable extractors (Cheragachi and Guruswami, TCC 2014) called as \textit{multi-source non-malleable extractors}. Multi-source non-malleable extractors are special independent source extractors which satisfy an additional non-malleability property. This property requires that the output of the extractor remains close to uniform even conditioned on its output generated by tampering {\it several sources together}. We formally define this primitive, give a construction that is secure against a wide class of tampering functions, and provide applications. More specifically, we obtain the following results: \begin{itemize} \item For any $s \geq 2$, we give an explicit construction of a $s$-source non-malleable extractor for min-entropy $\Omega(n)$ and error $2^{-n^{\Omega(1)}}$ in the {\it overlapping joint tampering model}. This means that each tampered source could depend on any strict subset of all the sources and the sets corresponding to each tampered source could be overlapping in a way that we define. Prior to our work, there were no known explicit constructions that were secure even against disjoint tampering (where the sets are required to be disjoint without any overlap). \item We adapt the techniques used in the above construction to give a $t$-out-of-$n$ non-malleable secret sharing scheme (Goyal and Kumar, STOC 2018) for any $t \leq n$ in the \emph{disjoint tampering model}. This is the first general construction of a threshold non-malleable secret sharing (NMSS) scheme in the disjoint tampering model. All prior constructions had a restriction that the size of the tampered subsets could not be equal. \item We further adapt the techniques used in the above construction to give a $t$-out-of-$n$ non-malleable secret sharing scheme (Goyal and Kumar, STOC 2018) for any $t \leq n$ in the \emph{overlapping joint tampering model}. This is the first construction of a threshold NMSS in the overlapping joint tampering model. \item We show that a stronger notion of $s$-source non-malleable extractor that is multi-tamperable against disjoint tampering functions gives a single round network extractor protocol (Kalai et al., FOCS 2008) with attractive features. Plugging in with a new construction of multi-tamperable, 2-source non-malleable extractors provided in our work, we get a network extractor protocol for min-entropy $\Omega(n)$ that tolerates an {\it optimum} number ($t = p-2$) of faulty processors and extracts random bits for {\it every} honest processor. The prior network extractor protocols could only tolerate $t = \Omega(p)$ faulty processors and failed to extract uniform random bits for a fraction of the honest processors. \end{itemize}

Garbled Circuits (GCs) represent fundamental and powerful tools in cryptography, and many variants of GCs have been considered since their introduction. An important property of the garbled circuits is that they can be evaluated securely if and only if exactly 1 key for each input wire is obtained: no less and no more. In this work we study the case when: 1) some of the wire-keys are missing, but we are still interested in computing the output of the garbled circuit and 2) the evaluator of the GC might have both keys for a constant number of wires. We start to study this question in terms of non-interactive multi-party computation (NIMPC) which is strongly connected with GCs. In this notion, there is a fixed number of parties (n) that can get correlated information from a trusted setup. Then these parties can send an encoding of their input to an evaluator, which can compute the output of the function. Similarly to the notion of ad hoc secure computation proposed by Beimel et al. [ITCS 2016], we consider the case when less than n parties participate in the online phase, and in addition we let these parties colluding with the evaluator. We refer to this notion as Threshold NIMPC. In addition, we show that when the number of parties participating in the online phase is a fixed threshold l <= n then it is possible to securely evaluate any l-input function. We build our result on top of a new secret-sharing scheme (which can be of independent interest) and on the results proposed by Benhamouda, Krawczyk and Rabin [Crypto 2017]. Our protocol can be used to compute any function in NC1 in the information-theoretic setting and any function in P assuming one-way functions. As a second (and main) contribution, we consider a slightly different notion of security in which the number of parties that can participate in the online phase is not specified, and can be any number c above the threshold l (in this case the evaluator cannot collude with the other parties). We solve an open question left open by Beimel, Ishai and Kushilevitz [Eurocrypt 2017] showing how to build a secure protocol for the case when c is constant, under the Learning with Errors assumption.

We initiate the study of multi-party computation for classical functionalities in the plain model, with security against malicious quantum adversaries. We observe that existing techniques readily give a polynomial-round protocol, but our main result is a construction of *constant-round* post-quantum multi-party computation. We assume mildly super-polynomial quantum hardness of learning with errors (LWE), and quantum polynomial hardness of an LWE-based circular security assumption. Along the way, we develop the following cryptographic primitives that may be of independent interest: 1.) A spooky encryption scheme for relations computable by quantum circuits, from the quantum hardness of (a circular variant of) the LWE problem. This immediately yields the first quantum multi-key fully-homomorphic encryption scheme with classical keys. 2.) A constant-round post-quantum non-malleable commitment scheme, from the mildly super-polynomial quantum hardness of LWE. To prove the security of our protocol, we develop a new straight-line non-black-box simulation technique against parallel sessions that does not clone the adversary's state. This technique may also be relevant to the classical setting.

It is well known that several cryptographic primitives cannot be achieved without a common reference string (CRS). Those include, for instance, non-interactive zero-knowledge for NP, or malicious secure computation in fewer than four rounds. The security of those primitives heavily rely upon on the assumption that the trusted authority, who generates the CRS, does not misuse the randomness used in the CRS generation. However, we argue that there is no such thing as an unconditionally trusted authority and every authority must be held accountable for any trust to be well-founded. Indeed, a malicious authority can, for instance, recover private inputs of honest parties given transcripts of the protocols executed with respect to the CRS it has generated. While eliminating trust in the trusted authority may not be entirely feasible, can we at least move towards achieving some notion of accountability? We propose a new notion in which, if the CRS authority releases the private inputs of protocol executions to others, we can then provide a publicly-verifiable proof that certifies that the authority misbehaved. We study the feasibility of this notion in the context of non-interactive zero knowledge and two-round secure two-party computation.

In this work, we address communication, computation, and round efficiency of unconditionally secure multi-party computation for arithmetic circuits in the honest majority setting. We achieve both algorithmic and practical improvements: - The best known result in the semi-honest setting has been due to Damgard and Nielsen (CRYPTO 2007). Over the last decade, their construction has played an important role in the progress of efficient secure computation. However despite a number of follow-up works, any significant improvements to the basic semi-honest protocol have been hard to come by. We show 33% improvement in communication complexity of this protocol. We show how to generalize this result to the malicious setting, leading to the best known unconditional honest majority MPC with malicious security. - We focus on the round complexity of the Damgard and Nielsen protocol and improve it by a factor of 2. Our improvement relies on a novel observation relating to an interplay between Damgard and Nielsen multiplication and Beaver triple multiplication. An implementation of our constructions shows an execution run time improvement compared to the state of the art ranging from 30% to 50%.

The best known n party unconditional multiparty computation protocols with an optimal corruption threshold communicates O(n) field elements per gate. This has been the case even in the semi-honest setting despite over a decade of research on communication complexity in this setting. Going to the slightly sub-optimal corruption setting, the work of Damgard, Ishai, and Kroigaard (EUROCRYPT 2010) provided the first protocol for a single circuit achieving communication complexity of O(log |C|) elements per gate. While a number of works have improved upon this result, obtaining a protocol with O(1) field elements per gate has been an open problem. In this work, we construct the first unconditional multi-party computation protocol evaluating a single arithmetic circuit with amortized communication complexity of O(1) elements per gate.

Consider a scenario where Alice stores some secret data $s$ on $n$ servers using a $t$-out-of-$n$ secret sharing scheme. Trudy (the collector) is interested in the secret data of Alice and is willing to pay for it. Trudy publishes an advertisement on the internet which describes an elaborate cryptographic scheme to collect the shares from the $n$ servers. Each server who decides to submit its share is paid a hefty monetary reward and is guaranteed ``immunity" from being caught or prosecuted in a court for violating its service agreement with Alice. Bob is one of the servers and sees this advertisement. On examining the collection scheme closely, Bob concludes that there is no way for Alice to prove anything in a court that he submitted his share. Indeed, if Bob is rational, he might use the cryptographic scheme in the advertisement and submit his share since there are no penalties and no fear of being caught and prosecuted. Can we design a secret sharing scheme which Alice can use to avoid such a scenario? We introduce a new primitive called as \textit{Traceable Secret Sharing} to tackle this problem. In particular, a traceable secret sharing scheme guarantees that a cheating server always runs the risk of getting traced and prosecuted by providing a valid evidence (which can be examined in a court of law) implicating its dishonest behavior. We explore various definitional aspects and show how they are highly non-trivial to construct (even ignoring efficiency aspects). We then give an efficient construction of traceable secret sharing assuming the existence of a secure two-party computation protocol. We also show an application of this primitive in constructing traceable protocols for multi-server delegation of computation.

We propose the first maliciously secure multi-party computation (MPC) protocol for general functionalities in two rounds, without any trusted setup. Since polynomial-time simulation is impossible in two rounds, we achieve the relaxed notion of superpolynomial-time simulation security [Pass, EUROCRYPT 2003]. Prior to our work, no such maliciously secure protocols were known even in the two-party setting for functionalities where both parties receive outputs. Our protocol is based on the sub-exponential security of standard assumptions plus a special type of non-interactive non-malleable commitment. At the heart of our approach is a two-round multi-party conditional disclosure of secrets (MCDS) protocol in the plain model from bilinear maps, which is constructed from techniques introduced in [Benhamouda and Lin, TCC 2020].

Oblivious transfer (OT) is a foundational primitive within cryptography owing to its connection with secure computation. One of the oldest constructions of oblivious transfer was from certified trapdoor permutations (TDPs). However several decades later, we do not know if a similar construction can be obtained from TDPs in general. In this work, we study the problem of constructing round optimal oblivious transfer from trapdoor permutations. In particular, we obtain the following new results (in the plain model) relying on TDPs in a black-box manner: – Three-round oblivious transfer protocol that guarantees indistinguishability-security against malicious senders (and semi-honest receivers). – Four-round oblivious transfer protocol secure against malicious adversaries with black-box simulation-based security. By combining our second result with an already known compiler we obtain the first round-optimal 2-party computation protocol that relies in a black-box way on TDPs. A key technical tool underlying our results is a new primitive we call dual witness encryption (DWE) that may be of independent interest.

We construct a four round secure multiparty computation (MPC) protocol in the plain model that achieves security against any dishonest majority. The security of our protocol relies only on the existence of four round oblivious transfer. This culminates the long line of research on constructing round-efficient MPC from minimal assumptions (at least w.r.t. black-box simulation).

We study the problem of achieving statistical privacy in interactive proof systems and oblivious transfer -- two of the most well studied two-party protocols -- when limited rounds of interaction are available. -- Statistical Zaps: We give the first construction of statistical Zaps, namely, two-round statistical witness-indistinguishable (WI) protocols with a public-coin verifier. Our construction achieves computational soundness based on the quasi-polynomial hardness of learning with errors assumption. -- Three-Round Statistical Receiver-Private Oblivious Transfer: We give the first construction of a three-round oblivious transfer (OT) protocol -- in the plain model -- that achieves statistical privacy for receivers and computational privacy for senders against malicious adversaries, based on polynomial-time assumptions. The round-complexity of our protocol is optimal. We obtain our first result by devising a public-coin approach to compress sigma protocols, without relying on trusted setup. To obtain our second result, we devise a general framework via a new notion of statistical hash commitments that may be of independent interest.

We study the communication complexity of unconditionally secure MPC with guaranteed output delivery over point-to-point channels for corruption threshold t < n/2, assuming the existence of a public broadcast channel. We ask the question: “is it possible to construct MPC in this setting s.t. the communication complexity per multiplication gate is linear in the number of parties?” While a number of works have focused on reducing the communication complexity in this setting, the answer to the above question has remained elusive until now. We also focus on the concrete communication complexity of evaluating each multiplication gate. We resolve the above question in the affirmative by providing an MPC with communication complexity O(Cn\phi) bits (ignoring fixed terms which are independent of the circuit) where \phi is the length of an element in the field, C is the size of the (arithmetic) circuit, n is the number of parties. This is the first construction where the asymptotic communication complexity matches the best-known semi-honest protocol. This represents a strict improvement over the previously best-known communication complexity of O(C(n\phi + \kappa) + D_Mn^2\kappa) bits, where \kappa is the security parameter and D_M is the multiplicative depth of the circuit. Furthermore, the concrete communication complexity per multiplication gate is 5.5 field elements per party in the best case and 7.5 field elements in the worst case when one or more corrupted parties have been identified. This also roughly matches the best-known semi-honest protocol, which requires 5.5 field elements per gate. The above also yields the first secure-with-abort MPC protocol with the same cost per multiplication gate as the best-known semi-honest protocol. Our main result is obtained by compiling the secure-with-abort MPC protocol into a fully secure one.

In this paper, we consider the setting where a party uses correlated random tapes across multiple executions of a cryptographic algorithm. We ask if the security properties could still be preserved in such a setting. As examples, we introduce the notion of correlated-tape zero knowledge, and, correlated-tape multi-party computation, where, the zero-knowledge property, and, the ideal/real model security must still be preserved even if a party uses correlated random tapes in multiple executions.Our constructions are based on a new type of randomness extractor which we call correlated-source extractors. Correlated-source extractors can be seen as a dual of non-malleable extractors, and, allow an adversary to choose several tampering functions which are applied to the randomness source. Correlated-source extractors guarantee that even given the output of the extractor on the tampered sources, the output on the original source is still uniformly random. Given (seeded) correlated-source extractors, and, resettably-secure computation protocols, we show how to directly get a positive result for both correlated-tape zero-knowledge and correlated-tape multi-party computation in the CRS model. This is tight considering the known impossibility results on cryptography with imperfect randomness.Our main technical contribution is an explicit construction of a correlated-source extractor where the length of the seed is independent of the number of tamperings. Additionally, we also provide a (non-explicit) existential result for correlated source extractors with almost optimal parameters.

We study the foundations of secure computation in the blockchain-hybrid model, where a blockchain – modeled as a global functionality – is available as an Oracle to all the participants of a cryptographic protocol. We demonstrate both destructive and constructive applications of blockchains:We show that classical rewinding-based simulation techniques used in many security proofs fail against blockchain-active adversaries that have read and post access to a global blockchain. In particular, we show that zero-knowledge (ZK) proofs with black-box simulation are impossible against blockchain-active adversaries.Nevertheless, we show that achieving security against blockchain-active adversaries is possible if the honest parties are also blockchain active. We construct an $$\omega (1)$$-round ZK protocol with black-box simulation. We show that this result is tight by proving the impossibility of constant-round ZK with black-box simulation.Finally, we demonstrate a novel application of blockchains to overcome the known impossibility results for concurrent secure computation in the plain model. We construct a concurrent self-composable secure computation protocol for general functionalities in the blockchain-hybrid model based on standard cryptographic assumptions. We develop a suite of techniques for constructing secure protocols in the blockchain-hybrid model that we hope will find applications to future research in this area.

We study the communication complexity of unconditionally secure MPC with guaranteed output delivery over point-to-point channels for corruption threshold $$t < n/3$$ . We ask the question: “is it possible to construct MPC in this setting s.t. the communication complexity per multiplication gate is linear in the number of parties?” While a number of works have focused on reducing the communication complexity in this setting, the answer to the above question has remained elusive for over a decade.We resolve the above question in the affirmative by providing an MPC with communication complexity $$O(Cn\kappa + n^3\kappa )$$ where $$\kappa $$ is the size of an element in the field, C is the size of the (arithmetic) circuit, and, n is the number of parties. This represents a strict improvement over the previously best known communication complexity of $$O(Cn\kappa +D_Mn^2\kappa +n^3\kappa )$$ where $$D_M$$ is the multiplicative depth of the circuit. To obtain this result, we introduce a novel technique called 4-consistent tuples of sharings which we believe to be of independent interest.

In this work, we explore the question of simultaneous privacy and soundness amplification for non-interactive zero-knowledge argument systems (NIZK). We show that any $$\delta _s-$$sound and $$\delta _z-$$zero-knowledge NIZK candidate satisfying $$\delta _s+\delta _z=1-\epsilon $$, for any constant $$\epsilon >0$$, can be turned into a computationally sound and zero-knowledge candidate with the only extra assumption of a subexponentially secure public-key encryption.We develop novel techniques to leverage the use of leakage simulation lemma (Jetchev-Peitzrak TCC 2014) to argue amplification. A crucial component of our result is a new notion for secret sharing $$\mathsf {NP}$$ instances. We believe that this may be of independent interest.To achieve this result we analyze following two transformations:Parallel Repetition: We show that using parallel repetition any $$\delta _s-$$sound and $$\delta _z-$$zero-knowledge $$\mathsf {NIZK}$$ candidate can be turned into (roughly) $$\delta ^n_s-$$sound and $$1-(1-\delta _{z})^n-$$zero-knowledge candidate. Here n is the repetition parameter.MPC based Repetition: We propose a new transformation that amplifies zero-knowledge in the same way that parallel repetition amplifies soundness. We show that using this any $$\delta _s-$$sound and $$\delta _z-$$zero-knowledge $$\mathsf {NIZK}$$ candidate can be turned into (roughly) $$1-(1-\delta _s)^n-$$sound and $$2\cdot \delta ^n_{z}-$$zero-knowledge candidate. Then we show that using these transformations in a zig-zag fashion we can obtain our result. Finally, we also present a simple transformation which directly turns any $$\mathsf {NIZK}$$ candidate satisfying $$\delta _s,\delta _z<1/3 -1/\mathsf {poly}(\lambda )$$ to a secure one.

Non-malleable codes (NMC) introduced by Dziembowski et al. [ICS’10] allow one to encode “passive” data in such a manner that when a codeword is tampered, the original data either remains completely intact or is essentially destroyed.In this work, we initiate the study of interactive non-malleable codes (INMCs) that allow for encoding “active communication” rather than passive data. An INMC allows two parties to engage in an interactive protocol such that an adversary who is able to tamper with the protocol messages either leaves the original transcript intact (i.e., the parties are able to reconstruct the original transcript) or the transcript is completely destroyed and replaced with an unrelated one.We formalize a tampering model for interactive protocols and put forward the notion of INMCs. Since constructing INMCs for general adversaries is impossible (as in the case of non-malleable codes), we construct INMCs for several specific classes of tampering functions. These include bounded state, split state, and fragmented sliding window tampering functions. We also obtain lower bounds for threshold tampering functions via a connection to interactive coding. All of our results are unconditional.

We devise a new partitioned simulation technique for MPC where the simulator uses different strategies for simulating the view of aborting adversaries and non-aborting adversaries. The protagonist of this technique is a new notion of promise zero knowledge (ZK) where the ZK property only holds against non-aborting verifiers. We show how to realize promise ZK in three rounds in the simultaneous-message model assuming polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We demonstrate the following applications of our new technique:We construct the first round-optimal (i.e., four round) MPC protocol for general functions based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity).We further show how to overcome the four-round barrier for MPC by constructing a three-round protocol for “list coin-tossing” – a slight relaxation of coin-tossing that suffices for most conceivable applications – based on polynomially hard DDH (or QR or N$$^{th}$$-Residuosity). This result generalizes to randomized input-less functionalities. Previously, four round MPC protocols required sub-exponential-time hardness assumptions and no multi-party three-round protocols were known for any relaxed security notions with polynomial-time simulation against malicious adversaries.In order to base security on polynomial-time standard assumptions, we also rely upon a leveled rewinding security technique that can be viewed as a polynomial-time alternative to leveled complexity leveraging for achieving “non-malleability” across different primitives.

Goyal and Kumar (STOC’18) recently introduced the notion of non-malleable secret sharing. Very roughly, the guarantee they seek is the following: the adversary may potentially tamper with all of the shares, and still, either the reconstruction procedure outputs the original secret, or, the original secret is “destroyed” and the reconstruction outputs a string which is completely “unrelated” to the original secret. Prior works on non-malleable codes in the 2 split-state model imply constructions which can be seen as 2-out-of-2 non-malleable secret sharing (NMSS) schemes. Goyal and Kumar proposed constructions of t-out-of-n NMSS schemes. These constructions have already been shown to have a number of applications in cryptography.We continue this line of research and construct NMSS for more general access structures. We give a generic compiler that converts any statistical (resp. computational) secret sharing scheme realizing any access structure into another statistical (resp. computational) secret sharing scheme that not only realizes the same access structure but also ensures statistical non-malleability against a computationally unbounded adversary who tampers each of the shares arbitrarily and independently. Instantiating with known schemes we get unconditional NMMS schemes that realize any access structures generated by polynomial size monotone span programs. Similarly, we also obtain conditional NMMS schemes realizing access structure in $$\mathbf {monotone \;P}$$ monotoneP (resp. $$\mathbf {monotone \;NP}$$ monotoneNP) assuming one-way functions (resp. witness encryption).Towards considering more general tampering models, we also propose a construction of n-out-of-n NMSS. Our construction is secure even if the adversary could divide the shares into any two (possibly overlapping) subsets and then arbitrarily tamper the shares in each subset. Our construction is based on a property of inner product and an observation that the inner-product based construction of Aggarwal, Dodis and Lovett (STOC’14) is in fact secure against a tampering class that is stronger than 2 split-states. We also show applications of our construction to the problem of non-malleable message transmission.