## CryptoDB

### Papers from TCC 2021

**Year**

**Venue**

**Title**

2021

TCC

Somewhere Statistical Soundness, Post-Quantum Security, and SNARGs
Abstract

The main conceptual contribution of this paper is a unification of two leading paradigms for constructing succinct argument systems, namely Kilian's protocol and the BMW (Biehl-Meyer-Wetzel) heuristic. We define the notion of a multi-extractable somewhere statistically binding (meSSB) hash family, an extension of the notion of somewhere statistically binding hash functions (Hubacek and Wichs, ITCS 2015), and construct it from LWE. We show that when instantiating Kilian's protocol with a meSSB hash family, the first two messages are simply an instantiation of the BMW heuristic. Therefore, if we also instantiate it with a PCP for which the BMW heuristic is sound, e.g., a computational non-signaling PCP, then the first two messages of the Kilian protocol is a sound instantiation of the BMW heuristic.
This leads us to two technical results. First, we show how to efficiently convert any succinct non-interactive argument (SNARG) for BatchNP into a SNARG for any language that has a computational non-signaling PCP. Put together with the recent and independent result of Choudhuri, Jain and Jin (Eprint 2021/808) which constructs a SNARG for BatchNP from LWE, we get a SNARG for any language that has a computational non-signaling PCP, including any language in P, but also any language in NTISP (non-deterministic bounded space), from LWE.
Second, we introduce the notion of a somewhere statistically sound (SSS) interactive argument, which is a hybrid between a statistically sound proof and a computationally sound proof (a.k.a. an argument), and
* prove that Kilian's protocol, instantiated as above, is an SSS argument;
* show that the soundness of SSS arguments can be proved in a straight-line manner, implying that they are also post-quantum sound if the underlying assumption is post-quantum secure; and
* conjecture that constant-round SSS arguments can be soundly converted into non-interactive arguments via the Fiat-Shamir transformation.

2021

TCC

On Derandomizing Yao’s Weak-to-Strong OWF Construction
Abstract

The celebrated result of Yao (Yao, FOCS'82) shows that concatenating n · p(n) copies of a weak one-way function f which can be inverted with probability 1 - 1/p(n) suffices to construct a strong one-way function g, showing that weak and strong one-way functions are black-box equivalent. This direct product theorem for hardness amplification of one-way functions has been very influential. However, the construction of Yao has severe efficiency limitations; in particular, it is not security-preserving (the input to g needs to be much larger than the input to f). Understanding whether this is inherent is an intriguing and long-standing open question.
In this work, we explore necessary features of constructions which achieve short input length by proving the following: for any direct product construction of strong OWF g from a weak OWF f, which can be inverted with probability 1-1/p(n), the input size of g must grow as Omega(p(n)). By direct product construction, we refer to any construction with the following structure: the construction g executes some arbitrary pre-processing function (independent of f) on its input, obtaining a vector (y_1 ,··· ,y_l ), and outputs f(y_1),··· ,f(y_l). Note that Yao's construction is obtained by setting the pre-processing to be the identity. Our result generalizes to functions g with post-processing, as long as the post-processing function is not too lossy. Thus, in essence, any weak-to-strong hardness amplification must either (1) be very far from security-preserving, (2) use adaptivity, or (3) must be very far from a direct-product structure (in the sense of having a very lossy post-processing of the outputs of f).
On a technical level, we use ideas from lower bounds for secret-sharing to prove the impossibility of derandomizing Yao in a black-box way. Our results are in line with Goldreich, Impagliazzo, Levin, Venkatesan, and Zuckerman (FOCS 1990) who derandomize Yao's construction for regular weak one-way functions by evaluating the OWF along a random walk on an expander graph---the construction is adaptive, since it alternates steps on the expander graph with evaluations of the weak one-way function.

2021

TCC

Information-Theoretically Secure MPC against Mixed Dynamic Adversaries
Abstract

In this work we consider information-theoretically secure MPC against an \emph{mixed} adversary who can corrupt $t_p$ parties passively, $t_a$ parties actively, and can make $t_f$ parties fail-stop.
With perfect security, it is known that every function can be computed securely if and only if $3t_a + 2t_p + t_f < n$,
for statistical security the bound is $2t_a + 2t_p + t_f < n$.
These results say that for each given set of parameters $(t_a, t_p, t_f)$ respecting the inequality, there exists a protocol secure against
this particular choice of corruption thresholds.
In this work we consider a \emph{dynamic} adversary. Here, the goal is a \emph{single} protocol that is secure, no matter which set of corruption thresholds $(t_a, t_p, t_f)$ from a certain class is chosen by the adversary. A dynamic adversary can choose a corruption strategy after seeing the protocol and so is much stronger than a standard adversary.
Dynamically secure protocols have been considered before for computational security. Also the information theoretic case has been studied, but only considering non-threshold adversaries, leading to inefficient protocols.
We consider threshold dynamic adversaries and information theoretic security.
For statistical security we show that efficient dynamic secure function evaluation (SFE) is possible if and only if $2t_a + 2t_p + t_f < n$, but any dynamically secure protocol must use
$\Omega(n)$ rounds, even if only fairness is required. Further, general reactive MPC is possible if we assume in addition that $2t_a+2t_f \leq n$,
but fair reactive MPC only requires $2t_a + 2t_p + t_f < n$.
For perfect security we show that both dynamic SFE and verifiable secret sharing (VSS) are impossible if we only assume
$3t_a + 2t_p + t_f < n$ and remain impossible even if we also assume $t_f=0$. In fact even SFE with security with abort is impossible in this case. On the other hand, perfect dynamic SFE with guaranteed output delivery (G.O.D.) is possible when either $t_p = 0$ or $t_a = 0$ i.e. if instead we assume $3t_a+t_f < n$ or $2t_p +t_f < n$. Further, perfect dynamic VSS with G.O.D. is possible under the stronger conditions
$3t_a + 3/2t_f \leq n$ or $2t_p + 2t_f \leq n$. These conditions are also sufficient for perfect reactive MPC. On the other hand, because perfect fair VSS only requires $3t_a+2t_p+t_f< n$, perfect reactive MPC is possible whenever perfect SFE is.

2021

TCC

Random-Index PIR and Applications
Abstract

Private information retrieval (PIR) lets a client retrieve an entry from a database without the server learning which entry was retrieved. Here we study a weaker variant that we call random-index PIR (RPIR), where the retrieved index is an output rather than an input of the protocol, and is chosen at random. RPIR is clearly weaker than PIR, but it suffices for some interesting applications and may be realized more efficiently than full-blown PIR.
We report here on two lines of work, both tied to RPIR but otherwise largely unrelated. The first line of work studies RPIR as a primitive on its own. Perhaps surprisingly, we show that RPIR is in fact equivalent to PIR when there are no restrictions on the number of communication rounds. On the other hand, RPIR can be implemented in a “noninteractive” setting (with preprocessing), which is clearly impossible for PIR. For two-server RPIR we show a truly noninteractive solution, offering information-theoretic security without any pre-processing.
The other line of work, which was the original motivation for our work, uses RPIR to improve on the recent work of Benhamouda et al. (TCC’20) for maintaining secret values on public blockchains. Their solution depends on a method for selecting many random public keys from a PKI while hiding most of the selected keys from an adversary. However, the method they proposed is vulnerable to a double-dipping attack, limiting its resilience. Here we observe that an RPIR protocol, where the client is implemented via secure MPC, can eliminate that vulnerability. We thus get a secrets-on-blockchain protocol (and more generally large-scale MPC), resilient to any fraction f < 1/2 of corrupted parties, resolving the main open problem left from the work of Benhamouda et al.
As the client in this solution is implemented via secure MPC, it really brings home the need to make it as efficient as possible. We thus strive to explore whatever efficiency gains we can get by using RPIR rather than PIR. We achieve more gains by using batch RPIR where multiple indexes are retrieved at once. Lastly, we observe that this application can make do with a weaker security guarantee than full RPIR, and show that this weaker variant can be realized even more efficiently. We discuss one protocol in particular, that may be attractive for practical implementations.

2021

TCC

Round-Efficient Byzantine Agreement and Multi-Party Computation with Asynchronous Fallback
Abstract

Protocols for Byzantine agreement (BA) and secure multi-party computation (MPC) can be classified according to the underlying communication model. The two most commonly considered models are the synchronous one and the asynchronous one. Synchronous protocols typically lose their security guarantees as soon as the network violates the synchrony assumptions. Asynchronous protocols remain secure regardless of the network conditions, but achieve weaker security guarantees even when the network is synchronous.
Recent works by Blum, Katz and Loss [TCC'19], and Blum, Liu-Zhang and Loss [CRYPTO'20] introduced BA and MPC protocols achieving security guarantees in both settings: security up to $t_s$ corruptions in a synchronous network, and up to $t_a$ corruptions in an asynchronous network, under the provably optimal threshold trade-offs $t_a \le t_s$ and $t_a + 2t_s < n$. However, current solutions incur a high synchronous round complexity when compared to state-of-the-art purely synchronous protocols. When the network is synchronous, the round complexity of BA protocols is linear in the number of parties, and the round complexity of MPC protocols also depends linearly on the depth of the circuit to evaluate.
In this work, we provide round-efficient constructions for both primitives with optimal resilience: fixed-round and expected constant-round BA protocols, and an MPC protocol whose round complexity is independent of the circuit depth.

2021

TCC

Ring-based Identity Based Encryption – Asymptotically Shorter MPK and Tighter Security
Abstract

This work constructs an identity based encryption from the
ring learning with errors assumption (RLWE), with shorter master public keys and tighter security analysis. To achieve this, we develop three new methods: (1) a new homomorphic equality test method using nice algebraic structures of the rings, (2) a new family of hash functions with natural homomorphic evaluation algorithms, and (3) a new insight for tighter reduction analyses. These methods can be used to improve other important cryptographic tasks, and thus are of general interests.
Particularly, our homomorphic equality test method can derive a new
method for packing/unpacking GSW-style encodings, showing a new
non-trivial advantage of RLWE over the plain LWE. Moreover, our new
insight for tighter analyses can improve the analyses of all the currently
known partition-based IBE designs, achieving the best of the both from
prior analytical frameworks of Waters (Eurocrypt ’05) and Bellare and
Ristenpart (Eurocrypt ’09).

2021

TCC

Forward Secret Encrypted RAM: Lower Bounds and Applications
Abstract

In this paper, we study forward secret encrypted RAMs (FS eRAMs) which enable clients to outsource the storage of an n-entry array to a server. In the case of a catastrophic attack where both client and server storage are compromised, FS eRAMs guarantee that the adversary may not recover any array entries that were deleted or overwritten prior to the attack. A simple folklore FS eRAM construction with O(logn) overhead has been known for at least two decades. Unfortunately, no progress has been made since then. We show the lack of progress is fundamental by presenting an \Omega(log n) lower bound for FS eRAMs proving that the folklore solution is optimal. To do this, we introduce the symbolic model for proving cryptographic data structures lower bounds that may be of independent interest.
Given this limitation, we investigate applications where forward secrecy may be obtained without the additional O(log n) overhead. We show this is possible for oblivious RAMs, memory checkers, and multicast encryption by incorporating the ideas of the folklore FS eRAM solution into carefully chosen constructions of the corresponding primitives.

2021

TCC

Cryptographic Shallots: A Formal Treatment of Repliable Onion Encryption
Abstract

Onion routing is a popular, efficient, and scalable method for enabling anonymous communications. To send a message m to Bob via onion routing, Alice picks several intermediaries, wraps m in multiple layers of encryption --- a layer per intermediary --- and sends the resulting “onion” to the first intermediary. Each intermediary “peels off'”a layer of encryption and learns the identity of the next entity on the path and what to send along; finally Bob learns that he is the recipient and recovers the message m.
Despite its wide use in the real world (e.g., Mixminion), the foundations of onion routing have not been thoroughly studied. In particular, although two-way communication is needed in most instances, such as anonymous Web browsing or anonymous access to a resource, until now no definitions or provably secure constructions have been given for two-way onion routing. Moreover, the security definitions that existed even for one-way onion routing were found to have significant flaws.
In this paper, we (1) propose an ideal functionality for a repliable onion encryption scheme; (2) give a game-based definition for repliable onion encryption and show that it is sufficient to realize our ideal functionality; and finally (3), our main result is a construction of repliable onion encryption that satisfies our definitions.

2021

TCC

Distributed Merkle's Puzzles
Abstract

Merkle's puzzles were proposed in 1974 by Ralph Merkle as a key agreement protocol
between two players based on symmetric-key primitives.
In order to agree on a secret key, each player
makes $T$ queries to a random function (oracle),
while any eavesdropping adversary has to make $\Omega(T^2)$ queries to the random oracle
in order to recover the key with high probability.
The quadratic gap between the query complexity of the honest players
and the eavesdropper was shown to be optimal by Barak and Mahmoody [CRYPTO`09].
We consider Merkle's puzzles in a distributed setting,
where the goal is to allow \emph{all} pairs among $M$ honest players
with access to a random oracle to agree on secret keys.
We devise a protocol in this setting, where each player makes $T$ queries
to the random oracle and communicates at most $T$ bits,
while any adversary has to make $\Omega(M \cdot T^2)$ queries to the random oracle
(up to logarithmic factors)
in order to recover \emph{any one} of the keys with high probability.
Therefore, the amortized (per-player) complexity of achieving
secure communication (for a fixed security level)
decreases with the size of the network.
Finally, we prove that the gap of $T \cdot M$
between the query complexity of each honest player
and the eavesdropper is optimal.

2021

TCC

Secure Quantum Computation with Classical Communication
Abstract

The study of secure multi-party computation (MPC) has thus far been limited to the following two settings: every party is fully classical, or every party has quantum capabilities. This paper studies a notion of MPC that allows some classical and some quantum parties to securely compute a quantum functionality over their joint private inputs.
In particular, we construct constant-round \emph{composable} protocols for blind and verifiable classical delegation of quantum computation, and give applications to secure quantum computation with classical communication. Assuming QLWE (the quantum hardness of learning with errors), we obtain the following (maliciously-secure) protocols for computing any BQP (bounded-error quantum polynomial-time) functionality.
- A six-round protocol between one quantum server and multiple classical clients in the CRS (common random string) model.
- A three-round protocol between one quantum server and multiple classical clients in the PKI (public-key infrastructure) + QRO (quantum random oracle) model.
- A two-message protocol between quantum sender and classical receiver (a quantum non-interactive secure computation protocol), in the QRO model.
To enable composability of classical verification of quantum computation, we require the notion of \emph{malicious blindness}, which stipulates that the prover does not learn anything about the verifier's delegated computation, even if it is able to observe whether or not the verifier accepted the proof. To construct a protocol with malicious blindness, we use a classical verification protocol for sampBQP computation (Chung et al., Arxiv 2020), which in general has inverse polynomial soundness error, to prove honest evaluation of QFHE (quantum fully-homomorphic encryption) ciphertexts with negligible soundness error. Obtaining a constant-round protocol requires a strong parallel repetition theorem for classical verification of quantum computation, which we show following the "nearly orthogonal projector" proof strategy (Alagic et al., TCC 2020).

2021

TCC

Black-Box Impossibilities of Obtaining 2-Round Weak ZK and Strong WI from Polynomial Hardness
Abstract

We study the problem of obtaining 2-round interactive arguments for NP with weak zero-knowledge (weak ZK) [Dwork et al., 2003] or with strong witness indistinguishability (strong WI) [Goldreich, 2001] under polynomially hard falsifiable assumptions. We consider both the delayed-input setting [Jain et al., 2017] and the standard non-delayed-input setting, where in the delayed-input setting, (i) prover privacy is only required to hold against delayed-input verifiers (which learn statements in the last round of the protocol) and (ii) soundness is required to hold even against adaptive provers (which choose statements in the last round of the protocol).
Concretely, we show the following black-box (BB) impossibility results by relying on standard cryptographic primitives.
1. It is impossible to obtain 2-round delayed-input weak ZK arguments under polynomially hard falsifiable assumptions if BB reductions are used to prove soundness. This result holds even when non-black-box techniques are used to prove weak ZK.
2. It is impossible to obtain 2-round non-delayed-input strong WI arguments and 2-round publicly verifiable delayed-input strong WI arguments under polynomially hard falsifiable assumptions if a natural type of BB reductions, called "oblivious" BB reductions, are used to prove strong WI.
3. It is impossible to obtain 2-round delayed-input strong WI arguments under polynomially hard falsifiable assumptions if BB reductions are used to prove both soundness and strong WI (the BB reductions for strong WI are required to be oblivious as above). Compared with the above result, this result no longer requires public verifiability in the delayed-input setting.

2021

TCC

Two-Round Maliciously Secure Computation with Super-Polynomial Simulation
Abstract

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].

2021

TCC

Continuously Non-Malleable Secret Sharing: Joint Tampering, Plain Model and Capacity
Abstract

We study non-malleable secret sharing against joint leakage and joint tampering attacks.
Our main result is the first threshold secret sharing scheme in the plain model achieving resilience to noisy-leakage and continuous tampering.
The above holds under (necessary) minimal computational assumptions (i.e., the existence of one-to-one one-way functions), and in a model where the adversary commits to a fixed partition of all the shares into non-overlapping subsets of at most t - 1 shares (where t is the reconstruction threshold), and subsequently jointly leaks from and tampers with the shares within each partition.
We also study the capacity (i.e., the maximum achievable asymptotic information rate) of continuously non-malleable secret sharing against joint continuous tampering attacks. In particular, we prove that whenever the attacker can tamper jointly with k > t/2 shares, the capacity is at most t - k.
The rate of our construction matches this upper bound.
An important corollary of our results is the first non-malleable secret sharing scheme against independent tampering attacks breaking the rate-one barrier (under the same computational assumptions as above).

2021

TCC

Tight Security Bounds for Micali’s SNARGs
Abstract

Succinct non-interactive arguments (SNARGs) in the random oracle model (ROM) have several attractive features: they are plausibly post-quantum; they can be heuristically instantiated via lightweight cryptography; and they have a transparent (public-coin) parameter setup.
The canonical construction of a SNARG in the ROM is due to Micali (FOCS 1994), who showed how to use a random oracle to compile any probabilistically checkable proof (PCP) with sufficiently-small soundness error into a corresponding SNARG. Yet, while Micali's construction is a seminal result, it has received little attention in terms of analysis in the past 25 years.
In this paper, we observe that prior analyses of the Micali construction are not tight and then present a new analysis that achieves tight security bounds. Our result enables reducing the random oracle's output size, and obtain corresponding savings in concrete argument size.
Departing from prior work, our approach relies on precisely quantifying the cost for an attacker to find several collisions and inversions in the random oracle, and proving that any PCP with small soundness error withstands attackers that succeed in finding a small number of collisions and inversions in a certain tree-based information-theoretic game.

2021

TCC

Secure Software Leasing from Standard Assumptions
Abstract

Secure software leasing (SSL) is a quantum cryptographic primitive that enables an authority to lease software to a user by encoding it into a quantum state. SSL prevents users from generating authenticated pirated copies of leased software, where authenticated copies indicate those run on legitimate platforms. Although SSL is a relaxed variant of quantum copy protection that prevents users from generating any copy of leased softwares, it is still meaningful and attractive. Recently, Ananth and La Placa proposed the first SSL scheme. It satisfies a strong security notion called infinite-term security. On the other hand, it has a drawback that it is based on public key quantum money, which is not instantiated with standard cryptographic assumptions so far. Moreover, their scheme only supports a subclass of evasive functions.
In this work, we present SSL schemes that satisfy a security notion called finite-term security based on the learning with errors assumption (LWE). Finite-term security is weaker than infinite-term security, but it still provides a reasonable security guarantee. Specifically, our contributions consist of the following.
- We construct a finite-term secure SSL scheme for pseudorandom functions from the LWE assumption against quantum adversaries.
- We construct a finite-term secure SSL scheme for a subclass of evasive functions from the LWE assumption against sub-exponential quantum adversaries.
- We construct finite-term secure SSL schemes for the functionalities above with classical communication from the LWE assumption against (sub-exponential) quantum adversaries.
SSL with classical communication means that entities exchange only classical information though they run quantum computation locally.
Our crucial tool is two-tier quantum lightning, which is introduced in this work and a relaxed version of quantum lighting. In two-tier quantum lightning schemes, we have a public verification algorithm called semi-verification and a private verification algorithm called full-verification. An adversary cannot generate possibly entangled two quantum states whose serial numbers are the same such that one passes the semi-verification, and the other also passes the full-verification. We show that we can construct a two-tier quantum lightning scheme from the LWE assumption.

2021

TCC

Simple Constructions from (Almost) Regular One-Way Functions
Abstract

Two of the most useful cryptographic primitives that can be constructed from one-way functions are pseudorandom generators (PRGs) and universal one-way hash functions (UOWHFs). In order to implement them in practice, the efficiency of such constructions must be considered. The three major efficiency measures are: the seed length, the call complexity to the one-way function, and the adaptivity of these calls. Still, the optimal efficiency of these constructions is not yet fully understood: there exist gaps between the known upper bound and the known lower bound for black-box constructions.
A special class of one-way functions called unknown-regular one-way functions is much better understood. Haitner, Harnik and Reingold (CRYPTO 2006) presented a PRG construction with semi-linear seed length and linear number of calls based on a method called randomized iterate. Ames, Gennaro and Venkitasubramaniam (TCC 2012) then gave a construction of UOWHF with similar parameters and using similar ideas. On the other hand, Holenstein and Sinha (FOCS 2012) and Barhum and Holenstein (TCC 2013) showed an almost linear call-complexity lower bound for black-box constructions of PRGs and UOWHFs from one-way functions. Hence Haitner et al. and Ames et al. reached tight constructions (in terms of seed length and the number of calls) of PRGs and UOWHFs from regular one-way functions. These constructions, however, are adaptive.
In this work, we present non-adaptive constructions for both primitives which match the optimal call-complexity given by Holenstein and Sinha and Barhum and Holenstein. Our constructions, besides being simple and non-adaptive, are robust also for almost-regular one-way functions.

2021

TCC

On the Impossibility of Purely Algebraic Signatures
Abstract

The existence of one-way functions implies secure digital sig- natures, but not public-key encryption (at least in a black-box setting). Somewhat surprisingly, though, efficient public-key encryption schemes appear to be much easier to construct from concrete algebraic assumptions (such as the factoring of Diffie-Hellman-like assumptions) than efficient digital signature schemes. In this work, we provide one reason for this apparent difficulty to construct efficient signature schemes. Specifically, we prove that a wide range of algebraic signature schemes (in which verification essentially checks a number of linear equations over a group) fall to conceptually surprisingly simple linear algebra attacks. In fact, we prove that in an algebraic signature scheme, sufficiently many signatures can be linearly combined to a signature of a fresh message. We present attacks both in known-order and hidden-order groups (although in hidden-order settings, we have to restrict our definition of algebraic signatures a little). More explicitly, we show:
– the insecurity of all algebraic signature schemes in Maurer’s generic group model, as long as the signature schemes do not rely on other cryptographic assumptions, such as hash functions.
– the insecurity of a natural class of signatures in hidden-order groups, where verification consists of linear equations over group elements.
We believe that this highlights the crucial role of public verifiability in digital signature schemes. Namely, while public-key encryption schemes do not require any publicly verifiable structure on ciphertexts, it is exactly this structure on signatures that invites attacks like ours and makes it hard to construct efficient signatures.

2021

TCC

Acyclicity Programming for Sigma-Protocols
Abstract

Cramer, Damgård, and Schoenmakers (CDS) built a proof system to demonstrate the possession of subsets of witnesses for a given collection of statements that belong to a prescribed access structure P by composing so-called sigma-protocols for each atomic statement. Their verifier complexity is linear in the size of the monotone span program
representation of P.
We propose an alternative method for combining sigma-protocols into a single non-interactive system for a compound statement in the random oracle model. In contrast to CDS, our verifier complexity is linear in the size of the acyclicity program representation of P, a complete model of monotone computation introduced in this work. We show that the acyclicity program size of a predicate is never larger than its de Morgan formula size and it is polynomially incomparable to its monotone span program size. We additionally present an extension of our proof system, with verifier complexity linear in the monotone circuit size of P, in the common reference string model.
Finally, considering the types of statement that naturally reduce to acyclicity programming, we discuss several applications of our new methods to protecting privacy in cryptocurrency and social networks.

2021

TCC

Statistical ZAPs from Group-Based Assumptions
Abstract

We put forth a template for constructing statistical ZAPs for NP. Our template compiles NIZKs for NP in the hidden bit model (which exist unconditionally) into statistical ZAPs using a new notion of interactive hidden-bit generator (IHBG), which adapts the notion of hidden-bit generator to the plain model by building upon the recent notion of statistically-hiding extractable commitments. We provide a construction of IHBG from the explicit hardness of the decision Diffie-Hellman assumption (where explicit refers to requiring an explicit upper bound on the advantage of any polynomial-time adversary against the assumption) and the existence of statistical ZAPs for a specific simple language, building upon the recent construction of dual-mode hidden-bit generator from (Libert et al., EUROCRYPT 2020). We provide two instantiations of the underlying simple ZAP:
1. Using the recent statistical ZAP for the Diffie-Hellman language of (Couteau and Hartmann, CRYPTO 2020), we obtain statistical ZAPs for NP assuming (the explicit hardness of) DDH in $G_1$ and kernel-DH in $G_2$ (a search assumption which is weaker than DDH), where $(G_1,G_2)$ are groups equipped with an asymmetric pairing. This improves over the recent work of (Lombardi et al., EUROCRYPT 2020) which achieved a relaxed variant of statistical ZAP for NP, under a stronger assumption.
2. Using the recent work of (Couteau et al., EUROCRYPT 2020), we obtain statistical ZAPs for NP assuming the explicit hardness of DDH, together with the assumption that no efficient adversary can break the key-dependent message one-wayness of ElGamal with respect to efficient functions over groups of size $2^\secpar$ with probability better than $\poly(\secpar)/2^{(c + o(1)) \cdot \secpar}$, denoted $2^{-c\secpar}$-\OWKDM, for a constant c = 1/2, in pairing-free groups.
Note that the latter is a search discrete-log-style falsifiable assumption, incomparable to DDH (in particular, it is not known to imply public-key encryption).

2021

TCC

Adaptive Security of Multi-Party Protocols, Revisited
Abstract

The goal of secure multi-party computation (MPC) is to allow a set of parties to perform an arbitrary computation task, where the security guarantees depend on the set of parties that are corrupted. The more parties are corrupted, the less is guaranteed, and typically the guarantees are completely lost when the number of corrupted parties exceeds a certain corruption bound.
Early and also many recent protocols are only statically secure in the sense that they provide no security guarantees if the adversary is allowed to choose adaptively which parties to corrupt. Security against an adversary with such a strong capability is often called adaptive security and a significant body of literature is devoted to achieving adaptive security, which is known as a difficult problem. In particular, a main technical obstacle in this context is the so-called ``commitment problem'', where the simulator is unable to consistently explain the internal state of a party with respect to its pre-corruption outputs. As a result, protocols typically resort to the use of cryptographic primitives like non-committing encryption, incurring a substantial efficiency loss.
This paper provides a new, clean-slate treatment of adaptive security in MPC, exploiting the specification concept of constructive cryptography (CC). A new natural security notion, called \cc-adaptive security, is proposed, which is technically weaker than standard adaptive security but nevertheless captures security against a fully adaptive adversary. Known protocol examples separating between adaptive and static security are also insecure in our notion. Moreover, our notion avoids the commitment problem and thereby the need to use non-committing or equivocal tools.
We exemplify this by showing that the protocols by Cramer, Damgard and Nielsen (EUROCRYPT'01) for the honest majority setting, and (the variant without non-committing encryption) by Canetti, Lindell, Ostrovsky and Sahai (STOC'02) for the dishonest majority setting, achieve \cc-adaptive security. The latter example is of special interest since all \uc-adaptive protocols in the dishonest majority setting require some form of non-committing encryption or equivocal tools.

2021

TCC

Generalized Proofs of Knowledge with Fully Dynamic Setup
Abstract

Proofs of knowledge (PoK) are one of the most fundamental notions in cryptography. The appeal of this notion is that it provides a general template that an application can suitably instantiate by choosing a specific relation.
Nonetheless, several important applications have been brought to light, including proofs-of-ownership of files or two-factor authentication, which do not fit the PoK template but naturally appear to be special cases of a more general notion of proofs of knowledge or possession. One would thus expect that their security properties, in particular privacy and soundness, are simply derived as concrete instantiation of a common generalized PoK concept with well understood security semantics. Unfortunately, such a notion does not exist, resulting in a variety of tailor-made security definitions whose plausibility must be checked on a case-by-case basis.
In this work, we close this gap by providing the theoretical foundations of a generalized notion of PoK that encompasses dynamic and setup-dependent relations as well as interactive statement derivations. This novel combination enables an application to directly specify relations that depend on an assumed setup, such as a random oracle, a database or ledger, and to have statements be agreed upon interactively and dynamically between parties based on the state of the setup.
Our new notion is called \emph{agree-and-prove} and provides clear semantics of correctness, soundness, and zero-knowledge in the above generalized scenario.
As an application, we first consider proofs-of-ownership of files for client-side file deduplication. We cast the problem and some of its prominent schemes in our agree-and-prove framework and formally analyze their security.
Leveraging our generic zero-knowledge formalization, we then devise a novel scheme that is provably the privacy-preserving analogue of the well-known Merkle-Tree based protocol. As a second application, we consider two-factor entity authentication to showcase how the agree-and-prove notion encompasses proofs of ability, such as proving the correct usage of an abstract hardware token.

2021

TCC

Multi-Party Functional Encryption
Abstract

We initiate the study of multi-party functional encryption (MPFE) which unifies and abstracts out various notions of functional encryption which support distributed ciphertexts or secret keys, such as multi-input FE, multi-client FE, decentralized multi-client FE, multi-authority FE, dynamic decentralized FE, adhoc multi-input FE and such others. Using our framework, we identify several gaps in the literature and provide some constructions to fill these:
1. Multi-Authority ABE with Inner Product Computation. The recent work of Abdalla et al. (ASIACRYPT’20) constructed a novel “composition” of Attribute Based Encryption (ABE) and Inner Product Functional Encryption (IPFE), namely functional encryption schemes that combine the access control functionality of attribute based encryption with the possibility of performing linear operations on the encrypted data. In this work, we extend the access control component to support the much more challenging multi-authority setting, i.e. “lift” the primitive of ABE in their construction to multi-authority ABE for the same class of access control policies (LSSS structures). This yields the first construction of a nontrivial multi-authority FE beyond ABE from simple assumptions on pairings to the best of our knowledge.
Our techniques can also be used to generalize the decentralized attribute based encryption scheme of Michalevsky and Joye (ESORICS’18) to support inner product computation on the message. While this scheme only supports inner product predicates which is less general than those supported by the Lewko-Waters (EUROCRYPT’11) construction, it supports policy hiding which the latter does not. Our extension inherits these features and is secure based on the k-linear assumption, in the random oracle model.
2. Function Hiding DDFE. The novel primitive of dynamic decentralized functional encryption (DDFE) was recently introduced by Chotard et al. (CRYPTO’20), where they also provided the first construction for inner products. However, the primitive of DDFE does not support function hiding, which is a significant limitation for several applications. In this work, we provide a new construction for inner product DDFE which supports function hiding. To achieve our final result, we define and construct the first function hiding multi-client functional encryption (MCFE) scheme for inner products, which may be of independent interest.
3. Distributed Ciphertext-Policy ABE. We provide a distributed variant of the recent ciphertext- policy attribute based encryption scheme, constructed by Agrawal and Yamada (EUROCRYPT’20). Our construction supports NC1 access policies, and is secure based on “Learning With Errors” and relies on the generic bilinear group model as well as the random oracle model.
Our new MPFE abstraction predicts meaningful new variants of functional encryption as useful targets for future work.

2021

TCC

On Treewidth, Separators and Yao’s Garbling
Abstract

We show that Yao’s garbling scheme is adaptively indistinguishable
for the class of Boolean circuits of size S and treewidth w
with only a S^{O(w)} loss in security. For instance, circuits with constant
treewidth are as a result adaptively indistinguishable with only a polynomial
loss. This (partially) complements a negative result of Applebaum
et al. (Crypto 2013), which showed (assuming one-way functions) that
Yao’s garbling scheme cannot be adaptively simulatable. As main technical
contributions, we introduce a new pebble game that abstracts out
our security reduction and then present a pebbling strategy for this game
where the number of pebbles used is roughly O(\delta w log(S)), \delta being the
fan-out of the circuit. The design of the strategy relies on separators, a
graph-theoretic notion with connections to circuit complexity.

2021

TCC

Policy-Compliant Signatures
Abstract

We introduce policy-compliant signatures (PCS). A PCS scheme can be used in a setting where a central authority determines a global policy and distributes public and secret keys associated with sets of attributes to the users in the system. If two users, Alice and Bob, have attribute sets that jointly satisfy the global policy, Alice can use her secret key and Bob's public key to sign a message. Unforgeability ensures that a valid signature can only be produced if Alice's secret key is known and if the policy is satisfied. Privacy guarantees that the public keys and produced signatures reveal nothing about the users' attributes beyond whether they satisfy the policy or not. PCS extends the functionality provided by existing primitives such as attribute-based signatures and policy-based signatures, which do not consider a designated receiver and thus cannot include the receiver's attributes in the policies. We describe practical applications of PCS which include controlling transactions in financial systems with strong privacy guarantees (avoiding additional trusted entities that check compliance), as well as being a tool for trust negotiations.
We introduce an indistinguishability-based privacy notion for PCS and present a generic and modular scheme based on standard building blocks such as signatures, non-interactive zero-knowledge proofs, and a (predicate-only) predicate encryption scheme. We show that it can be instantiated to obtain an efficient scheme that is provably secure under standard pairing-assumptions for a wide range of policies.
We further model PCS in UC by describing the goal of PCS as an enhanced ideal signature functionality which gives rise to a simulation-based privacy notion for PCS. We show that our generic scheme achieves this composable security notion under the additional assumption that the underlying predicate encryption scheme satisfies a stronger, fully adaptive, simulation-based attribute-hiding notion.

2021

TCC

Post-quantum Resettably-Sound Zero Knowledge
Abstract

We study post-quantum zero-knowledge (classical) protocols that are sound against quantum resetting attacks. Our model is inspired by the classical model of resetting provers (Barak-Goldreich-GoldwasserLindell, FOCS ‘01), providing a malicious efficient prover with oracle access to the verifier’s next-message-function, fixed to some initial random tape; thereby allowing it to effectively reset (or equivalently, rewind) the verifier. In our model, the prover has quantum access to the verifier’s function, and in particular can query it in superposition. The motivation behind quantum resettable soundness is twofold: First, ensuring a strong security guarantee in scenarios where quantum resetting may be possible (e.g., smart cards, or virtual machines). Second, drawing intuition from the classical setting, we hope to improve our understanding of basic questions regarding post-quantum zero knowledge. We prove the following results:
– Black-Box Barriers. Quantum resetting exactly captures the power of black-box zero knowledge quantum simulators. Accordingly, resettable soundness cannot be achieved in conjunction with black-box zero knowledge, except for languages in BQP. Leveraging this, we prove that constant-round public-coin, or three message, protocols cannot be black-box post-quantum zero-knowledge. For this, we show how to transform such protocols into quantumly resettably sound ones. The transformations are similar to classical ones, but their analysis is very different due to the essential difference between classical and quantum resetting.
– A Resettably-Sound Non-Black-Box Zero-Knowledge Protocol. Under the (quantum) Learning with Errors assumption and quantum fully-homomorphic encryption, we construct a post-quantum resettably sound zero knowledge protocol for NP. We rely on non-black-box simulation techniques, thus overcoming the black-box barrier for such protocols.
– From Resettable Soundness to The Impossibility of Quantum Obfuscation. Assuming one-way functions, we prove that any quantumly resettably-sound zero-knowledge protocol for NP implies the impossibility of quantum obfuscation. Combined with the above result, this gives an alternative proof to several recent results on quantum unobfuscatability.

2021

TCC

Simple and Efficient Batch Verification Techniques for Verifiable Delay Functions
Abstract

We study the problem of batch verification for verifiable delay functions (VDFs), focusing on proofs of correct exponentiation (PoCE), which underlie recent VDF constructions. We show how to compile any PoCE into a batch PoCE, offering significant savings in both communication and verification time. Concretely, given any PoCE with communication complexity $c$, verification time $t$ and soundness error $\delta$, and any pseudorandom function with key length ${\sf k}_{\sf prf}$ and evaluation time $ t_{\sf prf}$, we construct:
-- A batch PoCE for verifying $n$ instances with communication complexity $m\cdot c +{\sf k}_{\sf prf}$, verification time $m\cdot t + n\cdot m\cdot O(t_{\sf op} + t_{\sf prf})$ and soundness error $\delta + 2^{-m}$, where $\lambda$ is the security parameter, $m$ is an adjustable parameter that can take any integer value, and $t_{\sf op}$ is the time required to evaluate the group operation in the underlying group.
This should be contrasted with the naive approach, in which the communication complexity and verification time are $n \cdot c$ and $n \cdot t$, respectively. The soundness of this compiler relies only on the soundness of the underlying PoCE and the existence of one-way functions.
-- An improved batch PoCE based on the low order assumption. For verifying $n$ instances, the batch PoCE requires communication complexity $c +{\sf k}_{\sf prf}$ and verification time $t + n\cdot (t_{\sf prf} + \log(s)\cdot O(t_{\sf op}))$, and has soundness error $\delta + 1/s$. The parameter $s$ can take any integer value, as long as it is hard to find group elements of order less than $s$ in the underlying group.
We discuss instantiations in which $s$ can be exponentially large in the security parameter $\lambda$.
If the underlying PoCE is constant round and public coin (as is the case for existing protocols), then so are all of our batch PoCEs, implying that they can be made non-interactive using the Fiat-Shamir transform.
Additionally, for RSA groups with moduli which are the products of two safe primes, we show how to efficiently verify that certain elements are not of order $2$. This protocol, together with the second compiler above and any (single-instance) PoCE in these groups, yields an efficient batch PoCE in safe RSA groups. To complete the picture, we also show how to extend Pietrzak's protocol (which is statistically sound in the group $QR_N^+$ when $N$ is the product of two safe primes) to obtain a statistically-sound PoCE in safe RSA groups.

2021

TCC

On Actively-Secure Elementary MPC Reductions
Abstract

We introduce the notion of \emph{elementary MPC} reductions that allow us to securely compute a functionality $f$ by making a single call to a constant-degree ``non-cryptographic'' functionality $g$ without requiring any additional interaction. Roughly speaking, ``non-cryptographic'' means that $g$ does not make use of cryptographic primitives, though the parties can locally call such primitives.
Classical MPC results yield such elementary reductions in various cases including the setting of passive security with full corruption threshold $t<n$ (Yao, FOCS'86; Beaver, Micali, and Rogaway, STOC'90), the setting of full active security against a corrupted minority $t<n/2$ (Damg{\aa}rd and Ishai, Crypto'05), and, for NC1 functionalities, even for the setting of full active (information-theoretic) security with full corruption threshold of $t<n$ (Ishai and Kushilevitz, FOCS'00). This leaves open the existence of an elementary reduction that achieves full active security in the dishonest majority setting for all efficiently computable functions.
Our main result shows that such a reduction is unlikely to exist. Specifically, the existence of a computationally secure elementary reduction that makes black-box use of a PRG and achieves a very weak form of partial fairness (e.g., that holds only when the first party is not corrupted) would allow us to realize any efficiently-computable function by a \emph{constant-round} protocol that achieves a non-trivial notion of information-theoretic passive security. The existence of the latter is a well-known 3-decade old open problem in information-theoretic cryptography (Beaver, Micali, and Rogaway, STOC'90).
On the positive side, we observe that this barrier can be bypassed under any of the following relaxations: (1) non-black-box use of a pseudorandom generator; (2) weaker security guarantees such as security with identifiable abort; or (3) an additional round of communication with the functionality $g$.

2021

TCC

Succinct LWE Sampling, Random Polynomials, and Obfuscation
Abstract

We present a construction of indistinguishability obfuscation (iO) that relies on the learning with errors (LWE) assumption together with a new notion of succinctly sampling pseudo-random LWE samples. We then present a candidate LWE sampler whose security is related to the hardness of solving systems of polynomial equations. Our construction improves on the recent iO candidate of Wee and Wichs (Eurocrypt 2021) in two ways: first, we show that a much weaker and simpler notion of LWE sampling suffices for iO; and secondly, our candidate LWE sampler is secure based on a compactly specified and falsifiable assumption about random polynomials, with a simple error distribution that facilitates cryptanalysis.

2021

TCC

Laconic Private Set Intersection and Applications
Abstract

Consider a server with a \emph{large} set $S$ of strings $\{x_1,x_2\ldots,x_N\}$ that would like to publish a \emph{small} hash $h$ of its set $S$ such that any client with a string $y$ can send the server a \emph{short} message allowing it to learn $y$ if $y \in S$ and nothing otherwise. In this work, we study this problem of two-round private set intersection (PSI) with low (asymptotically optimal) communication cost, or what we call \emph{laconic} private set intersection ($\ell$PSI) and its extensions. This problem is inspired by the recent general frameworks for laconic cryptography [Cho et al. CRYPTO 2017, Quach et al. FOCS'18].
We start by showing the first feasibility result for realizing $\ell$PSI~ based on the CDH assumption, or LWE with polynomial noise-to-modulus ratio. However, these feasibility results use expensive non-black-box cryptographic techniques leading to significant inefficiency. Next, with the goal of avoiding these inefficient techniques, we give a construction of $\ell$PSI~schemes making only black-box use of cryptographic functions. Our construction is secure against semi-honest receivers, malicious senders and reusable in the sense that the receiver's message can be reused across any number of executions of the protocol. The scheme is secure under the $\phi$-hiding, decisional composite residuosity and subgroup decision assumptions.
Finally, we show natural applications of $\ell$PSI~to realizing a semantically-secure encryption scheme that supports detection of encrypted messages belonging to a set of ``illegal'' messages (e.g., an illegal video) circulating online.
Over the past few years, significant effort has gone into realizing laconic cryptographic protocols. Nonetheless, our work provides the first black-box constructions of such protocols for a natural application setting.

2021

TCC

Secure Software Leasing Without Assumptions
Abstract

Quantum cryptography is known for enabling functionalities that are unattainable using classical information alone. Recently, Secure Software Leasing (SSL) has emerged as one of these areas of interest. Given a target circuit C from a circuit class, SSL produces an encoding of C that enables a recipient to evaluate C, and also enables the originator of the software to verify that the software has been returned --- meaning that the recipient has relinquished the possibility of any further use of the software. Clearly, such a functionality is unachievable using classical information alone, since it is impossible to prevent a user from keeping a copy of the software. Recent results have shown the achievability of SSL using quantum information for a class of functions called compute-and-compare (these are a generalization of the well-known point functions). These prior works, however all make use of setup or computational assumptions. Here, we show that SSL is achievable for compute-and-compare circuits without any assumptions.
Our technique involves the study of quantum copy protection, which is a notion related to SSL, but where the encoding procedure inherently prevents a would-be quantum software pirate from splitting a single copy of an encoding for C into two parts, each of which enables a user to evaluate C. We show that point functions can be copy-protected without any assumptions, for a novel security definition involving one honest and one malicious evaluator; this is achieved by showing that from any quantum message authentication code, we can derive such an honest-malicious copy protection scheme. We then show that a generic honest-malicious copy protection scheme implies SSL; by prior work, this yields SSL for compute-and-compare functions.

2021

TCC

Oblivious Transfer from Trapdoor Permutations in Minimal Rounds
Abstract

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.

2021

TCC

Environmentally Friendly Composable Multi-Party Computation in the Plain Model from Standard (Timed) Assumptions
Abstract

Starting with the work of Rivest et al. in 1996, timed assumptions have found many applications in cryptography, building e.g. the foundation of the blockchain technology. They also have been used in the context of classical MPC, e.g. to enable fairness. We follow this line of research to obtain composable general MPC in the plain model.
This approach comes with a major advantage regarding environmental friendliness, a property coined by Canetti et al. (FOCS 2013). Informally, this means that our constructions do not “hurt” game-based security properties of protocols that hold against polynomial-time adversaries when executed alone.
As an additional property, our constructions can be plugged into any UC-secure protocol without loss of security.
Towards proving the security of our constructions, we introduce a variant of the UC security notion that captures timed cryptographic assumptions. Combining standard timed commitment schemes and standard polynomial-time hardness assumptions, we construct a composable commitment scheme in the plain model. As this construction is constant-round and black-box, we obtain the first fully environmentally friendly composable constant-round black-box general MPC protocol in the plain model from standard (timed) assumptions.

2021

TCC

Dory: Efficient, Transparent arguments for Generalised Inner Products and Polynomial Commitments
Abstract

This paper presents Dory, a transparent setup, public-coin interactive argument for inner-pairing products between committed vectors of elements of two source groups. For a product of vectors of length $n$, proofs are $6 \log n$ target group elements and $O(1)$ additional elements. Verifier work is dominated by an $O(\log n)$ multi-exponentiation in the target group and $O(1)$ pairings. Security is reduced to the standard SXDH assumption in the standard model.
We apply Dory to build a multivariate polynomial commitment scheme via the
Fiat-Shamir transform. For a dense polynomial with $n$ coefficients, Prover work to compute a commitment is dominated by a multi-exponentiation in one source group of size $n$. Prover work to show that a commitment to an evaluation is correct is $O(n^{\log{8}/\log{25}})$ in general ($O(n^{1/2})$ for univariate or multilinear polynomials); communication complexity and Verifier work are both $O(\log n)$. These asymptotics previously required trusted setup or concretely inefficient groups of unknown order. Critically for applications, these arguments can be batched, saving large factors on the Prover and improving Verifier asymptotics: to validate $\ell$ polynomial evaluations for polynomials of size at most $n$ requires $O(\ell + \log n)$ exponentiations and $O(\ell \log n)$ field operations.
Dory is also concretely efficient: Using one core and setting $n = 2^{20}$,
commitments are 192 bytes. Evaluation proofs are ~18kb, requiring ~3s to generate and ~25ms to verify. For batches at $n=2^{20}$, the marginal cost per evaluation is <1kb communication, ~300ms for the prover and ~1ms for the verifier.

2021

TCC

On Communication-Efficient Asynchronous MPC with Adaptive Security
Abstract

Secure multi-party computation (MPC) allows a set of $n$ parties to jointly compute an arbitrary computation over their private inputs. Two main variants have been considered in the literature according to the underlying communication model. Synchronous MPC protocols proceed in rounds, and rely on the fact that the communication network provides strong delivery guarantees within each round. Asynchronous MPC protocols achieve security guarantees even when the network delay is arbitrary.
While the problem of MPC has largely been studied in both variants with respect to both feasibility and efficiency results, there is still a substantial gap when it comes to communication complexity of adaptively secure protocols. Concretely, while adaptively secure synchronous MPC protocols with linear communication are known for a long time, the best asynchronous protocol communicates $\mathcal{O}(n^4 \kappa)$ bits per multiplication.
In this paper, we make progress towards closing this gap by providing two protocols. First, we present an adaptively secure asynchronous protocol with optimal resilience $t<n/3$ and $\mathcal{O}(n^2 \kappa)$ bits of communication per multiplication, improving over the state of the art protocols in this setting by a quadratic factor in the number of parties. The protocol has cryptographic security and follows the CDN approach [Eurocrypt'01], based on additive threshold homomorphic encryption.
Second, we show an optimization of the above protocol that tolerates up to $t<(1-\epsilon)n/3$ corruptions and communicates $\mathcal{O}(n\cdot \poly(\kappa))$ bits per multiplication under stronger assumptions.

2021

TCC

Efficient Perfectly Secure Computation with Optimal Resilience
Abstract

Secure computation enables $n$ mutually distrustful parties to compute a function over their private inputs jointly. In 1988 Ben-Or, Goldwasser, and Wigderson (BGW) demonstrated that any function can be computed with perfect security in the presence of a malicious adversary corrupting at most $t< n/3$ parties.
After more than 30 years, protocols with perfect malicious security, with round complexity proportional to the circuit's depth, still require sharing a total of $O(n^2)$ values per multiplication.
In contrast, only $O(n)$ values need to be shared per multiplication to achieve semi-honest security. Indeed sharing $\Omega(n)$ values for a single multiplication seems to be the natural barrier for polynomial secret sharing-based multiplication.
In this paper, we close this gap by constructing a new secure computation protocol with perfect, optimal resilience and malicious security that incurs sharing of only $O(n)$ values per multiplication, thus, matching the semi-honest setting for protocols with round complexity that is proportional to the circuit depth. Our protocol requires a constant number of rounds per multiplication. Like BGW, it has an overall round complexity that is proportional only to the multiplicative depth of the circuit.
Our improvement is obtained by a novel construction for {\em weak VSS for polynomials of degree-$2t$}, which incurs the same communication and round complexities as the state-of-the-art constructions for {\em VSS for polynomials of degree-$t$}.
Our second contribution is a method for reducing the communication complexity for any depth-1 sub-circuit to be proportional only to the size of the input and output (rather than the size of the circuit). This implies protocols with \emph{sublinear communication complexity} (in the size of the circuit) for perfectly secure computation for important functions like matrix multiplication.

2021

TCC

The Cost of Adaptivity in Security Games on Graphs
Abstract

The security of cryptographic primitives and protocols against adversaries that are allowed to make adaptive choices (e.g., which parties to corrupt or which queries to make) is notoriously difficult to establish. A broad theoretical framework was introduced by Jafargholi et al. [Crypto'17] for this purpose. In this paper we initiate the study of lower bounds on loss in adaptive security for certain cryptographic protocols considered in the framework. We prove lower bounds that almost match the upper bounds (proven using the framework) for proxy re-encryption, prefix-constrained PRFs and generalized selective decryption, a security game that captures the security of certain group messaging and broadcast encryption schemes. Those primitives have in common that their security game involves an underlying graph that can be adaptively built by the adversary.
Some of our lower bounds only apply to a restricted class of black-box reductions which we term "oblivious" (the existing upper bounds are of this restricted type), some apply to the broader but still restricted class of non-rewinding reductions, while our lower bound for proxy re-encryption applies to all black-box reductions. The fact that some of our lower bounds seem to crucially rely on obliviousness or at least a non-rewinding reduction hints to the exciting possibility that the existing upper bounds can be improved by using more sophisticated reductions.
Our main conceptual contribution is a two-player multi-stage game called the Builder-Pebbler Game. We can translate bounds on the winning probabilities for various instantiations of this game into cryptographic lower bounds for the above mentioned primitives using oracle separation techniques.

2021

TCC

ABE for DFA from LWE against Bounded Collusions, Revisited
Abstract

We present a new public-key ABE for DFA based on the LWE assumption,
achieving security against collusions of a-priori bounded size. Our
scheme achieves ciphertext size O~(L + B) for attributes
of length L and collusion size B. Prior LWE-based schemes has
either larger ciphertext size O~(L B), or are
limited to the secret-key setting. Along the way, we introduce a new
technique for lattice trapdoor sampling, which we believe would be
of independent interest. Finally, we present a simple candidate public-key ABE for DFA for the unbounded collusion setting.

2021

TCC

Non-Malleable Vector Commitments via Local Equivocability
Abstract

Vector commitments (VCs), enabling to commit to a vector and locally reveal any of its entries, play a key role in a variety of both classic and recently-evolving applications. However, security notions for VCs have so far focused on passive attacks, and non-malleability notions considering active attacks have not been explored. Moreover, existing frameworks that may enable to capture the non-malleability of VCs seem either too weak (non-malleable non-interactive commitments that do not account for the security implications of local openings) or too strong (non-malleable zero-knowledge sets that support both membership and non-membership proofs).
We put forward a rigorous framework capturing the non-malleability of VCs, striking a careful balance between the existing weaker and stronger frameworks: We strengthen the framework of non-malleable non-interactive commitments by considering attackers that may be exposed to local openings, and we relax the framework of non-malleable zero-knowledge sets by focusing on membership proofs. In addition, we strengthen both frameworks by supporting (inherently-private) updates to entries of committed vectors, and discuss the benefits of non-malleable VCs in the context of both UTXO-based and account-based stateless blockchains, and in the context of simultaneous multi-round auctions (that have been adopted by the US Federal Communications Commission as the standard auction format for selling spectrum ranges).
Within our framework we present a direct approach for constructing non-malleable VCs whose efficiency essentially matches that of the existing standard VCs. Specifically, we show that any VC can be transformed into a non-malleable one, relying on a new primitive that we put forth. Our new primitive, locally-equivocable commitments with all-but-one binding, is evidently both conceptually and technically simpler compared to multi-trapdoor mercurial trapdoor commitments (the main building block underlying existing non-malleable zero-knowledge sets), and admits more efficient instantiations based on the same number-theoretic assumptions.

2021

TCC

Non-Malleable Time-Lock Puzzles and Applications
Abstract

Time-lock puzzles are a mechanism for sending messages "to the future", by allowing a sender to quickly generate a puzzle with an underlying message that remains hidden until a receiver spends a moderately large amount of time solving it. We introduce and construct a variant of a time-lock puzzle which is non-malleable, which roughly guarantees that it is impossible to "maul" a puzzle into one for a related message without solving it.
Using non-malleable time-lock puzzles, we achieve the following applications:
- The first fair non-interactive multi-party protocols for coin flipping and auctions in the plain model without setup.
- Practically efficient fair multi-party protocols for coin flipping and auctions proven secure in the (auxiliary-input) random oracle model.
As a key step towards proving the security of our protocols, we introduce the notion of functional non-malleability, which protects against tampering attacks that affect a specific function of the related messages. To support an unbounded number of participants in our protocols, our time-lock puzzles satisfy functional non-malleability in the fully concurrent setting. We additionally show that standard (non-functional) non-malleability is impossible to achieve in the concurrent setting (even in the random oracle model).

2021

TCC

On Communication Models and Best-Achievable Security in Two-Round MPC
Abstract

Recently, a sequence of works have made strong advances in two-round (i.e., round-optimal) secure multi-party computation (MPC). In the {\em honest-majority} setting -- the focus of this work -- Ananth et al. [CRYPTO'18, EC'19], Applebaum et al. [TCC'18, EC'19] and Garg et al. [TCC'18] have established the feasibility of general two-round MPC in standard communication models involving broadcast ($\BC$) and private point-to-point ($\PTP$) channels.
In this work, we set out to understand what features of the communication model are necessary for these results, and more broadly the design of two-round MPC. Focusing our study on the plain model -- the most natural model for honest-majority MPC -- we obtain the following results:
1. {\bf Dishonest majority from Honest majority:}
In the two round setting, honest-majority MPC and dishonest-majority MPC are surprisingly close, and often {\em equivalent}. This follows from our results that the former implies 2-message oblivious transfer, in many settings. (i) We show that without private point-to-point ($\PTP$) channels, i.e., when we use only broadcast ($\BC$) channels, {\em honest-majority MPC implies 2-message oblivious transfer}. (ii) Furthermore, this implication holds even when we use both $\PTP$ and $\BC$, provided that the MPC protocol is robust against ``fail-stop'' adversaries.
2. {\bf Best-Achievable Security:} While security with guaranteed output delivery (and even fairness) against malicious adversaries is impossible in two rounds, nothing is known with regards to the ``next best'' security notion, namely, security with identifiable abort (\IA). We show that \IA\ is also {\em impossible} to achieve with honest-majority even if we use both $\PTP$ and $\BC$ channels. However, if we replace $\PTP$ channels with a ``bare'' (i.e., untrusted) public-key infrastructure ($\PKI$), then even security with guaranteed output delivery (and hence $\IA$) is possible to achieve.
\end{itemize}
These results ``explain'' that the reliance on $\PTP$ channels (together with $\BC$) in the recent two-round protocols in the plain model was in fact {\em necessary}, and that these protocols {\em couldn't} have achieved a stronger security guarantee, namely, $\IA$. Overall, our results (put together with prior works) fully determine the best-achievable security for honest-majority MPC in different communication models in two rounds. As a consequence, they yield the following hierarchy of communication models:
$\BC < \PTP < \BC+\PTP < \BC+\PKI$.
This shows that $\BC$ channel is the {\em weakest} communication model, and that $\BC+\PKI$ model is strictly stronger than $\BC+\PTP$ model.

2021

TCC

Concurrent Composition of Differential Privacy
Abstract

We initiate a study of the composition properties of interactive differentially private mechanisms. An interactive differentially private mechanism is an algorithm that allows an analyst to adaptively ask queries about a sensitive dataset, with the property that an adversarial analyst's view of the interaction is approximately the same regardless of whether or not any individual's data is in the dataset. Previous studies of composition of differential privacy have focused on non-interactive algorithms, but interactive mechanisms are needed to capture many of the intended applications of differential privacy and a number of the important differentially private primitives.
We focus on concurrent composition, where an adversary can arbitrarily interleave its queries to several differentially private mechanisms, which may be feasible when differentially private query systems are deployed in practice. We prove that when the interactive mechanisms being composed are pure differentially private, their concurrent composition achieves privacy parameters (with respect to pure or approximate differential privacy) that match the (optimal) composition theorem for noninteractive differential privacy. We also prove a composition theorem for interactive mechanisms that satisfy approximate differential privacy. That bound is weaker than even the basic (suboptimal) composition theorem for noninteractive differential privacy, and we leave closing the gap as a direction for future research, along with understanding concurrent composition for other variants of differential privacy.

2021

TCC

Grafting Key Trees: Efficient Key Management for Overlapping Groups
Abstract

Key trees are often the best solution in terms of transmission cost and storage requirements for managing keys in a setting where a group needs to share a secret key, while being able to efficiently rotate the key material of users (in order to recover from a potential compromise, or to add or remove users). Applications include multicast encryption protocols like LKH (Logical Key Hierarchies) or group messaging like the current IETF proposal TreeKEM.
A key tree is a (typically balanced) binary tree, where each node is identified with a key: leaf nodes hold users’ secret keys while the root is the shared group key. For a group of size N, each user just holds log(N) keys (the keys on the path from its leaf to the root) and its entire key material can be rotated by broadcasting 2log(N) ciphertexts (encrypting each fresh key on the path under the keys of its parents). In this work we consider the natural setting where we have many groups with partially overlapping sets of users, and ask if we can find solutions where the cost of rotating a key is better than in the trivial
one where we have a separate key tree for each group.
We show that in an asymptotic setting (where the number m of groups is fixed while the number N of users grows) there exist more general key graphs whose cost converges to the cost of a single group, thus saving a factor linear in the number of groups over the trivial solution.
As our asymptotic “solution” converges very slowly and performs poorly on concrete examples, we propose an algorithm that uses a natural heuristic to compute a key graph for any given group structure. Our algorithm combines two greedy algorithms, and is thus very efficient: it first converts the group
structure into a “lattice graph”, which then is turned into a key graph by repeatedly applying the algorithm for constructing a Huffman code.
To better understand how far our proposal is from an optimal solution, we prove lower bounds on the update cost of continuous group-key agreement and multicast encryption in a symbolic model admitting (asymmetric) encryption, pseudorandom generators, and secret sharing as building blocks.

2021

TCC

The Round Complexity of Quantum Zero-Knowledge
Abstract

We study the round complexity of zero-knowledge for QMA (the quantum analogue of NP). Assuming the quantum quasi-polynomial hardness of the learning with errors (LWE) problem, we obtain the following results:
- 2-Round statistical witness indistinguishable (WI) arguments for QMA.
- 4-Round statistical zero-knowledge arguments for QMA in the plain model, additionally assuming the existence of quantum fully homomorphic encryption. This is the first protocol for constant-round statistical zero-knowledge arguments for QMA.
- 2-Round computational (statistical, resp.) zero-knowledge for QMA in the timing model, additionally assuming the existence of post-quantum non-parallelizing functions (time-lock puzzles, resp.).
All of these protocols match the best round complexity known for the corresponding protocols for NP with post-quantum security. Along the way, we introduce and construct the notions of sometimes-extractable oblivious transfer and sometimes-simulatable zero-knowledge, which might be of independent interest.

2021

TCC

Rate-1 Quantum Fully Homomorphic Encryption
Abstract

Secure function evaluation (SFE) allows Alice to publish an encrypted version of her input m such that Bob (holding a circuit C) can send a single message that reveals C(m) to Alice, and nothing more. Security is required to hold against malicious parties, that may behave arbitrarily. In this work we study the notion of SFE in the quantum setting, where Alice outputs an encrypted quantum state |\psi> and learns C(|\psi>) after receiving Bob's message.
We show that, assuming the quantum hardness of the learning with errors problem (LWE), there exists an SFE protocol for quantum computation with communication complexity (||\psi>|+|C(|\psi>)|)(1+o(1)), which is nearly optimal. This result is obtained by two main technical steps, which might be of independent interest. Specifically, we show (i) a construction of a rate-1 quantum fully-homomorphic encryption and (ii) a generic transformation to achieve malicious circuit privacy in the quantum setting.

2021

TCC

Direct Product Hardness Amplification
Abstract

We revisit one of the most fundamental hardness amplification constructions, originally proposed by Yao (FOCS 1982). We present a hardness amplification theorem for the direct product of certain games that is simpler, more general, and stronger than previously known hardness amplification theorems of the same kind. Our focus is two-fold. First, we aim to provide close-to-optimal concrete bounds, as opposed to asymptotic ones. Second, in the spirit of abstraction and reusability, our goal is to capture the essence of direct product hardness amplification as generally as possible. Furthermore, we demonstrate how our amplification theorem can be applied to obtain hardness amplification results for non-trivial interactive cryptographic games such as MAC forgery or signature forgery games.

2021

TCC

Amortizing Rate-1 OT and Applications to PIR and PSI
Abstract

Recent new constructions of rate-1 OT [D\"ottling, Garg, Ishai, Malavolta, Mour, and Ostrovsky, CRYPTO 2019] have brought this primitive under the spotlight and the techniques have led to new feasibility results for private-information retrieval, and homomorphic encryption for branching programs. The receiver communication of this construction consists of a quadratic (in the sender's input size) number of group elements for a single instance of rate-1 OT. Recently [Garg, Hajiabadi, Ostrovsky, TCC 2020] improved the receiver communication to a linear number of group elements for a single string-OT. However, most applications of rate-1 OT require executing it multiple times, resulting in large communication costs for the receiver.
In this work, we introduce a new technique for amortizing the cost of multiple rate-1 OTs. Specifically, based on standard pairing assumptions, we obtain a two-message rate-1 OT protocol for which the amortized cost per string-OT is asymptotically reduced to only four group elements. Our results lead to significant communication improvements in PSI and PIR, special cases of SFE for branching programs.
1. PIR: We obtain a rate-1 PIR scheme with client communication cost of $O(\lambda\cdot\log N)$ group elements for security parameter $\lambda$ and database size $N$. Notably, after a one-time setup (or one PIR instance), any following PIR instance only requires communication cost $O(\log N)$ number of group elements.
2. PSI with unbalanced inputs: We apply our techniques to private set intersection with unbalanced set sizes (where the receiver has a smaller set) and achieve receiver communication of $O((m+\lambda) \log N)$ group elements where $m, N$ are the sizes of the receiver and sender sets, respectively. Similarly, after a one-time setup (or one PSI instance), any following PSI instance only requires communication cost $O(m \cdot \log N)$ number of group elements. All previous sublinear-communication non-FHE based PSI protocols for the above unbalanced setting were also based on rate-1 OT, but incurred at least $O(\lambda^2 m \log N)$ group elements.

2021

TCC

Unifying Presampling via Concentration Bounds
Abstract

Auxiliary-input (AI) idealized models, such as auxiliary-input random oracle model (AI-ROM) and auxiliary-input random permutation model (AI-PRM), play a critical role in assessing non-uniform security of symmetric key and hash function constructions. However, obtaining security bounds in these models is often much more challenging.
The presampling technique, introduced by Unruh (CRYPTO' 07), generically reduces security proofs in the auxiliary-input models to much simpler bit-fixing models. This technique has been further optimized by Coretti, Dodis, Guo, Steinberger (EUROCRYPT' 18), and generalized by Coretti, Dodis, Guo (CRYPTO' 18), resulting in powerful tools for proving non-uniform security bounds in various idealized models.
We study the possibility of leveraging the presampling technique to the quantum world. To this end,
(*) We show that such leveraging will {resolve a major open problem in quantum computing, which is closely related to the famous Aaronson-Ambainis conjecture (ITCS' 11).
(*) Faced with this barrier, we give a new but equivalent bit-fixing model and a simple proof of presampling techniques for arbitrary oracle distribution in the classical setting, including AI-ROM and AI-RPM. Our theorem matches the best-known security loss and unifies previous presampling techniques.
(*) Finally, we leverage our new classical presampling techniques to a novel ``quantum bit-fixing'' version of presampling. It matches the optimal security loss of the classical presampling. Using our techniques, we give the first post-quantum non-uniform security for salted Merkle-Damgard hash functions and reprove the tight non-uniform security for function inversion by Chung et al. (FOCS' 20).

2021

TCC

On the (Ir)Replaceability of Global Setups, or How (Not) to Use a Global Ledger
Abstract

In universally composable (UC) security, a global setup is intended to capture the ideal behavior of a primitive which is accessible by multiple protocols, allowing them to share state. A representative example is the Bitcoin ledger. Indeed, since Bitcoin---and more generally blockchain ledgers---are known to be useful in various scenarios, it has become increasingly popular to capture such ledgers as global setup. Intuitively, one would expect UC to allow us to make security statements about protocols that use such a global setup, e.g., a global ledger, which can then be automatically translated into the setting where the setup is replaced by a protocol implementing it, such as Bitcoin.
We show that the above reasoning is flawed and such a generic security-preserving replacement can only work under very (often unrealistic) strong conditions on the global setup and the security statement. For example, the UC security of Bitcoin for realizing a ledger proved by Badertscher {\em et al.} [CRYPTO'17] is {\em not} sufficient per se to allow us to replace the ledger by Bitcoin when used as a global setup. In particular, we cannot expect that all security statements in the global ledger-hybrid world would be preserved when using Bitcoin as a ledger.
On the positive side, we provide characterizations of security statements for protocols that make use of global setups, for which the replacement is sound. Our results can be seen as a first guide on how to navigate the very tricky question of what constitutes a ``good'' global setup and how to use it in order to keep the modular protocol-design approach intact.

2021

TCC

Disappearing Cryptography in the Bounded Storage Model
Abstract

In this work, we study disappearing cryptography in the bounded storage model. Here, a component of the transmission, say a ciphertext, a digital signature, or even a program, is streamed bit by bit. The stream is too large for anyone to store in its entirety, meaning the transmission effectively disappears once the stream stops.
We first propose the notion of online obfuscation, capturing the goal of disappearing programs in the bounded storage model. We give a negative result for VBB security in this model, but propose candidate constructions for a weaker security goal, namely VGB security. We then demonstrate the utility of VGB online obfuscation, showing that it can be used to generate disappearing ciphertexts and signatures. All of our applications are not possible in the standard model of cryptography, regardless of computational assumptions used.

2021

TCC

Trojan-Resilience without Cryptography
Abstract

Digital hardware Trojans are integrated circuits whose implementation differ from the specification in an arbitrary and malicious way. For example, the circuit can differ from its specified input/output behavior after some fixed number of queries (known as ``time bombs'') or on some particular input (known as ``cheat codes'').
To detect such Trojans, countermeasures using multiparty computation (MPC) or verifiable computation (VC), have been proposed. On a high level, to realize a circuit with specification $\cF$ one has more sophisticated circuits $\cF^\diamond$ manufactured (where $\cF^\diamond$ specifies a MPC or VC of $\cF$), and then embeds these $\cF^\diamond$'s into a \emph{master circuit} which must be trusted but is relatively simple compared to $\cF$. Those solutions have a significant overhead as $\cF^\diamond$ is significantly more complex than $\cF$ and also the master circuits are not exactly trivial either.
In this work, we show that in restricted settings, where $\cF$ has no evolving state and is queried on independent inputs, we can achieve a relaxed security notion using very simple constructions. In particular, we do not change the specification of the circuit at all (i.e., $\cF=\cF^\diamond$). Moreover the master circuit basically just queries a subset of its manufactured circuits and checks if they're all the same.
The security we achieve guarantees that, if the manufactured circuits are initially tested on up to $T$ inputs, the master circuit will catch Trojans that try to deviate on significantly more than a $1/T$ fraction of the inputs. This bound is optimal for the type of construction considered, and we provably achieve it using a construction where $12$ instantiations of $\cF$ need to be embedded into the master. We also discuss an extremely simple construction with just $2$ instantiations for which we conjecture that it already achieves the optimal bound.

2021

TCC

Vector and Functional Commitments from Lattices
Abstract

Vector commitment (VC) schemes allow one to commit concisely to an
ordered sequence of values, so that the values at desired positions
can later be proved concisely. In addition, a VC can be statelessly
updatable, meaning that commitments and proofs can be updated to
reflect changes to individual entries, using knowledge of just those
changes (and not the entire vector). VCs have found important
applications in verifiable outsourced databases, cryptographic
accumulators, and cryptocurrencies. However, to date there have been
relatively few post-quantum constructions, i.e., ones that are
plausibly secure against quantum attacks.
More generally, functional commitment (FC) schemes allow one to
concisely and verifiably reveal various functions of committed data,
such as linear functions (i.e., inner products, including evaluations
of a committed polynomial). Under falsifiable assumptions, all known
functional commitments schemes have been limited to ``linearizable''
functions, and there are no known post-quantum FC schemes beyond
ordinary VCs.
In this work we give post-quantum constructions of vector and
functional commitments based on the standard Short Integer Solution
lattice problem (appropriately parameterized):
\begin{itemize}
\item First, we present new statelessly updatable VCs with
significantly shorter proofs than (and efficiency otherwise similar
to) the only prior post-quantum, statelessly updatable construction
(Papamanthou \etal, EUROCRYPT 13). Our constructions use private-key
setup, in which an authority generates public parameters and then
goes offline.
\item Second, we construct functional commitments for \emph{arbitrary
(bounded) Boolean circuits} and branching programs. Under
falsifiable assumptions, this is the first post-quantum FC scheme
beyond ordinary VCs, and the first FC scheme of any kind that goes
beyond linearizable functions. Our construction works in a new model
involving an authority that generates the public parameters and
remains online to provide public, reusable ``opening keys'' for
desired functions of committed messages.
\end{itemize}

2021

TCC

Quantum Key-length Extension
Abstract

Should quantum computers become available, they will reduce the effective key length of basic secret-key primitives, such as blockciphers. To address this we will either need to use blockciphers with inherently longer keys or develop key-length extension techniques to amplify the security of a blockcipher to use longer keys.
We consider the latter approach and revisit the FX and double encryption constructions. Classically, FX was proven to be a secure key-length extension technique, while double encryption fails to be more secure than single encryption due to a meet-in-the-middle attack. In this work we provide positive results, with concrete and tight bounds, for the security of both of these constructions against quantum attackers in ideal models.
For FX, we consider a partially-quantum model, where the attacker has quantum access to the ideal primitive, but only classical access to FX. This is a natural model and also the strongest possible, since effective quantum attacks against FX exist in the fully-quantum model when quantum access is granted to both oracles. We provide two results for FX in this model. The first establishes the security of FX against non-adaptive attackers. The second establishes security against general adaptive attackers for a variant of FX using a random oracle in place of an ideal cipher. This result relies on the techniques of Zhandry (CRYPTO '19) for lazily sampling a quantum random oracle. An extension to perfectly lazily sampling a quantum random permutation, which would help resolve the adaptive security of standard FX, is an important but challenging open question. We introduce techniques for partially-quantum proofs without relying on analyzing the classical and quantum oracles separately, which is common in existing work. This may be of broader interest.
For double encryption, we show that it amplifies strong pseudorandom permutation security in the fully-quantum model, strengthening a known result in the weaker sense of key-recovery security. This is done by adapting a technique of Tessaro and Thiruvengadam (TCC '18) to reduce the security to the difficulty of solving the list disjointness problem and then showing its hardness via a chain of reductions to the known quantum difficulty of the element distinctness problem.

2021

TCC

Fully-succinct Publicly Verifiable Delegation from Constant-Size Assumptions
Abstract

We construct a publicly verifiable, non-interactive delegation scheme for any polynomial size arithmetic circuit with proof-size and verification complexity comparable to those of pairing based zk-SNARKS. Concretely, the proof consists of $O(1)$ group elements and verification requires $O(1)$ pairings and $n$ group exponentiations, where $n$ is the size of the input. While known SNARK-based constructions rely on non-falsifiable assumptions, our construction can be proven sound under any constant size ($k\geq 2$) $k$-Matrix Diffie-Hellman ($k$-MDDH) assumption. However, the size of the reference string as well as the prover's complexity are quadratic in the size of the circuit. This result demonstrates that we can construct delegation from very simple and well-understood assumptions. We consider this work a first step towards achieving practical delegation from standard, falsifiable assumptions.
Our main technical contributions are first, the introduction and construction of what we call "no-signaling, somewhere statistically binding commitment schemes". These commitments are extractable for any small part $x_S$ of an opening $x$, where $S\subseteq [n]$ is of size at most $K$. Here $n$ is the dimension of $x$ and $x_S=(x_i)_{i\in S}$. Importantly, for any $S'\subseteq S$, extracting $x_{S'}$ can be done independently of $S\setminus S'$. Second, we use of these commitments to construct more efficient "quasi-arguments"' with no-signaling extraction, introduced by Paneth and Rothblum (TCC 17). These arguments allow extracting parts of the witness of a statement and checking it against some local constraints without revealing which part is checked. We construct pairing-based quasi arguments for linear and quadratic constraints and combine them with the low-depth delegation result of Gonzáles et. al. (Asiacrypt 19) to construct the final delegation scheme.

2021

TCC

BKW Meets Fourier: New Algorithms for LPN with Sparse Parities
Abstract

We consider the Learning Parity with Noise (LPN) problem with a sparse secret, where the secret vector $\mathbf{s}$ of dimension $n$ has Hamming weight at most $k$. We are interested in algorithms with asymptotic improvement in the \emph{exponent} beyond the state of the art.
Prior work in this setting presented algorithms with runtime $n^{c \cdot k}$ for constant $c < 1$, obtaining a constant factor improvement over brute force search, which runs
in time ${n \choose k}$.
We obtain the following results:
- We first consider the \emph{constant} error rate setting, and in this case present a new algorithm that leverages a subroutine from the acclaimed BKW algorithm [Blum, Kalai, Wasserman, J.~ACM '03] as well as techniques from Fourier analysis for $p$-biased distributions. Our algorithm achieves asymptotic improvement in the exponent compared to prior work,
when the sparsity $k = k(n) = \frac{n}{\log^{1+ 1/c}(n)}$, where $c \in o(\log \log(n))$ and $c \in \omega(1)$. The runtime and sample complexity of this algorithm are approximately the same.
- We next consider the \emph{low noise} setting, where the error is subconstant. We present a new algorithm in this setting that requires only a \emph{polynomial}
number of samples and achieves asymptotic improvement in the exponent compared to prior work, when the sparsity $k = \frac{1}{\eta} \cdot \frac{\log(n)}{\log(f(n))}$ and noise rate of $\eta \neq 1/2$ and $\eta^2 = \left(\frac{\log(n)}{n} \cdot f(n)\right)$, for $f(n) \in \omega(1) \cap n^{o(1)}$. To obtain the improvement in sample complexity, we create subsets of samples using the \emph{design} of Nisan and Wigderson [J.~Comput.~Syst.~Sci. '94], so that any two subsets have a small intersection, while the number of subsets is large. Each of these subsets is used to generate a single $p$-biased sample for the Fourier analysis step. We then show that this allows us to bound the covariance of pairs of samples, which is sufficient for the Fourier analysis.
- Finally, we show that our first algorithm extends to the setting where the noise rate is very high $1/2 - o(1)$, and in this case can be used as a subroutine to obtain new algorithms for learning DNFs and Juntas. Our algorithms achieve asymptotic improvement in the exponent for certain regimes. For DNFs of size $s$ with approximation factor $\epsilon$ this regime is when $\log \frac{s}{\epsilon} \in \omega \left( \frac{c}{\log n \log \log c}\right)$, and $\log \frac{s}{\epsilon} \in n^{1 - o(1)}$, for $c \in n^{1 - o(1)}$. For Juntas of $k$ the regime is when $k \in \omega \left( \frac{c}{\log n \log \log c}\right)$, and $k \in n^{1 - o(1)}$, for $c \in n^{1 - o(1)}$.

2021

TCC

Computational Robust (Fuzzy) Extractors for CRS-dependent Sources with Minimal Min-entropy
Abstract

Robust (fuzzy) extractors are very useful for, e.g., authenticated key exchange from a shared weak secret and remote biometric authentication against active adversaries. They enable two parties to extract the same uniform randomness with a ``helper'' string. More importantly, they have an authentication mechanism built in that tampering of the ``helper'' string will be detected. Unfortunately, as shown by Dodis and Wichs, in the information-theoretic setting, a robust extractor for an $(n,k)$-source requires $k>n/2$, which is in sharp contrast with randomness extractors which only require $k=\omega(\log n)$. Existing works either rely on random oracles or introduce CRS and work only for CRS-independent sources (even in the computational setting).
In this work, we give a systematic study about robust (fuzzy) extractors for general CRS {\em dependent} sources. We show in the information-theoretic setting, the same entropy lower bound holds even in the CRS model; we then show we {\em can} have robust extractors in the computational setting for general CRS-dependent source that is only with minimal entropy. We further extend our construction to robust fuzzy extractors. Along the way, we propose a new primitive called $\kappa$-MAC, which is unforgeable with a weak key and hides all partial information about the key (both against auxiliary input); it may be of independent interests.

2021

TCC

Updatable Public Key Encryption in the Standard Model
Abstract

Forward security (FS) ensures that corrupting the current secret key in the system preserves the privacy or integrity of the prior usages of the system. Achieving forward security is especially hard in the setting of public-key encryption (PKE), where time is divided into periods, and in each period the receiver derives the next-period secret key from their current secret key, while the public key stays constant. Indeed, all current constructions of FS-PKE are built from hierarchical identity-based encryption (HIBE) and are rather complicated.
Motivated by applications to secure messaging, recent works of Jost et al. (Eurocrypt’19) and Alwen et al. (CRYPTO’20) consider a natural relaxation of FS-PKE, which they term *updatable* PKE (UPKE). In this setting, the transition to the next period can be initiated by any sender, who can compute a special update ciphertext. This ciphertext directly produces the next-period public key and can be processed by the receiver to compute the next-period secret key. If done honestly, future (regular) ciphertexts produced with the new public key can be decrypted with the new secret key, but past such ciphertexts cannot be decrypted with the new secret key. Moreover, this is true even if all other previous-period updates were initiated by untrusted senders.
Both papers also constructed a very simple UPKE scheme based on the CDH assumption in the random oracle model. However, they left open the question of building such schemes in the standard model, or based on other (e.g., post-quantum) assumptions, without using the heavy HIBE techniques. In this work, we construct two efficient UPKE schemes in the standard model, based on the DDH and LWE assumptions, respectively. Somewhat interestingly, our constructions gain their efficiency (compared to prior FS-PKE schemes from the same assumptions) by using tools from the area of circular-secure and leakage resilient public-key encryption schemes (rather than HIBE).

2021

TCC

Generalized Pseudorandom Secret Sharing and Efficient Straggler-Resilient Secure Computation
Abstract

Secure multiparty computation (MPC) enables $n$ parties, of which up to $t$ may be corrupted, to perform joint computations on their private inputs while revealing only the outputs. Optimizing the asymptotic and concrete costs of MPC protocols has become an important line of research. Much of this research focuses on the setting of an honest majority, where $n \ge 2t+1$, which gives rise to concretely efficient protocols that are either information-theoretic or make a black-box use of symmetric cryptography. Efficiency can be further improved in the case of a {\em strong} honest majority, where $n>2t+1$.
Motivated by the goal of minimizing the communication and latency costs of MPC with a strong honest majority, we make two related contributions.
\begin{itemize}[leftmargin=*]
\item {\bf Generalized pseudorandom secret sharing (PRSS).}
Linear correlations serve as an important resource for MPC protocols and beyond. PRSS enables secure generation of many pseudorandom instances of such correlations without interaction, given replicated seeds of a pseudorandom function.
We extend the PRSS technique of Cramer et al.\ (TCC 2015) for sharing degree-$d$ polynomials to new constructions leveraging a particular class of combinatorial designs. Our constructions yield a dramatic efficiency improvement when the degree $d$ is higher than the security threshold $t$, not only for standard degree-$d$ correlations but also for several useful generalizations. In particular, correlations for locally converting between slot configurations in ``share packing'' enable us to avoid the concrete overhead of prior works.
\item {\bf Cheap straggler resilience.}
In reality, communication is not fully synchronous: protocol executions suffer from variance in communication delays and occasional node or message-delivery failures. We explore the benefits of PRSS-based MPC with a strong honest majority toward robustness against such failures, in turn yielding improved latency delays. In doing so we develop a novel technique for defending against a subtle ``double-dipping'' attack, which applies to the best existing protocols, with almost no extra cost in communication or rounds.
\end{itemize}
Combining the above tools requires further work, including new methods for batch verification via distributed zero-knowledge proofs (Boneh et al., CRYPTO 2019) that apply to packed secret sharing.
Overall, our work demonstrates new advantages of the strong honest majority setting, and introduces new tools---in particular, generalized PRSS---that we believe will be of independent use within other cryptographic applications.

2021

TCC

Towards Tight Adaptive Security of Non-Interactive Key Exchange
Abstract

We investigate the quality of security reductions for non-interactive key
exchange (NIKE) schemes. Unlike for many other cryptographic building blocks
(like public-key encryption, signatures, or zero-knowledge proofs), all known
NIKE security reductions to date are non-tight, i.e., lose a factor of at least
the number of users in the system. In that sense, NIKE forms a particularly
elusive target for tight security reductions.
The main technical obstacle in achieving tightly secure NIKE schemes are
adaptive corruptions. Hence, in this work, we explore security notions and
schemes that lie between selective security and fully adaptive security.
Concretely:
- We exhibit a tradeoff between key size and reduction loss.
We show that a tighter reduction can be bought by larger public and secret NIKE
keys. Concretely, we present a simple NIKE scheme with a reduction loss of
O(N^2 log(\nu)/\nu^2), and public and secret keys of O(\nu) group
elements, where N denotes the overall number of users in the system, and
\nu is a freely adjustable scheme parameter.
Our scheme achieves full adaptive security even against multiple "test
queries" (i.e., adversarial challenges), but requires keys of size O(N) to
achieve (almost) tight security under the matrix Diffie-Hellman assumption.
Still, already this simple scheme circumvents existing lower bounds.
- We show that this tradeoff is inherent.
We contrast the security of our simple scheme with a lower bound for all NIKE
schemes in which shared keys can be expressed as an ``inner product in the
exponent''. This result covers the original Diffie-Hellman NIKE scheme, as well
as a large class of its variants, and in particular our simple scheme. Our
lower bound gives a tradeoff between the ``dimension'' of any such scheme
(which directly corresponds to key sizes in existing schemes), and the
reduction quality. For \nu = O(N), this shows our simple scheme and reduction
optimal (up to a logarithmic factor).
- We exhibit a tradeoff between security and key size for tight reductions.
We show that it is possible to circumvent the inherent tradeoff above by
relaxing the desired security notion. Concretely, we consider the natural
notion of semi-adaptive security, where the adversary has to commit to a single
test query after seeing all public keys. As a feasibility result, we bring
forward the first scheme that enjoys compact public keys and tight
semi-adaptive security under the conjunction of the matrix Diffie-Hellman and
learning with errors assumptions.
We believe that our results shed a new light on the role of adaptivity in NIKE
security, and also illustrate the special role of NIKE when it comes to tight
security reductions.

2021

TCC

Polynomial-time targeted attacks on coin-tossing for any number of corruptions
Abstract

Consider a coin tossing protocol in which n processors P_1,...,P_n agree on a random bit b in n rounds, where in round i P_i sends a single message w_i. Imagine a full-information adversary who prefers the output 1, and in every round i it knows all the finalized messages w_1,...,w_{i-1} so far as well as the prepared message w_i. A k-replacing attack will have a chance to replace the prepared w_i with its own choice w'_i \neq w_i in up to k rounds. Taking majority protocol over uniformly random bits w_i = b_i is robust in the following strong sense. Any k-replacing adversary can only increase the probability of outputting 1 by at most O(k/\sqrt{n}). In this work, we ask if the above simple protocol is tight.
For the same setting, but restricted to uniformly random bit messages, Lichtenstein, Linial, and Saks [Combinatorica'89] showed how to achieve bias \Omega(k/\sqrt{n}) for any k \in [n]. Kalai, Komargodski, and Raz [DISC'18, Combinatorica'21] gave an alternative polynomial-time attack when k \geq \Theta(\sqrt{n}). Etesami, Mahloujifar, and Mahmoody [ALT'19, SODA'20] extended the result of KKR18 to arbitrary long messages.
In this work, we resolve both of these problems.
- For arbitrary length messages, we show that k-replacing polynomial-time attacks can indeed increase the probability of outputting 1 by \Omega(k/\sqrt{n}) for any k, which is optimal up to a constant factor. By plugging in our attack into the framework of Mahloujifar Mahmoody [TCC'17] we obtain similar data poisoning attacks against deterministic learners when adversary is limited to changing k=o(\sqrt{n}) of the n training examples.
- For uniformly random bits b_1,...,b_n, we show that whenever Pr[b=1]=Pr[\sum b_i \geq t]=\beta[t]_n for t \in [n] is the probability of a Hamming ball, then online polynomial-time k-replacing attacks can increase Pr[b=1] from \beta[t]_n to \beta[t-k]_n , which is optimal due to the majority protocol. In comparison, the (information-theoretic) attack of LLS89 increased Pr[b=1] to \beta[t-k]_{n-k}, which is optimal for adaptive adversaries who cannot see the message before changing it. Thus, we obtain a computational variant of Harper's celebrated vertex isoperimetric inequality.

2021

TCC

On expected polynomial runtime in cryptography
Abstract

A common definition of black-box zero-knowledge considers strict polynomial time (PPT) adversaries but expected polynomial time (EPT) simulation. This is necessary for constant round black-box zero-knowledge in the plain model, and the asymmetry between simulator and adversary an accepted consequence. Consideration of EPT adversaries naturally leads to designated adversaries, i.e. adversaries which are only required to be efficient in the protocol they are designed to attack. They were first examined in Feige’s thesis [Fei90],
where obstructions to proving security are shown. Prior work on (designated) EPT adversaries by Katz and Lindell (TCC’05) requires superpolynomial hardness assumptions, whereas the work of Goldreich (TCC’07) postulates “nice” behaviour under rewinding.
In this work, we start from scratch and revisit the definition of efficient algorithms. We argue that the standard runtime classes, PPT and EPT, behave “unnatural” from a cryptographic perspective. Namely, algorithms can have indistinguishable runtime distributions, yet one is considered efficient while the other is not. Hence, classical runtime classes are not “closed under indistinguishability”, which causes problems. Relaxations of PPT which
are “closed” are (well-)known and used.
We propose computationally expected polynomial time (CEPT), the class of runtimes which are (computationally) indistinguishable from EPT, which is “closed”. We analyze CEPT in the setting of uniform complexity (following Goldreich (JC’93)) with designated adversaries, and provide easy-to-check criteria for zero-knowledge protocols with blackbox simulation in the plain model, which show that many (all known?) such protocols
handle designated CEPT adversaries in CEPT.

2021

TCC

Relationships between quantum IND-CPA notions
Abstract

An encryption scheme is called indistinguishable under chosen plaintext attack (short IND-CPA) if an attacker cannot distinguish the encryptions of two messages of his choice. There are other variants of this definition but they all turn out to be equivalent in the classical case.
In this paper, we give a comprehensive overview of these different variants of IND-CPA
for symmetric encryption schemes in the quantum setting.
We investigate the relationships between these notions
and prove various equivalences, implications, non-equivalences, and non-implications between these variants.

2021

TCC

Blockchains Enable Non-Interactive MPC
Abstract

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.

2021

TCC

Classical Binding for Quantum Commitments
Abstract

In classical commitments, statistical binding means that for almost any commitment transcript there is at most one possible opening. While quantum commitments (for classical messages) sometimes have benefits over their classical counterparts (e.g. in terms of assumptions), they provide a weaker notion of binding. Essentially that the sender cannot open a given commitment to a random value with probability noticeably greater than 1/2.
We introduce a notion of classical binding for quantum commitments which provides guarantees analogous to the classical case. In our notion, the receiver performs a (partial) measurement of the quantum commitment string, and the outcome of this measurement determines a single value that the sender may open. We expect that our notion can replace classical commitments in various settings, leaving the security proof essentially unchanged. As an example we show a soundness proof for the GMW zero-knowledge proof system.
We construct a non-interactive quantum commitment scheme which is classically statistically-binding and has a classical opening, based on the existence of any post-quantum one-way function. Prior candidates had inherently quantum openings and were not classically binding.
In contrast, we show that it is impossible to achieve classical binding for statistically hiding commitments, regardless of assumption or round complexity.
Our scheme is simply Naor's commitment scheme (which classically requires a common random string, CRS), but executed in superposition over all possible values of the CRS, and repeated several times. We hope that this technique for using quantum communication to remove a CRS may find other uses.

2021

TCC

Covert Learning: How to Learn with an Untrusted Intermediary
Abstract

We consider the task of learning a function via oracle queries, where the queries and responses are monitored (and perhaps also modified) by an untrusted intermediary. Our goal is twofold: First, we would like to prevent the intermediary from gaining any information about either the function or the learner's intentions (e.g. the particular hypothesis class the learner is considering). Second, we would like to curb the intermediary's ability to meaningfully interfere with the learning process, even when it can modify the oracles' responses.
Inspired by the works of Ishai et al. (Crypto 2019) and Goldwasser et al. (ITCS 2021), we formalize two new learning models, called Covert Learning and Covert Verifiable Learning, that capture these goals. Then, assuming hardness of the Learning Parity with Noise (LPN) problem, we show:
1. Covert Learning algorithms in the agnostic setting for parity functions and decision trees, where a polynomial time eavesdropping adversary that observes all queries and responses learns nothing about either the function, or the learned hypothesis.
2. Covert Verifiable Learning algorithms that provide similar learning and privacy guarantees, even in the presence of a polynomial-time adversarial intermediary that can modify all oracle responses. Here the learner is granted additional random examples and is allowed to abort whenever the oracles responses are modified.
Aside theoretical interest, our study is motivated by applications to the secure outsourcing of automated scientific discovery in drug design and molecular biology. It also uncovers limitations of current techniques for defending against model extraction attacks.

2021

TCC

Multi-party PSM, Revisited: Improved Communication and Unbalanced Communication
Abstract

We improve the communication complexity in the Private Simultaneous Messages (PSM) model, which is a minimal model of non-interactive information-theoretic multi-party computation. The state-of-the-art PSM protocols were recently constructed by Beimel, Kushilevitz and Nissim (EUROCRYPT 2018).
We present new constructions of $k$-party PSM protocols. The new protocols match the previous upper bounds when $k=2$ or $3$ and improve the upper bounds for larger $k$. We also construct $2$-party PSM protocols with unbalanced communication complexity.
More concretely,
- For infinitely many $k$ (including all $k \leq 20$), we construct $k$-party PSM protocols for arbitrary functionality $f:[N]^k\to\{0,1\}$, whose communication complexity is $O_k(N^{\frac{k-1}{2}})$. This improves the former best known upper bounds of $O_k(N^{\frac{k}{2}})$ for $k\geq 6$, $O(N^{7/3})$ for $k=5$, and $O(N^{5/3})$ for $k=4$.
- For all rational $0<\eta<1$ whose denominator is $\leq 20$, we construct 2-party PSM protocols for arbitrary functionality $f:[N]\times[N]\to\{0,1\}$, whose communication complexity is $O(N^\eta)$ for one party, $O(N^{1-\eta})$ for the other. Previously the only known unbalanced 2-party PSM has communication complexity $O(\log(N)), O(N)$.

2021

TCC

Unclonable Encryption, Revisited
Abstract

Unclonable encryption, introduced by Broadbent and Lord (TQC'20), is an encryption scheme with the following attractive feature: given a ciphertext, an adversary cannot create two ciphertexts both of which decrypt to the same message as the original ciphertext.
We revisit this notion and show the following:
-Reusability: The constructions proposed by Broadbent and Lord have the disadvantage that they either guarantee one-time security (that is, the encryption key can only be used once to encrypt the message) in the plain model or they guaranteed security in the random oracle model. We construct unclonable encryption schemes with semantic security. We present two constructions from minimal cryptographic assumptions: (i) a private-key unclonable encryption scheme assuming post-quantum one-way functions and, (ii) a public-key unclonable encryption scheme assuming a post-quantum public-key encryption scheme.
-Lower Bound and Generalized Construction: We revisit the information-theoretic one-time secure construction of Broadbent and Lord. The success probability of the adversary in their construction was guaranteed to be $0.85^n$, where $n$ is the length of the message. It was interesting to understand whether the ideal success probability of (negligibly close to) $0.5^n$ was unattainable. We generalize their construction to be based on a broader class of monogamy of entanglement games (while their construction was based on BB84 game). We demonstrate a simple cloning attack that succeeds with probability $0.71^n$ against a class of schemes including that of Broadbent and Lord. We also present a $0.75^n$ cloning attack exclusively against their scheme.
-Implication to Copy-Protection: We show that unclonable encryption, satisfying a stronger property, called unclonable-indistinguishability (defined by Broadbent and Lord), implies copy-protection for a simple class of unlearnable functions. While we currently don't have encryption schemes satisfying this stronger property, this implication demonstrates a new path to construct copy-protection.