## CryptoDB

### Giulio Malavolta

#### Publications

**Year**

**Venue**

**Title**

2024

EUROCRYPT

Time-Lock Puzzles with Efficient Batch Solving
Abstract

Time-Lock Puzzles (TLPs) are a powerful tool for concealing messages until a predetermined point in time.
When solving multiple puzzles, it becomes crucial to have the ability to \emph{batch-solve} puzzles, i.e., simultaneously open multiple puzzles while working to solve a \emph{single one}. Unfortunately, all previously known TLP constructions equipped for batch solving have been contingent upon super-polynomially secure indistinguishability obfuscation, rendering them impractical in real-world applications.
In light of this challenge, we present novel TLP constructions that offer batch-solving capabilities without using heavy cryptographic hammers. Our proposed schemes are characterized by their simplicity in concept and efficiency in practice, and they can be constructed based on well-established cryptographic assumptions based on pairings or learning with errors (LWE).
Along the way, we introduce new constructions of puncturable key-homomorphic PRFs both in the lattice and in the pairing setting, which may be of independent interest. Our analysis benefits from an interesting connection to Hall's marriage theorem and incorporates an optimized combinatorial approach, enhancing the practicality and feasibility of our TLP schemes.
Furthermore, we introduce the concept of ``rogue-puzzle attacks", wherein maliciously crafted puzzle instances may disrupt the batch-solving process of honest puzzles. In response, we demonstrate the construction of concrete and efficient TLPs designed to thwart such attacks.

2024

EUROCRYPT

Software with Certified Deletion
Abstract

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

2024

CRYPTO

Polynomial Commitments from Lattices: Post-Quantum Security, Fast Verification and Transparent Setup
Abstract

Polynomial commitment scheme allows a prover to commit to a polynomial $f \in \ring[X]$ of degree $L$, and later prove that the committed function was correctly evaluated at a specified point $x$; in other words $f(x)=u$ for public $x,u \in \ring$. Most applications of polynomial commitments, e.g. succinct non-interactive arguments of knowledge (SNARKs), require that (i) both the commitment and evaluation proof are succinct (i.e., polylogarithmic in the degree $L$) - with the latter being efficiently verifiable, and (ii) no pre-processing step is allowed.
Surprisingly, as far as plausibly quantum-safe polynomial commitments are concerned, the currently most efficient constructions only rely on weak cryptographic assumptions, such as security of hash functions. Indeed, despite making use of the underlying algebraic structure, prior lattice-based polynomial commitments still seem to be much behind the hash-based ones. Moreover, security of the aforementioned lattice constructions against quantum adversaries was never formally discussed.
In this work, we bridge the gap and propose the first (asymptotically and concretely) efficient lattice-based polynomial commitment with transparent setup and post-quantum security. Our interactive variant relies on the standard (Module-)SIS problem, and can be made non-interactive in the random oracle model using Fiat-Shamir transformation. In addition, we equip the scheme with a knowledge soundness proof against quantum adversaries which can be of independent interest. In terms of concrete efficiency, for $L=2^{20}$ our scheme yields proofs of size $2$X smaller than the hash-based \textsf{FRI} commitment (Block et al., Asiacrypt 2023), and $60$X smaller than the very recent lattice-based construction by Albrecht et al. (Eprint 2023/1469).

2024

CRYPTO

Robust Quantum Public-Key Encryption with Applications to Quantum Key Distribution
Abstract

Quantum key distribution (QKD) allows Alice and Bob to agree on a shared secret key, while communicating over a public (untrusted) quantum channel. Compared to classical key exchange, it has two main advantages: (i) The key is unconditionally hidden to the eyes of any attacker, and (ii) its security assumes only the existence of authenticated classical channels which, in practice, can be realized using Minicrypt assumptions, such as the existence of digital signatures. On the flip side, QKD protocols typically require multiple rounds of interactions, whereas classical key exchange can be realized with the minimal amount of two messages using public-key encryption. A long-standing open question is whether QKD requires more rounds of interaction than classical key exchange.
In this work, we propose a two-message QKD protocol that satisfies everlasting security, assuming only the existence of quantum-secure one-way functions. That is, the shared key is unconditionally hidden, provided computational assumptions hold during the protocol execution. Our result follows from a new construction of quantum public-key encryption (QPKE) whose security, much like its classical counterpart, only relies on authenticated classical channels.

2024

CRYPTO

Time-Lock Puzzles from Lattices
Abstract

Time-lock puzzles (TLP) are a cryptographic tool that allow one to encrypt a message into
the future, for a predetermined amount of time T . At present, we have only two constructions with provable security: One based on the repeated squaring assumption and the other based on indistinguishability obfuscation (iO). Basing TLP on any other assumption is a long-standing question, further motivated by the fact that know constructions are broken by quantum algorithms.
In this work, we propose a new approach to construct time-lock puzzles based on lattices,
and therefore with plausible post-quantum security. We obtain the following main results:
• In the preprocessing model, where a one-time public-coin preprocessing is allowed, we
obtain a time-lock puzzle with encryption time log(T ).
• In the plain model, where the encrypter does all the computation, we obtain a time-lock
puzzle with encryption time √T .
Both constructions assume the existence of any sequential function f , and the hardness of the circular small-secret learning with errors (LWE) problem.
At the heart of our results is a new construction of succinct randomized encodings (SRE)
for T-folded repeated circuits, where the complexity of the encoding is √T . This is the first
construction of SRE where the overall complexity of the encoding algorithm is sublinear in the runtime T , and which is not based on iO. Using our SRE we directly obtain the first non-
interactive RAM delegation scheme with sublinear complexity (in the number of steps T ), again without iO. Finally, we also propose a new heuristic construction of SREs, and consequently of TLPs, with fully-efficient encoding complexity log(T ). Our scheme is inspired by iO techniques, but carefully sidesteps the regime of zeroizing attacks that plague lattice-based iO candidates.

2024

JOFC

Multi-key and Multi-input Predicate Encryption (for Conjunctions) from Learning with Errors
Abstract

<jats:title>Abstract</jats:title><jats:p>We put forward two natural generalizations of predicate encryption (PE), dubbed <jats:italic>multi-key</jats:italic> and <jats:italic>multi-input</jats:italic> PE. More in details, our contributions are threefold.<jats:list list-type="bullet">
<jats:list-item>
<jats:p><jats:bold>Definitions.</jats:bold> We formalize security of multi-key PE and multi-input PE following the standard indistinguishability paradigm, and modeling security both against malicious senders (i.e., corruption of encryption keys) and malicious receivers (i.e., collusions).</jats:p>
</jats:list-item>
<jats:list-item>
<jats:p><jats:bold>Constructions.</jats:bold> We construct adaptively secure multi-key and multi-input PE supporting the conjunction of poly-many arbitrary single-input predicates, assuming the sub-exponential hardness of the learning with errors (LWE) problem.</jats:p>
</jats:list-item>
<jats:list-item>
<jats:p><jats:bold>Applications.</jats:bold> We show that multi-key and multi-input PE for expressive enough predicates suffices for interesting cryptographic applications, including non-interactive multi-party computation (NI-MPC) and matchmaking encryption (ME).</jats:p>
</jats:list-item>
</jats:list> In particular, plugging in our constructions of multi-key and multi-input PE, under the sub-exponential LWE assumption, we obtain the first ME supporting <jats:italic>arbitrary policies</jats:italic> with unbounded collusions, as well as robust (resp. non-robust) NI-MPC for so-called <jats:italic>all-or-nothing</jats:italic> functions satisfying a non-trivial notion of reusability and supporting a constant (resp. polynomial) number of parties. Prior to our work, both of these applications required much heavier tools such as indistinguishability obfuscation or compact functional encryption.</jats:p>

2024

ASIACRYPT

Traitor Tracing without Trusted Authority from Registered Functional Encryption
Abstract

Traitor-tracing systems allow identifying the users who contributed to building a rogue decoder in a broadcast environment. In a traditional traitor-tracing system, a key authority is responsible for generating the global public parameters and issuing secret keys to users. All security is lost if the \emph{key authority itself} is corrupt. This raises the question: Can we construct a traitor-tracing scheme, without a trusted authority?
In this work, we propose a new model for traitor-tracing systems where, instead of having a key authority, users could generate and register their own public keys. The public parameters are computed by aggregating all user public keys. Crucially, the aggregation process is \emph{public}, thus eliminating the need of any trusted authority. We present two new traitor-tracing systems in this model based on bilinear pairings. Our first scheme is proven adaptively secure in the generic group model. This scheme features a {\it transparent} setup, ciphertexts consisting of $6\sqrt{L}+4$ group elements, and a public tracing algorithm. Our second scheme supports a bounded collusion of traitors and is proven selectively secure in the standard model. Our main technical ingredients are new registered functional encryption (RFE) schemes for quadratic and linear functions which, prior to this work, were known only from indistinguishability obfuscation.
To substantiate the practicality of our approach, we evaluate the performance a proof of concept implementation. For a group of $L = 1024$ users, encryption and decryption take roughly 50ms and 4ms, respectively, whereas a ciphertext is of size 6.7KB.

2024

TCC

Key-Homomorphic and Aggregate Verifiable Random Functions
Abstract

A verifiable random function (VRF) allows one to compute a random-looking image, while at the same time providing a unique proof that the function was evaluated correctly. VRFs are a cornerstone of modern cryptography and, among other applications, are at the heart of recently proposed proof-of-stake consensus protocols. In this work we initiate the formal study of \emph{aggregate VRFs}, i.e., VRFs that allow for the aggregation of proofs/images into a small digest, whose size is \emph{independent} of the number of input proofs/images, yet it still enables sound verification. We formalize this notion along with its security properties and we propose two constructions: The first scheme is conceptually simple, concretely efficient, and uses (asymmetric) bilinear groups of prime order. Pseudorandomness holds in the random oracle model and aggregate pseudorandomness is proven in the algebraic group model. The second scheme is in the standard model and it is proven secure against the learning with errors (LWE) problem.
As a cryptographic building block of independent interest, we introduce the notion of \emph{key homomorphic VRFs}, where the verification keys and the proofs are endowed with a group structure. We conclude by discussing several applications of key-homomorphic and aggregate VRFs, such as distributed VRFs and aggregate proof-of-stake protocols.

2024

TCC

Unclonable Commitments and Proofs
Abstract

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

2023

TCC

Weakening Assumptions for Publicly-Verifiable Deletion
Abstract

We develop a simple compiler that generically adds publicly-verifiable deletion to a variety of cryptosystems. Our compiler only makes use of one-way functions (or one-way state generators, if we allow the public verification key to be quantum). Previously, similar compilers either relied on indistinguishability obfuscation along with any one-way function (Bartusek et. al., ePrint:2023/265), or on almost-regular one-way functions (Bartusek, Khurana and Poremba, CRYPTO 2023).

2023

PKC

Laconic Function Evaluation for Turing Machines
Abstract

Laconic function evaluation (LFE) allows Alice to compress a large circuit C into a small digest d. Given Alice’s digest, Bob can encrypt some input x under d in a way that enables Alice to recover C(x), without learning anything beyond that. The scheme is said to be laconic if the size of d, the runtime of the encryption algorithm, and the size of the ciphertext are all sublinear in the size of C.
Until now, all known LFE constructions have ciphertexts whose size depends on the depth of the circuit C, akin to the limitation of levelled homomorphic encryption. In this work we close this gap and present the first LFE scheme (for Turing machines) with asymptotically optimal parameters. Our scheme assumes the existence of indistinguishability obfuscation and somewhere statistically binding hash functions. As further contributions, we show how our scheme enables a wide range of new applications, including two previously unknown constructions:
– Non-interactive zero-knowledge (NIZK) proofs with optimal prover complexity.
– Witness encryption and attribute-based encryption (ABE) for Turing machines from falsifiable assumptions.

2023

PKC

Transparent Batchable Time-lock Puzzles and Applications to Byzantine Consensus
Abstract

Time-lock puzzles (TLP) are a fascinating type of cryptographic problem that is easy to generate, but takes a certain time to solve, even when arbitrary parallel speedup is allowed. TLPs have wide-ranging applications including fairness, round efficient computation, and more. To reduce the effort needed to solve large numbers of TLPs, prior work has proposed batching techniques to reduce the cost of solving. However, these proposals either require: (1) a trusted setup or (2) the puzzle size be linear in the maximum batch size, which implies setting an a priori bound on the maximum size of the batch. Any of these limitations restrict the utility of TLPs in decentralized and dynamic settings like permissionless blockchains. In this work, we demonstrate the feasibility and usefulness of a TLP that overcomes all the above limitations using indistinguishability obfuscation to show that there are no fundamental barriers to achieving such a TLP construction.
As a main application of our TLP, we show how to improve the resilience of consensus protocols toward network-level adversaries in the following settings: (1) We show a generic compiler that boosts the resilience of a Byzantine broadcast protocol $\Pi$ as follows: if $\Pi$ is secure against $t<n$ weakly adaptive corruptions, then the compiled protocol is secure against $t<n$ strongly adaptive corruptions. Here, `strong' refers to adaptively corrupting a party and deleting messages that it sent while still honest. Our compiler is round and communication preserving, and gives the first expected constant-round Byzantine broadcast protocol against a strongly adaptive adversary for the dishonest majority setting. (2) We adapt the Nakamoto consensus protocol to a weak model of synchrony where the adversary can adaptively create minority partitions in the network. Unlike prior works, we do not assume that all honest messages are delivered within a known upper bound on the message delay. This is the first work to show that it is possible to achieve consensus in the permissionless setting even after relaxing the standard synchrony assumption.

2023

EUROCRYPT

Efficient Laconic Cryptography from Learning With Errors
Abstract

Laconic cryptography is an emerging paradigm that enables cryptographic primitives with sublinear communication complexity in just two messages. In particular, a two-message protocol between Alice and Bob is called \emph{laconic} if its communication and computation complexity are essentially independent of the size of Alice's input. This can be thought of as a dual notion of fully-homomorphic encryption, as it enables ``Bob-optimized'' protocols. This paradigm has led to tremendous progress in recent years. However, all existing constructions of laconic primitives are considered only of \emph{theoretical interest}: They all rely on non-black-box cryptographic techniques, which are highly impractical.
This work shows that non-black-box techniques are not necessary for basic laconic cryptography primitives. We propose a \emph{completely algebraic} construction of laconic encryption, a notion that we introduce in this work, which serves as the cornerstone of our framework. We prove that the scheme is secure under the standard Learning With Errors assumption (with polynomial modulus-to-noise ratio). We provide proof-of-concept implementations for the first time for laconic primitives, demonstrating the construction is indeed practical: For a database size of $2^{50}$, encryption and decryption are in the order of single digit \emph{milliseconds}.
Laconic encryption can be used as a black box to construct other laconic primitives. Specifically, we show how to construct:
\begin{itemize}
\item Laconic oblivious transfer
\item Registration-based encryption scheme
\item Laconic private-set intersection protocol
\end{itemize}
All of the above have essentially optimal parameters and similar practical efficiency.
Furthermore, our laconic encryption can be preprocessed such that the online encryption step is entirely combinatorial and therefore much more efficient.
Using similar techniques, we also obtain identity-based encryption with an unbounded identity space and tight security proof (in the standard model).

2023

EUROCRYPT

Multi-key and Multi-input Predicate Encryption from Learning with Errors
Abstract

We put forward two natural generalizations of predicate encryption (PE), dubbed multi-key and multi-input PE. More in details, our contributions are threefold.
– Definitions. We formalize security of multi-key PE and multi-input PE following the standard indistinguishability paradigm, and modeling security both against malicious senders (i.e., corruption of encryption keys) and malicious receivers (i.e., collusions).
– Constructions. We construct adaptively secure multi-key and multi-input PE supporting the conjunction of poly-many arbitrary single-input predicates, assuming the sub-exponential hardness of the learning with errors (LWE) problem.
– Applications. We show that multi-key and multi-input PE for expressive enough predicates suffices for interesting cryptographic applications, including non-interactive multi-party computation (NI-MPC) and matchmaking encryption (ME).
In particular, plugging in our constructions of multi-key and multi-input PE, under the sub-exponential LWE assumption, we obtain the first ME supporting arbitrary policies with unbounded collusions, as well as robust (resp. non-robust) NI-MPC for so-called all-or-nothing functions satisfying a non-trivial notion of reusability and supporting a constant (resp. polynomial) number of parties. Prior to our work, both of these applications required much heavier tools such as indistinguishability obfuscation or compact functional encryption.

2023

CRYPTO

On Concurrent Multi-Party Quantum Computation
Abstract

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

2023

CRYPTO

Lattice-based Succinct Arguments from Vanishing Polynomials
Abstract

Succinct arguments allow a prover to convince a verifier of the validity of any statement in a language, with minimal communication and verifier's work. Among other approaches, lattice-based protocols offer solid theoretical foundations, post-quantum security, and a rich algebraic structure. In this work, we present some new approaches to constructing efficient lattice-based succinct arguments. Our main technical ingredient is a new commitment scheme based on \emph{vanishing polynomials}, a notion borrowed from algebraic geometry. We analyse the security of such a commitment scheme, and show how to take advantage of the additional algebraic structure to build new lattice-based succinct arguments. A few highlights amongst our results are:
\begin{enumerate}
\item The first recursive folding (i.e. Bulletproofs-like) protocol for linear relations with \emph{polylogarithmic} verifier runtime. Traditionally, the verifier runtime has been the efficiency bottleneck for such protocols (regardless of the underlying assumptions).
\item The first verifiable delay function (VDF) based on lattices, building on a recently introduced sequential relation.
\item The first lattice-based \emph{linear-time prover} succinct argument for NP, in the preprocessing model. The soundness of the scheme is based on (knowledge)-k-R-ISIS assumption [Albrecht et al., CRYPTO'22].
\end{enumerate}

2023

CRYPTO

Lattice-Based Timed Cryptography
Abstract

Timed cryptography studies primitives that retain their security only for a pre-determined amount of time, such as proofs of sequential work and time-lock puzzles. This feature has proven to be useful in a large number of practical applications, e.g., randomness generation, sealed-bid auctions, or fair multi-party computation. However, the current state of affairs in timed cryptography is unsatisfactory: Virtually all efficient constructions rely on a single sequentiality assumption, namely that repeated squaring in unknown order groups cannot be parallelized. This is a single point of failure in the classical setting and is even false against quantum adversaries.
In this work we put forward a new sequentiality assumption, which essentially says that a repeated application of the standard lattice-based hash function cannot be parallelized. We provide concrete evidence of the validity of this assumption and, to substantiate its usefulness, we show how it enables a new proof of sequential work, with a stronger sequentiality guarantee than prior hash-based schemes.

2023

JOFC

Candidate iO from Homomorphic Encryption Schemes
Abstract

We propose a new approach to construct general-purpose indistinguishability obfuscation (iO). Our construction is obtained via a new intermediate primitive that we call split fully homomorphic encryption (split FHE), which we show to be sufficient for constructing iO. Specifically, split FHE is FHE where decryption takes the following two-step syntactic form: (i) a secret decryption step that uses the secret key and produces a hint which is (asymptotically) shorter than the length of the encrypted message, and (ii) a public decryption step that only requires the ciphertext and the previously generated hint (and not the entire secret key) and recovers the encrypted message. In terms of security, the hints for a set of ciphertexts should not allow one to violate semantic security for any other ciphertexts. Next, we show a generic candidate construction of split FHE based on three building blocks: (i) A standard FHE scheme with linear decrypt-and-multiply (which can be instantiated with essentially all LWE-based constructions), (ii) a linearly homomorphic encryption scheme with short decryption hints (such as the Damgård-Jurik encryption scheme, based on the DCR problem), and (iii) a cryptographic hash function (which can be based on a variety of standard assumptions). Our approach is heuristic in the sense that our construction is not provably secure and makes implicit assumptions about the interplay between these underlying primitives. We show evidence that this construction is secure by providing an argument in an appropriately defined oracle model. We view our construction as a big departure from the state-of-the-art constructions, and it is in fact quite simple.

2023

ASIACRYPT

Registered (Inner-Product) Functional Encryption
Abstract

Registered encryption (Garg et al., TCC'18) is an emerging paradigm that tackles the key-escrow problem associated with identity-based encryption by replacing the private-key generator with a much weaker entity known as the key curator. The key curator holds no secret information, and is responsible to:
(i) update the master public key whenever a new user registers its own public key to the system;
(ii) provide helper decryption keys to the users already registered in the system, in order to still enable them to decrypt after new users join the system.
For practical purposes, tasks (i) and (ii) need to be efficient, in the sense that the size of the public parameters, of the master public key, and of the helper decryption keys, as well as the running times for key generation and user registration, and the number of updates, must be small.
In this paper, we generalize the notion of registered encryption to the setting of functional encryption (FE). As our main contribution, we show an efficient construction of registered FE for the special case of (attribute hiding) inner-product predicates, built over asymmetric bilinear groups of prime order. Our scheme supports a large attribute universe and is proven secure in the bilinear generic group model. We also implement our scheme and experimentally demonstrate the efficiency requirements of the registered settings. Our second contribution is a feasibility result where we build registered FE for P/poly based on indistinguishability obfuscation and somewhere statistically binding hash functions.

2023

ASIACRYPT

Two-Round Concurrent 2PC from Sub-Exponential LWE
Abstract

Secure computation is a cornerstone of modern cryptography and a rich body of research is devoted to understanding its round complexity. In this work, we consider two-party computation (2PC) protocols (where both parties receive output) that remain secure in the realistic setting where many instances of the protocol are executed in parallel (concurrent security). We obtain a two-round concurrent-secure 2PC protocol based on a single, standard, post-quantum assumption: The subexponential hardness of the learning-with-errors (LWE) problem. Our protocol is in the plain model, i.e., it has no trusted setup, and it is secure in the super-polynomial simulation framework of Pass (EUROCRYPT 2003). Since two rounds are minimal for (concurrent) 2PC, this work resolves the round complexity of concurrent 2PC from standard assumptions.
As immediate applications, our work establishes feasibility results for interesting cryptographic primitives such as the first two-round password authentication key exchange (PAKE) protocol in the plain model and the first two-round concurrent secure computation protocol for quantum circuits (2PQC).

2023

ASIACRYPT

Distributed Broadcast Encryption from Bilinear Groups
Abstract

Distributed broadcast encryption (DBE) improves on the
traditional notion of broadcast encryption by eliminating the key-escrow
problem: In a DBE system, users generate their own secret keys non-
interactively without the help of a trusted party. Then anyone can broad-
cast a message for a subset S of the users, in such a way that the resulting
ciphertext size is sublinear in (and, ideally, independent of) |S|. Unfor-
tunately, the only known constructions of DBE requires heavy crypto-
graphic machinery, such as general-purpose indistinguishability obfusca-
tion, or come without a security proof.
In this work, we formally show that obfuscation is not necessary for DBE,
and we present two practical DBE schemes from standard assumptions in
prime-order bilinear groups. Our constructions are conceptually simple,
satisfy the strong notion of adaptive security, and are concretely efficient.
In fact, their performance, in terms of number of group elements and
efficiency of the algorithms, is comparable with that of traditional (non
distributed) broadcast encryption schemes from bilinear groups.

2023

ASIACRYPT

Weak Zero-Knowledge via the Goldreich-Levin Theorem
Abstract

Obtaining three round zero-knowledge from standard cryptographic assumptions has remained a challenging open problem. Meanwhile, there has been exciting progress in realizing useful relaxations such as weak zero-knowledge, strong witness indistinguishability and witness hiding in two or three rounds. In particular, known realizations from generic assumptions obtain:
(1) security against {\em adaptive} verifiers assuming fully homomorphic encryption among other standard assumptions (Bitansky et. al., STOC 2019), and
(2) security against {\em non-adaptive} verifiers in the distributional setting from oblivious transfer (Jain et. al., Crypto 2017).
This work builds three round weak zero-knowledge for NP in the non-adaptive setting from
doubly-enhanced injective trapdoor functions. We obtain this result by developing a new distinguisher-dependent simulation technique that makes crucial use of the Goldreich-Levin list decoding algorithm, and may be of independent interest.

2023

TCC

Public-Key Encryption with Quantum Keys
Abstract

In the framework of Impagliazzo's five worlds, a distinction is often made between two worlds, one where public-key encryption exists (Cryptomania), and one in which only one-way functions exist (MiniCrypt). However, the boundaries between these worlds can change when quantum information is taken into account. Recent work has shown that quantum variants of oblivious transfer and multi-party computation, both primitives that are classically in Cryptomania, can be constructed from one-way functions, placing them in the realm of quantum MiniCrypt (the so-called MiniQCrypt). This naturally raises the following question:
Is it possible to construct a quantum variant of public-key encryption, which is at the heart of Cryptomania, from one-way functions or potentially weaker assumptions?
In this work, we initiate the formal study of the notion of quantum public-key encryption (qPKE), i.e., public-key encryption where keys are allowed to be quantum states. We propose new definitions of security and several constructions of qPKE based on the existence of one-way functions (OWF), or even weaker assumptions, such as pseudorandom function-like states (PRFS) and pseudorandom function-like states with proof of destruction (PRFSPD). Finally, to give a tight characterization of this primitive, we show that computational assumptions are necessary to build quantum public-key encryption. That is, we give a self-contained proof that no quantum public-key encryption scheme can provide information-theoretic security.

2022

TCC

Steganography-Free Zero-Knowledge
Abstract

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

2022

TCC

Quantum Rewinding for Many-Round Protocols
Abstract

We investigate the security of succinct arguments against quantum adversaries. Our main result is a proof of knowledge-soundness in the post-quantum setting for a class of multi-round interactive protocols, including those based on the recursive folding technique of Bulletproofs.
To prove this result, we devise a new quantum rewinding strategy, the first that allows for rewinding across many rounds. This technique applies to any protocol satisfying natural multi-round generalizations of special soundness and collapsing. For our main result, we show that recent Bulletproofs-like protocols based on lattices satisfy these properties, and are hence sound against quantum adversaries.

2022

PKC

A Note on the Post-Quantum Security of (Ring) Signatures
📺
Abstract

This work revisits the security of classical signatures and ring signatures in a quantum world. For (ordinary) signatures, we focus on the arguably preferable security notion of {\em blind-unforgeability} recently proposed by Alagic et al.\ (Eurocrypt'20). We present two {\em short} signature schemes achieving this notion: one is in the quantum random oracle model, assuming quantum hardness of SIS; and the other is in the plain model, assuming quantum hardness of LWE with super-polynomial modulus. Prior to this work, the only known blind-unforgeable schemes are Lamport's one-time signature and the Winternitz one-time signature, and both of them are in the quantum random oracle model.
For ring signatures, the recent work by Chatterjee et al.\ (Crypto'21) proposes a definition trying to capture adversaries with quantum access to the signer. However, it is unclear if their definition, when restricted to the classical world, is as strong as the standard security notion for ring signatures. They also present a construction that only {\em partially} achieves (even) this seeming weak definition, in the sense that the adversary can only conduct superposition attacks over the messages, but not the rings. We propose a new definition that does not suffer from the above issue. Our definition is an analog to the blind-unforgeability in the ring signature setting. Moreover, assuming the quantum hardness of LWE, we construct a compiler converting any blind-unforgeable (ordinary) signatures to a ring signature satisfying our definition.

2022

CRYPTO

Lattice-Based SNARKs: Publicly Verifiable, Preprocessing, and Recursively Composable
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Abstract

A succinct non-interactive argument of knowledge (SNARK) allows a prover to produce a short proof that certifies the veracity of a certain NP-statement. In the last decade, a large body of work has studied candidate constructions that are secure against quantum attackers. Unfortunately, no known candidate matches the efficiency and desirable features of (pre-quantum) constructions based on bilinear pairings.
In this work, we make progress on this question. We propose the first lattice-based SNARK that simultaneously satisfies many desirable properties: It (i) is tentatively post-quantum secure, (ii) is publicly-verifiable, (iii) has a logarithmic-time verifier and (iv) has a purely algebraic structure making it amenable to efficient recursive composition. Our construction stems from a general technical toolkit that we develop to translate pairing-based schemes to lattice-based ones. At the heart of our SNARK is a new lattice-based vector commitment (VC) scheme supporting openings to constant-degree multivariate polynomial maps, which is a candidate solution for the open problem of constructing VC schemes with openings to beyond linear functions. However, the security of our constructions is based on a new family of lattice-based computational assumptions which naturally generalises the standard Short Integer Solution (SIS) assumption.

2022

CRYPTO

Succinct Classical Verification of Quantum Computation
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Abstract

We construct a classically verifiable succinct interactive argument for quantum computation (BQP) with communication complexity and verifier runtime that are poly-logarithmic in the runtime of the BQP computation (and polynomial in the security parameter). Our protocol is secure assuming the post-quantum security of indistinguishability obfuscation (iO) and Learning with Errors (LWE). This is the first succinct argument for quantum computation in the plain model; prior work (Chia-Chung-Yamakawa, TCC ’20) requires both a long common reference string and non-black-box use of a hash function modeled as a random oracle.
At a technical level, we revisit the framework for constructing classically verifiable quantum computation (Mahadev, FOCS ’18). We give a self-contained, modular proof of security for Mahadev’s protocol, which we believe is of independent interest. Our proof readily generalizes to a setting in which the verifier’s first message (which consists of many public keys) is compressed. Next, we formalize this notion of compressed public keys; we view the object as a generalization of constrained/programmable PRFs and instantiate it based on indistinguishability obfuscation.
Finally, we compile the above protocol into a fully succinct argument using a (sufficiently composable) succinct argument of knowledge for NP. Using our framework, we achieve several additional results, including
– Succinct arguments for QMA (given multiple copies of the witness),
– Succinct non-interactive arguments for BQP (or QMA) in the quantum random oracle model, and
– Succinct batch arguments for BQP (or QMA) assuming post-quantum LWE (without iO).

2022

TCC

Candidate Trapdoor Claw-Free Functions from Group Actions with Applications to Quantum Protocols
Abstract

Trapdoor Claw-free Functions (TCFs) are two-to-one trapdoor functions where it is computationally hard to find a claw, i.e., a colliding pair of inputs. TCFs have recently seen a surge of renewed interest due to new applications to quantum cryptography: as an example, TCFs enable a classical machine to verify that some quantum computation has been performed correctly. In this work, we propose a new family of (almost two-to-one) TCFs based on conjectured hard problems on isogeny-based group actions. This is the first candidate construction that is not based on lattice-related problems and the first scheme (from any plausible post-quantum assumption) with a deterministic evaluation algorithm. To demonstrate the usefulness of our construction, we show that our TCF family can be used to devise a computational test of qubit, which is the basic building block used in general verification of quantum computations.

2022

JOFC

Everlasting UC Commitments from Fully Malicious PUFs
Abstract

Everlasting security models the setting where hardness assumptions hold during the execution of a protocol but may get broken in the future. Due to the strength of this adversarial model, achieving any meaningful security guarantees for composable protocols is impossible without relying on hardware assumptions (Müller-Quade and Unruh, JoC’10). For this reason, a rich line of research has tried to leverage physical assumptions to construct well-known everlasting cryptographic primitives, such as commitment schemes. The only known everlastingly UC secure commitment scheme, due to Müller-Quade and Unruh (JoC’10), assumes honestly generated hardware tokens. The authors leave the possibility of constructing everlastingly UC secure commitments from malicious hardware tokens as an open problem. Goyal et al. (Crypto’10) constructs unconditionally UC-secure commitments and secure computation from malicious hardware tokens, with the caveat that the honest tokens must encapsulate other tokens. This extra restriction rules out interesting classes of hardware tokens, such as physically uncloneable functions (PUFs). In this work, we present the first construction of an everlastingly UC-secure commitment scheme in the fully malicious token model without requiring honest token encapsulation. Our scheme assumes the existence of PUFs and is secure in the common reference string model. We also show that our results are tight by giving an impossibility proof for everlasting UC-secure computation from non-erasable tokens (such as PUFs), even with trusted setup.

2021

EUROCRYPT

Post-Quantum Multi-Party Computation
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Abstract

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

2021

EUROCRYPT

Unbounded Multi-Party Computation from Learning with Errors
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Abstract

We consider the problem of round-optimal *unbounded MPC*: in the first round, parties publish a message that depends only on their input. In the second round, any subset of parties can jointly and securely compute any function $f$ over their inputs in a single round of broadcast. We do not impose any a priori bound on the number of parties nor on the size of the functions that can be computed.
Our main result is a semi-honest two-round protocol for unbounded MPC in the plain model from the hardness of the standard learning with errors (LWE) problem. Prior work in the same setting assumes the hardness of problems over bilinear maps. Thus, our protocol is the first example of unbounded MPC that is post-quantum secure.
The central ingredient of our protocol is a new scheme of attribute-based secure function evaluation (AB-SFE) with *public decryption*. Our construction combines techniques from the realm of homomorphic commitments with delegation of lattice basis. We believe that such a scheme may find further applications in the future.

2021

PKC

A Geometric Approach to Homomorphic Secret Sharing
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Abstract

An (n,m,t)-homomorphic secret sharing (HSS) scheme allows n clients to share their inputs across m servers, such that the inputs are hidden from any t colluding servers, and moreover the servers can evaluate functions over the inputs locally by mapping their input shares to compact output shares. Such compactness makes HSS a useful building block for communication-efficient secure multi-party computation (MPC).
In this work, we propose a simple compiler for HSS evaluating multivariate polynomials based on two building blocks: (1) homomorphic encryption for linear functions or low-degree polynomials, and (2) information-theoretic HSS for low-degree polynomials. Our compiler leverages the power of the first building block towards improving the parameters of the second.
We use our compiler to generalize and improve on the HSS scheme of Lai, Malavolta, and Schröder [ASIACRYPT'18], which is only efficient when the number of servers is at most logarithmic in the security parameter. In contrast, we obtain efficient schemes for polynomials of higher degrees and an arbitrary number of servers. This application of our general compiler extends techniques that were developed in the context of information-theoretic private information retrieval (Woodruff and Yekhanin [CCC'05]), which use partial derivatives and Hermite interpolation to support the computation of polynomials of higher degrees.
In addition to the above, we propose a new application of HSS to MPC with preprocessing. By pushing the computation of some HSS servers to a preprocessing phase, we obtain communication-efficient MPC protocols for low-degree polynomials that use fewer parties than previous protocols based on the same assumptions. The online communication of these protocols is linear in the input size, independently of the description size of the polynomial.

2021

CRYPTO

Compact Ring Signatures from Learning With Errors
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Abstract

Ring signatures allow a user to sign a message on behalf of a ``ring'' of signers, while hiding the true identity of the signer. As the degree of anonymity guaranteed by a ring signature is directly proportional to the size of the ring, an important goal in cryptography is to study constructions that minimize the size of the signature as a function of the number of ring members.
In this work, we present the first compact ring signature scheme (i.e., where the size of the signature grows logarithmically with the size of the ring) from the (plain) learning with errors (LWE) problem. The construction is in the standard model and it does not rely on a trusted setup or on the random oracle heuristic. In contrast with the prior work of Backes
\etal~[EUROCRYPT'2019], our scheme does not rely on bilinear pairings, which allows us to show that the scheme is post-quantum secure assuming the quantum hardness of LWE.
At the heart of our scheme is a new construction of compact and statistically witness-indistinguishable ZAP arguments for NP $\cap$ coNP, that we show to be sound based on the plain LWE assumption. Prior to our work, statistical ZAPs (for all of NP) were known to exist only assuming \emph{sub-exponential} LWE. We believe that this scheme might find further applications in the future.

2021

ASIACRYPT

How to Build a Trapdoor Function from an Encryption Scheme
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Abstract

In this work we ask the following question: Can we transform any encryption scheme into a trapdoor function (TDF)? Alternatively stated, can we make any encryption scheme randomness recoverable? We propose a generic compiler that takes as input any encryption scheme with pseudorandom ciphertexts and adds a trapdoor to invert the encryption, recovering also the random coins. This universal TDFier only assumes in addition the existence of a hinting pseudorandom generator (PRG). Despite the simplicity, our transformation is quite general and we establish a series of new feasibility results:
- The first identity-based TDF [Bellare et al. EUROCRYPT 2012] from the CDH assumption in pairing-free groups (or from factoring), thus matching the state of the art for identity-based encryption schemes. Prior works required pairings or LWE.
- The first collusion-resistant attribute-based TDF (AB-TDF) for all ($NC^1$, resp.) circuits from LWE (bilinear maps, resp.). Moreover, the first single-key AB-TDF from CDH. To the best of our knowledge, no AB-TDF was known in the literature (not even for a single key) from any assumption. We obtain the same results for predicate encryption.
As an additional contribution, we define and construct a trapdoor garbling scheme: A simulation secure garbling scheme with a hidden ``trigger'' that allows the evaluator to fully recover the randomness used by the garbling algorithm. We show how to construct trapdoor garbling from the DDH or LWE assumption with an interplay of key-dependent message (KDM) and randomness-dependent message (RDM) techniques.
Trapdoor garbling allows us to obtain alternative constructions of (single-key) AB-TDFs with additional desirable properties, such as adaptive security (in the choice of the attribute) and projective keys. We expect trapdoor garbling to be useful in other contexts, e.g. in case where, upon successful execution, the evaluator needs to immediately verify that the garbled circuit was well-formed.

2021

TCC

Two-Round Maliciously Secure Computation with Super-Polynomial Simulation
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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

The Round Complexity of Quantum Zero-Knowledge
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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
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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.

2020

EUROCRYPT

Candidate iO From Homomorphic Encryption Schemes
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Abstract

We propose a new approach to construct general-purpose indistinguishability obfuscation (iO). Our construction is obtained via a new intermediate primitive that we call split fully-homomorphic encryption (split FHE), which we show to be sufficient for constructing iO. Specifically, split FHE is FHE where decryption takes the following two-step syntactic form: (i) A secret decryption step uses the secret key and produces a hint which is (asymptotically) shorter than the length of the encrypted message, and (ii) a public decryption step that only requires the ciphertext and the previously generated hint (and not the entire secret key), and recovers the encrypted message. In terms of security, the hints for a set of ciphertexts should not allow one to violate semantic security for any other ciphertexts.
Next, we show a generic candidate construction of split FHE based on three building blocks: (i) A standard FHE scheme with linear decrypt-and-multiply (which can be instantiated with essentially all LWE-based constructions), (ii) a linearly homomorphic encryption scheme with short decryption hints (such as the Damgard-Jurik encryption scheme, based on the DCR problem), and (iii) a cryptographic hash function (which can be based on a variety of standard assumptions). Our approach is heuristic in the sense that our construction is not provably secure and makes implicit assumptions about the interplay between these underlying primitives. We show evidence that this construction is secure by providing an argument in an appropriately defined oracle model.
We view our construction as a big departure from the state-of-the-art constructions, and it is in fact quite simple.

2020

EUROCRYPT

Statistical Zaps and New Oblivious Transfer Protocols
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Abstract

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

2020

TCC

Constant Ciphertext-Rate Non-Committing Encryption from Standard Assumptions
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Abstract

Non-committing encryption (NCE) is a type of public key encryption which comes with the ability to equivocate ciphertexts to encryptions of arbitrary messages, i.e., it allows one to find coins for key generation and encryption which ``explain'' a given ciphertext as an encryption of any message. NCE is the cornerstone to construct adaptively secure multiparty computation [Canetti et al. STOC'96] and can be seen as the quintessential notion of security for public key encryption to realize ideal communication channels.
A large body of literature investigates what is the best message-to-ciphertext ratio (i.e., the rate) that one can hope to achieve for NCE. In this work we propose a near complete resolution to this question and we show how to construct NCE with constant rate in the plain model from a variety of assumptions, such as the hardness of the learning with errors (LWE), the decisional Diffie-Hellman (DDH), or the quadratic residuosity (QR) problem. Prior to our work, constructing NCE with constant rate required a trusted setup and indistinguishability obfuscation [Canetti et al. ASIACRYPT'17].

2020

TCC

Multi-key Fully-Homomorphic Encryption in the Plain Model
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Abstract

The notion of multi-key fully homomorphic encryption (multi-key FHE) [Lopez-Alt, Tromer, Vaikuntanathan, STOC'12] was proposed as a generalization of fully homomorphic encryption to the multiparty setting. In a multi-key FHE scheme for $n$ parties, each party can individually choose a key pair and use it to encrypt its own private input. Given n ciphertexts computed in this manner, the parties can homomorphically evaluate a circuit C over them to obtain a new ciphertext containing the output of C, which can then be decrypted via a decryption protocol. The key efficiency property is that the size of the (evaluated) ciphertext is independent of the size of the circuit.
Multi-key FHE with one-round decryption [Mukherjee and Wichs, Eurocrypt'16], has found several powerful applications in cryptography over the past few years. However, an important drawback of all such known schemes is that they require a trusted setup.
In this work, we address the problem of constructing multi-key FHE in the plain model. We obtain the following results:
- A multi-key FHE scheme with one-round decryption based on the hardness of learning with errors (LWE), ring LWE, and decisional small polynomial ratio (DSPR) problems.
- A variant of multi-key FHE where we relax the decryption algorithm to be non-compact -- i.e., where the decryption complexity can depend on the size of C -- based on the hardness of LWE. We call this variant multi-homomorphic encryption (MHE). We observe that MHE is already sufficient for some of the applications of multi-key FHE.

2020

ASIACRYPT

A Combinatorial Approach to Quantum Random Functions
📺
Abstract

Quantum pseudorandom functions (QPRFs) extend the classical security of a PRF by allowing the adversary to issue queries on input superpositions. Zhandry [Zhandry, FOCS 2012] showed a separation between the two notions and proved that common construction paradigms are also quantum secure, albeit with a new ad-hoc analysis. In this work, we revisit the question of constructing QPRFs and propose a new method starting from small-domain (classical) PRFs: At the heart of our approach is a new domain-extension technique based on bipartite expanders. Interestingly, our analysis is almost entirely classical.
As a corollary of our main theorem, we obtain the first (approximate) key-homomorphic quantum PRF based on the quantum intractability of the learning with errors problem.

2020

ASIACRYPT

Multi-Client Oblivious RAM with Poly-Logarithmic Communication
📺
Abstract

Oblivious RAM enables oblivious access to memory in the single-client setting, which may not be the best fit in the network setting. Multi-client oblivious RAM (MCORAM) considers a collaborative but untrusted environment, where a database owner selectively grants read access and write access to different entries of a confidential database to multiple clients. Their access pattern must remain oblivious not only to the server but also to fellow clients. This upgrade rules out many techniques for constructing ORAM, forcing us to pursue new techniques.
MCORAM not only provides an alternative solution to private anonymous data access (Eurocrypt 2019) but also serves as a promising building block for equipping oblivious file systems with access control and extending other advanced cryptosystems to the multi-client setting.
Despite being a powerful object, the current state-of-the-art is unsatisfactory: The only existing scheme requires $O(\sqrt n)$ communication and client computation for a database of size $n$. Whether it is possible to reduce these complexities to $\mathsf{polylog}(n)$, thereby matching the upper bounds for ORAM, is an open problem, i.e., can we enjoy access control and client-obliviousness under the same bounds?
Our first result answers the above question affirmatively by giving a construction from fully homomorphic encryption (FHE). Our main technical innovation is a new technique for cross-key trial evaluation of ciphertexts.
We also consider the same question in the setting with $N$ non-colluding servers, out of which at most $t$ of them can be corrupt. We build multi-server MCORAM from distributed point functions (DPF), and propose new constructions of DPF via a virtualization technique with bootstrapping, assuming the existence of homomorphic secret sharing and pseudorandom generators in NC0, which are not known to imply FHE.

2019

PKC

Efficient Invisible and Unlinkable Sanitizable Signatures
Abstract

Sanitizable signatures allow designated parties (the sanitizers) to apply arbitrary modifications to some restricted parts of signed messages. A secure scheme should not only be unforgeable, but also protect privacy and hold both the signer and the sanitizer accountable. Two important security properties that are seemingly difficult to achieve simultaneously and efficiently are invisibility and unlinkability. While invisibility ensures that the admissible modifications are hidden from external parties, unlinkability says that sanitized signatures cannot be linked to their sources. Achieving both properties simultaneously is crucial for applications where sensitive personal data is signed with respect to data-dependent admissible modifications. The existence of an efficient construction achieving both properties was recently posed as an open question by Camenisch et al. (PKC’17). In this work, we propose a solution to this problem with a two-step construction. First, we construct (non-accountable) invisible and unlinkable sanitizable signatures from signatures on equivalence classes and other basic primitives. Second, we put forth a generic transformation using verifiable ring signatures to turn any non-accountable sanitizable signature into an accountable one while preserving all other properties. When instantiating in the generic group and random oracle model, the efficiency of our construction is comparable to that of prior constructions, while providing stronger security guarantees.

2019

EUROCRYPT

Incremental Proofs of Sequential Work
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Abstract

A proof of sequential work allows a prover to convince a verifier that a certain amount of sequential steps have been computed. In this work we introduce the notion of incremental proofs of sequential work where a prover can carry on the computation done by the previous prover incrementally, without affecting the resources of the individual provers or the size of the proofs.To date, the most efficient instance of proofs of sequential work [Cohen and Pietrzak, Eurocrypt 2018] for N steps require the prover to have $$\sqrt{N}$$N memory and to run for $$N + \sqrt{N}$$N+N steps. Using incremental proofs of sequential work we can bring down the prover’s storage complexity to $$\log N$$logN and its running time to N.We propose two different constructions of incremental proofs of sequential work: Our first scheme requires a single processor and introduces a poly-logarithmic factor in the proof size when compared with the proposals of Cohen and Pietrzak. Our second scheme assumes $$\log N$$logN parallel processors but brings down the overhead of the proof size to a factor of 9. Both schemes are simple to implement and only rely on hash functions (modelled as random oracles).

2019

CRYPTO

Subvector Commitments with Application to Succinct Arguments
📺
Abstract

We put forward the notion of subvector commitments (SVC): An SVC allows one to open a committed vector at a set of positions, where the opening size is independent of length of the committed vector and the number of positions to be opened. We propose two constructions under variants of the root assumption and the CDH assumption, respectively. We further generalize SVC to a notion called linear map commitments (LMC), which allows one to open a committed vector to its images under linear maps with a single short message, and propose a construction over pairing groups.Equipped with these newly developed tools, we revisit the “CS proofs” paradigm [Micali, FOCS 1994] which turns any arguments with public-coin verifiers into non-interactive arguments using the Fiat-Shamir transform in the random oracle model. We propose a compiler that turns any (linear, resp.) PCP into a non-interactive argument, using exclusively SVCs (LMCs, resp.). For an approximate 80 bits of soundness, we highlight the following new implications:1.There exists a succinct non-interactive argument of knowledge (SNARK) with public-coin setup with proofs of size 5360 bits, under the adaptive root assumption over class groups of imaginary quadratic orders against adversaries with runtime $$2^{128}$$. At the time of writing, this is the shortest SNARK with public-coin setup.2.There exists a non-interactive argument with private-coin setup, where proofs consist of 2 group elements and 3 field elements, in the generic bilinear group model.

2019

CRYPTO

Homomorphic Time-Lock Puzzles and Applications
📺
Abstract

Time-lock puzzles allow one to encrypt messages for the future, by efficiently generating a puzzle with a solution s that remains hidden until time $$\mathcal {T}$$ has elapsed. The solution is required to be concealed from the eyes of any algorithm running in (parallel) time less than $$\mathcal {T}$$. We put forth the concept of homomorphic time-lock puzzles, where one can evaluate functions over puzzles without solving them, i.e., one can manipulate a set of puzzles with solutions $$(s_1, \dots , s_n)$$ to obtain a puzzle that solves to $$f(s_1, \ldots , s_n)$$, for any function f. We propose candidate constructions under concrete cryptographic assumptions for different classes of functions. Then we show how homomorphic time-lock puzzles overcome the limitations of classical time-lock puzzles by proposing new protocols for applications of interest, such as e-voting, multi-party coin flipping, and fair contract signing.

2019

CRYPTO

Trapdoor Hash Functions and Their Applications
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Abstract

We introduce a new primitive, called trapdoor hash functions (TDH), which are hash functions $$\mathsf {H}: \{0,1\}^n \rightarrow \{0,1\}^\lambda $$ with additional trapdoor function-like properties. Specifically, given an index $$i\in [n]$$, TDHs allow for sampling an encoding key $$\mathsf {ek}$$ (that hides i) along with a corresponding trapdoor. Furthermore, given $$\mathsf {H}(x)$$, a hint value $$\mathsf {E}(\mathsf {ek},x)$$, and the trapdoor corresponding to $$\mathsf {ek}$$, the $$i^{th}$$ bit of x can be efficiently recovered. In this setting, one of our main questions is: How small can the hint value $$\mathsf {E}(\mathsf {ek},x)$$ be? We obtain constructions where the hint is only one bit long based on DDH, QR, DCR, or LWE.This primitive opens a floodgate of applications for low-communication secure computation. We mainly focus on two-message protocols between a receiver and a sender, with private inputs x and y, resp., where the receiver should learn f(x, y). We wish to optimize the (download) rate of such protocols, namely the asymptotic ratio between the size of the output and the sender’s message. Using TDHs, we obtain:1.The first protocols for (two-message) rate-1 string OT based on DDH, QR, or LWE. This has several useful consequences, such as:(a)The first constructions of PIR with communication cost poly-logarithmic in the database size based on DDH or QR. These protocols are in fact rate-1 when considering block PIR.(b)The first constructions of a semi-compact homomorphic encryption scheme for branching programs, where the encrypted output grows only with the program length, based on DDH or QR.(c)The first constructions of lossy trapdoor functions with input to output ratio approaching 1 based on DDH, QR or LWE.(d)The first constant-rate LWE-based construction of a 2-message “statistically sender-private” OT protocol in the plain model.2.The first rate-1 protocols (under any assumption) for n parallel OTs and matrix-vector products from DDH, QR or LWE.
We further consider the setting where f evaluates a RAM program y with running time $$T\ll |x|$$ on x. We obtain the first protocols with communication sublinear in the size of x, namely $$T\cdot \sqrt{|x|}$$ or $$T\cdot \root 3 \of {|x|}$$, based on DDH or, resp., pairings (and correlated-input secure hash functions).

2019

TCC

Leveraging Linear Decryption: Rate-1 Fully-Homomorphic Encryption and Time-Lock Puzzles
Abstract

We show how to combine a fully-homomorphic encryption scheme with linear decryption and a linearly-homomorphic encryption schemes to obtain constructions with new properties. Specifically, we present the following new results.
(1)Rate-1 Fully-Homomorphic Encryption: We construct the first scheme with message-to-ciphertext length ratio (i.e., rate) $$1-\sigma $$ for $$\sigma = o(1)$$. Our scheme is based on the hardness of the Learning with Errors (LWE) problem and $$\sigma $$ is proportional to the noise-to-modulus ratio of the assumption. Our building block is a construction of a new high-rate linearly-homomorphic encryption.One application of this result is the first general-purpose secure function evaluation protocol in the preprocessing model where the communication complexity is within additive factor of the optimal insecure protocol.(2)Fully-Homomorphic Time-Lock Puzzles: We construct the first time-lock puzzle where one can evaluate any function over a set of puzzles without solving them, from standard assumptions. Prior work required the existence of sub-exponentially hard indistinguishability obfuscation.

2019

ASIACRYPT

Rate-1 Trapdoor Functions from the Diffie-Hellman Problem
Abstract

Trapdoor functions (TDFs) are one of the fundamental building blocks in cryptography. Studying the underlying assumptions and the efficiency of the resulting instantiations is therefore of both theoretical and practical interest. In this work we improve the input-to-image rate of TDFs based on the Diffie-Hellman problem. Specifically, we present: (a)A rate-1 TDF from the computational Diffie-Hellman (CDH) assumption, improving the result of Garg, Gay, and Hajiabadi [EUROCRYPT 2019], which achieved linear-size outputs but with large constants. Our techniques combine non-binary alphabets and high-rate error-correcting codes over large fields.(b)A rate-1 deterministic public-key encryption satisfying block-source security from the decisional Diffie-Hellman (DDH) assumption. While this question was recently settled by Döttling et al. [CRYPTO 2019], our scheme is conceptually simpler and concretely more efficient. We demonstrate this fact by implementing our construction.

2018

ASIACRYPT

Homomorphic Secret Sharing for Low Degree Polynomials
Abstract

Homomorphic secret sharing (HSS) allows n clients to secret-share data to m servers, who can then homomorphically evaluate public functions over the shares. A natural application is outsourced computation over private data. In this work, we present the first plain-model homomorphic secret sharing scheme that supports the evaluation of polynomials with degree higher than 2. Our construction relies on any degree-k (multi-key) homomorphic encryption scheme and can evaluate degree-$$\left( (k+1)m -1 \right) $$ polynomials, for any polynomial number of inputs n and any sub-logarithmic (in the security parameter) number of servers m. At the heart of our work is a series of combinatorial arguments on how a polynomial can be split into several low-degree polynomials over the shares of the inputs, which we believe is of independent interest.

#### Program Committees

- TCC 2023
- Eurocrypt 2022
- PKC 2022
- Asiacrypt 2022
- PKC 2021

#### Coauthors

- Behzad Abdolmaleki (2)
- Amit Agarwal (2)
- Shweta Agrawal (1)
- Navid Alamati (1)
- Martin R. Albrecht (1)
- Prabhanjan Ananth (2)
- Saikrishna Badrinarayanan (1)
- Khashayar Barooti (1)
- James Bartusek (5)
- Zvika Brakerski (4)
- Pedro Branco (2)
- Xavier Bultel (1)
- Orestis Chardouvelis (2)
- Rahul Chatterjee (2)
- Sherman S. M. Chow (1)
- Kai-Min Chung (1)
- Valerio Cini (3)
- Nico Döttling (11)
- Jesko Dujmović (1)
- Katharina Fech (1)
- Rex Fernando (1)
- Nils Fleischhacker (2)
- Danilo Francati (3)
- Daniele Friolo (3)
- Phillip Gajland (1)
- Rachit Garg (1)
- Sanjam Garg (8)
- Vipul Goyal (7)
- Alex B. Grilo (1)
- Mohammad Hajiabadi (3)
- Loïs Huguenin-Dumittan (1)
- Yuval Ishai (2)
- Abhishek Jain (4)
- Zhengzhong Jin (3)
- Yael Tauman Kalai (1)
- Dakshita Khurana (6)
- Dimitris Kolonelos (2)
- Johannes Krupp (1)
- Pascal Lafourcade (1)
- Russell W. F. Lai (12)
- Xiaohui Liang (3)
- Chuanwei Lin (1)
- Kevin Liu (1)
- Alex Lombardi (1)
- Julian Loss (1)
- Fermi Ma (1)
- Bernardo Magri (1)
- Monosij Maitra (2)
- Giulio Malavolta (54)
- Tamer Mour (1)
- Kartik Nayak (1)
- Ngoc Khanh Nguyen (1)
- Rafail Ostrovsky (2)
- Omkant Pandey (1)
- Charalampos Papamanthou (1)
- Alexander Poremba (1)
- Sihang Pu (1)
- Ahmadreza Rahimi (5)
- Justin Raizes (2)
- Bhaskar Roberts (1)
- Amit Sahai (1)
- Or Sattath (1)
- Jonas Schneider (1)
- Dominique Schröder (5)
- Sina Shiehian (1)
- Mark Simkin (1)
- Nicholas Spooner (1)
- Shravan Srinivasan (1)
- Sri AravindaKrishnan Thyagarajan (1)
- Sri Aravinda Krishnan Thyagarajan (3)
- Kabir Tomer (1)
- Dominique Unruh (1)
- Vinod Vaikuntanathan (1)
- Daniele Venturi (3)
- Thomas Vidick (1)
- Quoc-Huy Vu (1)
- Michael Walter (3)
- Hoeteck Wee (2)
- Ivy K. Y. Woo (1)
- Lizhen Yang (1)
- Tianwei Zhang (1)