## CryptoDB

### Benoît Libert

#### Publications

Year
Venue
Title
2022
PKC
Range proofs allow a sender to convince a verifier that committed integers belong to an interval without revealing anything else. So far, all known non-interactive range proofs in the standard model rely on groups endowed with a bilinear map. Moreover, they either require the group order to be larger than the range of any proven statement or they suffer from a wasteful rate. Recently (Eurocrypt'21), Couteau et al. introduced a new approach to efficiently prove range membership by encoding integers as a modular ratio between small integers. We show that their technique can be transposed in the standard model under the Composite Residuosity (DCR) assumption. Interestingly, with this modification, the size of ranges is not a priori restricted by the common reference string. It also gives a constant ratio between the size of ranges and proofs. Moreover, we show that their technique of encoding messages as bounded rationals provides a secure standard model instantiation of the Naor-Yung CCA2 encryption paradigm under the DCR assumption. Keywords: Range proofs, NIZK, standard model, Naor-Yung.
2022
EUROCRYPT
The standard model security of the Fiat-Shamir transform has been an active research area for many years. In breakthrough results, Canetti {\it et al.} (STOC'19) and Peikert-Shiehian (Crypto'19) showed that, under the Learning-With-Errors (LWE) assumption, it provides soundness by applying correlation-intractable (CI) hash functions to so-called {\it trapdoor} $\Sigma$-protocols. In order to be compatible with CI hash functions based on standard LWE assumptions with polynomial approximation factors, all known such protocols have been obtained via parallel repetitions of a basic protocol with binary challenges. In this paper, we consider languages related to Paillier's composite residuosity assumption (DCR) for which we give the first trapdoor $\Sigma$-protocols providing soundness in one shot, via exponentially large challenge spaces. This improvement is analogous to the one enabled by Schnorr over the original Fiat-Shamir protocol in the random oracle model. Using the correlation-intractable hash function paradigm, we then obtain simulation-sound NIZK arguments showing that an element of $\mathbb{Z}_{N^2}^\ast$ is a composite residue, which opens the door to space-efficient applications in the standard model. As a concrete example, we build logarithmic-size ring signatures (assuming a common reference string) with the shortest signature length among schemes based on standard assumptions in the standard model. We prove security under the DCR and LWE assumptions, while keeping the signature size comparable with that of random-oracle-based schemes.
2021
EUROCRYPT
Over the development of modern cryptography, often, alternative cryptographic schemes are developed to achieve goals that in some important respect are orthogonal. Thus, we have to choose either a scheme which achieves the first goal and not the second, or vice versa. This results in two types of schemes that compete with each other. In the basic area of user privacy, specifically in anonymous (multi-use credentials) signing, such an orthogonality exists between anonymity and accountability. The conceptual contribution of this work is to reverse the above orthogonality by design, which essentially typifies the last 25 years or so, and to suggest an alternative methodology where the opposed properties are carefully folded into a single scheme. The schemes will support both opposing properties simultaneously in a bifurcated fashion, where: - First, based on rich semantics expressed over the message's context and content, the user, etc., the relevant property is applied point-wise per message operation depending on a predicate; and - Secondly, at the same time, the schemes provide what we call branch-hiding;'' namely, the resulting calculated value hides from outsiders which property has actually been locally applied. Specifically, we precisely define and give the first construction and security proof of a Bifurcated Anonymous Signature'' (BiAS): A scheme which supports either absolute anonymity or anonymity with accountability, based on a specific contextual predicate, while being branch-hiding. This novel signing scheme has numerous applications not easily implementable or not considered before, especially because: (i) the conditional traceability does 'not' rely on a trusted authority as it is (non-interactively) encapsulated into signatures; and (ii) signers 'know' the predicate value and can make a conscious choice at each signing time. Technically, we realize BiAS from homomorphic commitments for a general family of predicates that can be represented by bounded-depth circuits. Our construction is generic and can be instantiated in the standard model from lattices and, more efficiently, from bilinear maps. In particular, the signature length is independent of the circuit size when we use commitments with suitable efficiency properties.
2021
PKC
We consider threshold public-key encryption, where the decryption servers distributively hold the private key shares, and we need a threshold of these servers to decrypt the message (while the system remains secure when less than the threshold is corrupt). We investigate the notion of chosen-ciphertext secure threshold systems which has been historically hard to achieve. We further require the systems to be, both, adaptively secure (i.e., secure against a strong adversary making corruption decisions dynamically during the protocol), and non-interactive (i.e., where decryption servers do not interact amongst themselves but rather efficiently contribute, each, a single message). To date, only pairing-based implementations were known to achieve security in the standard security model without relaxation (i.e., without assuming the random oracle idealization) under the above stringent requirements. Here, we investigate how to achieve the above using other assumptions (in order to understand what other algebraic building blocks and mathematical assumptions are needed to extend the domain of encryption methods achieving the above). Specifically, we show realizations under the Decision Composite Residuosity (DCR) and Learning-With-Errors (LWE) assumptions.
2021
JOFC
In distributed pseudorandom functions (DPRFs), a PRF secret key SK is secret shared among N servers so that each server can locally compute a partial evaluation of the PRF on some input X . A combiner that collects t partial evaluations can then reconstruct the evaluation F ( SK ,  X ) of the PRF under the initial secret key. So far, all non-interactive constructions in the standard model are based on lattice assumptions. One caveat is that they are only known to be secure in the static corruption setting, where the adversary chooses the servers to corrupt at the very beginning of the game, before any evaluation query. In this work, we construct the first fully non-interactive adaptively secure DPRF in the standard model. Our construction is proved secure under the $\textsf {LWE}$ LWE assumption against adversaries that may adaptively decide which servers they want to corrupt. We also extend our construction in order to achieve robustness against malicious adversaries.
2020
EUROCRYPT
Non-interactive zero-knowledge proofs (NIZKs) are important primitives in cryptography. A major challenge since the early works on NIZKs has been to construct NIZKs with a statistical zero-knowledge guarantee against unbounded verifiers. In the common reference string (CRS) model, such "statistical NIZK arguments" are currently known from k-Lin in a pairing-group and from LWE. In the (reusable) designated-verifier model (DV-NIZK), where a trusted setup algorithm generates a reusable verification key for checking proofs, we also have a construction from DCR. If we relax our requirements to computational zero-knowledge, we additionally have NIZKs from factoring and CDH in a pairing group in the CRS model, and from nearly all assumptions that imply public-key encryption (e.g., CDH, LPN, LWE) in the designated-verifier model. Thus, there still remains a gap in our understanding of statistical NIZKs in both the CRS and the designated-verifier models. In this work, we develop new techniques for constructing statistical NIZK arguments. First, we construct statistical DV-NIZK arguments from the k-Lin assumption in pairing-free groups, the QR assumption, and the DCR assumption. These are the first constructions in pairing-free groups and from QR that satisfy statistical zero-knowledge. All of our constructions are secure even if the verification key is chosen maliciously (i.e., they are "malicious-designated-verifier" NIZKs), and moreover, they satisfy a "dual-mode" property where the CRS can be sampled from two computationally indistinguishable distributions: one distribution yields statistical DV-NIZK arguments while the other yields computational DV-NIZK proofs. We then show how to adapt our k-Lin construction in a pairing group to obtain new publicly-verifiable statistical NIZK arguments from pairings with a qualitatively weaker assumption than existing constructions of pairing-based statistical NIZKs. Our constructions follow the classic paradigm of Feige, Lapidot, and Shamir (FLS). While the FLS framework has traditionally been used to construct computational (DV)-NIZK proofs, we newly show that the same framework can be leveraged to construct dual-mode (DV)-NIZKs.
2020
PKC
Inner product functional encryption ( ${mathsf {IPFE}}$ ) [ 1 ] is a popular primitive which enables inner product computations on encrypted data. In ${mathsf {IPFE}}$ , the ciphertext is associated with a vector $varvec{x}$ , the secret key is associated with a vector $varvec{y}$ and decryption reveals the inner product $langle varvec{x},varvec{y} angle$ . Previously, it was known how to achieve adaptive indistinguishability ( $mathsf {IND}$ ) based security for ${mathsf {IPFE}}$ from the $mathsf {DDH}$ , $mathsf {DCR}$ and $mathsf {LWE}$ assumptions [ 8 ]. However, in the stronger simulation ( $mathsf {SIM}$ ) based security game, it was only known how to support a restricted adversary that makes all its key requests either before or after seeing the challenge ciphertext, but not both. In more detail, Wee [ 46 ] showed that the $mathsf {DDH}$ -based scheme of Agrawal et al. (Crypto 2016) achieves semi-adaptive simulation-based security, where the adversary must make all its key requests after seeing the challenge ciphertext. On the other hand, O’Neill showed that all $mathsf {IND}$ -secure ${mathsf {IPFE}}$ schemes (which may be based on $mathsf {DDH}$ , $mathsf {DCR}$ and $mathsf {LWE}$ ) satisfy $mathsf {SIM}$ based security in the restricted model where the adversary makes all its key requests before seeing the challenge ciphertext. In this work, we resolve the question of $mathsf {SIM}$ -based security for ${mathsf {IPFE}}$ by showing that variants of the ${mathsf {IPFE}}$ constructions by Agrawal et al. , based on $mathsf {DDH}$ , Paillier and $mathsf {LWE}$ , satisfy the strongest possible adaptive $mathsf {SIM}$ -based security where the adversary can make an unbounded number of key requests both before and after seeing the (single) challenge ciphertext. This establishes optimal security of the ${mathsf {IPFE}}$ schemes, under all hardness assumptions on which it can (presently) be based.
2020
ASIACRYPT
Electronic cash (e-cash) was introduced 40 years ago as the digital analogue of traditional cash. It allows users to withdraw electronic coins that can be spent anonymously with merchants. As advocated by Camenisch et al. (Eurocrypt 2005), it should be possible to store the withdrawn coins compactly (i.e., with logarithmic cost in the total number of coins), which has led to the notion of compact e-cash. Many solutions were proposed for this problem but the security proofs of most of them were invalidated by a very recent paper by Bourse et al. (Asiacrypt 2019). The same paper describes a generic way of fixing existing constructions/proofs but concrete instantiations of this patch are currently unknown in some settings. In particular, compact e-cash is no longer known to exist under quantum-safe assumptions. In this work, we resolve this problem by proposing the first secure compact e-cash system based on lattices following the result from Bourse et al. Contrarily to the latter work, our construction is not only generic, but we describe two concrete instantiations. We depart from previous frameworks of e-cash systems by leveraging lossy trapdoor functions to construct our coins. The indistinguishability of lossy and injective keys allows us to avoid the very strong requirements on the involved pseudo-random functions that were necessary to instantiate the generic patch proposed by Bourse et al.
2020
ASIACRYPT
The Naor-Yung paradigm is a well-known technique that constructs IND-CCA2-secure encryption schemes by means of non-interactive zero-knowledge proofs satisfying a notion of simulation-soundness. Until recently, it was an open problem to instantiate it under the sole Learning-With-Errors (LWE) assumption without relying on random oracles. While the recent results of Canetti et al. (STOC'19) and Peikert-Shiehian (Crypto'19) provide a solution to this problem by applying the Fiat-Shamir transform in the standard model, the resulting constructions are extremely inefficient as they proceed via a reduction to an NP-complete problem. In this paper, we give a direct, non-generic method for instantiating Naor-Yung under the LWE assumption outside the random oracle model. Specifically, we give a direct construction of an unbounded simulation-sound NIZK argument system which, for carefully chosen parameters, makes it possible to express the equality of plaintexts encrypted under different keys in Regev's cryptosystem. We also give a variant of our argument that provides tight security. As an application, we obtain an LWE-based public-key encryption scheme for which we can prove (tight) key-dependent message security under chosen-ciphertext attacks in the standard model.
2020
JOFC
In threshold cryptography, private keys are divided into n shares, each one of which is given to a different server in order to avoid single points of failure. In the case of threshold public-key encryption, at least $t \le n$ t ≤ n servers need to contribute to the decryption process. A threshold primitive is said robust if no coalition of t malicious servers can prevent remaining honest servers from successfully completing private key operations. Non-interactive schemes, considered the most practical ones, allow servers to contribute to decryption without interactions. So far, most non-interactive threshold cryptosystems were only proved secure against static corruptions. In the adaptive corruption scenario (where the adversary can corrupt servers at any time, based on its complete view), all existing robust threshold encryption schemes that also resist chosen-ciphertext attacks till recently require interaction in the decryption phase. A very specific method (in composite order groups) for getting rid of interaction was recently suggested, leaving the question of more generic frameworks and constructions with better security and, in particular, better flexibility (i.e., compatibility with distributed key generation). This paper advances the state of the art and describes a general construction of adaptively secure robust non-interactive threshold cryptosystems with chosen-ciphertext security. We define the novel notion of all-but-one perfectly sound threshold hash proof systems that can be seen as (threshold) hash proof systems with publicly verifiable and simulation-sound proofs. We show that this notion generically implies threshold cryptosystems combining the aforementioned properties. Then, we provide efficient instantiations under well-studied assumptions in bilinear groups (e.g., in such groups of prime order). These instantiations have a tighter security proof in the single-challenge setting and are indeed compatible with distributed key generation protocols.
2019
PKC
Lossy algebraic filters (LAFs) are function families where each function is parametrized by a tag, which determines if the function is injective or lossy. While initially introduced by Hofheinz (Eurocrypt 2013) as a technical tool to build encryption schemes with key-dependent message chosen-ciphertext (KDM-CCA) security, they also find applications in the design of robustly reusable fuzzy extractors. So far, the only known LAF family requires tags comprised of $\varTheta (n^2)$ group elements for functions with input space $\mathbb {Z}_p^n$, where p is the group order. In this paper, we describe a new LAF family where the tag size is only linear in n and prove it secure under simple assumptions in asymmetric bilinear groups. Our construction can be used as a drop-in replacement in all applications of the initial LAF system. In particular, it can shorten the ciphertexts of Hofheinz’s KDM-CCA-secure public-key encryption scheme by 19 group elements. It also allows substantial space improvements in a recent fuzzy extractor proposed by Wen and Liu (Asiacrypt 2018). As a second contribution, we show how to modify our scheme so as to prove it (almost) tightly secure, meaning that security reductions are not affected by a concrete security loss proportional to the number of adversarial queries.
2019
PKC
Zero-knowledge elementary databases (ZK-EDBs) are cryptographic schemes that allow a prover to commit to a set $\mathsf {D}$ of key-value pairs so as to be able to prove statements such as “x belongs to the support of $\mathsf {D}$ and $\mathsf {D}(x)=y$” or “x is not in the support of $\mathsf {D}$”. Importantly, proofs should leak no information beyond the proven statement and even the size of $\mathsf {D}$ should remain private. Chase et al. (Eurocrypt’05) showed that ZK-EDBs are implied by a special flavor of non-interactive commitment, called mercurial commitment, which enables efficient instantiations based on standard number theoretic assumptions. On the other hand, the resulting ZK-EDBs are only known to support proofs for simple statements like (non-)membership and value assignments. In this paper, we show that mercurial commitments actually enable significantly richer queries. We show that, modulo an additional security property met by all known efficient constructions, they actually enable range queries over keys and values – even for ranges of super-polynomial size – as well as membership/non-membership queries over the space of values. Beyond that, we exploit the range queries to realize richer queries such as $k$-nearest neighbors and revealing the $k$ smallest or largest records within a given range. In addition, we provide a new realization of trapdoor mercurial commitment from standard lattice assumptions, thus obtaining the most expressive quantum-safe ZK-EDB construction so far.
2019
ASIACRYPT
Multi-client functional encryption (MCFE) allows $\ell$ clients to encrypt ciphertexts $(\mathbf {C}_{t,1},\mathbf {C}_{t,2},\ldots ,\mathbf {C}_{t,\ell })$ under some label. Each client can encrypt his own data $X_i$ for a label t using a private encryption key $\mathsf {ek}_i$ issued by a trusted authority in such a way that, as long as all $\mathbf {C}_{t,i}$ share the same label t, an evaluator endowed with a functional key $\mathsf {dk}_f$ can evaluate $f(X_1,X_2,\ldots ,X_\ell )$ without learning anything else on the underlying plaintexts $X_i$. Functional decryption keys can be derived by the central authority using the master secret key. Under the Decision Diffie-Hellman assumption, Chotard et al. (Asiacrypt 2018) recently described an adaptively secure MCFE scheme for the evaluation of linear functions over the integers. They also gave a decentralized variant (DMCFE) of their scheme which does not rely on a centralized authority, but rather allows encryptors to issue functional secret keys in a distributed manner. While efficient, their constructions both rely on random oracles in their security analysis. In this paper, we build a standard-model MCFE scheme for the same functionality and prove it fully secure under adaptive corruptions. Our proof relies on the Learning-With-Errors ($\mathsf {LWE}$) assumption and does not require the random oracle model. We also provide a decentralized variant of our scheme, which we prove secure in the static corruption setting (but for adaptively chosen messages) under the $\mathsf {LWE}$ assumption.
2018
CRYPTO
We provide lattice-based protocols allowing to prove relations among committed integers. While the most general zero-knowledge proof techniques can handle arithmetic circuits in the lattice setting, adapting them to prove statements over the integers is non-trivial, at least if we want to handle exponentially large integers while working with a polynomial-size modulus q. For a polynomial L, we provide zero-knowledge arguments allowing a prover to convince a verifier that committed L-bit bitstrings x, y and z are the binary representations of integers X, Y and Z satisfying $Z=X+Y$ over $\mathbb {Z}$. The complexity of our arguments is only linear in L. Using them, we construct arguments allowing to prove inequalities $X<Z$ among committed integers, as well as arguments showing that a committed X belongs to a public interval $[\alpha ,\beta ]$, where $\alpha$ and $\beta$ can be arbitrarily large. Our range arguments have logarithmic cost (i.e., linear in L) in the maximal range magnitude. Using these tools, we obtain zero-knowledge arguments showing that a committed element X does not belong to a public set S using $\widetilde{\mathcal {O}}(n \cdot \log |S|)$ bits of communication, where n is the security parameter. We finally give a protocol allowing to argue that committed L-bit integers X, Y and Z satisfy multiplicative relations $Z=XY$ over the integers, with communication cost subquadratic in L. To this end, we use our protocol for integer addition to prove the correct recursive execution of Karatsuba’s multiplication algorithm. The security of our protocols relies on standard lattice assumptions with polynomial modulus and polynomial approximation factor.
2018
TCC
In distributed pseudorandom functions (DPRFs), a PRF secret key SK is secret shared among N servers so that each server can locally compute a partial evaluation of the PRF on some input X. A combiner that collects t partial evaluations can then reconstruct the evaluation F(SK, X) of the PRF under the initial secret key. So far, all non-interactive constructions in the standard model are based on lattice assumptions. One caveat is that they are only known to be secure in the static corruption setting, where the adversary chooses the servers to corrupt at the very beginning of the game, before any evaluation query. In this work, we construct the first fully non-interactive adaptively secure DPRF in the standard model. Our construction is proved secure under the $\mathsf {LWE}$ assumption against adversaries that may adaptively decide which servers they want to corrupt. We also extend our construction in order to achieve robustness against malicious adversaries.
2017
PKC
2017
CRYPTO
2017
ASIACRYPT
2017
ASIACRYPT
2017
JOFC
2016
EUROCRYPT
2016
CRYPTO
2016
ASIACRYPT
2016
ASIACRYPT
2015
PKC
2015
CRYPTO
2015
ASIACRYPT
2014
EUROCRYPT
2014
PKC
2014
PKC
2014
ASIACRYPT
2013
PKC
2013
PKC
2013
CRYPTO
2013
ASIACRYPT
2013
EUROCRYPT
2012
TCC
2012
EUROCRYPT
2012
CRYPTO
2012
PKC
2012
ASIACRYPT
2011
PKC
2011
PKC
2011
ASIACRYPT
2011
ASIACRYPT
2010
TCC
2010
PKC
2009
ASIACRYPT
2009
PKC
2008
PKC
2008
PKC
2007
PKC
2006
PKC
2005
ASIACRYPT
2004
PKC

Crypto 2022
PKC 2022
TCC 2021
Eurocrypt 2021
PKC 2020
Asiacrypt 2020
PKC 2019
Asiacrypt 2018
Eurocrypt 2017
TCC 2017
PKC 2016
PKC 2015
Eurocrypt 2015
PKC 2013
Asiacrypt 2013
Eurocrypt 2012
Eurocrypt 2011
PKC 2010