## CryptoDB

### Ron Steinfeld

#### Publications

Year
Venue
Title
2022
PKC
We introduce verifiable partially-decryptable commitments (VPDC), as a building block for constructing efficient privacy-preserving protocols supporting auditability by a trusted party. A VPDC is an extension of a commitment along with an accompanying proof, convincing a verifier that (i) the given commitment is well-formed and (ii) a certain part of the committed message can be decrypted using a (secret) trapdoor known to a trusted party. We first formalize VPDCs and then introduce a general decryption feasibility result that overcomes the challenges in relaxed proofs arising in the lattice setting. Our general result can be applied to a wide class of Fiat-Shamir based protocols and may be of independent interest. Next, we show how to extend the commonly used lattice-based Hashed-Message Commitment' (HMC) scheme into a succinct and efficient VPDC. In particular, we devise a novel gadget'-based Regev-style (partial) decryption method, compatible with efficient relaxed lattice-based zero-knowledge proofs. We prove the soundness of our VPDC in the setting of adversarial proofs, where a prover tries to create a valid VPDC output that fails in decryption. To demonstrate the effectiveness of our results, we extend a private blockchain payment protocol, MatRiCT, by Esgin et al. (ACM CCS '19) into a formally auditable construction, which we call MatRiCT-Au, with very low communication and computation overheads over MatRiCT.
2021
PKC
Several recent proposals of efficient public-key encryption are based on variants of the polynomial learning with errors problem (\textsf{PLWE}$^f$) in which the underlying \emph{polynomial} ring $\mZ_q[x]/f$ is replaced with the (related) modular \emph{integer} ring $\mZ_{f(q)}$; the corresponding problem is known as \emph{Integer Polynomial Learning with Errors} (\textsf{I-PLWE}$^f$). Cryptosystems based on \textsf{I-PLWE}$^f$ and its variants can exploit optimised big-integer arithmetic to achieve good practical performance, as exhibited by the \textsf{ThreeBears} cryptosystem. Unfortunately, the average-case hardness of \textsf{I-PLWE}$^f$ and its relation to more established lattice problems have to date remained unclear. We describe the first polynomial-time average-case reductions for the search variant of \textsf{I-PLWE}$^f$, proving its computational equivalence with the search variant of its counterpart problem \textsf{PLWE}$^f$. Our reductions apply to a large class of defining polynomials~$f$. To obtain our results, we employ a careful adaptation of R\'{e}nyi divergence analysis techniques to bound the impact of the integer ring arithmetic carries on the error distributions. As an application, we present a deterministic public-key cryptosystem over integer rings. Our cryptosystem, which resembles \textsf{ThreeBears}, enjoys one-way (OW-CPA) security provably based on the search variant of~\textsf{I-PLWE}$^f$.
2020
EUROCRYPT
We introduce a new technique called Measure-Rewind-Measure' (MRM) to achieve tighter security proofs in the quantum random oracle model (QROM). We first apply our MRM technique to derive a new security proof for a variant of the double-sided' quantum One-Way to Hiding Lemma (O2H) of Bindel et al. [TCC 2019] which, for the first time, avoids the square-root advantage loss in the security proof. In particular, it bypasses a previous `impossibility result' of Jiang, Zhang and Ma [IACR eprint 2019]. We then apply our new O2H Lemma to give a new tighter security proof for the Fujisaki-Okamoto transform for constructing a strong (INDCCA) Key Encapsulation Mechanism (KEM) from a weak (INDCPA) public-key encryption scheme satisfying a mild injectivity assumption.
2020
PKC
We revisit the method of designing public-key puncturable encryption schemes and present a generic conversion by leveraging the techniques of distributed key-distribution and revocable encryption. In particular, we first introduce a refined version of identity-based revocable encryption, named key-homomorphic identity-based revocable key encapsulation mechanism with extended correctness . Then, we propose a generic construction of puncturable key encapsulation mechanism from the former by merging the idea of distributed key-distribution. Compared to the state-of-the-art, our generic construction supports unbounded number of punctures and multiple tags per message, thus achieving more fine-grained revocation of decryption capability. Further, it does not rely on random oracles , not suffer from non-negligible correctness error, and results in a variety of efficient schemes with distinct features. More precisely, we obtain the first scheme with very compact ciphertexts in the standard model, and the first scheme with support for both unbounded size of tags per ciphertext and unbounded punctures as well as constant-time puncture operation. Moreover, we get a comparable scheme proven secure under the standard DBDH assumption, which enjoys both faster encryption and decryption than previous works based on the same assumption, especially when the number of tags associated with the ciphertext is large.
2020
PKC
We describe a digital signature scheme $mathsf {MPSign}$ , whose security relies on the conjectured hardness of the Polynomial Learning With Errors problem ( $mathsf {PLWE}$ ) for at least one defining polynomial within an exponential-size family (as a function of the security parameter). The proposed signature scheme follows the Fiat-Shamir framework and can be viewed as the Learning With Errors counterpart of the signature scheme described by Lyubashevsky at Asiacrypt 2016, whose security relies on the conjectured hardness of the Polynomial Short Integer Solution ( $mathsf {PSIS}$ ) problem for at least one defining polynomial within an exponential-size family. As opposed to the latter, $mathsf {MPSign}$ enjoys a security proof from $mathsf {PLWE}$ that is tight in the quantum-access random oracle model. The main ingredient is a reduction from $mathsf {PLWE}$ for an arbitrary defining polynomial among exponentially many, to a variant of the Middle-Product Learning with Errors problem ( $mathsf {MPLWE}$ ) that allows for secrets that are small compared to the working modulus. We present concrete parameters for $mathsf {MPSign}$ using such small secrets, and show that they lead to significant savings in signature length over Lyubashevsky’s Asiacrypt 2016 scheme (which uses larger secrets) at typical security levels. As an additional small contribution, and in contrast to $mathsf {MPSign}$ (or $mathsf {MPLWE}$ ), we present an efficient key-recovery attack against Lyubashevsky’s scheme (or the inhomogeneous $mathsf {PSIS}$ problem), when it is used with sufficiently small secrets, showing the necessity of a lower bound on secret size for the security of that scheme.
2019
CRYPTO
We devise new techniques for design and analysis of efficient lattice-based zero-knowledge proofs (ZKP). First, we introduce one-shot proof techniques for non-linear polynomial relations of degree $k\ge 2$, where the protocol achieves a negligible soundness error in a single execution, and thus performs significantly better in both computation and communication compared to prior protocols requiring multiple repetitions. Such proofs with degree $k\ge 2$ have been crucial ingredients for important privacy-preserving protocols in the discrete logarithm setting, such as Bulletproofs (IEEE S&P ’18) and arithmetic circuit arguments (EUROCRYPT ’16). In contrast, one-shot proofs in lattice-based cryptography have previously only been shown for the linear case ($k=1$) and a very specific quadratic case ($k=2$), which are obtained as a special case of our technique.Moreover, we introduce two speedup techniques for lattice-based ZKPs: a CRT-packing technique supporting “inter-slot” operations, and “NTT-friendly” tools that permit the use of fully-splitting rings. The former technique comes at almost no cost to the proof length, and the latter one barely increases it, which can be compensated for by tweaking the rejection sampling parameters while still having faster computation overall.To illustrate the utility of our techniques, we show how to use them to build efficient relaxed proofs for important relations, namely proof of commitment to bits, one-out-of-many proof, range proof and set membership proof. Despite their relaxed nature, we further show how our proof systems can be used as building blocks for advanced cryptographic tools such as ring signatures.Our ring signature achieves a dramatic improvement in length over all the existing proposals from lattices at the same security level. The computational evaluation also shows that our construction is highly likely to outperform all the relevant works in running times. Being efficient in both aspects, our ring signature is particularly suitable for both small-scale and large-scale applications such as cryptocurrencies and e-voting systems. No trusted setup is required for any of our proposals.
2018
JOFC
2017
CRYPTO
2017
CRYPTO
2015
FSE
2015
ASIACRYPT
2014
CRYPTO
2014
EUROCRYPT
2012
PKC
2012
JOFC
We study the natural problem of secure n-party computation (in the computationally unbounded attack model) of circuits over an arbitrary finite non-Abelian group (G,⋅), which we call G-circuits. Besides its intrinsic interest, this problem is also motivating by a completeness result of Barrington, stating that such protocols can be applied for general secure computation of arbitrary functions. For flexibility, we are interested in protocols which only require black-box access to the group G (i.e. the only computations performed by players in the protocol are a group operation, a group inverse, or sampling a uniformly random group element). Our investigations focus on the passive adversarial model, where up to t of the n participating parties are corrupted.Our results are as follows. We initiate a novel approach for the construction of black-box protocols for G-circuits based on k-of-k threshold secret-sharing schemes, which are efficiently implementable over any black-box (non-Abelian) group G. We reduce the problem of constructing such protocols to a combinatorial coloring problem in planar graphs. We then give three constructions for such colorings. Our first approach leads to a protocol with optimal resilience t<n/2, but it requires exponential communication complexity $O({\binom{2 t+1}{t}}^{2} \cdot N_{g})$ group elements and round complexity $O(\binom{2 t + 1}{t} \cdot N_{g})$, for a G-circuit of size Ng. Nonetheless, using this coloring recursively, we obtain another protocol to t-privately compute G-circuits with communication complexity $\mathcal{P}\mathit{oly}(n)\cdot N_{g}$ for any t∈O(n1−ϵ) where ϵ is any positive constant. For our third protocol, there is a probability δ (which can be made arbitrarily small) for the coloring to be flawed in term of security, in contrast to the first two techniques, where the colorings are always secure (we call this protocol probabilistic, and those earlier protocols deterministic). This third protocol achieves optimal resilience t<n/2. It has communication complexity O(n5.056(n+log δ−1)2⋅Ng) and the number of rounds is O(n2.528⋅(n+log δ−1)⋅Ng).
2011
EUROCRYPT
2010
ASIACRYPT
2009
ASIACRYPT
2008
FSE
2007
CRYPTO
2007
JOFC
2006
ASIACRYPT
2006
EUROCRYPT
2006
PKC
2005
PKC
2004
ASIACRYPT
2004
PKC
2003
ASIACRYPT
2002
PKC

Asiacrypt 2021
Crypto 2021
Asiacrypt 2020
Asiacrypt 2019
Asiacrypt 2017
Asiacrypt 2016
Crypto 2016
Eurocrypt 2016
Asiacrypt 2014
Eurocrypt 2014
PKC 2012
Asiacrypt 2012
Eurocrypt 2010
Asiacrypt 2010
Asiacrypt 2008
PKC 2006