## CryptoDB

### Geoffroy Couteau

#### Publications

Year
Venue
Title
2019
PKC
Anonymous credential ($\mathsf {AC}$) schemes are protocols which allow for authentication of authorized users without compromising their privacy. Of particular interest are non-interactive anonymous credential ($\mathsf {NIAC}$) schemes, where the authentication process only requires the user to send a single message that still conceals its identity. Unfortunately, all known $\mathsf {NIAC}$ schemes in the standard model require pairing based cryptography, which limits them to a restricted set of specific assumptions and requires expensive pairing computations. The notion of keyed-verification anonymous credential ($\mathsf {KVAC}$) was introduced in (Chase et al., CCS’14) as an alternative to standard anonymous credential schemes allowing for more efficient instantiations; yet, making existing $\mathsf {KVAC}$ non-interactive either requires pairing-based cryptography, or the Fiat-Shamir heuristic.In this work, we construct the first non-interactive keyed-verification anonymous credential ($\mathsf {NIKVAC}$) system in the standard model, without pairings. Our scheme is efficient, attribute-based, supports multi-show unlinkability, and anonymity revocation. We achieve this by building upon a combination of algebraic $\mathsf {MAC}$ with the recent designated-verifier non-interactive zero-knowledge ($\mathsf {DVNIZK}$) proof of knowledge of (Couteau and Chaidos, Eurocrypt’18). Toward our goal of building $\mathsf {NIKVAC}$, we revisit the security analysis of a $\mathsf {MAC}$ scheme introduced in (Chase et al., CCS’14), strengthening its guarantees, and we introduce the notion of oblivious non-interactive zero-knowledge proof system, where the prover can generate non-interactive proofs for statements that he cannot check by himself, having only a part of the corresponding witness, and where the proof can be checked efficiently given the missing part of the witness. We provide an efficient construction of an oblivious $\mathsf {DVNIZK}$, building upon the specific properties of the $\mathsf {DVNIZK}$ proof system of (Couteau and Chaidos, Eurocrypt’18).
2019
EUROCRYPT
Secure multiparty computation ( $\mathsf {MPC}$ MPC) addresses the challenge of evaluating functions on secret inputs without compromising their privacy. A central question in multiparty computation is to understand the amount of communication needed to securely evaluate a circuit of size s. In this work, we revisit this fundamental question in the setting of information-theoretically secure $\mathsf {MPC}$ MPC in the correlated randomness model, where a trusted dealer distributes correlated random coins, independent of the inputs, to all parties before the start of the protocol. This setting is of strong theoretical interest, and has led to the most practically efficient $\mathsf {MPC}$ MPC protocols known to date.While it is known that protocols with optimal communication (proportional to input plus output size) can be obtained from the LWE assumption, and that protocols with sublinear communication o(s) can be obtained from the DDH assumption, the question of constructing protocols with o(s) communication remains wide open for the important case of information-theoretic $\mathsf {MPC}$ MPC in the correlated randomness model; all known protocols in this model require O(s) communication in the online phase.In this work, we exhibit the first generic multiparty computation protocol in the correlated randomness model with communication sublinear in the circuit size, for a large class of circuits. More precisely, we show the following: any size-slayered circuit (whose nodes can be partitioned into layers so that any edge connects adjacent layers) can be evaluated with $O(s/\log \log s)$ O(s/loglogs) communication. Our results holds for both boolean and arithmetic circuits, in the honest-but-curious setting, and do not assume honest majority. For boolean circuits, we extend our results to handle malicious corruption.
2019
EUROCRYPT
We provide a generic construction of non-interactive zero-knowledge (NIZK) schemes. Our construction is a refinement of Dwork and Naor’s (FOCS 2000) implementation of the hidden bits model using verifiable pseudorandom generators (VPRGs). Our refinement simplifies their construction and relaxes the necessary assumptions considerably.As a result of this conceptual improvement, we obtain interesting new instantiations:A designated-verifier NIZK (with unbounded soundness) based on the computational Diffie-Hellman (CDH) problem. If a pairing is available, this NIZK becomes publicly verifiable. This constitutes the first fully secure CDH-based designated-verifier NIZKs (and more generally, the first fully secure designated-verifier NIZK from a non-generic assumption which does not already imply publicly-verifiable NIZKs), and it answers an open problem recently raised by Kim and Wu (CRYPTO 2018).A NIZK based on the learning with errors (LWE) assumption, and assuming a non-interactive witness-indistinguishable (NIWI) proof system for bounded distance decoding (BDD). This simplifies and improves upon a recent NIZK from LWE that assumes a NIZK for BDD (Rothblum et al., PKC 2019).
2019
CRYPTO
Secure multiparty computation (MPC) often relies on correlated randomness for better efficiency and simplicity. This is particularly useful for MPC with no honest majority, where input-independent correlated randomness enables a lightweight “non-cryptographic” online phase once the inputs are known. However, since the amount of randomness typically scales with the circuit size of the function being computed, securely generating correlated randomness forms an efficiency bottleneck, involving a large amount of communication and storage.A natural tool for addressing the above limitations is a pseudorandom correlation generator (PCG). A PCG allows two or more parties to securely generate long sources of useful correlated randomness via a local expansion of correlated short seeds and no interaction. PCGs enable MPC with silent preprocessing, where a small amount of interaction used for securely sampling the seeds is followed by silent local generation of correlated pseudorandomness.A concretely efficient PCG for Vector-OLE correlations was recently obtained by Boyle et al. (CCS 2018) based on variants of the learning parity with noise (LPN) assumption over large fields. In this work, we initiate a systematic study of PCGs and present concretely efficient constructions for several types of useful MPC correlations. We obtain the following main contributions:PCG foundations. We give a general security definition for PCGs. Our definition suffices for any MPC protocol satisfying a stronger security requirement that is met by existing protocols. We prove that a stronger security requirement is indeed necessary, and justify our PCG definition by ruling out a stronger and more natural definition.Silent OT extension. We present the first concretely efficient PCG for oblivious transfer correlations. Its security is based on a variant of the binary LPN assumption and any correlation-robust hash function. We expect it to provide a faster alternative to the IKNP OT extension protocol (Crypto 2003) when communication is the bottleneck. We present several applications, including protocols for non-interactive zero-knowledge with bounded-reusable preprocessing from binary LPN, and concretely efficient related-key oblivious pseudorandom functions.PCGs for simple 2-party correlations. We obtain PCGs for several other types of useful 2-party correlations, including (authenticated) one-time truth-tables and Beaver triples. While the latter PCGs are slower than our PCG for OT, they are still practically feasible. These PCGs are based on a host of assumptions and techniques, including specialized homomorphic secret sharing schemes and pseudorandom generators tailored to their structure.Multiparty correlations. We obtain PCGs for multiparty correlations that can be used to make the (input-dependent) online communication of MPC protocols scale linearly with the number of parties, instead of quadratically.
2018
EUROCRYPT
2018
ASIACRYPT
Local pseudorandom generators allow to expand a short random string into a long pseudo-random string, such that each output bit depends on a constant number d of input bits. Due to its extreme efficiency features, this intriguing primitive enjoys a wide variety of applications in cryptography and complexity. In the polynomial regime, where the seed is of size n and the output of size $n^{\textsf {s}}$ for $\textsf {s}> 1$ , the only known solution, commonly known as Goldreich’s PRG, proceeds by applying a simple d-ary predicate to public random size-d subsets of the bits of the seed.While the security of Goldreich’s PRG has been thoroughly investigated, with a variety of results deriving provable security guarantees against class of attacks in some parameter regimes and necessary criteria to be satisfied by the underlying predicate, little is known about its concrete security and efficiency. Motivated by its numerous theoretical applications and the hope of getting practical instantiations for some of them, we initiate a study of the concrete security of Goldreich’s PRG, and evaluate its resistance to cryptanalytic attacks. Along the way, we develop a new guess-and-determine-style attack, and identify new criteria which refine existing criteria and capture the security guarantees of candidate local PRGs in a more fine-grained way.
2017
EUROCRYPT
2016
CRYPTO
2015
EPRINT
2015
EPRINT
2015
CRYPTO

Eurocrypt 2020
TCC 2019