International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Pratik Sarkar

Publications

Year
Venue
Title
2021
ASIACRYPT
Reverse Firewalls for Adaptively Secure MPC without Setup
We study Multi-party computation (MPC) in the setting of subversion, where the adversary tampers with the machines of honest parties. Our goal is to construct actively secure MPC protocols where parties are corrupted adaptively by an adversary (as in the standard adaptive security setting), and in addition, honest parties' machines are compromised. The idea of reverse firewalls (RF) was introduced at EUROCRYPT'15 by Mironov and Stephens-Davidowitz as an approach to protecting protocols against corruption of honest parties' devices. Intuitively, an RF for a party $\mathcal{P}$ is an external entity that sits between $\mathcal{P}$ and the outside world and whose scope is to sanitize $\mathcal{P}$’s incoming and outgoing messages in the face of subversion of their computer. Mironov and Stephens-Davidowitz constructed a protocol for passively-secure two-party computation. At CRYPTO'20, Chakraborty, Dziembowski and Nielsen constructed a protocol for secure computation with firewalls that improved on this result, both by extending it to \textit{multi}-party computation protocol, and considering \textit{active} security in the presence of \textit{static} corruptions. In this paper, we initiate the study of RF for MPC in the \textit{adaptive} setting. We put forward a definition for adaptively secure MPC in the reverse firewall setting, explore relationships among the security notions, and then construct reverse firewalls for MPC in this stronger setting of adaptive security. We also resolve the open question of Chakraborty, Dziembowski and Nielsen by removing the need for a trusted setup in constructing RF for MPC. Towards this end, we construct reverse firewalls for adaptively secure augmented coin tossing and adaptively secure zero-knowledge protocols and obtain a constant round adaptively secure MPC protocol in the reverse firewall setting without setup. Along the way, we propose a new multi-party adaptively secure coin tossing protocol in the plain model, that is of independent interest.
2021
ASIACRYPT
Two-Round Adaptively Secure MPC from Isogenies, LPN, or CDH
We present a new framework for building round-optimal (two-round) adaptively secure MPC. We show that a relatively weak notion of OT that we call indistinguishability OT with receiver oblivious sampleability (r-iOT) is enough to build two-round, adaptively secure MPC against malicious adversaries in the CRS model. We then show how to construct r-iOT from CDH, LPN, or isogeny-based assumptions that can be viewed as group actions (such as CSIDH and CSI-FiSh). This yields the first concrete constructions of two-round adaptively secure MPC against malicious adversaries from CDH, LPN, or isogeny-based assumptions. We further extend our non-isogeny results to the plain model, achieving (to the best of our knowledge) the first construction of two-round adaptively secure MPC against semi-honest adversaries in the plain model from LPN. Our results allow us to build two-round adaptively secure MPC against malicious adversaries from essentially all of the well-studied assumptions in cryptography. In addition, our constructions from isogenies or LPN provide the first post-quantum alternatives to LWE-based constructions for round-optimal adaptively secure MPC. Along the way, we show that r-iOT also implies non-committing encryption (NCE), thereby yielding the first constructions of NCE from isogenies or LPN.
2020
PKC
Blazing Fast OT for Three-Round UC OT Extension 📺
Ran Canetti Pratik Sarkar Xiao Wang
Oblivious Transfer (OT) is an important building block for multi-party computation (MPC). Since OT requires expensive public-key operations, efficiency-conscious MPC protocols use an OT extension (OTE) mechanism [Beaver 96, Ishai et al. 03] to provide the functionality of many independent OT instances with the same sender and receiver, using only symmetric-key operations plus few instances of some base OT protocol. Consequently there is significant interest in constructing OTE friendly protocols, namely protocols that, when used as base-OT for OTE, result in extended OT that are both round-efficient and cost-efficient. We present the most efficient OTE-friendly protocol to date. Specifically: Our base protocol incurs only 3 exponentiations per instance. Our base protocol results in a 3 round extended OT protocol. The extended protocol is UC secure in the Observable Random Oracle Model (ROM) under the CDH assumption. For comparison, the state of the art for base OTs that result in 3-round OTE are proven only in the programmable ROM, and require 4 exponentiations under Interactive DDH or 6 exponentiations under DDH [Masney-Rindal 19]. We also implement our protocol and benchmark it against the Simplest OT protocol [Chou and Orlandi, Latincrypt 2015], which is the most efficient and widely used OT protocol but not known to suffice for OTE. The computation cost is roughly the same in both cases. Interestingly, our base OT is also 3 rounds. However, we slightly modify the extension mechanism (which normally adds a round) so as to preserve the number of rounds in our case.
2020
ASIACRYPT
Efficient and Round-Optimal Oblivious Transfer and Commitment with Adaptive Security 📺
Ran Canetti Pratik Sarkar Xiao Wang
We construct the most efficient two-round adaptively secure bit-OT in the Common Random String (CRS) model. The scheme is UC secure under the Decisional Diffie-Hellman (DDH) assumption. It incurs O(1) exponentiations and sends O(1) group elements, whereas the state of the art requires O(k^2) exponentiations and communicates poly(k) bits, where k is the computational security parameter. Along the way, we obtain several other efficient UC-secure OT protocols under DDH : - The most efficient yet two-round adaptive string-OT protocol assuming global programmable random oracle. Furthermore, the protocol can be made non-interactive in the simultaneous message setting, assuming random inputs for the sender. - The fi rst two-round string-OT with amortized constant exponentiations and communication overhead which is secure in the global observable random oracle model. - The first two-round receiver equivocal string-OT in the CRS model that incurs constant computation and communication overhead. We also obtain the first non-interactive adaptive string UC-commitment in the CRS model which incurs a sublinear communication overhead in the security parameter. Speci cally, we commit to polylog(k) bits while communicating O(k) bits. Moreover, it is additively homomorphic. We can also extend our results to the single CRS model where multiple sessions share the same CRS. As a corollary, we obtain a two-round adaptively secure MPC protocol in this model.
2018
PKC
Efficient Adaptively Secure Zero-Knowledge from Garbled Circuits
Zero-knowledge (ZK) protocols are undoubtedly among the central primitives in cryptography, lending their power to numerous applications such as secure computation, voting, auctions, and anonymous credentials to name a few. The study of efficient ZK protocols for non-algebraic statements has seen rapid progress in recent times, relying on secure computation techniques. The primary contribution of this work lies in constructing efficient UC-secure constant round ZK protocols from garbled circuits that are secure against adaptive corruptions, with communication linear in the size of the statement. We begin by showing that the practically efficient ZK protocol of Jawurek et al. (CCS 2013) is adaptively secure when the underlying oblivious transfer (OT) satisfies a mild adaptive security guarantee. We gain adaptive security with little to no overhead over the static case. A conditional verification technique is then used to obtain a three-round adaptively secure zero-knowledge argument in the non-programmable random oracle model (NPROM). Our three-round protocol yields a proof size that is shorter than the known UC-secure practically-efficient schemes in the short-CRS model with the right choice of security parameters.We draw motivation from state-of-the-art non-interactive secure computation protocols and leveraging specifics of ZK functionality show a two-round protocol that achieves static security. It is a proof, while most known efficient ZK protocols and our three round protocol are only arguments.