International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Matthew Green

Publications

Year
Venue
Title
2022
EUROCRYPT
Stacking Sigmas: A Framework to Compose Sigma-Protocols for Disjunctions
Zero-Knowledge (ZK) Proofs for disjunctive statements have been a focus of a long line of research. Classical results such as Cramer {\em et al.} [CRYPTO'94] and Abe {\em et al.} [AC'02] design generic compilers that transform certain classes of ZK proofs into ZK proofs for disjunctive statements. However, communication complexity of the resulting protocols in these results ends up being proportional to the total size of all the proofs in the disjunction. More recently, a series of works (e.g. Heath {\em et al.} [EC'20]) has exploited special properties of garbled circuits to construct efficient ZK proofs for disjunctions, where the proof size is only proportional to the length of the largest clause in the disjunction. However, these techniques do not appear to generalize beyond garbled circuits. In this work, we focus on achieving the best of both worlds. We design a \textit{general framework} that compiles a large class of {unmodified} $\Sigma$-protocols, each for an individual statement, into a new $\Sigma$-protocol that proves a disjunction of these statements. Our framework can be used both when each clause is proved with the same $\Sigma$-protocol and when different $\Sigma$-protocols are used for different clauses. The resulting $\Sigma$-protocol is concretely efficient and has communication complexity proportional to the communication required by the largest clause, with additive terms that are only logarithmic in the number of clauses. We show that our compiler can be applied to many well-known $\Sigma$-protocols, including classical protocols (\emph{e.g.} Schnorr and Guillou-Quisquater) and modern MPC-in-the-head protocols such as the recent work of Katz, Kolesnikov and Wang [CCS'18] and the Ligero protocol of Ames {\em et al.} [CCS'17] Finally, since all of the protocols in our class can be made non-interactive in the random oracle model using the Fiat-Shamir transform, our result yields the first generic non-interactive zero-knowledge protocol for disjunctions where the communication only depends on the size of the largest clause.
2021
EUROCRYPT
Abuse Resistant Law Enforcement Access Systems 📺
The increased deployment of end-to-end encryption has ignited a debate between technology firms and law enforcement agencies over the need for lawful access to encrypted communications. Unfortunately, existing solutions to this problem suffer from serious technical risks, such as the possibility of operator abuse and theft of escrow key material. In this work we investigate the problem of constructing law enforcement access systems that mitigate the possibility of unauthorized surveillance. We first define a set of desirable properties for an abuse-resistant law enforcement access system (ARLEAS), and motivate each of these properties. We then formalize these definitions in the Universal Composability framework, and present two main constructions that realize this definition. The first construction enables {\em prospective} access, allowing surveillance only if encryption occurs after a warrant has been issued and activated. The second, more powerful construction, allows {\em retrospective} access to communications that occurred prior to a warrant's issuance. To illustrate the technical challenge of constructing the latter type of protocol, we conclude by investigating the minimal assumptions required to realize these systems.
2021
CRYPTO
Fluid MPC: Secure Multiparty Computation with Dynamic Participants 📺
Existing approaches to secure multiparty computation (MPC) require all participants to commit to the entire duration of the protocol. As interest in MPC continues to grow, it is inevitable that there will be a desire to use it to evaluate increasingly complex functionalities, resulting in computations spanning several hours or days. Such scenarios call for a *dynamic* participation model for MPC where participants have the flexibility to go offline as needed and (re)join when they have available computational resources. Such a model would also democratize access to privacy-preserving computation by facilitating an ``MPC-as-a-service'' paradigm --- the deployment of MPC in volunteer-operated networks (such as blockchains, where dynamism is inherent) that perform computation on behalf of clients. In this work, we initiate the study of *fluid MPC*, where parties can dynamically join and leave the computation. The minimum commitment required from each participant is referred to as *fluidity*, measured in the number of rounds of communication that it must stay online. Our contributions are threefold: - We provide a formal treatment of fluid MPC, exploring various possible modeling choices. - We construct information-theoretic fluid MPC protocols in the honest-majority setting. Our protocols achieve *maximal fluidity*, meaning that a party can exit the computation after receiving and sending messages in one round. - We implement our protocol and test it in multiple network settings.
2018
EUROCRYPT
2017
EUROCRYPT
2015
CHES
2012
PKC
2011
PKC
Secure Blind Decryption
Matthew Green
2011
TCC
2009
PKC
2008
ASIACRYPT
2007
ASIACRYPT

Program Committees

Crypto 2017
PKC 2015
PKC 2012