Michele Ciampi, Rafail Ostrovsky, Hendrik Waldner, Vassilis Zikas Round-Optimal Byzantine Agreement Abstract Byzantine agreement is a fundamental primitive in cryptography and distributed computing, and minimizing its round complexity is of paramount importance. Whether we can match the lower bound agreement probability remains unknown. In this work, we resolve this long-standing open question.
We present a protocol that matches the lower bound up to constant factors. This is the first protocol that decreases the failure probability overall by a super-constant factor per round. The work by Chung et al. A natural game-theoretic formulation is to require that the honest protocol form a coalition-resistant Nash equilibrium, i.
Chung et al. Unfortunately, Chung et al. In this paper, we show that the impossibility of Chung et al. We give a complete characterization of the regime in which CSP-fair coin toss is possible, by providing a matching upper- and lower-bound.
Our complete characterization theorem also shows that the mathematical structure of game-theoretic fairness is starkly different from the de facto strong fairness notion in the multi-party computation literature. Our verifiability method is lightweight in two ways. Firstly, it is concretely efficient, making use of only symmetric key operations and no public key or MPC techniques are involved. Our performance is comparable with the state-of-the-art non-verifiable DPF constructions, and we outperform all prior DPF verification techniques in both computation and communication complexity, which we demonstrate with an implementation of our scheme.
Secondly, our verification procedure is essentially unconstrained. It will verify that distributed point function DPF shares correspond to some point function irrespective of the output group size, the structure of the DPF output, or the set of points on which the DPF must be evaluated. This is in stark contrast with prior works, which depend on at least one and often all three of these constraints.
In addition, our construction is the first DPF verification protocol that can verify general DPFs while remaining secure even if one server is malicious. Prior work on maliciously secure DPF verification could only verify DPFs where the non-zero output is binary and the output space is a large field. As an additional feature, our verification procedure can be batched so that verifying a polynomial number of DPF shares requires the exact same amount of communication as verifying one pair of DPF shares.
We combine this packed DPF verification with a novel method for packing DPFs into shares of a multi-point function where the evaluation time, verification time, and verification communication are independent of the number of non-zero points in the function. An immediate corollary of our results are two-server protocols for PIR and PSI that remain secure when any one of the three parties is malicious either the client or one of the servers.
The achieved security suffices for many applications, and, assuming DDH, can be cheaply compiled into full security. We use our compilers to obtain the following results: A two-round, two-party protocol secure against malicious adversaries in the random oracle model making black-box use of a two-round semi-honest secure protocol. Prior to our work, such a result was not known even considering special functionalities such as a two-round oblivious transfer. A three-round multiparty secure computation protocol in the random oracle model secure against malicious adversaries that is based on the black-box use of two-round semi-honest OT.
This protocol matches a known round complexity lower bound due to Applebaum et al. A two-round, multiparty secure computation protocol in the 1-out-of-2 OT correlations model that is secure against malicious adversaries and makes black-box use of cryptography.
This gives new round-optimal protocols for computing arithmetic branching programs that are statistically secure and makes black-box use of the underlying field. Due to their round-robin structure, protocols of this class inherently require n sequential broadcast rounds, where n is the number of participants.
Our compiled protocols guarantee output delivery against any dishonest majority. We show that in the context of the aforementioned applications, this bias is harmless. An important limitation of all currently known techniques for designing cryptographic protocols with security against subversion attacks is that they do not automatically guarantee security in the realistic setting where a protocol session may run concurrently with other protocols.
Both the core and the firewall can be subject to different flavors of corruption, modeling different kinds of subversion attacks. For instance, we capture the setting where a subverted core looks like the honest core to any efficient test, yet it may leak secret information via covert channels which we call specious subversion.
This yields a completeness theorem for maliciously secure MPC in the presence of specious subversion. Additionally, all our sanitized protocols are transparent, in the sense that communicating with a sanitized core looks indistinguishable from communicating with an honest core. Thanks to the composition theorem, our methodology allows, for the first time, to design subversion-resilient protocols by sanitizing different sub-components in a modular way. The main conclusion was that, under plausible assumptions, most cryptographic primitives can be realized with constant computational overhead.
We refer to this goal as asymptotically quasi-optimal AQO cryptography. Our protocol applies to OLE over small fields and relies on the near-exponential security of the ring learning with errors RLWE assumption. Leo de Castro, Carmit Hazay, Yuval Ishai, Vinod Vaikuntanathan, Muthu Venkitasubramaniam Round-Optimal Multi-party Computation with Identifiable Abort Abstract Secure multi-party computation MPC protocols that are resilient to a dishonest majority allow the adversary to get the output of the computation while, at the same time, forcing the honest parties to abort.
Aumann and Lindell introduced the enhanced notion of security with identifiable abort, which still allows the adversary to trigger an abort but, at the same time, it enables the honest parties to agree on the identity of the party that led to the abort. More recently, in Eurocrypt , Garg et al. Following Garg et al. In this work, we close this gap and show that four rounds of communication are also sufficient to securely realize any functionality with identifiable abort using standard and generic polynomial-time assumptions.
To achieve this result we introduce the new notion of bounded-rewind secure MPC that guarantees security even against an adversary that performs a mild form of reset attacks. We show how to instantiate this primitive starting from any MPC protocol and by assuming trapdoor-permutations.
The notion of bounded-rewind secure MPC allows for easier parallel composition of MPC protocols with other interactive cryptographic primitives. Therefore, we believe that this primitive can be useful in other contexts in which it is crucial to combine multiple primitives with MPC protocols while keeping the round complexity of the final protocol low. Additive key derivation is a simple mechanism for deriving many subkeys from a single master key, and is already widely used in cryptocurrency applications with the Hierarchical Deterministic Wallet mechanism standardized in Bitcoin Improvement Proposal 32 BIP Because of its linear nature, additive key derivation is also amenable to efficient implementation in the threshold setting.
With presignatures, the secret and public nonces used in the ECDSA signing algorithm are precomputed. Recent works have advocated for both of these variations, sometimes combined together. However, somewhat surprisingly, we are aware of no prior security proof for additive key derivation, let alone for additive key derivation in combination with presignatures.
In this paper, we provide a thorough analysis of these variations, both in isolation and in combination. Our analysis is in the generic group model GGM. Of independent interest, we also present a version of the GGM that is specific to elliptic curves. In addition to this analysis, we report security weaknesses in these variations that apparently have not been previously reported.
We also present two mitigations against these weaknesses: re-randomized presignatures and homogeneous key derivation. Crucially, the identity of the active branch must remain hidden from the protocol participants. While such circuits can be securely computed by evaluating each branch and then multiplexing the output, such an approach incurs a communication cost linear in the size of the entire circuit.
To alleviate this, a series of recent works have investigated the problem of reducing the communication cost of branching executions inside MPC without relying on fully homomorphic encryption. Presently, however, it is not known how to obtain similar communication improvements for secure computation involving more than two parties. In this work, we provide a generic framework for branching multi-party computation that supports any number of parties.
The communication complexity of our scheme is proportional to the size of the largest branch and the computation is linear in the size of the entire circuit. We provide an implementation and benchmarks to demonstrate practicality of our approach. Aarushi Goel, Mathias Hall-Andersen, Aditya Hegde, Abhishek Jain Secure Multiparty Computation with Sublinear Preprocessing Abstract A common technique for enhancing the efficiency of secure multiparty computation MPC with dishonest majority is via preprocessing: In an offline phase, parties engage in an input-independent protocol to securely generate correlated randomness.
Recent constructions of pseudorandom correlation generators Boyle et al. However, these techniques do not efficiently apply to authenticated triples, except in the case of secure two-party computation of arithmetic circuits over large fields. Many of the most active participants continue to attend each year, and attendance continues to grow at a healthy rate. The informal and collegial atmosphere and the beach side setting which contribute to the popularity of the event were again supported by flawless weather.
The absence of parallel sessions seemed to provide a welcome opportunity to keep abreast of developments in the various areas of activity. Each session of the meeting organized by the program committee is repre- sented by a section in the present volume. The papers were accepted by the program committee based on abstracts, and appear here without having been otherwise refereed.
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Many of the most active participants continue to attend each year, and attendance continues to grow at a healthy rate. The informal and collegial atmosphere and the beach side setting which contribute to the popularity of the event were again supported by flawless weather. The absence of parallel sessions seemed to provide a welcome opportunity to keep abreast of developments in the various areas of activity.
Each session of the meeting organized by the program committee is repre sented by a section in the present volume. The papers were accepted by the program committee based on abstracts, and appear here without having been otherwise refereed.
Jueneman investignte the security of DES when used in output feedback mode. The underlying theme of this section is that the security provided by a cryptographic algorithm is determined in part by the way the algorithm is used. Section Ill, "Protocols and Transaction Security," studies how protocols can be used to conduct various business transactions electronically.
In particular, protocols arc discussed for signing checks. Methods for proving the correctness of such protocols are also examined in detail. Section IV, "Applications," treats the key management aspects of a number of cryptographic applications, such as protecting personal data cards, controlling access to local networks, and implementing an electronic notary public.
Section V, "Cryptanalysis," investigates weaknesses ofknapsack ciphers. In what is perhaps the most significant unclassified cryptologic paper of the year, Adi Shamir Preface ix explains how to break the basic Merkle-Hellman knapsack public-key cryptosystem.
Gustavus J. Simmons el a!. Adleman describe related discoveries. During the conference, Adleman's prcsentntion was particularly notable for his use of an Apple II personal computer to solve an instance of the Graham-Shamir knapsack cipher. Jeff Legarins and Donald W. Dnvies also presented papers, but these papers were not received in time to be included in the proceedings. Davies's talk, which concluded the session, was a fascinating overview of techniques used by the Allies during WWII to break the Enigma cryptograph.
Section VI, "Rump Session," covers a potpourri of cryptologic topics including DES, multi-party protocols, pseudo-random number generators, threshold schemes, randomized stream ciphers. The papers in this section summarize brief impromptu talks given at an informal evening session of the conference.
A list of these papers appears immediately following the table of contents. During his opening remarks, David Chaum proposed the formation of an international organization that would further research in cryptology by coordinating and organizing meetings in the area, and by in forming its members of relevant events, publications, and work. The members were Henry J. Bcker, Ernest F. Jueneman, David Kahn, and Stephen Kent. At the end of the conference, the planning committee held its first meeting, at which it adopted the working title: 'The International Association for Cryptologic Research.
The editors would like to thank all of the authors, organizers, and other people who made these proceedings possible. We are grateful to Leonard M. Adleman, Dorothy Denning, Whitfield Diffie, and Stephen Kent, who served as session chairmen and members of the program committee.
We also thank Allen Gersho for his help on the program committee. Several other people made essential contributions to the conference: John Kowalchuk handled registration; Richard Kemmerer took care of local arrangements; Paul Eggert served as treasurer; and Thomas A. We would also like to express our appreciation to John Gordon, David Kahn, and Stephen Weinstein for helping organize the conference. Marchand of Plenum Press for their patient and cheerful assistance in preparing this book.
Hellman and Justin M. Ernest F.
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