Optimizing Authenticated Garbling for Faster Secure Two-Party Computation

Wang et al. (CCS 2017) recently proposed a protocol for malicious secure two-party computation that represents the state-of-the- art with regard to concrete efficiency in both the single-execution and amortized settings, with or without preprocessing. We show here several optimizations of their protocol that result in a significant improvement in the overall communication and running time. Specifically: - We show how to make the “authenticated garbling” at the heart of their protocol compatible with the half-gate optimization of Zahur et al. (Eurocrypt 2015). We also show how to avoid sending an information-theoretic MAC for each garbled row. These two optimizations give up to a 2.6x improvement in communication, and make the communication of the online phase essentially equivalent to that of state-of-the-art semi-honest secure computation. - We show various optimizations to their protocol for generating AND triples that, overall, result in a 1.5x improvement in the communication and a 2x improvement in the computation for that step

Actively Secure Garbled Circuits with Constant Communication Overhead in the Plain Model

We consider the problem of constant-round secure two-party computation in the presence of active (malicious) adversaries. We present the first protocol that has only a constant multiplicative communication overhead compared to Yao\u27s protocol for passive adversaries, and can be implemented in the plain model by only making a black-box use of (parallel) oblivious transfer and a pseudo-random generator. This improves over the polylogarithmic overhead of the previous best protocol. A similar result could previously be obtained only in an amortized setting, using preprocessing, or by assuming bit-oblivious-transfer as an ideal primitive that has a constant cost. We present two variants of this result, one which is aimed at minimizing the number of oblivious transfers and another which is aimed at optimizing concrete efficiency. Our protocols are based on a novel combination of previous techniques together with a new efficient protocol to certify that pairs of strings transmitted via oblivious transfer satisfy a global relation. Settling for ``security with correlated abort\u27\u27, the concrete communication complexity of the second variant of our protocol can beat the best previous protocols with the same kind of security even for realistic values of the circuit size and the security parameter. This variant is particularly attractive in the offline-online setting, where the online cost is dominated by a single evaluation of an authenticated garbled circuit, and can also be made non-interactive using the Fiat-Shamir heuristic

More Efficient MPC from Improved Triple Generation and Authenticated Garbling

Recent works on distributed garbling have provided highly efficient solutions for constant-round MPC tolerating an arbitrary number of corruptions. In this work, we improve upon state-of-the-art protocols in this paradigm for further performance gain. First, we propose a new protocol for generating authenticated AND triples, which is a key building block in many recent works. -- We propose a new authenticated bit protocol in the two-party and multi-party settings from bare IKNP OT extension, allowing us to reduce the communication by about 24% and eliminate many computation bottlenecks. We further improve the computational efficiency for multi-party authenticated AND triples with cheaper and fewer consistency checks and fewer hash function calls. -- We implemented our triple generation protocol and observe around 4x to 5x improvement compared to the best prior protocol in most settings. For example, in the two-party setting with 10 Gbps network and 8 threads, our protocol can generate more than 4 million authenticated triples per second, while the best prior implementation can only generate 0.8 million triples per second. In the multi-party setting, our protocol can generate more than 37000 triples per second over 80 parties, while the best prior protocol can only generate the same number of triples per second over 16 parties. We also improve the state-of-the-art multi-party authenticated garbling protocol. -- We take the first step towards applying half-gates in the multi-party setting, which enables us to reduce the size of garbled tables by 2\kappa bits per gate per garbler, where \kappa is the computational security parameter. This optimization is also applicable in the semi-honest multi-party setting. -- We further reduce the communication of circuit authentication from 4\rho bits to 1 bit per gate, using a new multi-party batched circuit authentication, where \rho is the statistical security parameter. Prior solution with similar efficiency is only applicable in the two-party setting. For example, in the three-party setting, our techniques can lead to roughly a 35% reduction in the size of a distributed garbled circuit

Blind Certificate Authorities

We explore how to build a blind certificate authority (CA). Unlike conventional CAs, which learn the exact identity of those registering a public key, a blind CA can simultaneously validate an identity and provide a certificate binding a public key to it, without ever learning the identity. Blind CAs would therefore allow bootstrapping truly anonymous systems in which no party ever learns who participates. In this work we focus on constructing blind CAs that can bind an email address to a public key. To do so, we first introduce secure channel injection (SCI) protocols. These allow one party (in our setting, the blind CA) to insert a private message into another party\u27s encrypted communications. We construct an efficient SCI protocol for communications delivered over TLS, and use it to realize anonymous proofs of account ownership for SMTP servers. Combined with a zero-knowledge certificate signing protocol, we build the first blind CA that allows Alice to obtain a X.509 certificate binding her email address [email protected] to a public key of her choosing without ever revealing ``alice\u27\u27 to the CA. We show experimentally that our system works with standard email server implementations as well as Gmail

Constant Round Maliciously Secure 2PC with Function-Independent Preprocessing using LEGO

Secure two-party computation (S2PC) allows two parties to compute a function on their joint inputs while leaking only the output of the function. At TCC 2009 Orlandi and Nielsen proposed the LEGO protocol for maliciously secure 2PC based on cut-and-choose of Yao\u27s garbled circuits at the gate level and showed that this is asymptotically more efficient than on the circuit level. Since then the LEGO approach has been improved upon in several theoretical works, but never implemented. In this paper we describe further concrete improvements and provide the first implementation of a protocol from the LEGO family. Our protocol is optimized for the offline/online setting and supports function-independent preprocessing using only a constant number of rounds. We have benchmarked our prototype and find that our protocol can compete with all existing implementations and that it is often more efficient. As an example, in a LAN setting we can evaluate an AES-128 with online latency down to 1.13 ms, while if evaluating 128 AES-128 in parallel the amortized cost is 0.09 ms per AES-128. This online performance does not come at the price of offline inefficiency as we achieve comparable performance to previous, less general protocols, and significantly better if we ignore the cost of the function-independent preprocessing. Also, as our protocol has an optimal 2-round online phase it is significantly more efficient than previous protocols\u27 when considering a high latency network