*O*(√n) Rounds for Round-Robin Sampling Protocols

We introduce a notion of *round-robin* secure sampling that captures several protocols in the literature, such as the “powers-of-tau” setup protocol for pairing-based polynomial commitments and zk-SNARKs, and certain verifiable mixnets.

Due to their round-robin structure, protocols of this class inherently require *n* sequential broadcast rounds, where *n* is the number of participants.

We describe how to compile them generically into protocols that require only *O*(√n) broadcast rounds. Our compiled protocols guarantee output delivery against *any* dishonest majority. This stands in contrast to prior techniques, which require *Ω*(*n*) sequential broadcasts in most cases (and sometimes many more). Our compiled protocols permit a certain amount of adversarial bias in the output, as all sampling protocols with guaranteed output must, due to Cleve's impossibility result (STOC'86). We show that in the context of the aforementioned applications, this bias is harmless.

We present a new multiparty protocol for the distributed generation of biprime RSA moduli, with security against any subset of maliciously colluding parties assuming oblivious transfer and the hardness of factoring.

Our protocol is highly modular, and its uppermost layer can be viewed as a template that generalizes the structure of prior works and leads to a simpler security proof. We introduce a combined sampling-and-sieving technique that eliminates both the inherent leakage in the approach of Frederiksen et al. (Crypto'18), and the dependence upon additively homomorphic encryption in the approach of Hazay et al. (JCrypt'19). We combine this technique with an efficient, privacy-free check to detect malicious behavior retroactively when a sampled candidate is not a biprime, and thereby overcome covert rejection-sampling attacks and achieve both asymptotic and concrete efficiency improvements over the previous state of the art.

Hiding memory access patterns is required for secure computation, but remains prohibitively expensive for many interesting applications. Prior work has either developed custom algorithms that minimize the need for data-dependant memory access, or proposed the use of Oblivious RAM (ORAM) to provide a general-purpose solution. However, most ORAMs are designed for client-server scenarios, and provide only asymptotic benefits in secure computation. Even the best prior schemes show concrete benefits over naïve linear scan only for array sizes greater than 100. This immediately implies each ORAM access is 100 times slower than a single access at a known location. Even then, prior evaluations ignore the substantial initialization cost of existing schemes.

We show how the classical square-root ORAM of Goldreich and Ostrovsky can be modified to overcome these problems, even though it is asymptotically worse than the best known schemes. Specifically, we show a design that has over 100 times lower initialization cost, and provides benefits over linear scan for just 8 blocks of data. For all benchmark applications we tried, including Gale-Shapley stable matching and the scrypt key derivation function, our scheme outperforms alternate approaches across a wide range of parameters, often by several orders of magnitude.

We design and implement a Distributed Oblivious Random Access Memory (DORAM) data structure that is optimized for use in two-party secure computation protocols. We improve upon the access time of previous constructions by a factor of up to ten, their memory overhead by a factor of one hundred or more, and their initialization time by a factor of thousands. We are able to instantiate ORAMs that hold 2^{34} bytes, and perform operations on them in seconds, which was not previously feasible with any implemented scheme.

Unlike prior ORAM constructions based on hierarchical hashing, permutation, or trees, our Distributed ORAM is derived from the new Function Secret Sharing scheme introduced by Boyle, Gilboa and Ishai. This significantly reduces the amount of secure computation required to implement an ORAM access, albeit at the cost of *O*(*n*) efficient *local* memory operations.

We implement our construction and find that, despite its poor *O*(*n*) asymptotic complexity, it still outperforms the fastest previously known constructions, Circuit ORAM and Square-root ORAM, for datasets that are 32 KiB or larger, and outperforms prior work on applications such as *stable matching* or *binary search* by factors of two to ten.

In the dishonest-majority setting, generic secure multiparty computation (MPC) protocols are fundamentally vulnerable to attacks in which malicious participants learn their outputs and then force the protocol to abort before outputs are delivered to the honest participants. In other words, generic MPC protocols typically guarantee *security with abort*.

This flavor of security permits denial-of-service attacks in many applications, unless the cheating participants who cause aborts are identified. At present, there is a substantial performance gap between the best known protocols that are secure with non-identifiable abort, and the best known protocols that achieve *security with identifiable abort* (IA). Known constructions with IA rely on generic zero-knowledge proofs, adaptively secure oblivious transfer (OT) protocols, or homomorphic primitives.

We present a novel approach for realizing functionalities with a weak form of *input-revealing* IA, which is based on delicate and selective revealing of committed input values. We refer to this new approach as *vindicating release*. When our approach is applied to several well-known protocols—including a variant of PVW OT, Softspoken OT extension, DKLs multiplication, and MASCOT generic MPC—the resulting protocols can be combined to realize any *sampling* functionality with (standard) IA. Such a realization is statistically secure given a variant of statically-corruptable ideal OT, and it differs minimally in terms of cost, techniques, and analysis from the equivalent realization (using the same well-known protocols, unmodified) that lacks identifiability.

Using our protocol to sample the correlated randomness of the IOZ compiler reduces the compiler's requirements from an adaptively secure OT protocol to a variant of statically-corruptable ideal OT.

When a group of individuals and organizations wish to compute a *stable matching*—for example, when medical students are matched to medical residency programs—they often outsource the computation to a trusted arbiter in order to preserve the privacy of participants' preferences. Secure multi-party computation offers the possibility of private matching processes that do not rely on any common trusted third party. However, stable matching algorithms have previously been considered infeasible for execution in a secure multi-party context on non-trivial inputs because they are computationally intensive and involve complex data-dependent memory access patterns.

We adapt the classic Gale-Shapley algorithm for use in such a context, and show experimentally that our modifications yield a lower asymptotic complexity and more than an order of magnitude in practical cost improvement over previous techniques. Our main improvements stem from designing new oblivious data structures that exploit the properties of the matching algorithms. We apply a similar strategy to scale the Roth-Peranson instability chaining algorithm, currently in use by the National Resident Matching Program. The resulting protocol is efficient enough to be useful at the scale required for matching medical residents nationwide, taking just over 18 hours to complete an execution simulating the 2016 national resident match with more than 35,000 participants and 30,000 residency slots.

The Elliptic Curve Digital Signature Algorithm (ECDSA) is one of the most widely used schemes in deployed cryptography. Through its applications in code and binary authentication, web security, and cryptocurrency, it is likely one of the few cryptographic algorithms encountered on a daily basis by the average person. However, its design is such that executing multi-party or threshold signatures in a secure manner is challenging: unlike other, less widespread signature schemes, secure multi-party ECDSA requires custom protocols, which has heretofore implied reliance upon additional cryptographic assumptions and primitives such as the Paillier cryptosystem.

We propose new protocols for multi-party ECDSA key-generation and signing with a threshold of two, which we prove secure against malicious adversaries in the random oracle model using only the Computational Diffie-Hellman Assumption and the assumptions already relied upon by ECDSA itself. Our scheme requires only two messages, and via implementation we find that it outperforms the best prior results in practice by a factor of 56 for key generation and 11 for signing, coming to within a factor of 18 of local signatures. Concretely, two parties can jointly sign a message in just over three milliseconds.

We investigate the conditions under which straight-line extractable NIZKs in the random oracle model (i.e. without a CRS) permit multiparty realizations that are black-box in the same random oracle. We show that even in the semi-honest setting, any MPC protocol to com- pute such a NIZK cannot make black-box use of the random oracle or a hash function instantiating it if security against all-but-one corruptions is desired, unless the size of the NIZK grows with the number of parties. This presents a fundamental barrier to constructing efficient protocols to securely distribute the computation of NIZKs (and signatures) based on MPC-in-the-head, PCPs/IOPs, and sigma protocols compiled with transformations due to Fischlin, Pass, or Unruh.

When the adversary is restricted to corrupt only a constant fraction of parties, we give a positive result by means of a tailored construction, which demonstrates that our impossibility does not extend to weaker corruptions models in general.

We propose a secure multiparty signing protocol for the BBS+ signature scheme; in other words, an anonymous credential scheme with threshold issuance. We prove that due to the structure of the BBS+ signature, simply verifying the signature produced by an otherwise semi-honest protocol is sufficient to achieve composable security against a malicious adversary. Consequently, our protocol is extremely simple and efficient: it involves a single request from the client (who requires a signature) to the signing parties, two exchanges of messages among the signing parties, and finally a response to the client; in some deployment scenarios the concrete cost bottleneck may be the client's local verification of the signature that it receives. Furthermore, our protocol can be extended to support the strongest form of blind signing and to serve as a distributed evaluation protocol for the Dodis-Yampolskiy Oblivious VRF. We validate our efficiency claims by implementing and benchmarking our protocol.

Cryptocurrency applications have spurred a resurgence of interest in the computation of ECDSA signatures using threshold protocols—that is, protocols in which the signing key is secret-shared among *n* parties, of which any subset of size *t* must interact in order to compute a signature. Among the resulting works to date, that of Doerner et al. requires the most natural assumptions while also achieving the best practical signing speed. It is, however, limited to the setting in which the threshold is two. We propose an extension of their scheme to arbitrary thresholds, and prove it secure against a malicious adversary corrupting up to one party less than the threshold under only the Computational Diffie-Hellman assumption in the Random Oracle model, an assumption strictly weaker than those under which ECDSA is proven.

We implement our scheme and evaluate it among groups of up to 256 of co-located and geographically-distributed parties, and among small groups of embedded devices. We find that in the LAN setting, our scheme outperforms all prior works by orders of magnitude, and that it is efficient enough for use even on smartphones or hardware tokens. In the WAN setting we find that, despite its logarithmic round count, our protocol outperforms the best constant-round protocols in realistic scenarios.

We present a three-round protocol for threshold ECDSA signing with malicious security against a dishonest majority, which information-theoretically UC-realizes a standard threshold signing functionality, assuming only ideal commitment and two-party multiplication primitives. Our protocol combines an intermediate representation of ECDSA signatures that was recently introduced by Abram et al. (Eurocrypt'22) with an efficient statistical consistency check reminiscent of the ones used by the protocols of Doerner et al. (S&P'18, S&P'19).

We show that shared keys for our signing protocol can be generated using a simple commit-release-and-complain procedure, without any proofs of knowledge, and to compute the intermediate representation or each signature, we propose a two-round vectorized multiplication protocol based on oblivious transfer that outperforms all similar constructions.

Our work fully subsumes the ECDSA protocols of Doerner et al., and we demonstrate that it is the most practically-efficient dishonest-majority threshold ECDSA protocol to date via concrete benchmarks of a complete implementation.