The problems of broadcast and multi-party computation are usually treated with respect to a small number of standard models. In this overview, with respect to two important such standard models, we address the following questions. Can the same problems still be solved when assuming less? What happens if we try to tolerate more corrupted players than provably tolerable? We conclude with a protocol for multi-party computation based on almost no setup assumptions at all
The problems of broadcast and multi-party computation are usually treated with respect to a small number of standard models. In this overview, with respect to two important such standard models, we address the following questions. Can the same problems still be solved when assuming less? What happens if we try to tolerate more corrupted players than provably tolerable? We conclude with a protocol for multi-party computation based on almost no setup assumptions at all
Secure Multi-party Computation (MPC) considers the problem where a set of mutually distrusting parties want to jointly compute a function over their private inputs, without revealing any extra information about these inputs other than what it can be inferred from the output of the function. This setting is well-motivated, and it has many real-world applications such as auction, voting, etc. MPC can be also seen as a generalization of many natural cryptographic primitives. For example, zero-knowledge (ZK) can be viewed as a special case of two-party secure computation. In ZK, a party, called prover aims to convince a second party, called verifier, that the prover’s private input witness w and a public input statement x belong to a relation R. An important research direction in secure computation is to find the trade-off between the required setup (e.g., the use of the broadcast channel, the use of common reference string (CRS) / public key infrastructure (PKI), the upper bound of the parties that can be corrupted, etc.), and the security guarantees that can be achieved. The setups can be viewed as some general assumptions that the protocol needs to satisfy, and they influence the usability of the protocol in real-world scenarios. In principle, having simpler (or no) setups mean that the protocol is more general and can be more useful in real-world scenarios. At the same time, having simpler setups may lead to weaker security guarantees. Therefore, finding the trade-off between setup and security guarantees is important and meaningful. In this thesis, we target MPC and ZK, and we focus on how to minimize the setup for MPC and ZK while still providing meaningful levels of security. More specifically: Regarding MPC, we focus on the dishonest majority (i.e., the adversary can corrupt all but one party), and we aim at 1) minimizing the use of broadcast channels. 2) studying the MPC with pre-processing when no setup is available. • Informally, a broadcast channel guarantees that when a message is sent, this reaches all the parties, without ambiguity. It also guarantees that if an honest party receives a message from a corrupted party, then it is guaranteed that all the honest parties have received the message. To realize broadcast, parties in the protocol could run the broadcast protocol, which may require many rounds of peer-to-peer communications. An alternative way is to rely on physical or external infrastructure such as blockchain. In both cases, broadcast is expensive, as such, we want to minimize its use. In particular, this thesis presents the following results: – When assuming no setup, we give a complete characterization with respect to the use of broadcast channels, and we obtain the optimal results. – We consider the same problem for the case that we only want to allow the black-box use (i.e., do not have access to the code of the algorithm) of the oblivious transfer protocol. We also give a characterization. • In the standard definition of MPC, the parties’ private inputs are fixed before the start of the protocol. However, there is another type of MPC named MPC with pre-processing, where the protocol can pre-compute some messages without using parties’ inputs, and these messages can accelerate computations in the online phase (i.e., other computations that require parties’ inputs). Since some expensive computations can be pre-computed, the online phase could be more lightweight. Therefore, we want to remove the dependency of the input from as many rounds as possible, so that we can do some pre-processing. In this direction, we explore the protocol with no setup. We provide a compiler that can turn a big class of MPC protocol that may require the inputs already to compute the first round, into a new protocol that needs the inputs only in the last two rounds. We also propose new MPC definitions that capture this delayed-input features. Regarding ZK, we do the following: • In standard single-theorem ZK definition, the security of the ZK protocol is guaranteed to hold only when one proof is issued. In the case where multiple zero-knowledge proofs need to be issued (i.e., to prove multiple NP statements), each new zero-knowledge proof requires a freshly generated setup. In the multi-theorem ZK definition, instead, one setup is sufficient for generating multiple zero-knowledge proofs for multiple instances. We propose a multi-theorem protocol (in the format of a compiler) that follows the Fiat-Shamir paradigm and relies on correlation intractable hash functions. Moreover, our protocol remains zero-knowledge and sound even against adversaries that choose the statement to be proven (and the witness for the case of zero-knowledge) adaptively on the key of the hash function. Prior works could achieve this adaptive security only inefficiently via NP reductions. • ZK protocols are secure only when all setups are correctly generated, but in real-world scenarios, some of the setups may not be correctly generated. For instance, to run a non-interactive zero-knowledge (NIZK) protocol, the setup CRS could be chosen with bias. In this case, the security of the NIZK protocol does not hold anymore. Instead of finding a secure ZK candidate, one alternative solution is to have multiple instantiations of ZK candidates and assume that only for a subset of them the setup is generated correctly. More formally, we consider the case where only a subset of the instances are secure. In more detail, given access to n candidate instantiations of a NIZK for some language, we want to have a construction that itself implements a NIZK for the same language without relying on any additional computational assumptions. We refer to this type of construction as combiner, and the combiner is secure assuming at least t of the given candidates are secure. In this work, we provide three different constructions of robust NIZK combiners and show that combiners are impossible to realize unless the majority of the input candidates are secure.
Secure multiparty computation (MPC) is perhaps the most popular paradigm in the area of cryptographic protocols. It allows several mutually untrustworthy parties to jointly compute a function of their private inputs, without revealing to each other information about those inputs. In the case of unconditional (information-theoretic) security, protocols are known which tolerate a dishonest minority of players, who may coordinate their attack and deviate arbitrarily from the protocol specification. It is typically assumed in these results that parties are connected pairwise by authenticated, private channels, and that in addition they have access to a “broadcast ” channel. Broadcast allows one party to send a consistent message to all other parties, guaranteeing consistency even if the broadcaster is corrupted. Because broadcast cannot be simulated on the point-to-point network when more than a third of the parties are corrupt, it is impossible to construct general MPC protocols in this setting without using a broadcast channel (or some equivalent addition to the model). A great deal of research has focused on increasing the efficiency of MPC, primarily in terms of round complexity and communication complexity. In this work we propose a refinement of the round complexity which we term broadcast complexity. We view the broadcast channel as an expensive resource
In this dissertation, we consider the design of broadcast and secure multi-party computation (MPC) protocols in the presence of adversarial faults.
Secure multi-party computation is the most generic problem in fault-tolerant distributed computing. In principle, a multi-party computation protocol can be used to solve any distributed cryptographic problem. Informally, the problem of multi-party computation is the following: suppose we have n parties P1, P2,…, Pn where each party Pi has a private input xi. Together, the parties want to compute a function of their inputs (y1, y2,…, yn) = f(x1,x2,…, xn). However, some parties can be corrupted and do not execute a prescribed protocol faithfully. Even worse, they may be controlled by an adversary and attack the protocol in a coordinated manner. Despite the presence of such an adversary, a secure MPC protocol should ensure that each (corrupted) party Pi learn only its output yi but nothing more.
Broadcast in the presence of adversarial faults is one of the simplest special cases of multi-party computation and important component of larger protocols. In short, broadcast allows a party to send the same message to all parties, and all parties to be assured they have received identical messages.
The contribution of this dissertation is twofold. First, we construct broadcast and secure multi-party computation protocols for honest majority in a point-to-point network whose round complexities improve significantly upon prior work. In particular, we give the first expected constant-round authenticated broadcast protocol for honest majority assuming only public-key infrastructure and signatures. Second, we initiate the study of broadcast in radio networks in the presence of adversarial faults. In radio networks, parties communicate through multicasting messages; a message can only be received by the parties within some radius from the sender. Feasibility and impossibility results are given, and our bounds are tight.
Two settings are traditionally considered for secure multiparty computation, depending on whether or not a majority of the parties are assumed to be honest. Protocols designed under this assumption provide “full security ” (and, in particular, guarantee output delivery and fairness) when this assumption holds; unfortunately, these protocols are completely insecure if this assumption is violated. On the other hand, protocols tolerating an arbitrary number of corruptions do not guarantee fairness or output delivery even if only a single party is dishonest. It is natural to wonder whether it is possible to achieve the “best of both worlds”: namely, a single protocol that simultaneously achieves the best possible security in both the above settings. Here, we rule out this possibility (at least for general functionalities) but show some positive results regarding what can be achieved.
We survey basic definitions and results concerning secure multi-party computations, where the two-party case is an important special case. In a nutshell, these results assert that, under a variety of reasonable settings and/or assumptions, it is possible to construct protocols for securely computing any desirable multi-party functionality. Confining ourselves to the very basics of this vast area of study, we focus on the stand-alone setting, while leaving the survey of the study of the security of concurrent executions to other surveys.
It is well-known that n players, connected only by pairwise secure channels, can achieve Byzantine agreement only if the number t of cheaters satisfies t < n/3, even with respect to computational security. However, for many applications it is sufficient to achieve detectable broadcast. With this primitive, broadcast is only guaranteed when all players are non-faulty ("honest"), but all non-faulty players always reach agreement on whether broadcast was achieved or not. We show that detectable broadcast can be achieved regardless of the number of faulty players (i.e., for all t < n). We give a protocol which is unconditionally secure, as well as two more efficient protocols which are secure with respect to computational assumptions, and the existence of quantum channels, respectively.These protocols allow for secure multi-party computation tolerating any t < n, assuming only pairwise authenticated channels. Moreover, they allow for the setup of public-key infrastructures that are consistent among all participants --- using neither a trusted party nor broadcast channels.Finally, we show that it is not even necessary for players to begin the protocol at the same time step. We give a "detectable Firing Squad" protocol which can be initiated by a single user at any time and such that either all honest players end up with synchronized clocks, or all honest players abort.