No abstract

Classical results on secure multi-party computation (MPC) imply that fully secure computation, including fairness (either all parties get output or none) and robustness (output delivery is guaranteed), is impossible unless a majority of the parties is honest. Recently, cryptocurrencies like Bitcoin where utilized to leverage the fairness loss in MPC against a dishonest majority. The idea is that when the protocol aborts in an unfair manner (i.e., after the adversary receives output) then honest parties get compensated by the adversarially controlled parties.Our contribution is three-fold. First, we put forth a new formal model of secure MPC with compensation and we show how the introduction of suitable ledger and synchronization functionalities makes it possible to express completely such protocols using standard interactive Turing machines (ITM) circumventing the need for the use of extra features that are outside the standard model as in previous works. Second, our model, is expressed in the universal composition setting with global setup and is equipped with a composition theorem that enables the design of protocols that compose safely with each other and within larger environments where other protocols with compensation take place; a composition theorem for MPC protocols with compensation was not known before. Third, we introduce the first robust MPC protocol with compensation, i.e., an MPC protocol where not only fairness is guaranteed (via compensation) but additionally the protocol is guaranteed to deliver output to the parties that get engaged and therefore the adversary, after an initial round of deposits, is not even able to mount a denial of service attack without having to su↵er a monetary penalty. Importantly, our robust MPC protocol requires only a constant number of (coin-transfer and communication) rounds.

Abstract. In synchronous networks, protocols can achieve security guarantees that are not possible in an asynchronous world: they can simultaneously achieve input completeness (all honest parties' inputs are included in the computation) and guaranteed termination (honest parties do not "hang" indefinitely). In practice truly synchronous networks rarely exist, but synchrony can be emulated if channels have (known) bounded latency and parties have loosely synchronized clocks. The widely-used framework of universal composability (UC) is inherently asynchronous, but several approaches for adding synchrony to the framework have been proposed. However, we show that the existing proposals do not provide the expected guarantees. Given this, we propose a novel approach to defining synchrony in the UC framework by introducing functionalities exactly meant to model, respectively, bounded-delay networks and loosely synchronized clocks. We show that the expected guarantees of synchronous computation can be achieved given these functionalities, and that previous similar models can all be expressed within our new framework.

Existing work on "rational cryptographic protocols" treats each party (or coalition of parties) running the protocol as a selfish agent trying to maximize its utility. In this work we propose a fundamentally different approach that is better suited to modeling a protocol under attack from an external entity. Specifically, we consider a two-party game between an protocol designer and an external attacker. The goal of the attacker is to break security properties such as correctness or privacy, possibly by corrupting protocol participants; the goal of the protocol designer is to prevent the attacker from succeeding.We lay the theoretical groundwork for a study of cryptographic protocol design in this setting by providing a methodology for defining the problem within the traditional simulation paradigm. Our framework provides ways of reasoning about important cryptographic concepts (e.g., adaptive corruptions or attacks on communication resources) not handled by previous game-theoretic treatments of cryptography. We also prove composition theorems that-for the first time-provide a sound way to design rational protocols assuming "ideal communication resources" (e.g., broadcast or authenticated channels) and then instantiate these resources using standard cryptographic tools.Finally, we investigate the problem of secure function evaluation in our framework, where the attacker has to pay for each party it corrupts. Our results demonstrate how knowledge of the attacker's incentives can be used to circumvent known impossibility results in this setting.

Protocols for secure multi-party computation (MPC) that resist a dishonest majority are susceptible to "denial of service" attacks, allowing even a single malicious party to force the protocol to abort. In this work, we initiate a systematic study of the more robust notion of security with identifiable abort, which leverages the effect of an abort by forcing, upon abort, at least one malicious party to reveal its identity.We present the first information-theoretic MPC protocol which is secure with identifiable abort (in short ID-MPC) using a correlated randomness setup. This complements a negative result of Ishai et al. (TCC 2012) which rules out information-theoretic ID-MPC in the OThybrid model, thereby showing that pairwise correlated randomness is insufficient for information-theoretic ID-MPC.In the standard model (i.e., without a correlated randomness setup), we present the first computationally secure ID-MPC protocol making black-box use of a standard cryptographic primitive, namely an (adaptively secure) oblivious transfer (OT) protocol. This provides a more efficient alternative to existing ID-MPC protocols, such as the GMW protocol, that make a non-black-box use of the underlying primitives.As a theoretically interesting sidenote, our black-box ID-MPC provides an example for a natural cryptographic task that can be realized using a black-box access to an OT protocol but cannot be realized unconditionally using an ideal OT oracle.