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. 1
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. 1
This work develops a composable notion of security in a synchronous communication network to analyze cryptographic primitives and protocols in a reliable network with guaranteed delivery. In such a synchronous model the abort of protocols must be handled explicitly. It is shown that a version of global bit commitment which allows to identify parties that did not give proper input cannot be securely realized with the primitives oblivious transfer and broadcast. This proves that the primitives oblivious transfer and broadcast are not complete in our synchronous model of security. In the synchronous model presented ideal functionalities as well as parties can be equipped with a “shell” which can delay communication until the adversary allows delivery or the number of rounds since the shell received the message exceeds a specified threshold. This additionally allows asynchronous specification of ideal functionalities and allows to model a network where messages are not necessarily delivered in the right order. If these latency times are chosen to be infinite the network is no more reliable and becomes completely asynchronous. In the full version [HMQ04] of this paper, it is shown that a large class of protocols which are secure in the asynchronous settings [Can01, CLOS02] can be transformed into secure realizations in the new model by choosing infinite latency times.
Abstract. The recently proposed universally composable (UC) security framework, for analyzing security of cryptographic protocols, provides very strong security guarantees. In particular, a protocol proven secure in this framework is guaranteed to maintain its security even when deployed in arbitrary multi-party, multi-protocol, multi-execution environments. Protocols for securely carrying out essentially any cryptographic task in a universally composable way exist, both in the case of an honest majority (in the plain model, i.e., without set-up assumptions) and in the case of no honest majority (in the common reference string model). However, in the plain model, little was known for the case of no honest majority and, in particular, for the important special case of two-party protocols. We study the feasibility of universally composable two-party function evaluation in the plain model. Our results show that very few functions can be computed in this model so as to provide the UC security guarantees. Specifically, for the case of deterministic functions, we provide a full characterization of the functions computable in this model. (Essentially, these are the functions that depend on at most one of the parties’ inputs, and furthermore are “efficiently invertible ” in a sense defined within.) For the case of probabilistic functions, we show that the only functions computable in this model are those where one of the parties can essentially uniquely determine the joint output. 1
Abstract. We introduce the notion of resource-fair protocols. Informally, this property states that if one party learns the output of the protocol, then so can all other parties, as long as they expend roughly the same amount of resources. As opposed to similar previously proposed definitions, our definition follows the standard simulation paradigm and enjoys strong composability properties. In particular, our definition is similar to the security definition in the universal composability (UC) framework, but works in a model that allows any party to request additional resources from the environment to deal with dishonest parties that may prematurely abort. In this model we specify the ideally fair functionality as allowing parties to “invest resources ” in return for outputs, but in such an event offering all other parties a fair deal. (The formulation of fair dealings is kept independent of any particular functionality, by defining it using a “wrapper.”) Thus, by relaxing the notion of fairness, we avoid a well-known impossibility result for fair multi-party computation with corrupted majority; in particular, our definition admits constructions that tolerate arbitrary number of corruptions. We also show that, as in the UC framework, protocols in our framework may be arbitrarily and concurrently composed. Turning to constructions, we define a “commit-prove-fair-open ” functionality and design an efficient resource-fair protocol that securely realizes it, using a new variant of a cryptographic primitive known as “timelines.” With (the fairly wrapped version of) this functionality we show that some of the existing secure multi-party computation protocols can be easily transformed into resource-fair protocols while preserving their security. 1
Universal composability and concurrent general composition consider a setting where secure protocols are run concurrently with each other and with arbitrary other possibly insecure protocols. Protocols that meet the definition of universal composability are guaranteed to remain secure even when run in this strongly adversarial setting. In the case of an honest majority, or where there is a trusted setup phase of some kind (like a common reference string or the keyregistration public-key infrastructure of Barak et al. in FOCS 2004), it has been shown that any functionality can be securely computed in a universally composable way. On the negative side, it has also been shown that in the plain model where there is no trusted setup at all, there are large classes of functionalities which cannot be securely computed in a universally composable way without an honest majority. In this paper we extend these impossibility results for universal composability. We study a number of public-key models and show for which models the impossibility results of universal composability hold and for which they do not. We also consider a setting where the inputs to the protocols running in the network are fixed before any execution begins. The majority of our results are negative and we show that the known impossibility results for universal composability in the case of no honest majority extend to many other settings.
Modeling security for protocols running in the complex network environment of the Internet can be a daunting task. Ideally, a security model for the Internet should provide the following guarantee: a protocol that implements a particular task specification will retain all the same security properties as the specification itself, even when an arbitrary set of protocols runs concurrently on the same network. This guarantee must hold even when other protocols are maliciously designed to interact badly with the analyzed protocol, and even when the analyzed protocol is composed with other protocols. The popular Universal Composability (UC) security framework aims to provide this guarantee.
Unfortunately, such strong security guarantees come with a price: they are impossible to achieve without the use of some trusted setup. Typically, this trusted setup is global in nature, and takes the form of a Public Key Infrastructure (PKI) and/or a Common Reference String (CRS). However, the current approach to modeling security in the presence of such setups falls short of providing expected security guarantees. A quintessential example of this phenomenon is the deniability concern: there exist natural protocols that meet the strongest known security notions (including UC) while failing to provide the same deniability guarantees that their task specifications imply they should provide.
We introduce the Generalized Universal Composability (GUC) framework to extend the UC security notion and enable the re-establishment of its original intuitive security guarantees even for protocols that use global trusted setups. In particular, GUC enables us to guarantee that secure protocols will provide the same level of deniability as the task specification they implement. To demonstrate the usefulness of the GUC framework, we first apply it to the analysis and construction of deniable authentication protocols. Building upon such deniable authentication protocols, we then prove a general feasibility result showing how to construct protocols satisfying our security notion for a large class of two-party and multi-party tasks (assuming the availability of some reasonable trusted setup). Finally, we highlight the practical applicability of GUC by constructing efficient protocols that securely instantiate two common cryptographic tasks: commitments and zero-knowledge proofs.
In this paper, we present a simpler and more restricted variant of the universally composable security (UC) framework that is suitable for “standard ” two-party and multiparty computation tasks. Many of the complications of the UC framework exist in order to enable more general tasks than classic secure computation. This generality may be a barrier to entry for those who are used to the stand-alone model of secure computation and wish to work with universally composable security but are overwhelmed by the differences. The variant presented here (called simplified universally composable security, or just SUC) is closer to the definition of security for multiparty computation in the stand-alone setting. The main difference is that a protocol in the SUC framework runs with a fixed set of parties who know each other’s identities ahead of time, and machines cannot be added dynamically to the execution. As a result, the definitions of polynomial time and protocol composition are much simpler. In addition, the SUC framework has authenticated channels built in, as is standard in previous definitions of security, and all communication is done via the adversary in order to enable arbitrary scheduling of messages. Due to these differences, not all cryptographic tasks can be expressed in the SUC framework.