We present a unified framework for obtaining Universally Composable (UC) protocols by relying on stand-alone secure non-malleable commitments. Essentially all results on concurrent secure computation--both in relaxed models (e.g., quasi-polynomial time simulation), or with trusted set-up assumptions (e.g., the CRS model, the imperfect CRS model, or the timing model)--are obtained as special cases of our framework. This not only leads to conceptually simpler solutions, but also to improved set-up assumptions, round-complexity, and computational assumptions.
We present a unified framework for obtaining Universally Composable (UC) protocols by relying on stand-alone secure non-malleable commitments. Essentially all results on concurrent secure computation--both in relaxed models (e.g., quasi-polynomial time simulation), or with trusted set-up assumptions (e.g., the CRS model, the imperfect CRS model, or the timing model)--are obtained as special cases of our framework. This not only leads to conceptually simpler solutions, but also to improved set-up assumptions, round-complexity, and computational assumptions.
We present a unified framework for obtaining general secure computation that achieves adaptive-Universally Composable (UC)-security. Our framework captures essentially all previous results on adaptive concurrent secure computation, both in relaxed models (e.g., quasi-polynomial time simulation), as well as trusted setup models (e.g., the CRS model, the imperfect CRS model). This provides conceptual simplicity and insight into what is required for adaptive and concurrent security, as well as yielding improvements to set-up assumptions and/or computational assumptions. Moreover, using our framework we provide first constructions of concurrent secure computation protocols that are adaptively secure in the timing model, and in the non-uniform simulation model. Conceptually, our framework can be viewed as an adaptive analogue to the recent work of Lin, Pass and Venkitasubramaniam [STOC ‘09], who considered only non-adaptive adversaries. Their main insight was that stand-alone non-malleability was sufficient for UC-security. A main conceptual contribution of this work is, quite surprisingly, that it is indeed the case even when considering adaptive security. A key element in our construction is a commitment scheme that satisfies a new notion of nonmalleability.
A desirable goal for cryptographic protocols is to guarantee security when the protocol is composed with other protocol instances. Universally composable (UC) protocols provide this guarantee in a strong sense: A protocol remains secure even when composed concurrently with an unbounded number of instances of arbitrary protocols. However, UC protocols for carrying out general tasks are known to exist only if a majority of the participants are honest, or in the common reference string (CRS) model where all parties are assumed to have access to a common string that is drawn from some pre-defined distribution. Furthermore, carrying out many interesting tasks in a UC manner and without honest majority or set-up assumptions is impossible, even if ideally authenticated communication is provided. A natural question is thus whether there exist more relaxed set-up assumptions than the CRS model that still allow for UC protocols. We answer this question in the affirmative: we propose alternative and relaxed set-up assumptions and show that they suffice for reproducing the general feasibility results for UC protocols in the CRS model. These alternative assumptions have the flavor of a "public-key infrastructure": parties have registered public keys, no single registration authority needs to be fully trusted, and no single piece of information has to be globally trusted and available. In addition, unlike known protocols in the CRS model, the proposed protocols guarantee some basic level of security even if the set-up assumption is violated.
A desirable goal for cryptographic protocols is to guarantee security when the protocol is composed with other protocol instances. Universally Composable (UC) security provides this guarantee in a strong sense: A UC-secure protocol maintains its security properties even when composed concurrently with an unbounded number of instances of arbitrary protocols. However, many interesting cryptographic tasks are provably impossible to realize with UC security in the standard, “plain” model of computation. Impossibility holds even if ideally authenticated communication channels are provided. In contrast, it has been demonstrated that general secure computation can be obtained in a number of idealized models. Each one of these models represents a form of trust that is put in some of the system’s components. This survey examines and compares some of these trust models, both from the point of view of their sufficiency for building UC secure protocols, and from the point of view of their practical realizability. We start with the common reference string (CRS) model, and then describe several relaxations and alternatives including the Defective CRS model, the key registration models, the hardware token model, the global and augmented CRS models, and a timing assumption. Finally, we briefly touch upon trust models for obtaining authenticated communication.
Cryptography today has evolved far beyond its traditional goal of secure message transmission. Through the notion of secure computation, a set of mutually dis- trustful agents can collaborate to accomplish a common goal while preserving each agent's privacy to a maximal extent. In the seminal works of Yao and Goldreich, Micali and Wigderson, it was shown that any computational task can be securely implemented through a protocol. Traditionally, the rules governing privacy for these protocols have been designed to work only when a single execution running in isolation. However, with the advent of the Internet, many transactions occur simultaneously, and the protocols designed for the single execution setting fail to remain secure in a concurrent setting. While both the need and definitions for concurrent security were realized in the early 90's, practical protocols that are concurrently secure are lacking. The protocols designed for concurrent security, thus far, have mostly relied on having a trusted setup or a relaxed definition of security. In this thesis, we put forward a unified framework for the construction of concurrently secure protocols both with and without trusted set-up. This framework not only provides a conceptually simple solution for essentially all previous results, but also significantly improves efficiency and reduces the requirements on the trusted setup used in these works. Moreover, in several setup models, our constructions are tight with respect to computational assumptions and efficiency.
Abstract. We propose a new security measure for commitment pro-tocols, called Universally Composable (UC) Commitment. The measure guarantees that commitment protocols behave like an \\ideal commitment service, " even when concurrently composed with an arbitrary set of pro-tocols. This is a strong guarantee: it implies that security is maintained even when an unbounded number of copies of the scheme are running concurrently, it implies non-malleability (not only with respect to other copies of the same protocol but even with respect to other protocols), it provides resilience to selective decommitment, and more. Unfortunately, two-party uc commitment protocols do not exist in the plain model. However, we construct two-party uc commitment proto-cols, based on general complexity assumptions, in the common reference string model where all parties have access to a common string taken from a predetermined distribution. The protocols are non-interactive, in the sense that both the commitment and the opening phases consist of a single message from the committer to the receiver.
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.