Contrary to prior belief, we show that there exist commitment, zero-knowledge and general function evaluation protocols with universally composable security, in a model where all parties and all protocols have access to a single, global, random oracle and no other trusted setup. This model provides significantly stronger composable security guarantees than the traditional random oracle model of Bellare and Rogaway [CCS'93] or even the common reference string model. Indeed, these latter models provide no security guarantees in the presence of arbitrary protocols that use the {\em same} random oracle (or reference string or hash function).
Contrary to prior belief, we show that there exist commitment, zero-knowledge and general function evaluation protocols with universally composable security, in a model where all parties and all protocols have access to a single, global, random oracle and no other trusted setup. This model provides significantly stronger composable security guarantees than the traditional random oracle model of Bellare and Rogaway [CCS'93] or even the common reference string model. Indeed, these latter models provide no security guarantees in the presence of arbitrary protocols that use the {\em same} random oracle (or reference string or hash function).
We show that there exist commitment, zero-knowledge and general function evaluation pro-tocols with universally composable security, in a model where all parties and all protocols have access to a single, global, random oracle and no other trusted setup. This model provides sig-nificantly stronger composable security guarantees than the traditional random oracle model of Bellare and Rogaway [CCS’93] or even the common reference string model. Indeed, these latter models provide no security guarantees in the presence of arbitrary protocols that use the same random oracle (or reference string or hash function). Furthermore, our protocols are highly efficient. Specifically, in the interactive setting, our commitment and general computation protocols are much more efficient than the best known ones due to Lindell [Crypto’11,’13] which are secure in the common reference string model. In the non-interactive setting, our protocols are slightly less efficient than the best known ones presented by Afshar et al. [Eurocrypt ’14] but do away with the need to rely on a non-global (programmable) reference string. 1
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.
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
Universally Composable (UC) Commitment is a strong notion that guarantees security even when the commitment protocol is composed with arbitrary protocols running many of their copies in parallel. It is impossible to implement a protocol that realizes UC Commitment without set-up assumptions. However, it has been implemented using such assumptions as common reference string, certified public keys and random oracles. In this paper we prove that the existence of a binary symmetric channel between the parties makes possible the accomplishment of UC Commitment.
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.