Introduced by Canetti in 2001, Universal Composability (UC) is a widely adopted security model that enables the specification and proof of security for a broad range of protocols, offering strong security guarantees. At its core lies the universal composition theorem (UC theorem), which ensures that protocols proven secure within the framework remain secure even when deployed in real-world environments with multiple instances of them. In this work, we present two key contributions. First, we identify several problems with the UC framework, in particular the UC Theorem. They include counterexamples, limitations that make it unusable for important classes of protocols, and weaknesses in its proof. These problems reveal flaws in nearly all the fundamental concepts of UC. Second, we propose a revised formulation of the main concepts of UC to address these issues. Although the resulting modifications are nontrivial, our updated definitions are designed to remain as faithful as possible to the structure and intent of the original model.
Introduced by Canetti in 2001, Universal Composability (UC) is a widely adopted security model that enables the specification and proof of security for a broad range of protocols, offering strong security guarantees. At its core lies the universal composition theorem (UC theorem), which ensures that protocols proven secure within the framework remain secure even when deployed in real-world environments with multiple instances of them. In this work, we present two key contributions. First, we identify several problems with the UC framework, in particular the UC Theorem. They include counterexamples, limitations that make it unusable for important classes of protocols, and weaknesses in its proof. These problems reveal flaws in nearly all the fundamental concepts of UC. Second, we propose a revised formulation of the main concepts of UC to address these issues. Although the resulting modifications are nontrivial, our updated definitions are designed to remain as faithful as possible to the structure and intent of the original model.
We put forward a framework for the modular design and analysis of multi-party protocols. Our framework is called “GNUC ” (with the recursive meaning “GNUC’s Not UC”), already alluding to the similarity to Canetti’s Universal Composability (UC) framework. In particular, like UC, we offer a universal composition theorem, as well as a theorem for composing protocols with joint state. We deviate from UC in several important aspects. Specifically, we have a rather different view than UC on the structuring of protocols, on the notion of polynomial-time protocols and attacks, and on corruptions. We will motivate our definitional choices by explaining why the definitions in the UC framework are problematic, and how we overcome these problems. Our goal is to offer a framework that is largely compatible with UC, such that previous results formulated in UC carry over to GNUC with minimal changes. We exemplify this by giving explicit formulations for several important protocol tasks, including authenticated and secure communication, as well as commitment and secure function evaluation. 1
The Universal Composability (UC) framework provides provable security guaranties for harsh application environment, where we want to construct protocols which keep security guarantees even when they are concurrently composed with arbitrary number of arbitrary (even hostile) protocols.This is a very strong guarantee.The UC-framework inherently supports the modular design, which allows secure composition of arbitrary number of UC-secure components with an arbitrary protocol.In contrast, traditional analysis and design is a stand alone analysis where security of a single instance is considered, i.e. an instance which is not in potential interaction with any concurrent instances.Furthermore, a typical traditional analysis is informal, i.e. without a formal proof.In spite of these facts, beyond the task of key-exchange this technology have not really took the attention of the community of applied cryptography.From practitioner's point of view the UC-world may seem more or less an academic interest of theoretical cryptographers.Accordingly we take a pragmatic approach, where we concentrate on meaningful compromises between the assumed adversarial strength, ideality wishes and realization complexity while keeping provable security guarantees within the UC-framework.We believe that even modest but provable goals (especially, if tunable to application scenarios) are interesting if a wider penetration of the UC-technology is desired into the daily-practice of protocol applications.
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.
Recently, Canetti and Krawczyk (Eurocrypt'2001) formu- lated a notion of security for key-exchange (ke) protocols, called SK- security, and showed that this notion suffices for constructing secure channels. However, their model and proofs do not suffice for proving more general composability properties of SK-secure ke protocols. We show that while the notion of SK-security is strictly weaker than a fully-idealized notion of key exchange security, it is sufficiently robust for providing secure composition with arbitrary protocols. In particular, SK-security guarantees the security of the key for any application that desires to set-up secret keys between pairs of parties. We also provide new definitions of secure-channels protocols with similarly strong com- posability properties, and show that SK-security suffices for obtaining these definitions. To obtain these results we use the recently proposed framework of uni- versally composable (UC) security. We also use a new tool, called non- information oracles, which will probably find applications beyond the present case. These tools allow us to bridge between seemingly limited indistinguishability-based definitions such as SK-security and more pow- erful, simulation-based definitions, such as UC security, where general composition theorems can be proven. Furthermore, based on such compo- sition theorems we reduce the analysis of a full-fledged multi-session key- exchange protocol to the (simpler) analysis of individual, stand-alone, key-exchange sessions.
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
We propose a modification to the framework of Universally Composable (UC) security [3]. Our new notion involves comparing the real protocol execution with an ideal execution involving ideal functionalities (just as in UC-security), but allowing the environment and adversary access to some super-polynomial computational power. We argue the meaningfulness of the new notion, which in particular subsumes many of the traditional notions of security. We generalize the Universal Composition theorem of [3] to the new setting. Then under new computational assumptions, we realize secure multi-party computation (for static adversaries) without a common reference string or any other set-up assumptions, in the new framework. This is known to be impossible under the UC framework.