It is well known that universally composable multiparty computation cannot, in general, be achieved in the standard model without setup assumptions when the adversary can corrupt an arbitrary number of players. One way to get around this problem is by having a trusted third party generate some global setup such as a common reference string (CRS) or a public key infrastructure (PKI). Recently, an alternative solution was proposed by Katz in [K07], suggesting that one may rely on physical assumptions rather than trusted third parties. Concretely, the solution assumed it physically possible to construct tamper proof hardware tokens which can be run in complete isolation from the surrounding environment. Here we improve upon the work of [K07] by constructing a scheme in which the tokens only need to be partially isolated and may have some limited communication with the environment. In addition we improve on Katz’s work by presenting a scheme which is secure against adaptive adversaries and is based on general cryptographic assumptions. We also consider an alternative scenario, in which there are some trusted third parties but no single such party is trusted by all of the players. This compromise allows us to limit the use of the physical set-up and hence might be preferred in practice.
It is well known that universally composable multiparty computation cannot, in general, be achieved in the standard model without setup assumptions when the adversary can corrupt an arbitrary number of players. One way to get around this problem is by having a trusted third party generate some global setup such as a common reference string (CRS) or a public key infrastructure (PKI). Recently, an alternative solution was proposed by Katz in [K07], suggesting that one may rely on physical assumptions rather than trusted third parties. Concretely, the solution assumed it physically possible to construct tamper proof hardware tokens which can be run in complete isolation from the surrounding environment. Here we improve upon the work of [K07] by constructing a scheme in which the tokens only need to be partially isolated and may have some limited communication with the environment. In addition we improve on Katz’s work by presenting a scheme which is secure against adaptive adversaries and is based on general cryptographic assumptions. We also consider an alternative scenario, in which there are some trusted third parties but no single such party is trusted by all of the players. This compromise allows us to limit the use of the physical set-up and hence might be preferred in practice.
The Universal Composability framework was introduced by Canetti to study the security of protocols which are concurrently executed with other protocols in a network environment. Unfortunately it was shown that in the so called plain model, a large class of functionalities cannot be securely realized. These severe impossibility results motivated the study of other models involving some sort of setup assumptions, where general positive results can be obtained. Until recently, all the setup assumptions which were proposed required some trusted third party (or parties). Katz recently proposed using a physical setup to avoid such trusted setup assumptions. In his model, the physical setup phase includes the parties exchanging tamper proof hardware tokens implementing some functionality. The tamper proof hardware is modeled so as to assume that the receiver of the token can do nothing more than observe its input/output characteristics. It is further assumed that the sender knows the program code of the hardware token which it distributed. Based on the DDH assumption, Katz gave general positive results for universally composable multi-party computation tolerating any number of dishonest
Universally composable (UC) protocols retain their security properties even when run concurrently alongside arbitrary other protocols. Unfortunately, it is known that UC multiparty computation (for general functionalities, and without assuming honest majority) is impossible without some form of setup. To circumvent this impossibility, various complete setup assumptions have been proposed. With only a few exceptions, past work has viewed these setup assumptions as being implemented by some ideal, incorruptible entity. Any such entity is thus a single point of failure, and security fails catastrophically in case the setup entity is subverted by an adversary. We propose here a clean, general, and generic approach for distributing trust among m arbitrary setups, by modeling potential corruption of setups within the UC framework, where such corruption might be fail-stop, passive, or arbitrary and is in addition to possible corruption of the parties themselves. We show several feasibility and impossibility results in this model, for different specifications of the corruptible sets. For example, we show that given m complete setups, up to t of which might be actively corrupted in an adaptive manner, general multiparty computation with no honest majority is possible if and only if t < m/2.
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.
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.
We show how to securely realize any multi-party functionality in a universally composable way, regardless of the number of corrupted participants. That is, we consider a multi-party network with open communication and an adversary that can adaptively corrupt as many parties as it wishes. In this setting, our protocols allow any subset of the parties (with pairs of parties being a special case) to securely realize any desired functionality of their local inputs, and be guaranteed that security is preserved regardless of the activity in the rest of the network. This implies that security is preserved under concurrent composition of an unbounded number of protocol executions, it implies non-malleability with respect to arbitrary protocols, and more. Our constructions are in the common reference string model and make general intractability assumptions.