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Universally Composable Commitments.

Ran Canetti, Marc Fischlin

Published Jan 1, 2001• 2 citations
OpenAlex

Abstract

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.

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Universally Composable Protocols with Relaxed Set-Up Assumptions

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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.

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Highly-Efficient Universally-Composable Commitments based on the DDH Assumption.

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Universal composability (or UC security) provides very strong security guarantees for protocols that run in complex real-world environments. In particular, security is guaranteed to hold when the protocol is run concurrently many times with other secure and possibly insecure protocols. Commitment schemes are a basic building block in many cryptographic constructions, and as such universally composable commitments are of great importance in constructing UC-secure protocols. In this paper, we construct highly efficient UC-secure commitments from the standard DDH assumption, in the common reference string model. Our commitment stage is non-interactive, has a common reference string with O(1) group elements, and has complexity of O(1) exponentiations for committing to a group element (to be more exact, the effective cost is that of 23 1 exponentiations overall, for both the commit and decommit stages). We present 3 a construction that is secure in the presence of static adversaries, and a construction that is secure in the presence of adaptive adversaries with erasures, where the latter construction has an effective additional cost of just 5 1 3 exponentiations. 1

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On Possibility of Universally Composable Commitments Based on Noisy Channels

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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.

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Obtaining Universally Composable Security: Towards the Bare Bones of Trust.

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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.

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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

•Jan 1, 2004•0 citations

Non-interactive and reusable non-malleable commitment schemes

Ivan Damgård, Jens Groth

We consider non-malleable (NM) and universally composable (UC) commitment schemes in the common reference string (CRS) model. We show how to construct non-interactive NM commitments that remain non-malleable even if the adversary has access to an arbitrary number of commitments from honest players - rather than one, as in several previous schemes. We show this is a strictly stronger security notion. Our construction is the first non-interactive scheme achieving this that can be based on the minimal assumption of existence of one-way functions. But it can also be instantiated in a very efficient version based on the strong RSA assumption. For UC commitments, we show that existence of a UC commitment scheme in the CRS model (interactive or not) implies key exchange and - for a uniform reference string - even implies oblivious transfer. This indicates that UC commitment is a strictly stronger primitive than NM. Finally, we show that our strong RSA based construction can be used to improve the most efficient known UC commitment scheme so it can work with a CRS of size independent of the number of players, without loss of efficiency.

ACM Symposium on Theory of Computing (STOC)•Jan 1, 2003•14 citations
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.
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.
Universal composability (or UC security) provides very strong security guarantees for protocols that run in complex real-world environments. In particular, security is guaranteed to hold when the protocol is run concurrently many times with other secure and possibly insecure protocols. Commitment schemes are a basic building block in many cryptographic constructions, and as such universally composable commitments are of great importance in constructing UC-secure protocols. In this paper, we construct highly efficient UC-secure commitments from the standard DDH assumption, in the common reference string model. Our commitment stage is non-interactive, has a common reference string with O(1) group elements, and has complexity of O(1) exponentiations for committing to a group element (to be more exact, the effective cost is that of 23 1 exponentiations overall, for both the commit and decommit stages). We present 3 a construction that is secure in the presence of static adversaries, and a construction that is secure in the presence of adaptive adversaries with erasures, where the latter construction has an effective additional cost of just 5 1 3 exponentiations. 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.
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
We consider non-malleable (NM) and universally composable (UC) commitment schemes in the common reference string (CRS) model. We show how to construct non-interactive NM commitments that remain non-malleable even if the adversary has access to an arbitrary number of commitments from honest players - rather than one, as in several previous schemes. We show this is a strictly stronger security notion. Our construction is the first non-interactive scheme achieving this that can be based on the minimal assumption of existence of one-way functions. But it can also be instantiated in a very efficient version based on the strong RSA assumption. For UC commitments, we show that existence of a UC commitment scheme in the CRS model (interactive or not) implies key exchange and - for a uniform reference string - even implies oblivious transfer. This indicates that UC commitment is a strictly stronger primitive than NM. Finally, we show that our strong RSA based construction can be used to improve the most efficient known UC commitment scheme so it can work with a CRS of size independent of the number of players, without loss of efficiency.