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Questions tagged [universal-composability]

The framework of universal composability (UC) is a general-purpose model for the analysis of cryptographic protocols.

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Session IDs in (simple) universal composability

In the simple UC framework paper, there is a brief discussion on session identifiers (sids) on page 13. It states The main protocol is given a session identifier sid but it is not entirely clear to ...
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How to write proofs for universal composable security?

Recently, I learn Ran Canetti's famous paper, "Universally Composable Security: A New Paradigm for Cryptographic Protocols". But I find it very difficult to grasp. When I read the paper that ...
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Failure of parallel composition

Is there a simple example of a cryptographic protocol that is secure when used on its own (with only one instance active at a time) but becomes insecure in a concurrent setting, i.e., a failure of ...
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2answers
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Why does the PAKE ideal functionallity allow the keys to be the same when the passwords differ?

My intuition for the security a symmetric PAKE is supposed to provide comes from the example of a login page. Both the user and the server know the password (assuming unhashed passwords), and the ...
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1answer
50 views

Is the following PAKE protocol UC secure?

Consider the following (simplification of a) PAKE protocol: Alice and Bob start with a pre-agreed password pw. To establish a new session key $k$, first Alice samples a random nonce $r$ and sends it ...
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320 views

What is universal composability guaranteeing, specifically? Where does it apply, and where does it not?

I don't have a proper computer science education, so bear with my misunderstandings. UC is supposed to "guarantee strong security properties". From what I stand, if you have some secure ...
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UC-secure commitment in the ROM

Looking at the question Is this a UC-secure commitment scheme in the ROM?, I was wondering, is the commitment scheme proposed in the answer (which uses $H(M||r)$ as commit) secure in the UC-model, ...
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2answers
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Simulation-based proofs and universal composability proofs

I recently read Ran Canetti's famous UC paper but I'm still trying to wrap my head around the concepts. I think this answer has me confused a bit, particularly where it says The stand-alone ...
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50 views

Universal composability: Can an ideal functionality call other ideal functionalities?

I'm new to universal composability. I'm trying to define a protocol, $\pi$, in UC. The protocol involves 3 parties: A, B, and a smart contract $C$. Parties A and B interact with each other and with $C$...
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2answers
158 views

What is the difference between a cryptographic primitive and a functionality?

In the context of papers using the UC framework, I have seen the same cryptographic tools refereed as cryptographic primitives and functionalities. Are the two terms interchangeable? An example is ...
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1answer
72 views

What is the border between an ideal functionality and a protocol?

In the context of the UC framework, what is the border between an ideal functionality and a protocol? It seems to me that it depends on the level of granularity required for proofs. For example, ...
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37 views

Abstract Cryptography: why local simulators?

In the paper that introduced Abstract Crypto [MauRen11], the Theorem 2 page 15 states that, basically, if there exists some local simulators that can be pluged on a resource $S$ such that this new ...
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1answer
83 views

Extract adversary's secret input in simulation based security proofs

I am trying to understand the simulation-based security proofs (as well as the UC framework), I find that there is a basic assumption when proving the security, i.e., the simulator could extract the ...
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1answer
87 views

Example of a problem for which Zero-Knowledge proof is not sequentially composable

There are notions that a Zero-knowledge proof with auxiliary input is necessary for sequential composition. However, the authors here give a very convoluted example of a system which is not ...
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1answer
92 views

Where was the stand-alone model for simulation-based MPC proposed first?

The stand-alone model and the UC model for simulation-based proofs in multiparty computation differ in the guarantees that they provide: The stand-alone model provides security under sequential ...
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140 views

Why is this functionality chosen for UC-NIZK?

A correct proof (a proof which is indistinguishable from previously sent proof) can be tagged as wrong by the ideal functionality here if the adversary sends no witness. And, let's say P is a party ...
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1answer
91 views

What does “constant rate” mean in universal composable commitment scheme?

I'm wondering what does the "constant rate" mean in universal composable commitment scheme? I have known the rate of a commitment scheme is message length divided by the communication complexity of ...
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Examples of protocols that are insecure when run concurrently

I was reading Canetti00 Universally Composable security paper. The first page of introduction says that there are some MPC protocols and Zero knowledge protocols that are insecure under concurrent ...
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1answer
114 views

Variants of universal composability in security proofs

Universal composability (UC) framework seems to be a powerful framework for proving security of protocols, which guarantees security even in the presence of concurrent composition. Though, I see there ...
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1answer
156 views

What are limitations of Universally Composable (UC) framework?

I am beginning to read some UC framework papers, and I guess it is similar to the simulation-based security analysis. I can understand the main advantage of UC is to assist us to analyze the ...
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45 views

Can ideal functionalities share variables

I am trying to define an ideal world for a protocol that requires two functionalities to interact with each other. Lets call them $\mathcal{F}_1$ and $\mathcal{F}_2$. Now $\mathcal{F}_1$ maintains a ...
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48 views

Efficient universally composable protocols for aggregate statistics

Let's say we have multiple parties $P_1, \dots P_n$ that gather personal data, and a party $C$ that is interested in aggregate statistics on these data, i.e. the average value. An ideal functionality ...
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1answer
106 views

UC-framework: Simulating specific cases (proof in PVW)

I am struggling to understand the proof of security in the UC-framework of the OT by PVW. The cases I don't understand are the cases where: i) Both parties are corrupted. In this case, they say ...
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1answer
122 views

Difference in the Powers of Simulator and Environment in Universal Composability

I know that in UC model, environment controls the real world adversaries but there is still a simulator wrapped around these real-world adversaries in the ideal world (which we sometimes collectively ...
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47 views

Proofs of security for anonymity in a multi-party setting

I'm looking for material that proves the anonymity of parties in a non-interactive setting. I would like to know if there are simulation based proofs (UC) and property based proofs (game based) for ...
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2answers
320 views

passive corruption ?= Byzantine corruption in the UC framework?

Notations: We follow the convention in the UC framework. We use $\mathcal{A}$ to denote the adversary, $\mathcal{P}$ to denote a party in the model. We focus on two types of corruption in the UC ...
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1answer
304 views

Is this a UC-secure commitment scheme in the ROM?

To prove UC-security (universally composable security) of a commitment scheme, we must show that a commitment scheme is extractable and equivocal. That is, we must construct a simulator that is able ...
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105 views

Are zkSTARKs simulation sound extractable in the random oracle model?

I've recently become interested in simulation sound extractable zero-knowledge proofs for use in UC-secure protocols. I've also become interested in the recent instantiation of PCIPs called zkSTARKs. ...
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46 views

Are theorems 2 and 3 in the Town Crier paper (Zhang et al, 2016) proven using the UC framework?

I am reading the paper ‘Town Crier: An Authenticated Data Feed for Smart Contracts’ by Zhang et al (2016), but there is something I just cannot wrap my head around. In Section 7 and Appendix C, it ...
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1answer
533 views

Does simulation extractability require that the protocol have a backdoor in it?

As I understand it, one of the properties required to prove UC-security is simulation extractability. One example is the paper by Kosba et al adapting NIZKs to the UC model, but in their construction ...
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2answers
267 views

Modelling broadcast functionality in Universally Composable Framework

I'm studying the UC framework by Canetti. While I understand the basic idea, there are still a lot of details that I need to work through. One thing that's puzzling me is how to achieve concurrency ...
3
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1answer
163 views

Why would someone prove security in the real-ideal model instead of Universal Composibility framework?

I am somewhat confused in the reasons why one would use the real-ideal model instead of the universal composibility framework. The UC framework seems to me, to provide a much stronger definition of ...
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1answer
247 views

Simplified universal composability (SUC): can I model the use of encryption? Can I prove fairness?

I have been reading Canetti, Cohen and Lindell's paper, "A Simpler Variant of Universally Composable Security for Standard Multiparty Computation", and there are some parts that I didn't understand. ...
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1answer
127 views

Is a composition of clocked entities a clocked entity in the UC model?

In the book titled "Secure Multiparty Computation and Secret Sharing" the authors define an interactive agent and an interactive system. Further, the author defines a clocked entity for considering ...
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1answer
816 views

What are the roles of the simulator in simulation based proofs

I'm trying to better understand simulation based proofs in the UC model - but the guidelines to construct a simulator confuse me. To my understanding, the simulator is activated in two ways: Outbound:...
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1answer
606 views

What's the principle to design the functionality under UC framework?

Though I can get the basic idea of UC Framework, namely let the environment cannot distinguish the real protocol with the real adversary and the functionality with the simulator, I am still puzzled in ...
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1answer
228 views

Protocol composition [closed]

I have been trying to wrap my head around different definitions of protocol security (stand alone, sequential composition, parallel composition, universal composition) and how the proofs of any of ...
2
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1answer
2k views

Security of cloud computational protocols in UC Framework?

The universal composability allows one to the analyze security of cryptographic protocols . But it does have some gaps when it comes to analyzing few protocols especially two party cases when there is ...
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55 views

Detail about reactive simulatability framework

I'm trying to understand the framework of reactive simulatability using *"PfWa2_00AsyncModel"* from this publication which is a tech report behind the paper A model for Asynchronous Reactive Systems ...