Two-Party Quantum Protocols Do Not Compose Securely Against Honest-But-Curious Adversaries

TL;DR

This paper demonstrates that in the model of two-party quantum computation, only trivial protocols are weakly self-composable against honest-but-curious adversaries, revealing limitations in secure quantum protocol composition.

Contribution

It introduces a framework to analyze composability in two-party quantum protocols and proves that non-trivial protocols cannot be securely composed in this setting.

Findings

Non-trivial protocols lack weak self-composability against honest-but-curious adversaries.

A payoff gap exists between coherent and local strategies, indicating composability limitations.

Only trivial protocols are weakly self-composable in the considered quantum model.

Abstract

In this paper, we build upon the model of two-party quantum computation introduced by Salvail et al. [SSS09] and show that in this model, only trivial correct two-party quantum protocols are weakly self-composable. We do so by defining a protocol \Pi, calling any non-trivial sub-protocol \pi N times and showing that there is a quantum honest-but-curious strategy that cannot be modeled by acting locally in every single copy of \pi. In order to achieve this, we assign a real value called "payoff" to any strategy for \Pi and show that that there is a gap between the highest payoff achievable by coherent and local strategies.