Position-Based Quantum Cryptography: Impossibility and Constructions

TL;DR

This paper explores the possibilities and limitations of position-based quantum cryptography, demonstrating that entanglement sharing by adversaries prevents secure verification, but security is possible without entanglement, raising questions about bounded entanglement scenarios.

Contribution

It proves the impossibility of secure position-verification with unlimited entanglement and presents a protocol for secure verification without entanglement, advancing understanding of quantum position-based cryptography.

Findings

Unlimited entanglement breaks all position-verification schemes.

Secure position-verification is possible without shared entanglement.

Potential for secure position-based cryptographic applications.

Abstract

In this work, we study position-based cryptography in the quantum setting. The aim is to use the geographical position of a party as its only credential. On the negative side, we show that if adversaries are allowed to share an arbitrarily large entangled quantum state, no secure position-verification is possible at all. We show a distributed protocol for computing any unitary operation on a state shared between the different users, using local operations and one round of classical communication. Using this surprising result, we break any position-verification scheme of a very general form. On the positive side, we show that if adversaries do not share any entangled quantum state but can compute arbitrary quantum operations, secure position-verification is achievable. Jointly, these results suggest the interesting question whether secure position-verification is possible in case of a bounded amount of entanglement. Our positive result can be interpreted as resolving this question in the simplest case, where the bound is set to zero. In models where secure positioning is achievable, it has a number of interesting applications. For example, it enables secure communication over an insecure channel without having any pre-shared key, with the guarantee that only a party at a specific location can learn the content of the conversation. More generally, we show that in settings where secure position-verification is achievable, other position-based cryptographic schemes are possible as well, such as secure position-based authentication and position-based key agreement.