# On the Security of Password-Authenticated Quantum Key Exchange

## TL;DR

This paper explores the possibility of unconditionally secure password-authenticated quantum key exchange, providing impossibility results for strong security and a positive construction achieving everlasting security in a realistic model.

## Contribution

It presents the first construction of quantum PAKE achieving everlasting security, addressing open questions about its feasibility.

## Key findings

- Impossibility results for very strong security in quantum PAKE
- A new quantum PAKE protocol with proven everlasting security
- The protocol is secure in the simulation-based model after execution

## Abstract

Motivated by the Quantum Key Distribution (QKD) protocol, introduced in 1984 in the seminal paper of Bennett and Brassard, we investigate in this paper the achievability of unconditionally secure password-authenticated quantum key exchange (quantum PAKE), where the authentication is implemented by the means of human-memorable passwords. We first show a series of impossibility results forbidding the achievement of very strong security, leaving open the feasibility of achieving a weaker security notion. We then answer this open question positively by presenting a construction for quantum PAKE that provably achieves everlasting security in the simulation-based model. Everlasting security is a security notion introduced by M\"uller-Quade and Unruh in 2007, which implies unconditional security after the execution of the protocol and only reduces the power of the adversary to be computational during the execution of the protocol, which seems quite a reasonable assumption for nowadays practical use-cases.