# Asymmetric delay attack on an entanglement-based bidirectional clock synchronization protocol

## TL;DR

This paper presents an attack on a quantum-based bidirectional clock synchronization protocol that exploits asymmetric delays introduced by optical circulators, causing errors in synchronization without detection.

## Contribution

It reveals a vulnerability in entanglement-based clock synchronization protocols by demonstrating how asymmetric delays can be exploited to induce errors.

## Key findings

- The attack successfully causes synchronization errors.
- The attack evades detection mechanisms.
- Optical circulators can be manipulated to break reciprocity.

## Abstract

We demonstrate an attack on a clock synchronization protocol that attempts to detect tampering of the synchronization channel using polarization-entangled photon pairs. The protocol relies on a symmetrical channel, where propagation delays do not depend on propagation direction, for correctly deducing the offset between clocks -- a condition that could be manipulated with optical circulators, which rely on static magnetic fields to break the reciprocity of propagating electromagnetic fields. Despite the polarization transformation induced within a set of circulators, our attack creates an error in time synchronization while successfully evading detection.