# Gaussian two-mode attacks in one-way quantum cryptography

## TL;DR

This paper analyzes the security of one-way continuous variable quantum key distribution against Gaussian two-mode coherent attacks, revealing that such attacks can significantly increase the eavesdropper's potential information gain.

## Contribution

It introduces a security analysis framework for two-mode Gaussian attacks in one-way quantum cryptography, showing these attacks can outperform single-mode attacks.

## Key findings

- Two-mode Gaussian correlations increase eavesdropper's information.
- Asymptotic secret key rates are higher under two-mode Gaussian attacks.
- Security analysis reduces to studying two-mode coherent attacks due to symmetry.

## Abstract

We investigate the asymptotic security of one-way continuous variable quantum key distribution against Gaussian two-mode coherent attacks. The one-way protocol is implemented by arranging the channel uses in two-mode blocks. By applying symmetric random permutations over these blocks, the security analysis is in fact reduced to study two-mode coherent attacks and, in particular, Gaussian ones, due to the extremality of Gaussian states. We explicitly show that the use of two-mode Gaussian correlations by an eavesdropper leads to asymptotic secret key rates which are strictly larger than the rate obtained under standard single-mode Gaussian attacks.