On the optimality of individual entangling-probe attacks against BB84 quantum key distribution

TL;DR

This paper demonstrates that an optimal individual attack on the BB84 quantum key distribution protocol can be explicitly constructed, achieving the Luetkenhaus bound and clarifying theoretical misconceptions.

Contribution

It adapts existing methods to include error correction, explicitly finds an optimal attack, and clarifies its consistency with established BB84 attack theory.

Findings

Optimal attack fills Luetkenhaus bound

Attack remains optimal with error correction

Clarifies no contradiction with old-style BB84 theory

Abstract

It is shown that an existing method to study ideal individual attacks on the BB84 QKD protocol using error discard can be adapted to reconciliation with error correction, and that an optimal attack can be explicitly found. Moreover, this attack fills Luetkenhaus bound, independently of whether error positions are leaked to Eve, proving that it is tight. In addition, we clarify why the existence of such optimal attacks is not in contradiction with the established ``old-style'' theory of BB84 individual attacks, as incorrectly suggested recently in a news feature.