Imperfect preparation and Trojan attack on the phase modulator in the decoy-state BB84 protocol

TL;DR

This paper analyzes the security vulnerabilities of decoy-state BB84 quantum key distribution protocols against Trojan horse attacks on phase modulators, considering imperfect state preparation and proposing countermeasures to maintain security over long distances.

Contribution

It introduces a generalized Trojan attack model for arbitrary probing pulses and derives conservative bounds on information leakage, enhancing security analysis of practical QKD systems.

Findings

Quantum coin imbalance remains low with new attack model.

Secure key rate is achievable over 100 km fiber channels.

Passive hardware countermeasures improve security robustness.

Abstract

Quantum key distribution (QKD) provides a theoretically secure method for cryptographic key exchange by leveraging quantum mechanics, but practical implementations face vulnerabilities such as Trojan horse attack on phase modulators. This work analyzes the security of QKD systems under such attacks, considering both ideal and imperfect state preparation scenarios. The Trojan attack model is generalized to arbitrary states of probing pulses and conservative bounds of information leakage through side-channel of special form are introduced. The quantum coin imbalance, a critical security parameter, remains low (on the order of $10^{-7}$ for ideal state preparation and $10^{-5}$ for imperfect preparation) with this new approach and presence additional hardware passive countermeasures. Numerical simulations confirm nonzero secure key rate at distances over 100 km through optical fiber channel.