After-gate attack on a quantum cryptosystem

TLDR

The authors propose a method to manipulate detection events in gated single‑photon detector QKD systems and suggest simple implementation changes to prevent such attacks. They use bright faked‑state pulses timed outside the detector gate to trigger detections, enabling an intercept‑resend attack, and experimentally test this on ID Quantique Clavis2 detectors to map the feasible parameter regime. The attack can evade detection by not raising the error rate, though afterpulses from accumulated carriers do increase errors, and the authors delineate the parameter regime where the attack remains viable.

Abstract

We present a method to control the detection events in quantum key distribution systems that use gated single-photon detectors. We employ bright pulses as faked states, timed to arrive at the avalanche photodiodes outside the activation time. The attack can remain unnoticed, since the faked states do not increase the error rate per se. This allows for an intercept–resend attack, where an eavesdropper transfers her detection events to the legitimate receiver without causing any errors. As a side effect, afterpulses, originating from accumulated charge carriers in the detectors, increase the error rate. We have experimentally tested detectors of the system id3110 (Clavis2) from ID Quantique. We identify the parameter regime in which the attack is feasible despite the side effect. Furthermore, we outline how simple modifications in the implementation can make the device immune to this attack.