  We demonstrate the experimental feasibility of a Trojan-horse attack that
remains nearly invisible to the single-photon detectors employed in practical
quantum key distribution (QKD) systems, such as Clavis2 from ID Quantique. We
perform a detailed numerical comparison of the attack performance against
Scarani-Acin-Ribordy-Gisin (SARG04) QKD protocol at 1924nm versus that at
1536nm. The attack strategy was proposed earlier but found to be unsuccessful
at the latter wavelength, as reported in N.~Jain et al., New J. Phys. 16,
123030 (2014). However at 1924nm, we show experimentally that the noise
response of the detectors to bright pulses is greatly reduced, and show by
modeling that the same attack will succeed. The invisible nature of the attack
poses a threat to the security of practical QKD if proper countermeasures are
not adopted.
