Trojan-horse attacks on quantum key distribution with classical Bob

Recently, Boyer et al. (Phys Rev Lett 99:140501, 2007) introduced a conceptually novel semi-quantum key distribution scheme (BKM07). Tan et al. (Phys Rev Lett 102:098901, 2009) showed that classical Bob is unable to detect Eve's eavesdropping by giving a special implementation of BKM07 protocol. In the reply, Boyer et al. (Phys Rev Lett 102:098902, 2009) gave a solution against the eavesdropping, i.e., Bob may place a filter that allows only photons with approximately specific frequency to pass just at the expected time . However, their improvement contradicts the descriptions about classical. If the assumption of classical is not considered, we give a delay-photon Trojan-horse attack on BKM07 protocol and its improvement and further present a possible improvement.