On the vulnerability of the swiss system of coherent quantum cryptography to an attack with repeated measurements

It has been shown that the coherent quantum cryptography protocol (Coherent One Way) and, correspondingly, fiber optic systems involving this protocol for quantum key distribution, are vulnerable to an attack with repeated measurements and do not guarantee the security of distributed keys in a communication channel with losses. The coherent quantum cryptography system is used in Switzerland as one of the key distribution channels in the framework of the network project SECOQC (SEcure COmmunications based on Quantum Cryptography). A critical attack with repeated measurements was missed when the cryptographic strength of this protocol was analyzed. The critical length of the communication channel has been determined; this is a value above which secure key distribution is certainly impossible. Beginning with the critical length, an eavesdropper knows the entire distributed key, does not introduce errors at the receiver end, and remains undetected. For typical parameters in a real system (the average photon number $\mu=0.5$ and the quantum efficiency of avalanche detectors $\eta=0.1$, see N. Gisin, G. Ribordy, H. Zbinden, et al., arXiv: quant-ph/0411022 and D. Stucki, C. Barreiro, S. Fasel, et al., arXiv: 0809.5264), the security of keys cannot be guaranteed even for a communication channel whose length is as small as wished.