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Optics Communications

Lower bounds for the security of modified coherent-one-way quantum key distribution against one-pulse-attack

Hong-Wei Li, Zhen−Qiang Yin, Shuang Wang, Wei Chen, Zheng‐Fu Han, Wan‐Su Bao, Guang‐Can Guo

Published Oct 15, 2010• 1 citation
Publisher
DOI
10.1016/j.optcom.2010.09.081

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Quantum key distribution (QKD) offers an unprecedented level of security, which, unlike its classical analog, is not based on computational assumptions. But tight lower and upper security bounds are still not provided for some QKD protocols. Here, we propose an effective attack strategy for coherent one-way (COW) QKD protocol. It is a generalized photon-number splitting (PNS) attack combined with sequential unambiguous state discrimination (USD) attack. We discuss the possibility of a generalized PNS attack because this attack demonstrates efficiency and does not introduce error, while previously the COW protocol was considered secure against PNS attack. To our knowledge, it is the most efficient attack on the COW protocol to date, hence we sharpen the upper security bound for this protocol.
The differential-phase-shift (DPS) and the coherent-one-way (COW) are among the most practical protocols for quantum cryptography, and are therefore the object of fast-paced experimental developments. The assessment of their security is also a challenge for theorists: the existing tools, that allow to prove security against the most general attacks, do not apply to these two protocols in any straightforward way. We present new upper bounds for their security in the limit of large distances ( d ≳50 km with typical values in optical fibers) by considering a large class of collective attacks, namely those in which the adversary attaches ancillary quantum systems to each pulse or to each pair of pulses. We introduce also two modified versions of the COW protocol, which may prove more robust than the original one.
Abstract Coherent-one-way (COW) quantum key distribution (QKD) held the promise of distributing secret keys over long distances with a simple experimental setup. Indeed, this scheme is currently used in commercial applications. Surprisingly, however, it has been recently shown that its secret key rate scales at most quadratically with the system’s transmittance and, thus, it is not appropriate for long distance QKD transmission. Such pessimistic result was derived by employing a so-called zero-error attack, in which the eavesdropper does not introduce any error, but still the legitimate users of the system cannot distill a secure key. Here, we present an improved zero-error attack against COW-QKD that significantly outperforms previous approaches, and provides stronger restrictions on its maximum achievable distance in the absence of errors. This translates into an upper bound on its secret key rate that is more than an order of magnitude lower than previously known upper bounds.