How much security does Y-00 protocol provide us?

Abstract

New quantum cryptography, often called Y-00 protocol, has much higher performance than the conventional quantum cryptographies. It seems that the conventional quantum cryptographic attacks are inefficient at Y-00 protocol as its security is based on the different grounds from that of the conventional ones. We have, then, tried to cryptoanalyze Y-00 protocol in the view of cryptographic communication system. As a result, it turns out that the security of Y-00 protocol is equivalent to that of classical stream cipher.PACS numbers: 03.67. Dd, 42.50.Lc, 42.50.Ar Quantum cryptography has appeared as a promising way to achieve security without depending on any computational complexity assumption. However, most of the proposed schemes up to date are based on single photon states (QCSPS) thus, presenting a well-known negative characteristic, namely, its bit rate is much slower than that of normal optical communication systems.Recently a quantum cryptography scheme which uses mesoscopic coherent states has been reported [1,2,3,4]. This scheme has much higher performance than conventional ones which are based on single photon states [5,6,7]. Because this scheme based on mesoscopic coherent states, often called "Y-00 protocol [8]," has an average photon number of 100-1,000 photons per pulse, its bit rate is expected to be 100-1,000 times faster than that of QCSPS. In addition, the required technical level to realize the protocol is supposed to be quite the same as in conventional optical systems. Y-00 protocol would be, then, a sufficiently fast and easily realizable quantum cryptography scheme, if it had actually perfect security.In this paper, we show that the Y-00 protocol does not provide perfect security, even against the simplest of cryptographic attacks, ciphertext-only ones. A usual cryptographic system consists of two channels ( Fig.1): an open channel for exchanging encrypted messages and a secure channel for key distribution. Quantum cryptography based protocols, including Y-00 protocol, provide a realization of the secure channel for key distribution. Let us underline that our attack targets are not only the secure channel, but also the open channel for messages.We also show that the security of Y-00 protocol is just equivalent to that of a classical stream cipher. In other words, we can safely say that Y-00 protocol has no perfect security and that its security depends on just a common computational complexity assumption, being then no better than currently used schemes. Y-00 protocol is a quantum key expansion (QKE) scheme. Both Alice and Bob must share a secret key K s in advance. Notice that, even conventional quantum cryptography can be regarded as quantum key expansion schemes, since a short key is needed for the authentication of the classical channel. Y-00 protocol uses the secret key for quantum modulation and de-modulation in a quantum channel for expansion of the key.Y-00 protocol has, then, 2M non-orthogonal coherent states called "qumodes" :where α ∈ C and k ∈ {0, . . . , 2M − 1}, orin the polar...

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