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arXiv:1807.04233 · Scientific Reports

Unconditionally secured classical cryptography using quantum superposition and unitary transformation

Byoung Seung HAM

Published Jul 15, 2020• 20 citations
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Abstract

Over decades quantum cryptography has been intensively studied for unconditionally secured key distribution in a quantum regime. Due to the quantum loopholes caused by imperfect single photon detectors and/or lossy quantum channels, however, the quantum cryptography is practically inefficient and even vulnerable to eavesdropping. Here, a method of unconditionally secured key distribution potentially compatible with current fiber-optic communications networks is proposed in a classical regime for high-speed optical backbone networks. The unconditional security is due to the quantum superposition-caused measurement indistinguishability between paired transmission channels and its unitary transformation resulting in deterministic randomness corresponding to the no-cloning theorem in a quantum key distribution protocol.

DOI
10.1038/s41598-020-68038-7

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Over decades quantum cryptography has been intensively studied for unconditionally secured key distribution in a quantum regime. Due to the quantum loopholes caused by imperfect single photon detectors and/or lossy quantum channels, however, the quantum cryptography is practically inefficient and even vulnerable to eavesdropping. Here, a method of unconditionally secured key distribution potentially compatible with current fiber-optic communications networks is proposed in a classical regime for high-speed optical backbone networks. The unconditional security is due to the quantum superposition-caused measurement indistinguishability between paired transmission channels and its unitary transformation resulting in deterministic randomness corresponding to the no-cloning theorem in a quantum key distribution protocol.
Based on the detection loophole-free photon key distribution (PKD) compatible with classical optical systems, an optical key distribution (OKD) protocol is presented for unconditionally secured cryptography in fiber-optic communications networks using addressable continuous phase basis, where each communication channel is composed of paired transmission lines. The unconditional security in OKD lies in quantum superposition between the paired lines of each channel. The continuous phase basis in OKD can be applied for one-time-pad optical cryptography in networks, whose network address capacity is dependent upon the robustness of OKD to channel noises.
Based on the addressability of quantum superposition and its unitary transformation, a network-compatible, unconditionally secured key distribution protocol is presented for arbitrary networking in a classical regime with potential applications of one-time-pad cryptography. The network capability is due to the addressable unitary transformation between arbitrary point-to-point connections in a network through commonly shared double transmission channels. The unconditional security is due to address-sensitive eavesdropping randomness via network authentication. The proposed protocol may offer a solid platform of unconditionally secured classical cryptography for mass-data communications in a conventional network, which would be otherwise impossible.
So far, unconditional security in key distribution processes has been confined to quantum key distribution (QKD) protocols based on the no-cloning theorem of nonorthogonal bases. Recently, a completely different approach, the unconditionally secured classical key distribution (USCKD), has been proposed for unconditional security in the purely classical regime. Unlike QKD, both classical channels and orthogonal bases are key ingredients in USCKD, where unconditional security is provided by deterministic randomness via path superposition-based reversible unitary transformations in a coupled Mach-Zehnder interferometer. Here, the first experimental demonstration of the USCKD protocol is presented.
The last two decades have witnessed an exciting advanced research field that stems from non-classical atomic theory, the quantum mechanics. This research promises an interesting applicability in computation, known as quantum computation, and also in secure data communications, known as quantum cryptography. Quantum cryptography capitalizes on the inherent random polarization state of single photons, which are associated with binary logic values. Because the polarization state of a photon is not reproducible by an eavesdropper between the source and the destination, polarized photons are used with an intelligent algorithm to disseminate the cryptographic key with high security from the source to the destination, a process known as quantum key distribution. However, although the polarization state of a photon remains intact in free-space propagation, it does not remain so in dielectric medium and thus quantum cryptography is not problem-free. In this paper we review quantum cryptography and we identify the various steps in the quantum key identification process. We then analyze and discuss issues related to quantum key distribution that rise in pragmatic fiber-optic transmission and in communication network topologies. In addition, we identify a major weakness of the method that is prone to attacking and which incapacitates quantum cryptography in fiber communications.
The unconditionally secured classical key distribution (USCKD) has been presented for an air turbulent free-space optical link. Even in the noisy air channels, the USCKD has shown nearly unconditionally secure communications in data transmission owing to double unitary transformation in a round-trip scheme, where the absolute phase noises in both air channels are replaced by a relative phase noise between them. Unlike classical cryptography, USCKD is based on basis randomness and thus resistant to the attack of quantum computers. Unlike quantum key distributions, USCKD is free from nonclassical light requirement such as a single photon, which is sensitive to quantum loopholes of channel loss and imperfect detectors.
The last two decades have witnessed an exciting advanced research field that stems from non-classical atomic theory, quantum mechanics. This field promises an important applicability in secure data communications, known as quantum cryptography. Quantum cryptography takes advantage of the inherent random polarization state of single photons, which is not reproducible by a third party or an eavesdropper located between the source and the destination. Thus, when polarization states of photons are associated with binary logic an algorithm may intelligently developed according to which a cryptographic key is disseminated by the source terminal to the destination. This is a process known as quantum key distribution. However, as photons propagate in a non-linear medium such as fiber, their polarization state does not remain intact and thus the quantum key distribution and quantum cryptography becomes vulnerable to attacks. In this paper we consider the applicability of quantum cryptography in a pragmatic fiber-optic medium and in a popular communication network topology. We identify major weaknesses for each step of the quantum key distribution process, and also potential attacks to incapacitate quantum cryptography in fiber communications, so that better countermeasure strategies can be developed.