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Quantum Information and Computation

Some attacks on quantum-based cryptographic protocols

LoHoi-Kwong, KoTsz-Mei

Published Jan 1, 2005• 1 citation
Publisher

Abstract

Quantum-based cryptographic protocols are often said to enjoy security guaranteed by the fundamental laws of physics. However, even carefully designed quantum-based cryptographic schemes may be sus...
DOI
10.5555/2011608.2011612

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Quantum-based cryptographic protocols are often said to enjoy security guaranteed by the fundamental laws of physics. However, even carefully designed quantum-based cryptographic schemes may be susceptible to subtle attacks that are outside the original design. As an example, we give attacks against a recently proposed ``secure communication using mesoscopic coherent states'', which employs mesoscopic states, rather than single-photon states. Our attacks can be used either as a known-plaintext attack or in the case where the plaintext has not been randomized. One of our attacks requires beamsplitters and the replacement of a lossy channel by a lossless one. It is successful provided that the original loss in the channel is so big that Eve can obtain $2^k$ copies of what Bob receives, where $k$ is the length of the seed key pre-shared by Alice and Bob. In addition, substantial improvements over such an exhaustive key search attack can be made, whenever a key is reused. Furthermore, we remark that, under the same assumption of a known or non-random plaintext, Grover's exhaustive key search attack can be applied directly to "secure communication using mesoscopic coherent states", whenever the channel loss is more than 50 percent. Therefore, as far as information-theoretic security is concerned, optically amplified signals necessarily degrade the security of the proposed scheme, when the plaintext is known or non-random. Our attacks apply even if the mesoscopic scheme is used only for key generation with a subsequent use of the key for one-time-pad encryption. Studying those attacks can help us to better define the risk models and parameter spaces in which quantum-based cryptographic schemes can operate securely. Finally, we remark that our attacks do not affect standard protocols such as Bennett-Brassard BB84 protocol or Bennett B92 protocol, which rely on single-photon signals.

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Quantum-based cryptographic protocols are often said to enjoy security guaranteed by the fundamental laws of physics. However, even carefully designed quantum-based cryptographic schemes may be susceptible to subtle attacks that are outside the original design. As an example, we give attacks against a recently proposed ``secure communication using mesoscopic coherent states'', which employs mesoscopic states, rather than single-photon states. Our attacks can be used either as a known-plaintext attack or in the case where the plaintext has not been randomized. One of our attacks requires beamsplitters and the replacement of a lossy channel by a lossless one. It is successful provided that the original loss in the channel is so big that Eve can obtain 2k2^k2k copies of what Bob receives, where kkk is the length of the seed key pre-shared by Alice and Bob. In addition, substantial improvements over such an exhaustive key search attack can be made, whenever a key is reused. Furthermore, we remark that, under the same assumption of a known or non-random plaintext, Grover's exhaustive key search attack can be applied directly to "secure communication using mesoscopic coherent states", whenever the channel loss is more than 50 percent. Therefore, as far as information-theoretic security is concerned, optically amplified signals necessarily degrade the security of the proposed scheme, when the plaintext is known or non-random. Our attacks apply even if the mesoscopic scheme is used only for key generation with a subsequent use of the key for one-time-pad encryption. Studying those attacks can help us to better define the risk models and parameter spaces in which quantum-based cryptographic schemes can operate securely. Finally, we remark that our attacks do not affect standard protocols such as Bennett-Brassard BB84 protocol or Bennett B92 protocol, which rely on single-photon signals.
The fundamental security of cryptographic protocols hinges on two intertwined pillars: secrecy and randomness. In classical cryptography, the relationship between these concepts is well-understood, often formalized through information theory and computational complexity. However, the advent of quantum computing poses a significant threat to many classical cryptographic primitives, necessitating a transition to post-quantum cryptography (PQC). This paper explores the critical notion of the secrecy-randomness equivalence within the context of post-quantum protocols. We delve into how the definitions and relationships between perfect secrecy, computational secrecy, true randomness, and pseudorandomness are impacted by quantum adversaries and quantum information theory. Specifically, we investigate whether the classical equivalence – where sufficient randomness can guarantee secrecy and robust secrecy often relies on high-quality randomness – holds or requires re-evaluation in a post-quantum landscape. We analyze the implications for the design and security proofs of various PQC families, including lattice-based, code-based, and hash-based schemes, considering the role of quantum random number generators (QRNGs) and the potential for quantum-enhanced attacks on randomness. Our findings suggest that while the core principles remain, the practical realization and theoretical guarantees of this equivalence become more nuanced, demanding careful consideration of quantum-resistant randomness sources and revised security models to ensure the long-term integrity of post-quantum cryptographic systems.