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Peer-reviewedCC BY 4.0PLOS One

Cryptanalysis of a controller-independent controlled bidirectional QSDC protocol

  • Chia-Wei Tsai1
  • Yao-Chung Chang2
  • Andy Su3
  • I-Chun Chen
  • Ying-Hsun Lai
  1. 1Department of Computer Science and Information Engineering, National Taichung University of Science and Technology, Taichung, Taiwan
  2. 2Department of Computer Science and Information Engineering, National Taitung University, Taitung, Taiwan
  3. 3Department of Industrial and Business Management, Open University of Kaohsiung, Kaohsiung, Taiwan
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Abstract

A recent protocol by Cai et al. [J. Korean Phys. Soc. (2026)] proposed a controller-independent controlled bidirectional quantum secure direct communication (CICBQSDC) scheme based on four-particle cluster states, claiming provable unconditional security via Wyner’s wiretap channel theory. In this paper, we demonstrate that the protocol is vulnerable to a quantum Trojan-horse attack that is entirely consistent with the threat model assumed by the original authors; namely, the controller Charlie is an untrusted party with the same capabilities as the strongest possible eavesdropper. Specifically, this paper shows that Charlie uses Trojan-horse probes to steal k0, k1, k2, and k3 independently, and then he combines the measurement results and initial states of the four-particle cluster states to completely recover all four secret message bits encoded by Alice and Bob. Crucially, this attack introduces no disturbance to the legitimate qubits and therefore produces no increase in the ZX or XZ error rates, rendering it undetectable by the protocol’s security-check mechanism. This paper concludes that the unconditional security claim of Cai et al. fails to hold because it relies on a critical blind spot: a fundamental contradiction between modeling the controller as a maximally powerful eavesdropper and implicitly restricting their capabilities to a side-channel-free, ideal single-photon framework. This result highlights the general inadequacy of wiretap-channel-based security proof in the absence of rigorous physical-layer constraints, underscoring the critical need for implementation security in future QSDC designs.

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Cite

BibTeXRIS
BibTeX
@article{tsai2026cryptanalysis,
  title = {{Cryptanalysis of a controller-independent controlled bidirectional QSDC protocol}},
  author = {Chia-Wei Tsai and Yao-Chung Chang and Andy Su and I-Chun Chen and Ying-Hsun Lai},
  journal = {PLOS One},
  year = {2026},
  month = aug,
  doi = {10.1371/journal.pone.0357035},
  url = {https://doi.org/10.1371/journal.pone.0357035},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Cryptanalysis of a controller-independent controlled bidirectional QSDC protocol
AU  - Chia-Wei Tsai
AU  - Yao-Chung Chang
AU  - Andy Su
AU  - I-Chun Chen
AU  - Ying-Hsun Lai
PY  - 2026
DA  - 2026/08/31
JO  - PLOS One
DO  - 10.1371/journal.pone.0357035
UR  - https://doi.org/10.1371/journal.pone.0357035
AB  - A recent protocol by Cai et al. [J. Korean Phys. Soc. (2026)] proposed a controller-independent controlled bidirectional quantum secure direct communication (CICBQSDC) scheme based on four-particle cluster states, claiming provable unconditional security via Wyner’s wiretap channel theory. In this paper, we demonstrate that the protocol is vulnerable to a quantum Trojan-horse attack that is entirely consistent with the threat model assumed by the original authors; namely, the controller Charlie is an untrusted party with the same capabilities as the strongest possible eavesdropper. Specifically, this paper shows that Charlie uses Trojan-horse probes to steal k0, k1, k2, and k3 independently, and then he combines the measurement results and initial states of the four-particle cluster states to completely recover all four secret message bits encoded by Alice and Bob. Crucially, this attack introduces no disturbance to the legitimate qubits and therefore produces no increase in the ZX or XZ error rates, rendering it undetectable by the protocol’s security-check mechanism. This paper concludes that the unconditional security claim of Cai et al. fails to hold because it relies on a critical blind spot: a fundamental contradiction between modeling the controller as a maximally powerful eavesdropper and implicitly restricting their capabilities to a side-channel-free, ideal single-photon framework. This result highlights the general inadequacy of wiretap-channel-based security proof in the absence of rigorous physical-layer constraints, underscoring the critical need for implementation security in future QSDC designs.
SN  - 1932-6203
ER  -