{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T18:20:38Z","timestamp":1788200438937,"version":"build-2803163510"},"reference-count":18,"publisher":"Public Library of Science (PLoS)","issue":"8","license":[{"start":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T00:00:00Z","timestamp":1788134400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100020595","name":"National Science and Technology Council","doi-asserted-by":"publisher","award":["114-2410-H-143-006-MY3"],"award-info":[{"award-number":["114-2410-H-143-006-MY3"]}],"id":[{"id":"10.13039\/100020595","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Ocean Affairs Council"},{"DOI":"10.13039\/100020595","name":"National Science and Technology Council","doi-asserted-by":"publisher","award":["114-2221-E-025 -006 -MY2"],"award-info":[{"award-number":["114-2221-E-025 -006 -MY2"]}],"id":[{"id":"10.13039\/100020595","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["www.plosone.org"],"crossmark-restriction":false},"short-container-title":["PLoS One"],"abstract":"\n A recent protocol by Cai et al. [\n J. Korean Phys. Soc.<\/jats:italic>\n (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\u2019s 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\n \n \n \n \n \n \n \n \n k<\/mml:mi>\n <\/mml:mrow>\n <\/mml:mrow>\n \n \n 0<\/mml:mtext>\n <\/mml:mrow>\n <\/mml:mrow>\n <\/mml:msub>\n ,<\/mml:mo>\n \u00a0<\/mml:mtext>\n \n \n \n k<\/mml:mi>\n <\/mml:mrow>\n <\/mml:mrow>\n \n \n 1<\/mml:mtext>\n <\/mml:mrow>\n <\/mml:mrow>\n <\/mml:msub>\n ,<\/mml:mo>\n \u00a0<\/mml:mtext>\n \n \n \n k<\/mml:mi>\n <\/mml:mrow>\n <\/mml:mrow>\n \n \n 2<\/mml:mtext>\n <\/mml:mrow>\n <\/mml:mrow>\n <\/mml:msub>\n <\/mml:mrow>\n <\/mml:math>\n <\/jats:alternatives>\n <\/jats:inline-formula>\n , and\n \n \n \n \n \n \n \n \n k<\/mml:mi>\n <\/mml:mrow>\n <\/mml:mrow>\n \n \n 3<\/mml:mtext>\n <\/mml:mrow>\n <\/mml:mrow>\n <\/mml:msub>\n <\/mml:mrow>\n <\/mml:math>\n <\/jats:alternatives>\n <\/jats:inline-formula>\n 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\u2019s 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.\n <\/jats:p>","DOI":"10.1371\/journal.pone.0357035","type":"journal-article","created":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T17:32:57Z","timestamp":1788197577000},"page":"e0357035","update-policy":"https:\/\/doi.org\/10.1371\/journal.pone.corrections_policy","source":"Crossref","is-referenced-by-count":0,"title":["Cryptanalysis of a controller-independent controlled bidirectional QSDC protocol"],"prefix":"10.1371","volume":"21","author":[{"given":"Chia-Wei","family":"Tsai","sequence":"first","affiliation":[{"name":"National Taichung University of Science and Technology","place":["Taichung, Taiwan"],"department":["Department of Computer Science and Information Engineering"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yao-Chung","family":"Chang","sequence":"additional","affiliation":[{"name":"National Taitung University","place":["Taitung, Taiwan"],"department":["Department of Computer Science and Information Engineering"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Andy","family":"Su","sequence":"additional","affiliation":[{"name":"Open University of Kaohsiung","place":["Kaohsiung, Taiwan"],"department":["Department of Industrial and Business Management"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"I-Chun","family":"Chen","sequence":"additional","affiliation":[{"name":"National Taitung University","place":["Taitung, Taiwan"],"department":["Department of Computer Science and Information Engineering"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0458-6435","authenticated-orcid":true,"given":"Ying-Hsun","family":"Lai","sequence":"additional","affiliation":[{"name":"National Taitung University","place":["Taitung, Taiwan"],"department":["Department of Computer Science and Information Engineering"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"340","published-online":{"date-parts":[[2026,8,31]]},"reference":[{"key":"pone.0357035.ref001","doi-asserted-by":"crossref","first-page":"7","DOI":"10.1016\/j.tcs.2014.05.025","article-title":"Quantum cryptography: public key distribution and coin tossing","volume":"560","author":"CH Bennett","year":"2014","journal-title":"Theor Comput Sci"},{"issue":"6","key":"pone.0357035.ref002","doi-asserted-by":"crossref","first-page":"661","DOI":"10.1103\/PhysRevLett.67.661","article-title":"Quantum cryptography based on Bell\u2019s theorem","volume":"67","author":"A Ekert","year":"1991","journal-title":"Phys Rev Lett"},{"issue":"3","key":"pone.0357035.ref003","doi-asserted-by":"crossref","first-page":"1829","DOI":"10.1103\/PhysRevA.59.1829","article-title":"Quantum secret sharing","volume":"59","author":"M Hillery","year":"1999","journal-title":"Phys Rev A"},{"issue":"18","key":"pone.0357035.ref004","doi-asserted-by":"crossref","first-page":"1149","DOI":"10.1049\/el:20045183","article-title":"Quantum key agreement protocol","volume":"40","author":"N Zhou","year":"2004","journal-title":"Electron Lett"},{"issue":"6","key":"pone.0357035.ref005","doi-asserted-by":"crossref","first-page":"062309","DOI":"10.1103\/PhysRevA.64.062309","article-title":"Quantum authentication of classical messages","volume":"64","author":"M Curty","year":"2001","journal-title":"Phys Rev A"},{"issue":"3","key":"pone.0357035.ref006","doi-asserted-by":"crossref","first-page":"032302","DOI":"10.1103\/PhysRevA.65.032302","article-title":"Theoretically efficient high-capacity quantum-key-distribution scheme","volume":"65","author":"GL Long","year":"2002","journal-title":"Phys Rev A"},{"issue":"4","key":"pone.0357035.ref007","doi-asserted-by":"crossref","first-page":"042317","DOI":"10.1103\/PhysRevA.68.042317","article-title":"Two-step quantum direct communication protocol using the Einstein-Podolsky-Rosen pair block","volume":"68","author":"FG Deng","year":"2003","journal-title":"Phys Rev A"},{"issue":"5","key":"pone.0357035.ref008","doi-asserted-by":"crossref","first-page":"052319","DOI":"10.1103\/PhysRevA.69.052319","article-title":"Secure direct communication with a quantum one-time pad","volume":"69","author":"FG Deng","year":"2004","journal-title":"Phys Rev A"},{"issue":"1","key":"pone.0357035.ref009","doi-asserted-by":"crossref","first-page":"6","DOI":"10.1016\/j.physleta.2004.06.009","article-title":"Quantum dialogue","volume":"328","author":"BA Nguyen","year":"2004","journal-title":"Phys Lett A"},{"issue":"5","key":"pone.0357035.ref010","doi-asserted-by":"crossref","first-page":"893","DOI":"10.1088\/1009-1963\/14\/5\/006","article-title":"Controlled quantum teleportation and secure direct communication","volume":"14","author":"T Gao","year":"2005","journal-title":"Chin Phys"},{"issue":"6","key":"pone.0357035.ref011","doi-asserted-by":"crossref","first-page":"1927","DOI":"10.1007\/s10773-019-04087-7","article-title":"A controller-independent quantum dialogue protocol with four-particle states","volume":"58","author":"L Zhang","year":"2019","journal-title":"Int J Theor Phys"},{"issue":"28","key":"pone.0357035.ref012","doi-asserted-by":"crossref","first-page":"2050228","DOI":"10.1142\/S0217732320502284","article-title":"Controlled bidirectional quantum secure direct communication protocol based on Grover\u2019s algorithm","volume":"35","author":"A Yin","year":"2020","journal-title":"Mod Phys Lett A"},{"issue":"10","key":"pone.0357035.ref013","doi-asserted-by":"crossref","first-page":"1032","DOI":"10.1007\/s40042-025-01532-1","article-title":"Controller independent controlled bidirectional quantum secure direct communication protocol based on four-particle cluster states","volume":"88","author":"M Cai","year":"2026","journal-title":"J Korean Phys Soc."},{"issue":"3","key":"pone.0357035.ref014","doi-asserted-by":"crossref","first-page":"168","DOI":"10.1109\/JSTQE.2014.2365585","article-title":"Risk analysis of trojan-horse attacks on practical quantum key distribution systems","volume":"21","author":"N Jain","year":"2015","journal-title":"IEEE J Select Topics Quantum Electron"},{"issue":"11","key":"pone.0357035.ref015","doi-asserted-by":"crossref","first-page":"1892","DOI":"10.1134\/S0030400X20110077","article-title":"Risk analysis of countermeasures against the Trojan-horse attacks on quantum key distribution systems in 1260\u20131650 nm spectral range","volume":"128","author":"AV Borisova","year":"2020","journal-title":"Opt Spectrosc"},{"issue":"12","key":"pone.0357035.ref016","doi-asserted-by":"crossref","first-page":"123030","DOI":"10.1088\/1367-2630\/16\/12\/123030","article-title":"Trojan-horse attacks threaten the security of practical quantum cryptography","volume":"16","author":"N Jain","year":"2014","journal-title":"New J Phys"},{"issue":"3","key":"pone.0357035.ref017","first-page":"031030","article-title":"Practical security bounds against the Trojan-horse attack in quantum key distribution","volume":"5","author":"M Lucamarini","year":"2015","journal-title":"Phys Rev X"},{"issue":"1","key":"pone.0357035.ref018","doi-asserted-by":"crossref","first-page":"1978","DOI":"10.1038\/s41598-017-01884-0","article-title":"Quantum key distribution with hacking countermeasures and long term field trial","volume":"7","author":"AR Dixon","year":"2017","journal-title":"Sci Rep"}],"container-title":["PLOS One"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/dx.plos.org\/10.1371\/journal.pone.0357035","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,8,31]],"date-time":"2026-08-31T17:33:00Z","timestamp":1788197580000},"score":1,"resource":{"primary":{"URL":"https:\/\/dx.plos.org\/10.1371\/journal.pone.0357035"}},"subtitle":[],"editor":[{"given":"Chun-Wei","family":"Yang","sequence":"first","affiliation":[{"name":"China Medical University Taiwan","place":["TAIWAN"]}],"role":[{"vocabulary":"crossref","role":"editor"}]}],"short-title":[],"issued":{"date-parts":[[2026,8,31]]},"references-count":18,"journal-issue":{"issue":"8","published-online":{"date-parts":[[2026,8,31]]}},"URL":"https:\/\/doi.org\/10.1371\/journal.pone.0357035","relation":{},"ISSN":["1932-6203"],"issn-type":[{"value":"1932-6203","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,8,31]]}}}